Encyclopedia of the Solar System [2 ed.] 0120885891, 9780120885893

Citation preview

CLBE001-ESS2E

November 15, 2006

16:55

i

CLBE001-ESS2E

November 15, 2006

16:55

View From the Martian Rover Spirit: October 26, 2006 This was Spirit's view on its 1000th Martian-day (sol) of what was planned to be a 90-sol mission. The robotic Spirit rover has stayed alive so long on Mars that it needed a place to wait out its second cold and dim Martian winter. Earth scientists selected Low Ridge Hill, a place with sufficient slant to give Spirit's solar panels enough sunlight to keep it powered and making scientific observations. From its winter haven, Spirit photographed the above panorama, which has been digitally altered to exaggerate colors. Spirit's track through the Martian hills can be seen at the center of the image. (Courtesy of NASA)

ii

CLBE001-ESS2E

November 15, 2006

16:55

o a I

iii

CLBE001-ESS2E

November 15, 2006

16:55

Cover: Images courtesy of NASA/JPL-Caltech and Planet Art Academic Press is an imprint of Elsevier 525 B Street, Suite 1900, San Diego, CA 92101-4495, USA 84 Theobald’s Road, London WC1X 8RR, UK Radarweg 29, PO BOX 211, 1000 AE Amsterdam, The Netherlands 30 Corporate Drive, Suite 400, Burlington, MA 01803, USA First edition 1999 Second edition 2007 Copyright  2007 Elsevier Inc. All rights reserved C

No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher Permissions may be sought directly from Elsevier’s Science & Technology Rights Department in Oxford, UK: phone (+44) (0) 1865 843830; fax (+44) (0) 1865 853333; email: [email protected]. Alternatively you can submit your request online by visiting the Elsevier web site at http://elsevier.com/locate/permissions, and selecting Obtaining permission to use Elsevier material Notice No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Catalog Number: 2006937972 ISBN-13: 978-0-12-088589-3 ISBN-10: 0-12-088589-1 For information on all Academic Press publications visit our website at books.elsevier.com Printed and bound in Canada 07

08

09

10

10

9

8

7

6

5

4

3

2

1

iv

CLBE001-ESS2E

November 15, 2006

100-C

25-C

50-C

75-C

16:55

C+M

50-C+M

C+Y

50-C+Y

M+Y

50-M+Y

100-M

25-M

50-M

75-M

100-Y

25-Y

50-Y

75-Y

100-K

25-K

25-19-19

50-K

50-40-40

75-K

75-64-64

Contents

CONTRIBUTORS ix ABOUT THE EDITORS xiii FOREWORD xv PREFACE TO THE SECOND EDITION xvii PREFACE TO THE FIRST EDITION xix

CHAPTER 9

Earth as a Planet: Atmosphere and Oceans 169 Timothy E. Dowling and Adam P. Showman CHAPTER 10

CHAPTER 1

The Solar System and its Place in the Galaxy 1

Earth as a Planet: Surface and Interior 189 David C. Pieri and Adam M. Dziewonski

Paul R. Weissman

CHAPTER 11

CHAPTER 2

Janet G. Luhmann and Stanley C. Solomon

Alex N. Halliday and John E. Chambers

CHAPTER 12

CHAPTER 3

Stuart Ross Taylor

David Leverington

CHAPTER 13

CHAPTER 4

Michael E. Lipschutz and Ludolf Schultz

Markus J. Aschwanden

CHAPTER 14

CHAPTER 5

Lucy A. McFadden and Richard P. Binzel

John T. Gosling

CHAPTER 15

The Sun–Earth Connection 213

The Origin of the Solar System 29 The Moon 227

A History of Solar System Studies 53 Meteorites 251

The Sun 71 Near-Earth Objects 283

The Solar Wind 99 CHAPTER 6

Mercury 117

Mars Atmosphere: History and Surface Interactions 301 David C. Catling and Conway Leovy

Robert G. Strom CHAPTER 16 CHAPTER 7

Venus: Atmosphere 139

Mars: Surface and Interior 315 Michael H. Carr

Donald M. Hunten CHAPTER 17 CHAPTER 8

Venus: Surface and Interior 149

Mars: Landing Site Geology, Mineralogy, and Geochemistry 331

Suzanne E. Smrekar and Ellen R. Stofan

Matthew P. Golombek and Harry Y. McSween, Jr.

C 2007 by Academic Press. All rights of reproduction in any form reserved. Encyclopedia of the Solar System 2e 

v

CLBE001-ESS2E

November 15, 2006

16:55

vi Contents CHAPTER 18

CHAPTER 32

Daniel T. Britt, Guy Colsolmagno, and Larry Lebofsky

Alessandro Morbidelli and Harold F. Levison

CHAPTER 19

CHAPTER 33

Main-Belt Asteroids 349

Planetary Satellites 365 Bonnie J. Buratti and Peter C. Thomas CHAPTER 20

Kuiper Belt: Dynamics 589

Kuiper Belt Objects: Physical Studies 605 Stephen C. Tegler

Atmospheres of the Giant Planets 383

CHAPTER 34

Robert A. West

¨ Eberhard Grun

CHAPTER 21

Interiors of the Giant Planets 403 Mark S. Marley and Jonathan J. Fortney

Solar System Dust 621 CHAPTER 35

X-Rays in the Solar System 637 Anil Bhardwaj and Carey M. Lisse

CHAPTER 22

Io: The Volcanic Moon 419 Rosaly M. C. Lopes CHAPTER 23

Europa 431 Louise M. Prockter and Robert T. Pappalardo CHAPTER 24

Ganymede and Callisto 449 Geoffrey Collins and Torrence V. Johnson

CHAPTER 36

The Solar System at Ultraviolet Wavelengths 659 Amanda R. Hendrix, Robert M. Nelson, and Deborah L. Domingue CHAPTER 37

Infrared Views of the Solar System from Space 681 Mark V. Sykes

CHAPTER 25

Titan 467 Athena Coustenis

CHAPTER 38

The Solar System at Radio Wavelengths 695 Imke de Pater and William S. Kurth

CHAPTER 26

Triton 483 William B. McKinnon and Randolph L. Kirk CHAPTER 27

Planetary Rings 503 Carolyn C. Porco and Douglas P. Hamilton CHAPTER 28

Planetary Magnetospheres 519 Margaret Galland Kivelson and Fran Bagenal

CHAPTER 39

New Generation Ground-Based Optical/ Infrared Telescopes 719 Alan T. Tokunaga and Robert Jedicke CHAPTER 40

Planetary Radar 735 Steven J. Ostro CHAPTER 41

CHAPTER 29

Remote Chemical Sensing Using Nuclear Spectroscopy 765

S. Alan Stern

Thomas H. Prettyman

CHAPTER 30

CHAPTER 42

Pluto 541

Physics and Chemistry of Comets 557 John C. Brandt

Solar System Dynamics: Regular and Chaotic Motion 787 Jack J. Lissauer and Carl D. Murray

CHAPTER 31

Comet Populations and Cometary Dynamics 575

CHAPTER 43

Harold F. Levison and Luke Dones

Richard A. F. Grieve, Mark J. Cintala and, Roald Tagle

Planetary Impacts 813

CLBE001-ESS2E

November 15, 2006

16:55

Contents

CHAPTER 44

CHAPTER 47

Lionel Wilson

Michael Endl and William D. Cochran

CHAPTER 45

APPENDIX

903

GLOSSARY

919

Planetary Volcanism 829

Astrobiology 849 Christopher P. McKay and Wanda L. Davis CHAPTER 46

Planetary Exploration Missions 869 James D. Burke

Extrasolar Planets 887

INDEX

939

vii

CLBE001-ESS2E

November 15, 2006

16:55

viii

CLBE001-ESS2E

November 15, 2006

100-C

25-C

50-C

75-C

16:55

C+M

50-C+M

C+Y

50-C+Y

M+Y

50-M+Y

100-M

25-M

50-M

75-M

100-Y

25-Y

50-Y

75-Y

100-K

25-K

25-19-19

50-K

50-40-40

75-K

75-64-64

Contributors

Markus J. Aschwanden The Sun Lockheed-Martin ATC, Solar and Astrophysics Laboratory Palo Alto, California Fran Bagenal Planetary Magnetospheres University of Colorado Boulder, Colorado Anil Bhardwaj X-Rays in the Solar System Space Physics Laboratory Vikram Sarabhai Space Centre Trivandrum, India Richard P. Binzel Near-Earth Objects Massachusetts Institute of Technology Cambridge, Massachusetts John Brandt Physics and Chemistry of Comets Institute for Astrophysics, Department of Physics and Astronomy University of New Mexico Albuquerque, New Mexico Daniel T. Britt Main-Belt Asteroids Department of Physics University of Central Florida Orlando, Florida Bonnie J. Buratti Planetary Satellites Jet Propulsion Laboratory California Institute of Technology Pasadena, California

James D. Burke Planetary Exploration Missions Jet Propulsion Laboratory California Institute of Technology Pasadena, California Michael H. Carr Mars: Surface and Interior U.S. Geological Survey Menlo Park, California David C. Catling Mars Atmosphere: History and Surface Interaction University of Washington Seattle, Washington John E. Chambers The Origin of the Solar System Carnegie Institution of Washington Washington, D.C. Mark J. Cintala Planetary Impacts NASA Johnson Space Center Houston, Texas William D. Cochran Extra-Solar Planets Department of Astronomy, McDonald Observatory University of Texas Austin, Austin, Texas Geoffrey Collins Ganymede and Callisto Wheaton College Norton, Massachusetts Athena Coustenis Titan Observatoire de Paris-Meudon Meudon, France

C 2007 by Academic Press. All rights of reproduction in any form reserved. Encyclopedia of the Solar System 2e 

ix

CLBE001-ESS2E

November 15, 2006

16:55

x Contributors Guy Colsolmagno Main-Belt Asteroids Specola Vaticana Castel Gandolfo, Italy

Richard A. F. Grieve Planetary Impacts Natural Resources Canada Ottawa, Canada

Wanda L. Davis Astrobiology NASA Ames Research Center, Space Science Division Moffett Field, California

Eberhard Grun ¨ Solar System Dust Max Planck Institute of Nuclear Physics Heidelberg, Germany

Imke de Pater The Solar System at Radio Wavelengths Department of Astronomy University of California, Berkeley Berkeley, California

Alex N. Halliday The Origin of the Solar System Department of Earth Sciences, University of Oxford Oxford, United Kingdom

Luke Dones Comet Populations and Cometary Dynamics Southwest Research Institute Boulder, Colorado Deborah L. Domingue The Solar System at Ultraviolet Wavelengths Johns Hopkins University Applied Physics Laboratory Laurel, Maryland Timothy E. Dowling Earth as a Planet: Atmosphere and Oceans Department of Mechanical Engineering University of Louisville Louisville, Kentucky Adam M. Dziewonski Earth as a Planet: Surface and Interior Harvard University Cambridge, Massachusetts Michael Endl Extra-Solar Planets Department of Astronomy, McDonald Observatory University of Texas, Austin Austin, Texas Jonathan Fortney Interiors of the Giant Planets NASA Ames Research Center Moffett Field, California Matthew P. Golombek Mars: Landing Site Geology, Mineralogy, and Geochemistry Jet Propulsion Laboratory Mars Exploration Program Pasadena, California John T. Gosling The Solar Wind Laboratory for Atmospheric and Space Physics University of Colorado Boulder, Colorado

Douglas P. Hamilton Planetary Rings Department of Astronomy University of Maryland College Park, Maryland Amanda R. Hendrix The Solar System at Ultraviolet Wavelengths Jet Propulsion Laboratory California Institute of Technology Pasadena, California Donald M. Hunten Venus: Atmosphere Department of Planetary Sciences Lunar and Planetary Laboratory, University of Arizona Tucson, Arizona Robert Jedicke New Generation Optical/Infrared Telescopes Institute for Astronomy University of Hawaii Honolulu, Hawaii Torrence V. Johnson Ganymede and Calllisto Jet Propulsion Laboratory California Institute of Technology Pasadena, California Randolph L. Kirk Triton U. S. Geological Survey Flagstaff, Arizona William S. Kurth The Solar System at Radio Wavelengths University of Iowa Iowa City, Iowa Margaret Galland Kivelson Planetary Magnetospheres Department of Earth and Space Sciences University of California, Los Angeles Los Angeles, California

CLBE001-ESS2E

November 15, 2006

16:55

Contributors

Larry Lebofsky Main-Belt Asteroids Lunar and Planetary Laboratory, University of Arizona Tucson, Arizona

Christopher P. McKay Astrobiology NASA Ames Research Center, Space Science Division Moffett Field, California

Conway Leovy Mars: Atmosphere: History and Surface Interaction University of Washington Seattle, Washington

William B. McKinnon Triton Department of Earth and Planetary Sciences Washington University St. Louis, Missouri

David Leverington A History of Solar System Studies Author and Consultant Stoke Lacy, Herefordshire, United Kingdom Harold F. Levison Comet Populations and Cometary Dynamics Southwest Research Institute Boulder, Colorado Michael E. Lipschutz Meteorites Department of Chemistry, Purdue University West Lafayette, Indiana Jack J. Lissauer Solar System Dynamics: Regular and Chaotic Motion Space Science Division NASA Ames Research Center Moffett Field, California Carey M. Lisse X-Rays in the Solar System Johns Hopkins University Applied Physics Laboratory Laurel, Maryland Rosaly M. C. Lopes Io: The Volcanic Moon Jet Propulsion Laboratory California Institute of Technology Pasadena, California

Harry Y. McSween, Jr. Mars: Landing Site Geology, Mineralogy, and Geochemistry University of Tennessee, Knoxville Knoxville, Tennessee Alessandro Morbidelli Kuiper Belt: Dynamics ¨ d’Azur Observatoirs de La Cote Nice, France Carl D. Murray Solar System Dynamics: Regular and Chaotic Motion Queen Mary, University of London London, United Kingdom Robert M. Nelson The Solar System at Ultraviolet Wavelengths Jet Propulsion Laboratory California Institute of Technology Pasadena, California Steven J. Ostro Planetary Radar Jet Propulsion Laboratory California Institute of Technology Pasadena, California Robert T. Pappalardo Europa University of Colorado Boulder, Colorado

Janet G. Luhmann The Sun-Earth Connection Space Sciences Laboratory University of California, Berkeley Berkeley, California

David C. Pieri Earth as a Planet: Surface and Interior Jet Propulsion Laboratory California Institute of Technology Pasadena, California

Mark S. Marley Interiors of the Giant Planets NASA Ames Research Center Moffett Field, California

Carolyn C. Porco Planetary Rings Space Science Institute Boulder, Colorado

Lucy A. McFadden Near-Earth Objects Department of Astronomy University of Maryland College Park, Maryland

Thomas H. Prettyman Remote Elemental Sensing Using Nuclear Spectroscopy Los Alamos National Laboratory, Space and Atmospheric Sciences Los Alamos, New Mexico

xi

CLBE001-ESS2E

November 15, 2006

16:55

xii Contributors Louise M. Prockter Europa Planetary Exploration Group Johns Hopkins University Applied Physics Laboratory Laurel, Maryland Ludolf Schultz Meterorites ¨ Chemie Max-Plank-Institut fur Mainz, Germany Adam Showman Earth as a Planet: Atmosphere and Oceans University of Arizona Tucson, Arizona Suzanne E. Smrekar Venus: Surface and Interior Earth and Space Sciences Division Jet Propulsion Laboratory California Institute of Technology Pasadena, California Stanley C. Solomon High Altitude Observatory National Center for Atmospheric Research Boulder, Colorado S. Alan Stern Pluto Southwest Research Institute Boulder, Colorado Ellen R. Stofan Venus: Surface and Interior Proxemy Research Rectortown, Maryland Robert G. Strom Mercury Lunar and Planetary Laboratory Department of Planetary Sciences University of Arizona Tucson, Arizona Mark V. Sykes Infrared Views of the Solar System from Space Planetary Science Institute Tucson, Arizona

Roald Tagle Planetary Impacts Humboldt University Berlin, Germany Stuart Ross Taylor The Moon Department of Earth and Marine Sciences, Emeritus Australian National University Canberra, Australia Stephen C. Tegler Kuiper Belt Objects: Physical Studies Department of Physics and Astronomy Northern Arizona University Flagstaff, Arizona Peter C. Thomas Planetary Satellites Cornell University Ithaca, New York Alan T. Tokunaga New Generation Optical/Infrared Telescopes Institute for Astronomy University of Hawaii Honolulu, Hawaii Paul R. Weissman The Solar System and its Place in the Galaxy Jet Propulsion Laboratory California Institute of Technology Pasadena, California Robert A. West Atmospheres of the Giant Planets Jet Propulsion Laboratory California Institute of Technology Pasadena, California Lionel Wilson Planetary Volcanism Planetary Science Research Group Institute of Environmental and Natural Sciences Environmental Science Department Lancaster, United Kingdom

CLBE001-ESS2E

November 15, 2006

16:55

About the Editors

Lucy-Ann McFadden is a planetary scientist at the University of Maryland. She was the founding director of the College Park Scholars Program, Science, Discovery, and the Universe. She has published over 75 articles in refereed journals and has been co-investigator on NASA’s NEAR, Deep Impact, and Dawn missions, exploring asteroids and comets. McFadden has served on committees on solar system exploration for the National Academy of Sciences and on the editorial board of Icarus.

Paul R. Weissman is a Senior Research Scientist at the Jet Propulsion Laboratory, specializing in comets. He is the author of over 100 scientific papers and 30 popular articles. He is also co-author, along with Alan Harris, of a children’s book on the Voyager mission. Dr. Weissman received his doctorate in planetary and space physics from the University of California, Los Angeles. His work includes both theoretical and observational studies of comets, investigating their orbital motion, their physical make-up, and the threat they pose due to possible impacts on the Earth. Dr. Weissman is an Interdisciplinary Scientist on ESA’s Rosetta mission to comet Churyumov-Gerasimenko.

Torrence V. Johnson is a specialist on icy satellites in the solar system. He has written over 130 papers for scientific journals. He received a Ph.D. in planetary science from the California Institute of Technology and is now the Chief Scientist for Solar System Explorations at the Jet Propulsion Laboratory. Johnson was the Project Scientist for the Galileo mission and is currently an investigator on the Cassini mission. He is the recipient of two NASA Exceptional Scientific Achievement Medals and the NASA Outstanding Leadership Medal and has an honorary doctorate from the University of Padua, where Galileo made his first observations of the solar system.

xiii

CLBE001-ESS2E

November 15, 2006

16:55

xiv

CLBE001-ESS2E

November 15, 2006

100-C

25-C

50-C

75-C

16:55

C+M

50-C+M

C+Y

50-C+Y

M+Y

50-M+Y

100-M

25-M

50-M

75-M

100-Y

25-Y

50-Y

75-Y

100-K

25-K

25-19-19

50-K

50-40-40

75-K

75-64-64

Foreword

The solar system has become humankind’s new backyard. It is the playground of robotic planetary spacecraft that have surveyed just about every corner of this vast expanse in space. Nowadays, every schoolchild knows what even the farthest planets look like. Fifty years ago, these places could only be imagined, and traveling to them was the realm of fiction. In just this short time in the long history of the human species we have leapt off the surface of our home planet and sent robotic extensions of our eyes, ears, noses, arms, and legs to the far reaches of the solar system and beyond. In the early days of the 20th century, we were using airplanes to extend our reach to the last unexplored surface regions of our own planet. Now 100 years later, at the beginning of the 21st century, we are using spacecraft to extend our reach from the innermost planet Mercury to the outmost planet Neptune, and we have a spacecraft on the way to Pluto and the Kuiper Belt. Today, there are telescopes beyond imagination 100 or even 50 years ago that can image Pluto and detect planets around other stars! Now, Sol’s planets can say “we are not alone”; there are objects just like us elsewhere in the universe. As humanity’s space technology improves, perhaps in the next 100 years or so human beings also may be able to say “we are not alone.” When I was a kid 50 years ago, I was thrilled by the paintings of Chesley Bonestell and others who put their imagination on canvas to show us what it might be like “out there.” Werner Von Braun’s Collier’s magazine articles of 1952–1954 superbly illustrated how we would go to the Moon and Mars using new rocket technologies. Reading those fabulous articles crystallized thoughts in my young mind about what to do with my life. I wanted to be part of the adventure to find out what these places were like. Not so long after the Collier’s articles appeared, we did go to the Moon, and pretty much as illustrated, although perhaps not in such a grand manner. We have not sent humans to Mars— at least we haven’t yet—but we have sent our robots to Mars and to just about every other place in the solar system as well.

This book is filled with the knowledge about our solar system that resulted from all this exploration, whether by spacecraft or by telescopes both in space and earth-bound. It could not have been written 50 years ago as almost everything in this Encyclopedia was unknown back then. All of this new knowledge is based on discoveries made in the interim by scientist-explorers who have followed their inborn human imperative to explore and to understand. Many old mysteries, misunderstandings, and fears that existed 50 years ago about what lay beyond the Earth have been eliminated. We now know the major features of the landscape in our cosmic backyard and can look forward to the adventure, excitement, and new knowledge that will result from more in-depth exploration by today’s spacecraft, such as those actually exploring the surface of these faraway places, including the Huygens Titan lander and the Mars Exploration rovers, doing things that were unimaginable before the Space Age began. The Encyclopedia of the Solar System is filled with images, illustrations, and charts to aid in understanding. Every object in the solar system is covered by at least one chapter. Other chapters are devoted to the relationships among the objects in the solar system and with the galaxy beyond. The processes that operate on solar system objects, in their atmospheres, on their surfaces, in their interiors, and interactions with space itself are all described in detail. There are chapters on how we explore and learn about the solar system and about the investigations used to make new discoveries. And there are chapters on the history of solar system exploration and the missions that have carried out this enterprise. All written by an international set of world-class scientists using rigorous yet easy-to-understand prose. Everything you want to know about the solar system is here. This is your highway to the solar system. It is as much fun exploring this Encyclopedia as all the exploration it took to get the information that it contains. Let your fingers be the spacecraft as you thumb through this book visiting all the planets, moons and other small objects in the solar

C 2007 by Academic Press. All rights of reproduction in any form reserved. Encyclopedia of the Solar System 2e 

xv

CLBE001-ESS2E

November 15, 2006

16:55

xvi Foreword system. Experience what it is like to look at our solar system with ultraviolet eyes, infrared eyes, radio eyes, and radar eyes. It has been seven years since the first edition. The exploration of space has continued at a rapid pace since then, and many missions have flown in the interim. New discoveries are being made all the time. This second edition will catch you up on all that has happened since the first, including several new chapters based on information from our latest missions. I invite you to enjoy a virtual exploration of the solar system by flipping through the pages in this volume. This

book deserves a place in any academic setting and wherever there is a need to understand the cosmos beyond our home planet. It is the perfect solar system reference book, lavishly illustrated and well written. The editors and authors have done a magnificent job. We live in a wonderful time of exploration and discovery. Here is your window to the adventure. WESLEY T. HUNTRESS Geophysical Laboratory Carnegie Institution of Washington Washington, D.C.

CLBE001-ESS2E

November 15, 2006

100-C

25-C

50-C

75-C

16:55

C+M

50-C+M

C+Y

50-C+Y

M+Y

50-M+Y

100-M

25-M

50-M

75-M

100-Y

25-Y

50-Y

75-Y

100-K

25-K

25-19-19

50-K

50-40-40

75-K

75-64-64

Preface to the Second Edition

“Knowledge is not static. Science is a process, not a product. Some of what is presented in this volume will inevitably be out of date by the time you read it.” From the Preface to the first edition, 1999.

Written on the eve of the new millennium, the statement above was our acknowledgment that one cannot simply “freeze” our knowledge of the solar system we inhabit, box it up, and display it like a collection of rare butterflies in a 19th century “cabinet of curiosities.” Rather our goal was to provide our readers with an introduction to understanding the solar system as an interacting system, shaped by its place in the universe, its history, and the chemical and physical processes that operate from the extreme pressures and temperatures of the Sun’s interior to the frigid realm of the Oort cloud. We aimed to provide a work that was useful to students, professionals, and serious amateurs at a variety of levels, containing both detailed technical material and clear expositions of general principles and findings. With the help of our extremely talented colleagues who agreed to author the chapters, we humbly believe we achieved at least some of these ambitious goals. How to decide when to update a work whose subject matter is in a constant, exuberant state of flux? Difficult. Waiting for our knowledge of the solar system to be “complete” was deemed impractical, since our thesis is that this will never happen. Picking an anniversary date (30 years since this, or 50 years after that) seemed arbitrary. We compromised on taking an informal inventory of major events and advances in knowledge since that last edition whenever we got together at conferences and meetings. When we realized that virtually every chapter in the first edition needed major revisions and that new chapters would be called for to properly reflect new material, we decided to undertake the task of preparing a second edition with the encouragement and help from our friends and colleagues at Academic Press. Consider how much has happened in the relatively short time since the first edition, published in 1999. An international fleet of spacecraft is now in place around Mars and two

rovers are roaming its surface, with more to follow. Galileo ended its mission of discovery at Jupiter with a spectacular fiery plunge into the giant planet’s atmosphere. We have reached out and touched one comet with the Deep Impact mission and brought back precious fragments from another with Stardust. Cassini is sending back incredible data from the Saturn system and the Huygens probe descended to the surface of the giant, smog-shrouded moon Titan, revealing an eerily Earth-like landscape carved by methane rains. NEAR and Hayabusa each orbited and then touched down on the surface of near-Earth asteroids Eros and Itokawa, respectively. Scientists on the Earth are continually improving the capabilities of telescopes and instruments, while laboratory studies and advances in theory improve our ability to synthesize and understand the vast amounts of new data being returned. What you have before you is far more than a minor tweak to add a few new items to a table here or a figure there. It is a complete revamping of the Encyclopedia to reflect the solar system as we understand it today. We have attempted to capture the excitement and breadth of all this new material in the layout of the new edition. The authors of existing chapters were eager to update them to reflect our current state of knowledge, and many new authors have been added to bring fresh perspectives to the work. To all of those authors who contributed to the second edition and to the army of reviewers who carefully checked each chapter, we offer our sincere thanks and gratitude. The organization of the chapters remains based on the logic of combining individual surveys of objects and planets, reviews of common elements and processes, and discussions of the latest techniques used to observe the solar system. Within this context you will find old acquaintances and many new friends. The sections on our own home planet have been revised and a new chapter on the Sun-Earth connection added to reflect our growing understanding of the intimate relationship between our star and conditions here on Earth. The treatment of Mars has been updated and a new chapter included incorporating the knowledge gained

C 2007 by Academic Press. All rights of reproduction in any form reserved. Encyclopedia of the Solar System 2e 

xvii

CLBE001-ESS2E

November 15, 2006

16:55

xviii Preface to the Second Edition from the rovers Spirit and Opportunity and new orbital exploration of the red planet. Galileo’s remarkable discovery of evidence for subsurface oceans on the icy Galilean satellites is treated fully in new chapters devoted to Europa and to Ganymede and Callisto. New information from the Deep Impact mission and the Stardust sample return is included as well. We continue to find out more and more about the denizens of the most distant reaches of the solar system, and have expanded the discussion of the Kuiper belt with a new chapter on physical properties. The area of observational techniques and instrumentation has been expanded to include chapters covering the X-ray portion of the spectrum, new generation telescopes, and remote chemical analysis. Finally, nothing exemplifies the dynamic character of our knowledge than the area of extra-solar planets, which completes the volume. In the first edition the chapter on extra-solar planets contained a section entitled, “What is a Planet?” which concluded with this: “The reader is cautioned that these definitions are not uniformly accepted.” The chapter included a table of nineteen objects cautiously labeled “Discovered Substellar Companions.” As this work goes to press, more than 200 extra-solar planets are known, many in multi-planet systems, with more being discovered every day. And at the 2006 General Assembly of the International Astronomical Union, the question of the definition of “planet” was still being hotly debated. The current IAU definition is discussed in the introductory chapter by one of us (PRW) and other views concerning the status of Pluto may be found in the chapter on that body. In addition to the energy and hard work of all of our authors, this edition of the Encyclopedia is greatly enhanced by the vision and talents of our friends at Academic Press. Specifically, we wish to thank Jennifer Hele, ´ our Publishing Editor, who oversaw the project and learned the hard truth that herding scientists and herding cats are one and the same thing. Jennifer was the task master who made us realize that we could not just keep adding exciting new results to the volume, but one day had to stop and actually publish it. Francine Ribeau was our very able Marketing Manager and Deena Burgess, our Publishing Services Manager in the U.K., handled all of the last minute loose ends and made

certain that the book was published without a hitch yet on a very tight schedule. Frank Cynar was our Publishing Editor for the first edition and for the beginning of the second, assisted by Gail Rice who was the Developmental Editor early on for the second edition. At Techbooks, Frank Scott was the Project Manager who oversaw all the final chapter and figure submissions and proof checking. Finally, also at Techbooks, was Carol Field, our Developmental Editor, simply known as Fabulous Carol, who seemed to work 30-hour days for more than a year to see the volume through to fruition, while still finding time to get married in the midst of it all. This Encyclopedia would not exist without the tireless efforts of all of these extremely talented and dedicated individuals. To all of them we offer our eternal thanks. Extensive use of color and new graphic designs have made the Encyclopedia even more beautiful and enhanced its readability while at the same time allowing the authors to display their information more effectively. The Encyclopedia you have before you is the result of all these efforts and we sincerely hope you will enjoy reading it as much as we enjoyed the process of compiling it. Which brings us back to the quotation at the start of the Preface. We sincerely hope that this edition of the Encyclopedia will indeed also be out of date by the time you read it. The New Horizons spacecraft is on its way to the Pluto/Charon system, MESSENGER is on its way to Mercury, Rosetta is en route to a rendezvous with periodic comet Churyumov-Gerasimenko, new spacecraft are probing Venus and Mars, many nations are refocusing on exploration of the Moon, plans are being laid to study the deep interior of Jupiter and return to Europa, while the results from the Saturn system, Titan and Enceladus, have sparked a multitude of ideas for future exploration. We hope this Encyclopedia will help you, the reader, appreciate and enjoy this on-going process of discovery and change as much as we do. Lucy-Ann McFadden Paul R. Weissman Torrence V. Johnson November 1, 2006

CLBE001-ESS2E

November 15, 2006

100-C

25-C

50-C

75-C

16:55

C+M

50-C+M

C+Y

50-C+Y

M+Y

50-M+Y

100-M

25-M

50-M

75-M

100-Y

25-Y

50-Y

75-Y

100-K

25-K

25-19-19

50-K

50-40-40

75-K

75-64-64

Preface to the First Edition

“This is what hydrogen atoms can accomplish after four billion years of evolution.” —CARL SAGAN, COSMOS, 1981

The quote above comes from the final episode of the public television series “Cosmos,” which was created by Carl Sagan and several colleagues in 1981. Carl was describing the incredible accomplishments of the scientists and engineers who made the Voyager 1 and 2 missions to Jupiter and Saturn possible. But he just as easily could have been describing the chapters in this book. This Encyclopedia is the product of the many scientists, engineers, technicians, and managers who produced the spacecraft missions which have explored our solar system over the past four decades. It is our attempt to provide to you, the reader, a comprehensive view of all we have learned in that 40 years of exploration and discovery. But we cannot take credit for this work. It is the product of the efforts of thousands of very talented and hard-working individuals in a score of countries who have contributed to that exploration. And it includes not only those involved directly in space missions, but also the many ground-based telescopic observers (both professional and amateur), laboratory scientists, theorists, and computer specialists who have contributed to creating that body of knowledge called solar system science. To all of these individuals, we say thank you. Our goal in creating this Encyclopedia is to provide an integrated view of all we have learned about the solar system, at a level that is useful to the advanced amateur or student, to teachers, to non-solar system astronomers, and to professionals in other scientific and technical fields. What we present here is an introduction to the many different specialties that constitute solar system science, written by the world’s leading experts in each field. A reader can start at the beginning and follow the course we have laid out, or delve into the volume at almost any point and pursue his or her own personal interests. If the reader wishes to go further, the lists of recommended reading at the end of each article

provide the next step in learning about any of the subjects covered. Our approach is to have the reader understand the solar system not only as a collection of individual and distinct bodies, but also as an integrated, interacting system, shaped by its initial conditions and by a variety of physical and chemical processes. The Encyclopedia begins with an overview chapter which describes the general features of the solar system and its relationship to the Milky Way galaxy, followed by a chapter on the origin of the system. Next we proceed from the Sun outward. We present the terrestrial planets (Mercury, Venus, Earth, Mars) individually with separate chapters on their atmospheres and satellites (where they exist). For the giant planets (Jupiter, Saturn, Uranus, Neptune) our focus shifts to common areas of scientific knowledge: atmospheres, interiors, satellites, rings, and magnetospheres. In addition, we have singled out three amazing satellites for individual chapters: Io, Titan, and Triton. Next is a chapter on the planetary system’s most distant outpost, Pluto, and its icy satellite, Charon. From there we move into discussing the small bodies of the solar system: comets, asteroids, meteorites, and dust. Having looked at the individual members of the solar system, we next describe the different view of those members at a variety of wavelengths outside the normal visual region. From there we consider the important processes that have played such an important role in the formation and evolution of the system: celestial dynamics, chaos, impacts, and volcanism. Last, we look at three topics which are as much in our future as in our past: life on other planets, space exploration missions, and the search for planets around other stars. A volume like this one does not come into being without the efforts of a great number of very dedicated people. We express our appreciation to the more than 50 colleagues who wrote chapters, sharing their expertise with you, the reader. In addition to providing chapters that captured the excitement of their individual fields, the authors have endured revisions, rewrites, endless questions, and unforeseen delays.

C 2007 by Academic Press. All rights of reproduction in any form reserved. Encyclopedia of the Solar System 2e 

xix

CLBE001-ESS2E

November 15, 2006

16:55

xx Preface to the First Edition For all of these we offer our humble apologies. To ensure the quality and accuracy of each contribution, at least two independent reviewers critiqued each chapter. The peer review process maintains its integrity through the anonymity of the reviewers. Although we cannot acknowledge them by name, we thank all the reviewers for their time and their conscientious efforts. We are also deeply indebted to the team at Academic Press. Our executive editor, Frank Cynar, worked tirelessly with us to conceptualize and execute the encyclopedia, while allowing us to maintain the highest intellectual and scientific standards. We thank him for his patience and for his perseverance in seeing this volume through to completion. Frank’s assistants, Daniela Dell’Orco, Della Grayson, Linda McAleer, Cathleen Ryan, and Suzanne Walters, kept the entire process moving and attended to the myriad of details and questions that arise with such a large and complex volume. Advice and valuable guidance came from Academic Press’ director of major reference works, Chris Morris. Lori

Asbury masterfully oversaw the production and copy editing. To all of the people at Academic Press, we give our sincere thanks. Knowledge is not static. Science is a process, not a product. Some of what is presented in this volume will inevitably be out of date by the time you read it. New discoveries seem to come every day from our colleagues using Earth-based and orbiting telescopes, and from the fiotilla of new small spacecraft that are out there adding to our store of knowledge about the solar system. In this spirit we hope that you, the reader, will benefit from the knowledge and understanding compiled in the following pages. The new millennium will surely add to the legacy presented herein, and we will all be the better for it. Enjoy, wonder, and keep watching the sky. Paul R. Weissman Lucy-Ann McFadden Torrence V. Johnson

CLBE001-ESS2E

November 8, 2006

100-C

25-C

50-C

75-C

11:17

C+M

50-C+M

C+Y

50-C+Y

M+Y

50-M+Y

100-M

25-M

50-M

75-M

100-Y

25-Y

50-Y

75-Y

100-K

25-K

25-19-19

50-K

50-40-40

75-K

75-64-64

The Solar System and Its Place in the Galaxy Paul R. Weissman Jet Propulsion Laboratory California Institute of Technology Pasadena, California

CHAPTER

1

1. Introduction 2. The Definition of a Planet 3. The Architecture of the Solar System

5. The Solar System’s Place in the Galaxy 6. The Fate of the Solar System 7. Concluding Remarks

4. The Origin of the Solar System

Bibliography

1. Introduction The origins of modern astronomy lie with the study of our solar system. When ancient humans first gazed at the skies, they recognized the same patterns of fixed stars rotating over their heads each night. They identified these fixed patterns, now called constellations, with familiar objects or animals, or stories from their mythologies and their culture. But along with the fixed stars, there were a few bright points of light that moved each night, slowly following similar paths through a belt of constellations around the sky (the Sun and Moon also appeared to move through the same belt of constellations). These wandering objects were the planets of our solar system. Indeed, the name “planet” derives from the Latin planeta, meaning wanderer. The ancients recognized five planets that they could see with their naked eyes. We now know that the solar system consists of eight planets, at least three dwarf planets, plus a myriad of smaller objects: satellites, asteroids, comets, rings, and dust. Discoveries of new objects and new classes of objects are continuing even today. Thus, our view of the solar system is constantly changing and evolving as new data and new theories to explain (or anticipate) the data become available. The solar system we see today is the result of the complex interaction of physical, chemical, and dynamical processes that have shaped the planets and other bodies. By studying

each of the planets and other bodies individually as well as collectively, we seek to gain an understanding of those processes and the steps that led to the current solar system. Many of those processes operated most intensely early in the solar system’s history, as the Sun and planets formed from an interstellar cloud of dust and gas, 4.56 billion years ago. The first billion years of the solar system’s history was a violent period as the planets cleared their orbital zones of much of the leftover debris from the process of planet formation, flinging small bodies into planet-crossing, and often planet-impacting, orbits or out to interstellar space. In comparison, the present-day solar system is a much quieter place, though many of these processes continue today on a lesser scale. Our knowledge of the solar system has exploded in the past four decades as interplanetary exploration spacecraft have provided close-up views of all of the planets, as well as of a diverse collection of satellites, asteroids, and comets. Earth-orbiting telescopes have provided an unprecedented view of the solar system, often at wavelengths not accessible from the Earth’s surface. Ground-based observations have also continued to produce exciting new discoveries through the application of a variety of new technologies such as charge-coupled device (CCD) cameras, infrared detector arrays, adaptive optics, and powerful planetary radars. Theoretical studies have also contributed significantly to our understanding of the solar system, largely through the

C 2007 by Academic Press. All rights of reproduction in any form reserved. Encyclopedia of the Solar System 2e 

1

CLBE001-ESS2E

November 8, 2006

11:17

2 Encyclopedia of the Solar System use of advanced computer codes and high-speed, dedicated computers. Serendipity has also played an important role in many new discoveries. Along with this increased knowledge have come numerous additional questions as we attempt to explain the complexity and diversity that we observe on each newly encountered world. The increased spatial and spectral resolution of the observations, along with in situ measurements of atmospheres, surface materials, and magnetospheres, have revealed that each body is unique, the result of a different combination of physical, chemical, and dynamical processes that formed and shaped it, as well as its different initial composition. Yet, at the same time, there are broad systematic trends and similarities that are clues to the collective history that the solar system has undergone. We have now begun an exciting new age of discovery with the detection of numerous planet-sized bodies around nearby stars. Although the properties and placement of these extra-solar planets appear to be very different from those in our solar system, they are likely the prelude to the discovery of other planetary systems that may more closely resemble our own. We may also be on the brink of discovering evidence for life on other planets, in particular, Mars. There is an ongoing debate as to whether biogenic materials have been discovered in meteorites that were blasted off the surface of Mars and have found their way to Earth. Although still very controversial, this finding, if confirmed, would have profound implications for the existence of life elsewhere in the solar system and the galaxy. The goal of this chapter is to provide the reader with an introduction to the solar system. It seeks to provide a broad overview of the solar system and its constituent parts, to note the location of the solar system in the galaxy, and to describe the local galactic environment. Detailed discussions of each of the bodies that make up the solar system, as well as the processes that have shaped those bodies and the techniques for observing the planetary system are provided in the following chapters of this Encyclopedia. The reader is referred to those chapters for more detailed discussions of each of the topics introduced. Some brief notes about planetary nomenclature will likely be useful. The names of the planets are all taken from Greek and Roman mythology (with the exception of Earth), as are the names of their satellites, with the exception of the Moon and the Uranian satellites, the latter being named after Shakespearean characters. The Earth is occasionally referred to as Terra, and the Moon as Luna, each the Latin version of their names. The naming system for planetary rings is different at each planet and includes descriptive names of the structures (at Jupiter), letters of the Roman alphabet (at Saturn), Greek letters and Arabic numerals (at Uranus), and the names of scientists associated with the discovery of Neptune (at Neptune).

Asteroids were initially named after Greek and Roman goddesses. As their numbers have increased, asteroids have been named after the family members of the discoverers, after observatories, universities, cities, provinces, historical figures, scientists, writers, artists, literary figures, and, in at least one case, the astronomer’s cat. Initial discoveries of asteroids are designated by the year of their discovery and a letter/number code. Once the orbits of the asteroids are firmly established, they are given official numbers in the asteroid catalog: over 136,500 asteroids have been numbered (as of September 2006). The discoverer(s) of an asteroid are given the privilege of suggesting its name, if done so within 10 years from when it was officially numbered. Comets are generally named for their discoverers, though in a few well-known cases such as comets Halley and Encke, they are named for the individuals who first computed their orbits and linked several apparitions. Because some astronomers have discovered more than one short-period comet, a number is added at the end of the name in order to differentiate them, though this system is not applied to long-period comets. Comets are also designated by the year of their discovery and a letter code (a recently abandoned system used lowercase Roman letters and Roman numerals in place of the letter codes). The naming of newly discovered comets, asteroids, and satellites, as well as surface features on solar system bodies, is overseen by several working groups of the International Astronomical Union (IAU).

2. The Definition of A Planet No formal definition of a planet existed until very recently. Originally, the ancients recognized five planets that could be seen with the naked eye, plus the Earth. Two more jovian planets, Uranus and Neptune, were discovered telescopically in 1781 and 1846, respectively. The largest asteroid, Ceres, was discovered in 1801 in an orbit between Mars and Jupiter and was hailed as a new planet because it fit into Bode’s law (see discussion later in this chapter). However, it was soon recognized that Ceres was much smaller than any of the known planets. As more and more asteroids were discovered in similar orbits between Mars and Jupiter, it became evident that Ceres was simply the largest body of a huge swarm of bodies between Mars and Jupiter that we now call the Asteroid Belt. A new term was coined, “minor planet,” to describe these bodies. Searches for planets beyond Neptune continued and culminated in the discovery of Pluto in 1930. As with Ceres, it was soon recognized that Pluto was much smaller than any of the neighboring jovian planets. Later, measurements of Pluto’s diameter by stellar occultations showed that it was also smaller than any of the terrestrial planets, in fact, smaller even than the Earth’s Moon. As a result, Pluto’s status as a planet was called into question.

CLBE001-ESS2E

November 8, 2006

11:17

The Solar System and Its Place in the Galaxy

In the 1980s, dynamical calculations suggested the existence of a belt of many small objects in orbits beyond Neptune. In the early 1990s the first of these objects, 1992 QB1 was discovered at a distance of 40.9 astronomical units (AU). More discoveries followed and over 1000 bodies have now been found in the trans-Neptunian zone. They are collectively known as the Kuiper belt. All of these bodies were estimated to be smaller than Pluto, though a few were found that were about half the diameter of Pluto. The existence of the Kuiper belt suggested that Pluto, like Ceres, was simply the largest body among a huge swarm of bodies beyond Neptune, again calling Pluto’s status into question. Then came the discovery of Eris (2003 UB313 ), a Kuiper belt object in a distant orbit, which turned out to be slightly larger than Pluto. In response, the IAU, the governing body for astronomers worldwide, formed a committee to create a formal definition of a planet. The definition was presented at the IAU’s triennial gathering in Prague in 2006, where it was revised several times by the astronomers at the meeting. Eventually the IAU voted and passed a resolution that defined a planet. That resolution states that a planet must have three qualities: (1) it must be round, indicating its interior is in hydrostatic equilibrium; (2) it must orbit the Sun; and (3) it must have gravitationally cleared its zone of other debris. The last requirement means that a planet must be massive enough to be gravitationally dominant in its zone in the solar system. Any round body orbiting the Sun that fails condition (3) is labeled a “dwarf planet” by the IAU. The outcome left the solar system with the eight major planets discovered through 1846, and reclassified Ceres, Pluto, and Eris as dwarf planets. Other large objects in the asteroid and Kuiper belts may be added to the list of dwarf planets if observations show that they too are round. Although most astronomers have accepted the new IAU definition, there are some who have not, and who are actively campaigning to change it. There are weaknesses in the definition, particularly in condition (3), which are likely to be modified by an IAU committee tasked with improving the definition. However, the likelihood of the definition being changed sufficiently to again classify Pluto as a planet is small. In this chapter we will use the new IAU definition of a planet. For an alternative view of the new definition, the reader is directed to the chapter Pluto.

3. The Architecture of the Solar System The solar system consists of the Sun at its center, eight planets, three dwarf planets, 165 known natural satellites (or moons) of planets and dwarf planets (as of September 2006), four ring systems, approximately one million asteroids (greater than 1 km in diameter), trillions of comets

3

(greater than 1 km in diameter), the solar wind, and a large cloud of interplanetary dust. The arrangement and nature of all these bodies are the result of physical and dynamical processes during their origin and subsequent evolution, and their complex interactions with one another. At the center of the solar system is the Sun, a rather ordinary, main sequence star. The Sun is classified spectrally as a G2 dwarf, which means that it emits the bulk of its radiation in the visible region of the spectrum, peaking at yellow-green wavelengths. The Sun contains 99.86% of the mass in the solar system, but only about 0.5% of the angular momentum. The low angular momentum of the Sun results from the transfer of momentum to the accretion disk surrounding the Sun during the formation of the planetary system, and to a slow spin-down due to angular momentum being carried away by the solar wind. The Sun is composed of hydrogen (70% by mass), helium (28%), and heavier elements (2%). The Sun produces energy through nuclear fusion at its center, hydrogen atoms combining to form helium and releasing energy that eventually makes its way to the Sun’s surface as visible sunlight. The central temperature of the Sun where fusion takes place is 15.7 million kelvins, while the temperature at the visible surface, the photosphere, is ∼6400 K. The Sun has an outer atmosphere called the corona, which is only visible during solar eclipses, or through the use of specially designed telescopes called coronagraphs. A star like the Sun is believed to have a typical lifetime of 9 billion to 10 billion years on the main sequence. The present age of the Sun (and the entire solar system) is estimated to be 4.56 billion years, so it is about halfway through its nominal lifetime. The age estimate comes from radioisotope dating of meteorites.

3.1 Dynamics The planets all orbit the Sun in roughly the same plane, known as the ecliptic (the plane of the Earth’s orbit), and in the same direction, counterclockwise as viewed from the north ecliptic pole. Because of gravitational torques from the other planets, the ecliptic is not inertially fixed in space, and so dynamicists often use the invariable plane, which is the plane defined by the summed angular momentum vectors of all of the planets. To first order, the motion of any body about the Sun is governed by Kepler’s laws of planetary motion. These laws state that (1) each planet moves about the Sun in an orbit that is an ellipse, with the Sun at one focus of the ellipse; (2) the straight line joining a planet and the Sun sweeps out equal areas in space in equal intervals of time; and (3) the squares of the sidereal periods of the planets are in direct proportion to the cubes of the semimajor axes of their orbits. The laws of planetary motion, first set down by J. Kepler in 1609 and 1619, are easily shown to be the result of the inverse-square law of gravity with the Sun as the

CLBE001-ESS2E

November 8, 2006

11:17

4 Encyclopedia of the Solar System

TABLE 1

Orbits of the Planets and Dwarf Planetsa

Name

Semimajor Axis (AU)

Eccentricity

Inclination (◦ )

Period (years)

0.38710 0.72333 1.00000 1.52366 2.7665 5.20336 9.53707 19.1913 30.0690 39.4817 68.1461

0.205631 0.006773 0.016710 0.093412 0.078375 0.048393 0.054151 0.047168 0.008586 0.248808 0.432439

7.0049 3.3947 0.0000 1.8506 10.5834 1.3053 2.4845 0.7699 1.7692 17.1417 43.7408

0.2408 0.6152 1.0000 1.8808 4.601 11.862 29.457 84.018 164.78 248.4 562.55

Mercury Venus Earth Mars Ceresb Jupiter Saturn Uranus Neptune Plutob Eris (2003 UB313 )b a b

J2000, Epoch: January 1, 2000. Dwarf planet.

central body, and the conservation of angular momentum and energy. Parameters for the orbits of the eight planets and three dwarf planets are listed in Table 1. Because the planets themselves have finite masses, they exert small gravitational tugs on one another, which cause their orbits to depart from perfect ellipses. The major effects of these long-term or “secular” perturbations are to cause the perihelion point of each orbit to precess (rotate counterclockwise) in space, and the line of nodes (the intersection between the planet’s orbital plane and the ecliptic plane) of each orbit to regress (rotate clockwise). Additional effects include slow oscillations in the eccentricity and inclination of each orbit, and the inclination of the planet’s rotation pole to the planet’s orbit plane (called the obliquity). For the Earth, these orbital oscillations have periods of 19,000 to 100,000 years. They have been identified with long-term variations in the Earth’s climate, known as Milankovitch cycles, though the linking physical mechanism is not well understood. Relativistic effects also play a small but detectable role. They are most evident in the precession of the perihelion of the orbit of Mercury, the planet deepest in the Sun’s gravitational potential well. General relativity adds 43 arcsec/ century to the precession rate of Mercury’s orbit, which is 574 arcsec/century. Prior to Einstein’s theory of general relativity in 1916, it was thought that the excess in the precession rate of Mercury was due to a planet orbiting interior to it. This hypothetical planet was given the name Vulcan, and extensive searches were conducted for it, primarily during solar eclipses. No planet was detected. A more successful search for a new planet occurred in 1846. Two celestial mechanicians, U. J. J. Leverrier and J. C. Adams, independently used the observed deviations

of Uranus from its predicted orbit to successfully predict the existence and position of Neptune. Neptune was found by J. G. Galle on September 23, 1846, using Leverrier’s prediction. More complex dynamical interactions are also possible, in particular when the orbital period of one body is a smallinteger ratio of another’s orbital period. This is known as a mean-motion resonance and can have dramatic effects. For example, Pluto is locked in a 2:3 mean-motion resonance with Neptune, and although the orbits of the two bodies cross in space, the resonance prevents them from ever coming within 14 AU of each other. Also, when two bodies have identical perihelion precession rates or nodal regression rates, they are said to be in a secular resonance, and similarly interesting dynamical effects can result. In many cases, mean-motion and secular resonances can lead to chaotic motion, driving a body onto a planet-crossing orbit, which will then lead to its being dynamically scattered among the planets, and eventually either ejected from the solar system or impacted on the Sun or a planet. In other cases, such as Pluto and some asteroids, the mean-motion resonance is actually a stabilizing factor for the orbit. Chaos has become a very exciting topic in solar system dynamics in the past 25 years and has been able to explain many features of the planetary system that were not previously understood. It should be noted that the dynamical definition of chaos is not always the same as the general dictionary definition. In celestial mechanics, the term “chaos” is applied to describe systems that are not perfectly predictable over time. That is, small variations in the initial conditions, or the inability to specify the initial conditions precisely, will lead to a growing error in predictions of the long-term behavior of the system. If the error grows

CLBE001-ESS2E

November 8, 2006

11:17

The Solar System and Its Place in the Galaxy

exponentially, then the system is said to be chaotic. However, the chaotic zone, the allowed area in phase space over which an orbit may vary, may still be quite constrained. Thus, although studies have found that the orbits of the planets are chaotic, this does not mean that Jupiter may one day become Earth-crossing, or vice versa. It means that the precise position of the Earth or Jupiter in their orbits is not predictable over very long periods of time. Because this happens for all the planets, the long-term secular perturbations of the planets on one another are also not perfectly predictable and can vary. On the other hand, chaos can result in some extreme changes in orbits, with sudden increases in eccentricity that can throw small bodies onto planet-crossing orbits. One well-recognized case occurs near mean-motion resonances in the asteroid belt, which causes small asteroids to be thrown onto Earth-crossing orbits, allowing for the delivery of meteoroids to the Earth. The natural satellites of the planets and their ring systems (where they exist) are governed by the same dynamical laws of motion. Most major satellites and all ring systems are deep within their planets’ gravitational potential wells and so they move, to first order, on Keplerian ellipses. The Sun, planets, and other satellites all act as perturbers on the satellite and ring particle orbits. Additionally, the equatorial bulges of the planets, caused by the planets’ rotation, act as a perturber on the orbits. Finally, the satellites raise tides on the planets (and vice versa), and these result in yet another dynamical effect, causing the planets to transfer rotational angular momentum to the satellite orbits in the case of direct or prograde orbits (satellites in retrograde orbits lose angular momentum). As a result, satellites may slowly move away from their planets into larger orbits (or smaller orbits in the case of retrograde satellites). The mutual gravitational interactions can be quite complex, particularly in multisatellite systems. For example, the

TABLE 2 Planet

Mercury Venus Earth Mars Ceres a Jupiter Saturn Uranus Neptune Plutoa a

Dwarf planet.

three innermost Galilean satellites of Jupiter (so named because they were discovered by Galileo in 1610)—Io, Europa, and Ganymede—are locked in a 4:2:1 meanmotion resonance with one another. In other words, Ganymede’s orbital period is twice that of Europa and four times that of Io. At the same time, the other jovian satellites (primarily Callisto), the Sun, and Jupiter’s oblateness perturb the orbits, forcing them to be slightly eccentric and inclined to one another, while the tidal interaction with Jupiter forces the orbits to evolve outward. These competing dynamical processes result in considerable energy deposition in the satellites, which manifests itself as volcanic activity on Io, as a possible subsurface ocean on Europa, and as past tectonic activity on Ganymede. This illustrates an important point in understanding the solar system. The bodies in the solar system do not exist as independent, isolated entities, with no physical interactions between them. Even these “action at a distance” gravitational interactions can lead to profound physical and chemical changes in the bodies involved. To understand the solar system as a whole, one must recognize and understand the processes that were involved in its formation and its subsequent evolution, and that continue to act today. An interesting feature of the planetary orbits is their regular spacing. This is described by Bode’s law, first discovered by J. B. Titius in 1766 and brought to prominence by J. E. Bode in 1772. The law states that the semimajor axes of the planets in astronomical units can be roughly approximated by taking the sequence 0, 3, 6, 12, 24, . . . adding 4, and dividing by 10. The values for Bode’s law and the actual semimajor axes of the planets and two dwarf planets are listed in Table 2. It can be seen that the law works very well for the planets as far as Uranus, but it then breaks down. It also predicts a planet between Mars and Jupiter, the current location of the asteroid belt. Yet Bode’s law predates

Bode’s Law, a1 = 0.4, an = 0.3 × 2n−2 + 0.4 Semimajor Axis (AU)

0.387 0.723 1.000 1.524 2.767 5.203 9.537 19.19 30.07 39.48

5

n

1 2 3 4 5 6 7 8 9 10

Bode’s Law

0.4 0.7 1.0 1.6 2.8 5.2 10.0 19.6 38.8 77.2

CLBE001-ESS2E

November 8, 2006

11:17

6 Encyclopedia of the Solar System the discovery of the first asteroid by 35 years, as well as the discovery of Uranus by 15 years. The reason why Bode’s law works so well is not understood. H. Levison has recently suggested that, at least for the giant planets, it is a result of their spacing themselves at distances where they are equally likely to scatter a smaller body inward or outward to the next planet in either direction. However, it has also been argued that Bode’s law may just be a case of numerology and not reflect any real physical principle at all. Computer-based dynamical simulations have shown that the spacing of the planets is such that a body placed in a circular orbit between any pair of neighboring planets will likely be dynamically unstable. It will not survive over the history of the solar system unless protected by some dynamical mechanism such as a mean-motion resonance with one of the planets. Over the history of the solar system, the planets have generally cleared their zones of smaller bodies through gravitational scattering. The larger planets, in particular Jupiter and Saturn, are capable of throwing small bodies onto hyperbolic orbits, allowing the objects to escape to interstellar space. In the course of doing this, the planets themselves “migrate” moving either closer or farther from the Sun as a result of the angular momentum exchange with many smaller bodies. Thus, the comets and asteroids we now see in planetcrossing orbits must have been introduced into the planetary system relatively recently from storage locations either outside the planetary system, or in protected, dynamically stable reservoirs. Because of its position at one of the Bode’s law locations, the asteroid belt is a relatively stable reservoir. However, the asteroid belt’s proximity to Jupiter’s substantial gravitational influence results in some highly complex dynamics. Mean-motion and secular resonances, as well as mutual collisions, act to remove objects from the asteroid belt and throw them into planet-crossing orbits. The failure of a major planet to grow in the asteroid belt is generally attributed to the gravitational effects of Jupiter disrupting the slow growth by accretion of a planetary-sized body in the neighboring asteroid belt region. It is generally believed that comets originated as icy planetesimals in the outer regions of the solar nebula, at the orbit of Jupiter and beyond. Those proto-comets with orbits between the giant planets were gravitationally ejected, mostly to interstellar space. However, a fraction of the proto-comets were flung into distant but still bound orbits; the Sun’s gravitational sphere of influence extends ∼2 × 105 AU, or about 1 parsec (pc). These orbits were sufficiently distant from the Sun that they were perturbed by random passing stars and by the tidal perturbation from the galactic disk. The stellar and galactic perturbations raised the perihelia of the comet orbits out of the planetary region. Additionally, the stellar perturbations randomized the inclinations of the comet orbits, forming a spherical cloud of comets around the planetary system and extending halfway

to the nearest stars. This region is now called the Oort cloud, after J. H. Oort who first suggested its existence in 1950. The current population of the Oort cloud is estimated at several times 1012 comets, with a total mass of about 15 Earth masses of material. Between 50 and 80% of the Oort cloud population is in a dense core within ∼104 AU of the Sun. Long-period comets (those with orbital periods greater than 200 years) observed passing through the planetary region come from the Oort cloud. Some of the short-period comets (those with orbital periods less than 200 years), such as comet Halley, may be long-period comets that have evolved to short-period orbits due to repeated planetary perturbations. A second reservoir of comets is the Kuiper belt beyond the orbit of Neptune, named after G. P. Kuiper who in 1951 was one of the first to suggest its existence. Because no large planet grew beyond Neptune, there was no body to scatter away the icy planetesimals formed in that region. (The failure of a large planet to grow beyond Neptune is generally attributed to the increasing timescale for planetary accretion with increasing heliocentric distance.) This belt of remnant planetesimals may terminate at ∼50 AU or may extend out several hundred AU from the Sun, analogous to the disks of dust that have been discovered around main sequence stars such as Vega and Beta Pictoris (Fig. 1). The Kuiper belt actually consists of two different dynamical populations. The classical Kuiper belt is the population in low-inclination, low-eccentricity orbits beyond Neptune. Some of this population, including Pluto, is trapped in mean-motion resonances with Neptune at both the 3:2 and 2:1 resonances. The second population is objects in more eccentric and inclined orbits, typically with larger semimajor axes, called the scattered disk. These objects all have perihelia relatively close to Neptune’s orbit, such that they continue to gravitationally interact with Neptune. The Kuiper belt may contain many tens of Earth masses of comets, though the mass within 50 AU is currently estimated as ∼0.1 Earth mass. A slow gravitational erosion of comets from the Kuiper belt, in particular from the scattered disk, due to the perturbing effect of Neptune, causes these comets to “leak” into the planetary region. Eventually, some fraction of the comets evolves due to gravitational scattering by the jovian planets into the terrestrial planets region where they are observed as short-period comets. Short-period comets from the Kuiper belt are often called Jupiter-family or ecliptic comets because most are in orbits that can have close encounters with Jupiter, and also are in orbits with inclinations close to the ecliptic plane. Based on the observed number of ecliptic comets, the number of comets in the Kuiper belt between 30 and 50 AU has been estimated at ∼109 objects larger than 1 km diameter, with a roughly equal number in the scattered disk. Current studies suggest that the Kuiper belt has been collisionally

CLBE001-ESS2E

November 8, 2006

11:17

The Solar System and Its Place in the Galaxy

7

FIGURE 1 False color images of the dust disk around the star ß Pictoris, discovered by the IRAS satellite in 1983. The disk is viewed nearly edge on and is over 900 AU in diameter. The gaps in the center of each image are where the central star image has been removed. The top image shows the full disk as imaged with the Wide Field Planetary Camera 2 (WFPC2) camera onboard the Hubble Space Telescope (HST). The lower image shows the inner disk as viewed by the Space Telescope Imaging Spectrograph (STIS) instrument on HST. The orbits of the outer planets of our solar system, including the dwarf planet Pluto, are shown to scale for comparison. There is evidence of a warping of the ß Pic disk, possibly caused by perturbations from a passing star. Infrared data show that the disk does not extend all the way in to the star, but that it has an inner edge at about 30 AU from ß Pic. The disk interior to that distance may have been swept up by the accretion of planets in the nebula around the star. This disk is a possible analog for the Kuiper belt around our own solar system.

eroded out to a distance of ∼100 AU from the Sun, but that considerably more mass may still exist in orbits beyond that distance. Although gravity is the dominant force in determining the motion of bodies in the solar system, other forces do come into play in special cases. Dust grains produced by asteroid collisions or liberated from the sublimating icy surfaces of comets are small enough to be affected by radiation pressure forces. For submicron grains, radiation pressure from sunlight is sufficient to blow the grains out of the solar system. For larger grains, radiation pressure causes the

grains to depart from Keplerian orbits. Radiation effects can also cause larger grains to slowly spiral in toward the Sun through the Poynting–Robertson effect, and meter- to kilometer-sized bodies to spiral either inward or outward due to the Yarkovsky effect. Electromagnetic forces play a role in planetary magnetospheres where ions are trapped and spiral back and forth along magnetic field lines, and in cometary Type I plasma tails where ions are accelerated away from the cometary coma by the solar wind. Dust grains trapped in planetary magnetospheres and in interplanetary space also respond to

CLBE001-ESS2E

November 8, 2006

11:17

8 Encyclopedia of the Solar System electromagnetic forces, though to a lesser extent than ions because of their much lower charge-to-mass ratios.

3.2 Nature and Composition The solar nebula, the cloud of dust and gas out of which the planetary system formed, almost certainly exhibited a strong temperature gradient with heliocentric distance, hottest near the forming proto-Sun at its center, and cooling as one moved outward through the planetary region. This temperature gradient is reflected in the compositional arrangement of the planets and their satellites versus heliocentric distance. Parts of the gradient are also preserved in the asteroid belt between Mars and Jupiter and likely in the Kuiper belt beyond Neptune. Physical parameters for the planets and dwarf planets are given in Table 3. The planets fall into two major compositional groups. The terrestrial or Earth-like planets are Mercury, Venus, Earth, and Mars and are shown in Fig. 2. The terrestrial planets are characterized by predominantly silicate compositions with iron cores. This results from the fact that they all formed close to the Sun where it was too warm for ices to condense. Also, the modest masses of the terrestrial planets and their closeness to the Sun did not allow them to capture and retain gas directly from the solar nebula. The terrestrial planets all have solid surfaces that are modified to varying degrees by both cratering and internal processes (tectonics, weather, etc.). Mercury is the most heavily cratered because it has no appreciable atmosphere to protect it from impacts or weather to erode the cratered terrain, and also because encounter velocities with Mercury are very high that close to the Sun. Additionally, tectonic processes on Mercury appear to have been modest at best. Mars is next in degree of cratering, in large part

TABLE 3

Physical Parameters for the Sun, Planets, and Dwarf Planets

Name

Sun Mercury Venus Earth Mars Ceresa Jupiter Saturn Uranus Neptune Plutoa Eris (2003 UB313 )a a

Dwarf planet.

because of its proximity to the asteroid belt. Also, Mars’ thin atmosphere affords little protection against impactors. However, Mars also displays substantial volcanic and tectonic features, and evidence of erosion by wind and flowing water, the latter presumably having occurred early in the planet’s history. The surface of Venus is dominated by a wide variety of volcanic terrains. The degree of cratering on Venus is less than on Mercury or Mars for two reasons: (1) Venus’ thick atmosphere (surface pressure = 94 bar) breaks up smaller asteroids and comets before they can reach the surface, and (2) vulcanism on the planet has covered over the older craters on the planet surface. The surface of Venus is estimated to be 500 million to 800 million years in age. The Earth’s surface is dominated by plate tectonics, in which large plates of the crust can move about the planet, and whose motions are reflected in such features as mountain ranges (where plates collide) and volcanic zones (where one plate dives under another). The Earth is the only planet with the right combination of atmospheric surface pressure and temperature to permit liquid water on its surface, and some 70% of the planet is covered by oceans. Craters on the Earth are rapidly erased by its active geology and weather, though the atmosphere only provides protection against very modest size impactors, on the order of 100 m diameter or less. Still, 172 impact craters or their remnants have been found on the Earth’s surface or under its oceans. The terrestrial planets each have substantially different atmospheres. Mercury has a tenuous atmosphere arising from its interaction with the solar wind. Hydrogen and helium ions are captured directly from the solar wind, whereas oxygen, sodium, and potassium are likely the product of sputtering. In contrast, Venus has a dense CO2 atmosphere with a surface pressure 94 times the pressure at the Earth’s

Mass (kg)

Equatorial Radius (km)

Density (g cm−3 )

1.989 × 1033 3.302 × 1023 4.869 × 1024 5.974 × 1024 6.419 × 1023 9.47 × 1020 1.899 × 1027 5.685 × 1026 8.662 × 1025 1.028 × 1026 1.314 × 1022 1.5 × 1022

695,500 2,440 6,052 6,378 3,397 474 71,492 60,268 25,559 24,764 1,151 1,200

1.41 5.43 5.24 5.52 3.94 2.1 1.33 0.70 1.30 1.76 2.0 2.1

Rotation Period

25.38–35. 56.646 d. 243.018 d. 23.934 h. 24.623 h. 9.075 h. 9.925 h. 10.656 h. 17.24 h. 16.11 h. 6.387 d.

Obliquity (◦)

7.25 0. 177.33 23.45 25.19 3.08 26.73 97.92 28.80 119.6

Escape Velocity (km sec−1 )

617.7 4.25 10.36 11.18 5.02 0.52 59.54 35.49 21.26 23.53 1.23 1.29

CLBE001-ESS2E

November 8, 2006

11:17

The Solar System and Its Place in the Galaxy

FIGURE 2 The terrestrial planets: the heavily cratered surface of Mercury as photographed by the Mariner 10 spacecraft in 1974 (top left); false color image of clouds on the night side of Venus, backlit by the intense infrared radiation from the planet’s hot surface, as seen by the Galileo Near-Infrared Mapping Spectrometer (NIMS) instrument in 1990 (top right); South America and Antarctica as imaged by the Galileo spacecraft during a gravity assist flyby of the Earth in 1990 (bottom left); Valles Marineris, a 3000 km long canyon on Mars as photographed by the Viking 1 orbiter in 1980 (bottom right).

9

CLBE001-ESS2E

November 8, 2006

11:17

10 Encyclopedia of the Solar System surface. Nitrogen is also present in the Venus atmosphere at a few percent relative to CO2 . The dense atmosphere results in a massive greenhouse on the planet, heating the surface to a mean temperature of 735 K. The middle and upper atmosphere contain thick clouds composed of H2 SO4 and H2 O, which shroud the surface from view. However, thermal radiation from the surface does penetrate the clouds, making it possible to view surface features through infrared “windows.” The Earth’s atmosphere is unique because of its large abundance of free oxygen, which is normally tied up in oxidized surface materials on other planets. The reason for this unusual state is the presence of life on the planet, which traps and buries CO2 as carbonates and also converts the CO2 to free oxygen. Still, the bulk of the Earth’s atmosphere is nitrogen (78%), with oxygen making up 21% and argon about 1%. The water vapor content of the atmosphere varies from about 1 to 4%. Various lines of evidence suggest that the composition of the Earth’s atmosphere has evolved considerably over the history of the solar system and that the original atmosphere was denser than the present-day atmosphere and dominated by CO2 . Mars has a relatively modest CO2 atmosphere with a mean surface pressure of only 6 mbar. The atmosphere also contains a few percent of N2 and argon. Mineralogic and isotopic evidence and geologic features suggest that the past atmosphere of Mars may have been much denser and warmer, allowing liquid water to flow across the surface in massive floods. The volatiles in the terrestrial planets’ atmospheres (and the Earth’s oceans) may have been contained in hydrated minerals in the planetesimals that originally formed the planets, and/or may have been added later due to asteroid and comet bombardment as the planets dynamically cleared their individual zones of leftover planetesimals. It appears most likely that all these reservoirs contributed some fraction of the volatiles on the terrestrial planets. The jovian or Jupiter-like planets are Jupiter, Saturn, Uranus, and Neptune and are shown in Fig. 3. The jovian planets are also referred to as the gas giants. They are characterized by low mean densities and thick hydrogen– helium atmospheres, presumably captured directly from the solar nebula during the formation of these planets. The composition of the jovian planets is similar to that of the Sun, though more enriched in heavier elements. Because of their primarily gaseous composition and their high internal temperatures and pressures, the jovian planets do not have solid surfaces. However, they may each have silicate–iron cores of several to tens of Earth masses of material. Because they formed at heliocentric distances where ices could condense, the giant planets may have initially had a much greater local density of solid material to grow from. This may, in fact, have allowed them to form before the terrestrial planets interior to them. Studies of the

dissipation of nebula dust disks around nearby solar-type protostars suggest that the timescale for the formation of giant planets is on the order of 10 million years or less. This is very rapid as compared with the ∼100 million year timescale currently estimated for the formation of the terrestrial planets (though questions have now been raised as to the correctness of that accretionary timescale). Additionally, the higher uncompressed densities of Uranus and Neptune (0.5 g cm−3 ) versus Jupiter and Saturn (0.3 g cm−3 ), suggest that the outer two giant planets contain a significantly lower fraction of gas captured from the nebula. This may mean that the outer pair formed later than the inner two giant planets, consistent with the increasing timescale for planetary accretion at larger heliocentric distances. Because of their heliocentric arrangement, the terrestrial and jovian planets are occasionally called the inner and outer planets, respectively, though sometimes the term “inner planets” is used only to denote Mercury and Venus, the planets interior to the Earth’s orbit. Among the dwarf planets, Ceres has a surface composition and density similar to carbonaceous chondrite meteorites. This is a primitive class of meteorites that shows only limited processing during and since formation. Water frost has also been detected on the surface of Ceres. Because of its large size, the interior of Ceres is likely differentiated. Pluto and its largest satellite Charon are shown in Fig. 4. Pluto bears a strong resemblance to Triton, Neptune’s large icy satellite (which is slightly larger than Pluto) and to other large icy planetesimals in the Kuiper belt beyond the orbit of Neptune. Pluto has a thin, extended atmosphere, probably methane and nitrogen, which is slowly escaping because of Pluto’s low gravity. This puts it in a somewhat intermediate state between a freely outflowing cometary coma and a bound atmosphere. Spectroscopic evidence shows that methane frost covers much of the surface of Pluto, whereas its largest satellite Charon appears to be covered with water frost. Nitrogen frost has also been detected on Pluto. The density of Pluto is ∼2 g cm−3 , suggesting that the rocky component of the dwarf planet accounts for about 70% of its total mass. The third dwarf planet, Eris, is a Kuiper belt object in a distant orbit that ranges from 37.8 to 97.5 AU from the Sun. It is slightly larger than Pluto, has a similar bulk density, and also displays evidence for methane frost on its surface. There has been considerable speculation as to the existence of a major planet beyond Neptune, often dubbed “Planet X.” The search program that found Pluto in 1930 was continued for many years afterward but failed to detect any other distant planet, even though the limiting magnitude was considerably fainter than Pluto’s visual magnitude of ∼13.5. Other searches have been carried out, most notably by the Infrared Astronomical Satellite (IRAS) in 1983–1984. An automated algorithm was used to search for a distant planet in the IRAS data; it successfully “discovered” Neptune, but nothing else. Telescopic searches for Kuiper

CLBE001-ESS2E

November 8, 2006

11:17

The Solar System and Its Place in the Galaxy

FIGURE 3 The jovian planets: the complex, belted atmosphere of Jupiter with the Great Red Spot at the lower center, as photographed by Cassini during its gravity assist flyby in 2000 (top left); Saturn and its beautiful ring system, as photographed by Cassini in 2005 (top right); the featureless atmosphere of Uranus, obscured by a high-altitude methane haze, as imaged by Voyager 2 in 1986 (bottom left); several large storm systems and a banded structure, similar to that of Jupiter, in Neptune’s atmosphere, as photographed by Voyager 2 in 1989 (bottom right).

11

CLBE001-ESS2E

November 8, 2006

11:17

12 Encyclopedia of the Solar System

FIGURE 4 Hubble Space Telescope image of the dwarf planet Pluto (center) with its large moon Charon (just above and to the right of Pluto), and two newly discovered small satellites (top). A NASA spacecraft mission, New Horizons, was launched in 2006 and will fly by Pluto and Charon in 2015. (Courtesy of NASA and the Space Telescope Science Institute.)

belt objects have found objects comparable to Pluto in size, but not significantly larger. Gravitational analyses of the orbits of Uranus and Neptune show no evidence of an additional perturber at greater heliocentric distances. Studies of the trajectories of the Pioneer 10 and 11 and Voyager 1 and 2 spacecraft have also yielded negative results. Analyses of the spacecraft trajectories do provide an upper limit on the unaccounted mass within the orbit of Neptune of < 3 × 10−6 solar masses (M ), equal to about one Earth mass. The compositional gradient in the solar system is perhaps best visible in the asteroid belt, whose members range from stony bodies in the inner belt (inside of ∼2.6 AU), to volatile-rich carbonaceous bodies in the outer main belt (out to about 3.3 AU). (See Fig. 5.) There also exist thermally processed asteroids, such as Vesta, whose surface material resembles a basaltic lava flow, and iron–nickel objects, presumably the differentiated cores of larger asteroids that were subsequently disrupted by collisions. The thermal gradient that processed the asteroids appears to be very steep and likely cannot be explained simply by the individual distances of these bodies from the forming proto-Sun. Rather, various special mechanisms such as magnetic induction, short-lived radioisotopes, or massive solar flares have been invoked to explain the heating event that so strongly processed the inner half of the asteroid belt. The largest asteroid is Ceres, now classified as a dwarf planet, at a mean distance of 2.77 AU from the Sun. Ceres was the first asteroid discovered, by G. Piazzi on January 1, 1801. Ceres is 948 km in diameter, rotates in 9.075 hours, and appears to have a surface composition similar to that of carbonaceous chondrite meteorites. The second largest as-

teroid is Pallas, also a carbonaceous type with a diameter of 532 km. Pallas is also at 2.77 AU, but its orbit has an unusually large inclination of 34.8◦ . Over 136,500 asteroids have had their orbits accurately determined and have been given official numbers in the asteroid catalog (as of September 2006). Another 204,700 asteroids have been observed well enough to obtain preliminary orbits, 137,300 of them at more than one opposition. Note that these numbers include all objects nominally classified as asteroids: main-belt, nearEarth, Trojans, Centaurs, and Kuiper belt objects (including Pluto and Eris). As a result of the large number of objects in the asteroid belt, impacts and collisions are frequent. Several “families” of asteroids have been identified by their closely grouped orbital elements and are likely fragments of larger asteroids that collided. Spectroscopic studies have shown that the members of these families often have very similar surface compositions, further evidence that they are related. The largest asteroids such as Ceres and Pallas are likely too large to be disrupted by impacts, but most of the smaller asteroids have probably been collisionally processed. Increasing evidence suggests that many asteroids may be “rubble piles,” that is, asteroids that have been broken up but not dispersed by previous collisions, and that now form a single but poorly consolidated body. Beyond the main asteroid belt there exist small groups of asteroids locked in dynamical resonances with Jupiter. These include the Hildas at the 3:2 mean-motion resonance, the Thule group at the 4:3 resonance, and the Trojans, which are in a 1:1 mean-motion resonance with Jupiter. The effect of the resonances is to prevent these asteroids from making close approaches to Jupiter, even though many of the asteroids are in Jupiter-crossing orbits. The Trojans are particularly interesting. They are essentially in the same orbit as Jupiter, but they librate about points 60◦ ahead and 60◦ behind the planet in its orbit, known as the Lagrange L4 and L5 points. These are pseudostable points in the three-body problem (Sun–Jupiter– asteroid) where bodies can remain dynamically stable for extended periods of time. Some estimates have placed the total number of objects in the Jupiter L4 and L5 Trojan swarms as equivalent to the population of the main asteroid belt. Trojan-type 1:1 librators have also been found for the Earth and Mars (one each), and for Neptune (four). Searches at the L4 and L5 points of the other giant planets have been negative so far. Interestingly, the Saturnian satellites Dione and Tethys also have small satellites locked in Trojan-type librations in their respective orbits. Much of what we know about the asteroid belt and about the early history of the solar system comes from meteorites recovered on the Earth. It appears that the asteroid belt is the source of almost all recovered meteorites. A modest number of meteorites that are from the Moon and from Mars, presumably blasted off of those bodies by asteroid and/or comet impacts, have been found. Cometary meteoroids are thought to be too fragile to survive atmospheric entry. In addition, cometary meteoroids typically encounter

CLBE001-ESS2E

November 8, 2006

11:17

The Solar System and Its Place in the Galaxy

13

FIGURE 5 A sampling of main-belt and near-Earth asteroids: 243 Ida along with its small satellite Dactyl (top left), 951 Gaspra (top right), 253 Mathilde (bottom left), and 25143 Itokawa (bottom right). All these asteroids, with the exception of Mathilde, are stony types; Mathilde is a carbonaceous asteroid. Most of the asteroids exhibit heavily cratered surfaces, but Itokawa is an exception, appearing to be a complete rubble pile. Ida is 54 × 24 × 15 km in diameter, Dactyl is 1.5 km in diameter Gaspra is 18 × 10 × 9 km, Mathilde is roughly 53 km in diameter and Itokawa is only 550 × 300 × 260 m. The asteroids were photographed by the Galileo spacecraft while it was en route to Jupiter, in 1993 and 1991, the NEAR spacecraft while enroute to Eros in 1997, and the Hayabusa spacecraft while in orbit in 2005, respectively. Ida’s tiny satellite, Dactyl, was an unexpected discovery of two of Galileo’s remote sensing instruments, the Near Infrared Mapping Spectrometer and the Solid State Imaging system, during the flyby.

the Earth at higher velocities than asteroidal debris and thus are more likely to fragment and burn up during atmospheric entry. However, we may have cometary meteorites in our sample collections and simply not yet be knowledgeable enough to recognize them. Recovered meteorites are roughly equally split between silicate and carbonaceous types, with a few percent being iron–nickel meteorites. The most primitive meteorites (i.e., the meteorites which appear to show the least processing in the solar nebula) are the volatile-rich carbonaceous

chondrites. However, even these meteorites show evidence of some thermal processing and aqueous alteration (i.e., processing in the presence of liquid water). Study of carbonaceous and ordinary (silicate) chondrites provides significant information on the composition of the original solar nebula, on the physical and chemical processes operating in the solar nebula, and on the chronology of the early solar system. The other major group of primitive bodies in the solar system is the comets. Because comets formed farther from

CLBE001-ESS2E

November 8, 2006

11:17

14 Encyclopedia of the Solar System the Sun than the asteroids, in colder environments, they contain a significant fraction of volatile ices. Water ice is the dominant and most stable volatile. Typical comets also contain modest amounts of CO, CO2 , CH4 , NH3 , H2 CO, and CH3 OH, most likely in the form of ices, but possibly also contained within complex organic molecules and/or in clathrate hydrates. Organics make up a significant fraction of the cometary nucleus, as well as silicate grains. F. Whipple described this icy-conglomerate mix as “dirty snowballs,” though the term “frozen mudball” may be more appropriate since the comets are more than 60% organics and silicates. It appears that the composition of comets is very similar to the condensed (solid) grains and ices observed in dense interstellar cloud cores, with little or no evidence of processing in the solar nebula. Thus, comets appear to be the most primitive bodies in the solar system. As a result, the study of comets is extremely valuable for learning about the origin of the planetary system and the conditions in the solar nebula 4.56 billion years ago. Four cometary nuclei—periodic comets Halley, Borrelly, Wild 2, and Tempel 1—have been encountered by interplanetary spacecraft and imaged (Fig. 6). These irregular nuclei range from about 4 to 12 km in mean diameter and have low albedos, only 3–4%. The nuclei exhibit a variety of complex surface morphologies unlike any other bodies in the solar system. It has been suggested that cometary nuclei are weakly bound conglomerations of smaller dirty snowballs, assembled at low velocity and low temperature in the outer regions of the solar nebula. Thus, comets may be “primordial rubble piles,” in some ways similar to the asteroids. Recent studies have suggested that cometary nuclei, like the asteroids, may have undergone intense collisional evolution, either while resident in the Kuiper belt, or in the giant planets region prior to their dynamical ejection to the Oort cloud. Subtle and not-so-subtle differences in cometary compositions have been observed. However, it is not entirely clear if these differences are intrinsic or due to the physical evolution of cometary surfaces over many close approaches to the Sun. Because the comets that originated among the giant planets have all been ejected to the Oort cloud or to interstellar space, the compositional spectrum resulting from the heliocentric thermal profile is not spatially preserved as it has been in the asteroid belt. Although comets in the classical Kuiper belt are likely located close to their formation distances, physical studies of these distant objects are still in an early stage. There is an observed compositional trend, but it is associated with orbital eccentricity and inclination, rather than semimajor axis.

3.3 Satellites, Rings, and Things The natural satellites of the planets, listed in the appendix to this volume, show as much diversity as the planets they orbit (see Fig. 7). Among the terrestrial planets, the only

known satellites are the Earth’s Moon and the two small moons of Mars, Phobos and Deimos. The Earth’s Moon is unusual in that it is so large relative to its primary. The Moon has a silicate composition similar to the Earth’s mantle and a very small iron core. It is now widely believed that the Moon formed as a result of a collision between the proto-Earth and another protoplanet about the size of Mars, late in the accretion of the terrestrial planets. Such “giant impacts” are now recognized as being capable of explaining many of the features of the solar system, such as the unusually high density of Mercury and the large obliquities of several of the planetary rotation axes. In the case of the Earth, the collision with another protoplanet resulted in the cores of the two planets merging, while a fraction of the mantles of both bodies was thrown into orbit around the Earth where some of the material reaccreted to form the Moon. The tidal interaction between the Earth and Moon then slowly evolved the orbit of the Moon outward to its present position, at the same time slowing the rotation of both the Earth and the Moon. The giant-impact hypothesis is capable of explaining many of the features of the Earth–Moon system, including the similarity in composition between the Moon and the Earth’s mantle, the lack of a significant iron core within the Moon, and the high angular momentum of the Earth–Moon system. Like most large natural satellites, the Moon has tidally evolved to where its rotation period matches its revolution period in its orbit. This is known as synchronous rotation. It results in the Moon showing the same face to the Earth at all times, though there are small departures from this because of the eccentricity of the Moon’s orbit. The Moon’s surface displays a record of the intense bombardment all the planets have undergone over the history of the solar system. Returned lunar samples have been agedated based on decay of long-lived radioisotopes. This has allowed the determination of a chronology of lunar bombardment by comparing the sample ages with the crater counts on the lunar plains where the samples were collected. The lunar plains, or maria, are the result of massive eruptions of lava during the first billion years of the Moon’s history. The revealed chronology shows that the Moon experienced a massive bombardment between 4.2 billion and 3.8 billion years ago, known as the Late Heavy Bombardment. This time period is relatively late as compared with the 100 million to 200 million years required to form the terrestrial planets and to clear their orbital zones of most interplanetary debris. Similarities in crater size distributions on the Moon, Mercury, and Mars suggest that the Late Heavy Bombardment swept over all the terrestrial planets. Recent explanations for the Late Heavy Bombardment have focused on the possibility that it came from the clearing of the outer planets zones of their cometary debris. However, the detailed dynamical calculations of the timescales for that process are still being determined.

CLBE001-ESS2E

November 8, 2006

11:17

The Solar System and Its Place in the Galaxy

15

FIGURE 6 Four cometary nuclei photographed by flyby spacecraft: Halley’s comet in 1986 (Giotto, top left), Borrelly in 2001 (Deep Space 1, top right), Wild 2 in 2004 (Stardust, bottom left), and Tempel 1 in 2005 (Deep Impact, bottom right). The nuclei show considerable diversity both in shape and in surface topography. The Halley nucleus is about 15 × 8 km in diameter, the Borrelly nucleus is 8 × 3.2 km, the Wild 2 nucleus is 5.2 × 4.0 km, and the Tempel 2 nucleus is 7.6 × 4.9 km. The Halley image shows bright dust jets emanating from active areas on the nucleus surface. The other three nuclei were also active during their respective flybys but the activity was too faint to show in these images.

Like almost all other satellites in the solar system, the Moon has no substantial atmosphere. There is a transient atmosphere due to helium atoms in the solar wind striking the lunar surface and being captured. Argon has been detected escaping from surface rocks and being temporarily cold-

trapped during the lunar night. Also, sodium and potassium have been detected, likely the result of sputtering of surface materials due to solar wind particles, as on Mercury. Unlike the Earth’s Moon, the two natural satellites of Mars are both small, irregular bodies and in orbits relatively

CLBE001-ESS2E

November 8, 2006

11:17

16 Encyclopedia of the Solar System FIGURE 7 A sampling of satellites in the solar system: the dusty surface of the Earth’s Moon, still the only other celestial body visited by humans (top row, left); Phobos, the larger of Mars’ two moons showing the large crater Stickney at left (top row, right); the innermost Galilean satellite, Io, displays active vulcanism on its sulfur-rich surface (second row, left); the outermost Galilean satellite, Callisto, displays a heavily cratered surface, likely dating back to the origin of the solar system (second row, right); one of Saturn’s smaller satellites, Hyperion, is irregularly shaped, in chaotic rotation, and displays a very unusual surface morphology (third row, left); Saturn’s satellite Enceladus is one of several in the solar system that has active geysers on its surface (third row, right); another small Saturnian satellite, Mimas, displays an immense impact crater on one hemisphere (fourth row, left); Saturn’s satellite Iapetus is black on one hemisphere and white on the other, and has a high ridge circling it at the equator (fourth row, right); Uranus’ outermost major satellite, Miranda, has a complex surface morphology suggesting that the satellite was disrupted and reaccreted (bottom row, left); Neptune’s one large satellite, Triton, displays a mix of icy terrains and ice vulcanism (bottom, right).

CLBE001-ESS2E

November 8, 2006

11:17

The Solar System and Its Place in the Galaxy

close to the planet. In fact, Phobos, the larger and closer satellite, orbits Mars faster than the planet rotates. Both of the martian satellites have surface compositions that appear to be similar to carbonaceous chondrites in composition. This has resulted in speculation that the satellites are captured asteroids. A problem with this hypothesis is that Mars is located close to the inner edge of the asteroid belt, where silicate asteroids dominate the population, and where carbonaceous asteroids are relatively rare. Also, both satellites are located very close to the planet and in near-circular orbits, which is unusual for captured objects. In contrast to the satellites of the terrestrial planets, the satellites of the giant planets are numerous and are arranged in complex systems. Jupiter has four major satellites, easily visible in small telescopes from Earth, and 58 known lesser satellites. The discovery of the four major satellites by Galileo in 1610, now known as the Galilean satellites, was one of the early confirmations of the Copernican theory of a heliocentric solar system. The innermost Galilean satellite, Io, is about the same size as the Earth’s Moon and has active vulcanism on its surface as a result of Jupiter’s tidal perturbation and the gravitational interaction with Europa and Ganymede (see Section 2.1). The next satellite outward is Europa, somewhat smaller than Io, which appears to have a thin ice crust overlying a possible liquid water ocean, also the result of tidal heating by Jupiter and the satellite–satellite interactions. Estimates of the age of the surface of Europa, based on counting impact craters, are very young, suggesting that the thin ice crust may repeatedly break up and reform. The next satellite outward from Jupiter is Ganymede, the largest satellite in the solar system, even larger than the planet Mercury. Ganymede is another icy satellite and shows evidence of tectonic activity and of being partially resurfaced at some time(s) in its past. The final Galilean satellite is Callisto, another icy satellite that appears to preserve an impact record of comets and asteroids dating back to the origin of the solar system. As previously noted, the orbits of the inner three Galilean satellites are locked into a 4:2:1 mean-motion resonance. The lesser satellites of Jupiter include 4 within the orbit of Io, and 54 at very large distance from the planet. The latter are mostly in retrograde orbits, which suggest that they are likely captured comets and asteroids. The orbital parameters of many of these satellites fall into several tightly associated groups. This suggests that each group consists of fragments of a larger object that was disrupted, most likely by a collision with another asteroid or comet. Possibly, the collision occurred within the gravitational sphere of Jupiter, which then could have led to the dynamical capture of the fragments. All of the close-orbiting jovian satellites (out to the orbit of Callisto) appear to be in synchronous rotation with Jupiter. However, rotation periods have been determined

17

for two of the outer satellites, Himalia and Elara, and these appear to be around 10 to 12 hours, much shorter than their ∼250 day periods of revolution about the planet. Saturn’s satellite system is very different from Jupiter’s in that it contains only one large satellite, Titan, comparable in size to the Galilean satellites, 8 intermediate-sized satellites, and 47 smaller satellites. Titan is the only satellite in the solar system with a substantial atmosphere. Clouds of organic residue in its atmosphere prevent easy viewing of the surface of that moon, though the Cassini spacecraft has had success in viewing the surface at infrared and radar wavelengths. The atmosphere is primarily nitrogen and also contains methane and possibly argon. The surface temperature on Titan has been measured at 94 K, and the surface pressure is 1.5 bar. Cassini has revealed on Titan a complex surface morphology that includes rivers, lakes, and possible cryo-vulcanism. The intermediate and smaller satellites of Saturn all appear to have icy compositions and have undergone substantial processing, possibly as a result of tidal heating and also due to collisions. Orbital resonances exist between several pairs of satellites, and most are in synchronous rotation with Saturn. An interesting exception is Hyperion, which is a highly nonspherical body and which appears to be in chaotic rotation. Another moon, Enceladus, has a ring of material in its orbit that likely has come from the satellite, either as a result of a recent massive impact or as a result of active vulcanism on the icy satellite; Cassini has found ice geysers near Enceladus’ south pole. Two other satellites, Dione and Tethys, have companion satellites in the same orbit, which oscillate about the Trojan-libration points for the Saturn–Dione (1 companion) and Saturn–Tethys (2 companions) systems, respectively. Yet another particularly interesting satellite of Saturn is Iapetus, which is dark on one hemisphere and bright on the other and has a narrow ridge circling the satellite at its equator. The reason(s) for the unusual dichotomy in surface albedos or the equatorial ridge are not known. Saturn has one very distant, intermediate-sized satellite, Phoebe, which is in a retrograde orbit and which is suspected of being a captured early solar system planetesimal, albeit a very large one. Phoebe is not in synchronous rotation, but rather has a period of about 10 hours. The 47 known small satellites of Saturn include 10 embedded in or immediately adjacent to the planet’s ring system, the three Trojan-type librators, and 34 in distant orbits. As with Jupiter, the majority of these distant objects are in retrograde orbits and some are in groups, which suggests that they are collisional fragments. The Uranian system consists of five intermediate-sized satellites and 22 smaller ones. Again, these are all icy bodies. These satellites also exhibit evidence of past heating and possible tectonic activity. The satellite Miranda is particularly unusual in that it exhibits a wide variety of complex

CLBE001-ESS2E

November 8, 2006

11:17

18 Encyclopedia of the Solar System terrains. It has been suggested that Miranda, and possibly many other icy satellites, were collisionally disrupted at some time in their history, and the debris then reaccreted in orbit to form the currently observed satellites, but preserved some of the older morphology. Such disruption/reaccretion phases may have even reoccurred on several occasions for a particular satellite over the history of the solar system. Of the smaller Uranian satellites, 13 are embedded in the ring system and 9 are in distant, mostly retrograde orbits. Again, these are likely captured objects. Neptune’s satellite system consists of one large icy satellite, Triton, and 12 smaller ones. Triton is somewhat larger than Pluto and is unusual in that it is in a retrograde orbit. As a result, the tidal interaction with Neptune is causing the satellite’s orbit to decay, and eventually Triton will be torn apart by the planet’s gravity when it passes within the Roche limit. The retrograde orbit is often cited as evidence that Triton must have been captured from interplanetary space and did not actually form in orbit around the planet. Despite its tremendous distance from the Sun, Triton’s icy surface displays a number of unusual terrain types that strongly suggest thermal processing and possibly even current activity. The Voyager 2 spacecraft photographed what appeared to be plumes from “ice volcanoes” on Triton. The lesser satellites of Neptune include 6 that are either in or adjacent to the ring system and 6 in distant orbits, evenly split between direct and retrograde. Among the dwarf planets, Ceres has no known satellites. Pluto has one very large satellite, Charon, which is slightly more than half the size of Pluto, and two smaller satellites, Nix and Hydra, each estimated to be ∼40–60 km in diameter. The Pluto–Charon system is fully tidally evolved. This means that Pluto and Charon each rotate with the same period, 6.38723 days, which is also the revolution period of the satellite in its orbit. As a result, Pluto and Charon always show the same faces to each other. It is suspected that the Pluto–Charon system was formed by a giant impact between two large Kuiper belt objects. The third dwarf planet, Eris, also has an intermediate-sized satellite, Dysnomia, about 300–400 km in diameter. In addition to their satellite systems, all of the jovian planets have ring systems (Fig. 8). As with the satellite systems, each ring system is distinctly different from its neighbors. Jupiter has a single ring at 1.72–1.81 planetary radii, discovered by the Voyager 1 spacecraft. The ring has several components, related to the four small satellites in or close to the ring. The micron-sized ring particles appear to be sputtered material off the embedded satellites. Saturn has an immense, broad ring system extending between 1.11 and 2.27 planetary radii, easily seen in a small telescope from Earth. The ring system consists of three major rings, known as A, B, and C ordered from the outside in toward the planet, a diffuse ring labeled D inside the C ring and extending down almost to the top of the Saturnian atmosphere, and several other narrow, individual rings.

Closer examination by the Voyager spacecraft revealed that the A, B, and C rings were each composed of thousands of individual ringlets. This complex structure is the result of mean-motion resonances with the many Saturnian satellites, as well as with small satellites embedded within the rings themselves. Some of the small satellites act as gravitational “shepherds,” focusing the ring particles into narrow ringlets. Additional narrow and diffuse rings are located outside the main ring system. The Uranian ring system was discovered accidentally in 1977 during observation of a stellar occultation by Uranus. A symmetric pattern of five narrow dips in the stellar signal was seen on both sides of the planet. Later observations of other stellar occultations found an additional five narrow rings. Voyager 2 detected several more, fainter, diffuse rings and provided detailed imaging of the entire ring system. The success with finding Uranus’ rings led to similar searches for a ring system around Neptune using stellar occultations. Rings were detected but were not always symmetric about the planet, suggesting gaps in the rings. Subsequent Voyager 2 imaging revealed large azimuthal concentrations of material in one of the six detected rings. All of the ring systems are within the Roche limits of their respective planets, at distances where tidal forces from the planet will disrupt any solid body, unless it is small enough and strong enough to be held together by its own material strength. This has led to the general belief that the rings are disrupted satellites, or possibly material that could never successfully form into satellites. Ring particles have typical sizes ranging from micron-sized dust to meter-sized objects and appear to be made primarily of icy materials, though in some cases contaminated with carbonaceous materials. Jupiter’s ring is an exception because it appears to be composed of carbonaceous and silicate materials, with no ice. Another component of the solar system is the zodiacal dust cloud, a huge, continuous cloud of fine dust extending throughout the planetary region and generally concentrated toward the ecliptic plane. The cloud consists of dust grains liberated from comets as the nucleus ices sublimate and from collisions between asteroids. Comets are estimated to account for about two thirds of the total material in the zodiacal cloud, with asteroid collisions providing the rest. Dynamical processes tend to spread the dust uniformly around the Sun, though some structure is visible as a result of the most recent asteroid collisions. These structures, or bands as they are also known, are each associated with specific asteroid collisional families. Dust particles will typically burn up due to friction with the atmosphere when they encounter the Earth, appearing as visible meteors. However, particles less than about 50 μm in radius have sufficiently large area-to-mass ratios that they can be decelerated high in the atmosphere at an altitude of about 100 km and can radiate away the energy generated by friction without vaporizing the particles. These particles then settle slowly through the atmosphere and

CLBE001-ESS2E

November 8, 2006

11:17

The Solar System and Its Place in the Galaxy

19

FIGURE 8 The ring systems of the jovian planets: Jupiter’s single ring photographed in forward scattered light while the Galileo spacecraft was in eclipse behind the giant planet: the lit circle is sunlight filtering through the atmosphere of Jupiter (top); Saturn’s rings break up into hundreds of ringlets when viewed at high resolution, as in this Cassini mosaic (middle); Uranus’ system of narrow rings as viewed in forward scattered light by Voyager 2 as it passed behind the planet (bottom left); two of Neptune’s rings showing the unusual azimuthal concentrations, as photographed by Voyager 2 as it passed behind the planet; the greatly overexposed crescent of Neptune is visible at lower right in the image (bottom right).

are eventually incorporated into terrestrial sediments. In the 1970s, NASA began experimenting with collecting interplanetary dust particles (IDPs, also known as Brownlee particles because of the pioneering work of D. Brownlee) using high altitude U2 reconnaissance aircraft. Terrestrial sources of particulates in the stratosphere are rare and consist largely of volcanic aerosols and aluminum oxide particles from solid rocket fuel exhausts, each of which are readily distinguishable from extraterrestrial materials. The composition of the IDPs reflects the range of source bodies that produce them and include ordinary and carbonaceous chondritic material and suspected cometary par-

ticles. Because the degree of heating during atmospheric deceleration is a function of the encounter velocity, recovered IDPs are strongly biased toward asteroidal particles from the main belt, which approach the Earth in lower eccentricity orbits. Nevertheless, suspected cometary particles are included in the IDPs. The cometary IDPs show a random, “botryoidal” (cluster-of-grapes) arrangement of submicron silicate grains similar in size to interstellar dust grains, intimately mixed in a carbonaceous matrix. Voids in IDPs may have once been filled by cometary ices. In 2006, the Stardust spacecraft returned samples of cometary dust collected during a flyby of comet Wild 2; these will provide

CLBE001-ESS2E

November 8, 2006

11:17

20 Encyclopedia of the Solar System

FIGURE 9 A suspected cometary interplanetary dust particle. The IDP is a highly porous, apparently random collection of submicron silicate grains embedded in a carbonaceous matrix. The particle is ∼10 μm across. The voids in the IDP may have once been filled with cometary ices.

an important comparison with the IDPs collected by highflying aircraft. An example of a suspected cometary IDP is shown in Fig. 9. Extraterrestrial particulates are also collected on the Earth in Antarctic ice cores, in melt-ponds in Greenland, and as millimeter-sized silicate and nickel–iron melt products in sediments. The IDP component in terrestrial sediments can be determined by measuring the abundance of 3 He. 3 He has normal abundances in terrestrial materials of 10−6 or less. The 3 He is implanted in the IDP grains during their exposure to the solar wind. Using this technique, one can look for variations in the infall rate of extraterrestrial particulates over time, and such variations are seen, sometimes correlated with impact events on the Earth. A largely unseen part of the solar system is the solar wind, an ionized gas that streams continuously into space from the Sun. The solar wind is composed primarily of protons (hydrogen nuclei) and electrons with some alpha particles (helium nuclei) and trace amounts of heavier ions. It is accelerated to supersonic speed in the solar corona and streams outward at a typical velocity of 400 km sec−1 . The solar wind is highly variable, changing with both the solar rotation period of ∼25 days and with the 22 year solar cycle, as well as on much more rapid time scales. As the solar wind expands outward, it carries the solar magnetic field with it in a spiral pattern caused by the rotation of the Sun. The solar wind was first inferred in the late 1940s by L. Biermann based on observations of cometary plasma tails. The theory of the supersonic solar wind was first described by E. N. Parker in 1958, and the solar wind itself was detected in

1962 by the Explorer 10 spacecraft in Earth orbit, and the Mariner 2 spacecraft while en route to a flyby of Venus. The solar wind interaction with the planets and the other bodies in the solar system is also highly variable, depending primarily on whether or not the body has its own intrinsic magnetic field. For bodies without a magnetic field, such as Venus and the Moon, the solar wind impinges directly on the top of the atmosphere or on the solid surface, respectively. For bodies like the Earth or Jupiter, which do have magnetic fields, the field acts as a barrier and deflects the solar wind around it. Because the solar wind is expanding at supersonic speeds, a shock wave, or bow shock, develops at the interface between the interplanetary solar wind and the planetary magnetosphere or ionosphere. The planetary magnetospheres can be quite large, extending out ∼12 planetary radii upstream (sunward) of the Earth, and 50–100 radii from Jupiter. Solar wind ions can leak into the planetary magnetospheres near the poles, and these can result in visible aurora, which have been observed on the Earth, Jupiter (Fig. 10), and Saturn. As it flows past the planet, the interaction of the solar wind with the planetary magnetospheres results in huge magnetotail structures that often extend over interplanetary distances. All the jovian planets, as well as the Earth, have substantial magnetic fields and thus planetary magnetospheres. Mercury has a weak magnetic field, but Venus has no detectable field. Mars has a patchy field, indicative of a past magnetic field at some point in the planet’s history, but it has no organized magnetic field at this time. The Galileo spacecraft detected a magnetic field associated with Ganymede, the largest of the Galilean satellites. However, no magnetic field was detected for Europa or Callisto. The Earth’s Moon has no magnetic field. The most visible manifestation of the solar wind is cometary plasma tails, which result when the evolving gases in the cometary comae are ionized by sunlight and by charge exchange with the solar wind and then accelerated by the

FIGURE 10 The auroral ring over the north polar region of Jupiter, as imaged by the Hubble Space Telescope. Several of the bright spots correspond to “footprints” of the Galilean satellites and their interaction with Jupiter’s magnetosphere.

CLBE001-ESS2E

November 8, 2006

11:17

The Solar System and Its Place in the Galaxy

21

FIGURE 11 Artist’s concept of the major boundaries predicted for the heliosphere and the trajectories of the two Voyager spacecraft. Voyager 1 crossed the termination shock in 2004.

solar magnetic field. The ions stream away from the cometary comae at high velocity in the antisunward direction. Structures in the tail are visible as a result of fluorescence by CO+ and other ions. At some distance from the Sun, far beyond the orbits of the planets, the solar wind reaches a point where the ram pressure from the wind is equal to the external pressure from the local interstellar medium flowing past the solar system. A termination shock will develop upstream of that point, and the solar wind will be decelerated from supersonic to subsonic. Recently Voyager 1 detected the termination shock at 94 AU. Beyond this distance is a region still dominated by the subsonic solar plasma, extending out another 30–50 AU or more. The outer boundary of this region is known as the heliopause and defines the limit between solar system–dominated plasma and the interstellar medium. It is not currently known if the flow of interstellar medium past the solar system is supersonic or subsonic. If it is supersonic, then there must additionally be a bow shock beyond the heliopause, where the interstellar medium encounters the obstacle presented by the heliosphere. A di-

agram of the major features of the heliosphere is shown in Fig. 11. The Voyager 1 and 2 spacecraft, which are currently leaving the planetary region on hyperbolic trajectories, continue to study the outermost regions of the heliosphere. Voyager 1 is currently at 100.4 AU (as of September 2006) and Voyager 2 is at 80.7 AU. The Voyager spacecraft are expected to continue to send measurements until the year 2015, when they are expected to be at about 130 and 106 AU from the Sun, respectively. To many planetary scientists, the heliopause defines the boundary of the solar system because it marks the changeover from a solar wind to an interstellar medium dominated space. However, as already noted, the Sun’s gravitational sphere of influence extends out much farther, to ∼2 × 105 AU, and there are bodies in orbit around the Sun at those distances. These include the Kuiper belt and scattered disk, which may each extend out to ∼103 AU (possibly even farther for the scattered disk), and the Oort cloud which is populated to the limits of the Sun’s gravitational field.

CLBE001-ESS2E

November 8, 2006

11:17

22 Encyclopedia of the Solar System 4. The Origin of the Solar System Our knowledge of the origin of the Sun and the planetary system comes from two sources: study of the solar system itself and study of star formation in nearby giant molecular clouds. The two sources are radically different. In the case of the solar system, we have an abundance of detailed information on the planets, their satellites, and numerous small bodies. But the solar system we see today is highly evolved and has undergone massive changes since it first condensed from the natal interstellar cloud. We must learn to recognize which qualities reflect that often violent evolution and which truly record conditions at the time of solar system formation. In contrast, when studying even the closest star-forming regions (which are about 140 pc from the Sun), we are handicapped by a lack of adequate resolution and detail. In addition, we are forced to take a “snapshot” view of many young stars at different stages in their formation, and from that attempt to generate a time-ordered sequence of those different stages and processes involved. When we observe the formation of other stars, we also need to recognize that some of the observed processes or events may not be applicable to the formation of our own Sun and planetary system. Still, a coherent picture has emerged of the major events and processes in the formation of the solar system. That picture assumes that the Sun is a typical star and that it formed in a similar way to many of the low-mass protostars we see today. The birthplace of stars is giant molecular clouds in the galaxy. These huge clouds of molecular hydrogen have masses of 105 –106 M . Within these clouds are denser regions or cores where star formation actually takes place. Some process, perhaps the shockwave from a nearby supernova, triggers the gravitational collapse of a cloud core. Material falls toward the center of the core under its own self-gravity and a massive object begins to grow at the center of the cloud. Heated by the gravitational potential energy of the infalling matter, the object becomes self-luminous and is then described as a protostar. Although central pressures and temperatures are not yet high enough to ignite nuclear fusion, the protostar begins to heat the growing nebula around it. The timescale of the infall of the cloud material for a solar-mass cloud is about 106 years. The infalling cloud material consists of both gas and dust. The gas is mostly hydrogen (75% by mass) and helium (22%). The dust (2%) is a mix of interstellar grains, including silicates, organics, and condensed ices. A popular model suggests that the silicate grains are coated with icyorganic mantles. As the dust grains fall inward, they experience a pressure from the increasing density of gas toward the center of the nebula. This slows and even halts the inward radial component of their motion. However, the dust grains can still move vertically with respect to the central plane of the nebula, as defined by the rotational angular

momentum vector of the original cloud core. As a result, the grains settle toward the central plane. As the grains settle, they begin to collide with one another. The grains stick and quickly grow from microscopic to macroscopic objects, perhaps meters in size (initial agglomerations of grains may look very much like the suspected cometary IDP in Fig. 9). This process continues and even increases as the grains reach the denser environment at the central plane of the nebula. The meter-sized bodies grow to kilometer-sized bodies and the kilometer-sized bodies grow to 100 km-sized bodies. These bodies are known as planetesimals. As a planetesimal begins to acquire significant mass, its cross section for accretion grows beyond its physical cross section because it is now capable of gravitationally deflecting smaller planetesimals toward it. These larger planetesimals then “run away” from the others, growing at an ever increasing rate. The actual process is far more complex than described here, and there are many details of this scenario that still need to be worked out. For example, the role of turbulence in the nebula is not well quantified. Turbulence would tend to slow or even prevent the accretion of grains into larger objects. Also, the role of electrostatic and magnetic effects in the nebula is not understood. Nevertheless, it appears that accretion in the central plane of the solar nebula can account for the growth of planets from interstellar grains. An artist’s concept of the accretion disk in the solar nebula is shown in Fig. 12. In the inner region of the solar nebula, close to the forming Sun, the higher temperatures would vaporize icy and organic grains, leaving only silicate grains to form the planetesimals, which eventually merged to form the terrestrial planets. At larger distances where the nebula was cooler, organic and icy grains would condense, and these would combine with the silicates to form the cores of the giant planets. Because the total mass of ice and organics may have been several times the mass of silicates, the cores of the giant planets may actually have grown faster than the terrestrial planets interior to them. At some point, the growing cores of the giant planets became sufficiently massive to begin capturing hydrogen and helium directly from the nebula gas. Because of the lower temperatures in the outer planets zone, the giant planets were able to retain the gas and continue to grow even larger. The terrestrial planets close to the Sun may have acquired some nebula gas, but likely they could not hold on to it at their higher temperatures. Observations of protostars in nearby molecular clouds have found substantial evidence for accretionary disks and gas nebulae surrounding these stars. The relative ages of these protostars can be estimated by comparing their luminosity and color with theoretical predictions of their location in the Hertzsprung–Russell diagram. One of the more interesting observations is that the nebula dust and gas around solar-mass protostars seem to dissipate after about

CLBE001-ESS2E

November 8, 2006

11:17

The Solar System and Its Place in the Galaxy

23

FIGURE 12 Artist’s concept of the accretion disk in the solar nebula, showing dust, orbiting planetesimals and the proto-Sun at the center. (Painting by William Hartmann.)

107 years. It appears that the nebula and dust may be swept away by mass outflows, essentially super-powerful solar winds, from the protostars. If the Sun formed similarly to the protostars we see today, then these observations set strong limits on the likely formation times of Jupiter and Saturn. An interesting process that must have occurred during the late stages of planetary accretion is “giant impacts” (i.e., collisions between very large protoplanetary objects). As noted in Section 2.3, a giant impact between a Mars-size protoplanet and the proto-Earth is now the accepted explanation for the origin of the Earth’s Moon. Although it was previously thought that such giant impacts were low probability events, they are now recognized to be a natural consequence of the final stages of planetary accretion. Another interesting process late in the accretion of the planets is the clearing of debris from the planetary zones. At some point in the growth of the planets, their gravitational spheres of influence grew sufficiently large that an encounter with a planetesimal would more likely lead to the planetesimal being scattered into a different orbit, rather than an actual collision. This would be particularly true for the massive jovian planets, both because of their stronger gravitational fields and because of their larger distances from the Sun. Because it is just as likely that a planet will scatter objects inward as outward, the clearing of the planetary zones resulted in planetesimals being flung throughout the solar system and in a massive bombardment of all planets and

satellites. Many planetesimals were also flung out of the planetary system to interstellar space or to distant orbits in the Oort cloud. Although the terrestrial planets are generally too small to eject objects out of the solar system, they can scatter objects to Jupiter-crossing orbits where Jupiter will quickly dispose of them. The clearing of the planetary zones has several interesting consequences. The dynamical interaction between the planets and the remaining planetesimals results in an exchange of angular momentum. Computer-based dynamical simulations have shown that this causes the semimajor axes of the planets to migrate. In general, Saturn, Uranus, and Neptune are expected to first move inward and then later outward as the ejection of material progresses. Jupiter, which ejects the most material because of its huge mass, migrates inward but by only a few tenths of an astronomical unit. This migration of the giant planets has significant consequences for the populations of small bodies in the planetary region. As the planets move, the locations of their mean-motion and secular resonances will move with them. This will result in some small bodies being captured into resonances while others will be thrown into chaotic orbits, leading to their eventual ejection from the system or possibly to impacts on the planets and the Sun. The radial migration of the giant planets has been invoked both in the clearing of the outer regions of the main asteroid belt, and the inner regions of the Kuiper belt.

CLBE001-ESS2E

November 8, 2006

11:17

24 Encyclopedia of the Solar System FIGURE 13 An image of the entire sky at infrared wavelengths, constructed from ground-based data by the 2MASS survey. The Milky Way galaxy is visible as the bright horizontal band through the image, with the galactic bulge at the center of the image. Lanes of interstellar dust obscure the view of the galactic center. The Magellanic clouds, two small, irregular companion galaxies to the Milky Way are visible below and to the right of the galactic center.

Another consequence of the clearing of the planetary zones is that rocky planetesimals formed in the terrestrial planets zone will be scattered throughout the jovian planets region, and vice versa for icy planetesimals formed in the outer planets zone. The bombardment of the terrestrial planets by icy planetesimals is of particular interest, both as an explanation for the Late Heavy Bombardment and as a means of delivering the volatile reservoirs of the terrestrial planets. Isotopic studies suggest that some fraction of the water in the Earth’s oceans may have come from comets and/or volatile-rich asteroids, though not all of it. Also, the discovery of an asteroidal-appearing object, 1996 PW, on a long-period comet orbit has provided evidence that asteroids may indeed have been ejected to the Oort cloud, where they may make up 1–3% of the population there.

massive black holes. The age of the galaxy is estimated to be 13 billion years, equal to the age of the universe. The Milky Way galaxy consists of four major structures: the galactic disk, the central bar, the halo, and the corona (Fig. 13). As the name implies, the disk is a highly flattened, rotating structure about 15–25 kpc in radius and about 0.5– 1.3 kpc thick, depending on which population of stars is used to trace the disk. The disk contains relatively young stars and interstellar clouds, arranged in a multiarm spiral structure (Figs. 14 and 15). At the center of the disk is the bar, a prolate spheroid about 3 kpc in radius in the plane of

5. The Solar System’s Place in the Galaxy The Milky Way is a large, spiral galaxy, about 30 kpc in diameter. Some parts of the galactic disk can be traced out to 25 kpc from the galactic center, and the halo can be traced to 50 kpc. The galaxy contains approximately 1011 , stars and the total mass of the galaxy is estimated to be about 4 × 1011 solar masses (M ). Approximately 25% of the mass of the galaxy is estimated to be in visible stars, about 15% in stellar remnants (white dwarfs, neutron stars, and black holes), 25% in interstellar clouds and interstellar material, and 35% in “dark matter.” Dark matter is a general term used to describe unseen mass in the galaxy, which is needed to explain the observed dynamics of the galaxy (i.e., stellar motions, galactic rotation) but which has not been detected through any available means. There is considerable speculation about the nature of the dark matter, which includes everything from exotic nuclear particles to brown dwarfs (substellar objects, not capable of nuclear burning) and dark stars (the burned out remnants of old stars) to

FIGURE 14 Messier 33, a large spiral galaxy in the constellation Triangulum, as photographed by the Galex spacecraft. M33 is part of the local group of nearby galaxies. The Milky Way galaxy may appear similar to this.

CLBE001-ESS2E

November 8, 2006

11:17

The Solar System and Its Place in the Galaxy

25

FIGURE 15 The spiral structure of the Milky Way galaxy as inferred from the positions of HII regions (clouds of ionized hydrogen) in the galaxy. The Sun and solar system are located at the upper center, as indicated by the  symbol. (Reprinted with kind permission from Kluwer Academic Publishers, Forbes and Shuter, in “Kinematics, Dynamics, and Structure of the Milky Way,” p. 221, Fig. 3, copyright  1983.) C

the disk, and with a radius of about 1.5 kpc perpendicular to the disk. The bar rotates more slowly than the disk and consists largely of densely packed older stars and interstellar clouds. It does not display spiral structure. At the center of the bar is the nucleus, a complex region only 4–5 pc across, which appears to have a massive black hole at its center. The mass of the central black hole has been estimated at 2.6 million M . The halo surrounds both of these structures and extends ∼20–30 kpc from the galactic center. The halo has an oblate spheroid shape and contains older stars and globular clusters of stars. The corona appears to be a yet more distant halo extending 60–100 kpc and consists of dark matter, unobservable except for the effect it has on the dynamics of observable bodies in the galaxy. The corona may be several times more massive than the other three galactic components combined. The galactic disk is visible in the night sky as the Milky Way, a bright band of light extending around the celestial sphere. When examined with a small telescope, the Milky

Way is resolved into thousands or even millions of individual stars and numerous nebulae and star clusters. The direction to the center of the galaxy is in the constellation Sagittarius (best seen from the southern hemisphere in June), and the disk appears visibly wider in that direction, which is the view of the central bulge and bar. The disk is not perfectly flat; there is evidence for warping in the outer reaches of the disk, between 15 and 25 kpc. The warp may be the result of gravitational perturbations due to encounters with other galaxies and/or with the Magellanic clouds, two nearby, irregular dwarf galaxies that appear to be in orbit around the Milky Way. In addition, the Milky Way’s central bar appears to be tilted relative to the plane of the galactic disk. The nonspherical shape of the bar and the tilt have important implications for understanding stellar dynamics and the long-term evolution of the galaxy. Stars in the galactic disk have different characteristic velocities as a function of their stellar classification, and hence age. Low mass, older stars, like the Sun, have relatively high random velocities and, as a result, can move farther out of

CLBE001-ESS2E

November 8, 2006

11:17

26 Encyclopedia of the Solar System the galactic plane. Younger, more massive stars have lower mean velocities and thus smaller scale heights above and below the plane. Giant molecular clouds, the birthplace of stars, also have low mean velocities and thus are confined to regions relatively close to the galactic plane. The galactic disk rotates clockwise as viewed from “galactic north,” at a relatively constant velocity of 160–220 km sec−1 . This motion is distinctly non-Keplerian, the result of the very nonspherical mass distribution. The rotation velocity for a circular galactic orbit in the galactic plane defines the Local Standard of Rest (LSR). The LSR is then used as the reference frame for describing local stellar dynamics. The Sun and the solar system are located approximately 8.5 kpc from the galactic center (though some estimates put it closer at ∼7 kpc), and 10–20 pc above the central plane of the galactic disk. The circular orbit velocity at the Sun’s distance from the galactic center is 190–220 km sec−1 , and the Sun and the solar system are moving at approximately 17 to 22 km sec−1 relative to the LSR. The Sun’s velocity vector is currently directed toward a point in the constellation of Hercules, approximately at right ascension 18h 0m , and declination +30◦ , known as the solar apex. Because of this motion relative to the LSR, the solar system’s galactic orbit is not circular. The Sun and planets move in a quasi-elliptical orbit between about 8.4 and 9.7 kpc from the galactic center, with a period of revolution of about 240 million years. The solar system is currently close to and moving inward toward “perigalacticon,” the point in the orbit closest to the galactic center. In addition, the solar system moves perpendicular to the galactic plane in a harmonic fashion, with an estimated period of 52 million to 74 million years, and an amplitude of ±49–93 pc out of the galactic plane. (The uncertainties in the estimates of the period and amplitude of the motion are caused by the uncertainty in the amount of dark matter in the galactic disk.) The Sun and planets passed through the galactic plane about 2 million to 3 million years ago, moving “northward.” The Sun and solar system are located at the inner edge of one of the spiral arms of the galaxy, known as the Orion or local arm. Nearby spiral structures can be traced by constructing a 3-dimensional map of stars, star clusters, and interstellar clouds in the solar neighborhood. Two welldefined neighboring structures are the Perseus arm, farther from the galactic center than the local arm, and the Sagittarius arm, toward the galactic center. The arms are about 0.5 kpc wide, and the spacing between the spiral arms is ∼1.2–1.6 kpc. The local galactic spiral arm structure is illustrated in Fig. 15. The Sun’s velocity relative to the LSR is low as compared with other G-type stars, which have typical velocities of 40–45 km sec−1 relative to the LSR. Stars are accelerated by encounters with giant molecular clouds in the galactic disk. Thus, older stars can be accelerated to higher mean velocities, as noted earlier. The reason(s) for the Sun’s low velocity is not known. Velocity-altering encounters with gi-

ant molecular clouds occur with a typical frequency of once every 300 million to 500 million years. The local density of stars in the solar neighborhood is about 0.11 pc−3 , though many of the stars are in binary or multiple star systems. The local density of binary and multiple star systems is 0.086 pc−3 . Most of these are lowmass stars, less massive and less luminous than the Sun. The nearest star to the solar system is Proxima Centauri, which is a low-mass (M  0.1 M ), distant companion to Alpha Centauri, which itself is a double star system of two closeorbiting solar-type stars. Proxima Centauri is currently about 1.3 pc from the Sun and about 0.06 pc (1.35 × 104 AU) from the Alpha Centauri pair it is orbiting. The second nearest star is Barnard’s star, a fast-moving red dwarf at a distance of 1.83 pc. The brightest star within 5 pc of the Sun is Sirius, an A1 star (M  2 M ) about 2.6 pc away. Sirius also is a double star, with a faint, white dwarf companion. The stars in the solar neighborhood are shown in Fig. 16. The Sun’s motion relative to the LSR, as well as the random velocities of the stars in the solar neighborhood, will occasionally result in close encounters between the Sun and other stars. Using the value above for the density of stars in the solar neighborhood, one can predict that ∼12 star systems (single or multiple stars) will pass within 1 pc of the Sun per million years. The total number of stellar encounters scales as the square of the encounter distance. This rate has been confirmed in part by data from the Hipparcos astrometry satellite, which measured the distances and proper motions of ∼118,000 stars, and which was used to reconstruct the trajectories of stars in the solar neighborhood. Based on this rate, the closest stellar approach over the lifetime of the solar system would be expected to be at ∼900 AU. Such an encounter would result in a major perturbation of the Oort cloud and would eject many comets to interstellar space. It would also send a shower of comets into the planetary region, raising the impact rate on the planets for a period of about 2 million to 3 million years, and having other effects that may be detectable in the stratigraphic record on the Earth or on other planets. A stellar encounter at 900 AU could also have a substantial perturbative effect on the orbits of comets in the Kuiper belt and scattered disk and would likely disrupt the outer regions of those populations. Obviously, the effect that any such stellar passage will have is a strong function of the mass and velocity of the passing star. Because the Sun likely formed in a star cluster, and because the Sun will move through denser regions of the galactic disk (in particular, the spiral arms), the encounter rate mentioned above is likely a lower limit and was higher in the past. That also means that the closest stellar encounters may have been even closer to the planetary system. The advent of space-based astronomy, primarily through Earth-orbiting ultraviolet and X-ray telescopes, has made it possible to study the local interstellar medium surrounding the solar system. The structure of the local interstellar

CLBE001-ESS2E

November 8, 2006

11:17

The Solar System and Its Place in the Galaxy

27

FIGURE 16 A 3-dimensional representation of the stars in the solar neighborhood. Horizontal lines indicate the relative distance of the stars north (to the right) or south (to the left) of the celestial equator. The size of the dot representing each star denotes its relative brightness. (From G. F. Gilmore, in “Astronomy and Astrophysics Encyclopedia,” S. P. Maran, Ed. Copyright  1992 John Wiley & Sons, New York. Reprinted by permission of John Wiley & Sons, Inc.) C

medium has turned out to be quite complex. The solar system appears to be on the edge of an expanding bubble of hot plasma about 120 pc in radius, which appears to have originated from multiple supernovae explosions in the Scorpius-Centaurus OB association. The Sco-Cen association is a nearby star-forming region that contains many young, high-mass O- and B-type stars. Such stars have relatively short lifetimes and end their lives in massive supernova explosions, before collapsing into black holes. The expanding shells of hot gas blown off the stars in the supernova explosions are able to “sweep” material before them, leaving a low density “bubble” of hot plasma. Within this bubble, known as the Local Bubble, the solar system is at this time within a small interstellar cloud, perhaps 2–5 pc across, known as the Local Interstellar Cloud. That cloud is apparently a fragment of the expanding shells of gas from the supernova explosions, and there appear to be a number of such clouds within the local solar neighborhood.

6. The Fate of the Solar System Stars like the Sun are expected to have lifetimes on the main sequence of about 1010 years. The main sequence lifetime refers to the time period during which the star produces energy through hydrogen fusion in its core. As the hydrogen fuel in the core is slowly depleted over time, the core contracts to maintain the internal pressure. This raises the central temperature and as a result, the rate of nuclear fusion also increases and the star slowly brightens. Thus, temperatures throughout the solar system will slowly increase over time. Presumably, this slow brightening has already been going on since the formation of the Sun and solar system.

A 1 M star like the Sun is expected to run out of hydrogen at its core in about 1010 years. As the production of energy declines, the core again contracts. The rising internal temperature and pressure are then able to ignite hydrogen burning in a shell surrounding the depleted core. The hydrogen burning in the shell heats the surrounding mass of the star and causes it to expand. The radius of the star increases and the surface temperature drops. The luminosity of the star increases dramatically, and it becomes a red giant. Eventually the star reaches a brightness about 103 times more luminous than the present-day Sun, a surface temperature of 3000 K, and a radius of 100–200 solar radii. One hundred solar radii is equal to 0.46 AU, larger than the orbit of Mercury. Two hundred radii is just within the orbit of the Earth. Thus, Mercury and likely Venus will be incorporated into the outer shell of the red giant Sun and will be vaporized. The increased solar luminosity during the red giant phase will result in a fivefold rise in temperatures throughout the solar system. At the Earth’s orbit this temperature increase will vaporize the oceans and roast the planet at a temperature on the order of ∼1400 K or more. At Jupiter’s orbit it will melt the icy Galilean satellites and cook them at a more modest temperature of about 600 K, about the same as current noon-time temperatures on the surface of Mercury. Typical temperatures at the orbit of Neptune will be about the same as they are today at the orbit of the Earth. Comets in the inner portion of the Kuiper belt will be warmed sufficiently to produce visible comae. The lowered gravity at the surface of the greatly expanded Sun will result in a substantially increased solar wind, and the Sun will slowly lose mass from its outer envelope. Meanwhile, the core of the Sun will continue to contract until the central temperature and pressure are great

CLBE001-ESS2E

November 8, 2006

11:17

28 Encyclopedia of the Solar System enough to ignite helium burning in the core. During this time, hydrogen burning continues in a shell around the core. Helium burning continues during the red giant phase until the helium in the core is also exhausted. The star again contracts, and this permits helium burning to ignite in a shell around the core. This is an unstable situation, and the star can undergo successive contractions and reignition pulses, during which it will blow off part or all of its outer envelope into space. These huge mass ejections produce an expanding nebula around the star, known as a planetary nebula (because it looks somewhat like the disk of a jovian planet through a telescope). For a star with the mass of the Sun, the entire red giant phase lasts about 7 × 108 years. As the Sun loses mass in this fashion, the orbits of the surviving planets will slowly spiral outward. This will also be true for comets in the Kuiper belt and Oort cloud. The gravitational sphere of influence of the Sun will shrink as a result of the Sun’s decreasing mass, so comets will be lost to interstellar space at the outer limits of the Oort cloud. As a red giant star loses mass, its core continues to contract. However, for an initially 1 M star like the Sun, the central pressure and temperature cannot rise sufficiently to ignite carbon burning in the core, the next phase in nuclear fusion. With no way of producing additional energy other than gravitational contraction, the luminosity of the star plunges. The star continues to contract and cool, until the contraction is halted by degenerate electron pressure in the super-dense core. At this point, the mass of the star has been reduced to about 70% of its original mass and the diameter is about the same as the present-day Earth. Such a star is known as a white dwarf. The remnants of the previously roasted planets will be plunged into a deep freeze as the luminosity of the white dwarf slowly declines.

The white dwarf star will continue to cool over a period of about 109 years, to the point where its luminosity drops below detectable levels. Such a star is referred to as a black dwarf. A nonluminous star is obviously very difficult to detect. There is some suggestion that they may have been found through an observing technique known as micro-lensing events. Dark stars provide one of the possible explanations for the dark matter in the galaxy.

7. Concluding Remarks This chapter has provided an introduction to the solar system and its varied members, viewing them as components of a large and complex system. Each of them (the Sun, the planets, their satellites, the comets and asteroids, etc.) is also a fascinating world in its own right. The ensuing chapters provide more detailed descriptions of each of these members of the solar system, as well as descriptions of important physical and dynamical processes, discussions of some of the more advanced ways we study the solar system, the search for life elsewhere in the solar system, and finally, the search for planetary systems around other stars.

Bibliography Lewis, J. S. (2004). “Physics and Chemistry of the Solar System,” 2nd Ed. Elsevier Academic Press, San Diego. von Steiger, R., Lallement, R., and Lee, M. A., eds. (1996). “The Heliosphere in the Local Interstellar Medium.” Kluwer, Dordrecht, The Netherlands. Sparke, L. S., and Gallagher, J. S. (2000) “Galaxies in the Universe: An Introduction.” Cambridge University Press, Cambridge, UK.

CLBE001-ESS2E

November 8, 2006

100-C

25-C

50-C

75-C

16:33

C+M

50-C+M

C+Y

50-C+Y

M+Y

50-M+Y

100-M

25-M

50-M

75-M

100-Y

25-Y

50-Y

75-Y

100-K

25-K

25-19-19

50-K

50-40-40

75-K

75-64-64

The Origin of the Solar System John E. Chambers Carnegie Institution of Washington Washington, D.C.

Alex N. Halliday University of Oxford Oxford, United Kingdom

CHAPTER

2

1. Introduction 2. Star Formation and Protoplanetary Disks 3. Meteorites and the Origin of the Solar System

7. The Asteroid Belt 8. Growth of Gas and Ice Giant Planets 9. Planetary Satellites

4. Nucleosynthesis and Short-Lived Isotopes 5. Early Stages of Planetary Growth

10. Extrasolar Planets 11. Summary and Future Prospects

6. Formation of Terrestrial Planets

Bibliography

1. Introduction The origin of the solar system has long been a fascinating subject posing difficult questions of deep significance. It takes one to the heart of the question of our origins, of how we came to be here and why our surroundings look the way they do. Unfortunately, we currently lack a self-consistent model for the origin of the solar system and other planetary systems. The early stages of planet formation are obscure, and we have only a modest understanding of how much the orbits of planets change during and after their formation. At present, we cannot say whether terrestrial planets similar to the Earth are commonplace or highly unusual. Nor do we know the source of the water that makes our planet habitable. In the face of such uncertainty, one might ask whether we will ever understand how planetary systems form. In fact, the last 10 years have seen rapid progress in almost every area of planetary science, and our understanding of the origin of the solar system and other planetary systems has improved greatly as a result. Planetary science today is as exciting as it has been at any time since the Apollo landings on the Moon, and the coming decade looks set to continue this trend. C 2007 by Academic Press. All rights of reproduction in any form reserved. Encyclopedia of the Solar System 2e 

Some key recent developments follow: 1. A decade ago, the first planet orbiting another Sunlike star was discovered. Since then, new planets have been found at an astounding rate, and roughly 200 objects are known today. Most of these planets appear to be gas giants similar to Jupiter and Saturn. Recently, several smaller planets have been found, and these may be akin to Uranus and Neptune, or possibly large analogs of terrestrial planets like Earth. 2. In the last 10 years, there have been a number of highly successful space missions to other bodies in the solar system, including Mars, Saturn, Titan, and several asteroids and comets. Information and images returned from these missions have transformed our view of these objects and greatly enhanced our understanding of their origin and evolution. 3. The discovery that one can physically separate and analyze star dust–presolar grains that can be extracted from meteorites and that formed in the envelopes of other stars—has meant that scientists can for the first time test decades of theory on how stars work. The parallel development of methods for extracting isotopic information at the submicron scale has opened

29

CLBE001-ESS2E

November 8, 2006

16:33

30 Encyclopedia of the Solar System up a new window to the information stored in such grains. 4. The development of multiple collector inductively coupled plasma mass spectrometry has made it possible to use new isotopic systems for determining the mechanisms and timescales for the growth of bodies early in the solar system. 5. Our theoretical understanding of planet formation has advanced substantially in several areas, including new models for the rapid growth of giant planets, a better understanding of the physical and chemical evolution of protoplanetary disks, and the growing realization that planets can migrate substantially during and after their formation. 6. The recent development of powerful new computer codes and equations of state has facilitated realistic, high-resolution simulations of collisions between planet-sized bodies. Scientists are discovering that the resolution of their models significantly changes the outcome, and the race is on to find reliable solutions. Today, the formation of the solar system is being studied using three complementary approaches. • Astronomical observations of protoplanetary disks around young stars are providing valuable information about probable conditions during the early history of the solar system and the timescales involved in planet formation. The discovery of new planets orbiting other stars is adding to the astonishing diversity of possible planetary systems and providing additional tests for theories of how planetary systems form. • Physical, chemical, and isotopic analyses of meteorites and samples returned by space missions are generating important information about the formation and evolution of objects in the solar system and their constituent materials. This field of cosmochemistry has taken off in several important new directions in recent years, including the determination of timescales involved in the formation of the terrestrial planets and asteroids, and constraints on the origin of the materials that make up the solar system. • Theoretical calculations and numerical simulations are being used to examine every stage in the formation of the solar system. These provide valuable insights into the complex interplay of physical and chemical processes involved, and help to fill in some of the gaps when astronomical and cosmochemical data are unavailable. In this chapter, we will describe what we currently know about how the solar system formed and highlight some of the main areas of uncertainty that await future discoveries.

2. Star Formation and Protoplanetary Disks The solar system formed 4.5–4.6 billion years (Ga) ago by collapse of a portion of a molecular cloud of gas and dust rather like the Eagle or Orion Nebulae. Some of the star dust from that ancient Solar Nebula has now been isolated from primitive meteorites. Their isotopic compositions are vastly different from those of our own solar system and provide fingerprints of nearby stars that preceded our Sun. These include red giants, asymptotic giant branch stars, supernovae, and novae. It has also become clear from studying modern molecular clouds that stars like our Sun can form in significant numbers in close proximity to each other. Such observation also provide clues as to how own solar system formed because they have provided us with images of circumstellar disks—the environments in which planetary objects are born. Observations from space-based infrared telescopes such as the Infrared Astronomical Satellite (IRAS) have shown that many young stars give off more infrared radiation than would be expected for blackbodies of the same size. This infrared excess comes from micron-sized grains of dust orbiting the star in an optically thick (opaque) disk. Dark, dusty disks can be seen with the Hubble Space Telescope surrounding some young stars in the Orion Nebula (Fig. 1). These disks have been dubbed proplyds, short for protoplanetary disks. It is thought that protoplanetary disks are mostly composed of gas, and in a few cases this gas has been detected, although gas is generally much harder to see than dust. The fraction of stars having a massive disk declines with stellar age, and large infrared excesses are rarely seen in stars older than 107 years. In some cases, such as the disk surrounding the star HR 4796A, there are signs that the inner portion of a disk has been cleared of dust (Fig. 2), perhaps due to the presence of one or more planets.

FIGURE 1 Proplyds are young stellar objects embedded in an optically dense envelope of gas and dust. The objects shown here are from the Orion Nebula.

CLBE001-ESS2E

November 8, 2006

16:33

The Origin of the Solar System

31

FIGURE 2 The circumstellar disk surrounding HR 4796A as revealed by interferometry measurements of the infrared excess. Note the area close into the star swept clear of dust, which has presumably been incorporated into planetary objects.

Roughly half of stars up to a few hundred million years old have low-mass, optically thin (nearly transparent) disks containing some dust but apparently little or no gas. In a few cases, such as the star Beta Pictoris, a disk can be seen at visible wavelengths when the light from the star itself is blocked. Dust grains in these disks will be quickly accelerated outward by radiation pressure or spiral inward due to Poynting–Robertson drag caused by collisions with photons from the central star. This dust should be either removed from the disk or destroyed in high-speed collisions with other dust grains on a timescale that is short compared to the age of the star. For this reason, the dust in these disks is thought to be second-generation material formed by collisions between asteroids or sublimation from comets orbiting these stars in more massive analogs of the Kuiper Belt in our own solar system. These are often referred to as debris disks as a result. In the solar system, the planets all orbit the Sun in the same direction, and their orbits are very roughly coplanar. This suggests the solar system originated from a disk-shaped region of material referred to as the solar nebula, an idea going back more than 2 centuries to Kant and later Laplace. The discovery of disks of gas and dust around many young stars provides strong support for this idea and implies that planet formation is associated with the formation of stars themselves. Stars typically form in clusters of a few hundred to a few thousand objects in dense regions of the interstellar medium called molecular clouds (see Fig. 3). The gas in molecular clouds is cold (roughly 10 K) and dense compared to that in other regions of space (roughly 104 atoms/cm3 ) but still much more tenuous than the gas in a typical laboratory “vacuum.” Stars in these clusters are typically separated by about 0.1 pc (0.3 lightyears), much less than the distance between stars in the Sun’s neighborhood.

FIGURE 3 This Hubble Space Telescope image of the Orion Nebula shows molecular clouds of gas and dust illuminated by radiation from young stars. Some early stars appear shrouded in dusty disks (see Fig. 1). Scientists think that our solar system formed by collapse of a portion of a similar kind of molecular cloud leading to formation of a new star embedded in a dusty disk. How that collapse occurred is unclear. It may have been triggered by a shock wave carrying material being shed from another star such as an AGB star or supernova.

It is unclear precisely what causes the densest portions of a molecular cloud (called molecular cloud cores) to collapse to form stars. It may be that contraction of a cloud core is inevitable sooner or later due to the core’s own gravity, or an external event may cause the triggered collapse of a core. The original triggered collapse theory was based on the sequencing found in the ages of stars in close proximity to one another in molecular clouds. This suggests that the formation and evolution of some stars triggered the formation of additional stars in neighboring regions of the cloud. However, several other triggering mechanisms are possible, such as energetic radiation and mass loss from other newly formed stars, the effects of a nearby, pulsating asymptotic giant branch (AGB) star, or a shock wave from the supernova explosion of a massive star. Gas in molecular cloud cores is typically moving. When a core collapses, the gas has too much angular momentum for all the material to form a single, isolated star. In many cases, a binary star system forms. In others cases, a single protostar forms (called a T Tauri star or pre-main sequence star),

CLBE001-ESS2E

November 8, 2006

16:33

32 Encyclopedia of the Solar System while a significant fraction of the gas goes into orbit about the star forming a disk that is typically 100 astronomical units (AU) in diameter. Temperatures in T Tauri stars are initially too low for nuclear reactions to take place. However T Tauri stars are much brighter then older stars like the Sun due to the release of gravitational energy as the star contracts. The initial collapse of a molecular cloud core takes roughly 105 years, and material continues to fall onto both the star and its disk until the core is depleted. The spectra of T Tauri stars contain strong ultraviolet and visible emission lines caused by hot gas falling onto the star. This provides evidence that disks lose mass over time as material moves inward through the disk and onto the star, a process called viscous accretion. This process provides one reason why older stars do not have disks, another reason being planet formation itself. Estimated disk accretion rates range from 10−6 to 10−9 solar masses per year. The mechanism responsible for viscous accretion is unclear. A promising candidate is magneto-rotational instability (MRI), in which partially ionized gas in the disk becomes coupled to the local magnetic field. Because stars rotate, the magnetic field sweeps around rapidly, increasing the orbital velocity of material that couples strongly to it and moving it outward. Friction causes the remaining material to move inward. As a result, a disk loses mass to its star and spreads outwards over time. This kind of disk evolution explains why the planets currently contain only 0.1% of the mass in the solar system but have retained more than 99% of its angular momentum. MRI requires a certain fraction of the gas to be ionized, and it may not be effective in all portions of a disk. Disks are also eroded over time by photoevaporation. In this process, gas is accelerated when atoms absorb ultraviolet photons from the central star or nearby, energetic stars, until the gas is moving fast enough to escape into interstellar space. T Tauri stars often have jets of material moving rapidly away from the star perpendicular to the plane of the disk. These jets are powered by the inward accretion of material through the disk coupled with the rotating magnetic field. Outward flowing winds also arise from the inner portions of a disk. It is possible that a wind arising from the very inner edge of the disk (called the x-wind) can entrain small solid particles with it. These objects will be heated strongly as they emerge from the disk’s shadow. Many of these particles will return to the disk several AU from the star, and may drift inward again to repeat the process. Some of these particles may be preserved today in meteorites. T Tauri stars are strong emitters of X-rays, generating fluxes up to 104 times greater than that of the Sun during the strongest solar flares. Careful sampling of large populations of young solar mass stars in the Orion Nebula shows that this is normal behavior in young stars. This energetic flare activity is strongest in the first million years and declines at later times, persisting for up to 108 years. From this it has been concluded that the young Sun generated

FIGURE 4 Pie chart showing the bulk composition of the Earth. Most of the iron (Fe), nickel (Ni), and sulfur (S) are in Earth’s core, while the silicate Earth mostly contains magnesium (Mg), silicon (Si), and oxygen (O) together with some iron.

105 times as many energetic protons as today. It is thought that reactions between these protons and material in the disk may have provided some of the short-lived isotopes whose daughter products are seen today in meteorites although the formation of nearly all of these predate that of the solar system. (See Section 4.) The minimum mass of material that passed through the solar nebula can be estimated from the total mass of the planets, asteroids, and comets in the solar system. However, all of these objects are depleted in hydrogen and helium relative to the Sun. Ninety percent of the mass of the terrestrial planets is made up of oxygen, magnesium, silicon, and iron (Fig. 4), and although Jupiter and Saturn are mostly composed of hydrogen and helium, they are enriched in the heavier elements compared to the Sun. When the missing hydrogen and helium is added, the minimum-mass solar nebula (MMSN) turns out to be 1–2% of the Sun’s mass. The major uncertainties in this number come from the fact that the interior compositions of the giant planets and the initial mass of the Kuiper Belt are poorly known. Not all of this mass necessarily existed in the nebula at the same time, but it must have been present at some point. Current theoretical models predict that planet formation is an inefficient process, with some mass falling into the Sun or being ejected into interstellar space, so the solar nebula was probably more massive than the MMSN. Gas in the solar nebula was heated as it viscously accreted toward the Sun, releasing gravitational energy. The presence of large amounts of dust meant the inner portions of the nebula were optically thick to infrared radiation so these regions became hot. Numerical disk models show that temperatures probably exceeded 1500 K in the terrestrialplanet forming region early in the disk’s history. Viscous heating mainly took place at the disk midplane where most of the mass was concentrated. The surfaces of the disk would have been much cooler. The amount of energy generated by viscous accretion declined rapidly with distance from the Sun. In the outer nebula, solar irradiation was the more

CLBE001-ESS2E

November 8, 2006

16:33

The Origin of the Solar System

important effect. Protoplanetary disks are thought to be flared, so that their vertical thickness grows more rapidly than their radius. As a result, the surface layers are always irradiated by the central star. For this reason, the surface layers of the outer solar nebula may have been warmer than the midplane. The nebula cooled over time as the viscous accretion rate declined and dust was swept up by larger bodies, reducing the optical depth. In the inner nebula, cooling was probably rapid. Models show that at the midplane at 1 AU, the temperature probably fell to about 300 K after 105 years. Because the energy generated by viscous accretion and solar irradiation declined with distance from the Sun, disk temperatures also declined with heliocentric distance. At some distance from the Sun, a location referred to as the ice line, temperatures became low enough for water ice to form. Initially, the ice line may have been 5–6 AU from the Sun, but it moved inward over time as the nebula cooled. Some asteroids contain hydrated minerals formed by reactions between water ice and dry rock. This suggests water ice was present when these asteroids formed, in which case the ice line would have been no more than 2–3 AU from the Sun at the time. Meter-sized icy bodies drifted rapidly inward through the solar nebula due to gas drag (see Section 5). When these objects crossed the ice line, they would have evaporated, depositing water vapor in the nebular gas. As a result, the inner nebula probably became more oxidizing over time as the level of oxygen from water increased. When the flux of drifting particles dwindled, the inner nebula may have become chemically reducing again, as water vapor diffused outward across the ice line, froze to form ice, and became incorporated into growing planets.

33

Petrography is the detailed examination of mineralogical and textural features. Geochemistry uses the isotopic and chemical compositions. This combined approach to these fascinating archives has provided a vast amount of information on our Sun and solar system and how they formed. We know about the stars and events that predated formation of the Sun, the nature of the material from which the planets were built, the solar nebula the timescales for planetary accretion, and the interior workings and geological histories of other planets. Not only these, meteorites provide an essential frame of reference for understanding how our own planet Earth formed and differentiated. The geochemistry of meteorites and IDPs provides evidence that the Sun’s protoplanetary disk as well as the planets it seeded had a composition that was similar in some respects to that of the Sun itself (Fig. 5). In other respects however, it is clear the disk was a highly modified residuum that generated a vast range of planetary compositions. The composition of the Sun can be estimated from the depths of lines associated with each element in the Sun’s spectra (although this is problematic for the lightest elements and the noble gases). The Sun contains almost 99.9% of the total mass of the solar system. A sizable fraction of this material passed through the solar nebula at some point, which tells us that the composition of the original nebula would have

3. Meteorites and the Origin of the Solar System Much of the above is based on theory and observations of other stars. To find out how our own solar system formed, it is necessary to study meteorites and interplanetary dust particles (IDPs). These are fragments of rock and metal from other bodies in the solar system that have fallen to Earth and survived passage through its atmosphere. Meteorites and IDPs tend to have broadly similar compositions, and the difference is mainly one of size. IDPs are much the smaller of the two, typically 10–100 μm in diameter, while meteorites can range up to several meters in size. Most such objects are quite unlike any objects formed on Earth. Therefore, we cannot readily link them to natural present-day processes as earth scientists do when unraveling past geological history. Yet the approaches that are used are in some respects very similar. The research conducted on meteorites and IDPs is dominated by two fields: petrography and geochemistry.

FIGURE 5 The abundances of elements in our Sun and solar system are estimated from the spectroscopic determination of the composition of the Sun and the laboratory analysis of primitive meteorites called carbonaceous chondrites—thought to represent unprocessed dust and other solid debris from the circumsolar disk. To compare the abundances of different elements, it is customary to scale the elements relative to one million atoms of silicon. The pattern provides powerful clues to how the various elements were created. See text for details. (Based on a figure in W. S. Broecker “How to Build a Habitable Planet,” with kind permission.)

CLBE001-ESS2E

November 8, 2006

16:33

34 Encyclopedia of the Solar System been similar to that of the Sun today. The challenge is therefore to explain how it is possible that a disk that formed gas giant objects like Jupiter and Saturn, also generated rocky terrestrial planets like the Earth (Fig. 4). Most meteorites are thought to come from parent bodies in the Main Asteroid Belt that formed during the first few million years of the solar system. As a result, these objects carry a record of processes that occurred in the solar nebula during the formation of the planets. In a few cases, the trajectories of falling meteorites have been used to establish that they arrived on orbits coming from the Asteroid Belt. Most other meteorites are deduced to come from asteroids based on their age and composition. IDPs are thought to come from both asteroids and comets. A few meteorites did not originate in the Asteroid Belt. The young ages and noble-gas abundances of the Shergottite– Nakhlite–Chassignite (SNC) meteorites suggest they come from Mars. A few dozen SNC meteorites have been found to date, and a comparable number of lunar meteorites from the Moon are also known. The Earth is currently accumulating meteoritic material at the rate of about 5 × 107 kg/year. At this rate, it would take more than 1017 years to obtain the Earth’s current mass of 5.97 × 1024 kg, which is much longer than the age of the universe. Even though it is thought that the Earth did form as the result of the accumulation of smaller bodies, it is clear that the rate of impacts was much higher while the planets were forming than it is today. Broadly speaking, meteorites can be divided into three types: chondrites, achondrites, and irons, which can be distinguished as follows:

1. Chondrites are mixtures of grains from submicronsized dust to millimeter- to centimeter-sized particles of rock and metal, apparently assembled in the solar nebula. Most elements in chondrites are present in broadly similar ratios to those in the Sun, with the exception of carbon, nitrogen, hydrogen, and the noble gases, which are all highly depleted. For this reason, chondrites have long been viewed as representative of the dust and debris in the circumstellar disk from which the planets formed. So, for example, refractory elements that would have resided in solid phases in the solar nebula have chondritic (and therefore solar) relative proportions in the Earth, even though the volatile elements are vastly depleted. The nonmetallic components of chondrites are mostly silicates such as olivine and pyroxene. Chondrules are a major component of most chondrites (see Fig. 6). These are roughly millimeter-sized rounded beads of rock that formed by melting, either partially or completely. Their mineral-grain textures suggest they cooled over a period of a few hours, presumably in the nebula, with the heating possibly caused by passage

FIGURE 6 Chondrules are spherical objects, sometimes partly flattened and composed of mafic silicate minerals, metal, and oxides. They are thought to form by sudden (flash) heating in the solar nebula. Some formed as much as 3 Ma after the start of the solar system. (Photograph courtesy of Drs. M. Grady and S. Russell and the Natural History Museum, London.)

through shock waves in the nebular gas. Some chondrules are thought to have formed later in collisions between planetary objects. Most chondrites also contain calcium-aluminum-rich inclusions (CAIs, see Fig. 7), which have chemical compositions similar to those predicted for objects that condensed from a gas of roughly solar composition at very high temperatures. It is possible that CAIs formed in the very

FIGURE 7 Calcium–aluminum refractory inclusions are found in chondrite meteorites and are thought to be the earliest objects that formed within our solar system. They have a chemical composition consistent with condensation from a hot gas of solar composition. How they formed exactly is unclear, but some have suggested they were produced close in to the Sun and then scattered across the disk. (Photograph courtesy of Drs. M. Grady and S. Russell and the Natural History Museum, London.)

CLBE001-ESS2E

November 8, 2006

16:33

The Origin of the Solar System

FIGURE 8 The current best estimates for the timescales over which very early inner solar system objects and the terrestrial planets formed. The approximated mean life of accretion is the time taken to achieve 63% growth at exponentially decreasing rates of growth. The dashed lines indicate the mean lives for accretion deduced for the Earth based on W isotopes. (Based on a figure that first appeared in A. N. Halliday and T. Kleine, 2006, Meteorites and the timing, mechanisms and conditions of terrestrial planet accretion and early differentiation, in “Meteorites and the Early Solar System II” (D. Lauretta, L. Leshin, and H. MacSween., eds.), pp. 775–801, Univ. Arizona Press, Tucson.)

innermost regions of the solar nebula close to the Sun. Dating based on radioactive isotopes suggest that CAIs are the oldest surviving materials to have formed in the solar system. CAIs in the Efremovka chondrite are 4.5672 ± 0.0006 Ga old based on the 235/238 U–207/206 Pb system, and this date is often used to define the canonical start to the solar system. The oldest chondrules appear to have formed at about the same time, but most chondrules are 1–3 million years (Ma) younger than this (Fig. 8). The space between the chondrules and CAIs in chondrites is filled with fine-grained dust called matrix.Most chondrites are variably depleted in moderately volatile elements like potassium (K) and rubidium (Rb) (Fig. 9). This depletion is more a feature of the chondrules and CAIs rather than the matrix. Chondrites are subdivided into groups of like objects thought to come originally from the same parent body. Currently, about 15 groups are firmly established, 8 of which are collectively referred to as carbonaceous chondrites. These tend to be richer in highly volatile elements such as carbon and nitrogen compared to other chondrites, although as with all meteorites these elements are less abundant than they are in the Sun. Ordinary chondrites are more depleted in volatile elements than carbonaceous chondrites, and are largely made of silicates and metal grains. Enstatite chondrites are similar but

35

FIGURE 9 Comparison between the K/U and Rb/Sr ratios of the Earth and other differentiated objects compared with chondrites. The alkali elements K and Rb are both relatively volatile compared with U and Sr, which are refractory. Therefore, these trace element ratios provide an indication of the degree of volatile element depletion in inner solar system differentiated planets relative to chondrites, which are relatively primitive. It can be seen that the differentiated objects are more depleted in moderately volatile elements than are chondrites. (Based on a figure that first appeared in A. N. Halliday and D. Porcelli, 2001, In search of lost planets—The paleocosmochemistry of the inner solar system, Earth Planet. Sci. Lett. 192, 545–559.)

highly reduced. Chondrules are absent from the most primitive, volatile-rich group of carbonaceous chondrites (the CI group), either because their parent body formed entirely from matrix-like material or because chondrule structures have been erased by subsequent reactions with water in the parent body. Chondrites also contain presolar grains, which are submicron grains that are highly anomalous isotopically and have compositions that match those predicted to form by condensation in the outer envelopes of various stars. These represent a remarkable source of information on stellar nucleosyntheis and can be used to test theoretical models. 2. Achondrites are silicate-rich mafic and ultramafic igneous rocks not too dissimilar from those forming on Earth but with slightly different chemistry and isotopic compositions. They clearly represent the near-surface rocks of planets and asteroids that have melted and differentiated. A few achondrites come from asteroids that appear to have undergone only partial differentiation. In principle, it is possible to group achondrites and distinguish which planet or asteroid they came from. The oxygen isotopic composition of a meteorite is particularly useful in this respect. Isotopically, oxygen is extremely heterogeneous in the solar system, and planets that formed

CLBE001-ESS2E

November 8, 2006

16:33

36 Encyclopedia of the Solar System

FIGURE 10 Oxygen isotopic composition of various bodies in the solar system. The x and y axes show increasing 170 and 180 abundances, respectively. The oxygen isotopic composition of the components in chondrites, in particular CAIs, is highly heterogeneous for reasons that are unclear. The net result of this variability is that different planets possess distinct oxygen isotopic compositions that define as individual mass fractionation lines as shown here for eucrites, howardites, and diogenites, which come from Vesta and SNC meteorites, thought to come from Mars. The Moon is thought to have formed from the debris produced in a giant impact between the proto-Earth when 90% formed and an impacting Mars-sized planet sometimes named “Theia.” The fact that the data for lunar samples are collinear with the terrestrial fractionation line could mean that the Moon formed from the Earth, or the planet from which it was created was formed at the same heliocentric distance, or it could mean that the silicate reservoirs of the two planets homogenized during the impact process, for example by mixing in a vapor cloud from which lunar material condensed. (From A. N. Halliday, 2003, The origin and earliest history of the Earth, in “Meteorites, Comets and Planets” (A. M. Davis, ed.), Vol. 1, “Treatise of Geochemistry” (H. D. Holland and K. K. Turekian, eds.), pp. 509–557, Elsevier-Pergamon, Oxford.

in different parts of the nebula seem to have specific oxygen isotope compositions. This makes it possible to link all of martian meteorites together for example (Fig. 10). These meteorites are specifically linked to Mars because nearly all of them are too young to have formed on any asteroid; they had to come from an object that was large enough to be geologically active in the recent past. This was confirmed by a very close match between the composition of the atmosphere measured with the Viking lander and that measured in fluids trapped in alteration products in martian meteorites. In fact, martian meteorites provide an astonishing archive of information into how Mars formed and evolved as discussed in Section 6. To date, only

FIGURE 11 Iron meteorites are the most abundant kind of meteorite found because they are distinctive and because they survive long after other kinds of meteorites are destroyed by weathering. In contrast, chondrites are the most abundant class of meteorite observed to fall. Some iron meteorites are thought to represent disrupted fragments of planetesimal cores. Others appear to have formed at low pressures, probably as metal-rich pools formed from impacts on asteroids. The Henbury meteorite shown here is a type IIIAB magmatic iron that fell near Alice Springs, Australia, about 5000 years ago. The texture shown on the sawn face are Windmanstatten patterns formed by slow cooling, consistent with an origin from a core located deep within a meteorite parent body. (Photograph courtesy of Drs. M. Grady and S. Russell and the Natural History Museum, London.)

one asteroidal source has been positively identified: Vesta, whose spectrum and orbital location strongly suggest it is the source of the howardite, eucrite, and diogenite (HED) meteorites. 3. Irons (see Fig. 11) are largely composed of iron, nickel (about 10% by mass), and sulfides, together with other elements that have a chemical affinity for iron, called siderophile elements. Like chondrites, irons can be grouped according to their likely parent body, and several dozen groups or unique irons have been found. The textures of mineral grains in iron meteorites have been used to estimate how quickly their parent bodies cooled, and thus the depth at which they formed. It appears that most irons are samples of metallic cores of small asteroidal parent bodies, 10–100 km in radius. These appear to have formed very early, probably within a million years of CAIs, when there was considerable heat available from decay of short-lived radioactive isotopes (see Section 6). Other irons appear to have formed by impact melting at the surface of asteroids, and these formed later. A rare class of stony-iron meteorite (amounting to about 5% of all nonchondritic meteorites) called pallasites contains an intricate mixture of metal and silicate (Fig. 12). It is thought these come from the core– mantle boundary regions of differentiated asteroids that broke up during collisions.

CLBE001-ESS2E

November 8, 2006

16:33

The Origin of the Solar System

FIGURE 12 The pallasite Esquel is a mixture of silicate (olivine) and iron metal that may have formed at a planetary core-mantle boundary. (Photograph courtesy of Drs. M. Grady and S. Russell and the Natural History Museum, London.)

Note that there are no clear examples of mantle material within meteorite collections. The isotopic compositions of some elements in irons reveal that they have been exposed to cosmic rays for long periods—up to hundreds of millions of years. This means their parent bodies broke up a long time ago. Because they are extremely hard, they survived the collisions that destroyed their parent body as well as any subsequent impacts. In contrast, fragments of mantle material (as with samples excavated by volcanoes on the Earth) are extremely friable and would not survive collisions. Survivability is also an issue for meteorites entering Earth’s atmosphere and being recovered in recognizable form. Chondrites and achondrites are mainly composed of silicates that undergo physical and chemical alteration on the surface of Earth more rapidly than the material in iron meteorites. Furthermore, iron meteorites are highly distinctive, so they are easier to recognize than stony meteorites. For this reason, most meteorites found on the ground are irons, whereas most meteorites that are seen to fall from the sky (referred to as falls) are actually chondrites. Most falls are ordinary chondrites, which probably reflects the fact that they survive passage through the atmosphere better than the weaker carbonaceous chondrites. The parent bodies of ordinary chondrites may also have orbits in the Asteroid Belt that favor their delivery to Earth. IDPs are less prone to destruction during passage through the atmosphere than meteorites so they probably provide a less biased sample of the true population of interplanetary material. Most IDPs are compositionally similar to carbonaceous rather than ordinary chondrites and this suggests that the Asteroid Belt is dominated by carbonaceous-chondrite-like material.

37

Mass spectrometric measurements on meteorites and lunar samples provide evidence that the isotopes of most elements are present in similar proportions in the Earth, Moon, Mars, and the asteroids. The isotopes of elements heavier than hydrogen and helium were made by nucleosynthesis in stars that generate extremely varied isotopic compositions. Since the solar nebula probably formed from material from a variety of sources, the observed isotopic homogeneity was originally interpreted as indicative that the inner solar nebula was very hot and planetary material condensed from a ∼2000 K gas of solar composition. However, a variety of observations including the preservation of presolar grains in chondrites suggest that the starting point of planet formation was cold dust and gas. This homogeneity is therefore nowadays interpreted as indicating that the inner nebula was initially turbulent, allowing dust to become thoroughly mixed. CAIs sometimes contain nucleosynthetic isotopic anomalies. This suggests that CAIs sampled varied proportions of the isotopes of the elements before they became homogenized in the turbulent disk. With improved mass spectrometric measurements evidence has been accumulating for small differences in isotopic composition in some elements between certain meteorites and those of the Earth and Moon. This area of study that searches for nucleosythetic isotopic heterogeneity in the solar system is ongoing and is now providing a method for tracking the provenance of different portions of the disk. However, oxygen and the noble gases are very different in this respect. Extreme isotopic variations have been found for these elements. The different oxygen and noble gas isotope ratios provide evidence of mixing between compositions of dust and those of volatile (gaseous) components. Some of this mixing may have arisen later when the nebula cooled, possibly because large amounts of isotopically distinct material are thought to have arrived from the outer nebula in the form of water ice. There are also possibilities for generating some of the heterogeneity in oxygen by irradiation within the solar nebula itself. The terrestrial planets and asteroids are not just depleted in nebular gas relative to the Sun. They are also depleted in moderately volatile elements (elements such as lead, potassium, and rubidium that condense at temperatures in the range 700–1350 K) (Figs. 9 and 13). In chondritic meteorites, the degree of depletion becomes larger as an element’s condensation temperature decreases. It was long assumed that this is the result of the loss of gas from a hot nebula before it cooled. For example, by the time temperatures became cool enough for lead to condense, much of the lead had already accreted onto the Sun as a gas. However, it is clear that moderately volatile elements are depleted in chondrites at least in part because they contain CAIs and chondrules that lost volatiles by evaporation during heating events. The least depleted chondrites (CI carbonaceous chondrites) contain no CAIs or chondrules. Another mechanism for losing moderately volatile elements is planetary

CLBE001-ESS2E

November 8, 2006

16:33

38 Encyclopedia of the Solar System

FIGURE 13 The estimated composition of the silicate portion of the Earth as a function of the calculated temperature at which half the mass of the element would have condensed. The concentrations of the various elements are normalized to the average composition of the solid matter in the disk as represented by CI carbonaceous chondrites. Open circles: lithophile elements; shaded squares: chalcophile elements; shaded triangles: moderately siderophile elements; solid diamonds: highly siderophile elements. It can be seen that refractory lithophile elements are enriched relative to CI concentrations. This is because of core formation and volatile losses compared with CI chondrites. The moderately volatile lithophile elements like K are depleted because of loss of volatiles. Siderophile elements are depleted by core formation. However, the pattern of depletion is not as strong as expected given the ease with which these elements should enter the core. The explanation is that there was addition of a late veneer of chondritic material to the silicate Earth after core formation. (From A. N. Halliday, 2003, The origin and earliest history of the Earth, in “Meteorites, Comets and Planets” (A. M. Davis, ed.), Vol. 1, “Treatise of Geochemistry” (H. D. Holland and K. K. Turekian, eds.), pp 509–557, Elsevier-Pergamon, Oxford.)

collisions. Energetic collisions between large bodies would have generated high temperatures and could have caused further loss of moderately volatile elements. For this reason, the terrestrial planets have compositions that differ from one another and also from chondritic meteorites. The Moon is highly depleted in moderately volatile elements (Fig. 9) and is thought to be the product of such an energetic planetary collision.

4. Nucleosynthesis and Short-lived Isotopes With the exception of hydrogen and helium the elements were mainly made by stellar nucleosynthesis. If one examines Fig. 5, seven rather striking features stand out. • The estimated abundances of the elements in the Sun and the solar nebula span a huge range of 13 orders of

magnitude. For this reason, they are most easily compared by plotting on a log scale such that the number of atoms of Si is 106 . • Hydrogen and helium are by far the most abundant elements in the Sun as they are elsewhere in the universe. These two elements were made in the Big Bang. • The abundances of the heavier elements generally decrease with increasing atomic number. This is because most of the elements are themselves formed from lighter elements by stellar nucleosyntheis. • Iron is about 1000 times more abundant than its neighbors in the periodic table because of a peak in the binding energy providing enhanced stability during nucleosynthesis. • Lithium, beryllium, and boron are all relatively underabundant compared to other light elements because they are unstable in stellar interiors. • A saw-toothed variability is superimposed on the overall trend reflecting the relatively high stability of evennumbered isotopes compared to odd-numbered ones. • All the elements in the periodic table are present in the solar system except those with no long-lived or stable isotopes, namely technetium (Tc), promethium (Pm), and the trans-uranic elements. Those elements lighter than Fe can be made by fusion because the process of combining two nuclei to make a heavier nuclide releases energy. This produces the energy in stars and is activated when the pressure exceeds a critical threshold (i.e., when a star reaches a certain mass). Larger stars exert more pressure on their cores such that fusion reactions proceed more quickly. When a star has converted all of the hydrogen in its center to helium, it will either die out if it is small or proceed to the next fusion cycle such as the conversion of helium to carbon if it is sufficiently massive to drive this reaction. Lithium, beryllium, and boron are unstable at the temperatures and pressures of stellar interiors, hence the drop in abundance in Fig. 5. They are made by spallation reactions from heavier elements by irradiation in the outer portions of stars. Nearly all nuclides heavier than Fe must be made by neutron irradiation because their synthesis via fusion would consume energy. Neutron addition continues until an unstable isotope is made; it will decay to an isotope of another element, which then receives more neutrons until another unstable nuclide is made and so forth. These are s-process isotopes (produced by a slow burst of neutrons). However, some of these isotopes cannot be made simply by adding a neutron to a stable nuclide because there is no stable isotope with a suitable mass. Such nuclides are instead created with a very high flux of neutrons such that unstable nuclides produced by neutron irradiation receive additional neutrons before they have time to decay, jumping the gap to very heavy nuclides. These are r-process isotopes (produced by

CLBE001-ESS2E

November 8, 2006

16:33

The Origin of the Solar System

39

Because it is easier to measure these effects using isotopic ratios rather than absolute numbers of atoms, we divide by another isotope of Mg:  26

Mg 24 Mg

 =

 26

today

Mg 24 Mg



 + original

26

Al 24 Mg

 (2) original

However, the 26 Al is no longer extant and so cannot be measured. For this reason, we convert Eq. (2) to a form that includes a monitor of the amount of 26 Al that would be determined from the amount of Al today. Aluminum has only one stable nuclide 27 Al. Hence, Eq. (2) becomes  26 FIGURE 14 Most solar system nuclides heavier than hydrogen and helium were produced in stars over the history of our galaxy. This schematic figure shows the difference between nuclides that are stable, those that have very long half-lives (such as 238 U used for determining the ages of geological events and the solar system itself), and those that have short half-lives of 238U/204Pbsolar system~0.14 Core Rich in W and Pb Poor in Hf and U

Silicate Earth or Primitive Mantie Rich in Hf and U Poor in W and Pb

180

Hf/184WSilicate Earth = 15−20 180

Hf/184Wcore = 0

238

U/204PbSilicate Earth = 8−9 238

U/204Pbcore = 0

FIGURE 19 Hafnium–tungsten chronometry provides insights into the rates and mechanisms of formation of the solar system whereas U–Pb chronometry provides us with an absolute age of the solar system. In both cases the radioactive parent/radiogenic daughter element ratio is fractionated by core formation, an early planetary process. It is this fractionation that is being dated. The Hf/W ratio of the total Earth is chondritic (average solar system) because Hf and W are both refractory elements. The U/Pb ratio of the Earth is enhanced relative to average solar system because approximately >80% of the Pb was lost by volatilization or incomplete condensation mainly at an early stage of the development of the circumstellar disk. The fractionation within the Earth for Hf/W and U/Pb is similar. In both cases, the parent (Hf or U) prefers to reside in the silicate portion of the Earth. In both cases the daughter (W or Pb) prefers to reside in the core.

collided with Earth. Most of the xenon produced by radioactive decay of plutonium (half-life 83 Ma) and 129 I has been lost, which implies that Earth’s atmosphere was still being eroded 100 Ma after the start of the solar system, possibly by impacts. Radioactive isotopes can be used to place constraints on the timing of planet formation. The hafnium–tungsten system is particularly useful in this respect because the parent nuclide 182 Hf is lithophile (tending to reside in silicate mantles) while the daughter nuclide 182 W is siderophile (tending to combine with iron during core formation) (Fig. 19). Isotopic data can be used in a variety of ways to define a timescale for planetary accretion. The simplest method uses a model age calculation, which corresponds to the calculated time when an object or sample would have needed to form from a simple average solar system reservoir, as represented by chondrites, in order generate its isotopic composition. For the 182 Hf–182 W system, this time is given as tCHUR =

 182  1 Hf ln 180 λ Hf BSSI ⎛  182 W 184 W SAMPLE

−

Hf 184 W SAMPLE

−

× ⎝  180

 182

⎞⎤

W

184 W

 180 CHONDRITES ⎠⎦ Hf

184 W

CHONDRITES

(13)

CLBE001-ESS2E

November 8, 2006

16:33

46 Encyclopedia of the Solar System where tCHUR is the time of separation from a CHondritic Uniform Reservoir, λ = (ln 2/half-life) is the decay constant for 182 Hf (0.078 per million years) and (182 Hf/180 Hf)BSSI is the bulk solar system initial ratio of 182 Hf to 180 Hf. Tungsten-182 excesses have been found in Earth, Mars, and the HED meteorites, which are thought to come from asteroid Vesta, indicating that all these bodies differentiated while some 182 Hf was still present. Iron meteorites, which come from the cores of differentiated planetesimals, have low Hf/W ratios and are deficient in 182 W. This means these planetesimals must have formed at a very early stage before most of the 182 Hf had decayed. New, very precise 182 Hf–182 W chronometry has shown that some of these objects formed within the first few hundred thousand years of the solar system (Fig. 8). New modeling of the latest 182 Hf–182 W data for martian meteorites also provides evidence that Mars grew and started differentiating within about 1 Ma of the start of the solar system. This short timescale is consistent with runaway growth described earlier. So far, isotopic data for other silicate objects has not been so readily explicable in terms of very rapid growth. However, asteroid Vesta certainly formed within about 3 Ma of the start of the solar system (Fig. 8). The existence of meteorites from differentiated asteroids suggests that core formation began early, and this is confirmed by 182 Hf–182 W chronometry. Therefore, most planetary embryos would have been differentiated when they collided with one another. Although Mars grew extremely rapidly, Earth does not appear to have reached its current size until the giant impact that was associated with the formation of the Moon (see Section 8). 182 Hf–182 W chronometry for lunar samples shows that this took place 35–50 Ma after the start of the solar system. Geochemical evidence has been used to argue that the formation of the Moon probably happened near the end of Earth’s accretion, and this is consistent with the results of Moon-forming impact simulations. This is also consistent with the W isotopic composition of the silicate Earth itself (Fig. 20). This shows that the Earth accreted at least half of its mass within the first 3 × 107 years of the solar system. However, the data are fully consistent with the final stage of accretion being around the time of the Moon-forming impact. Because the Earth accreted over a protracted period rather than in a single event, it is simplest to model the W isotope data in terms of an exponentially decreasing rate of growth (Fig. 20). F = 1 − e −(1/τ )×t

(14)

where F is the mass fraction of the Earth that has accumulated, τ is the mean life for accretion in millions of years (Fig. 20) and t is time in millions of years. This is consistent with the kinds of curves produced by the late George Wetherill who modeled the growth of the terrestrial planets using Monte Carlo simulations. The W isotope data are consistent with a mean life of between 10 and

FIGURE 20 The mean life of accretion of the Earth (τ ) is the inverse of the time constant for exponentially decreasing oligarchic growth from stochastic collisions between planetary embryos and planets. The growth curves corresponding to several such mean lives are shown including the one that most closely matches the calculation made by the late George Wetherill based on Monte Carlo simulations. The mean life determined from tungsten isotopes (Fig. 8) is in excellent agreement with Wetherill’s predictions.

15 Ma, depending on the exact parameters used. This is fully consistent with the timescales proposed by Wetherill. From these protracted timescales, it is clear that Earth took much longer to approach its current size than Mars or Vesta, which probably formed from different mechanisms (Fig. 8).

7. The Asteroid Belt The Asteroid Belt currently contains only enough material to make a planet 2000 times less massive than Earth, even though the spatial extent of the belt is huge. It seems likely that this region once contained much more mass than it does today. A smooth interpolation of the amount of solid material needed to form the inner planets and the gas giants would place about 2 Earth-masses in the Asteroid Belt. Even if most of this mass was lost at an early stage, the surface density of solid material must have been at least 100 times higher than it is today in order to grow bodies the size of Ceres and Vesta (roughly 900 and 500 km in diameter, respectively) in only a few million years. Several regions of the Asteroid Belt contain clusters of asteroids with similar orbits and similar spectral features, suggesting they are made of the same material. These clusters are fragments from the collisional breakup of larger asteroids. There are relatively few of these asteroid families, which implies that catastrophic collisions are quite rare. This suggests the Asteroid Belt has contained relatively little

CLBE001-ESS2E

November 8, 2006

16:33

The Origin of the Solar System

mass for most of its history. The spectrum of asteroid Vesta, located 2.4 AU from the Sun, shows that it has a basaltic crust. The HED meteorites, which probably come from Vesta, show this crust formed only a few million years after the solar system, according to several isotopic systems. The survival of Vesta’s crust suggests that the crust formed the impact rate in the belt has never been much higher than it is today. For these reasons, it is thought that most of the Asteroid Belt’s original mass was removed at a very early stage by a dynamical process rather than by collisional erosion. The Asteroid Belt currently contains a number of orbital resonances associated with the giant planets. Resonances occur when either the orbital period or precession period of an asteroid has a simple ratio with the corresponding period for one of the planets. Many resonances induce large changes in orbital eccentricity, causing asteroids to fall into the Sun, or to come close to Jupiter, leading to close encounters and ejection from the solar system. For this reason, there are very few asteroids that orbit the Sun twice every time Jupiter orbits the Sun once, for example. When the nebular gas was still present, small asteroids moving on eccentric orbits would have drifted inward rapidly due to gas drag. After the giant planets had formed, a combination of resonances and gas drag may have transferred most objects smaller than a few hundred kilometers from the Asteroid Belt into the terrestrial-planet region. Larger planetary embryos would not have drifted very far. However, once oligarchic growth ceased, embryos began to gravitationally scatter one another across the belt. Numerical simulations show that most or all of these bodies would eventually enter a resonance and be removed, leaving an Asteroid Belt greatly depleted in mass and containing no objects bigger than Ceres. The timescale for the depletion of the belt depends sensitively on the orbital eccentricities of the giant planets at the time, which are poorly known. The belt may have been cleared in only a few million years, but it may have required as much as several hundred million years if the giant planets had nearly circular orbits. The albedos and spectral features of asteroids vary widely from one body to another, but clear trends are apparent as one moves across the Asteroid Belt. S-type asteroids, which generally lie in the inner Asteroid Belt, appear to be more thermally processed than the C-type asteroids that dominate the middle belt. These may include the parent bodies of ordinary and carbonaceous chondrites respectively. C-types in turn seem more processed than the P-type asteroids that mostly lie in the outer belt. These differences may reflect differences in the composition of solid materials in different parts of the nebula, or differences in the time at which asteroids formed. Ordinary and enstatite chondrites, which probably come from the inner Asteroid Belt, tend to be dry, while carbonaceous chondrites from the middle and outer belt contain up to 10% water by mass in the form of hydrated minerals. This suggests that temperatures were cold enough in the outer Asteroid Belt for water ice to form

47

and become incorporated into asteroids where it reacted with dry rock. Temperatures were apparently too high for water ice to condense in the inner Asteroid Belt. It is possible that some of the objects currently in the Asteroid Belt formed elsewhere. For example, it has been proposed that many of the parent bodies of the iron meteorites, and possibly Vesta, formed in the terrestrial-planet region and were later gravitationally scattered outward to their current orbits. Iron meteorites from the cores of melted asteroids are common, whereas meteorites from the mantles of these asteroids are rarely seen. This suggests that a substantial amount of collisional erosion took place at an early stage, with only the strong, iron-rich cores of many bodies surviving. A number of other meteorites also show signs that their parent asteroids experienced violent collisions early in their history. Chondrites presumably formed somewhat later than the differentiated asteroids, when the main radioactive heat sources had mostly decayed. Chondrites are mostly composed of chondrules, which typically formed 1–3 Ma after CAIs. Chondrite parent bodies cannot be older than the youngest chondrules they contain, so they must have formed several million years after the start of the solar system. For this reason, it appears that the early stages of planet formation were prolonged in the Asteroid Belt. Chondrites have experienced some degree of thermal processing, but their late formation meant that their parent bodies never grew hot enough to melt, which has allowed chondrules, CAIs, and matrix grains to survive.

8. Growth of Gas and Ice Giant Planets Jupiter and Saturn are mostly composed of hydrogen and helium. These elements do not condense at temperatures and pressures found in protoplanetary disks, so they must have been gravitationally captured from the gaseous component of the solar nebula. Observations of young stars indicate that protoplanetary disks survive for only a few million years, and this sets an upper limit for the amount of time required to form giant planets. Uranus and Neptune also contain significant amounts of hydrogen and helium (somewhere in the range 3–25%), and so they probably also formed quickly, before the solar nebula dispersed. Jupiter and Saturn also contain elements heavier than helium and they are enriched in these elements compared to the Sun. The gravitational field of Saturn strongly suggests it has a core of dense material at its center, containing roughly one fifth of the planet’s total mass. Jupiter may also have a dense core containing a few Earth masses of material. The interior structure of Jupiter remains quite uncertain because we lack adequate equations of state for the behavior of hydrogen at the very high pressures found in the planet’s interior. The upper atmospheres of both planets are enriched in elements such as carbon, nitrogen, sulfur,

CLBE001-ESS2E

November 8, 2006

16:33

48 Encyclopedia of the Solar System and argon, compared to the Sun. It is thought likely that these enrichments extend deep into the planets’ interiors, but this remains uncertain. Giant planets may form directly by the contraction and collapse of gravitationally unstable regions of a protoplanetary disk. This disk instability is analogous to the gravitational instabilities that may have formed planetesimals, but instead the instability takes place in nebula gas rather than the solid component of the disk. Instabilities will occur if the Toomre stability criterion Q becomes close to or lower than 1, where Q=

Msun cs πa 2 vkep

(15)

where vkep is the Keplerian velocity, cs is the sound speed, and  is the local surface density of gas in the disk. Gas in an unstable region quickly becomes much denser than the surrounding material. Disk instability requires high surface densities and low sound speeds (cold gas), so it is most likely to occur in the outer regions of a massive protoplanetary disk. Numerical calculations suggest instabilities will occur beyond about 5 AU in a nebula a few times more massive than the minimum-mass solar nebula. What happens to an unstable region depends on how quickly the gas cools as it contracts, and this is the subject of much debate. If the gas remains hot, the dense regions will quickly become sheared out and destroyed by the differential rotation of the disk. If cooling is efficient, simulations show that gravitationally bound clumps will form in a few hundred years, and these may ultimately contract to form giant planets. Initially, such planets would be homogeneous and have the same composition as the nebula. Their structure and composition may change subsequently due to gravitational settling of heavier elements to the center and capture of rocky or icy bodies such as comets. The evidence for dense cores at the centers of Jupiter and Saturn suggests to many scientists that giant planets form by core accretion rather than disk instability. In this model, the early stages of giant-planet formation mirror the growth of rocky planets, beginning with the formation of planetesimals, followed by runaway and oligarchic growth. However, planetary embryos would have grown larger in the outer solar system for two reasons. First, feeding zones here are larger because the Sun’s gravity is weaker, so each embryo gravitationally holds sway over a larger region of the nebula. Second, temperatures here were cold enough for volatile materials such as tars, water ice, and other ices to condense, so more solid material was available to build large embryos. In the outer solar system, bodies roughly ten times more massive than Earth would have formed via oligarchic growth in a million years, provided the disk was a few times more massive than the minimum-mass solar nebula. Bodies that grew larger than Mars would have captured substantial atmospheres of gas from the nebula. Such atmospheres

remain in equilibrium due to a balance between an embryo’s gravity and an outward pressure gradient. However, there is a critical core mass above which an embryo can no longer support a static atmosphere. Above this limit, the atmosphere begins to collapse onto the planet forming a massive gas envelope that increases in mass over time as more gas is captured from the nebula. As gas falls toward the planet, it heats up as gravitational potential energy is released. The rate at which a planet grows depends on how fast this heat can be radiated away. The critical core mass depends on the opacity of the envelope and the rate at which planetesimals collide with the core, but calculations suggest it is in the range 3–20 Earth masses. The growth of the envelope is slow at first, but speeds up rapidly once an embryo reaches 20–30 Earth masses. Numerical simulations show that Jupiter-mass planets can form this way in 1–5 Ma. Such planets are mostly composed of hydrogen-rich nebular gas, but are also enriched in heavier elements due to the presence of a solid core. As with the disk instability, the planet’s envelope may be further enriched in heavy elements by collisions with comets. Measurements by the Galileo spacecraft showed that Jupiter’s upper atmosphere is enriched in carbon, nitrogen, sulfur, and the noble gases argon, krypton, and xenon by factors of 2–3 compared to the Sun. If these enrichments are typical of Jupiter’s envelope as a whole, it suggests the planet captured a huge number of comets. Argon can be trapped in cometary ices but only if these ices form at temperatures below about 30 K. Temperatures at Jupiter’s current distance from the Sun were probably quite a lot higher than this. This suggests either that the comets came from colder regions of the nebula or that Jupiter itself migrated inward over a large distance. However, the fact that relatively refractory elements such as sulfur are present in the same enrichment as the noble gases suggests these elements may all have been captured as gases from the nebula along with hydrogen and helium. If so, Jupiter’s envelope must be nonhomogeneous, with the lower layers depleted in heavy elements, perhaps due to exclusion from high pressure phases of hydrogen, while the upper layers are enriched. It is unclear why Jupiter and Saturn stopped growing when they reached their current masses. These planets are sufficiently massive that they would continue to grow very rapidly if a supply of gas was available nearby. It is possible, but unlikely, that they stopped growing because the nebula happened to disperse at this point. A more likely explanation is that the growth of these planets slowed because they each became massive enough to clear an annular gap in the nebula around their orbit. Gap clearing happens when a planet’s Hill radius becomes comparable to the vertical thickness of the gas disk, which would have been the case for Jupiter and Saturn. Gas orbiting a little further from the Sun than Jupiter would have been sped up by the planet’s gravitational pull, moving the gas away from the Sun. Gas orbiting closer to the Sun than Jupiter was slowed down, causing it to move inward. These forces open up a gap in

CLBE001-ESS2E

November 8, 2006

16:33

The Origin of the Solar System

the disk around Jupiter’s orbit, balancing viscous forces that would cause gas to flow back into the gap. Numerical simulations show that generally gaps are not cleared completely, and some gas continues to cross a gap and accrete onto a planet. However, the accretion rate declines as a planet becomes more massive. Uranus and Neptune are referred to as ice giant planets because they contain large amounts of materials such as water and methane that form ices at low temperatures. They contain some hydrogen and helium, but they did not acquire the huge gaseous envelopes that Jupiter and Saturn possess. This suggests the nebula gas had largely dispersed in the region where Uranus and Neptune were forming before they became massive enough to undergo rapid gas accretion. This may be because they formed in the outer regions of the protoplanetary disk, where embryo growth rates were slowest. It is also possible that the nebula dispersed more quickly in some regions than others. In particular, the outer regions of the nebula may have disappeared at an early stage as the gas escaped the solar system due to photoevaporation by ultraviolet radiation. The presence of a gap modifies planetary migration. Planets massive enough to open a gap still generate spiral density waves in the gas beyond the gap, but these waves are located further away from the planet as a result, so migration is slower. As a planet with a gap migrates inward, gas tends to pile up at the inner edge of the gap and become rarified at the outer edge, slowing migration as a result. The migration of the planet now becomes tied to the inward viscous accretion of the gas toward the star. The planet, its gap, and the nebular gas all move inward at the same rate, given by da = −1.5α dt



cs vkep

2 vkep

(16)

where α = νvkep /(acs 2 ) and ν is the viscosity of the nebular gas. This is called type-II migration. Type-II migration slows when a planet’s mass becomes comparable to that of the nebula, and migration ceases as the nebular gas disperses. Giant planets in the solar system experienced another kind of migration as they interacted gravitationally with planetesimals moving on orbits between the giant planets and in the primordial Kuiper Belt. One consequence of this process was the formation of the Oort cloud of comets. Once Jupiter approached its current mass, many planetesimals that came close to the planet would have been flung far beyond the outer edge of the protoplanetary disk. Some were ejected from the solar system altogether, but others remained weakly bound to the Sun. Over time, gravitational interactions with molecular clouds, other nearby stars, and the galactic disk circularized the orbits of these objects so they no longer passed through the planetary system. Many of these objects are still present orbiting far from the Sun in the Oort cloud. The ultimate source of angular momentum for these objects came at the expense of Jupiter’s orbit,

49

which shrank accordingly. Saturn, Uranus, and Neptune ejected some planetesimals, but they also perturbed inward many objects, which were then ejected by Jupiter. As a result, Saturn, Uranus, and Neptune probably moved outward rather than inward. As Neptune migrated outward, it interacted dynamically with the primordial Kuiper Belt of comets orbiting in the very outer region of the nebula. Some of these comets were ejected from the solar system or perturbed inward toward Jupiter. Others were perturbed onto highly eccentric orbits with periods of hundreds or thousands of years, and now form the scattered disk,a region that extends out beyond the Kuiper Belt but whose objects are gradually being removed by close encounters with Neptune. A sizable fraction of the objects in the region beyond Neptune were trapped in external mean-motion resonances and migrated outward with the planet. Pluto, currently located in the 3:2 meanmotion resonance with Neptune, probably represents one of these objects. As the giant planets migrated, it is possible that they passed through orbital resonances with one another. In particular, if Jupiter and Saturn passed through the 2:1 mean-motion resonance, their orbital eccentricities would have increased significantly, with important consequences throughout the solar system. The eccentricities of Uranus and Neptune would have briefly become large until they were damped by dynamical friction with the primordial Kuiper Belt. Many comets would have been perturbed into the inner solar system as a result. In addition, the changing orbits of the giant planets would have perturbed many mainbelt asteroids into unstable resonances, also leading to a flux of asteroids into orbits crossing the inner planets. Currently, it is unclear whether Jupiter and Saturn passed through the 2:1 resonance, or when this may have happened. It has been proposed that passage through this resonance was responsible for the late heavy bombardment of the inner planets, which occurred 600–700 Ma after the start of the solar system and left a clear record of impacts on the Moon, Mars, and Mercury.

9. Planetary Satellites Earth’s moon possesses a number of unusual features. It has a low density compared to the inner planets, and it has only a very small core. The Moon is highly depleted in volatile materials such as water. In addition, the Earth– Moon system has a large amount of angular momentum per unit mass. If they were combined into a single body, the object would rotate once every 4 hours! All these features can be understood if the Moon formed as the result of an oblique impact between Earth and another large, differentiated body, sometimes referred to as Theia, late in Earth’s formation. Numerical simulations of this giant impact show that much of Theia’s core would have sunk through Earth’s

CLBE001-ESS2E

November 8, 2006

16:33

50 Encyclopedia of the Solar System mantle to coalesce with Earth’s core. Molten and vaporized mantle material from both bodies was ejected outward. Gravitational torques from the highly nonspherical distribution of matter during the collision gave some of this mantle material enough angular momentum to go into orbit about Earth. This material quickly formed into a disk, from which the Moon accreted. Certain features of the Moon’s composition are very similar to those of the Earth, which means that either ((1)) Theia was formed from similar material, (2) the resulting vapor and debris that condensed to form the Moon totally equilibrated with the outer portions of the Earth, or (3) the Moon is mostly composed of material from Earth rather than Theia, although numerical simulations tend to find that the opposite is true in this case. The impact released huge amounts of energy, heating the disk sufficiently that many volatile materials escaped. As a result, the Moon formed mostly from volatile-depleted mantle materials, explaining its current composition. The simulations suggest Theia probably had a mass similar to Mars, which has roughly one tenth the mass of Earth. We know little about Theia’s composition except that, like Mars, it seems to have been rich in geochemical volatile elements such as rubidium compared to Earth (Fig. 9). The Earth and the Moon have identical oxygen isotope characteristics (Fig. 10). It was once thought that this meant Earth and Theia had a similar isotopic composition, but this similarity now appears to be the result of exchange of material between the Earth and the protolunar disk while the Moon was forming. The satellites of the giant planets are much smaller relative to their parent planet than the Moon is compared to the Earth. Whereas the Moon is roughly 1/80 of the mass of the Earth, the satellite systems of Jupiter, Saturn, and Uranus each contain about 1/10,000 of the mass of their respective planet. The satellites of the giant planets can be divided into two classes with different properties. Those close to their parent planet tend to have nearly circular orbits in the same plane as the planet’s equator and orbiting in the same direction as the planet spins. These are referred to as regular satellites. Satellites orbiting further from the planet tend to have highly inclined and eccentric orbits, and these are called irregular satellites as a result. The regular satellites tend to be larger and include the Galilean satellites of Jupiter and Saturn’s largest satellite Titan. The orbits of the regular satellites suggest they formed from gas-rich circumplanetary disks orbiting each planet, while the irregular satellites are thought to have been captured later. Large satellites would have moved rapidly inward through a circumplanetary disk due to type-I migration, on a timescale that was short compared to the lifetime of the solar nebula. For this reason, it is likely that multiple generations of satellites formed, with the satellites we see today being the last to form. The satellites probably formed from planetesimals originating in the solar nebula that were slowed and captured when they passed through the relatively dense gas in the circumplanetary disk.

Orbital resonances involving two or more satellites are common. For example, the inner three Galilean satellites— Io, Europa, and Ganymede—have orbital periods in the ratio 1:2:4. This contrasts with the absence of resonances between the planets except for Neptune and Pluto. The ubiquity of satellite resonances suggests many of the satellites migrated considerable distances during or after their formation, becoming captured in a resonance en route. Some resonances may have arisen as the growing satellites migrated inward through their planet’s accretion disk. Others could have arisen later as tidal interactions between a planet and its satellite caused the satellites to move outward at different rates. The Neptunian satellite system is different from those of the other giant planets, having relatively few moons with most mass contained in a single large satellite Triton, which is larger than Pluto. Triton is unusual in that its orbit is retrograde, unlike all the other large satellites in the solar system. This suggests it was captured rather than forming in situ. Several capture mechanisms have been proposed, but most are low-probability events, which makes them unlikely to explain the origin of Triton. A more plausible idea is that Triton was once part of a binary planet like the Pluto–Charon system, orbiting around the Sun. During a close encounter with Neptune, the binary components were parted. Triton’s companion remained in orbit about the Sun, taking with it enough kinetic energy to leave Triton in a bound orbit about Neptune. Triton’s orbit would have been highly eccentric initially, but tidal interactions with Neptune caused its orbit to shrink and become more circular over time. As Triton’s orbit shrank, it would have disturbed the orbits of smaller satellites orbiting Neptune, leading to their destruction by mutual collisions. This is presumably the reason for the paucity of regular satellites orbiting Neptune today.

10. Extrasolar Planets At the time of writing, about 200 planets are known orbiting stars other than the Sun. These are referred to as extrasolar planets or exoplanets. Most of these objects have been found using the Doppler radial velocity technique. This makes use of the fact that the gravitational pull of a planet causes its star to move in an ellipse with the same period as the orbital period of the planet. As the star moves toward and away from the observer, lines in its spectra are alternately blue- and red-shifted by the Doppler effect, indicating the planet’s presence. Current levels of precision allow the detection of gas giant planets and also ice giants in some cases, but not Earth-mass planets. The planet’s orbital period P can be readily identified from the radial velocity variation. The mean radius of the planet’s orbit a can then be found using Kepler’s third law if the star’s mass M∗ is known: a3 =

P 2 GM∗ 4π 2

(17)

CLBE001-ESS2E

November 8, 2006

16:33

The Origin of the Solar System

Fraction of Stars with Planets

Unfortunately, the Doppler method determines only one component of the star’s velocity, so the orientation of the orbital plane is not known in general. This means one can obtain only a lower limit on the planet’s mass. For randomly oriented orbits however, the true mass of the planet is most likely to lie within 30% of its minimum value. Some extrasolar planets have been detected when they transit across the face of their star, typically causing the star to dim by 1–2% for a few hours. Only a small fraction of extrasolar planets generate a transit since their orbital plane must be almost edge on as seen from the Earth. When a planet is observed using both the Doppler and transit methods, its true mass can be obtained since the orientation of the orbital plane is known. If the stellar radius is also known, the degree of dimming yields the planet’s radius and hence its density. The densities of extrasolar planets observed this way are generally comparable to that of Jupiter and substantially lower than that of Earth. This suggests these planets are composed mainly of gas rather than rock or ice. In one case, hydrogen has been detected escaping from an extrasolar planet. A few objects have been found whose minimum masses are below 15 Earth masses, and it is plausible that these are more akin to ice giants or even terrestrial planets than gas giants. Stars with known extrasolar planets tend to have high metallicities; that is, they are enriched in elements heavier than helium compared to most stars in the Sun’s neighborhood (Fig. 21). (The Sun also has a high metallicity.) The meaning of this correlation is hotly debated, but it is consistent with the formation of giant planets via core accretion (see Section 8). When a star has a high metallicity, its disk will contain large amounts of the elements needed to form a solid core, promoting rapid growth and increasing the likelihood that a gas giant can form before the gas disk disperses. Both the Doppler velocity and transit techniques are biased toward finding massive planets since these generate a

30 25 20 15 10 5 0 0.63 1.0 1.58 2.51 0.40 Stellar Metallicity (Sun = 1)

FIGURE 21 The fraction of stars that have planets as a function of the stellar metallicity (the abundance of elements heavier than helium compared to the Sun). Here the iron-to-hydrogen ratio relative to the Sun is used a proxy for metallicity.

51

stronger signal. Both are also biased toward detecting planets lying close to their star. In the case of transits, the probability of suitable orbital alignment declines with increasing orbital distance, while for the Doppler velocity method, one generally needs to observe a planet for at least a full orbital period to obtain a firm detection. Despite these biases, it is clear that at least 10% of Sun-like stars have planets, and this fraction may be much higher. The fraction of planets with a given mass increases as the planetary mass grows smaller, despite the strong observational bias working in the opposite direction. Roughly 10% of known extrasolar planets have orbital periods of only a few days, which implies their orbits are several times smaller than Mercury’s orbit about the Sun. These planets are often referred to as hot Jupiters due to their likely high temperatures. Theoretical models of planet formation suggest it is unlikely that planets will form this close to a star. Instead, it is thought that these planets formed at larger distances and moved inward due to type-I and/or type-II migration. Alternatively, they may have been scattered onto highly eccentric orbits following close encounters with other planets in the same system. In this case, subsequent tidal interactions with the star will circularize a planet’s orbit and cause the orbit to shrink. Roughly 20 stars are known to have two or more planets. In a sizable fraction of these cases, the planets are involved in orbital resonances where either the ratio of the orbital periods or precession periods of two planets is close to the ratio of two integers, such as 2:1. This state of affairs has a low probability of occurring by chance, which suggests these planets have been captured into a resonance when the orbits of one or both planets migrated inwards.

11. Summary and Future Prospects Thanks to improvements in isotopic chronology, we now know the timescales over which the Earth, Moon, Mars, and some asteroids formed. Terrestrial-planet accretion started soon after the solar system formed, leading to the growth of some Mars-sized and smaller objects within the first few million, and in some cases only a few hundred thousand, years. This early accretionary phase was accompanied by widespread melting due to heat generated by short-lived isotopes and the formation of planetary cores. The Moon formed relatively late, 30–55 Ma after the start of the solar system, with the most likely date being 40–50 Ma. This was the last major event in Earth’s formation. These isotopic timescales are consistent with theoretical models that predict rapid runaway and oligarchic growth at early times, to form asteroid-to-Mars-sized bodies within a million years, while predicting that Earth took tens of millions of years to grow to its final size. The presence in Earth’s mantle of nonnegligible amounts of siderophile elements such as platinum and osmium argues that roughly 1% of Earth’s mass arrived after its core

CLBE001-ESS2E

November 8, 2006

16:33

52 Encyclopedia of the Solar System had finished forming. For some time it has been postulated that Earth formed in a very dry environment and that its water was delivered along with these siderophile elements in a late veneer. This now appears unlikely given the composition of Earth’s mantle. Instead, Earth probably acquired its water earlier, perhaps from carbonaceous–chondrite-like asteroids, before core formation was complete. This implies that the planet somehow held onto much of its water during the giant impact that led to the formation of the Moon. It now seems that chondrites, the most primitive meteorites in our collection both physically and chemically, actually formed at a rather late stage, long after the parent bodies of the iron meteorites had formed. Chondrites escaped melting because the potent heat sources 26 Al and 60 Fe had largely decayed by that point. For a long time, it has been thought that chondrites, or something similar, provided the basic building blocks of Earth and the other terrestrial planets, but it now seems that the parent bodies of the iron meteorites provide a better analog in this respect. Currently, we do not have good dynamical or cosmochemical models for how chondrites and their constituents formed. Chondrules, CAIs, matrix grains, and presolar grains all survived in the nebula for several million years, undergoing different degrees of thermal processing, and then were collected together into large bodies. The refractory CAIs may have formed close to the Sun prior to being scattered across the disk, perhaps by an x-wind. Supporting evidence for this hypothesis comes from the recent discovery of high-temperature condensates in samples from comet Wild 2 returned by the Stardust mission. Where chondrules formed remains unclear, but these objects would have been highly mobile as long as nebular gas was present, and they may have drifted radially over large distances. The origin of giant planets remains a subject of debate, but the observed correlation between stellar metallicity and the presence of giant planets, and the recent discovery of a Saturn-mass extrasolar planet that appears to have a very massive core, lend weight to the core accretion model. Recent simulations using plausible envelope opacities have found that giant planets can form within the typical lifetime of a protoplanetary disk, overcoming a longstanding obstacle for core accretion. It is becoming apparent that planetary migration is an important feature in the formation and early evolution of planetary systems. This presumably explains the fact that extrasolar planets are seen to orbit

their stars at a wide range of distances. Planets also migrate when they clear away residual planetesimals. This may have led to a dramatic episode early in the history of the solar system associated with the late heavy bombardment of comets and asteroids onto the Moon and inner planets. It is impressive to look back on the past 10 years of discovery in planetary science partly because the breakthroughs have involved so many diverse areas of research. Technology has been a key driver, be it in the form of more powerful computers, mass spectrometers, instrumentation for planetary missions, or new telescopes and detectors. The near future looks equally exciting. The Atacama Large Millimeter Array (ALMA) promises to transform our knowledge of protoplanetary disks with very high spatial resolution able to observe features as small as 1 AU in size and sufficient sensitivity to detect many new molecules including organic materials. Space missions will continue to expand our survey of the solar system, with the Messenger and New Horizons probes en route to Mercury and Pluto, and the Rosetta spacecraft heading for comet Churyumov–Gerasimenko. In addition to NASA and ESA, space agencies in Japan, China, and India are also becoming active players in space exploration. The Doppler radial velocity and transit techniques continue to be refined and are set to expand the catalogue of known extrasolar planets. The relatively new micro-lensing technique is opening up the possibility of finding Earthmass planets. Within the next few years, the Kepler and COROT space missions should finally answer the question of whether Earth-sized planets are common or relatively rare. Here on Earth, continuing analysis of dust samples from comet Wild 2 returned by the Stardust mission, and solar wind samples from the Genesis mission, will enhance our understanding of the cosmochemical evolution of the solar system. New isotopic measurement techniques and a new generation of nanosims ion probes are sure to generate exciting discoveries at a rapid pace. All in all, we have much to look forward to.

Bibliography de Pater, I., and Lissauer, J. J. (2001). “Planetary Sciences.” Cambridge Univ. Press, New York, NY. Lewis, J. S. (2004). “Physics and Chemistry of the Solar System.” Academic Press, San Diego, CA. Reipurth, B., Jewitt, D., and Keil, K. (2006). “Protostars and Planets V.” Univ. Arizona Press, Tucson.

CLBE001-ESS2E

November 11, 2006

100-C

25-C

50-C

75-C

4:27

C+M

50-C+M

C+Y

50-C+Y

M+Y

50-M+Y

100-M

25-M

50-M

75-M

100-Y

25-Y

50-Y

75-Y

100-K

25-K

25-19-19

50-K

50-40-40

75-K

75-64-64

A History of Solar System Studies David Leverington BAE Systems, England (retired)

CHAPTER

1. Babylonians and Greeks 2. Copernicus and Tycho 3. Kepler and Galileo

4. Second Half of the 17th Century 5. The 18th Century 6. The 19th Century

This chapter gives a brief overview of the history of solar system research from the earliest times up to the start of the space age.

1. Babylonians and Greeks Many early civilizations studied the heavens, but it was the Babylonians of the first millennium b.c. who first used mathematics to try to predict the positions of the Sun, Moon, and visible planets (Mercury, Venus, Mars, Jupiter, and Saturn) in the sky. In this they differed from the Greeks, as the Babylonians were priests trying to predict the movement of the heavenly bodies for religious purposes, whereas the Greeks were philosophers trying to understand why they moved in the way they did. The Babylonians were fascinated by numbers, whereas the Greeks were more interested in geometrical figures. The accuracy of the Babylonian predictions in the 2nd century b.c. is remarkable. For example, their estimate of the length of the sidereal year was within 6 minutes of its true value, and that of the average anomalistic month was within 3 seconds. In addition, Jupiter’s sidereal and synodic periods were within 0.01% of their correct values. Pythagoras (c. 580–500 b.c.) was a highly influential early Greek philosopher who set up a school of philosophers, now known as the Pythagoreans. None of Pythagoras’ original writings survive, but later evidence suggests that the

3

7. The 20th Century prior to the Space Age

Pythagoreans were probably the first to believe that the Earth is spherical, and that the planets all move in separate orbits inclined to the celestial equator. But the Pythagorean spherical Earth did not spin and was surrounded by a series of concentric, crystalline spheres supporting the Sun, Moon, and individual planets. Each had its own sphere, which revolved around the Earth at different speeds, producing a musical sound, the ”music of the spheres,” as they went past each other. Hicetus of Syracuse (fl. 5th century b.c.) was the first person to specifically suggest that the Earth spun on its axis, at the center of the universe. This model was further developed by Heracleides who proposed that Mercury and Venus orbited the Sun as it orbited the Earth. Then Aristarchus (c. 310–230 b.c.), who was one of the last of the Pythagoreans, went one step further and proposed a heliocentric (i.e., Sun-centered) universe in which the planets orbit the Sun in the (correct) order of Mercury, Venus, Earth, Mars, Jupiter and Saturn, with the Moon orbiting a spinning Earth. This was 1700 years before Copernicus came up with the same idea. Aristarchus was also the first to produce a realistic estimate for the Earth–Moon distance, although his estimate of the Earth–Sun distance was an order of magnitude too low. While the Pythagoreans were developing their ideas, Plato (c. 427–347 b.c.) was developing a completely different school of thought. Plato, who was a highly respected philosopher, was not too successful with his geocentric (i.e., Earth-centered) model of the universe. His main legacy to

C 2007 by Academic Press. All rights of reproduction in any form reserved. Encyclopedia of the Solar System 2e 

53

CLBE001-ESS2E

November 11, 2006

4:27

54 Encyclopedia of the Solar System astronomy was his teaching that all heavenly bodies must be spherical, as that is the perfect shape, and that they must move in uniform circular orbits, for the same reason. Aristotle (384–322 b.c.), a follower of Plato, was one of the greatest of Greek philosophers. His ideas were to hold sway in Europe until well into the Middle Ages. However, his geocentric model of the universe was highly complex, requiring a total of 56 spheres to explain the motions of the Sun, Moon, and planets. Unfortunately, many of its predictions were wrong, and it soon fell into disuse. Hipparchus (c. 185–120 b.c.), who was the first person to quantify the precession of the equinoxes, was aware that the Sun’s velocity along the ecliptic was not linear. This was known to the Babylonians and to Callippus of Cyzicus, but they did not seek an explanation. Hipparchus, on the other hand, in adopting Plato’s philosophy of uniform circular motion in a geocentric universe, realized that this phenomenon could only be explained if the Sun was orbiting an off-center Earth. However, his estimate of the off-center amount was far too large, although his apogee position was in error by only 35 . The mathematician Apollonius of Perga (c. 265–190 b.c.) appears to have been the first to examine the properties of epicycles. These were later adopted by Ptolemy (c. a.d. 100–170) in his geocentric model of the universe. In Ptolemy’s scheme (Fig. 1), the Moon, Sun, and planets

each describe a circular orbit called an epicycle, the center of which goes in a circle, called a deferent, around a nonspinning Earth. Because the inferior planets, Mercury and Venus, each appear almost symmetrically on both sides of the Sun at maximum elongation, he assumed that the centers of their epicycles were always on a line joining the Earth and Sun. For the superior planets he assumed that the lines linking these with the center of their epicycles were always parallel to the Earth–Sun line. Unfortunately, this simple system did not provide accurate enough position estimates, and so Ptolemy introduced a number of modifications. In the case of the Moon, he made the center of the Moon’s deferent describe a circle whose center was the Earth. For the planets he introduced the concept of an equant, which was a point in space equidistant with the Earth from the center of the deferent (Fig. 2). The equant was the point about which the planet’s angular velocity appeared to be uniform. Other modifications were also required, but by the time he had finished, he was able to make accurate position estimates for all but the Moon and Mercury. In addition, assuming that there were no gaps between the furthest part of one epicycle and the nearest part of the next, he was able to produce an estimate for the size of the solar system of about 20,000 times the radius of the Earth (or about 120 million km). Although this was a gross underestimate, it gave, for the first time, an idea of how large the solar system really was.

FIGURE 1 Ptolemy’s model of the universe in which all bodies, except the Sun (and stars), describe epicycles, the centers of which orbit the Earth in deferents. He assumed that there were no gaps between the circle enclosing the furthest distance of one planet, and that just touching the epicycle of the next planet out from the Earth.

CLBE001-ESS2E

November 11, 2006

4:27

A History of Solar System Studies

55

FIGURE 2 Ptolemy modified his epicycle theory for the superior planets by moving the Earth O from the center M of the deferent, and by defining an equant point E such that the distance EM = MO. He then assumed that the angular velocity of C, the center of the epicycle, is uniform about the equant point E, rather than about the center M of the deferent.

Epicycle

C Planet

Deferent E Equant point M Centre of deferent O Earth

2. Copernicus and Tycho There was virtually no progress in astronomy over the next one thousand years, and during this time many of the Greek texts had been lost in Europe. But in the 12th century Arab translations found their way to Europe, mainly via Islamic Spain. Then in the 14th century Ibn al-Sh¯atir (1304–1375), working in Damascus, improved Ptolemy’s model by modifying his epicycles and deleting his equant. Interestingly, al-Sh¯atir’s system was very much like Copernicus’ later system, but with the Earth, not the Sun, at the center. Copernicus’ heliocentric theory of the universe (Fig. 3) was published in his De Revolutionibus Orbium Caelestium in 1543, the year of his death. Interestingly, in the light of Galileo’s later problems with the Church, the book was well received. This is probably because of the Foreword, which had been written by the theologian Andreas Osiander and explained that the book described a mathematical model of the universe, rather than the universe itself. Copernicus (1473–1543) acknowledged that his idea of a spinning Earth in a heliocentric universe was not new, having been proposed by Aristarchus. In addition, Copernicus’ theory was based on circular motion and still depended on epicycles, although he deleted the equant. But he had resurrected the heliocentric theory, which had not been seriously considered for almost two thousand years, at the height of the Renaissance, which was eager for new ideas. In the Middle Ages, Aristotle’s ideas were taught at all the European universities. But now Copernicus had broken with the Aristotelian concept of a nonspinning Earth at the center of the universe. Then in 1577 Tycho Brahe (1546–

1601) disproved another of Aristotle’s ideas. Aristotle had believed that comets are in the Earth’s atmosphere, but Tycho was unable to measure any clear parallax for the comet of that year. Finally, Tycho, in his book of 1588, rejected another of Aristotle’s ideas, that the heavenly bodies are carried in their orbits on crystalline spheres. This is because,

FIGURE 3 Copernicus’ heliocentric universe, as described in his De Revolutionibus, in which the planets orbit the Sun (Sol) and the Moon orbits the Earth (Terra).

CLBE001-ESS2E

November 11, 2006

4:27

56 Encyclopedia of the Solar System in Tycho’s new model of the universe, all the planets, except the Earth, orbit the Sun as the Sun orbits the Earth. This meant that the sphere that carried Mars around the Sun would intercept that which carried the Sun around the Earth, which was clearly impossible if they were crystalline.

3. Kepler and Galileo Johannes Kepler (1571–1630) looked at the universe in an entirely different way than his predecessors. The Babylonians had examined it arithmetically, and the Greeks and later astronomers had considered it in geometrical terms. Kepler, on the other hand, tried to understand the structure of the solar system by considering physical forces. Kepler conceived of a force emanating from the Sun that pushed the planets around their orbit of the Sun such that planetary movement would stop if the force stopped. The magnitude of his force, and hence the linear velocity of the planets, decreased linearly with distance. This should have resulted in the period of the planets varying as their distance squared, but Kepler made a mathematical error and came up with another relationship. Fortuitously, however, his analysis produced remarkably accurate results. Although Kepler was having some success with this and other theories, he thought he could improve them if he had access to Tycho Brahe’s accurate observational data. So Kepler went to see Tycho; a visit that ended with him joining Tycho and eventually succeeding him after his death. Tycho had initially asked Kepler to analyze Mars’ orbit, a task that he continued well after Tycho’s death. Kepler published his results in 1609 in his book Astronomia Nova, in which he reintroduced the equant, previously deleted by Copernicus. In Kepler’s model, all the planets orbited the Sun in a circle, with the Sun off-center, but he could not find a suitable circle to match Mars’ observations, even with an equant. So he decided to reexamine the Earth’s orbit, as the Earth was the platform from which the observations had been made. Copernicus had proposed that the Earth moved around the Sun in a circle at a uniform speed, with the Sun offcenter. So there had been no need for an equant. But Kepler found that an equant was required to explain the Earth’s orbit. However, even adding this, he could not fit a circle, or even a flattened circle to Mars’ orbit. And so in desperation he tried an ellipse, with the Sun at one focus, and, much to his surprise, it worked. Kepler now considered what type of force was driving the planets in their orbits, and concluded that the basic circular motion was produced by vortices generated by a rotating Sun. Magnetic forces then made the orbits elliptical. So Kepler thought that the Sun rotated on its axis, and that the planets and Sun were magnetic. Initially, Kepler had only shown that Mars moved in an ellipse, but in his Epitome of 1618–1621 he showed that this was the case for all the planets, as well as the Moon

and the satellites of Jupiter. He also stated what we now know as his third law, that the square of the periods of the planets are proportional to the cubes of their mean distances from the Sun. Finally, in his Rudolphine Tables, he listed detailed predictions for planetary positions and predicted the transits of Mercury and Venus across the Sun’s disc. Galileo Galilei (1564–1642) made his first telescopes in 1609 and started his first telescopic observations of the Moon in November of that year. He noticed that the terminator had a very irregular shape and concluded that this was because the Moon had mountains and valleys. It was quite unlike the pure spherical body of Aristotle’s cosmology. Galileo undertook a series of observations of Jupiter in January 1610 and found that it had four moons that changed their positions from night to night (Fig. 4). Galileo presented his early Moon and Jupiter observations in his Sidereus Nuncius published in March 1610. By 1612, he had determined the periods of Jupiter’s moons to within a few minutes. Galileo’s Sidereus Nuncius created quite a stir, with many people suggesting that Galileo’s images of Jupiter’s moons were an illusion. Kepler, who was in communication with Galileo, first saw the moons himself in August 1610 and supported Galileo against his doubters. The month before, Galileo had also seen what he took to be two moons on either side of Saturn, but for some reason they did not move. Finally in late 1610 he observed the phases of Venus, finally proving that Ptolemy’s structure of the solar system was incorrect. As a result, Galileo settled on the Copernican heliocentric system. Sunspots had been seen from time to time in antiquity, but most people took them to be something between the

FIGURE 4 Galileo’s observations of the moons of Jupiter on consecutive nights from 7 to 13 January (excluding 9 January) 1610, as shown in his book Sidereus Nuncius.

CLBE001-ESS2E

November 11, 2006

4:27

A History of Solar System Studies

Earth and Sun. Although Thomas Harriot and Galileo had both seen sunspots telescopically in 1610, it was Johann Fabricius who first published his results in June 1611. He concluded that they were on the surface of the Sun, and that their movement indicated that the Sun was rotating. This was completely against Aristotle’s teachings that the Sun was a perfect body. In the meantime, Galileo had visited the Jesuits of the Roman College to get their support for his work and, in particular, their support for Copernicus’ heliocentric cosmology. His reception was very warm, and he was even received in audience by the pope. But, although the Roman Catholic Church did not argue with his observations, outlined above, there was considerable unease at his interpretation. Initially, the Church was prepared to tolerate Galileo’s support of the Copernican cosmology, provided he presented this cosmology as a working hypothesis, rather than as a universal truth. But Galileo was stubborn and tried to take on the Church in its interpretation of theology. In this he could not win, of course, and the Church put him on trial, where he was treated very well. Nevertheless, he was forced in 1633 to recant his views and was then placed under house arrest for the remaining nine years of his life.

4. Second Half of the 17th Century

57

4.2 Saturn Christiaan Huygens (1629–1695) and his brother Constantyn finished building a state-of-the-art telescope in early 1655. Shortly afterwards Christiaan discovered Saturn’s first Moon, Titan, which he announced in his De Saturni of 1656. The next four moons of Saturn were discovered by Gian Domenico Cassini (1625–1712); Iapetus in 1671, Rhea in 1672, and both Tethys and Dione in 1684. Huygens had also mentioned in De Saturni that he had solved the problem of Saturn’s two “moons” observed by Galileo. In fact, the behavior of these moons had been very odd, as they had both completely disappeared in November 1612, reappearing again in mid 1613. Since then, their shape had gradually changed. In 1650, Francesco Grimaldi discovered Saturn’s polar flattening, but still the behavior of the moons, then called ansae, was unexplained. Finally, Huygens announced, in his Systema Saturnium of 1659, that the ansae were actually a thin, flat, solid ring, which was inclined to the ecliptic, and so changed its appearance with time. Then in 1675 Cassini noticed that Saturn’s ring was divided in two by a dark line, now called the Cassini Division, going all the way around the planet. Cassini speculated that the two rings were not solid but composed of swarms of small satellites. Other major observational discoveries of this period are listed in Table 1.

4.1 The Moon Thomas Harriot (1560–1621) was the first astronomer to record what we now know as the libration in latitude of the Moon, which has a period of one month. This occurs because the Moon’s spin axis is not perpendicular to its orbit. A little later Galileo detected a libration in longitude, which he thought had a period of one day. In fact, it has a period of one month and is caused by the eccentricity of the Moon’s orbit. Although Galileo thought that the Moon has an atmosphere, he concluded that there was very little water on the surface as there were no clouds. His early telescopes were not sufficiently powerful, however, to show much surface detail. But over the next few decades, maps of the Moon were produced by a number of astronomers. The most definitive of which were published in 1647 by Johannes Hevelius (1611–1687). They were the first to show the effect of libration. By midcentury, it was clear that there were numerous craters on the Moon, and in 1665 Robert Hooke (1635– 1703) speculated on their cause in his Micrographia. He undertook laboratory-like experiments and noted that if round objects were dropped into a mixture of clay and water, features that resemble lunar craters were produced. But he could not think of the source of large objects hitting the Moon. However, he also found that he could produce crater-like features if he boiled dry alabaster powder in a container. As a result, he concluded that lunar craters are produced by the collapsed blisters of warm viscous lava.

4.3 Newton Kepler had thought that the planets were being pushed around their orbits by a vortex emanating from the Sun but attributed the tides on Earth to the combined attraction of the Sun and Moon by a gravitational force. It seems strange to us that he did not think of this attractive force as having some effect on the orbits of the planets. Rene´ Descartes (1596–1650) also developed a vortex theory to explain the motion of the planets. In his theory, the vortices are in the ether, which is a frictionless fluid filling the universe. In his Principia of 1644, Descartes stated that each planet had two “tendencies”: one tangential to its orbit and one away from the orbit’s center. It is the pressure in the vortex that counterbalances the latter and keeps the planet in its orbit. In 1664, Isaac Newton (1642–1727) started to consider the motion of a body in a circle. In the following year, he proved that the force acting radially on such a body is proportional to its mass multiplied by its velocity squared, and divided by the radius of the circle (i.e., mv2 /r). From this, he was able to prove that the force on a planet moving in a circular orbit is inversely proportional to the square of its distance from the center. Newton realized that this outward centrifugal force on a planet must be counterbalanced by an equal and opposite centripetal force, but it was not obvious at that time that this force was gravity.

CLBE001-ESS2E

November 11, 2006

4:27

58 Encyclopedia of the Solar System

TABLE 1

Key Solar System Discoveries and Observations, 1630–1700

Sun-Earth distance 1672 Richer, Cassini, and Picard deduce a solar parallax of 9.5 minutes of arc from observations of the parallax of Mars. John Flamsteed independently deduces a similar value. This implied a Sun-Earth distance of about 22,000 earth radii, or 140 million km. Moon See main text Mercury 1631 1639 Venus 1639 1646 1667

First observation of a transit of Mercury by Gassendi, Remus, and Cysat—all independently. It occurred on the date predicted by Kepler. Phases of Mercury first observed by Zupus. First observation of a transit of Venus by Horrocks and Crabtree. Fontana observes that Venus’ terminator is uneven, attributing the cause to high mountains. (This is now known to be incorrect; Venus is covered in dense clouds.) Cassini deduces a rotation period of about 24 hours. (This is now known to be incorrect).

Mars 1659 1672

Huygens observes Syrtis Major and deduces a planetary rotation period of about 24 hours. Huygens first unambiguously records the south polar cap.

Jupiter c. 1630 1643 1663 1665 1690 1691

Fontana, Torricelli, and Zucchi independently observe the main belts. Riccioli observes the shadows of the Galilean satellites on Jupiter’s disc. Cassini deduces a Jupiter rotation period of 9 h 56 min. Cassini observes a prominent spot that may be an early appearance of the Great Red Spot. Cassini observes the differential rotation of Jupiter. Cassini observes Jupiter’s polar flattening, which he estimates to be about 7%.

Saturn See main text

At this time, it was known that gravity acted on objects on the Earth’s surface, but it was not known how far from Earth gravity extended. To get a better understanding of this, Newton devised his so-called Moon test. In this test, he compared the force acting on the Moon, because of its motion in a circle, with the force of the Earth’s gravity at the Moon’s orbit and found that they were not the same. The difference was not large, but it was sufficient to cause Newton to stop work on gravity. In fact, at that time, Newton appears to have thought that the centripetal force was a mixture of the gravitational force and the force created by vortices in the ether, so he may not have been too surprised by his result. Newton was finally prompted to return to the subject of gravity by an exchange of letters with Robert Hooke in 1679. In the following year, Newton proved that, assuming an inverse square law of attraction, planets and moons will orbit a central body in an ellipse, with the central body at one focus. Then in 1684 he finally rejected the idea of etherial vortices and started to develop his theory of universal gravitation. It was during this period that the comet of 1680 appeared. At that time, most astronomers, including New-

ton, believed that comets described rectilinear orbits. John Flamsteed (1646–1719), on the other hand, believed that comets described closed orbits, and he suggested, in a letter to Edmond Halley (1656–1742), that the 1680 comet had passed in front of the Sun. Newton, who had been sent a copy of this letter, thought, like a number of astronomers, that there had been two comets, one approaching the Sun and one retreating. Further communications between Flamsteed and Newton in 1681 did not resolve their disagreements, causing Newton to drop the subject of cometary orbits. Eventually, Newton returned to the subject, and by 1686 he had changed his position entirely, as he proved that cometary orbits are highly elliptical or parabolic, to a first approximation. So the 1680 comet had been one comet after all. Newton now felt, having solved the problem of cometary orbits, that he could complete his Principia, which was published in 1687. Newton developed his universal theory of gravitation in his Principia, which ran to three editions. For example, he used Venus to “weigh” the Sun, and planetary moons to weight their parent planets, and by the third edition he had deduced the masses and densities for the Earth, Jupiter, and Saturn relative to the Sun (Table 2).

CLBE001-ESS2E

November 11, 2006

4:27

A History of Solar System Studies

TABLE 2

59

A Comparison of Newton’s Results (Relative to the Sun) with Modern Values Mass

Sun Earth Jupiter Saturn

Density

Principia

Modern Value

1 1/169,282 1/1,067 1/3,021

1 1/332,980 1/1,047 1/3,498

Newton realized that if gravity was really universal, then not only would the Sun’s gravity affect the orbit of a planet, and the planet’s gravity affect the orbit of its moons, but the Sun would also affect the orbits of the moons, and one planet would affect the orbits of other planets. In particular, Newton calculated that Jupiter, at its closest approach to Saturn, would have about 1/217 times the gravitational attraction of the Sun. So he was delighted when Flamsteed told him that Saturn’s orbit did not seem to fit exactly the orbit that it should if it was only influenced by the Sun. Gravity really did appear to be universal. Richer, Cassini, and Picard had found evidence in 1672 that the Earth had an equatorial bulge. Newton was able to use his new gravitational theory to calculate a theoretical value for this oblateness of 1/230 (modern value 1/298). He then considered the gravitational attraction of the Moon and Sun on the oblate Earth and calculated that the Earth’s spin axis should precess at a rate of about 50 .0 per annum (modern value 50 .3). This explained the precession of the equinoxes.

5. The 18th Century 5.1 Halley’s Comet Halley used Newton’s methodology to determine the orbits of 24 comets that had been observed between 1337 and 1698. None of them appeared to be hyperbolic, and so the comets were all clearly permanent members of the solar system. Halley also concluded that the comets of 1531, 1607, and 1682 were successive appearances of the same comet as their orbital elements were very similar. But the time intervals between successive perihelia were not the same; a fact he attributed to the perturbing effect of Jupiter. Taking this into account, he predicted in 1717 that the comet would return in late 1758 or early 1759. Shortly before the expected return of this comet, which we now called Halley’s comet, Alexis Clairaut (1713–1765) attempted to produce a more accurate prediction of its perihelion date. He used a new approximate solution to the three-body problem that allowed him to take account of planetary perturbations. This showed that the return would be delayed by 518 days due to Jupiter and 100 days due to

Principia

100 400 94.5 67

Modern Value

100 392 94.2 49

Saturn. As a result, he predicted that Halley’s comet would reach perihelion on about 15 April 1759 ± 1 month. It did so on 13 March 1759, so Clairaut was just 33 days out with his estimate.

5.2 The 1761 and 1769 Transits of Venus James Gregory (1638–1675) had suggested in 1663 that observations of a transit of Mercury could be used to determine the solar parallax, and hence the distance of the Sun from Earth. Such a determination required observations from at least two different places on Earth, separated by as large a distance as possible. In 1677, Edmond Halley observed such a transit when he was on St. Helena observing the southern sky. But, when he returned, he found that Jean Gallet in Avignon seemed to have been the only other person who had recorded the transit. Unfortunately, there were too many problems in comparing their results, which resulted in a highly inaccurate solar parallax. In 1678, Halley reviewed possible methods of measuring the solar parallax and suggested that transits of Venus would produce the most accurate results. The problem was, however, that these occur in pairs, 8 years apart, only every 120 years. The next pair were due almost one hundred years later, in 1761 and 1769. Joseph Delisle (1688–1768) took up Halley’s suggestion and tried to motivate the astronomical community to undertake coordinated observations of the 1761 transit. After much discussion, the French Academy of Sciences sent observers to Vienna, Siberia, India, and an island in the Indian Ocean, while other countries sent observers to St. Helena, Indonesia, Newfoundland, and Norway. Unfortunately, precise timing of the planetary contacts proved much more difficult than expected, resulting in solar parallaxes ranging from 8 .3 to 10 .6. Interestingly, several observers noticed that Venus appeared to be surrounded by a luminous ring when the planet was partially on the Sun. Mikhail Lomonsov (1711–1765) correctly concluded that this showed that Venus was surrounded by an extensive atmosphere. The lessons learned from the 1761 transit were invaluable in observing the next transit in 1769. This was undertaken from over 70 different sites, and analysis of all the results eventually yielded a best estimate of 8 .6 (modern value 8 .79) for the solar parallax.

CLBE001-ESS2E

November 11, 2006

4:27

60 Encyclopedia of the Solar System 5.3 The Discovery of Uranus On 13 March 1781, William Herschel (1738–1822), whilst looking for double stars, noticed what he thought was a comet. Four days later, when he next saw the object, it had clearly moved, confirming Herschel’s suspicion that it was a comet. He then wrote to Nevil Maskelyne (1732–1811), the Astronomer Royal, notifying him of his discovery. As a result, Maskelyne observed the object on a number of occasions, but he was unsure as to whether it was a comet or a new planet. Over the next few weeks a number of astronomers observed the object and calculated its orbit, which was found to be essentially circular. So it was a planet, now called Uranus. It was the first planet to be discovered since ancient times, and its discovery had a profound effect on the astronomical community, indicating that there may yet be more undiscovered planets in the solar system. A few years later Herschel discovered the first two of Uranus’ satellites, now called Titania and Oberon, with orbits at a considerable angle to orbit.

of 1772. However, what is now known as the Titius–Bode series was not considered of any particular significance, until Uranus was found with an orbital radius of 18.9 AU. This was very close to the 19.6 AU required by the series. In 1800, a group of astronomers, who came to be known as the Celestial Police, agreed to undertake a search for the missing planet. But before they could start Giuseppe Piazzi (1746–1826) found a likely candidate by accident in January 1801. Unfortunately, although he observed the object for about 6 weeks, he was unable to fit an orbit, and wondered if it was a comet. But Karl Gauss (1777–1855) had derived a new method of determining orbits from a limited amount of information, and in November of that year he was able to fit an orbit. It was clearly a planet, now called Ceres, at almost exactly the expected distance from the Sun. But it was much smaller than any other planet. Then in March 1802 Heinrich Olbers (1758–1840) found another, similar object, now called Pallas, at a similar distance from the Sun. At first Olbers thought that these two objects may be the remnants of an exploded planet. But he dropped the idea after the discovery of the fourth such asteroid, as they are now called, in 1807, because its orbit was inconsistent with his theory.

5.4 Origin of the Solar System Immanuel Kant (1724–1804) outlined his theory of the origin of the solar system in his Universal Natural History of 1755. In this he suggested that the solar system had condensed out of a nebulous mass of gas, which had developed into a flat rotating disc as it contracted. As it continued to contract, it spun faster and faster, throwing off masses of gas that cooled to form the planets. However, Kant had difficulty in explaining how a nebula with random internal motions could start rotating when it started to contract. Forty years later, Laplace (1749–1827) independently produced a similar but more detailed theory. In his theory, the mass of gas was rotating before it started contracting. As it contracted, it spun faster, progressively throwing from its outer edge rings of material that condensed to form the planets. Laplace suggested that the planetary satellites formed in a similar way from condensing rings of material around each of the protoplanets. Saturn’s rings did not condense to form a satellite because they were too close to the planet. At face value, the theory seemed plausible, but it became clear in the 19th century that the original solar nebula did not have enough angular momentum to spin off the required material.

5.5 The First Asteroids A number of astronomers had wondered why there was such a large gap in the solar system between the orbits of Mars and Jupiter. Then in 1766 Johann Titius (1729–1796) produced a numerical series that indicated that there should be an object orbiting the Sun with an orbital radius of 2.8 astronomical units (AUs). Johann Elert Bode (1747–1826) was convinced that this was correct and mentioned it in his book

6. The 19th Century 6.1 The Sun Sunspots were still an enigma in the 19th century. Many astronomers thought that they were holes in the photosphere, but because the Sun was presumably hotter beneath the photosphere, the Sunspots should appear bright rather than dark. Then in 1872 Angelo Secchi suggested that matter was ejected from the surface of the Sun at the edges of a sunspot. This matter then cooled and fell back into the center of the spot, so producing its dark central region. In 1843, Heinrich Schwabe found that the number of sunspots varied with a period of about 10 years. A little later Rudolf Wolf analyzed historical records that showed periods ranging from 7 to 17 years, with an average of 11.1 years. Then in 1852, Sabine, Wolf, and Gautier independently concluded that there was a correlation between sunspots and disturbances in the Earth’s magnetic field. There were also various unsuccessful attempts to link the sunspot cycle to the Earth’s weather. But toward the end of the century, Walter Maunder pointed out that there had been a lack of sunspots between about 1645 and 1715. He suggested that this period, now called the Maunder Minimum, could have had a more profound effect on the Earth’s weather than the 11-year solar cycle. In 1858, Richard Carrington discovered that the latitude of sunspots changed over the solar cycle. In the following year, he found that sunspots near the solar equator moved faster than those at higher latitudes, showing that the Sun did not rotate as a rigid body. This so-called differential rotation of the Sun was interpreted by Secchi as indicating

CLBE001-ESS2E

November 11, 2006

4:27

A History of Solar System Studies

that the Sun was gaseous. In the same year, Carrington and Hodgson independently observed two white light solar flares moving over the surface of a large sunspot. About 36 hours later, this was followed by a major geomagnetic storm. Astronomy was revolutionized in the 19th century by Kirchoff’s and Bunsen’s development of spectroscopy in the early 1860s, which, for the first time, enabled astronomers to determine the chemical composition of celestial objects. Kirchoff measured thousands of dark Fraunhofer lines in the solar spectrum and recognized the lines of sodium and iron. By the end of the century, about 40 different elements had been discovered on the Sun. Solar prominences had been observed during a total solar eclipse in 1733, but it was not until 1860 that they were proved to be connected with the Sun rather than the Moon. Spectroscopic observations during and after the 1868 total eclipse showed that prominences were composed of hydrogen and an element that produced a bright yellow line. This was initially attributed to sodium, but Norman Lockyer suggested that it was caused by a new element that he called helium. This was confirmed when helium was found on Earth in 1895.

6.2 Vulcan Newton’s gravitational theory had been remarkably accurate in explaining the movement of the planets, but by the 19th century there appeared to be something wrong with the orbit of Mercury. In 1858, Le Verrier analyzed data from a number of transits and concluded that the perihelion of Mercury’s orbit was precessing at about 565 /century, which was 38 /century more than could be accounted for using Newton’s theory. As a result, Le Verrier suggested that there was an unknown planet called Vulcan, inside the orbit of Mercury, causing the extra precession. A number of astronomers reported seeing such a planet, but none of the observations stood up to detailed scrutiny, and the idea was eventually dropped. Einstein finally solved the problem of Mercury’s perihelion precession in 1915 with his general theory of relativity. No extra planets were required.

6.3 Mercury There was considerable disagreement among astronomers in the 19th century on what could be seen on Mercury. Some thought that they could see an atmosphere around the planet, but others could not. Hermann Vogel detected water vapor lines in its spectrum, and Angelo Secchi saw clouds in its atmosphere. However, Friedrich Zollner ¨ concluded, from his photometer measurements, that Mercury was more like the Moon with, at most, a very thin atmosphere. A number of astronomers detected markings on Mercury’s disc in the middle of the 19th century and concluded that the planet’s period is about 24 hours. On the other hand, Daniel Kirkwood maintained that it should have a

61

synchronous rotation period because of tidal effects of the Sun on its crust. In the 1880s, Giovanni Schiaparelli confirmed this synchronous rotation observationally, and in 1897 Percival Lowell came to the same conclusion. So at the end of the century, synchronous rotation was thought to be the most likely.

6.4 Venus In the 18th century, Venus was thought to have an axial rotation rate of about 24 hours. In fact, a 24-hour period was generally accepted until in 1890 Schiaparelli and others concluded that it, like Mercury, has a synchronous rotation period. Spectroscopic observations of Venus yielded conflicting results in the 19th century. A number of astronomers detected oxygen and water vapor lines in its atmosphere; however, W. W. Campbell, who used the powerful Lick telescopes, could find no such lines.

6.5 The Moon The impact theory for the formation of lunar craters was resurrected at the start of the 19th century, after the discovery of the first asteroids and a number of meteorites. There now seemed to be a ready source of impacting bodies, which Hooke had been unaware of when he had abandoned his impact hypothesis. But both the impact and volcanic theories still had problems. Most meteorites would not hit the lunar surface vertically, and so the craters should be elliptical, but they were mostly circular. Also, as Grove K. Gilbert pointed out, the floors of lunar craters are generally below the height of their surrounding area, whereas on Earth the floors of volcanic craters are generally higher than their surroundings. Edmond Halley had discovered in 1693 that the Moon’s position in the sky was in advance of where it should be based on ancient eclipse records. This so-called secular acceleration of the Moon could be because the Moon was accelerating in its orbit, and/or because the Earth’s spin rate was slowing down. In 1787, Laplace had shown that the observed effect, which was about 10 /century2 , could be completely explained by planetary perturbations. But in 1853, John Couch Adams included some of Laplace’s secondorder terms, which Laplace had omitted, so reducing the calculated figure from 10 /century2 to just 6 /century2 . Charles Delaunay suggested that the missing amount was probably due to tidal friction, but it was impossible at that time to produce a reasonably accurate estimate of the effect. In the early 20th century, Taylor and Jeffreys produced the necessary calculations, showing that Delaunay was correct. In 1879, George Darwin developed a theory of the origin of the Moon. In this the proto-Earth had gradually contracted and increased its spin rate as it cooled. Then, when the spin rate had reached about 3 hours per revolution, it had broken into two unequal parts: the Earth and the Moon.

CLBE001-ESS2E

November 11, 2006

4:27

62 Encyclopedia of the Solar System After breakup, tidal forces had caused the Earth’s spin rate to slow down and the Moon’s orbit to gradually increase in size. A major problem with this theory was that the Earth would have had a tendency to break up the Moon shortly after separation. It was not clear whether the Moon could have passed through the danger zone before this could have happened.

6.6 The Earth Karl Friedrich Kustner ¨ undertook precise position measurements of a number of stars in 1884 and 1885 from the Berlin Observatory. When he analyzed his results, however, he found that the latitude of the observatory had apparently decreased by about 0.20 in a year. Intrigued, the International Commission for Geodesy (ICG) decided to organize a series of observations around the world to define the effect more precisely. These results indicated that the Earth’s spin axis was moving, relative to its surface, with a period of about 12 or 13 months. Seth Chandler had also noticed slight variations in the latitude of the Harvard College Observatory, at about the same time as Kustner ¨ was making his measurements, but Chandler had not taken the matter further. Galvanized by Kustner’s ¨ and the ICG’s results, however, he undertook a thorough review of all available data. As a result, he concluded that the observed effect had two components. One had a period of 14 months, and was due to the nonrigid Earth not spinning around its shortest diameter. The other, which had a period of a year, was due to the seasonal movement of water and air from one hemisphere to the other and back.

6.7 Mars The first systematic investigation of Mars’ polar caps had been undertaken in the 18th century by Giacomo Maraldi, who found that the south polar cap had completely disappeared in late 1719, only to reappear later. William Herschel

suggested that this was because it consisted of ice and snow that melted in the southern summer. At the end of the 18th century, most astronomers thought that the reddish color of Mars was due to its atmosphere. But in 1830, John Herschel suggested that it was the true color of its surface. Camille Flammarion, on the other hand, hypothesized that it was the color of its vegetation. It was generally believed by astronomers in the mid-19th century that there must be some form of life on Mars, even if it was only plant life, because the planet clearly had an atmosphere and a surface that exhibited seasonal effects. The polar caps were apparently made of ice or snow, and there were dark areas on the surface that may be seas. Schiaparelli produced a map of Mars, following its 1877 opposition, that showed a network of linear features that he called canali. This was translated incorrectly into English as canals, which implied that they had been built by intelligent beings. Schiaparelli and others saw more canali in subsequent years (Fig. 5), but other, equally competent observers could not see them at all. Percival Lowell then went further than Schiaparelli in not only observing many canali, but interpreting them to be a network of artificial irrigation channels. At the end of the century, the debate as to whether these canali really existed was still in full swing. Spectroscopic observations of Mars in the late 19th century yielded conflicting results. Some astronomers detected oxygen and water vapor lines, whereas Campbell at the Lick Observatory could find none. There was also a problem with the polar caps: Calculations showed that the average temperature of Mars should be about −34◦ C, yet both polar caps clearly melted substantially in summer, which they should not have done if they had been made of water ice or snow. In 1898, Ranyard and Stoney suggested that the caps could be made of frozen carbon dioxide. But there appeared to be a melt band at the edge of the caps in spring, yet carbon dioxide should sublimate directly into gas on Mars. Two satellites of Mars, now called Phobos and Deimos, were discovered by Asaph Hall in 1877. Their orbits were extremely close to the planet, and the satellites were both very small. As a result, they were thought to be captured asteroids. FIGURE 5 Schiaparelli’s map of Mars produced following the 1881 opposition. A large number of canali are seen, many of them double. (From Robert Ball, 1897, “The Story of the Heavens,” Plate XVIII.)

CLBE001-ESS2E

November 11, 2006

4:27

A History of Solar System Studies

63

6.8 Jupiter The Great Red Spot (GRS) was first clearly observed in the 1870s. Then in 1880 an unusually bright, white equatorial spot appeared; it rotated around Jupiter over 5 minutes faster than the GRS. This gave a differential velocity of about 400 km/h. But the rotation rates of both the white spot and the GRS were not constant, indicating that neither could be surface features as some astronomers had supposed. White and dark spots were continuously appearing and disappearing on Jupiter, suggesting that they were probably clouds. But the GRS was completely different because, although it changed its appearance and size over time, it was still there at the end of the century. This longevity led astronomers to wonder if it could really be a cloud system. In 1778, Leclerc, Compte de Buffon, had suggested that rapid changes in Jupiter’s appearance showed that it had not completely cooled down since its formation. In the 19th century, Jupiter’s differential rotation and low density, which were both similar in nature to those of the Sun, caused some astronomers to go even further and wonder if Jupiter was self-luminous. Although this was considered unlikely, the idea had not been completely ruled out by the end of the century. William Herschel had concluded in 1797 that the axial rotation rates of the four Galilean satellites were synchronous. However, it was not until the 1870s that Engelmann and Burton independently confirmed this for Callisto and the 1890s that Pickering and Douglass confirmed it for Ganymede. The rotation rates of Io and Europa were still unclear. In 1892, Edward Barnard discovered Jupiter’s fifth satellite, now called Amalthea, very close to the planet, when he was observing Jupiter visually through the 36-in. Lick refractor. Amalthea was very small compared to the four Galilean satellites. It was the last satellite of any planet to be discovered visually.

6.9 Saturn In 1837, Johann Encke found that the A ring was divided into two by a clear gap, now called the Encke Division. Then in 1850 W. C. and G. P. Bond discovered a third ring, now called the C ring, inside the B ring. The new ring was very dark (Fig. 6) and partly transparent. In 1867, Kirkwood pointed out that any particles in the Cassini Division would have periods of about one-half that of Mimas, one-third that of Enceladus, one-quarter that of Tethys, and one-sixth that of Dione. He concluded that these resonances had created the Cassini Division, which would be clear of particles. The true nature of Saturn’s rings had been a complete mystery in the 18th century. Cassini had thought that they may be composed of many small satellites, and Laplace had suggested that they were made of a number of thin

FIGURE 6 Trouvelot’s 1874 drawing of Saturn. It clearly shows the dark C ring extending from the inner edge of the B ring to about half-way to the planet. (From Edmund Ledger, 1882, “The Sun: Its Planets and Their Satellites,” Plate IX.)

solid rings. Others thought that they may be liquid. But in 1857, James Clerk Maxwell proved mathematically that they could not be solid or liquid. Instead, he concluded that they were composed of an indefinite number of small particles. Two new satellites were found in the 19th century: Hyperion by G. P. Bond in 1848 and Phoebe by William Pickering 50 years later. Phoebe was the first satellite in the solar system to be discovered photographically. It was some 13 million kilometers from Saturn, in a highly eccentric, retrograde orbit. So it appeared to be a captured object.

6.10 Uranus Little was know about Uranus in the 19th century. William Herschel had noticed that Uranus had a polar flattening, its orientation indicating that its axis of rotation was perpendicular to the plane of its satellites. But observations of apparent surface features produced very different orientations. Uranus’ spectrum appeared to be clearly different from those of Jupiter and Saturn, but it was very difficult to interpret. There was even confusion about the discovery of new satellites. It was not until 1851 that William Lassell could be sure that he had discovered two new satellites, now called Ariel and Umbriel within the orbit of Titania. He had, in fact, seen them both some years before, but his earlier observations had been too infrequent to produce clear orbits.

6.11 The Discovery of Neptune In 1821, Alexis Bouvard tried to produce an orbit for Uranus using both prediscovery and postdiscovery observations. But he could not find a single orbit to fit them. The best he could manage was an orbit based on only the postdiscovery observations; he published the result but admitted that it was less than ideal. However, it did not take long for Uranus

CLBE001-ESS2E

November 11, 2006

4:27

64 Encyclopedia of the Solar System to deviate more and more from even this orbit. One possible explanation was that Uranus was being disturbed by yet another planet, and if the Titius–Bode series was correct it would be about 38.8 AU from the Sun. In 1843, the Englishman John Couch Adams set out to try to calculate the orbit of the planet that seemed to be disturbing the orbit of Uranus. By September 1845, he had calculated its orbital elements and its expected position in the sky, and over the next year, he progressively updated this prediction. Unfortunately, these predictions varied wildly, making it impossible to use them for a telescopic search of the real planet. In parallel, and unknown to both men, Urbain Le Verrier, a French astronomer, undertook the same task. He published his final results in August 1846 and asked Johann Galle of the Berlin Observatory if he would undertake a telescope search for it. Galle and his assistant d’Arrest found the planet within an hour of starting the search on 23 September 1846. There then followed a monumental argument between the English and French astronomical establishments on the priority for the orbital predictions. But much of the evidence on the English side was never published, and an “official line” was agreed. That evidence has recently come to light, however, and it is currently being analyzed to establish the exact sequence of events. What is clear, however, is that when Neptune’s real orbit was calculated, it turned out to be quite different from either of the orbits predicted by Le Verrier or Adams. So its discovery had been somewhat fortuitous. Less than a month after Neptune’s discovery, William Lassell observed an object close to Neptune, which he thought may be a satellite. It was not until the following July that he was able to confirm his discovery of Neptune’s first satellite, now called Triton. Triton was later found to have a retrograde orbit inclined at approximately 30◦ to the ecliptic.

6.12 Asteroids The fourth asteroid, Vesta, had been discovered in 1807, but it was not until 1845 that the fifth asteroid was found. Then the discovery rate increased rapidly so that nearly 500 asteroids were known by the end of 1900. As the number of asteroids increased, Kirkwood noticed that there were none with certain fractional periods of Jupiter’s orbital period. This he attributed to resonance interactions with Jupiter. All the early asteroids had orbits between those of Mars and Jupiter, and even as late as 1898 astronomers had discovered only one that had part of its orbit inside that of Mars. But in 1898, Eros was found with an orbit that came very close to that of the Earth, with the next closest approach expected in 1931. This could be used to provide an accurate estimate of solar parallax. In 1906, two asteroids were found at the Lagrangian points, 60◦ in front of and behind Jupiter in its orbit. They were the first of the so-called Trojan asteroids to be discovered.

6.13 Comets Charles Messier discovered a comet that passed very close to the Earth in 1770. Anders Lexell was the first to fit an orbit to it, showing that it had a period of just 5.6 years. With such a short period it should have been seen a number of times before, but it had not. As Lexell explained, this comet had not been seen because it had passed very close to Jupiter in 1767, which had radically changed its orbit. In the late 19th century, Hubert Newton examined the effect of such planetary perturbations on the orbits of comets and found that, for a random selection of comets, they were remarkably inefficient. Lexell’s comet appeared to be an exception. Jean Louis Pons in 1818 discovered a comet that, on further investigation, proved to have been seen near previous perihelia. In the following year, Johann Encke showed that the comet, which now bears his name, has an orbit that takes it inside the orbit of Mercury. When the comet returned in 1822, Encke noticed that it was a few hours early and suggested that it was being affected by some sort of resistive medium close to the Sun. In 1882, however, a comet passed even closer to the Sun and showed no effect of Encke’s medium. Then in 1933, Wolf’s comet was late, rather than early. The problem of these cometary orbits was finally solved in 1950 when Fred Whipple showed that the change in period was caused by jetlike, vaporization emissions from the rotating cometary nucleus. The first successful observation of a cometary spectrum was made by Giovanni Donati in 1864. When the comet was near the Sun, it had three faint luminous bands, indicating that it was self-luminous. Then four years later, William Huggins found that the bands were similar to those emitted by hydrocarbon compounds in the laboratory. Quite a number of cometary spectra were recorded over the next 20 years. When they were first found, they generally exhibited a broad continuous spectrum like that of the Sun indicating that they were scattering sunlight. As they got closer to the Sun, however, the hydrocarbon bands appeared. Then in 1882 Wells’ comet approached very close to the Sun. Near perihelion its bandlike structure disappeared to be replaced by a bright, double sodium line. In the second comet of 1882, this double sodium line was also accompanied by several iron lines when the comet was very near the Sun. As the comet receded, these lines faded and the hydrocarbon bands returned.

6.14 Meteor Showers A spectacular display of shooting stars was seen in November 1799, and again in November 1833. They seemed to originate in the constellation Leo. In the following year, Denison Olmsted pointed out the similarities between these two meteor showers and a less intense one in 1832. These so-called Leonid meteors seemed to be an annual event occurring on or about 12 November. Olmsted explained that the radiant in Leo was due to a perspective

CLBE001-ESS2E

November 11, 2006

4:27

A History of Solar System Studies

65

FIGURE 7 Paths of the Leonid meteors showing their apparent origin from a common radiant due to parallax. (From Simon Newcomb, 1898, “Popular Astronomy,” p. 403.)

effect (Fig. 7). A similar effect was then observed for a meteor shower on 8 August 1834, which appeared to have a radiant in Perseus. Shortly afterward, Lambert Quetelet showed that these were also an annual event. In 1839, Adolf Erman suggested that both the Leonid and Perseid meteor showers were produced by the Earth passing through swarms of small particles that were orbiting the Sun and spread out along Earth’s orbit. But it was still unclear as to the size of the orbit. In 1864, Hubert Newton found that the node of the Leonids’ orbit was precessing at about 52 /year. John Couch Adams then showed that only a particle in a 33.25-year orbit would have this nodal precession. So the Leonids were orbiting the Sun in a diffuse cloud every 33.25 years, which explained why the most intense showers occurred with this frequency. The stragglers all around the orbit explained why we saw the Leonids on an annual basis. In 1867, Carl Peters recognized that the source of the Leonid meteor stream was a periodic comet called Tempel–Tuttle. This was just after Schiaparelli had linked the Perseids to another periodic comet, Swift– Tuttle.

7. The 20th Century Prior to the Space Age 7.1 The Sun In the 19th century, most physicists had thought that heat was transported from the interior to the exterior of the Sun by convection. But in 1894, R. A. Sampson suggested that the primary mechanism was radiation. Then, 30 years later, Arthur Eddington used the concept of radiative equilibrium to calculate the temperature at the center of the Sun and found it to be about 39 million K. At about the same time,

Cecilia Payne showed that hydrogen and helium were the most abundant elements in the stars. Although this idea was initially rejected, it was soon accepted for both the Sun and stars. As a result, in 1935 Eddington reduced his temperature estimate for the center of the Sun to 19 million K. However, Eddington’s calculations made no assumption on how the Sun’s heat was produced, which was still unknown at the time. Earlier, in 1920, Eddington himself had proposed two alternative mechanisms. The heat could be produced either by the mutual annihilation of protons and electrons or by the fusion of hydrogen atoms into helium atoms in some unknown manner. There were other mechanisms suggested by other physicists, but the issue could not be resolved at the time because nuclear physics was still in its infancy. The breakthrough came in 1938 when Charles Critchfield explained how energy could be produced at high temperatures by a chain reaction starting with proton–proton collisions and ending with the synthesis of helium nuclei. Hans Bethe then collaborated with Critchfield to develop this idea. But Bethe also examined an alternative mechanism that relied on carbon as a catalyst to produce helium from hydrogen, in the so-called carbon cycle. Carl von Weizs¨acker independently developed this same scheme. Which mechanism was predominant in the Sun depended crucially on temperature, and it was not until the 1950s that it became clear that the proton–proton chain is dominant in the Sun. In the 19th century, the corona had been found to have a faint continuous spectrum crossed by Fraunhofer absorption lines, but the conditions in the corona were unclear. Of particular interest was a bright green emission line in the coronal spectrum; Young and Harkness found it in 1869 and originally attributed it to iron. In 1898, however, it was found to have a slightly different wavelength than the iron

CLBE001-ESS2E

November 11, 2006

4:27

66 Encyclopedia of the Solar System line. Because no known element generated the required line, it was attributed to a new element called coronium. At that time, it was assumed that the temperature of the Sun and its corona gradually reduced from the center moving outwards. But in the early part of the 20th century, competing theories were put forward, one for a lowtemperature corona and another for a high-temperature one. In 1934, Walter Grotrian analyzed the coronal spectrum and concluded that the temperature was an astonishing 350,000 K. A few years later Bengt Edlen, ´ in a seminal paper, showed that coronal lines are produced by highly ionized iron, calcium, and nickel at a temperature of at least 2 million K. The “coronium” line, in particular, was the product of highly ionized iron. How the temperature of the corona could be so high, when the photosphere temperature is only of the order of 6,000 K, was a mystery, which has not been completely resolved even today. Charles Young discovered in 1894 that, at very high dispersions, many absorption lines in sunspot spectra appeared to have a sharp bright line in their centers. In 1908, George Ellery Hale and Walter Adams found that photographs of the Sun taken in the light of the 656.3-nm hydrogen line showed patterns that looked like iron filings in a magnetic field. This caused Hale to examine sunspot spectra in detail. He found that the Young effect was actually caused by Zeeman splitting of spectral lines in a magnetic field, which was of the order of 3,000 gauss. So sunspots were the home of very high magnetic fields. Hale then started to examine the polarities of sunspots, and found that spots generally occur in pairs, with the polarity of the lead spot, as they crossed the disc, being different in the two hemispheres. This pattern was well established by 1912 when the polarities were found to be reversed at the solar minimum. They reversed yet again at the next solar minimum in 1923. So the solar cycle was really 22 years, not 11. Walter Maunder found in 1913 that large magnetic storms on Earth start about 30 hours after a large sunspot crosses the center of the solar disc. Later work showed that the most intense storms were often associated with solar flares. In 1927, Chree and Stagg found that smaller storms, which did not seem to be associated with sunspots, tended to recur at the Sun’s synodic period of 27 days. Julius Bartels called the invisible source on the Sun of these smaller storms, M regions. Both the so-called flare storms and the M storms were assumed to be caused by particles ejected from the Sun. In 1951, Ludwig Biermann suggested that, to explain the behavior of cometary ion tails, there must be a continuous stream of charged particles emitted by the Sun. Then in 1957, Eugene Parker proposed his theory of the solar wind, which was later confirmed by early spacecraft. Marconi noticed in 1927 that interference with radio signals in September and October of that year coincided with the appearance of large sunspots and intense aurorae. In the late 1930s, Howard Dellinger carried out a detailed examination of the timing of shortwave radio fadeouts, at numerous receiving stations, and solar flares. He found a

reasonable but by no means perfect correlation. The fadeouts seemed to start almost instantaneously after the flare was seen, and they only occurred when the receiving station was in daylight. So Dellinger concluded that they were caused by some form of electromagnetic radiation from the Sun, rather than particles.

7.2 Mercury The synchronous rotation period of Mercury was gradually accepted as a fact in the 20th century. But in 1962, W. E. Howard found that Mercury’s dark side seemed to be warmer than it should be if it were permanently in shadow. Then 3 years later, Dyce and Pettengill found, using radar, that Mercury’s rotation period was not synchronous, but represented two-thirds of its orbital rotation period.

7.3 Venus There was considerable confusion in the first half of the 20th century about Venus’ rotation period. All sorts of periods were proposed between about 24 hours and synchronous (225 days). Then in 1957 Charles Boyer found a distinctive V-shaped pattern of Venus’ clouds that had a 4-day period. In 1962, however, Carpenter and Goldstein deduced a period of about 250 days retrograde using radar, which was modified to 243 days in 1965 for the rotation period of Venus’ surface. So Venus has a 243-day period, whilst its clouds have a period of about 4 days, both periods being retrograde. In 1932, Adams and Dunham concluded that there was no oxygen or water vapor on Venus, but carbon dioxide was clearly present. A few years later, Rupert Wildt calculated that the greenhouse heating of the latter could produce a surface temperature as high as 400 K. Then in 1956, Mayer, McCullough, and Sloanaker deduced a surface temperature of about 600 K by analyzing Venus’ thermal radio emissions. The suggestion that Venus’ surface temperature could be so high was naturally treated with caution. Shortly afterward, Carl Sagan estimated that the surface atmospheric pressure was an equally incredible 100 bar.

7.4 The Moon The idea that there may be life on the Moon had fascinated people for centuries. Even respected astronomers like William Herschel had thought that there would be “lunarians” as he called them. But by the start of the 20th century, it was thought that the most complex lifeforms would be some sort of plant life. However, by the 1960s, when the Americans were planning their lunar landings, even this concept had been rejected. Nevertheless, it was thought that there may be some sort of very elemental life, like bacteria, on the Moon. Bernard Lyot had concluded in 1929, from polarization measurements, that the Moon was probably covered by volcanic ash. Then in the 1950s, Thomas Gold suggested

CLBE001-ESS2E

November 11, 2006

4:27

A History of Solar System Studies

that the Moon may be covered with dust up to a few meters deep. If this was so, it would have provided a major problem for the manned Apollo missions. At the end of the 19th century, the key objection to the impact theory for the formation of lunar craters had been that the craters were generally circular, when they should have been elliptical, because most of the impacts would not be vertical. However, after the First World War it was realized that the shape of the lunar craters resembled shell craters. The shell craters were formed by the shock wave of the impact or explosion, so a nonvertical impact could still produce a circular crater. Nevertheless, not all lunar craters have the same general appearance. So, by the start of the space age it was still unclear if they had been produced by volcanic action, meteorite impact, or both.

7.5 The Earth It was known in the 19th century that temperatures in deep mines on Earth increased with depth. That, together with the existence of volcanoes, clearly indicated that the Earth has a molten interior. Calculations indicated that the rocks would be molten at a depth of only about 40 km. In 1897, Emil Wiechert suggested that the Earth has a dense metallic core, mostly of iron, surrounded by a lighter rocky layer, now called the mantle. A little later, Richard Oldham found clear evidence for the existence of the core from earthquake data. Then in 1914, Beno Gutenberg showed that the interface between the mantle and the core, now called the Wiechert–Gutenberg discontinuity, is at about 0.545r from the center of the Earth (where r is its radius). A little earlier, Andrija Mohoroviˇci´c had discovered the boundary between the crust and mantle, now called the Mohoroviˇci´c discontinuity, by analyzing records of the Croatian earthquake of 1909. The depth of this discontinuity was later found to vary from about 70 km under some mountains to only about 5 km under the deep oceans. A number of theories were proposed to try to define and explain the internal structure of the Earth. In particular, Harold Jeffreys produced a theory that assumed that all the terrestrial planets and the Moon have a core of liquid metals, mostly iron, and a silicate mantle. But it could not explain how those planets with the smallest cores could have retained a higher percentage of lighter material in their mantles. In 1948, William Ramsey solved this problem when he proposed that the whole of the interior of the terrestrial planets consists of silicates, with the internal pressure in the largest planets causing the silicates near the center to become metallic. Unfortunately this idea became unviable when Eugene Rabe found in 1950 that Mercury’s density was much higher than originally thought. It was even higher than that of Venus and Mars, which were much larger planets. In the mid-20th century, most astronomers believed that the planets had been hot when first formed from the solar

67

nebula, but in 1949 Harold Urey suggested that the nebula had been cold. According to Urey, the Earth had been heating up since it was formed because of radioactive decay. Internal convection had then started as iron had gradually settled into the core. Urey believed that the Moon was homogenous because it was relatively small. At the turn of the 19th century, it was thought that radio waves generally traveled in a straight line. So it was a great surprise when Marconi showed in 1901 that radio waves could be successfully transmitted across the Atlantic. Refraction could have caused them to bend to a limited degree, but not enough to cross the ocean. In the following year, Heaviside and Kennelly independently suggested that the waves were being reflected off an electrically conducting layer in the upper atmosphere. The structure of what we now call the E or Heaviside layer, and of other layers in the ionosphere, was gradually clarified over the next 20 years or so. The 80 km high D layer was found to largely disappear at night, and the higher E layer was found to maintain its reflectivity for only 4 or 5 hours after sunset. In addition, it was found that solar flares can cause a major disruption to the ionosphere (see Section 7.1). However, it was not until after the Second World War that the cause of these effects could be examined in detail by first sounding rockets and then by spacecraft. The first major discovery was made by Herbert Friedman in 1949 when he showed that the Sun emits X-rays, which have a major effect on the Earth’s ionosphere.

7.6 Mars There was a great deal of uncertainty about the surface of Mars in the first half of the 20th century. It was thought unlikely that the linear markings called canali really existed, but they were still recorded from time to time by respected observers. In addition, some astronomers thought that the bluish green areas on Mars were vegetation, while others thought that they were volcanic lava. There was also considerable uncertainty about the spectroscopic observations of Mars. Some observers recognized water vapor and oxygen lines, whereas others found none. But in 1947 Gerard Kuiper clearly found evidence for a small amount of carbon dioxide, and in 1963 Andouin Dollfus found a trace amount of water vapor. Estimates of the surface atmospheric pressure varied from about 25 to 120 millibars. Then in 1963, shortly before the first spacecraft reached Mars, a figure of 25 ± 15 millibars was estimated by Kaplan, Munch, ¨ and Spinrad. It seemed clear that the yellow clouds seen on Mars were dust. In 1909, Fournier and Antoniadi found that they appeared to cover the whole planet for a while. Later Antoniadi found that they tended to occur around perihelion when the solar heating is greatest, and so appeared to be produced by thermally generated winds. Thirty years later, De Vaucouleurs measured the wind velocities as being

CLBE001-ESS2E

November 11, 2006

4:27

68 Encyclopedia of the Solar System typically in the range of 60 to 90 km/h when the clouds first formed.

7.7 Internal Structures of the Giant Planets It was known in the 19th century that the densities of Jupiter, Saturn, Uranus, and Neptune were similar to that of the Sun, and were much less than that of the terrestrial planets. At that time, it was thought that Jupiter, and probably Saturn, had not yet fully cooled down since their formation. As a result, they were probably emitting more energy than they received from the Sun. In 1923, Donald Menzel found that the cloud top temperatures of Jupiter and Saturn were about 160 K. This compares with temperatures of 120 and 90 K for Jupiter and Saturn, respectively, that would be maintained solely by incident solar radiation. Three years later, Menzel produced modified observed temperatures of 140, 120, and 100 K, for Jupiter, Saturn, and Uranus. So any internally generated heat would be rather low. In 1923, Harold Jeffreys pointed out that the ratio of the densities of Io and Europa, the innermost of Jupiter’s large satellites, to that of Jupiter, was about the same as the ratio of the density of Titan, Saturn’s largest satellite, to that of Saturn. He then assumed that the density of the cores of Jupiter and Saturn were the same as these their large satellites. In that case, the thickness of the planetary atmospheres would be about 20% of their radii. In the following year, Jeffreys included consideration of the moments of inertia of Jupiter and Saturn in his analysis and concluded that their atmospheres would have depths of 0.09RJ and 0.23RS , respectively (where RJ and RS are the radii of Jupiter and Saturn, respectively). He assumed that beneath their atmospheres there was a layer of ice and solid carbon dioxide, which in turn was surrounded a rocky core. Various schemes were then produced by a number of physicists, of which those of Rupert Wildt in 1938 and William Ramsey in 1951 were probably the most significant. Wildt, who was particularly interested in internal pressures, wanted to find out if matter at the core of the large planets was degenerate. His calculations indicated that it was not. Ramsey, on the other hand, developed his theory assuming that the giant planets were made of hydrogen. He then added helium and other ingredients until their densities and moments of inertia were correct. On this basis, he concluded that Jupiter and Saturn were composed of 76% and 62% hydrogen, by mass, respectively, with central pressures of 32 and 6 × 106 bar. At these pressures, most of the hydrogen would be metallic. The structures of Uranus and Neptune were a problem in Ramsey’s analysis because the heavier planet, Neptune, was the smaller. So their constituents could not be the same. Then in 1961 William Porter produced a model that seemed to fit; in this model, Neptune had 74% ammonia and 26% heavier elements, whereas Uranus had less heavy elements and a small amount of hydrogen.

7.8 Atmospheres of the Giant Planets Vesto Slipher undertook a detailed investigation of the spectra of Jupiter, Saturn, Uranus, and Neptune in the early decades of the 20th century. He recorded numerous bands for all the planets but had trouble interpreting them. In 1932, Rupert Wildt deduced that a number of the bands in all four planets were due to ammonia and methane. However, subsequent work by Mecke, Dunham, Adel, and Slipher showed that some of the lines had been misattributed, so there was no ammonia in the atmospheres of Uranus and Neptune. This was, presumably, because it had been frozen out at their lower temperatures. Adel and Slipher also concluded that the methane concentration reduced in going from Neptune to Uranus to Saturn to Jupiter.

7.9 Jupiter In 1955, Burke and Franklin made the unexpected discovery that Jupiter was emitting radio waves at 22.2 MHz. Subsequently, it was found that Jupiter emitted energy at many radio frequencies. Some of it was thermal energy, with an effective temperature of 145 K, but some was clearly nonthermal. The latter was taken to indicate that Jupiter had an intense magnetic field, with radiation belts similar to those that had, by then, been found around the Earth. Our knowledge of Jupiter’s Galilean satellites changed little in the 20th century before the space age. In 1900, Bernard had observed that the poles of Io appeared to be reddish in color. Then in 1914 Paul Guthnik showed that all four Galilean satellites exhibited synchronous rotation. In the 19th century, it was thought that all four satellites probably had atmospheres, but this was considered more and more unlikely as the 20th century progressed.

7.10 Saturn A prominent white equatorial spot had been observed on Saturn in 1876. Then in 1903 Edward Barnard discovered another temporary prominent white spot at about 36◦ N, but its rotation period around Saturn was some 25 minutes slower. Another equatorial spot that had a similar period to the 1876 equatorial spot appeared in 1933, and another spot that had a similar period to the 1903 spot was observed at about 60◦ N in 1960. The velocities of these spots showed that there was an equatorial current on Saturn, similar to that on Jupiter. But the one on Saturn had a velocity of about 1400 km/h, compared with just 400 km/h for Jupiter. It was unclear why Saturn, which is farther from the Sun, and so receives less heat than Jupiter, should have a much faster equatorial current. Markings on Saturn’s rings were seen by a number of observers in the late 19th and early 20th centuries, including the respected observers Etienne Trouvelot and Eugene ` Antoniadi. In 1955, Guido Ruggieri noticed clear radial streaks at both ansae of the A ring, but after further investigation

CLBE001-ESS2E

November 11, 2006

4:27

A History of Solar System Studies

he concluded that they were an optical illusion. It is unclear whether any of these observations were early observations of spokes, of the sort discovered by the Voyager spacecraft on the B ring, or not. In the winter of 1943–1944, Gerard Kuiper photographed the spectrum of the ten largest satellites of the solar system and found evidence for an atmosphere on Titan and possibly Triton. He could find no such evidence for the Galilean satellites of Jupiter, however.

Lowell Pluto Pickering

Neptune

Uranus

7.11 Uranus and Neptune In the 19th century, Triton had been found to orbit Neptune in a retrograde sense, and it was unclear at the time whether Neptune’s spin was also retrograde. But in 1928 Moore and Menzel found, by observing the Doppler shift of its spectral lines, that Neptune’s spin was direct or prograde. So Neptune’s largest satellite was orbiting the planet in the opposite sense to the planet’s spin. This phenomenon had not been observed before in the solar system for a major satellite. Kuiper discovered Uranus’ fifth satellite, now called Miranda, in 1948. It was orbiting the planet in an approximately circular orbit inside that of the other four satellites. Then in the following year he discovered Neptune’s second satellite, now called Nereid, orbiting Neptune in the opposite sense to Triton. Nereid was in a highly elliptical orbit well outside the orbit of Triton. So Nereid was the “normal” satellite in orbiting Neptune direct or prograde, whereas the larger Triton, which was nearer to Neptune in an almost circular orbit, appeared to be the abnormal one.

7.12 The Discovery of Pluto The discoveries of Uranus and Neptune made astronomers realize that there may well be planets even farther out from the Sun. As Neptune had only been discovered in 1846, and as it was moving very slowly, its orbit was not very well known in the second half of the 19th century. However astronomers had much better information on Uranus’ orbit, and so they reexamined it to see if there were any unexplained deviations that might indicate the whereabouts of a new planet. Such deviations were soon found, and a number of possible locations for the new planet proposed by various astronomers, including Percival Lowell. A photographic search for the new planet was started at Lowell’s observatory, but this was abandoned when Lowell died in 1916. In 1929, Vesto Slipher, the new director of Lowell’s observatory, recruited Clyde Tombaugh to undertake a search for the new planet using a photographic refractor that had been specifically purchased for the task. Tombaugh photographed the whole of the zodiac, and used a blink comparator to find objects that had moved over time. The task was very tedious, but he discovered Pluto in February 1930 after working for 10 months. However, although the planet’s orbit was very similar to that predicted by Lowell (Fig. 8), it

69

Sun

FIGURE 8 A comparison between the true orbit of Pluto and that predicted by Lowell and Pickering. Although Lowell’s orbit was reasonably close to that of Pluto, the agreement was fortuitous. (The open circles show the positions of the planets in 1900, and the closed circles represent those in 1930.)

was far too small to have perturbed Uranus in the way that Lowell had estimated. Over the years, the estimated mass of Pluto has gradually reduced from 6.6 ME (ME is the mass of the Earth) predicted by Lowell, to 0.7 ME (maximum) at the time of its discovery, to 0.002 ME now. Its orbit is highly eccentric, and it has the largest inclination of the traditional planets. In 1955, Walker and Hardie deduced a rotation period of 6d 9h 17min from regular fluctuations in Pluto’s intensity. Little more was known about the planet when the space age started.

7.13 Asteroids In 1918, Kiyotsugu Hirayama identified families of asteroids based on their orbital radius, eccentricity, and inclination. Initially, he identified three families, Themis (22 members), Eos (21 members), and Koronis (13 members). Hirayama suggested that the three families were each the remnants of a larger asteroid that had fractured. This resurrected, in modified form, the theories of Thomas Wright and Wilhelm Olbers, in the 18th and 19th centuries. They both believed that there had been a planet between the orbits of Mars and Jupiter that had broken up. In the 19th century, Eros had been discovered with a perihelion of 1.13 AU. In 1932, another asteroid, now called Amor, was found that had an orbit that came even closer to

CLBE001-ESS2E

November 11, 2006

4:27

70 Encyclopedia of the Solar System that of the Earth than Eros. Then, just 6 weeks later, the first asteroid, now called Apollo, whose orbit crossed that of the Earth, was discovered. The names of Amor and Apollo have now been given to families of asteroids with similar orbital characteristics.

7.14 Comets Huggins had shown in the 19th century that there were hydrocarbon compounds in the heads of comets, but he was not able to specify exactly which hydrocarbons were involved. Molecular carbon, C2 , was first identified in the head of a comet just after the turn of the century, and by the mid1950s C3 , CH, CN, OH, NH, and NH2 , had been found in the heads of comets. Molecular bands were observed in the tail of Daniel’s comet by Deslandres, Bernard, and Evershed in 1907 and in the tail of Morehouse’s comet by Deslandres and Bernard the following year. These bands were later identified by Alfred Fowler as those of ionized carbon monoxide, (CO+ ) and N2 + . Later CO2 + as also found in the tail of a comet. In the 1930s, Karl Wurm observed that many of the molecules found in comets were chemically very active, and so they cannot have been present there for very long. He suggested, instead, that they had come from the more stable so-called parent molecules (CN)2 , H2 O, and CH4 (methane). In 1948, Pol Swings, in his study of Encke’s comet, concluded that the parent molecules were water, methane, ammonia (NH3 ), nitrogen, carbon monoxide and carbon dioxide, all of which had been in the form of ice before being heated by the Sun. In 1950 and 1951, Fred Whipple proposed his icyconglomerate model (better known as his dirty snowball theory) in which the nucleus is composed of ices, such as methane, with meteoric material embedded within it. Unfortunately, some of the parent molecules were highly volatile. But in 1952 Delsemme and Swings suggested that these highly volatile elements would be able to resist solar heating better if they were trapped within the crystalline structure of water ice, in what are known as clathrate hydrates. It was difficult to determine the orbits of long-period comets because they were only observed for the fraction of their orbit when they were close to the Sun. However, a survey of about 400 cometary orbits observed up to 1910 showed that only a tiny minority appeared to be hyperbolic. Stromgren ¨ and Fayet then showed that none of these comets had hyperbolic orbits before they passed Saturn or Jupiter on their approach to the Sun. So the long-period comets appeared to be members of the solar system. ¨ In 1932, Ernst Opik concluded, from an analysis of stellar perturbations, that comets could remain bound to the Sun at distances of up to 106 AU. Some years later, Adrianus Van Woerkom showed that there must be a continuous source of

new, near-parabolic comets to explain the relative numbers observed. Then in 1950 Jan Oort showed that the orbits of 10 comets, with near parabolic orbits, had an average aphelion distance of about 100,000 AU. As a result, he suggested that all long-period comets originate in what is now called the Oort cloud about 50,000 to 150,000 AU from the Sun.

7.15 The Origin of the Solar System In the early decades of the 20th century, theories of the origin of the solar system generally focused on the effect of collisions, and close encounters of another star to the Sun. But all the theories were found to have significant problems, so Laplace’s theory of a condensing nebula was reconsidered. Laplace’s theory had been rejected in the 19th century because the original solar nebula did not appear to have had enough angular momentum. However, in the 1930s, McCrea showed that this would not be a problem if the original nebula had been turbulent. In 1943, Carl von Weizs¨acker produced a theory where cells of circulating convection currents, or vortices, formed in the solar nebula after the Sun had condensed. These vortices produced planetesimals that grew to form planets by accretion. Unfortunately, as Chandrasekhar and Kuiper showed, the vortices would not be stable enough to allow condensation to take place. Kuiper then produced his own theory, as did Safronov and others, with the common theme of planetesimals merging to form planets, but none was fully satisfactory.

Bibliography Hoskin, M., ed. (1997). “Cambridge Illustrated History of Astronomy.” Cambridge Univ. Press, Cambridge, England. Hufbauer, K. (1993). “Exploring the Sun; Solar Science Since Galileo.” Johns Hopkins Univ. Press, Baltimore. Koestler, A. (1990). “The Sleepwalkers: A History of Man’s Changing Vision of the Universe.” Penguin Books. Leverington, D. (2003). “Babylon to Voyager and Beyond: A History of Planetary Astronomy.” Cambridge Univ. Press, Cambridge, England. North, J. (1995). “The Norton History of Astronomy and Cosmology.” Norton, New York. Pannekoek, A. (1961). “A History of Astronomy.” Interscience, New York (Dover reprint 1989). Taton, R., and Wilson, C., eds. (1989). “The General History of Astronomy, Vol. 2, Planetary Astronomy from the Renaissance to the Rise of Astrophysics: Part A, Tycho Brahe to Newton.” Cambridge Univ. Press, Cambridge, England. Taton, R. and Wilson, C., eds. (1995). “The General History of Astronomy, Vol. 2, Planetary Astronomy from the Renaissance to the Rise of Astrophysics: Part B, The Eighteenth and Nineteenth Centuries.” Cambridge Univ. Press, Cambridge, England.

CLBE001-ESS2E

November 11, 2006

100-C

25-C

50-C

75-C

11:37

C+M

50-C+M

C+Y

50-C+Y

M+Y

50-M+Y

100-M

25-M

50-M

75-M

100-Y

25-Y

50-Y

75-Y

100-K

25-K

25-19-19

50-K

50-40-40

75-K

75-64-64

The Sun

Markus J. Aschwanden Lockheed Martin ATC Solar and Astrophysics Laboratory Palo Alto, Callifornia

CHAPTER

4

1. Introduction 2. The Solar Interior 3. The Photosphere

5. The Corona 6. Solar Flares and Coronal Mass Ejections 7. Final Comments

4. The Chromosphere and Transition Region

Bibliography

1. Introduction The Sun is the central body and energy source of our solar system. The Sun is our nearest star, but otherwise it represents a fairly typical star in our galaxy, classified as G2-V spectral type, with a radius of r◦ ≈ 700,000 km, a mass of m◦ ≈ 2 × 1033 g, a luminosity of L◦ ≈ 3.8 × 1026 W, and an age of t◦ ≈ 4.6 × 109 years (Table 1). The distance from the Sun to our Earth is called an astronomical unit (AU) and amounts to ∼150 × 106 km. The Sun lies in a spiral arm of our galaxy, the Milky Way, at a distance of 8.5 kiloparsecs from the galactic center. Our galaxy contains ∼1012 individual stars, many of which are likely to be populated with similar solar systems, according to the rapidly increasing detection of extrasolar planets over the last years; the binary star systems are very unlikely to harbor planets because of their unstable, gravitationally disturbed orbits. The Sun is for us humans of particular significance, first because it provides us with the source of all life, and second because it furnishes us with the closest laboratory for astrophysical plasma physics, magneto-hydrodynamics (MHD), atomic physics, and particle physics. The Sun still represents the only star from which we can obtain spatial images, in many wavelengths. The basic structure of the Sun is sketched in Fig. 1. The Sun and the solar system were formed together from an interstellar cloud of molecular hydrogen some 5 billion years

ago. After gravitational contraction and subsequent collapse, the central object became the Sun, with a central temperature hot enough to ignite thermonuclear reactions, the ultimate source of energy for the entire solar system. The chemical composition of the Sun consists of 92.1% hydrogen and 7.8% helium by number (or 27.4% He by mass), and 0.1% of heavier elements (or 1.9% by mass, mostly C, N, O, Ne, Mg, Si, S, Fe). The central core, where hydrogen burns into helium, has a temperature of ∼15 million K (Fig. 1). The solar interior further consists of a radiative zone, where energy is transported mainly by radiative diffusion, a process where photons with hard X-ray (keV) energies get scattered, absorbed, and reemitted. The outer third of the solar interior is called the convective zone, where energy is transported mostly by convection. At the solar surface, photons leave the Sun in optical wavelengths, with an energy that is about a factor of 105 lower than the original hard X-ray photons generated in the nuclear core, after a random walk of ∼105 –106 years. The irradiance spectrum of the Sun is shown in Fig. 2, covering all wavelengths from gamma rays, hard X-rays, soft X-rays, extreme ultraviolet (EUV), ultraviolet, white light, infrared, to radio wavelengths. The quiet Sun irradiates most of the energy in visible (white-light) wavelengths, to which our human eyes have developed the prime sensitivity during the evolution. Emission in extreme ultraviolet is dominant in the solar corona because it is produced

C 2007 by Academic Press. All rights of reproduction in any form reserved. Encyclopedia of the Solar System 2e 

71

CLBE001-ESS2E

November 11, 2006

11:37

72 Encyclopedia of the Solar System

TABLE 1

Basic Physical Properties of the Sun

Physical Parameter

Solar radius, R Solar mass, m Mean density, ρ Gravity at solar surface, g Escape velocity at solar surface, v Synodic rotation period, P Sidereal rotation period, P Mean distance from Earth Solar luminosity, L Solar age, t Temperature at Sun center, Tc Temperature at solar surface, Tph

Numerical Value

695,500 km 1.989 × 1033 g 1.409 g cm−3 274.0 m s−2 617.7 km s−1 P = 27.3 days (equator) P = 25.4 days (equator) P = 35.0 days (at latitude ± 70◦ ) 1 AU = 149,597,870 km 3.844 × 1026 W (or 1033 ergs s−1 ) 4.57 × 109 years 15.7 × 106 K 6400 K

Source: Cox, 2000.

FIGURE 1 A cutaway view of the Sun, showing the three internal (thermonuclear, radiative, and convective) zones, the solar surface (photosphere), the lower (chromosphere) and upper atmosphere (corona), and a number of phenomena associated with the solar activity cycle (filaments, prominences, flares). (Courtesy of Calvin J. Hamilton and NASA/ESA.)

CLBE001-ESS2E

November 11, 2006

11:37

The Sun

73

100 MHz

1 GHz

10 GHz

102 GHz

103 GHz

1013 Hz

1014 Hz

1015 Hz

1016 Hz

1017 Hz

1018 Hz

1019 Hz

107

1020 Hz

1021 Hz

FREQUENCY

10-5

VISIBLE GAMMA RAY

106

X RAY

ULTRA VIOLET

RADIO

INFRARED

10-6

QUIET SUN

10-7

105 LARGEST BURST REPORTED (NONTHERMAL)

103

10-8 10-9

2B FLARE GRADUAL BURST (NONTHERMAL)

100

LARGE BURST 3B FLARE, 8/1/72 (NONTHERMAL) 10-10

POSTFLARE

10

10-11

1

SLOWLY-VARYING S-COMPONENT 10-12 ACTIVE REGIONS (THERMAL)

1B FLARE

10-1

10-13

NON-FLARE CONDITIONS

10-2

10-14 LARGE STORM (NONTHERMAL)

10-3

10-15

10-4

102∝m

10 ∝m

1 ∝m

103 A

102 A

10 A

QUIET SUN (THERMAL)

1A

10-3 A

10-6

10-17

LARGE BURST (3B FLARE, 8/4/72) (NONTHERMAL)

10-1 A

10-5

10-16

10-2 A

SPECTRAL IRRADIANCE (erg cm-2 s-1 ∝m-1)

104

10-18 10-19

10 m

10-20 1m

10 cm

1 cm

1 mm

WAVELENGTH

FIGURE 2 The solar irradiance spectrum from gamma rays to radio waves. The spectrum is shifted by 12 orders of magnitude in the vertical axis at λ = 1 mm to accommodate for the large dynamic range in spectral irradiance. (Courtesy of H. Malitson and NASA/NSSDC.)

by ionized plasma in the coronal temperature range of ∼1–2 million K. Emissions in shorter wavelengths require higher plasma temperatures and thus occur during flares only. Flares also accelerate particles to nonthermal energies, which cause emission in hard X-rays, gamma rays, and radio wavelengths, but to a highly variable degree.

2. The Solar Interior The physical structure of the solar interior is mostly based on theoretical models that are constrained (1) by global quantities (age, radius, luminosity, total energy output; see Table 1); (2) by the measurement of global oscillations

CLBE001-ESS2E

November 11, 2006

11:37

74 Encyclopedia of the Solar System (helioseismology); and (3) by the neutrino flux, which now constrains for the first time elemental abundances in the solar interior, since the neutrino problem has been solved in the year 2001.

2.1 Standard Models There are two types of models of the solar interior: (1) hydrostatic equilibrium models and (2) time-dependent numerical simulations of the evolution of the Sun, starting from an initial gas cloud to its present state today, after ∼8% of the hydrogen has been burned into helium. The standard hydrostatic model essentially calculates the radial run of temperature, pressure, and density that fulfill the conservation of mass, momentum, and energy in all internal spherical layers of the Sun, constrained by the boundary conditions of radius, temperature, and radiation output (luminosity) at the solar surface, the total mass, and the chemical composition. Furthermore, the ideal gas law and thermal equilibrium is assumed, and thus the radiation is close to that of an ideal black body. The solar radius has been measured by triangulation inside the solar system (e.g., during a Venus transit) and by radar echo measurements. The mass of the Sun has been deduced from the orbital motions of the planets (Kepler’s laws) and from precise laboratory measurements of the gravitational constant. The solar luminosity is measured by the heat flux received at Earth. From these standard models, a central temperature of ∼15 million K, a central density of ∼150 g cm−3 , and a central pressure of 2.3 × 1017 dyne cm−2 have been inferred. Fine-tuning of the standard model is obtained by including convective transport and by varying the (inaccurately known) helium abundance.

2.2 Thermonuclear Energy Source The source of solar energy was understood in the 1920s, when Hans Bethe, George Gamow, and Carl Von Weizs¨acker identified the relevant nuclear chain reactions that generate solar energy. The main nuclear reaction is the transformation of hydrogen into helium, where 0.7% of the mass is converted into radiation (according to Einstein’s energy equivalence, E = mc2 ), the so-called p- p chain, which starts with the fusion of two protons into a nucleus of deuterium (2 He), and, after chain reactions involving 3 He, 7 Be, and 7 Li, produces helium (4 He), p + p → 2 He + e + + νe 2 He + p → 3 He + γ 3 He + 3 He → 4 He + p + p or 3

He + 4 He → 7 Be + γ 7 Be + e − → 7 Li + νe 7 Li + p → 8 Be + γ → 4 He + 4 He

One can estimate the Sun’s lifetime by dividing the available mass energy by the luminosity, t◦ ≈ 0.1 × 0.007 m◦ c 2 /L◦ ≈ 1010 years where we assumed that only about a fraction of 0.1 of the total solar mass is transformed because only the innermost core of the Sun is sufficiently hot to sustain nuclear reactions. An alternative nuclear chain reaction occurring in the Sun and stars is the carbon–nitrogen–oxygen (CNO) cycle, 12

C + p → 13 N + γ N → 13 C + e + + νe 13 C + p → 14 N + γ 14 N + p → 15 O + γ 15 O → 15 N + e + + νe 15 N + p → 12 C + 4 He 13

The p- p chain produces 98.5% of the solar energy, and the CNO cycle produces the remainder, but the CNO cycle is faster in stars that are more massive than the Sun.

2.3 Neutrinos Neutrinos interact very little with matter, unlike photons, and thus most of the electronic neutrinos (ν e ), emitted by the fusion of hydrogen to helium in the central core, escape the Sun without interactions and a very small amount is detected at Earth. Solar neutrinos have been detected since 1967, pioneered by Raymond Davis, Jr., using a chlorine tank in the Homestake Gold Mine in South Dakota, but the observed count rate was about a third of the theoretically expected value, causing the puzzling neutrino problem that persisted for the next 35 years. However, Pontecorvo and Gribov predicted already in 1969 that low-energy solar neutrinos undergo a “personality disorder” on their travel to Earth and oscillate into other atomic flavors of muonic neutrinos (ν μ ) (from a process involving a muon particle) and tauonic neutrinos (ν t ) (from a process involving a tauon particle), which turned out to be the solution of the missing neutrino problem for detectors that are only sensitive to the highest-energy (electronic) neutrinos, such as the Homestake chlorine tank and the gallium detectors GALLEX in Italy and SAGE in Russia. Only the Kamiokande and Super-Kamiokande-I pure-water experiments and the Sudbury Neutrino Observatory (SNO; Ontario, Canada) heavy-water experiments are somewhat sensitive to the muonic and tauonic neutrinos. It was the SNO that measured in 2001 for the first time all three lepton flavors and, in this way, brilliantly confirmed the theory of neutrino (flavor) oscillations. Today, after the successful solution of the neutrino problem, the measured neutrino fluxes are sufficiently accurate to constrain the helium abundance and heavy element abundances in the solar interior.

CLBE001-ESS2E

November 11, 2006

11:37

The Sun

2.4 Helioseismology In the decade of 1960–1970, global oscillations were discovered on the solar surface in visible light, which became the field of helioseismology. Velocity oscillations were first measured by R. Leighton, and then interpreted in 1970 as standing sound waves in the solar convection zone by R. Ulrich, C. Wolfe, and J. Leibacher. These acoustic oscillations, also called p-modes (pressure-driven waves), are detectable from fundamental up to harmonic numbers of ∼1000 and are most conspicuous in dispersion diagrams, ω(k), where each harmonic shows up as a separate ridge, when the oscillation frequency (ω) is plotted as function of the wavelength λ (i.e., essentially the solar circumference divided by the harmonic number). Frequencies of the pmode correspond to periods of ∼5 minutes. An example of a p-mode standing wave is shown in Fig. 3 (left), which appears like a standing wave on a drum skin. Each mode is characterized by the number of radial, longitudinal, and latitudinal nodes, corresponding to the radial quantum number n, the azimuthal number m, and the degree l of spherical harmonic functions. Since the density and temperature increase monotonically with depth inside the Sun, the sound speed varies as a function of radial distance from the Sun center. P-mode waves excited at the solar surface propagate

75

downward and are refracted toward the surface, where the low harmonics penetrate very deep, whereas high harmonics are confined to the outermost layers of the solar interior. By measuring the frequencies at each harmonic, the sound speed can be inverted as a function of the depth; in this way, the density and temperature profile of the solar interior can be inferred and unknown parameters of theoretical standard models can be constrained, such as the abundance of helium and heavier elements. By exploiting the Doppler effect, frequency shifts of the p-mode oscillations can be used to measure the internal velocity rates as a function of depth and latitude, as shown in Fig. 3 (right). A layer of rapid change in the internal rotation rate was discovered this way at the bottom of the convection zone, the so-called tachocline (at 0.693 ± 0.002 solar radius, with a thickness of 0.039 ± 0.013 solar radius). Besides the p-mode waves, gravity waves (g-modes), where buoyancy rather than pressure supplies the restoring force, are suspected in the solar core. These gravity waves are predicted to have long periods (hours) and very small velocity amplitudes, but they have not yet been convincingly detected. Global helioseismology detects p-modes as a pattern of standing waves that encompass the entire solar surface;

FIGURE 3 Left: A global acoustic p-mode wave is visualized: The radial order is n = 14, the angular degree is l = 20, the angular order is m = 16, and the frequency is ν = 2935.88 ± 0.1 μHz with SoHO/MDI (Michelson Doppler Image). The red and blue zones show displacement amplitudes of opposite sign. Right: The internal rotation rate is shown with a color code, measured with SoHO/MDI during May 1996–April 1997. The red zone shows the fastest rotation rates (P ≈ 25 days), dark blue the slowest (P ≈ 35 days). Note that the rotation rate varies in latitude differently in the radiative and convective zones. (Courtesy of SoHO/MDI and NASA.)

CLBE001-ESS2E

November 11, 2006

11:37

76 Encyclopedia of the Solar System however, local deviations of the sound speed can also be detected beneath sunspots and active regions, a diagnostic that is called local helioseismology. Near sunspots, p-modes are found to have oscillation periods in the order of 3 minutes, compared to 5 minutes in active region plages and quiet-Sun regions.

2.5 Solar Dynamo The Sun is governed by a strong magnetic field (much stronger than those on planets), which is generated with a magnetic field strength of B ≈ 105 G in the tachocline, the thin shear layer sandwiched between the radiative and the convective zone. Buoyant magnetic flux tubes rise through the convection zone (due to the convective instability obeying the Schwartzschild criterion) and emerge at the solar surface in active regions, where they form sunspots with magnetic field strengths of B ≈ 103 G and coronal loops with field strengths of B ≈102 G at the photospheric footpoints, and B ≈ 10 G in larger coronal heights. The differential rotation on the solar surface is thought to wind up the surface magnetic field, which then fragments under the magnetic stress, circulates meridionally to the poles, and reorients from the toroidally stressed state (with field lines oriented in east–west direction) at solar maximum into a poloidal dipole field (connecting the North with the South Pole) in the solar minimum. This process is called the solar dynamo, which flips the magnetic polarity of the Sun every ∼11 years (the solar cycle), or returns to the same magnetic configuration every ∼22 years (the Hale cycle). The solar cycle controls the occurrence rate of all solar activity phenomena—from sunspot numbers, active regions, to flares, and coronal mass ejections (CMEs).

3. The Photosphere The photosphere is a thin layer at the solar surface that is observed in white light. The irradiance spectrum in Fig. 2 shows the maximum at visible wavelengths, which can be fitted with a black-body spectrum with a temperature of ˚ which is the T ≈ 6400 K at wavelengths of λ ≥ 2000 A, solar surface temperature. The photosphere is defined as the range of heights from which photons directly escape, which encompasses an optical depth range of 0.1 ≤ τ ≤ 3 and translates into a height range of h ≈ 300 km for the visible wavelength range.

3.1 Granulation and Convection The photospheric plasma is only partially ionized, there are fewer than 0.001 electrons per hydrogen atom at the photo˚ These spheric temperature of T = 6400 K at λ = 5000 A.

few ionized electrons come mostly from less abundant elements with a low ionization potential, such as magnesium, while hydrogen and helium are almost completely atomic. The magnetic field is frozen in to the gas under these conditions. However, the temperature is rapidly increasing below the photospheric surface, exceeding the hydrogen ionization temperature of T = 11, 000 K at a depth of 50 km, where the number of ionized electrons increases to 0.1 electrons per hydrogen atom, and the opacity increases by a similar factor. The high opacity of the partially ionized plasma impedes the heat flow. Moreover, a stratification with a temperature gradient steeper than an adiabatic gradient is unstable to convection (Schwartzschild criterion). Thus the partially ionized photosphere of the Sun, as well as of other low-mass stars (with masses m < 2m◦ are therefore convective. The observational manifestation of subphotospheric convection is the granulation pattern (Fig. 4, right), which contains granules with typical sizes of ∼1000 km and lifetimes of τ ≈ 7 min. The subphotospheric gas flows up in the bright centers of granulation cells, cools then by radiating away some heat at the optically thin photospheric surface, and, while cooling, becomes denser and flows down in the intergranular lanes. This convection process can now be reproduced with numerical simulations that include hydrodynamics, radiative transfer, and atomic physics of ionization and radiative processes (Fig. 4, left). The convection process is also organized on larger scales, exhibiting cellular patterns on scales of ∼5,000–10,000 km (mesogranulation) and on scales of ∼20,000 km (supergranulation).

3.2 Photospheric Magnetic Field Most of what we know about the solar magnetic field is inferred from observations of the photospheric field, from the Zeeman effect of spectral lines in visible wavelengths ˚ From two-dimensional (2D) maps of the (e.g., Fe 5250 A). photospheric magnetic field strength, we extrapolate the coronal three-dimensional (3D) magnetic field, or try to trace the subphotospheric origin from emerging magnetic flux elements. The creation of magnetic flux is thought to happen in the tachocline at the bottom of the convection zone, from where it rises upward in form of buoyant magnetic flux tubes and emerges at the photospheric surface. The strongest fields emerge in sunspots, amounting to several kilogauss field strengths, and fields with strengths of several 100 G emerge also all over in active regions, often in the form of a leading sunspot trailed by following groups of opposite magnetic polarity. Due to the convective motion, small magnetic flux elements that emerge in the center of granulation cells are then swept to the intergranular lanes, where often unresolved small concentrations are found, with sizes of less than a few 100 km. The flow velocities due to photospheric convection are on the order of

CLBE001-ESS2E

November 11, 2006

11:37

The Sun

77

FIGURE 4 (Left) Numerical simulation of cellular convection at the solar surface, performed by Fausto Cattaneo and Andrea Malagoli; (Right) High-resolution observation of the granulation pattern in the solar photosphere. A granule has a typical size of 1000 km, representing the surface of an elementary convection cell. The large black area represents a sunspot, where the temperature is cooler than the surroundings. (This image was taken by Tom Berger with the Swedish Solar Observatory.)

∼1 km s−1 . In the quiet Sun, away from active regions, the mean photospheric magnetic field amounts to a few Gauss.

3.3 Sunspots Sunspots are the areas with the strongest magnetic fields, and therefore a good indicator of the solar activity (Fig. 5, bottom). The butterfly diagram shows that sunspots (or active regions) appear first at higher latitudes early in the solar cycle and then drift equatorward toward the end of the solar cycle (Fig. 5, top). Since all solar activity phenomena are controlled by the magnetic field, they have a similar solar cycle dependence as sunspots, such as the flare rate, active region area, global soft X-ray brightness, and radio emission. The appearance of dark sunspots lowers the total luminosity of the Sun only by about 0.15% at sunspot maximum, and thus the variation of the sunlight has a negligible effect on the Earth’s climate. The variation of the EUV emission, which affects the ionization in the Earth’s ionosphere, however, has a more decisive impact on the Earth’s climate.

An individual sunspot consists of a very dark central umbra, surrounded by a brighter, radially striated penumbra. The darkness of sunspots is attributed to the inhibition of convective transport of heat, emitting only about 20% of the average solar heat flux in the umbra and being significantly cooler (∼4500 K) than the surroundings (∼6000 K). Their diameters range from 3600 to 50,000 km, and their lifetime ranges from a week to several months. The magnetic field in the umbra is mostly vertically oriented, but it is strongly inclined over the penumbra, nearly horizontally. Current theoretical models explain the interlocking comb structure of the filamentary penumbra with outward submerged field lines that are pumped down by turbulent, compressible convection of strong descending plumes. Sunspots are used to trace the surface rotation, since Galileo in 1611. The average sidereal differential rotation rate is ω = 14.522 − 2.84 sin2  deg/day where  is the heliographic latitude. The rotation rate of an individual feature, however, can deviate from this average

CLBE001-ESS2E

November 11, 2006

11:37

78 Encyclopedia of the Solar System DAILY SUNSPOT AREA AVERAGED OVER INDIVIDUAL SOLAR ROTATIONS

90N

> 0.0% > 0.1% > 1.0%

SUNSPOT AREA IN EQUAL AREA LATITUDE STRIPS (% OF STRIP AREA)

30N EQ 30S 90S 1870

1880

1890

1900

1910

1920

1930

1940

1950

1960

1970

1980

1990

2000

FIGURE 5 Top: Butterfly diagram of sunspot appearance, which marks the heliographic latitude of sunspot locations as a function of time, during the solar cycles 12–23 (covering the years 1880–2000). Bottom: Sunspot area as a function of time, which is a similar measure of the solar cycle activity as the sunspot number. (Courtesy of D. Hathaway and NASA/MSFC.)

2010

DATE

0.5

AVERAGE DAILY SUNSPOT AREA (% OF VISIBLE HEMISPHERE)

0.4 0.3 0.2 0.1 0.0 1870

12 1880

13 1890

14 1900

1910

15 1920

16 1930

17

18

19

20

21

22

1940

1950

1960

1970

1980

1990

23 2000

2010

DATE

by a few percent because it depends on the anchor depth to which the feature is rooted, since the solar internal differential rate varies radially (Fig. 3, right).

4. The Chromosphere and Transition Region 4.1 Basic Physical Properties The chromosphere (from the Greek word χρωμoσ , color) is the lowest part of the solar atmosphere, extending to an average height of ∼2000 km above the photosphere. The first theoretical concepts conceived the chromosphere as a spherical layer around the solar surface (in the 1950s; Fig. 6, left), while later refinements included the diverging magnetic fields (canopies) with height (in the 1980s; Fig. 6, middle), and finally ended up with a very inhomogeneous mixture of cool gas and hot plasma, as a result of the extremely dynamic nature of chromospheric phenomena (in the 2000s; Fig. 6, right). According to hydrostatic standard models assuming local thermodynamic equilibrium (LTE), the temperature reaches first a temperature minimum of T = 4300 K at a height of h ≈ 500 km above the photosphere, and rises then suddenly to ∼10,000 K in the upper chromosphere at h ≈ 2000 km, but the hydrogen density drops by about a factor of 106 over the same chromospheric height range. These hydrostatic models have been criti-

cized because they neglect the magnetic field, horizontal inhomogeneities, dynamic processes, waves, and non-LTE conditions. Beyond the solar limb (without having the photosphere in the background), the chromospheric spectrum is characterized by emission lines; these lines appear dark on the disk as a result of photospheric absorption. The principal lines of the photospheric spectrum are called the Fraunhofer lines, including, for example, hydrogen lines (H I; with the ˚ Hß 4861 A, ˚ Hγ 4341 A, ˚ Hδ Balmer series Hα (6563 A, ˚ ˚ ˚ 4102 A), calcium lines (Ca II; K 3934 A, H 3968 A), and ˚ helium lines (He I; D3 5975 A).

4.2 Chromospheric Dynamic Phenomena The appearance and fine structure of the chromosphere varies enormously depending on which spectral line, wavelength, and line position (core, red wing, blue wing) is used because of their sensitivity to different temperatures (and thus altitudes) and Doppler shifts (and thus velocity ranges). In the H and K lines of Ca II, the chromospheric images show a bright network surrounding supergranulation cells, which coincide with the large-scale subphotospheric convection cells. In the Ca II K2 or in ultraviolet contin˚ the network and internetwork appear uum lines (1600 A), grainier. The so-called bright grains have a high contrast in

CLBE001-ESS2E

November 11, 2006

11:37

The Sun

79

FIGURE 6 Cartoon of geometric concepts of the solar chromosphere, transition region, and corona: gravitationally stratified layers in the 1950s (left), vertical fluxtubes with chromospheric canopies in the 1980s (middle), and a fully inhomogeneous mixing of photospheric, chromospheric, and coronal zones by dynamic processes such as heated upflows, cooling downflows, intermittent heating (ε), nonthermal electron beams (e), field line motions and reconnections, emission from hot plasma, absorption and scattering in cool plasma, acoustic waves, and shocks (right). (Courtesy of Carolus J. Schrijver.)

wavelengths that are sensitive to the temperature minimum (4300 K), with an excessive temperature of 30–360 K, and with spatial sizes of ∼1000 km. The bright points in the network are generally associated with magnetic elements that collide, which then heat the local plasma after magnetic reconnection. In the intranetwork, bright grains result from chromospheric oscillations that produce shock waves. There are also very thin spaghetti-like elongated fine structures visible in Hα spectroheliograms (Fig. 7, left), which are called fibrils around sunspots. More vertically oriented fine structures are called mottles on the disk, or spicules above the limb. Mottles appear as irregular threads, localized in groups around and above supergranules, at altitudes of 700–3000 km above the photosphere, with lifetimes of 12–20 min, and are apparently signatures of upward and downward motions of plasmas with temperatures of T = 8000–15,000 K and velocities of v ≈ 5– 10 km s−1 . Spicules (Fig. 7, right) are jet-like structures of plasma with temperatures of T ≈ 10,000 K that rise to a maximum height of h ≈ 10,000 km into the lower corona, with velocities of v ≈ 20 km s−1 . They carry a maximum flux of 100 times the solar wind into the low corona. Recent numerical simulations by DePontieu and Erdelyi ´ show that global (helioseismic) p-mode oscillations leak sufficient energy from the global resonant cavity into the chromosphere to power shocks that drive upward flows and form spicules. There is also the notion that mottles, fibrils, and spicules could be unified, being different manifestations of the same

physical phenomenon at different locations (quiet Sun, active region, above the limb), in analogy to the unification of filaments (on the disk) and prominences (above the limb).

FIGURE 7 High-resolution image of Active Region 10380 on June 16, 2003, located near the limb, showing chromospheric spicules in the right half of the image. The image was taken with the Swedish 1-m Solar Telescope (SST) on La Palma, Spain, using a tunable filter, tuned to the blue-shifted line wing of the Hα 6563 A˚ line. The spicules are jets of moving gas, flowing upward in the chromosphere with a speed of ∼15 km s−1 . The scale of the image is 65,000 × 45,000 km. (Courtesy of Bart DePontieu.)

CLBE001-ESS2E

November 11, 2006

11:37

80 Encyclopedia of the Solar System 5. The Corona It is customary to subdivide the solar corona into three zones, which all vary their size during the solar cycle: (1) active regions, (2) quiet-Sun regions, and (3) coronal holes.

5.1 Active Regions Active regions are located in areas of strong magnetic field concentrations, visible as sunspot groups in optical wavelengths or magnetograms. Sunspot groups typically exhibit a strongly concentrated leading magnetic polarity, followed by a more fragmented trailing group of opposite polarity. Because of this bipolar nature, active regions are mainly made up of closed magnetic field lines. Due to the permanent magnetic activity in terms of magnetic flux emergence, flux cancellation, magnetic reconfigurations, and magnetic reconnection processes, a number of dynamic processes such as plasma heating, flares, and CMEs occur in active regions. A consequence of plasma heating in the chromosphere are upflows into coronal loops, which give active regions the familiar appearance of numerous filled loops, which are hotter and denser than the background corona, producing bright emission in soft X-rays and EUV wavelengths. In the EUV image shown in Fig. 8, active regions appear in white.

5.2 Quiet-Sun Regions Historically, the remaining areas outside of active regions were dubbed quiet-Sun regions. Today, however, many dynamic processes have been discovered all over the solar surface, so that the term quiet Sun is considered to be a misnomer, only justified in relative terms. Dynamic processes in the quiet Sun range from small-scale phenomena such as network heating events, nanoflares, explosive events, bright points, and soft X-ray jets, to large-scale structures, such as transequatorial loops or coronal arches. The distinction between active regions and quiet-Sun regions becomes more and more blurred because most of the largescale structures that overarch quiet-Sun regions are rooted in active regions. A good working definition is that quietSun regions encompass all closed magnetic field regions (excluding active regions), which demarcates the quiet-Sun territory from coronal holes (that encompass open magnetic field regions).

FIGURE 8 The multitemperature corona, recorded with the EIT (Extreme-ultraviolet Imaging Telescope) instrument on board the SoHO spacecraft. The representation shown here is a false-color composite of three images all taken in extreme ultraviolet light. Each individual image highlights a different temperature regime in the upper solar atmosphere and was assigned a specific color; red at 2 million, green at 1.5 million, and blue at 1 million degrees K. The combined image shows active regions in white color (according to Newton’s law of color addition), because they contain many loops with different temperatures. Also, nested regions above the limb appear in white, because they contain a multitude of loops with different temperatures along a line-of-sight, while isolated loops on the disk show a specific color according to their intrinsic temperature. (Courtesy of EIT/SoHO and NASA.)

inated by open magnetic field lines, which act as efficient conduits for flushing heated plasma from the corona into the solar wind, whenever they are fed by chromospheric upflows at their footpoints. Because of this efficient transport mechanism, coronal holes are empty of plasma most of the time, and thus appear much darker than the quiet Sun, where heated plasma flowing upward from the chromosphere remains trapped, until it cools down and precipitates back to the chromosphere. A coronal hole is visible in Fig. 8 at the North Pole, where the field structures point radially away from the Sun and show a cooler temperature (T ≤ 1.0 MK; dark blue in Fig. 8) than the surrounding quiet-Sun regions.

5.4 Hydrostatics of Coronal Loops 5.3 Coronal Holes The northern and southern polar zones of the solar globe have generally been found to be darker than the equatorial zones during solar eclipses. Max Waldmeier thus ¨ dubbed those zones as coronal holes (i.e., Koronale Locher in German). Today it is fairly clear that these zones are dom-

Coronal loops are curvilinear structures aligned with the magnetic field. The cross section of a loop is essentially defined by the spatial extent of the heating source because the heated plasma distributes along the coronal magnetic field lines without cross-field diffusion, since the thermal pressure is much less than the magnetic pressure in the solar

CLBE001-ESS2E

November 11, 2006

11:37

The Sun

81

corona. The solar corona consists of many thermally isolated loops, where each one has its own gravitational stratification, depending on its plasma temperature. A useful quantity is the hydrostatic pressure scale height λp , which depends only on the electron temperature Te , λp (Te ) =

2k B Te Te ≈ 47,000 μm H g 1 MK

(km).

Observing the solar corona in soft X-rays or EUV, which are both optically thin emissions, the line-of-sight integrated brightness intercepts many different scale heights, leading to a hydrostatic weighting bias toward systematically hotter temperatures in larger altitudes above the limb. The observed height dependence of the density needs to be modeled with a statistical ensemble of multihydrostatic loops. Measuring a density scale height of a loop requires careful consideration of projection effects, loop plane inclination angles, cross-sectional variations, line-ofsight integration, and the instrumental response functions. Hydrostatic solutions have been computed from the energy balance between the heating rate, the radiative energy loss, and the conductive loss. The major unknown quantity is the spatial heating function, but analysis of loops in high-resolution images indicate that the heating function is concentrated near the footpoints, say at altitudes of h ≤ 20,000 km. Of course, a large number of coronal loops are found to be not in hydrostatic equilibrium, while nearly hydrostatic loops have been found preferentially in the quiet corona and in older dipolar active regions. An example of an active region [recorded with the Transition Region and Coronal Explorer (TRACE) about 10 hours after a flare] is shown in Fig. 9, which clearly shows superhydrostatic loops where the coronal plasma is distributed over up to four times larger heights than expected in hydrostatic equilibrium (Fig. 9, bottom).

5.5 Dynamics of the Solar Corona Although the Sun appears lifeless and unchanging to our eyes, except for the monotonic rotation that we can trace from the sunspot motions, there are actually numerous vibrant dynamic plasma processes continuously happening in the solar corona, which can be detected mainly in EUV and soft X-rays. There is currently a paradigm shift stating that most of the apparently static structures seen in the corona are probably controlled by plasma flows and intermittent heating. It is, however, not easy to measure and track these flows with our remote sensing methods, like the apparently motionless rivers seen from an airplane. For slow flow speeds, the so-called laminar flows, there is no feature to track, while the turbulent flows may be easier to detect because they produce whirls and vortices that can be tracked. A similar situation happens in the solar corona. Occasionally, a moving plasma blob is detected in a coronal

FIGURE 9 An active region with many loops that have an extended scale height of λp /λT ≤ 3–4 (top) has been scaled to the hydrostatic thermal scale height of T = 1 MK (bottom). The pressure scale height of the 1 MK plasma is λT = 47,000 km, but the observed flux is proportional to the emission measure (F → EM → n2e ), which has the half pressure scale height λT /2 = 23,000 km.

loop; it can be used as a tracer. Most of the flows in coronal loops seem to be subsonic (like laminar flows) and thus featureless. Occasionally, we observe turbulent flows, which clearly reveal motion, especially when cool and hot plasma becomes mixed by turbulence and thus yields contrast by emission and absorption in a particular temperature filter. Motion can also be detected with Doppler shift measurements, but this yields only the flow component along the line-of-sight. There is increasing evidence that flows are ubiquitous in the solar corona. There are a number of theoretically expected dynamic processes. For instance, loops at coronal temperatures are thermally unstable when the radiative cooling time is shorter than the conductive cooling time, or when the heating scale height falls below one third of a loop half length. Recent observations show ample evidence for the presence of flows in coronal loops, as well as evidence for impulsive

CLBE001-ESS2E

November 11, 2006

11:37

82 Encyclopedia of the Solar System heating with subsequent cooling, rather than a stationary hydrostatic equilibrium. High-resolution observations of coronal loops reveal that many loops have a superhydrostatic density scale height, far in excess of hydrostatic equilibrium solutions (Fig. 9, top). Time-dependent hydrodynamic simulations are still in a very exploratory phase, and hydrodynamic modeling of the transition region, coronal holes, and the solar wind remains challenging due to the number of effects that cannot easily be quantified by observations, such as unresolved geometries, inhomogeneities, time-dependent dynamics, and MHD effects. The coronal plasma is studied with regard to hydrostatic equilibria in terms of fluid mechanics (hydrostatics), with regard to flows in terms of fluid dynamics (hydrodynamics), and including the coronal magnetic field in terms of magneto-hydrodynamics (MHD). The coronal magnetic field has many effects on the hydrodynamics of the plasma. It can play a passive role in the sense that the magnetic geometry does not change (e.g., by channeling particles, plasma flows, heat flows, and waves along its field lines or by maintaining a thermal insulation between the plasmas of neighboring loops or fluxtubes). On the other hand, the magnetic field can play an active role (where the magnetic geometry changes), such as exerting a Lorentz force on the plasma, building up and storing nonpotential energy, triggering an instability, changing the topology (by various types of magnetic reconnection), and accelerating plasma structures (filaments, prominences, CMEs).

5.6 The Coronal Magnetic Field The solar magnetic field controls the dynamics and topology of all coronal phenomena. Heated plasma flows along

magnetic field lines and energetic particles can only propagate along magnetic field lines. Coronal loops are nothing other than conduits filled with heated plasma, shaped by the geometry of the coronal magnetic field, where cross-field diffusion is strongly inhibited. Magnetic field lines take on the same role for coronal phenomena as do highways for street traffic. There are two different magnetic zones in the solar corona that have fundamentally different properties: open-field and closed-field regions. Open-field regions (white zones above the limb in Fig. 10), which always exist in the polar regions, and sometimes extend toward the equator, connect the solar surface with the interplanetary field and are the source of the fast solar wind (∼800 km s−1 ). A consequence of the open-field configuration is efficient plasma transport out into the heliosphere, whenever chromospheric plasma is heated at the footpoints. Closed-field regions (gray zones in Fig. 10), in contrast, contain mostly closed-field lines in the corona up to heights of about one solar radius, which open up at higher altitudes and connect eventually to the heliosphere, but produce a slow solar wind component of ∼400 km s−1 . It is the closed-field regions that contain all the bright and overdense coronal loops, produced by filling with chromospheric plasma that stays trapped in these closed-field lines. For loops reaching altitudes higher than about one solar radius, plasma confinement starts to become leaky, because the thermal plasma pressure exceeds the weak magnetic field pressure that decreases with height (plasma-ß parameter < 1). The magnetic field on the solar surface is very inhomogeneous. The strongest magnetic field regions are in sunspots, reaching field strengths of B = 2000–3000 G. Sunspot groups are dipolar, oriented in an east–west direction (with the leading spot slightly closer to the equator)

FIGURE 10 The global coronal magnetic field can be subdivided into open-field regions (mostly near the polar regions) and into closed-field regions (mostly in latitudes of  ≤ 70◦ ). The analytical magnetic field model shown here, a multipole-current sheet coronal model of Banaszkiewicz, approximately outlines the general trends. The high-speed solar wind originates and leaves the Sun in the unshaded volume.

CLBE001-ESS2E

November 11, 2006

11:37

The Sun

and with opposite leading polarity in both hemispheres, reversing every 11-year cycle (Hale’s laws). Active regions and their plages comprise a larger area around sunspots, with average photospheric fields of B ≈ 100–300 G, containing small-scale pores with typical fields of B ≈ 1000 G. The background magnetic field in the quiet Sun and in coronal holes has a net field of B ≈ 0.1 – 0.5 G, while the absolute field strengths in resolved elements amount to B = 10–50 G. Our knowledge of the solar magnetic field is mainly based on measurements of Zeeman splitting in spectral lines, whereas the coronal magnetic field is reconstructed by extrapolation from magnetograms at the lower boundary, using a potential or force-free field model. The extrapolation through the chromosphere and transition region is, however, uncertain due to unknown currents and non-force-free conditions. The fact that coronal loops exhibit generally much less expansion with height than potential-field models underscores the inadequacy of potential-field extrapolations. Direct measurements of the magnetic field in coronal heights are still in their infancy.

5.7 MHD Oscillations of Coronal Loops Much like the discovery of helioseismology four decades ago, it was recently discovered that also the solar corona contains an impressively large ensemble of plasma structures that are capable of producing sound waves and harmonic oscillations. Thanks to the high spatial resolution, image contrast, and time cadence capabilities of the Solar and Heliospheric Observatory (SoHO) and TRACE spacecraft, oscillating loops, prominences, or sunspots, and propagating waves have been identified and localized in the corona and transition region, and studied in detail since 1999. These new discoveries established a new discipline that became known as coronal seismology. Even though the theory of MHD oscillations was developed several decades earlier,

only the new imaging observations provide diagnostics on length scales, periods, damping times, and densities that allow a quantitative application of the theoretical dispersion relations of MHD waves. The theory of MHD oscillations has been developed for homogeneous media, single interfaces, slender slabs, and cylindrical fluxtubes. There are four√basic speeds in fluxtubes: (1) the ´ speed v A = √ Alfven B0 / 4πρ0 , (2) the sound speed cs = γ P0 /ρ0 , (3) the cusp or tube speed c T = (1/cs2 + 1/v 2A )−1/2 , and (4) the kink or mean Alfven ´ speed ck = [(ρ0 v 2A + ρe v 2Ae )/(ρ0 + ρe )]1/2 . For coronal conditions, the dispersion relation reveals a slowmode branch (with acoustic phase speeds) and a fast-mode branch of solutions (with Alfven ´ speeds). For the fast-mode branch, a symmetric (sausage) mode and an asymmetric (kink) mode can be distinguished. The fast kink mode produces transverse amplitude oscillations of coronal loops, which have been detected with TRACE (Fig. 11), having periods in the range of P = 2–10 min, and can be used to infer the coronal magnetic field strength, thanks to its nondispersive nature. The fast sausage mode is highly dispersive and is subject to a long-wavelength cutoff, so that standing wave oscillations are only possible for thick and high-density (flare and postflare) loops, with periods in the range of P ≈ 1 s to 1 min. Fast sausage-mode oscillations with periods of P ≈ 10 s have recently been imaged for the first time with the Nobeyama radioheliograph, and there are numerous earlier reports on nonimaging detections with periods of P ≈ 0.5–5 s. Finally, slow-mode acoustic oscillations have been detected in flare-like loops with Solar Ultraviolet Measurements of Emitted Radiation (SUMER) having periods in the range of P ≈ 5–30 min. All loop oscillations observed in the solar corona have been found to be subject to strong damping, typically with decay times of only one or two periods. The relevant damping mechanisms are resonant absorption for fast-mode oscillations (or alternatively phase mixing, although requiring an extremely low Reynolds number), and thermal conduction for

FIGURE 11 The transverse amplitude of a kink-mode oscillation measured in one loop of a postflare loop arcade observed with TRACE on April 15, 2001, 21:58:44 UT. The amplitudes are fitted by a damped sine plus a linear function, a(t) = a0 + a1 sin (2π ∗ (t − t0 )/P ) exp (−t/τD ) + a2∗ t, with a period of P = 365 s and a damping time of tD = 1000 s. (Courtesy of Ed DeLuca and Joseph Shoer.)

5000 2001-Apr-15, 2158:44 UT Amplitude a[km]

4500 4000 3500 3000 2500 0

200

400

600

800

1000

83

1200

1400

CLBE001-ESS2E

November 11, 2006

11:37

84 Encyclopedia of the Solar System FIGURE 12 TRACE 171 A˚ observation of a slow-mode (acoustic) wave recorded on June 13, 2001, 06:46 UT. (Left) The diverging fan-like loop structures emerge near a sunspot, where the acoustic waves are launched and propagate upward. (Right) A running difference plot is shown for the loop segment marked in the left frame, with time running upward in the plot. Note the diagonal pattern, which indicates propagating disturbances. (Courtesy of Ineke De Moortel.)

slow-mode acoustic oscillations. Quantitative modeling of coronal oscillations offers exciting new diagnostics on physical parameters.

5.8 MHD Waves in Solar Corona In contrast to standing modes (with fixed nodes), also propagating MHD waves (with moving nodes) have been discovered in the solar corona recently. Propagating MHD waves result mainly when disturbances are generated impulsively, on time scales faster than the Alfvenic ´ or acoustic travel time across a structure. Propagating slow-mode MHD waves (with acoustic speed) have been recently detected in coronal loops with TRACE and SoHO/EIT (Fig. 12); they are usually being launched with 3-minute periods near sunspots, or with 5minute periods in plage regions. These acoustic waves propagate upward from a loop footpoint and are quickly damped; they have never been detected in downward direction at the opposite loop side. Propagating fast-mode MHD waves (with Alfvenic ´ speeds) have recently been discovered in a loop in optical [Solar Eclipse Coronal Imaging System (SECIS) eclipse] data, as well as in (Nobeyama) radio images. Besides from coronal loops, slow-mode MHD waves have also been detected in plumes in open-field regions in coronal holes, while fast-mode MHD waves have not yet been detected in open-field structures. However, spectroscopic observations of line broadening in coronal holes provide strong support for the detection of Alfven ´ waves, based on the agreement with the theoretically predicted height-dependent scaling between line broadening and density, v(h) ∝ ne (h)−1/4 . The largest manifestation of propagating MHD waves in the solar corona are global waves that spherically propagate after a flare and/or CME over the entire solar surface. These

global waves were discovered earlier in Hα, called Moreton waves, and recently in EUV, called EIT waves (Fig. 13), usually accompanied with a coronal dimming behind the wave front, suggesting evacuation of coronal plasma by the CME. The speed of Moreton waves is about three times faster than that of EIT waves, which still challenges dynamic MHD models of CMEs.

5.9 Coronal Heating When Bengt Edlen ´ and Walter Grotrian identified Fe IX (nine-times ionized iron) and Ca XIV (14-times ionized calcium) lines in the solar spectrum in 1943, a coronal temperature of T ≈ 1 MK was first inferred from the formation temperature of these highly ionized atoms. A profound consequence of this measurement is the implication that the corona then consists of a fully ionized hydrogen plasma. Comparing this coronal temperature with the photospheric temperature of 6400 K, we are confronted with the puzzle of how the 200 times hotter coronal temperature can be maintained, the so-called coronal heating problem. Of course, there is also a chromospheric heating problem and a solar wind heating problem. If only thermal conduction were at work, the temperature in the corona should steadily drop down from the chromospheric value with increasing distance, according to the second law of thermodynamics. Moreover, since we have radiative losses by EUV emission, the corona would just cool off in a matter of hours to days, if the plasma temperature could not be maintained continuously by some heating source. The coronal heating problem has been narrowed down by substantial progress in theoretical modeling with MHD codes, new high-resolution imaging with the SXT (Yohkoh Soft X-ray Telescope), EIT, TRACE, and Hinode telescopes, and with more sophisticated data analysis using automated pattern recognition codes. The total energy losses

CLBE001-ESS2E

November 11, 2006

11:37

The Sun

85

FIGURE 13 Two global wave events observed with SoHO/EIT ˚ on April 7, 1997 (top row) 195 A, and May 12, 1997 (bottom row). The intensity images (right) were recorded before the eruption, while the difference images (left and middle) show differences between the subsequent images, enhancing emission measure increases (white areas) and dimming (black areas). (Courtesy of Yi-Ming Wang.)

in the solar corona range from F = 3 × 105 erg cm−2 s−1 in quiet-Sun regions to F ≈ 107 erg cm−2 s−1 in active regions. Two main groups of DC (direct current) and AC (alternating current) models involve as a primary energy source chromospheric footpoint motion or upward leaking Alfven ´ waves, which are dissipated in the corona by magnetic reconnection, current cascades, MHD turbulence, Alfven ´ resonance, resonant absorption, or phase mixing. There is also strong observational evidence for solar wind heating by cyclotron resonance, while velocity filtration seems not to be consistent with EUV data. Progress in theoretical models has mainly been made by abandoning homogeneous fluxtubes, but instead including gravitational scale heights and more

realistic models of the transition region, and taking advantage of numerical simulations with 3D MHD codes (by Boris Gudiksen and Aake Nordlund). From the observational side we can now unify many coronal small-scale phenomena with flare-like characteristics, subdivided into microflares (in soft X-rays) and nanoflares (in EUV) solely by their energy content. Scaling laws of the physical parameters corroborate their unification. They provide a physical basis to understand the frequency distributions of their parameters and allow estimation of their energy budget for coronal heating. Synthesized data sets of microflares and nanoflares in EUV and soft X-rays have established that these impulsive small-scale phenomena match the radiative

CLBE001-ESS2E

November 11, 2006

11:37

86 Encyclopedia of the Solar System 105

Flare frequency N(E) [10-50 s-1 cm-2 erg-1]

A

P K

Quiet Sun Nanoflares -1.54+ N(E)~E-1.54_0.03

100 171 195

10-5

Active Region Transient brigthenings S -1.55+ N(E)~E-1.55_0.05 C

Active Region Hard X-ray Flares -1.53+ N(E)~E-1.53_0.02

10-10

1024

1026

1028 Flare energy E [erg]

1030

1032

FIGURE 14 Compilation of frequency distributions of thermal energies from nanoflare statistics in the quiet Sun, active region transient brightenings, and hard X-ray flares. The overall slope of the synthesized nanoflare distribution, N(E) ∝ E−1.54±0.03 , is similar to that of transient brightenings and hard X-ray flares. The grey area indicates the coronal heating requirement of F = 3 × 105 erg cm−2 s−1 for quiet-Sun regions. Note that the observed distribution of nanoflare energies, which only includes the radiative losses, accounts for about one third of the heating rate requirement of the quiet Sun.

loss of the average quiet-Sun corona (Fig. 14), which points to small-scale magnetic reconnection processes in the transition region and lower corona as primary heating sources.

6. Solar Flares and Coronal Mass Ejections Rapidly varying processes in the solar corona, which result from a loss of magnetic equilibrium, are called eruptive phenomena, such as flares, CMEs, or eruptive filaments and prominences. The fundamental process that drives all these phenomena is magnetic reconnection.

6.1 Magnetic Reconnection The solar corona has dynamic boundary conditions: (1) The solar dynamo in the interior of the Sun constantly generates new magnetic flux from the bottom of the convection zone (i.e., the tachocline) which rises by buoyancy and emerges through the photosphere into the corona; (2) the differential rotation as well as convective motion at the solar surface continuously wrap up the coronal field; and (3) the connectivity to the interplanetary field has constantly to

break up to avoid excessive magnetic stress. These three dynamic boundary conditions are the essential reasons why the coronal magnetic field is constantly stressed and has to adjust by restructuring the large-scale magnetic field by topological changes, called magnetic reconnection processes. Of course, such magnetic restructuring processes occur wherever magnetic stresses build up (e.g., in filaments, in twisted sigmoid-shaped loops, and along sheared neutral lines). Topological changes in the form of magnetic reconnection always liberate free nonpotential energy, which is converted into heating of plasma, acceleration of particles, and kinematic motion of coronal plasma. Magnetic reconnection processes can occur in a slowly changing quasi-steady way, which may contribute to coronal heating (Section 5.9), but more often happen as sudden violent processes that are manifested as flares and CMEs. Theory and numerical simulations of magnetic reconnection processes in the solar corona have been developed for steady 2D reconnection (Fig. 15, left), bursty 2D reconnection, and 3D reconnection. Only steady 2D reconnection models can be formulated analytically; they provide basic relations for inflow speed, outflow speed, and reconnection rate, but represent oversimplifications for most (if not all) observed flares. A more realistic approach seems to be bursty 2D reconnection models (Fig. 15, right), which involve the tearing-mode and coalescence instability and can reproduce the sufficiently fast temporal and small spatial scales required by solar flare observations. The sheared magnetic field configurations and the existence or coronal and chromospheric nullpoints, which are now inferred more commonly in solar flares, require ultimately 3D reconnection models, possibly involving nullpoint coalescence, spine reconnection, fan reconnection, and separator reconnection. Magnetic reconnection operates in two quite distinct physical parameter domains: in the chromosphere during magnetic flux emergence, magnetic flux cancellation, and so-called explosive events and under coronal conditions during microflares, flares, and CMEs.

6.2 Filaments and Prominences Key elements in triggering flares and/or CMEs are erupting filaments. A filament is a current system above a magnetic neutral line that builds up gradually over days and erupts during a flare or CME process. The horizontal magnetic field lines overlying a neutral line (i.e., the magnetic polarity inversion line) of an active region are filled with cool gas (of chromospheric temperature), embedded in the much hotter tenuous coronal plasma. On the solar disk, these cool dense features appear dark in Hα or EUV images, in absorption against the bright background, and are called filaments, while the same structures appear bright above the limb, in emission against the dark sky background, where they are called prominences. Thus, filaments and prominences

CLBE001-ESS2E

November 11, 2006

11:37

The Sun

87

FIGURE 15 Left: Geometry of the Sweet–Parker (top) and Petschek reconnection model (bottom). The geometry of the diffusion region (gray box) is a long thin sheet ( d) in the Sweet–Parker model, but much more compact ( ≈ d) in the Petschek model. The Petschek model also considers slow-mode MHD shocks in the outflow region. Right: Numeric MHD simulation of a magnetic reconnection process in a sheared arcade. The grayscale represents the mass density difference ratio, and the dashed lines show the projected magnetic field lines in the vicinity of the reconnection region, at two particular times of the reconnection process. The location a corresponds to a thin compressed region along the slowly rising inner separatrix, b to a narrow downflow stream outside of the left outer separatrix, and c indicates a broader upflow that follows along the same field lines. (Courtesy of Judith Karpen.)

are identical structures physically, while their dual name just reflects a different observed location (inside or outside the disk). A further distinction is made regarding their dynamic nature: Quiescent filaments/prominences are longlived stable structures that can last for several months, while eruptive filaments/prominences are usually associated with flares and CMEs (see example in Fig. 16).

6.3 Solar Flare Models A flare process is associated with a rapid energy release in the solar corona, believed to be driven by stored nonpotential magnetic energy and triggered by an instability in the magnetic configuration. Such an energy release process results in acceleration of nonthermal particles and in heating of coronal/chromospheric plasma. These processes emit radiation in almost all wavelengths: radio,

white light, EUV, soft X-rays, hard X-rays, and even gamma rays during large flares. The energy range of flares extends over many orders of magnitude. Small flares that have an energy content of 10−6 to 10−9 of the largest flares fall into the categories of microflares and nanoflares (Fig. 14), which are observed not only in active regions but also in quiet-Sun regions. Some of the microflares and nanoflares have been localized above the photospheric network and are thus also dubbed network flares or network heating events. There are also a number of small-scale phenomena with rapid time variability for which it is not clear whether they represent miniature flare processes (e.g., active region transients, explosive events, blinkers). It is conceivable that some are related to photospheric or chromospheric magnetic reconnection processes, in contrast to flares that always involve coronal magnetic reconnection processes.

CLBE001-ESS2E

November 11, 2006

11:37

88 Encyclopedia of the Solar System FIGURE 15 (Continued )

FIGURE 16 Erupting filament observed with TRACE at 171 A˚ on July 19, 2000, 23:30 UT, in Active Region 9077. The dark filament mass has temperatures around 20,000 K, while the hot kernels and threads contain plasma with temperatures of 1.0 MK or hotter. The erupting structure extends over a height of 75,000 km here. (Courtesy of TRACE and NASA.)

The best known flare/CME models entail magnetic reconnection processes that are driven by a rising filament/ prominence, flux emergence, converging flows, or shear motion along the neutral line. Flare scenarios with a driver perpendicular to the neutral line (rising prominence, flux emergence, convergence flows) are formulated as 2D reconnection models, while scenarios that involve shear along the neutral line (tearing-mode instability, quadrupolar flux transfer, the magnetic breakout model, sheared arcade interactions) require 3D descriptions. A 2D reconnection model involving a magnetic X-point is shown in Fig. 17 (left); a generalized 3D version involving a highly sheared neutral line is sketched in Fig. 17 (right). There are more complex versions like the magnetic breakout model, where a second arcade triggers reconnection above a primary arcade. Observational evidence for magnetic reconnection in flares includes the 3D geometry, reconnection inflows, outflows, detection of shocks, jets, ejected plasmoids, and secondary effects like particle acceleration, conduction fronts, and chromospheric evaporation processes. Flare images in soft X-rays often show the cusp-shaped geometry of

CLBE001-ESS2E

November 11, 2006

11:37

The Sun

a)

b)

c)

d)

e)

f)

89

FIGURE 17 Left: A version of the standard 2D X-type reconnection model for two-ribbon flares, pioneered by Carmichael, Sturrock, Hirayama, and Kopp-Pneumann (CSHKP), which also includes the slow and fast shocks in the outflow region, the upward-ejected plasmoid, and the locations of the soft X-ray bright flare loops. (Courtesy of Saku Tsuneta.) Right: 3D version of the two-ribbon flare model, based on the observed evolution during the Bastille Day (July 14, 2000) flare: (a) low-lying, highly sheared loops above the neutral line first become unstable; (b) after loss of magnetic equilibrium the filament jumps upward and forms a current sheet according to the model by Forbes and Priest. When the current sheet becomes stretched, magnetic islands form and coalescence of islands occurs at locations of enhanced resistivity, initiating particle acceleration and plasma heating; (c) the lowest lying loops relax after reconnection and become filled due to chromospheric evaporation (loops with thick linestyle); (d) reconnection proceeds upward and involves higher lying, less sheared loops; (e) the arcade gradually fills up with filled loops; ( f ) the last reconnecting loops have no shear and are oriented perpendicular to the neutral line. At some point, the filament disconnects completely from the flare arcade and escapes into interplanetary space.

reconnecting field lines (Fig. 18, top), while EUV images invariably display the relaxed postreconnection field lines after the flare loops cooled down to EUV temperatures in the postflare phase (Fig. 18, middle and bottom).

6.4 Flare Plasma Dynamics The flare plasma dynamics and associated thermal evolution during a flare consists of a number of sequential processes: plasma heating in coronal reconnection sites, chromospheric flare plasma heating (either by precipitating nonthermal particles and/or downward propagating heat conduction fronts), chromospheric evaporation in the form

of upflowing heated plasma, and cooling of postflare loops. The initial heating of the coronal plasma requires anomalous resistivity because Joule heating with classical resistivity is unable to explain the observed densities, temperatures, and rapid timescales in flare plasmas. Other forms of coronal flare plasma heating, such as slow shocks, electron beams, proton beams, or inductive currents, are difficult to constrain with currently available observables. The second stage of chromospheric heating is more thoroughly explored, based on the theory of the thick-target model, with numeric hydrodynamic simulations, and with particle-incell simulations. Important diagnostics on chromospheric heating are also available from Hα, white light, and UV emission, but quantitative modeling is still quite difficult

CLBE001-ESS2E

November 11, 2006

11:37

90 Encyclopedia of the Solar System FIGURE 18 Soft X-ray and EUV images of flare loops and flare arcades with bipolar structure. Yohkoh/SXT observed flares (March 18, 1999, 16:40 UT, and June 7, 2000, 14:49 UT) with “candle-flame”-like cusp geometry during ongoing reconnection, while TRACE sees postflare loops once they cooled down to 1–2 MK, when they already relaxed into a near-dipolar state. Examples are shown for a small flare (the April 19, 2001, 13:31 UT, GOES class M2 flare), and for two large flares with long arcades, seen at the limb (September 30, 1998, 14:30 UT) and on the disk (the July 14, 2000, 10:59 UT, X5.7 flare). (Courtesy of Yohkoh/ISAS and TRACE/NASA.)

because of the chromospheric opacities and partial ionization. The third stage of chromospheric evaporation has been extensively explored with hydrodynamic simulations, in particular to explain the observed Doppler shifts in soft X-ray lines, while application of spatial models to imaging data is quite sparse. Also certain types of slow-drifting radio bursts seem to contain information on the motion of chromospheric evaporation fronts. The fourth stage of postflare loop cooling is now understood to be dominated by thermal conduction initially and by radiative cooling later on. How-

ever, spatiotemporal temperature modeling of flare plasmas (Fig. 19) has not yet been fitted to observations in detail.

6.5 Particle Acceleration and Kinematics Particle acceleration in solar flares is mostly explored by theoretical models because neither macroscopic nor microscopic electric fields are directly measurable by remotesensing methods. The motion of particles can be described in terms of acceleration by parallel electric fields, drift

CLBE001-ESS2E

November 11, 2006

11:37

The Sun

91

FIGURE 19 2D numerical MHD simulation of a solar flare with chromospheric evaporation and anisotropic heat conduction in the framework of a 2D magnetic reconnecting geometry. The temporal evolution of the plasma temperature (top row) and density (bottom row) is shown. The temperature and density scale is shown in the bars on the right side. The simulation illustrates the propagation of thermal conduction fronts and the upflows of chromospheric plasma in response. (Courtesy of Takaaki Yokoyama and Kazunari Shibata.)

velocities caused by perpendicular forces (i.e., E × Bdrifts), and gyromotion caused by the Lorentz force of the magnetic field. Theoretical models of particle acceleration in solar flares can be broken down into three groups: (1) DC electric field acceleration, (2) stochastic or second-order Fermi acceleration, and (3) shock acceleration. In the models of the first group, there is a paradigm shift from large-

scale DC electric fields (of the size of flare loops) to smallscale electric fields (of the size of magnetic islands produced by the tearing mode instability). The acceleration and trajectories of particles is studied more realistically in the inhomogeneous and time-varying electromagnetic fields around magnetic X-points and O-points of magnetic reconnection sites, rather than in static, homogeneous, large-scale

CLBE001-ESS2E

November 11, 2006

11:37

92 Encyclopedia of the Solar System Parker-type current sheets. The second group of models entails stochastic acceleration by gyroresonant wave-particle interactions, which can be driven by a variety of electrostatic and electromagnetic waves, supposed that wave turbulence is present at a sufficiently enhanced level and that the MHD turbulence cascading process is at work. The third group of acceleration models includes a rich variety of shock acceleration models, which is extensively explored in magnetospheric physics and could cross-fertilize solar flare models. Two major groups of models are studied in the context of solar flares (i.e., first-order Fermi acceleration or shockdrift acceleration, and diffusive shock acceleration). New aspects are that shock acceleration is now applied to the outflow regions of coronal magnetic reconnection sites, where first-order Fermi acceleration at the standing fast shock is a leading candidate. Traditionally, evidence for shock acceleration in solar flares came mainly from radio type II bursts. New trends in this area are the distinction of different acceleration sites that produce type II emission: flare blast waves, the leading edge of CMEs (bowshock), and shocks in internal and lateral parts of CMEs. In summary, we can say that (1) all three basic acceleration mechanisms seem to play a role to a variable degree in some parts of solar flares and CMEs, (2) the distinctions among the three basic models become more blurred in more realistic (stochastic) models, and (3) the relative importance and efficiency of various acceleration models can only be assessed by including a realistic description of the electromagnetic fields, kinetic particle distributions, and MHD evolution of magnetic reconnection regions pertinent to solar flares. Particle kinematics, the quantitative analysis of particle trajectories, has been systematically explored in solar flares by performing high-precision energy-dependent time delay measurements with the large-area detectors of the Compton Gamma-Ray Observatory (CGRO). There are essentially five different kinematic processes that play a role in the timing of nonthermal particles energized during flares: (1) acceleration, (2) injection, (3) free-streaming propagation, (4) magnetic trapping, and (5) precipitation and energy loss. The time structures of hard X-ray and radio emission from nonthermal particles indicate that the observed energydependent timing is dominated either by free-streaming propagation (obeying the expected electron time-of-flight dispersion) or by magnetic trapping in the weak-diffusion limit (where the trapping times are controlled by collisional pitch angle scattering). The measurements of the velocity dispersion from energy-dependent hard X-ray delays allows then to localize the acceleration region, which was invariably found in the cusp of postflare loops (Fig. 20).

6.6 Hard X-Ray Emission Hard X-ray emission is produced by energized electrons via collisional bremsstrahlung, most prominently in the form

of thick-target bremsstrahlung when precipitating electrons hit the chromosphere. Thin-target bremsstrahlung may be observable in the corona for footpoint-occulted flares. Thermal bremsstrahlung dominates only at energies of ≤15 keV. Hard X-ray spectra can generally be fitted with a thermal spectrum at low energies and with a single- or doublepowerlaw nonthermal spectrum at higher energies. Virtually all flares exhibit fast (subsecond) pulses in hard Xrays, which scale proportionally with flare loop size and are most likely spatiotemporal signatures of bursty magnetic reconnection events. The energy-dependent timing of these fast subsecond pulses exhibit electron time-of-flight delays from the propagation between the coronal acceleration site and the chromospheric thick-target site. The inferred acceleration site is located about 50% higher than the soft X-ray flare loop height, most likely near X-points of magnetic reconnection sites (Fig. 20). The more gradually varying hard X-ray emission exhibits an energy-dependent time delay with opposite sign, which corresponds to the timing of the collisional deflection of trapped electrons. In many flares, the time evolution of soft X-rays roughly follows the integral of the hard X-ray flux profile, which is called the Neupert effect. Spatial structures of hard X-ray sources include: (1) footpoint sources produced by thicktarget bremsstrahlung, (2) thermal hard X-rays from flare looptops, (3) above-the-looptop (Masuda-type) sources that result from nonthermal bremsstrahlung from electrons that are either trapped in the acceleration region or interact with reconnection shocks, (4) hard X-ray sources associated with upward soft X-ray ejecta, and (5) hard X-ray halo or albedo sources due to backscattering at the photosphere. In spatially extended flares, the footpoint sources assume ribbonlike morphology if mapped with sufficient sensitivity. The monthly hard X-ray flare rate varies about a factor of 20 during the solar cycle, similar to magnetic flux variations implied by the monthly sunspot number, as expected from the magnetic origin of flare energies.

6.7 Gamma-Ray Emission The energy spectrum of flares (Fig. 21) in gamma-ray wavelengths (0.5 MeV–1 GeV) is more structured than in hard X-ray wavelengths (20–500 keV) because it exhibits both continuum emission as well as line emission. There are at least six different physical processes that contribute to gamma-ray emission: (1) electron bremsstrahlung continuum emission, (2) nuclear deexcitation line emission, (3) neutron capture line emission at 2.223 MeV, (4) positron annihilation line emission at 511 keV, (5) pion-decay radiation at ≥50 MeV, and (6) neutron production. The ratio of continuum to line emission varies from flare to flare, and gamma-ray lines can completely be overwhelmed in electron-rich flares or flare phases. When gamma-ray lines are present, they provide a diagnostic of the elemental abundances, densities, and temperatures of the ambient plasma

CLBE001-ESS2E

November 11, 2006

11:37

The Sun

93

FIGURE 20 Top: The geometry of the acceleration region inferred from direct detections of above-the-looptop hard X-ray sources with Yohkoh/HXT (Hard X-Ray Telescope) (contours) and simultaneous modeling of electron time-of-flight distances based on energy-dependent time delays of 20–200 keV hard X-ray emission measured with Burst and Transient Source Experiment, BATSE/CGRO (crosses marked with ACC). Soft X-rays detected with Yohkoh/SXT or thermal hard X-ray emission from the low-energy channel of Yohkoh/HXT/Lo are shown in colors, outlining the flare loops. Bottom: The observations in the left panel show a Yohkoh/HXT 23–33 keV image (thick contours) and Be119 SXT image (thin contours) of the Masuda flare, January 13, 1992, 17:28 UT. The interpretation of the above-the-looptop source is that temporary trapping occurs in the acceleration region in the cusp region below the reconnection point (bottom right).

in the chromosphere, as well as of the directivity and pitch angle distribution of the precipitating protons and ions that have been accelerated in coronal flare sites, presumably in magnetic reconnection regions. Critical issues that have been addressed in studies of gamma-ray data are the maximum energies of coronal acceleration mechanisms, the ion/electron ratios (because selective acceleration of ions indicate gyroresonant interactions), the ion/electron timing (to distinguish between simultaneous or second-step acceleration), differences in ion/electron transport (e.g.,

neutron sources were recently found to be displaced from electron sources), and the first ionization potential (FIP) effect of chromospheric abundances (indicating enhanced abundances of certain ions that could be preferentially accelerated by gyroresonant interactions). Although detailed modeling of gamma-ray line profiles provides significant constraints on elemental abundances and physical properties of the ambient chromospheric plasma, as well as on the energy and pitch angle distribution of accelerated particles, little information or constraints could be retrieved about the

CLBE001-ESS2E

November 11, 2006

11:37

94 Encyclopedia of the Solar System FIGURE 21 Composite photon spectrum of a large flare, extending from soft X-rays (1–10 keV), hard X-rays (10 keV–1 MeV), to gamma-rays (1 MeV–100 GeV). The energy spectrum is dominated by different processes: by thermal electrons (in soft X-rays), bremsstrahlung from nonthermal electrons (in hard X-rays), nuclear deexcitation lines (in ∼0.5–8 MeV gamma-rays), bremsstrahlung from high-energetic electrons (in ∼10–100 MeV gamma-rays), and pion-decay (in ≥100 MeV gamma rays). Note also the prominent electron-positron annihilation line (at 511 keV) and the neutron capture line (at 2.2 MeV).

10

10

Nonthermal electrons

High-energetic High-energetic ions electrons

Pions

105

e+

10-5

n

2.2 MeV

100

511 keV

Flux (photons cm-2 s-1 keV-1)

Thermal electrons

π SOFT X-RAYS 10

HARD X-RAYS

GAMMA-RAYS

-10

100

101

102

103 Photon energy (keV)

104

timescales and geometry of the acceleration mechanisms, using gamma-ray data. Nevertheless, the high spectral and imaging resolution of the recently launched Ramaty HighEnergy Spectroscopic Solar Imager (RHESSI) spacecraft facilitates promising new data for a deeper understanding of ion acceleration in solar flares.

6.8 Radio Emission Radio emission in the solar corona is produced by thermal, nonthermal, up to high-relativistic electrons, and thus provides useful diagnostics complementary to EUV, soft X-rays, hard X-rays, and gamma rays. Thermal or Maxwellian distribution functions produce in radio wavelengths either free-free emission (bremsstrahlung) for low magnetic field strengths or gyroresonant emission in locations of high magnetic field strengths, such as above sunspots, which are both called incoherent emission mechanisms. Since EUV and soft X-ray emission occurs in the optically thin regime, the emissivity adds up linearly along the line-of-sight. Free-free radio emission is somewhat more complicated because the optical thickness depends on the frequency, which allows direct measurement of the electron temperature in optically thick coronal layers in metric and decimetric frequencies up to ν ≤ 1 GHz. Above ∼2 GHz, free-free emission becomes optically thin in the corona, but gyroresonance emission at harmonics of s ≈ 2, 3, 4 dominates in strong-field regions. In flares, high-relativistic electrons are produced that emit gyrosynchrotron emission, which allows for detailed modeling of precipitating and trapped electron populations in time profiles recorded at different microwave frequencies. Unstable non-Maxwellian particle velocity distributions, which have a positive gradient in parallel (beams) or

105

106

perpendicular (losscones) direction to the magnetic field, drive gyroresonant wave-particle interactions that produce coherent wave growth, detectable in the form of coherent radio emission. Two natural processes that provide these conditions are dispersive electron propagation (producing beams) and magnetic trapping (producing losscones). The wave-particle interactions produce growth of Langmuir waves, upper-hybrid waves, and electron-cyclotron maser emission, leading to a variety of radio burst types (type I, II, III, IV, V, DCIM; Fig. 22), which have been mainly explored from (nonimaging) dynamic spectra, while imaging observations have been rarely obtained. Although there is much theoretical understanding of the underlying wave-particle interactions, spatiotemporal modeling of imaging observations is still in its infancy. A solar-dedicated, frequency-agile imager with many frequencies (FASR) is in planning stage and might provide more comprehensive observations.

6.9 Coronal Mass Ejections As a result of phenomena in the atmosphere, every star is losing mass, caused by dynamic phenomena in its atmosphere, which accelerate plasma or particles beyond the escape speed. Inspecting the Sun, our nearest star, we observe two forms of mass loss: the steady solar wind outflow and the sporadic ejection of large plasma structures, or CMEs. The solar wind outflow amounts to ∼2 × 10−10 (g cm−2 s−1 ) in coronal holes, and to ≤4 × 10−11 (g cm−2 s−1 ) in active regions. The phenomenon of a CME occurs with a frequency of about one event per day, carrying a mass in the range of mCME ≈ 1014 – 1016 g, which corresponds to an average mass loss rate of mCME /( t · 4π R2◦ ≈ 2) × 10−14 –2 × 10−12 (g cm−2 s−1 ), which is ≤1% of the solar wind mass loss in

CLBE001-ESS2E

November 11, 2006

11:37

The Sun

Dynamic Spectrum

ne=108 cm-3 (νp= 100 MHz)

Closed Magnetic Fields

ne=109 cm-3 (νp= 300 MHz)

N

Open Magnetic Fields Interplanetary Beams

U

Frequency ν

dν/dt0

Time t

95

Downward Beams

ne=1010 cm-3 (νp= 1 GHz)

RS MW

ne=1011 cm-3 (νp= 3 GHz)

SXR

DCIM HXR

Chromospheric Evaporation

HXR

Chromosphere

FIGURE 22 Radio burst types in the framework of the standard flare scenario: The acceleration region is located in the reconnection region above the soft X-ray–bright flare loop, accelerating electron beams in the upward direction (type III, U, N bursts) and in the downward direction (type RS, DCIM bursts). Downward moving electron beams precipitate to the chromosphere (producing hard X-ray emission and driving chromospheric evaporation), or remain transiently trapped, producing microwave (MW) emission. Soft X-ray loops become subsequently filled up, with increasing footpoint separation as the X-point rises. The insert shows a dynamic radio spectrum (ETH Zurich) of the September 6, 1992, 1154 UT, flare, showing a separatrix between type III and type RS bursts at ∼600 MHz, probably associated with the acceleration region.

coronal holes, or ≤10% of the solar wind mass in active regions. The transverse size of CMEs can cover a fraction up to more than a solar radius, and the ejection speed is in the range of ν CME ≈ 102 –103 (km s−1 ). A CME structure can have the geometric shape of a fluxrope, a semishell, or a bubble (like a light bulb, see Fig. 24), which is the subject of much debate, because of ambiguities from line-of-sight projection effects and the optical thinness. There is a general consensus that a CME is associated with a release of magnetic energy in the solar corona, but its relation to the flare phenomenon is controversial. Even big flares [at least Geostationary Orbiting Earth Satellite (GOES) M-class] have no associated CMEs in 40% of the cases. A long-standing debate focused on the question of whether a CME is a byproduct of the flare process or vice versa. This question has been settled in the view that flares and CMEs are two aspects of a large-scale magnetic energy release, but the two terms evolved historically from two different observational manifestations (i.e., flares, which mainly denote the emission in hard X-rays, soft X-rays, and radio waves, and CMEs, which refer to the white-light emission of the erupting mass in the outer corona and heliosphere). Recent studies, however, clearly established the coevolution of both processes

triggered by a common magnetic instability. A CME is a dynamically evolving plasma structure, propagating outward from the Sun into interplanetary space, carrying a frozen-in magnetic flux and expanding in size. If a CME structure travels toward the Earth, which is mostly the case when launched in the western solar hemisphere, due to the curvature of the Parker spiral interplanetary magnetic field, such an Earth-directed event can engulf the Earth’s magnetosphere and generate significant geomagnetic storms. Obviously such geomagnetic storms can cause disruptions of global communication and navigation networks, can cause failures of satellites and commercial power systems, and thus are the subject of high interest. Theoretical models include five categories: (1) thermal blast models, (2) dynamo models, (3) mass loading models, (4) tether release models, and (5) tether straining models. Numerical MHD simulations of CMEs are currently produced by combinations of a fine-scale grid that entails the corona and a connected large-scale grid that encompasses propagation into interplanetary space, which can reproduce CME speeds, densities, and the coarse geometry. The trigger that initiates the origin of a CME seems to be related to previous photospheric shear motion and subsequent kink

CLBE001-ESS2E

November 11, 2006

11:37

96 Encyclopedia of the Solar System

FIGURE 23 Numerical MHD simulation of the evolution of a CME, driven by turbulent diffusion. The four panels correspond to the times (a) t = 850, (b) t = 950, (c) t = 1050, and (d) t = 1150, where viscous relaxation is started at t = 850, triggering a global disruption involving opening, reconnection through the overlying arcade and below, and the formation of a current sheet, associated with a high dissipation of magnetic energy and a strong increase of kinetic energy. (Courtesy of T. Amari.)

instability of twisted structures (Fig. 23). The geometry of CMEs is quite complex, exhibiting a variety of topological shapes from spherical semishells to helical fluxropes (Fig. 24), and the density and temperature structure of CMEs is currently investigated with multiwavelength imagers. The height-time, velocity, and acceleration profiles of CMEs seem to establish two different CME classes: gradual CMEs associated with propagating interplanetary shocks and impulsive CMEs caused by coronal flares. The total energy of CMEs (i.e., the sum of magnetic, kinetic, and gravi-

tational energy) seems to be conserved in some events, and the total energy of CMEs is comparable to the energy range estimated from flare signatures. A phenomenon closely associated with CMEs is coronal dimming (Fig. 13), which is interpreted in terms of an evacuation of coronal mass during the launch of a CME. The propagation of CMEs in interplanetary space provides diagnostic information on the heliospheric magnetic field, the solar wind, interplanetary shocks, solar energetic particle (SEP) events, and interplanetary radio bursts.

CLBE001-ESS2E

November 11, 2006

11:37

The Sun

97

FIGURE 24 Large Angle Solar COronagraph (LASCO) C3 image of a halo CME of May 6, 1998 (top); an erupting prominence of June 2, 1998, 13:31 UT (bottom left); and a large CME of November 6, 1997, 12:36 UT (bottom right). (Courtesy of SoHO/LASCO and NASA.)

7. Final Comments The study of the Sun, our nearest Star, is systematically moving from morphological observations (sunspots, active regions, filaments, flares, CMEs) to a more physics-based

modeling and theoretical understanding, in terms of nuclear physics, magneto-convection, magneto-hydrodynamics, magnetic reconnection, and particle physics processes. The major impact of physics-based modeling came from the multiwavelength observations from solar-dedicated

CLBE001-ESS2E

November 11, 2006

11:37

98 Encyclopedia of the Solar System space-based (Hinode, SMM, Yohkoh, CGRO, SoHO, TRACE, RHESSI) and ground-based instruments (in radio, Hα, and white-light wavelengths). Major achievements over the last decades are the advancement of new disciplines such as helioseismology and coronal seismology, and the solution of the neutrino problem; however, there are still unsolved outstanding problems such as the coronal heating problem and particle acceleration mechanisms. We can optimistically expect substantial progress from future solar-dedicated space missions [Solar TErrestrial RElationships Observatory (STEREO), Solar Dynamics Observatory (SDO), Solar Orbiter, Solar Probe] and ground-based instruments [Synoptic Optical Long-Term Investigations of the Sun (SOLIS), The Advanced Technology Solar Telescope (ATST) FASR].

Bibliography Aschwanden, M. J. (2004). “Physics of the Solar Corona—An Introduction.” Praxis Publishing Ltd., Chichester, England, and Springer: New York. Benz, A. O. (2003). “Plasma Astrophysics, Kinetic Processes

in Solar and Stellar Coronae,” 2nd edition. Kluwer Acad. Publ., Dordrecht, Netherlands. Cox, A. N., ed. (2000). “Allen’s Astrophysical Quantities,” 4th edition. American Institute of Physics Press/Springer, New York. Dwivedi, B. N. (2003). “The Dynamic Sun.” Cambridge University Press, Cambridge, England. Foukal, P. V. (2003). “Solar Astrophysics,” 2nd edition. John Wiley and Sons, New York. Golub, L., and Pasachoff, J. M. (2001). “Nearest Star: The Surprising Science of Our Sun.” Harvard Univ. Press, Cambridge, Massachusetts. Golub, L., and Pasachoff, J.M. (1997). “The Solar Corona.” Cambridge Univ. Press, Cambridge, Massachusetts. Lang, K. R. (2001). “The Cambridge Encyclopedia of the Sun.” Cambridge Univ. Press, Cambridge, England. Murdin, P. (ed.) 2000, “Encyclopedia of Astronomy and Astrophysics,” Institute of Physics Publishing/Grove’s Dictionaries, New York. Schrijver, C. J, and Zwaan, C. (2000). “Solar and Stellar Magnetic Activity.” Cambridge Univ. Press, Cambridge, England. Stix, M. (1989, 2002). “The Sun,” 2nd edition. Springer, New York. Zirker, J. B. (2002). “Journey from the Center of the Sun.” Princeton Univ. Press, Princeton, New Jersey.

CLBE001-ESS2E

November 11, 2006

100-C

25-C

50-C

75-C

0:14

C+M

50-C+M

C+Y

50-C+Y

M+Y

50-M+Y

100-M

25-M

50-M

75-M

100-Y

25-Y

50-Y

75-Y

100-K

25-K

25-19-19

50-K

50-40-40

75-K

75-64-64

The Solar Wind

John T. Gosling University of Colorado Boulder, Colorado

CHAPTER

5

1. Discovery 2. Statistical Properties in the Ecliptic Plane at 1 AU 3. Nature of the Heliospheric Magnetic Field

9. Termination of the Solar Wind 10. Kinetic Properties of the Plasma 11. Heavy Ion Content

4. Coronal and Solar Wind Stream Structure 5. The Heliospheric Current Sheet and Solar Latitude Effects

12. Energetic Particles 13. Waves and Turbulence

6. Evolution of Stream Structure with Heliocentric Distance 7. Coronal Mass Ejections and Transient Solar Wind Disturbances

14. Conclusion Bibliography

8. Variation with Distance from the Sun

T

he Solar Wind is a plasma, that is, an ionized gas, that permeates interplanetary space. It exists as a consequence of the supersonic expansion of the Sun’s hot outer atmosphere, the solar corona. The solar wind consists primarily of electrons and protons, but alpha particles and many other ionic species are also present at low abundance levels. At the orbit of Earth, 1 astronomical unit (AU) from the Sun, typical solar wind densities, flow speeds, and temperatures are on the order of 8 protons cm−3 , 440 km/s, and 1.2 × 105 K, respectively; however, the solar wind is highly variable in both space and time. A weak magnetic field embedded within the solar wind plasma is effective both in excluding some low-energy cosmic rays from the solar system and in channeling energetic particles from the Sun into the heliosphere. The solar wind plays an essential role in shaping and stimulating planetary magnetospheres and the ionic tails of comets. [See Planetary Magnetospheres.]

1. Discovery 1.1 Early Indirect Observations In 1859, R. Carrington made one of the first white light observations of a solar flare. He noted that a major geomagnetic storm began approximately 17 hours after the flare and tentatively suggested that a causal relationship might exist between the solar and geomagnetic events. Subsequent observations revealed numerous examples of associations between solar flares and large geomagnetic storms. In the early 1900s, F. Lindemann suggested that this could be explained if large geomagnetic storms result from an interaction between the geomagnetic field and plasma clouds ejected into interplanetary space by solar activity. Early studies of geomagnetic activity also noted that some geomagnetic storms tend to recur at the ∼27 day rotation period of the Sun as observed from Earth, particularly during

C 2007 by Academic Press. All rights of reproduction in any form reserved. Encyclopedia of the Solar System 2e 

99

CLBE001-ESS2E

November 11, 2006

0:14

100 Encyclopedia of the Solar System declining years of solar activity. This observation led to the suggestion that certain regions on the Sun, commonly called M (for magnetic)-regions, occasionally produce long-lived charged particle streams in interplanetary space. Further, because some form of auroral and geomagnetic activity is almost always present at high geomagnetic latitudes, it was inferred that charged particles from the Sun almost continuously impact and perturb the geomagnetic field. Observations of modulations in galactic cosmic rays (highly energetic charged particles that originate outside the solar system) in the 1930s also suggested that plasma and magnetic fields are ejected from the Sun during intervals of high solar activity. For example, S. Forbush noted that cosmic ray intensity often decreases suddenly during large geomagnetic storms and then recovers slowly over a period of several days. Moreover, cosmic ray intensity varies in a cycle of ∼11 years, but roughly 180◦ out of phase with the solar activity cycle. One possible explanation of these observations was that magnetic fields embedded in plasma clouds from the Sun sweep cosmic rays away from the vicinity of Earth. In the early 1950s, L. Biermann concluded that there must be a continuous outflow of charged particles from the Sun to explain the fact that ionic tails of comets always point away from the Sun. He estimated that a continuous particle flux on the order of 1010 protons cm−2 s−1 was needed at 1 AU to explain the comet tail observations. He later revised his estimate downward to a value of ∼109 protons cm−2 s−1 , closer to the average observed solar wind proton flux of ∼3.8 × 108 protons cm−2 s−1 at 1 AU.

1.2 Parker’s Solar Wind Model Apparently inspired by these diverse observations and interpretations, E. Parker, in 1958, formulated a radically new model of the solar corona in which the solar atmosphere is continually expanding outward. Prior to Parker’s work most theories of the solar atmosphere treated the corona as static and gravitationally bound to the Sun except for sporadic outbursts of material into space at times of high solar activity. S. Chapman had constructed a model of a static solar corona in which heat transport was dominated by electron thermal conduction. For a 106 K corona, Chapman found that even a static solar corona must extend far out into space. Parker realized, however, that a static model leads to pressures at large distances from the Sun that are seven to eight orders of magnitude larger than estimated pressures in the interstellar plasma. Because of this mismatch in pressure at large heliocentric distances, he reasoned that the solar corona could not be in hydrostatic equilibrium and must therefore be expanding. His consideration of the hydrodynamic (i.e., fluid) equations for mass, momentum, and energy conservation for a hot solar corona led him to unique solutions for the coronal expansion that depended on the coronal temperature close to the surface of the Sun. Parker’s model

produced low flow speeds close to the Sun, supersonic flow speeds far from the Sun, and vanishingly small pressures at large heliocentric distances. In view of the fluid character of the solutions, Parker called this continuous, supersonic, coronal expansion the solar wind. The region of space filled by the solar wind is now known as the heliosphere.

1.3 First Direct Observations of the Solar Wind Several Russian and American space probes in the 1959– 1961 era penetrated interplanetary space and found tentative evidence for a solar wind. Firm proof of the wind’s existence was provided by C. Snyder and M. Neugebauer, who flew a plasma experiment on Mariner 2 during its epic 3-month journey to Venus in late 1962. Their experiment detected a continual outflow of plasma from the Sun that was highly variable, being structured into alternating streams of high- and low-speed flows that lasted for several days each. Several of the high-speed streams recurred at roughly the rotation period of the Sun. Average solar wind proton densities (normalized for a 1 AU heliocentric distance), flow speeds, and temperatures during this 3-month interval were 5.4 cm−3 , 504 km/s, and 1.7 × 105 K, respectively, in essential agreement with Parker’s predictions. The Mariner 2 observations also showed that helium, in the form of alpha particles, is present in the solar wind in variable amounts; the average alpha particle abundance relative to protons of 4.6% is about a factor of 2 lower than estimates of the helium abundance within the Sun. Finally, measurements made by Mariner 2 confirmed that the solar wind carried a magnetic field whose strength and orientation in the ecliptic plane were much as predicted by Parker (see Section 3). Despite the good agreement of observations with Parker’s model, we still do not fully understand the processes that heat the solar corona and accelerate the solar wind. Parker simply assumed that the corona is heated to a very high temperature, but he did not explain how the heating was accomplished. Moreover, it is now known that electron heat conduction is insufficient to power the coronal expansion. Present models for heating the corona and accelerating the solar wind generally fall into two classes: (1) heating and acceleration by waves generated by convective motions below the photosphere and (2) bulk acceleration and heating associated with transient events in the solar atmosphere such as magnetic reconnection. Present observations are incapable of distinguishing among these and other alternatives.

2. Statistical Properties in the Ecliptic Plane at 1 AU Table 1 summarizes a number of statistical solar wind properties derived from spacecraft measurements in the

CLBE001-ESS2E

November 11, 2006

0:14

The Solar Wind

TABLE 1

101

Statistical Properties of the Solar Wind at 1 AU

Parameter

Mean

STD

Most Probable

Median

5–95% Range

n (cm–3 ) Vsw (km/s) B (nT) A(He) Tp (×105 K) Te (×105 K) Tα (×105 K) Te /Tp Tα /Tp nVsw (×108 /cm2 s) Cs (km/s) CA (km/s)

8.7 468 6.2 0.047 1.2 1.4 5.8 1.9 4.9 3.8 63 50

6.6 116 2.9 0.019 0.9 0.4 5.0 1.6 1.8 2.4 15 24

5.0 375 5.1 0.048 0.5 1.2 1.2 0.7 4.8 2.6 59 50

6.9 442 5.6 0.047 0.95 1.33 4.5 1.5 4.7 3.1 61 46

3.0–20.0 320–710 2.2–9.9 0.017–0.078 0.1–3.0 0.9–2.0 0.6–15.5 0.37–5.0 2.3–7.5 1.5–7.8 41–91 30–100

ecliptic plane at 1 AU. The table includes mean values, standard deviations about the mean values, most probable values, median values, and the 5–95% range limits for the proton number density (n), the flow speed (Vsw ), the magnetic field strength (B), the alpha particle abundance relative to protons [A(He)], the proton temperature (Tp ), the electron temperature (Te ), the alpha particle temperature (Ta ), the ratio of the electron and proton temperatures (Te /Tp ), the ratio of alpha particle and proton temperatures (Ta /Tp ), the number flux (nVsw ), the sound speed (Cs ), and the Alfven ´ speed (CA ) (the speed at which small amplitude perturbations in the magnetic field propagate through the plasma). All solar wind parameters exhibit considerable variability; moreover, variations in solar wind parameters are often coupled to one another. Proton temperatures are considerably more variable than electron temperatures, and alpha particle temperatures are almost always higher than electron and proton temperatures. Alpha particles and the protons tend to have nearly equal thermal speeds and therefore temperatures that differ by a factor of ∼4. The solar wind flow is usually both supersonic and super-Alfvenic. ´ Finally, we note that the Sun yearly loses ∼6.8 × 1019 g to the solar wind, a very small fraction of the total solar mass of ∼2 × 1033 g.

3. Nature of the Heliospheric Magnetic Field In addition to being a very good thermal conductor, the solar wind plasma is an excellent electrical conductor. The electrical conductivity of the plasma is so high that the solar magnetic field is “frozen” into the solar wind flow as it expands away from the Sun. Because the Sun rotates, magnetic field lines in the equatorial plane of the Sun are bent into spirals (Fig. 1) whose inclinations relative to the radial direction depend on heliocentric distance and the speed of

B

V sw

FIGURE 1 Configuration of the heliospheric magnetic field in the ecliptic plane for a uniform, radial solar wind flow.

the wind. At 1 AU, the average field line in the equatorial plane is inclined ∼45◦ to the radial direction. In Parker’s simple model, the magnetic field lines out of the equatorial plane take the form of helices wrapped about the rotation axis of the Sun. These helices are ever more elongated at higher solar latitudes and eventually approach radial lines over the solar poles. The equations describing Parker’s model of the magnetic field far from the Sun are Br (r, φ, θ ) = B(r0 , φ0 , θ)(r0 /r)2 Bφ (r, φ, θ ) = −B(r0 , φ0 , θ)(ωr02 /Vswr) sin θ Bθ = 0

CLBE001-ESS2E

November 11, 2006

0:14

102 Encyclopedia of the Solar System

The solar corona is highly nonuniform, being structured by the complex solar magnetic field into arcades, rays, holes (regions relatively devoid of material), and streamers. [See The Sun.] The strength of the Sun’s magnetic field falls off sufficiently rapidly with height above the solar surface that it is incapable of containing the coronal expansion at altitudes above ∼0.5–1.0 solar radii. The resulting solar wind outflow produces the “combed-out” appearance of coronal structures above those heights in eclipse photographs. The solar wind is also highly nonuniform. In the ecliptic plane, it tends to be organized into alternating streams of high- and low-speed flows. Figure 2, which shows solar wind flow speed, flow azimuth, the radial component of the heliospheric magnetic field, and the field strength at 1 AU for a 50-day interval in 2004, illustrates certain characteristic aspects of this stream structure. Four high-speed streams with flows exceeding 700 km/s are clearly evident in the figure. The third and fourth streams were actually reencounters with the first and second streams, respectively, on the following solar rotation. Each high-speed stream was asymmetric with the speed rising more rapidly than it fell, and each stream was essentially unipolar in the sense that Br was either positive or negative throughout the stream. Reversals in field polarity occurred in the low-speed flows between the streams. Those polarity reversals correspond to crossings of the heliospheric current sheet (discussed in more detail in the following section) that separates solar wind regions of opposite magnetic polarity. The magnetic field and plasma density (not shown) peaked on the leading edges of the streams, and the solar wind flow there was deflected first westward (positive flow azimuth) and then eastward. This pattern of variability is highly repeatable from

Sp e e d ( k m /s)

600 400

Fl o w A z i m u t h ( De g s)

200 10

0

-10

10 0 r

B ( nT)

4. Coronal and Solar Wind Stream Structure

800

-10 -20 20 B ( nT)

Here r, φ, and θ are radial distance, longitude, and latitude in a Sun-centered spherical coordinate system, Br , Bφ , and Bθ are the magnetic field components, ω is the Sun’s angular velocity (2.9 × 10−6 radians sec−1 ), Vsw is the flow speed (assumed constant with distance from the Sun), and φ0 is an initial longitude at a reference distance r0 from Sun center. This model is in reasonably good agreement with suitable averages of the heliospheric magnetic field measured over a wide range of heliocentric distances and latitudes. However, the instantaneous orientation of the field usually deviates substantially from that of the model field at all distances and latitudes. Moreover, there is evidence that the magnetic field lines wander in latitude as they extend out into the heliosphere. This appears to be a result of field line foot point motions associated with differential solar rotation (the surface of the Sun rotates at different rates at different latitudes) and convective motions in the solar atmosphere.

10

0 40

50

60 70 Day 2004

80

90

FIGURE 2 1-hr average solar wind speed, flow azimuth, radial component of the heliospheric magnetic field, and the field magnitude at 1 AU for a 50-day interval in 2004.

one stream to the next and is the inevitable consequence of the evolution of the streams as they progress outward from the Sun (see Section 6). Recurrent high-speed streams originate primarily in coronal holes, which are large, nearly unipolar regions in the solar atmosphere having relatively low density. Low-speed flows, on the other hand, tend to originate in the coronal streamers that straddle regions of magnetic field polarity reversals in the solar atmosphere. Both coronal and solar wind stream structure evolve considerably from one solar rotation to the next as the solar magnetic field, which controls that structure, continuously evolves. It is now clear that the mysterious M-regions, hypothesized long before the era of satellite X-ray observations of the Sun, are to be identified with coronal holes, and the long-lived particle streams responsible for recurrent geomagnetic activity are

CLBE001-ESS2E

November 11, 2006

0:14

The Solar Wind

103

FIGURE 3 Right, schematic illustrating the configuration of the heliospheric current sheet when the solar magnetic dipole is tilted substantially relative to the rotation axis of the Sun. The heliospheric current sheet separates magnetic fields of opposite magnetic polarity and is the heliospheric extension of the solar magnetic equator. Left, schematic illustrating the changing tilt of coronal structure and the solar magnetic dipole relative to the rotation axis of the Sun as a function of the phase of the solar activity cycle. [Adapted from J. R. Jokipii and B.Thomas, 1981, Astrophys. J. 243, 1115, and from A. J. Hundhausen, 1977, in “Coronal Holes and High Speed Wind Streams” (J. Zirker, ed.), Colorado Associated University Press, Boulder, Colorado.]

to be identified with high-speed solar wind streams. [See Sun–Earth Connection.]

5. The Heliospheric Current Sheet and Solar Latitude Effects 5.1 The Sun’s Large-Scale Magnetic Field and the Ballerina Skirt Model On the declining phase of the solar activity cycle and near solar activity minimum, the Sun’s large-scale magnetic field well above the photosphere appears to be approximately that of a dipole. The solar magnetic dipole is tilted with respect to the Sun’s rotation axis; this tilt changes with the advance of the solar cycle. As illustrated in the left-hand side of Fig. 3, near the solar activity minimum the solar magnetic dipole tends to be aligned nearly with the rotation axis, whereas on the declining phase of the activity cycle it is generally inclined at a considerable angle relative to the rotation axis. Near the solar maximum, the Sun’s large-scale field is probably not well approximated by a dipole. When the solar magnetic dipole and the solar rotation axis are closely aligned, the heliospheric current sheet, which is effectively the extension of the solar magnetic equator into the solar wind, coincides roughly with the solar equatorial plane. On the other hand, at times when the dipole is tilted substantially, the heliospheric current sheet is warped and resembles a ballerina’s twirling skirt, as illustrated in the right-hand side of Fig. 3. Successive outward

ridges in the current sheet (folds in the skirt) correspond to successive solar rotations and are separated radially by about 4.7 AU when the flow speed at the current sheet is 300 km/s. The maximum solar latitude of the current sheet in this simple picture is equal to the tilt angle of the magnetic dipole axis relative to the rotation axis.

5.2 Solar Latitude Effects On the declining phase of the solar activity cycle and near the solar activity minimum, stream structure and solar wind variability are largely confined to a relatively narrow latitude band centered on the solar equator. This is illustrated in the upper left portion of Fig. 4, which shows solar wind speed as a function of solar latitude measured by Ulysses on the declining phase of the most recent solar cycle. (Ulysses is in a solar orbit that takes it to solar latitudes of ±80◦ in its ∼5.5-year journey about the Sun.) At this phase of the solar cycle, the solar wind is dominated by stream structure at low latitudes, but it flows at a nearly constant speed of ∼850 km/s at high latitudes. This latitude effect is a consequence of the following: (1) Solar wind properties change rapidly with distance from the heliospheric current sheet, with flow speed being a minimum in the vicinity of the current sheet; and (2) the heliospheric current sheet is commonly tilted relative to the solar equator but is usually found within about ±30◦ of it during this phase of the solar cycle. The width of the band of solar wind variability changes as the solar magnetic dipole tilt changes. The upper right portion of Fig. 4 demonstrates that, in contrast, in the years surrounding the

CLBE001-ESS2E

November 11, 2006

0:14

104 Encyclopedia of the Solar System FIGURE 4 Solar wind speed as a function of heliographic latitude as measured by the Ulysses space probe during the declining phase of the solar activity cycle (left) and near solar activity maximum (right). The speed data are color-coded according to the polarity of the magnetic field and are superimposed on representative images of the solar corona at those phases of the solar cycle. Smoothed sunspot numbers are shown at the bottom. The S and N labels on the latter indicate the solar hemisphere that Ulysses was in at those times. (From D. J. McComas et al., 2003, Geophys. Res. Lett. 30, 10 10.1029.2003GL017136.)

solar activity maximum, the band of solar wind variability extends up to the highest latitudes sampled by Ulysses.

6. Evolution of Stream Structure with Heliocentric Distance 6.1 Kinematic Stream Steepening and the Dynamic Response Because the coronal expansion is spatially variable, alternately slow and fast plasma is directed outward along any radial line from the Sun as the Sun rotates (with a period of 27 days as seen from Earth). Faster-moving plasma overtakes slower-moving plasma ahead while outrunning slower-moving plasma behind. The result is that the leading edges of high-speed streams steepen with increasing distance from the Sun, producing the asymmetric stream profiles obvious in Fig. 2. Material within the streams is rearranged as the streams steepen; plasma and field on the leading edge of a stream are compressed, causing an increase in plasma density, temperature, field strength, and pressure there, while plasma and field on the trailing edge become increasingly rarefied. The buildup of pressure on the leading edge of a stream produces forces that accelerate

the low-speed wind ahead and decelerate the high-speed wind within the stream itself. The net result is a transfer of momentum and energy from the fast-moving wind to the slow-moving wind.

6.2 Shock Formation As long as the amplitude of a high-speed solar wind stream is sufficiently small, it gradually dampens with increasing heliocentric distance in the manner just described. However, when the difference in flow speed between the crest of a stream and the trough ahead is greater than about twice the local fast mode speed, Cf [the fast mode speed is the characteristic speed with which small amplitude pressure signals propagate in a plasma: Cf = (Cs 2 + CA 2 )0.5 ], ordinary pressure signals do not propagate sufficiently fast to move the slow wind out of the path of the oncoming high-speed stream. In that case, the pressure eventually increases nonlinearly, and shock waves form on either side of the highpressure region (see Fig. 5). The leading shock, known as a forward shock, propagates into the low-speed wind ahead, and the trailing shock, known as a reverse shock, propagates back through the stream. Both shocks are, however, convected away from the Sun by the high bulk flow of the wind. The major accelerations and decelerations associated with

CLBE001-ESS2E

November 11, 2006

0:14

The Solar Wind

105

ture is damped out. The dominant structure in the solar equatorial plane in the outer heliosphere is the expanding compression region where most of the plasma and magnetic field are concentrated.

6.3 Stream Evolution in Two and Three Dimensions When the coronal expansion is spatially variable but timestationary, a steady flow pattern such as that sketched in Fig. 6 develops in the equatorial plane. This entire pattern corotates with the Sun, and the compression regions are known as corotating interaction regions (CIRs); however, only the pattern rotates—each parcel of solar wind plasma moves outward nearly radially as indicated by the black arrows. The region of high pressure associated with a CIR is nearly aligned with the magnetic field line spirals in the equatorial plane, and the pressure gradients are thus nearly perpendicular to those spirals. Consequently, at 1 AU, the pressure gradients that form on the rising speed portions of high-speed streams have transverse as well as radial components. In particular, not only is the low-speed plasma ahead of a high-speed stream accelerated to a higher speed, but it is also deflected in the direction of solar rotation. In contrast, the high-speed plasma near the crest of the stream is both decelerated and deflected in the direction opposite

FIGURE 5 Snapshots of solar wind flow speed (above) and pressure (below) as functions of heliocentric distance at different times during the evolution of a large-amplitude, high-speed solar wind stream as calculated from a simple one-dimensional numerical model. (Adapted from A. J. Hundhausen, 1973, J. Geophys. Res. 78, 1528.)

stream evolution occur discontinuously at the shocks, giving a stream speed profile the appearance of a double saw-tooth wave. The stream amplitude decreases and the compression region expands with increasing heliocentric distance as the shocks propagate. Observations indicate that the shocks typically do not form until the streams are well beyond 1 AU. Nevertheless, because Cf generally decreases with increasing heliocentric distance, virtually all large-amplitude solar wind streams steepen into shock wave structures at heliocentric distances beyond ∼3 AU. At heliocentric distances beyond the orbit of Jupiter (∼5.4 AU) a large fraction of the mass in the solar wind is found within compression regions bounded by shock waves on the rising portions of damped high-speed streams. The basic structure of the solar wind in the solar equatorial plane in the distant heliosphere thus differs considerably from that observed at 1 AU. Stream amplitudes are severely reduced, and short wavelength struc-

FIGURE 6 Schematic illustrating two-dimensional, quasi-stationary stream structure in the ecliptic plane in the inner heliosphere. The compression region on the leading edge of a stream is nearly aligned with the spiral magnetic field, and the forces associated with the pressure gradients have transverse as well as radial components. (From V. J. Pizzo, 1978, J. Geophys. Res. 83, 5563.)

CLBE001-ESS2E

November 11, 2006

0:14

106 Encyclopedia of the Solar System to solar rotation. These transverse deflections produce the systematic west–east flow direction changes observed near the leading edges of quasi-stationary, high-speed streams (see Fig. 2). There is an interesting three-dimensional aspect to stream evolution, ultimately associated with the fact that the solar magnetic dipole is tilted relative to the solar rotation axis. That tilt causes CIRs in the northern and southern solar hemispheres to have opposed meridional tilts that, particularly in the outer heliosphere, can be discerned in plasma data as systematic north–south deflections of the flow at CIRs. The meridional tilts are such that the forward waves in both hemispheres initially propagate equatorward, whereas the reverse waves in both hemispheres propagate poleward. As a result, forward shocks in the outer heliosphere near the solar minimum are generally confined to the low-latitude band of solar wind variability, whereas the reverse shocks are commonly observed both within the band of variability and poleward of it. However, the reverse waves seldom reach latitudes more than ∼15◦ above the low-latitude band of variability.

7. Coronal Mass Ejections and Transient Solar Wind Disturbances 7.1 Coronal Mass Ejections The solar corona evolves on a variety of time scales closely connected with the evolution of the coronal magnetic field. [See The Sun.] The most rapid and dramatic evolution in the corona occurs in events known as coronal mass ejections, or CMEs (Fig. 7a). CMEs originate in closed field regions in the corona where the magnetic field normally is sufficiently strong to constrain the coronal plasma from expanding outward. Typically these closed field regions are found in the coronal streamer belt that encircles the Sun and that underlies the heliospheric current sheet. The outer edges of CMEs often have the optical appearance of closed loops such as the event shown in Fig. 7a. Few, if any, CMEs ever appear to sever completely their magnetic connection with the Sun. During a typical CME, somewhere between 1015 and 1016 g of solar material is ejected into the heliosphere. Ejection speeds near the Sun range from less than

FIGURE 7 (a) A coronal mass ejection as imaged by the LASCO/C3 coronagraph on SOHO on April 20, 1998. The Sun, indicated by the white circle, has been occulted within the instrument. The field of view of the image is 30 solar diameters. [The SOHO/LASCO data are produced by a consortium of the Naval Research Laboratory (USA), Max-Planck-Institut fur Sonnensystemforshung (Germany), Laboratoire d’Astronomie (France), and the University of Birmingham (UK). SOHO is a project of international cooperation between ESA and NASA.] (b) A sketch of a solar wind shock disturbance produced by a fast ICME directed toward Earth. Red and magenta arrows indicate the ambient magnetic field and that threading the ICME, respectively. Blue arrows indicate the suprathermal electron strahl flowing away from the Sun along the magnetic field. The ambient magnetic field is compressed by its interaction with the ICME and is forced to drape around the ICME. [To appear in T. H. Zurbuchen and I. G. Richardson, 2006, in “Coronal Mass Ejections” (H. Kunow et al., eds.), Kluwer academic Publishers, Dordrecht.]

CLBE001-ESS2E

November 11, 2006

0:14

The Solar Wind

107

50 km/s in some of the slowest events to greater than 2500 km/s in the fastest ones. The average CME speed at ∼5 solar radii is close to the median ecliptic solar wind speed of ∼440 km/s. Since observed solar wind speeds near 1 AU are never less than ∼280 km/s, the slowest CMEs are further accelerated enroute to 1 AU.

7.2 Origins, Associations with Other Forms of Solar Activity, and Frequency of Occurrence The processes that trigger CMEs and that determine their sizes and outward speeds are only poorly understood; there is presently no consensus on the physical processes responsible for initiating or accelerating these events, although it is clear that stressed magnetic fields are the underlying cause of these events and that CMEs play a fundamental role in the long-term evolution of the structure of the solar corona. They appear to be an essential part of the way the corona responds to the evolution of the solar magnetic field associated with the advance of the solar activity cycle. Indeed, the release of a CME is one way that the solar atmosphere reconfigures itself in response to changes in the solar magnetic field. CMEs are commonly, but not always, observed in association with other forms of solar activity such as eruptive prominences and solar flares. From a historical perspective, one might be led to expect that large solar flares are the prime cause of CMEs; however, it is now clear that flares and CMEs are separate, but closely related, phenomena associated with magnetic disturbances on the Sun. Like other forms of solar activity, CMEs occur with a frequency that varies in a cycle of ∼11 years. On average, the Sun emits about 3.5 CMEs/day near the peak of the solar activity cycle, but only about 0.1 CMEs/day near solar activity minimum.

7.3 Heliospheric Disturbances Driven by Fast Coronal Mass Ejections As illustrated in Fig. 7b, fast CMEs produce transient solar wind disturbances that, in turn, often are the cause of large, geomagnetic storms. [See Sun–Earth Connections.] Figure 8 shows calculated radial speed and pressure profiles of a simulated solar wind disturbance driven by a fast CME at the time the disturbance first reaches 1 AU. As indicated by the insert in the top portion of the figure, the disturbance was initiated at the inner boundary of the onedimensional fluid calculation by abruptly raising the flow speed from 275 to 980 km/s, sustaining it at this level for 6 hours, and then returning it to its original value of 275 km/s. The initial disturbance thus mimics a uniformly fast, spatially limited CME with an internal pressure equal to that of the surrounding solar wind plasma. A region of high pressure develops on the leading edge of the disturbance as the CME overtakes the slower wind ahead. This region of

FIGURE 8 Solar wind speed and pressure as functions of heliocentric distance for a simple, one-dimensional gas-dynamic simulation of a CME-driven disturbance. The dashed line indicates the steady state prior to introduction of the temporal variation in flow speed imposed at the inner boundary of 0.14 AU and shown at the top of the figure. The hatching identifies material that was introduced with a speed of 980 km/s at the inner boundary, and therefore identifies the CME in the simulation. [Adapted originally from A. J. Hundhausen, 1985, in “Collisionless Shocks in the Heliosphere: A Tutorial Review” (R. G. Stone and B. T. Tsurutani, eds.), American Geophysical Union, Washington, D.C.].

higher pressure is bounded by a forward shock on its leading edge that propagates into the ambient solar wind ahead and by a reverse shock on its trailing edge that propagates backward into and eventually through the CME. Both shocks are, however, carried away from the Sun by the highly supersonic flow of the solar wind. Observations and more detailed calculations indicate, however, that reverse shocks in CME-driven disturbances are ordinarily present only near the central portions of the disturbances.

CLBE001-ESS2E

November 11, 2006

0:14

108 Encyclopedia of the Solar System Except for the reverse shock, the simple calculation shown in Fig. 8 is consistent with observations of many solar wind disturbances obtained near 1 AU in the ecliptic plane and illustrates to first order the radial and temporal evolution of an interplanetary disturbance driven by a fast CME (now commonly called an interplanetary coronal mass ejection, ICME, when observed in the solar wind). The leading edge of the disturbance is a shock that stands off ahead of the ICME (see also Fig. 7b). The ambient solar wind ahead of the ICME is compressed, heated, and accelerated as the shock passes by, and the leading portion of the ICME is compressed, heated, and slowed as a result of the interaction. In the example illustrated, the ICME slows from an initial speed of 980 km/s to less than 600 km/s by the time the leading edge of the disturbance reaches 1 AU. This slowing is a result of momentum transfer to the ambient solar wind ahead and proceeds at an everslower rate as the disturbance propagates outward. Figure 9 displays selected plasma and magnetic field data from a solar wind disturbance driven by an ICME observed near 1 AU. The shock is distinguished in the data by discontinuous increases in flow speed, density, temperature, and field strength. The plasma identified as the ICME had a higher flow speed than the ambient solar wind ahead of the shock. In this case, it was also distinguished by counterstreaming suprathermal electrons (indicative of a closed magnetic field topology, see Section 10.3), anomalously low proton temperatures, somewhat elevated helium abundance, and a strong, smoothly rotating magnetic field that indicates the magnetic field topology was that of a nested helical structure (i.e., a flux rope; see Fig. 7b).

7.4 Characteristics of Interplanetary Coronal Mass Ejections The identification of ICMEs in solar wind plasma and field data is still something of an art; however, shocks serve as useful fiducials for identifying fast ICMEs. Table 2 provides a summary of plasma and field signatures that qualify as being unusual compared to the normal solar wind, but that are commonly observed a number of hours after shock passage. Most of these anomalous signatures are also observed elsewhere in the solar wind where, presumably, they serve to identify those numerous, relatively low-speed ICMEs that do not drive shock disturbances. Few ICMEs at 1 AU exhibit all of these characteristics, and some of these signatures are more commonly observed than are others. Most ICMEs expand as they propagate outward through the heliosphere. ICME radial thicknesses are variable; at 1 AU the typical ICME has a radial width of ∼0.2 AU, whereas at Jupiter’s orbit ICMEs can have radial widths as large as 2.5 AU. Approximately one third of all ICMEs

FIGURE 9 A solar wind disturbance associated with moderately fast ICME observed by the Advanced Composition Explorer in October 1998. From top to bottom the quantities plotted are color-coded pitch angle distributions of 256–288 eV electrons, proton density, proton temperature, bulk flow speed, alpha-proton density ratio, and magnetic field strength, azimuth, and polar angle in solar ecliptic coordinates. The color scale for f (v) extends from 5 × 10−32 (dark purple) to 2 × 10−29 s3 cm−6 (dark red). Dashed and solid vertical lines respectively mark the shock and the edges of the ICME. (From J. T. Gosling et al., 2002, Geophys. Res. Lett. 29, 12 10.1029/2001GL013949.)

in the ecliptic plane have sufficiently high speeds relative to the ambient solar wind to drive shock disturbances at 1 AU; the remainder do not and simply coast along with the rest of the solar wind. Typically, ICMEs cannot be distinguished from the normal solar wind at 1 AU on the basis of either their speed or density (the event in Fig. 9 is an example). Near the solar activity maximum, ICMEs account for 15–20% of the solar wind in the ecliptic plane at 1 AU, while they account for less than 1% near the solar activity minimum. The Earth intercepts about 72 ICMEs/year near the solar activity maximum and ∼8 ICMEs/year near the solar activity minimum. ICMEs are much less common at high heliographic latitudes, particularly near activity minimum

CLBE001-ESS2E

November 11, 2006

0:14

The Solar Wind

TABLE 2

109

Characteristics of Interplanetary Coronal Mass Ejections at 1 AU

Common signatures: Counterstreaming (along the field) suprathermal (energy >70 eV) electrons Counterstreaming (along the field) energetic (energy >20 keV) protons Helium abundance enhancement Anomalously low proton and electron temperatures Strong magnetic field Low plasma beta Low magnetic field strength variance Anomalous field rotation (flux rope) Anomalous ionic composition (e.g., Fe16+ , He+ ) Cosmic ray depression Average radial thickness: 0.2 AU Range of speeds: 300–2000 km/s Single point occurrence frequency: ∼72 events/year at solar activity maximum ∼8 events/year at solar activity minimum Magnetic field topology: Predominantly closed magnetic loops rooted in Sun Fraction of events driving shocks: ∼1/3 Fraction of earthward-directed events producing large geomagnetic storms: ∼1/6

when ICMEs are confined largely to the low-latitude band of solar wind variability.

7.5 The Magnetic Field Topology of ICMEs and the Problem of Magnetic Flux Balance The coronal expansion carries the solar magnetic field outward to form the heliospheric magnetic field. In the quiescent wind, these field lines are usually “open” in the sense that they connect to field lines of the opposite polarity only in the very distant heliosphere. CMEs, on the other hand, originate in the corona in closed field regions that have not previously participated directly in the solar wind expansion and carry new magnetic flux into the heliosphere. The magnetic flux that each CME apparently adds to the heliosphere must be balanced by removal of magnetic flux elsewhere since the overall heliospheric magnetic field strength is roughly (within a factor of 2) constant in time. Magnetic reconnection within the magnetic “legs” of a CME close to the Sun appears to be the prime way that this balance is achieved. Figure 10 illustrates that such reconnection is inherently three-dimensional in nature and initially produces helical magnetic field lines that are partially disconnected from the Sun (see, also, Fig. 7b). Sustained three-dimensional magnetic reconnection eventually produces open and disconnected field lines threading an ICME, both of which are sometimes observed. All of the types of reconnection illustrated in Fig. 10 reduce the amount of magnetic flux permanently added to the

heliosphere by an ICME. However, it is not presently clear what mix of reconnections within the magnetic legs of ICMEs and elsewhere in the solar atmosphere (e.g., at the heliospheric current sheet) is actually responsible for maintaining a rough long-term balance of magnetic flux in the heliosphere.

7.6 Field Line Draping About Fast Interplanetary Coronal Mass Ejections Because the closed field nature of ICMEs effectively prevents any substantial interpenetration between the plasma within an ICME and that in the surrounding wind, the ambient plasma and magnetic field ahead must be deflected away from the path of a fast ICME. Figure 7b illustrates that such deflections cause the ambient magnetic field to drape about the ICME. The degree of draping and the resulting orientation of the field ahead of an ICME depend upon the relative speed between the ICME and the ambient plasma, the shape of the ICME, and the original orientation of the magnetic field in the ambient plasma. Draping plays an important role in reorienting the magnetic field ahead of a fast ICME. On the other hand, conditions and processes back at the Sun largely determine the field orientation within ICMEs. As a final point of interest, Figure 7b also illustrates that, just as the bow wave in front of a boat moving through water is considerably broader in extent than is the boat that produces it, so too is the shock in front of a fast ICME somewhat broader in extent than is the

CLBE001-ESS2E

November 11, 2006

0:14

110 Encyclopedia of the Solar System FIGURE 10 Sketches of successive steps in three-dimensional reconnection in the corona beneath a departing CME. The sketches are not to scale and are intended only to illustrate successive changes in CME magnetic topologies resulting from reconnection. (From J. T. Gosling et al., 1995, Geophys. Res. Lett. 22, 869.)

a

b

c

d

ICME that drives it. As a result, spacecraft often encounter ICME-driven shocks without also encountering the ICMEs that drive them.

8. Variation with Distance from the Sun For a structureless solar wind, the speed remains nearly constant beyond the orbit of Earth, the density falls off with heliocentric distance (r, as r –2 ), and the magnetic field decreases with distance as described by the equations in Section 2. The temperature also decreases with increasing heliocentric distance due to the spherical expansion of the plasma; however, the precise nature of the decrease depends upon particle species and the relative importance of such things as collisions and heat conduction (e.g., protons and electrons have different temperatures and evolve differently with increasing heliocentric distance). For an adiabatic expansion of an isotropic plasma, the temperature falls off as r −4/3 ; for a plasma dominated by heat conduction, the temperature falls as r −2/7 . Of course, the solar wind is not structureless. The continual interaction of high- and low-speed flows with increasing heliocentric distance produces a radial variation of speed that differs considerably from that predicted for a structureless wind. High-speed flows decelerate and low-speed flows accelerate with increasing heliocentric distance as a result of momentum transfer (see Sections 6 and 7). Consequently, near the solar equatorial plane far from the Sun (beyond ∼15 AU) the solar wind flows at 400 to 500 km/s most of the time (Fig. 11). Only rarely are substantial speed perturbations observed at these distances; these rare events usually are associated with disturbances driven by very large and fast ICMEs that require a greater-than-usual distance to share their momentum with the low-speed wind.

With increasing heliocentric distance an ever-greater fraction of the plasma and magnetic field in the wind becomes concentrated within the compression regions on the rising speed portions of high-speed flows; extended rarefaction regions relatively devoid of plasma and field follow these compressions. Thus, at low heliographic latitudes, solar wind density and magnetic field strength tend to vary over a wider range in the outer heliosphere than near the orbit of the Earth, although the average density falls roughly as r −2 , and the average magnetic field falls off roughly as predicted by the equations in Section 2. On the other hand, plasma heating associated with the compression regions causes the solar wind temperature to fall off with increasing heliocentric distance more slowly than it otherwise would. Observations reveal that both proton and electron temperatures decrease with distance somewhere between the adiabatic and conduction-dominated extremes.

9. Termination of the Solar Wind Interstellar space is filled with a dilute gas of neutral and ionized particles and is threaded by a weak magnetic field. In the absence of the solar wind, the interstellar plasma would penetrate deep into the solar system. However, the interstellar and solar wind plasmas cannot interpenetrate one another because of the magnetic fields embedded in both. The result is that the solar wind creates a cavity in the interstellar plasma. The details of the solar wind’s interaction with the interstellar plasma are still somewhat speculative largely because, until recently, we lacked direct observations of this interaction. Figure 12 shows what are believed to be the major elements of the interaction. The Sun and heliosphere

CLBE001-ESS2E

November 11, 2006

0:14

The Solar Wind

111

FIGURE 11 Solar wind speed as a function of time as measured by Voyager 2 during a 1.5-year interval when the spacecraft was beyond 18 AU from the Sun. Because stream amplitudes are severely damped at large distances from the Sun, the solar wind speed there generally varies within a very narrow range of values. Compare with the speed variations evident in Fig. 2 that were obtained at 1 AU during a comparable period of the solar cycle. [Adapted from A. J. Lazarus and J. Belcher, 1988, in “Proceedings of the Sixth International Solar Wind Conference” (V. J. Pizzo, T. E. Holzer, and D. G. Sime, eds.), National Center for Atmospheric Research, Boulder, Colorado.]

move at a speed of ∼23 km/s relative to the interstellar medium. If this relative motion exceeds the fast mode speed, Cf , in the interstellar plasma, then a bow shock must stand in the interstellar plasma upstream of the heliosphere to initiate the slowing and deflection of the interstellar

plasma around the heliosphere. The heliopause, which is the outermost boundary of the heliosphere, separates the interstellar and solar wind plasmas. Sunward of the heliopause is a termination shock where the solar wind flow becomes subsonic so that it can be turned to flow roughly

FIGURE 12 Simulated structure of the solar wind’s interaction with the interstellar plasma. Colorcoding represents the proton temperature and arrows indicate the direction of the solar wind and interplanetary plasma flows. (Courtesy of G. P. Zank and H. R. Mueller.)

CLBE001-ESS2E

November 11, 2006

0:14

112 Encyclopedia of the Solar System parallel to the heliopause. The shape of the heliosphere is asymmetric because of its motion relative to the interstellar gas; it is compressed in the direction of that motion and is greatly elongated in the opposite direction. Observations in the outer heliosphere suggest that the termination shock is constantly in motion relative to the Sun owing to an ever-changing solar wind momentum flux; it may never truly achieve an equilibrium position. The size and shape of the heliosphere depend on the momentum flux carried by the solar wind, the pressure of the interstellar plasma, and the motion of the heliosphere relative to the interstellar medium. Voyager 1 recently verified the existence of the termination shock, having crossed it in December 2004 at a heliocentric distance of 94 AU roughly in the direction of the heliosphere’s motion relative to the interstellar medium. It is currently believed that the heliopause lies at a heliocentric distance of 115–150 AU and should be encountered by Voyager within the next decade.

10. Kinetic Properties of the Plasma 10.1 The Solar Wind as a Marginally Collisional Plasma On a large scale, the solar wind behaves like a compressible fluid and is capable of supporting relatively thin structures such as shocks. It is perhaps not obvious why the solar wind should exhibit this fluid-like behavior since the wind is a dilute plasma in which collisions are relatively rare. For example, using values given in Table 1, we find that the time between collisions for a typical solar wind proton at 1 AU is several days. (These collisions do not result from direct particle impacts such as colliding billiard balls, but rather from the long distance Coulomb interactions characteristic of charged particles.) The time between collisions is thus comparable to the time for the solar wind to expand from the vicinity of the Sun to 1 AU; this is the basis for statements that the solar wind is a marginally collisional plasma. There are several reasons why the solar wind behaves like a fluid even in the absence of particle collisions to effect fluid-like behavior. First, when the temperature is low and the density is high, collisions are more frequent than noted previously. Second, the presence of the heliospheric magnetic field causes charged particles to gyrate about the field, and they thus do not travel in straight lines between collisions. For typical conditions at 1 AU, solar wind electrons and protons have gyro radii of ∼1.4 and ∼60 km, respectively, which are small compared to the scale size of most structures in the solar wind. Third, the solar wind is subject to a variety of instabilities that are triggered whenever particle distribution functions depart significantly from thermal distributions (see Section 10.2). These instabilities produce collective interactions that mimic the effects of particle collisions. Finally, because the magnetic field is frozen

into the solar wind flow, parcels of plasma originating from different positions on the Sun cannot interpenetrate one another.

10.2 Kinetic Aspects of Solar Wind Ions Collisional gases can usually be described by a single isotropic (i.e., the same in all directions) temperature (T ) with the distribution of particle speeds (v) obeying f (v) ∼exp[−m(v − V0 )2 2kT], where f is the number of particles per unit volume of velocity space, k is Boltzman’s constant (1.38 × 10−16 erg/deg), m is the particle mass, and V0 is the bulk speed of the gas. In contrast, proton distribution functions in the solar wind are usually anisotropic because of the paucity of collisions and because the magnetic field provides a preferred direction in space. At 1 AU, the proton temperature parallel to the field is generally greater than the temperature perpendicular to the field, on average by a factor of ∼1.4. Moreover, solar wind proton and alpha particle distributions often exhibit significant nonMaxwellian features such as the double-peaked distributions illustrated in Fig. 13a. The secondary proton and alpha particle peaks are associated with beams streaming relative to the main solar wind component along the heliospheric magnetic field. The relative streaming speed of such beams is usually comparable to or less than the local Alfven ´ speed, suggesting that the streaming is limited by a kinetic instability. Closer to the Sun, where the Alfven ´ speed is higher, relative streaming speeds between the beams and the main components can be as large as several hundred km/s. Secondary proton beams are common in the solar wind in both low- and high-speed flows and may play a fundamental role in the overall acceleration and heating of the wind; however, their origin in solar and/or heliospheric processes is presently uncertain. Figure 13b illustrates that solar wind ion distributions in the low-speed wind also commonly have extended nonthermal tails of uncertain origin. Particles in these extended tails are easily accelerated to much higher energy when they encounter shocks (see Section 7).

10.3 Kinetic Aspects of Solar Wind Electrons Electron distributions in the solar wind consist of a relatively cold and dense thermal “core” population that is electrically bound to the solar wind ion population and a much hotter and freer-running suprathermal population that becomes collisionless close to the Sun. At 1 AU, the breakpoint between these populations typically occurs at ∼70 eV (Fig. 14a). This breakpoint moves steadily to lower energies with increasing heliocentric distance as the core population cools. Typically the core contains about 95% of the electrons, and at 1 AU has a temperature of ∼1.3 × 105 . The core electrons typically are mildly anisotropic, with the temperature parallel to the field exceeding the temperature

CLBE001-ESS2E

November 11, 2006

0:14

The Solar Wind

113

108 106 104 102

(b)

He++ He+

100 10- 2 98.055-120 Quiet periods only

10

(a)

H+

ACE Quiet 98 55-120

F(W) Phase Space Density (s3 /k m6 )

SWICS ACE

-4

1

1

0

W Ion Speed/Solar Wind Speed

FIGURE 13 (a) A cut through a solar wind ion count spectrum parallel to the magnetic field. The first two peaks are protons, and the second two peaks are alpha particles. (The velocity scale for the alpha particles has been increased by a factor of 1.4.) Both the proton and alpha particle spectra show clear evidence for a secondary beam of particles streaming along the field relative to the main solar wind beam at about the Alfven ´ speed. Such secondary beams, not always well resolved, are common in both the low and the high-speed wind. (From J. R. Asbridge et al., 1974, Solar Phys. 37, 451.) (b) Solar wind speed distributions of H+ , He2+ , and He+ observed in the low-speed solar wind at 1 AU, averaged over a 65-day period in 1998 and excluding intervals of shocks and other disturbances. Such extended suprathermal tails appear to be ubiquitous in the low-speed solar wind. The He+ ions are primarily of interstellar origin. (From G. Gloeckler et al., Acceleration and transport of energetic particles observed in the heliosphere, in Mewaldt et al., 2000.)

FIGURE 14 (a) One-dimensional cut through a solar wind electron distribution showing the thermal and suprathermal populations. (b) Suprathermal electron pitch angle distribution (relative to the magnetic field) showing the field-aligned strahl and the nearly isotropic halo components.

(a)

(b)

CLBE001-ESS2E

November 11, 2006

0:14

114 Encyclopedia of the Solar System perpendicular to the field by a factor of ∼1.1 on average at 1 AU. However, the temperature anisotropy for core electrons varies systematically with density such that at very low densities (10 cm−3 ) the temperature ratio is often slightly less than 1.0. Such systematic variations of core electron temperature anisotropy with plasma density reflect the marginally collisional nature of the thermal electrons and their nearly adiabatic expansion in the spiral magnetic field. The suprathermal electrons consist of a beam of variable width and intensity, known as the strahl, directed outward from the Sun along the heliospheric magnetic field and a more tenuous and roughly isotropic “halo” (Fig. 14b). The angular width of the strahl results from a competition between focusing associated with conservation of magnetic moment in the diverging heliospheric magnetic field and defocusing associated with particle scattering. The strahl carries the solar wind electron heat flux; variations in strahl intensity largely reflect spatial variations in the corona from which it arises. In addition, brief (hours) strahl intensifications often occur during solar electron bursts associated with solar activity (see Section 7). The strahl serves as an effective tracer of magnetic field topology in the interplanetary medium since its usual unidirectional nature arises because field lines in the normal solar wind are “open” (see Section 7.5) and are thus effectively connected to the solar corona at only one end. In contrast, field lines threading ICMEs are often attached to the Sun at both ends (see Section 7.4 and 7.5), and counterstreaming strahls are commonly observed there. Indeed, counterstreaming strahls are one of the more reliable signatures of ICMEs (see Figs. 7b and 9 and Table 2). Finally, the nearly isotropic electron halo results primarily from the scattering out of the strahl at distances beyond 1 AU and the subsequent reflection of those backscattered electrons inside 1 AU by the stronger magnetic fields that reside there.

11. Heavy Ion Content Although the solar wind consists primarily of protons (hydrogen), electrons, and alpha particles (doubly ionized helium), it also contains traces of ions of a number of heavier elements. Table 3 provides estimates of the relative abundances of some of the more common solar wind elements summed over all ionization states. After hydrogen and helium, the most abundant elements are carbon and oxygen. The ionization states of all solar wind ions are “frozen in” close to the Sun because the characteristic times for ionization and recombination are long compared to the solar wind expansion time. Commonly observed ionization states include He2+ , C5+ , C6+ , O6+ to O8+ , Si7+ to Si10+ , and Fe8+ to Fe14+ . Ionization state temperatures in the low-speed wind are typically in the range 1.4 to 1.6 × 106 K, whereas ionization state temperatures in the high-speed wind are

TABLE 3 Element

Average Elemental Abundances in the Solar Wind Abundance Relative to Oxygen

H He C N O Ne Mg Si Ar Fe

1900 ± 400 75 ± 20 0.67 ± 0.10 0.15 ± 0.06 1.00 0.17 ± 0.02 0.15 ± 0.02 0.19 ± 0.04 0.0040 ± 0.0010 019 + 0.10, − 0.07

typically in the range 1.0 to 1.2 × 106 K. Unusual ionization states such as Fe+16 and He+1 , which are not common in the normal solar wind, are often abundant within ICMEs, reflecting the unusual coronal origins of those events. The relative abundance values in Table 3 are longterm averages; however, abundances vary considerably with time. Such variations have been extensively studied for the He2+ /H+ ratio, A(He), but are less well established for heavier elements. The most probable A(He) value is ∼0.045, but the A(He) ranges from less than 0.01 to values of 0.35 on occasion. The average A(He) is about half that commonly attributed to the solar interior, for reasons presently unknown. Much of the variation in A(He) and in the abundance of heavier elements is related to the largescale structure of the wind. For example, Fe/O and Mg/O ratios are systematically lower in high-speed streams than in low-speed flows. A(He) tends to be relatively constant at ∼0.045 within quasi-stationary, high-speed streams but tends to be highly variable within low-speed flows. Particularly low (75 km depending on location) at the end of late heavy bombardment. The 1300-km-diameter Caloris impact basin is the largest well-preserved impact structure (Fig. 7), although the much more degraded Borealis Basin is larger (1530 km). The floor structure of the Caloris Basin is like no other basin floor structure in the solar system. It consists of closely spaced ridges and troughs arranged in both a concentric and radial pattern (Fig. 8a and 8b). The ridges are probably due to contraction, while the troughs are probably extensional grabens that postdate the ridges. The fractures get progressively deeper and wider toward the center of the basin. Near the edge of the basin there are very few fractures. This pattern may have been caused by subsidence and subsequent uplift of the basin floor. 6.1.2 HILLY AND LINEATED TERRAIN

Directly opposite the Caloris Basin on the other side of Mercury (the antipodal point of Caloris) is the unusual hilly and lineated terrain that disrupts preexisting landforms, particularly crater rims (Fig. 9a and 9b). The hills are 5–10 km wide and about 0.1–1.8 km high. Linear depressions that are probably extensional fault troughs form a roughly orthogonal pattern. Geologic relationships suggest that the age of this terrain is the same as that of the Caloris Basin. Similar, but smaller, terrains occur at the antipodes of the Imbrium and Orientale impact basins on the Moon. The hilly and lineated terrain is thought to be the result of shock waves generated by the Caloris impact and focused at the antipodal region (Fig. 10). Computer simulations of shock wave propagation indicate that focused shock waves from an impact of this size can cause vertical ground motions of about 1 km or more and tensile failure to depths of tens of kilometers below the antipode. Although the lunar Imbrium Basin (1400 km diameter) is larger than the Caloris Basin, the disrupted terrain at its antipode is much smaller than that at the Caloris antipode. The larger disrupted terrain on Mercury may be the result of enhanced shock wave focusing due to the large iron core. 6.1.3 INTERCRATER PLAINS

Mercury’s two plains units have been interpreted as either impact basin ejecta or as lava plains. The older intercrater plains are the most extensive terrain on Mercury (Figs. 11

FIGURE 7 Photomosaic of the 1300-km-diameter Caloris impact basin showing the highly ridged and fractured nature of its floor. (Courtesy of NASA.)

and 12). They both partially fill and are superimposed by craters in the heavily cratered uplands. Furthermore, they have probably been responsible for obliterating a significant number of craters as evidenced by the paucity of craters less than about 40 km diameter compared to the highlands of the Moon. Therefore, intercrater plains were emplaced over a range of ages contemporaneous with the period of late heavy bombardment. There are no definitive features diagnostic of their origin. Because intercrater plains were emplaced during the period of late heavy bombardment, they are probably extensively fragmented and do not retain any signature of their original surface morphology. Although no landforms diagnostic of volcanic activity have been discovered, there are also no obvious source basins to provide ballistically emplaced ejecta. The global distribution of intercrater plains and the lack of source basins for ejecta deposits are indirect evidence for a volcanic origin. Additional evidence for a volcanic origin is recent Mariner 10 enhanced color images showing color boundaries that coincide with geologic unit boundaries of some intercrater plains (Fig. 13). If intercrater plains are volcanic, then they are

CLBE001-ESS2E

November 2, 2006

21:11

FIGURE 8 Map of the (a) fractures and (b) ridges on the floor of the Caloris Basin. The basin interior is shown in brown, and the dash-dot line to the northeast of the main ring is a faint outer ring. The floor fractures and ridges have both radial and concentric components. The red spots are post-Caloris craters, and the blue ones are pre-Caloris craters partly covered with basin ejecta. The lines radial to the basin are lineations due to the basin ejecta, and the thick black lines are lobate scarps. The small green spots at the eastern edge of the basin are rimless volcanic collapse pits. (From Strom et al., 1975, J. Geophys. Res. 80, 2478–2507.)

(a)

(b)

FIGURE 9 (a) A portion of the hilly and lineated terrain antipodal to the Caloris impact basin. The image is 543 km across. (b) Detail of the hilly and lineated terrain. The largest crater in (b) is 31 km in diameter. (Courtesy of NASA.)

126

CLBE001-ESS2E

November 2, 2006

21:11

Mercury

127

FIGURE 10 Diagrammatic representation of the formation of the hilly and lineated terrain by focused seismic waves from the Caloris impact. (From P. Schultz and D. Gault, 1975, “The Moon,” 12, pp. 159–177.)

FIGURE 11 View of the intercrater plains surrounding clusters of craters in the Mercurian highlands. Several lobate scarps (thrust faults) can also be seen. (Courtesy of NASA.)

CLBE001-ESS2E

November 2, 2006

21:11

128 Encyclopedia of the Solar System FIGURE 12 High-resolution view of the intercrater plains. The chains and clusters of small craters are secondaries from younger craters. The 90-km-diameter crater in the upper right-hand corner has been embayed by intercrater plains. The lobate scarp that diagonally crosses the image is a thrust fault. (Courtesy of NASA.)

probably lava flows erupted from fissures early in Mercurian history. Intercrater plains are probably ≥3.9 billions years old. 6.1.4 SMOOTH PLAINS

The younger smooth plains cover almost 40% of the total area imaged by Mariner 10. About 90% of the regional exposures of smooth plains are associated with large impact basins. They also fill smaller basins and large craters. The largest occurrence of smooth plains fill and surround the Caloris Basin (Fig. 7), and occupy a large circular area in the north polar region that is probably an old impact basin (Borealis Basin). They are similar in morphology and mode of occurrence to the lunar maria. Craters within the Borealis, Goethe, Tolstoy, and other basins have been flooded by smooth plains (Fig. 14). This indicates the plains are younger than the basins they occupy. This is supported by the fact that the density of craters superimposed on the smooth plains that surround the Caloris Basin is substantially less than the density of craters superimposed on the floors of all major basins including Caloris. Furthermore, several irregular rimless depressions that are probably of volcanic origin occur in smooth plains on the floors of the Caloris and the Tolstoy basins. The smooth plains’ youth relative to the basins they occupy, their great areal extent, and other stratigraphic relationships suggest they are vol-

canic deposits erupted relatively late in Mercurian history. Mariner 10 enhanced color images show the boundary of smooth plains within the Tolstoy Basin is also a color boundary, further strengthening the volcanic interpretation for the smooth plains. Based on the shape and density of the size/frequency distribution of superimposed craters, the smooth plains probably formed near the end of late heavy bombardment. They may have an average age of about 3.8 billion years as indicated by crater densities. If so, they are, in general, older than the lava deposits that constitute the lunar maria. Three large radar-bright anomalies have been identified on the unimaged side of Mercury. They are designated as A (347◦ W longitude, −34◦ latitude), B (343◦ W longitude, 58◦ longitude), and C (246◦ W longitude, 11◦ N latitude). All features are relatively fresh impact craters with radar-bright ejecta blankets and/or rays similar to Kuiper crater (60 km diameter) on the imaged portion of Mercury. Feature A is 85 km in diameter with an extensive ray system and a rough radar-bright floor, consistent with a fresh impact crater (Figure 15). Feature B is 95 km diameter with radar-bright rays and a radar-dark floor (Figure 15). Unlike feature A, the radar-dark floor indicates it is smooth at the 12.6 cm wavelength of the image. Feature C is a fresh crater about 125 kilometers in diameter. Water-rich comets or asteroids responsible for one or more

CLBE001-ESS2E

November 2, 2006

21:11

Mercury

129

FIGURE 14 Photomosaic of the Borealis Basin showing numerous craters (arrows) that have been flooded by smooth plains. The largest crater is the Goethe Basin 340 km in diameter. (Courtesy NASA.)

FIGURE 13 Enhanced color mosaic of a portion of the incoming side of Mercury as viewed by Mariner 10. The area at F has a sharp boundary that coincides with an intercrater plains boundary and may have a different composition. The relatively dark and blue unit at D is consistent with enhanced titanium content. The bright orange unit at B may represent primitive crustal material, and Kuiper crater at K shows a yellowish color representing fresh material excavated from a subsurface unit that may have an unusual composition. (Courtesy of Mark Robinson, Northwestern Univ., Evanston, Illinois.)

of these craters could be the source of the polar water-ice deposits.

6.2 Surface Composition Little is known about the surface composition of Mercury. If the plains units (intercrater and smooth) are lava flows, then they must have been very fluid with viscosities similar to fluid flood basalts on the Moon, Mars, Venus, and Earth.

The way in which light is reflected from the surface is very similar to that of the Moon. However, at comparable phase angles and wavelengths in the visible part of the spectrum, Mercury appears to have systematically higher albedos than the Moon. Mercurian albedos range from 0.09 to 0.36 at 5◦ phase angle. The higher albedos are usually associated with rayed craters. However, the highest albedo (0.36) on Mariner 10 images is not associated with a bright-rayed crater: It is a floor deposit in Tyagaraja Crater at 3◦ N latitude and 149◦ longitude. The lunar highlands/mare albedo ratio is almost a factor of 2 on the Moon, but it is only a factor of 1.4 on Mercury. Furthermore, at ultraviolet wavelengths (58–166 nm) Mercury’s albedo is about 65% lower than the Moon’s at comparable wavelengths. These differences in albedo suggest that there are systematic differences in the surface composition between the two bodies. A recalibration and color ratioing of Mariner 10 images have been used to derive the FeO abundance, the opaque mineral content, and the soil maturity over the region viewed by Mariner 10. The probably volcanic smooth plains have a FeO content of 10 GPa) to whole-rock melting (50– 100 GPa). Above about 150 GPa, the rocks are vaporized. Vapor masses of a few times projectile mass and melt masses about 100 times the projectile mass may be formed. Impact melts compose 30–50% of all samples returned from the lunar highlands.

6.2 Lunar Cratering History and the Lunar Cataclysm

FIGURE 11 The transition between central-peak craters and peak-ring craters. The large central basin is Schrodinger ¨ (320 km in diameter), which has a well-developed peak ring. Antoniadi (135 km in diameter), southeast from Schrodinger, ¨ has both a central peak and a peak ring. The small crater immediately southwest of Antoniadi has a central peak only. (Courtesy of NASA, Orbiter IV-8M.)

The intense cratering of the lunar highlands and the absence of a similar heavily cratered surface on the Earth were long recognized as due to an early “pregeological” bombardment. In contrast, the lightly cratered basaltic mare surfaces, on which the cratering rate is about 200 times less, had escaped this catastrophe and were clearly much younger. The ages of the mare surfaces, dated from the sample return to be between 3.3 and 3.8 billion years old, showed that the cratering flux was similar, within a factor of 2, to that observed terrestrially. It also established that the intense cratering of the highlands occurred more than 3.8 billion years ago. Most highland samples have ages in the range 3.8–4.3 billion years. The radiometric ages of the ejecta blankets from the large collisions tend to cluster around 3.9 billion years, with the dates for the Imbrium collision being 3.85 billion years and that for Nectaris, 3.90 or 3.92 billion years. This is a surprisingly narrow range and indicates a rapid increase in the cratering flux just before 3.8 billion years. This clustering has led to the concept of a “lunar cataclysm” or a spike in the collisional history at that time.

CLBE001-ESS2E

November 10, 2006

17:37

238 Encyclopedia of the Solar System

FIGURE 12 A mosaic of 1500 Clementine UV VIS images centered on the South Pole, showing the heavily cratered south polar region of the Moon. The Schrodinger ¨ Basin (320 km in diameter), which is the freshest peak basin on the Moon, is at four o’clock. Schrodinger ¨ is slightly older than Orientale. Note the small volcanic cone in the bottom left-hand sector. It is possible that some ice (from cometary impacts?) is trapped in the permanently shadowed craters at the South Pole. (LP1 Clementine press release.)

FIGURE 13 Orientale is a classic example of a multiring basin. The diameter of the outer mountain ring (Montes Cordillera) is 930 km, about the size of France. Note the radial structures resulting from the impact. It is the youngest major impact basin on the Moon. This structure was formed about 3800 million years ago in a few minutes following the impact of a planetsimal or asteroid about 50–100 km in diameter. Basalt has flooded the center of the Orientale Basin. The small circular patch of mare basalt northeast of Orientale is Grimaldi. The western edge of Oceanus Procellarum fills the northeastern horizon. (Courtesy NASA, Orbiter IV-187M.)

CLBE001-ESS2E

November 10, 2006

17:37

The Moon

239

FIGURE 14 The distribution of major impact basins on (a) the nearside and (b) the farside of the Moon. (Courtesy of D. E. Williams.)

The noncataclysmic explanation is that the Imbrium and Orientale Basins formed during the tail end of the accretion of the planets and so represent the final sweep-up of large objects. The problem with this scenario is that extrapolation from the rate at 3.8 billion years back to 4.5 billion years results in the accretion of a Moon several orders of magnitude larger than observed. It seems probable that accretion of the Moon was essentially complete and that the Moon was at its present size by about 4450 million years ago, at the time of the crystallization of the feldspathic highland crust. Other arguments in favor of the cataclysm include the scarcity of impact melts older than 4 billion years and the lead isotope data, which indicate a major resetting of the lead ages at 3.86 billion years. Although it is often argued that the sampling from the Apollo missions is dominated by Imbrium ejecta, lunar meteorites have provided fresh insights. These provide a random sampling of the surface but display no impact melts older than 3.92 billion years, supporting the notion of a “cataclysm” although the storage for several hundred million years and supply of the massive impactors poses some interesting problems. Figure 15 shows a reconstruction of the lunar crater production rate with time.

7. The Maria The maria make up the prominent dark areas that form the features of “the man in the Moon” (Figs. 1 and 16).

FIGURE 15 The production rate over geological time for lunar craters greater than 4 km in diameter. This illustrates the very high cratering prior to 3.8 billion years ago, but whether this represents the tail end of accretion or spike (cataclysm), as preferred here, in the bombardment history is unclear. The terrestrial rate is for the past 200 million years. (Updated from the Basaltic Volcanism Study Project, 1981.)

CLBE001-ESS2E

November 10, 2006

17:37

240 Encyclopedia of the Solar System FIGURE 16 The distinction between the maria and highlands is clearly shown in this view of the lunar farside. The large circular crater, filled with dark mare basalt, is Thomson (112 km in diameter), within the partly visible Mare Ingenii (370 km in diameter, 34◦ S, 164◦ E). The large crater in the highland terrain in the right foreground is Zelinskiy (54 km in diameter). The stratigraphic sequence, from oldest to youngest, is (a) formation of the white highland crust, (b) excavation of the Ingenii Basin, (c) excavation of the Thomson Crater within the Ingenii Basin, (d) excavation of Zelinskiy Crater in the highland crust, (e) flooding of Ingenii Basin and Thomson Crater with mare basalt, and (f) excavation of small craters, including a probable chain of secondary craters, on the mare basalt surface. (Courtesy of NASA, AS15-87-11724.)

After centuries of speculation during which the maria were thought to be composed of sediments, dried lake beds, asphalt, or other unlikely materials, they were conclusively identified following the Apollo 11 sample return in 1969 as being formed of basaltic lavas. This conclusion had already been reached by earlier workers such as R. B. Baldwin and G. Kuiper and was strongly suggested by the data from the Surveyor landers. These vast plains cover 17% (6.4 × 106 km2 ) of the surface of the Moon, and they are exceedingly smooth, with slopes of 1:500 to 1:200 and elevation differences of only 150 m over distances of 500 km. This smoothness and the lack of volcanic constructional forms, which litter many terrestrial volcanic fields, remind one of plateau basalts on Earth and are probably due to several factors. These include a combination of high eruption rates and the low viscosity of the lunar lavas, which is about an order of magnitude lower than that of their terrestrial counterparts and is close to that of engine oil at room temperature. The lava flows (Fig. 17) are thin (10–40 m) and up to 1200 km long, a consequence of the low viscosity and probable long duration of the eruption. Flow fronts are generally less than about 15 m in height. Occasional small volcanic domes and cones occur on the mare surface. The classic example is the region of the Marius Hills. The maria are not all at the same level, and this is indicative of independent eruptions from diverse sources at differing depths in the interior. They are mostly subcircular in form owing to their filling of the multiring basins, originally excavated by impact. The dark basaltic lavas that fill

FIGURE 17 Mare basalt flows in southwestern Mare Imbrium. Flow thicknesses are in the range 10–30 m. The source of the flow is about 200 km southwest of crater La Hire (5 km in diameter), seen at right center. This crater is superimposed on Mount La Hire (about 30 m long at its base), a highland remnant that is partially submerged by lavas. Note the prominent concentric wrinkle ridges on Mare Imbrium. (Courtesy of NASA, AS15-1556.)

CLBE001-ESS2E

November 10, 2006

17:37

The Moon

the basins form the maria, as in Mare Imbrium (see Figs. 1 and 14) or Mare Ingenii (see Fig. 16). The basins, as in the Imbrium Basin, were formed much earlier by impact and have nothing to do with the generation of the mare basalts. Thus, the mare basalt, which fills many basins, is unrelated to the formation of the basins, a common misconception; instead, it is derived from the deep lunar interior and merely floods into the low-lying depressions much later. Some impact melt, distinct in composition from the lavas, may be formed at the time of the impact, but it should not be confused with the basaltic mare lavas, which differ both in composition and age. Oceanus Procellarum (see Fig. 1) is the type example of an irregular mare, where the lavas have flooded widely over the highland crust. However, this mare may be filling parts of an old, large, and very degraded Procellarum Basin (3200 km in diameter), although the existence of this basin is questionable. The mare lavas reach the surface because of the density difference between the melt and that of the overlying column of rock. The scarcity of maria on the farside of the Moon (see Fig. 3) is due to the greater crustal thickness. An exception is part of the area of the deep depression of the South Pole–Aitken multiring basin (2500 km in diameter), on which is superimposed the Ingenii impact basin (650 km in diameter), now occupied in part by the lavas of Mare Ingenii (see Fig. 16). However, most of the South Pole–Aitken Basin, which is deeper than the nearside maria, is not flooded with lava. This argues for mantle heterogeneity and localized sources for the mare basalts, rather than some moonwide melting of the interior, with consequent flooding of lava to a uniform level. Dark mantle deposits, which represent pyroclastic deposits formed probably by “fire fountaining” during lunar eruptions, occur, for example, around the southern borders of Mare Serenitatis. These pyroclastic deposits are composed mainly of glass droplets and fragments and can be distinguished from the ubiquitous glasses of impact origin by their uniformity, homogeneous composition, and absence of meteoritic contamination. Over 25 distinct compositions have been recognized. They commonly have a superficial coating of volatile elements such as Pb, Zn, Cl, and F, derived from volcanic vapors during the eruption. The dominant gas, however, was probably CO. The source of the volatile elements is uncertain. They are rare in the lunar samples, and the Moon is generally thought to be strongly depleted in them. Possibly they come from local cumulate sources, and so do not imply an enrichment of the deep lunar interior in volatile elements. However, they may have originated at a greater depth than the crystalline mare basalts. Some areas of mare basalts (so-called cryptomaria) are covered by ejecta blankets of highland material from multiring basins; their presence is revealed by the haloes of dark basalt ejected from impact craters that have punched through the light-colored highland plains

241

units of anorthositic composition into the underlying basalts. Although they are prominent visually on the Moon, the maria typically form a thin veneer, mostly less than 1–2 km thick, except in the centers of the circular maria where they may reach maximum of 5 km as in the middle of Mare Imbrium. The basalt thickness in Orientale is estimated to be only 0.6 km. The total volume of mare basalt is usually estimated at between 6 and 7 × 106 km3 or about 0.1% of lunar volume. Cooling rates for mare basalts range from 0.1◦ C to 30◦ C per hour, indicative of fast cooling in thin lava flows. Sinuous rilles occur widely near the edges of the maria and are either lava channels or collapsed lava tubes. They have eroded into the surrounding lavas by a combination of thermal and mechanical erosion. The classic example is Hadley Rille (Fig. 18), visited by the Apollo 15 mission. The rille is 135 km long and averages 1.2 km in width and 370 m in depth. Massive lava bedrock is exposed in the rille wall at the Apollo 15 site. The sinuous rilles should not be confused with the straight or arcuate rilles, which are grabens of tectonic origin.

7.1 Mare Basalt Ages The oldest ages for returned lunar mare basalts are from Apollo 14 breccias; aluminous low-Ti basaltic clasts in these

FIGURE 18 Hadley Rille, a typically sinuous rille, about 1 km wide, at the Apollo 15 landing site, close to the base of the Apennine Mountains. (Courtesy of NASA, Lunar Orbiter IV-102H3.)

CLBE001-ESS2E

November 10, 2006

17:37

242 Encyclopedia of the Solar System breccias range in age from 3.9 to 4.3 billion years. The oldest basalt from a visible maria is Apollo sample number 10003, a low-K basalt from Mare Tranquilitatis with an age of 3.86 ± 0.03 billion years. This gives a younger limit for the age of the Imbrium collision because the lavas of Mare Tranquilitatis overlie the Imbrium ejecta blanket. The youngest dated sample is number 12022, an ilmenite basalt with an age of 3.08 ± 0.05 billion years, although some doubtful younger ages are in the literature. Low-Ti basalts are generally younger than high-Ti basalts. Stratigraphically younger flows, some of which appear to embay young ray craters, may be as young as 1 billion years but are of very limited extent. The most voluminous period of eruption of lavas appears to have been between about 3.8 and 3.1 billion years ago. Isotopic measurements show that the mare basalt source regions formed at about 4.4 billion years, and this age must represent the solidification of much of the magma ocean.

7.2 Composition of the Mare Basalts The basic classification is chemical, with finer subdivisions based on mineral composition. The basalts are divided into low-Ti, high-Ti, and high-Al basalts. The low-Ti basalts include VLT (very-low-Ti), olivine, pigeonite, and ilmenite basalts. The high-Ti basalts include high-K, low-K, and VHT (very-high-Ti) basalts. The Clementine data suggest that there is a continuous variation in Ti contents. The major minerals are pyroxene, olivine (Mg-rich), plagioclase (Ca-rich), and opaques, mainly ilmenite. The basalts are highly reduced, with oxygen fugacities of 10−14 at 1100◦ C or about a factor of 106 lower than those of terrestrial basalts at any given temperature. Ferric iron is effectively absent, and 90% of Cr and 70% of Eu in the Moon is divalent. An alloy of FeNi metal is a common late-stage crystallizing phase. In comparison with terrestrial basalts, the silica contents of mare basalts are low (37–45%), and the lavas are ironrich (18–22% FeO). The lunar basalts are notably high in Ti, Cr, and Fe/Mg ratios and low in Ni, Al, Ca, Na, and K compared with terrestrial counterparts (Table 2). They are depleted in volatile (e.g., K, Rb, Pb, Bi) and siderophile (e.g., Ni, Co, Ir, Au) elements. The ratio of volatile (e.g., K) to refractory elements (e.g., U) is low. Thus, lunar K/U ratios average about 2500 compared to terrestrial values of about 12,000. The rare earth elements (REEs) display a characteristic depletion in divalent Eu or europium anomaly (Fig. 19). This is one of the several pieces of evidence that the mare basalts come from a previously differentiated interior, rather than being melted from a primitive undifferentiated lunar composition. Even the lunar glasses that may come from deeper show the tell-tale evidence of depletion in Eu, indicating that they too come from a differentiated interior.

The differences in composition of the mare basalts are mostly due to source region heterogeneity, with only minor evidence for near-surface fractionation. Variations in the amount of partial melting from a uniform source, subsequent fractional crystallization, or assimilation cannot account for the observed diversity. Some mare basalts are vesicular, evidence for a now-vanished gas phase, usually thought to be CO.

7.3 Origin of the Mare Basalts Mare basalts originate by partial melting, at temperatures of about 1200◦ C, deep in the lunar interior (see Fig. 8), probably at depths between 200 and 400 km. The lunar volcanic glasses appear to come from greater depths, but still from a differentiated source. The basalts are derived from the zones and piles of cumulate minerals developed, at various depths, during crystallization of the magma ocean. The isotopic systematics of the mare basalts indicate that the source region had crystallized by 4.4 billion years. Partial melting occurred in these diverse mineral zones some hundreds of millions of years later due to the slow buildup of heat from the presence of the radioactive elements K, U, and Th. The melting was not extensive. Over 25 distinct types of mare basalt were erupted over an interval of more than 1 billion years, but the total amount of melt so generated amounted to only about 0.1% of the volume of the Moon. This forms a stark contrast to the state of the Moon at accretion, when it may have been entirely molten.

8. Lunar Highland Crust Most of the rocks returned from the highlands are polymict breccias, pulverized by the massive bombardment. However, some monomict breccias have low siderophile element contents. These are considered to be “pristine” rocks that represent the original igneous components making up the highland crust. Three pristine constituents make up the lunar highland crust, namely, ferroan anorthosites that are the dominant component, the Mg suite, and KREEP.

8.1 Ferroan Anorthosite Ferroan anorthosite is the single most common pristine highland rock type, making up probably 80% of the highland crust. The pristine clasts in lunar meteorites are mostly ferroan anorthosites. The major component (95%) is highly calcium-rich plagioclase, typically An95–97 with a pronounced enrichment in Eu (Eu/Eu∗ = 50). LowMg pyroxene is the next most abundant mineral, but the mafic minerals are only minor constitutents in this nearly monomineralic feldspathic rock. The anorthosites are typically coarsely crystalline with cumulate textures. Reliable

CLBE001-ESS2E

November 10, 2006

17:37

The Moon

TABLE 2

243

Elemental Abundancesa

Oxide (weight percent)

SiO2 TiO2 Al2 O3 FeO MgO CaO Na2 O K 2 O Volatile elements K (ppm) Rb (ppm) Cs (ppb)

Cl

Earth Mantle + Crust

Bulk Moon

34.2 0.11 2.44 35.8 23.7 1.89 0.98 0.10 99.2

49.9 0.16 3.64 8.0 35.1 2.89 0.34 0.02 100.1

47 0.3 6.0 13.0 29 4.5 0.09 0.01 99.9

854 3.45 279

180 0.55 18

83 0.28 12

Highlands

Low-Ti Basalt

High-Ti Basalt

45.0 0.56 24.6 6.6 6.8 15.8 0.45 0.03 100

43.6 2.60 7.87 21.7 14.9 8.26 0.23 0.05 100.4

37.8 13.0 8.85 19.7 8.44 10.7 0.36 0.05 99.5

200 0.7 20

420 1.0 40

500 1.2 30

Moderately volatile element Mn (ppm) 2940

1000

1200

570

2150

2080

Moderately refractory element Cr (ppm) 3975

3000

4200

800

5260

3030

Refractory elements Sr (ppm) 11.9 U (ppb) 12.2 La (ppm) 0.367 Eu (ppm) 0.087 V (ppm) 85 Siderophile elements Ni (ppm) 16500 Ir (ppb) 710 Mo (ppb) 1380 Ge (ppm) 48.3

17.8 18 0.55 0.13 128 2000 3.2 59 1.2

30 30 0.90 0.21 150 400 0.01 1.4 0.0035

130 80 2.0 1.0 30 100 — 5 0.02

101 220 6.0 0.84 175 64 0.02 50 0.003

121 130 5.22 1.37 50 2 0.04 50 0.003

a Elemental abundances in Cl chondrites (volatile = primitive solar nebula). Earth mantle + crust = primitive Earth mantle; bulk Moon; average lunar highland crust; low-Ti mare basalt (12002) and high-Ti mare basalt (70215). Both of these later samples are probably primary basaltic magmas. Data sources from Taylor (1982) and Hartmann et al. (1986).

ages of 4440 ± 20 million years and 4460 ± 40 million years have been obtained, and their average of 4450 million years is taken to represent the crystallization age of ferroan anorthosites from the lunar magma ocean and the flotation of the feldspathic highland crust as “rockbergs.” Alternatively, this date may represent the “isotopic closure age” during cooling of the crust.

8.2 Mg Suite The Mg suite comprises norites, troctolites, dunites, spinel troctolites, and gabbroic anorthosites. They are characterized by higher, and so more primitive, Mg/ (Mg + Fe2+ ) ratios compared to the ferroan anorthosites. They range in

age from about 4.44 billion years down to about 4.2 billion years, but typical ages are 100–200 million years younger than those of the ferroan anorthosites. The Mg suite is petrographically distinct from the older ferroan anorthosites and does not appear to be related directly to the crystallization from the magma ocean. It makes up a minor but significant proportion (perhaps 10%) of the highland crust and has two distinct and contradictory components in terms of conventional petrology. It is Mg-rich, and so primitive in terms of igneous differentiation, but also contains high concentrations of incompatible elements, typical or highly evolved or differentiated igneous systems. These characteristics point to an origin by mixing of these two distinct components.

CLBE001-ESS2E

November 10, 2006

17:37

244 Encyclopedia of the Solar System contains Mg-rich orthopyroxene, a mineral that is lacking in mare basalts. Clearly the Mg suite originates in a location distinct from the source region of the mare basalts. During crystallization of the magma ocean, Mg-rich minerals (e.g., olivine and orthopyroxene) are among the first to crystallize and accumulate on the bottom of the magma chamber, in this case at depths exceeding 400 km. It is sometimes suggested that massive overturning has occurred to bring these within reach of the surface. However, the magma ocean had completed crystallization by 4400 million years with only the KREEP component remaining liquid until about 4360 million years; it was solid at the time of the formation of the Mg suite. There is no obvious source of energy for remelting early refractory Mg-rich cumulates. Such material may have been derived from a late infall of planetesimals that might provide both the primitive component and the energy for melting. Subsequent melting to produce mare basalts took place in more differentiated cumulates and produced lavas with a different mineralogy (e.g., lacking orthopyroxene), without the primitive characteristics of the Mg suite.

8.3 Alkali Suite

FIGURE 19 The abundances of rare earth elements in the source regions of the mare basalts, the highland crust, and KREEP, relative to bulk moon concentrations. These patterns result from the preferential entry of divalent europium (similar radius to strontium) into plagioclase feldspar. This mineral floats to form the highland crust, and so depletes the interior of Eu. Mare basalts that subsequently erupted from this region deep within the Moon bear the signature of this early depletion. KREEP is the final residue of the crystallization of the magma ocean. It is strongly depleted in Eu owing to prior crystallization of plagioclase and is enriched in the other rare earth elements (e.g., K, U, Th, Ba, Rb, Cs, Zr, P) that are excluded from olivine, pyroxene, and ilmenite during the crystallization of the major mineral phases of the magma ocean.

The source of the highly evolved component is clearly KREEP. The source of the “primitive” Mg-rich component is less obvious. If the primitive component came from deep cumulates, the concentrations of Ni in olivine of the Mg suite are low, not high as predicted. Conventional theories propose that the Mg suite arose as separate plutons that intruded the crust as separate igneous intrusions. However, all Mg suite rocks have parallel REE patterns, a characteristic compatible with mixing, but not expected to occur in separate igneous intrusions. This is a major constraint on the concept that the lunar highland crust formed through “serial magmatism.” Furthermore, it is of interest that the Mg suite

A rare component of the highlands crust is the Alkali suite. The largest sample is 1.6 gm and they seem to have undergone severe thermal metamorphism but their origin is not well understood. They are commonly 85% plagioclase feldspar, the remainder being mostly pyroxene. Their significant feature is an enrichment in the alkali elements so that they contain Na-rich rather than Ca-rich feldspar. They are probably related to KREEP as the trace element patterns are similar.

8.4 Kreep KREEP is enriched in potassium, rare earth elements, and phosphorus, hence the name. It is commonly applied as an adjective to refer to highland rocks with an enhanced and characteristic trace element signature. KREEP originated as the final 2% or so melt phase from the crystallization of the magma ocean and is strongly enriched in those “incompatible” trace elements excluded from the major mineral phases (olivine, orthopyroxene, clinopyroxene, plagioclase, ilmenite) during crystallization of the bulk of the magma ocean. This residual phase was the last to crystallize, at about 4.36 billion years, and apparently pervaded the crust, with which it was intimately mixed by cratering. Its presence tends to dominate the trace element chemistry of the highland crust. Extreme REE enrichment up to 1000 times the chondritic (or solar nebula abundances) are known (see Fig. 19). This extreme concentration of trace elements amounts to a significant part of the total lunar budget and so provides strong evidence for the magma ocean hypothesis and for large-scale lunar melting.

CLBE001-ESS2E

November 10, 2006

17:37

The Moon

8.5 Kreep Basalt KREEP basalt, an enigmatic rock type with only a few undisputed examples, is highly enriched in incompatible elements (KREEP) but has a more primitive Mg/ (Mg + Fe2+ ) ratio. This combination of primitive and evolved components suggests that they are derived, like the members of the Mg suite, from different sources and may be impact melts. Probably the Apennine Bench formation is composed of KREEP basalt. This formation appears to have formed close in time to the excavation of the Imbrium Basin.

8.6 Breccias A consequence of the massive bombardment that pulverized the lunar highlands is that the rocks returned from the lunar highlands are breccias, usually consisting of rock fragments or clasts set in a fine-grained matrix. Lunar breccias are usually divided into monomict, dimict, and polymict breccias, consisting, respectively, of a single rock type, two distinct components, and a variety of rock types and impact melts. Polymict breccias, usually involving several generations of breccias, are the most common rock type returned from the lunar highlands. They are further subdivided into fragmental breccias, glassy melt breccias, crystalline melt breccias (or impact melt breccias), clast-poor impact melts, granulitic breccias, and regolith breccias.

8.7 The Magma Ocean The geochemical evidence is clear that at least half and possibly the whole Moon was molten at accretion. This stupendous mass of molten rock is referred to as the “magma ocean,” and a very energetic mode of origin of the Moon, such as provided by the giant impact hypothesis, is required to account for it. The crystallization of such a body is difficult to constrain, or even imagine, from our limited terrestrial experience. A possible scenario is that initial crystallization of olivine and orthopyroxene formed deep cumulates. As the Al and Ca content of the magma increased, plagioclase crystallized and floated in the bone-dry melt, forming rockbergs that eventually coalesced to form the lunar highland crust, around 4450 ± 20 million years ago. The first-order variation in thickness from nearside to farside is probably a relic of primordial convection currents in the magma ocean. Excavation by large basin impacts has subsequently imposed additional substantial variations in crustal thickness. Plagioclase was a very early phase to crystallize, as all lavas derived from the interior bear the signature of prior removal of Eu (and Sr) (see Fig. 19). Accordingly, the magma ocean was probably enriched in Ca and Al over typical terrestrial values, a conclusion reinforced by the more recent Galileo, Clementine, and Lunar Prospector data. The

245

implication is that the Moon was enriched in these and other refractory elements compared to our estimates of the terrestrial mantle. Continued crystallization of the magma ocean eventually produced KREEP, which appears to have pervaded and has been intimately mixed into the highland crust on the nearside. The crystallization of the magma ocean was probably asymmetric, as shown by the variations in crustal thickness and the apparent concentration of the residual KREEP melt under the nearside. Crystallization of the main phases was complete by 4400 million years ago, and the final KREEP residue was solid by about 4360 million years ago. The crystallization sequence portrayed here was far from peaceful. During all this time, the outer portions of the Moon were subjected to a continuing bombardment, which broke up and mixed the various components of the highland crust. Perhaps coeval with these events was the intrusion into the crust of the Mg suite. Probably some local overturning of the deeper cumulate pile may have occurred, but such events did not homogenize the interior that later produced a wide variety of mare basalt compositions.

8.8 Lunar Crustal Terranes Geochemical mapping carried out by the Clementine and Lunar Prospector missions has resulted in a significant advance in our understanding of the detailed structure of the lunar highland crust. Based on the FeO and Th abundances measured by the Clementine and Lunar Prospector missions, the crust can be divided into three major terranes: (1) the Feldspathic Highland Terrane (FHT), (2) the Procellarum KREEP Terrane (PKT), and (3) the South Pole– Aitken Terrane (SPAT) (Fig. 20). The Feldspathic Highland Terrane constitutes the feldspathic lunar crust formed by flotation from the magma ocean. The Procellarum KREEP Terrane results from the intrusion (or mixing in) of the residual KREEP liquid from the last stages of crystallization of the magma ocean. The South Pole–Aitken Terrane is the result of the subsequent excavation of the 2500 km diameter South Pole–Aitken Basin, that stripped off most of the upper crust over that region and whose ejecta contributed significantly to the thickness of the farside anorthositic crust, north of the basin. The interior of the South Pole–Aitken Basin, the deepest basin on the Moon, has a more mafic (Fe- and Mg-rich) composition relative to the more feldspathic lunar highlands, but it is not clear that the impact has uncovered the lunar mantle. It would be of great interest to study this area in detail, as no excavated mantle samples have ever been identified in the returned Apollo samples. The South Pole–Aitken Basin, where most of the upper crust is missing, has been preserved for over 4.1 billion years without significant isostatic compensation occurring. As this is the oldest and largest recognized lunar basin, the

CLBE001-ESS2E

November 10, 2006

17:37

246 Encyclopedia of the Solar System FIGURE 20 Major lunar crustal terranes. The Procellarum KREEP Terrane (PKT) is on the nearside, with Th greater than 3.5 ppm. The South Pole–Aitken Terrane (SPAT) has an outer region corresponding to basin ejecta. The Feldspathic Highland Terrane (FHT) corresponds to the thickest part of the crust, concentrated on the lunar farside. FHT,O consists of those regions where basin ejecta or cryptomare obscure the feldspathic surface. (Courtesy of Brad Joliff, Washington University.)

lack of compensation indicates that the crust and interior have been strong enough to support this structure for most of lunar history. There is little sign of the residual KREEP component in this location, despite the depth of excavation. This reinforces the notion that the residual KREEP melt was not uniformly distributed. Figure 21 shows the mantle uplift beneath the South Pole–Aitken Basin as well as that partially superimposed later uplift resulting from the excavation of the Apollo basin.

NE

9. Lunar Composition The Moon is bone-dry and highly reduced, no indigenous H2 O having been detected at ppb levels, and lacks ferric iron. It is strongly depleted to volatile elements (e.g., K, Pb, Bi) by a factor of about 50 compared to the Earth, or 200 relative to primordial solar nebula abundances. Compared to the Earth, the most striking difference is in the abundance of iron that is reflected in the low lunar density. The Earth

CENTER

SE South Pole-Aitken Basin Rim

Apollo Basin Maria

Cryptomare Basin Ejecta Crater Ejecta

Megaregolith

Upper Crust Fall-back Breccia Mantle Uplift

Mantle Uplifted by Later Apollo Event FIGURE 21 The South Pole–Aitken Basin (2500 km diameter and 12 km deep) on which are superimposed two later impact basins.

Lower Crust

CLBE001-ESS2E

November 10, 2006

17:37

The Moon

contains about 25% metallic Fe; the Moon, less than about 2–3%. However, the bulk Moon contains 12–13% FeO, 50% more than in current estimates of 8% FeO in the terrestrial mantle. Along with its depletion in iron, the Moon also has a low abundance of siderophile elements that are depleted in order of their metal-silicate distribution coefficients. This observation indicates that these elements have been segregated into a metallic core. However, this pattern may have been established in precursor planetesimals or in the impactor from which most of the Moon appears to have been derived, rather than, or as well as, into a lunar core. The other major element abundances are mostly modeldependent. Si/Mg ratios are commonly assumed to be chondritic (CI), although the Earth and many meteorite classes differ from this value. The lunar Mg value is generally estimated to be about 0.80, lower than that of the terrestrial mantle value of 0.89. The Moon is probably enriched in refractory elements such as Ti, U, Al, and Ca, a conclusion consistent with geophysical studies of the lunar interior. This conclusion is reinforced by the data from the Galileo, Clementine, and Lunar Prospector missions, which indicate that the highland crust is dominated by anorthositic rocks. This requires that the bulk lunar composition contains about 5–6% Al2 O3 , compared with a value of about 3.6% for the terrestrial mantle and so is probably enriched in refractory elements (e.g., Ca, Al, Ti, U) by a factor of about 1.5 compared to the Earth. In the light of the caveats already given, the bulk composition of the Moon is only known to a first approximation. Data for the bulk composition of the Moon are given in Table 2 compared to CI, the terrestrial mantle abundances and to the bulk Earth. Both the Cr and O isotopic compositions are identical in the Earth and Moon, probably indicating an origin in the same part of the nebula, consistent with the single impact hypothesis that derives most of the Moon from the silicate mantle of the impactor, Theia. Clearly the Moon has a composition that cannot be made by any single-stage process from the material of the primordial solar nebula. The compositional differences from that of the primitive solar nebula, from the Earth, from Phobos and Deimos (almost certainly of carbonaceous chondritic composition), and from the satellites of the outer planets (rock-ice mixtures with the exception of lo) thus call for a distinctive mode of origin.

9.1 Lunar Minerals Only about 100 minerals have been identified in lunar samples, in contrast to the several thousand species that have been identified on Earth. This lunar paucity is due to the dry nature of the Moon and the depletion in volatile and siderophile elements. Extensive summaries of lunar

247

mineralogy can be found in Frondel (1975), Heiken et al. (1991), and Papike et al. (1999).

9.2 Lunar Meteorites Our understanding of the lunar crust has been aided by the discovery of lunar meteorites of which about 20 are known. From their feldspar-rich and KREEP-poor composition, many appear to be from the lunar farside; they are distinct from the nearside highland samples returned by Apollo 14, 15, 16, and 17 and Luna 20. However, their major element composition is close to that of estimates of the average highland crust. They confirm, as do the Galileo, Clementine, and Lunar Prospector missions, the essentially anorthositic nature of the lunar highland crust.

9.3 Tektites The notion that tektites were derived from the Moon enjoyed considerable support before the Apollo missions. However, the controversy that had raged, particularly in the 1960s, over a lunar versus a terrestrial origin was settled in favor of the latter source by the first sample return from the Moon in 1969. It has been decisively established from isotopic and chemical evidence that tektites are derived from the surface of the Earth by meteoritic or asteroidal impact. Because the debate still surfaces occasionally, readers interested in these glassy objects will find a useful review of the evidence for a terrestrial origin in Koeberl (1994).

10. The Origin of the Moon 10.1 The Nature of the Problem Hypotheses for the origin of the Moon must explain the high value for the angular momentum of the Earth–Moon system, the strange lunar orbit inclined at 5.09◦ to the plane of the ecliptic, the high mass relative to that of its primary planet and the low bulk density of the Moon, much less than that of the Earth or the other inner planets. The chemical age and isotopic data revealed by the returned lunar samples added additional complexities to these classic problems because the lunar composition is unusual by either cosmic or terrestrial standards. It is perhaps not surprising that previous theories for the origin of the Moon failed to account for this diverse set of properties and that only recently has something approaching a consensus been reached. Hypotheses for lunar origin can be separated into five categories: 1. 2. 3. 4.

Capture from an independent orbit Formation as a double planet Fission from a rapidly rotating Earth Disintegration of incoming planetesimals

5. Earth impact by a Mars-sized planetesimal and capture of the resulting debris into Earth orbit

CLBE001-ESS2E

November 10, 2006

17:37

248 Encyclopedia of the Solar System These are not all mutually exclusive, and elements of some hypotheses occur in others. For example: 1. Capture of an already formed Moon from an independent orbit has been shown to be highly unlikely on dynamic grounds. The hypothesis provides no explanation for the peculiar composition of our satellite. In addition, it could be expected that the Moon might be an example of a common and primitive early solar system object, similar to the captured rock-ice satellites of the outer planets. This indeed had been the expectation of Harold Urey, based on the similarity of the lunar density to that of primitive carbonaceous chondrites. It would be an extraordinary coincidence if the Earth had captured an object with a unique composition, in contrast to the many examples of icy satellites captured by the giant planets. 2. Formation of the Earth and the Moon in association as a double-planet system immediately encounters the problems of differing density and composition of the two bodies. Various attempts to overcome the density problem led to coaccretion scenarios in which disruption of incoming differentiated planetesimals formed from a ring of low-density silicate debris. Popular models to provide this ring involved the breakup of differentiated planetesimals as they come within a Roche limit (about 3 Earth radii). The denser and tougher metallic cores of the planetesimals survived and accreted to the Earth, while their mantles formed a circumterrestrial ring of broken-up silicate debris from which the Moon could accumulate. This attractive scenario has been shown to be flawed because the proposed breakup of planetesimals close to the Earth is unlikely to occur. It is also difficult to achieve the required high value for the angular momentum in this model. Such a process might be expected to have been common during the formation of the terrestrial planets, and Venus, in particular, could be expected to have a satellite. 3. In 1879, George Darwin proposed that the Moon was derived from the terrestrial mantle by rotational fission. Such fission hypotheses have been popular since they produced a low-density, metal-poor Moon. However, the angular momentum of the Earth–Moon system, although large, is insufficient by a factor of about 4 to allow for rotational fission. If the Earth had been spinning fast enough for fission to occur, there is no available mechanism for removing the excess angular momentum following lunar formation. The lunar sample return provided an opportunity to test these hypotheses because they predicted that the bulk composition of the Moon should provide some identifiable signature of the terrestrial mantle. The O and Cr isotopic compositions are similar, and this is sometimes

used to argue for a lunar origin from the Earth’s mantle. However, the enstatite chondrites also have identical O isotopic compositions in both bodies; however, both bodies differ significantly in major and trace element contents. Similarity does not constitute identity. Fission hypotheses failed to account for significant chemical differences between the compositions of the Moon and that of the terrestrial mantle or to provide a unique terrestrial signature in the lunar samples. The Moon contains, for example, 50% more FeO and has distinctly different trace siderophile element signatures. It also contains higher concentrations of refractory elements (e.g., Al, U) and lower amounts of volatile elements (e.g., Bi, Pb). The Moon and the Earth have distinctly different siderophile element patterns. The similarity in V, Cr, and Mn abundances in the Moon and the Earth is nonunique since CM, CO, and CV chondrites show the same pattern. These differences between the chemical compositions of the Earth’s mantle and the Moon are fatal to theories that wish to derive the Moon from the Earth. 4. One proposed modification of the fission hypothesis uses multiple small impacts to place terrestrial mantle material into orbit. It is exceedingly difficult to obtain the required high angular momentum by such processes because multiple impacts should average out. Most of these Moon-forming hypotheses should be general features of planetary and satellite formation and should produce Moon-like satellites around the other terrestrial planets. They either fail to account for the unique nature of the Earth–Moon system and the peculiar bone-dry composition of the Moon, or they do not account for the differences between the lunar composition and that of the terrestrial mantle. These earlier theories accounted neither for the lunar orbit nor for the high angular momentum, relative to the other terrestrial planets, of the Earth–Moon system, a rock on which all older hypotheses foundered.

10.2 The Single-Impact Hypothesis The single-impact hypothesis was developed by A. G. W. Cameron basically to solve the angular momentum problem, but, in the manner of successful hypotheses, it has accounted for other parameters as well and has become virtually a consensus. The theory proposes that, during the final stages of accretion of the terrestrial planets, a body about the size of Mars collided with the Earth and spun out a disk of material from which the Moon formed. This giant impact theory resolves many of the problems associated with the origin of the Moon and its orbit. The following scenario is one of several possible, although restricted, variations on the theme.

CLBE001-ESS2E

November 10, 2006

17:37

The Moon

In the closing stages of the accretion of the terrestrial planets 50–100 million years after T0 (4567 million years ago), the Earth suffered a grazing impact with an object (named Theia) of about 0.10 Earth mass. This body is assumed to have differentiated into a silicate mantle and a metallic core. It came from the same general region of the nebula as the Earth (the oxygen and chromium signatures of Earth and Moon are identical and the impact velocities are required to be low in the models). Theia was disrupted by the collision and mostly went into orbit about the Earth. Gravitational torques, due to the asymmetrical shape of the Earth following the impact, assisted in accelerating material into orbit. Expanding gases from the vaporized part of the impactor also promoted material into orbit. Following the impact, the mantle material from Theia was accelerated, but its metallic core remained as a coherent mass and was decelerated relative to the Earth, so that it fell into the Earth within about 4 hours. A metal-poor mass of silicate, mostly from the mantle of Theia, remained in orbit. In some variants of the hypothesis, this material immediately coalesced to form a totally molten Moon. In others, it broke up into several moonlets that subsequently accreted to form a partly molten Moon. This highly energetic event accounts for the geochemical evidence that indicates that at least half the Moon was molten shortly after accretion. Figure 22 illustrates several stages of a computer simulation of the formation of the Moon according to one version of the single giant impact hypothesis. Although the giant impact event vaporized much of the material, the material now in the Moon does not seem to have condensed from vapor. The extreme depletion of very volatile elements and the bone-dry nature of the Moon may be inherited from Theia and so have been a general feature of the early inner solar nebula (all primary meteorite minerals are anhydrous) with volatiles and water added later to the Earth from near Jupiter. Unique events are notoriously difficult to accommodate in most scientific disciplines. An obvious requirement in this model is that a suitable population of impactors existed in the early solar system. Evidence in support of the previous existence of large objects in the early solar system comes from the ubiquitous presence of heavily cratered ancient planetary surfaces, from the large number of impact basins with diameters up to 2000 km or so, and from the obliquities or tilts of the planets, all of which demand collisions with large objects in the final stages of accretion. The extreme example is that an encounter between Uranus and an Earthsized body is required to tip that planet on its side. Thus, the possibility of many large collisions in the early solar system is well established, one of which had the right parameters to form the Moon. The single impact scenario is thus consistent with the planetesimal hypothesis for the formation of the planets from a hierarchical sequence of smaller bodies.

249

FIGURE 22 A computer simulation of the origin of the Moon by a glancing impact of a body larger than Mars with the early Earth. This event occurred about 4500 million years ago during the final stages of accretion of the terrestrial planets. Both the impactor and the Earth have differentiated into a metallic core and rocky silicate mantle. Following the collision, the mantle of the impactor is ejected into orbit. The metallic core of the impactor clumps together and falls into the Earth within about 4 hours in this simulation. Most terrestrial mantle material ejected by the impact follows a ballistic trajectory and is reaccreted by the Earth. The metal-poor, low-density Moon is thus derived mainly from the silicate mantle of the impactor. (Courtesy A. G. W. Cameron.)

This research was conducted in part at the Lunar and Planetary Institute, which is operated by the USRA under contract CAN-NCC5-679 with NASA. This is LPI Contribution 1260.

Bibliography Basaltic Volcanism Study Project (1981). “Basaltic Volcanism on the Terrestrial Planets.” Pergamon, New York.

CLBE001-ESS2E

November 10, 2006

17:37

250 Encyclopedia of the Solar System Canup, R. M. (2004). Dynamics of lunar formation. Annu. Rev. Astron. Astrophys. 42, 441–475. Canup, R. M., and Righter, K. (2000). “Origin of the Earth and Moon.”Arizona Univ. Press, Tucson. Frondel, J. W. (1975). “Lunar Mineralogy.” John Wiley & Sons, New York. Fuller, M. J., and Cisowski, S. M. (1987). Lunar paleomagnetism. In “Geomagnetism 2” (J. A. Jacobs, ed.), pp. 307–455. Academic Press, San Diego. Hartman, W. R., Phillips, R. J., and Taylor, G. L., eds. (1986). “Origin of the Moon.” Lunar and Planetary Institute, Houston. Heiken, G., Vaniman, D., and French, B. M. (1991). “The Lunar Sourcebook.” Cambridge Univ. Press, Cambridge, England. Jolliff, B. L., et al. (2000). Major lunar crustal terranes. J. Geophys. Res. 105, 4197–4216.

Khan, A., and Mosegaard, K. (2001). New information on the deep lunar interior from an inversion of lunar free oscillation periods. Geophys. Res. Lett. 28, 1791–1794. Koeberl, C. (1994). “Tektite Origin by Hypervelocity Asteroidal or Cometary Impact: Target Rocks, Source Craters and Mechanisms,” Geol. Soc. Am. Spec. Paper 293, 133–151. Papike, J. J., ed. (1999). “Planetary Materials,” Miner. Soc. Amer. Rev. Miner. 36. Taylor, S. R. (1982). “Planetary Science: A Lunar Perspective.” Lunar and Planetary Institute, Houston. Taylor, S. R. (2001). ”Solar System Evolution: A New Perspective,” 2nd Ed. Cambridge Univ. Press, Cambridge, England. Wilhelms, D. E. (1987). “The Geologic History of the Moon,” U.S. Geol. Surv. Prof. Paper No. 1348. U.S. Geological Survey, Washington, D.C.

CLBE001-ESS2E

November 14, 2006

100-C

25-C

50-C

75-C

3:54

C+M

50-C+M

C+Y

50-C+Y

M+Y

50-M+Y

100-M

25-M

50-M

75-M

100-Y

25-Y

50-Y

75-Y

100-K

25-K

25-19-19

50-K

50-40-40

75-K

75-64-64

Meteorites

Michael E. Lipschutz Purdue University West Lafayette, Indiana

Ludolf Schultz Max-Planck-Institut fur ¨ Chemie, Mainz, Germany

CHAPTER

1. Introduction 2. Meteorite Classification 3. Meteorites of Asteroidal Origin and Their Parent Bodies

13

5. Chemical and Isotopic Constituents of Meteorites 6. Meteorite Chronometry Bibliography

4. Meteorites from Larger Bodies

M

eteorites, the “Poor Man’s Space Probe,” are important because they contain the oldest solar system materials for research and sample a wide range of parent body—exteriors and interiors—some primitive, some highly evolved. Meteorites record certain solar and galactic effects and yield otherwise unobtainable data relevant to the genesis, evolution, and composition of the Earth, other major planets, satellites, asteroids, and the Sun. Some contain inclusions created before solar system formation; others contain organic matter produced on grain boundaries in the early nebula and/or in giant interstellar clouds. Meteorites also constitute important “ground truth” in a chemical and physical sense, critical to interpreting planetary data obtained by remote sensing. Most importantly, meteorites are on Earth, available for laboratory study by the simplest to the most sophisticated analytical techniques. If one picture is worth 10,000 words, then one sample is worth 10,000 pictures. Even though meteorites are only tiny source-fragments, proper integration of data from them can better describe their sources, just as a more complete mosaic can be deduced from a few tesserae.

∗

1. Introduction 1.1 General In the Western world, 1492 marked the discovery of the New World by the Old, the Spanish Expulsion, and, the oldest documented, preserved, and scientifically studied meteorite fall—a 127 kg (LL6) stone that fell at Ensisheim in Alsace. [A meteorite is named for the nearest post office or geographic feature. The chemical-petrologic classification is the scheme by which Ensisheim, for example, is classified as an LL6 chondrite (see Section 1.2).] The oldest preserved meteorite fall might be Nogata (Japan), an L6, which allegedly fell in 861 (but all associated documentation is more recent) and is in a Shinto shrine there. Recovered meteorites, whose fall was unobserved, are finds, some having been discovered (occasionally artificially reworked) in archaeological excavations in such Old World locations as Ur, Egypt, and Poland, and in New World burial sites. Obviously, prehistoric and early historic man recognized meteorites as unusual, even venerable, objects.

Actual meteriorite (chondrite) dust is embedded in the stamp reproduced above. The stamp was issued by the Austrian postal service in 2006.

C 2007 by Academic Press. All rights of reproduction in any form reserved. Encyclopedia of the Solar System 2e 

251

CLBE001-ESS2E

November 14, 2006

3:54

252 Encyclopedia of the Solar System

TABLE 1

Numbers of Classified Non-Antarctic Meteorite Falls and Finds, Including Those from Hot and Cold Deserts. Fallsa

Findsa,b

ANSMETc

797 5 17 18 17 316 350 72 2

>814 0(2) 6(85) 14(225) 8(307) 405 350 30 1(34)

2925 (11557) 0(0) 51(200) 58(182) 43(103) 1048(4194) 1140(4562) 574(2299) 11(17)

Irons

40

>690(34)

47(97)

Stony-Irons Mesosiderites Pallasites

12 7 5

>61 21(10) 40(4)

13 11(29) 2(11)

Meteorite

Chondrites CI1 CM/C2 C other E H L LL Other

Meteorite

Achondrites Acapulcoites Lodranites Winonaites Angrites Aubrites Howardites Eucrites Diogenites Ureilites Lunar Martian Other

Fallsa

Findsa,b

ANSMETc

81 1 1 0 1 9 20 29 11 5 0 4 0

>73 1 }(14) 0 3(10) 1(3) 3(2) 4(58) 12(137) 0(81) 3(90) 1(22) 3(17) 3(9)

184 5(12) 4(4) 1(1) 2(2) 7(38) 26(44) 66(124) 22(24) 34(47) 8(15) 8(8) 1(4)

a Data from Grady (2000) updated to Nov. 2004 (J. N. Grossman, USGS, personal communication). These do not include 41 unclassified stony or 8 unclassified iron meteorites. b Except for Lunar and Martian meteorites, numbers in parentheses indicate fragments (uncorrected for pairing) recovered as meteorite clusters from hot and cold deserts (ANSMET data not included): These are not combined with corresponding non-desert-cluster finds (Grady, 2000; Grossman, personal communication). The ∼16,500 JARE samples are incompletely classified and, except for lunar and martian meteorites, are not included in this table: Ordinary chondrites from hot and other cold deserts (other than ANSMET) are also omitted Because of their special importance, numbers of lunar and martian meteorites (cf. http://epsc.wustl.edu/admin/resources/ meteorites/moon meteorites.html and http://curator.jsc.nasa.gov/curator/antmet/ marsmets/contents.htm, respectively) in parentheses are the meteorite falls corrected for pairing. c Antarctic Search for Meteorites (ANSMET) recoveries from West Antarctica. Numbers in parentheses are fragments recovered: Associated numbers are corrected for known pairings or by estimating (italics) four fragments per fall. (Data from K. Righter, NASA—JSC.)

Despite this history, and direct evidence for meteorite falls, scientists generally began to accept them as genuine samples of other planetary bodies only at the beginning of the 19th century. Earlier, acceptance of meteorites as being extraterrestrial and, thus, of great scientific interest, was spotty. One might laboriously assemble a meteorite collection only to have someone later dispose of this invaluable material. This occurred, for example, when the noted mineralogist, Ignaz Edler von Born, discarded the imperial collection in Vienna as “useless rubbish” in the latter part of the 18th century. With the recognition that meteorites sample extraterrestrial planetary bodies, collections of them proved particularly important. In 1943, with the imminent invasion of Germany, the Russian government planned for “trophy brigades” to accompany their armies and collect artistic, scientific, and production materials as restitution for Russian property seized or destroyed by Nazi armies during their occupation of parts of Russia. Meteorites that fell in Russia, fragments of which were acquired by and housed in German collections, were explicitly identified as material to be seized. In late 2004, the price for a meteorite from Mars was at least $4000/g (the current price of gold is $14/g). Apart from its recovery and preservation, Ensisheim is a typical fall. For finds, some peculiarity must promote recognition—hence, the high proportion of high-density,

iron meteorites outside of Antarctica (Table 1). Observed falls are taken to best approximate the contemporary population of near-Earth meteoroids. Of course, bias may affect the fall population. Some data suggest that highly friable meteoroids are largely or totally disaggregated during atmospheric passage. The initial entry velocities of meteorites range from 11 to 70 km/s, average 15 km/s, and cause surface material to melt and ablate by frictional heating during atmospheric passage. Heat generation and ablation rates are rapid and nearly equivalent, so detectable heat effects only affect a few millimeters below the surface: The meteorite’s interior is preserved in its cool, preterrestrial state. Ablation and fragmentation—causing substantial (∼90%) mass loss and deceleration, often to terminal velocity—leave a dark brown-to-black, sculpted fusion crust as the surface, diagnostic of a meteorite on Earth (Fig. 1a). If it is appropriately shaped perhaps by ablation, a meteoroid may assume a quasi-stable orientation late in its atmospheric traversal. In this case, material ablated from the front can redeposit as delicate droplets or streamlets on its sides and rear (Fig. 1b). The delicate droplets on Lafayette’s fusion crust would have been erased in a few days’ weathering: It must have been recovered almost immediately after it fell. Yet, when Lafayette was recognized as meteoritic during a 1931 visit to Purdue

CLBE001-ESS2E

November 14, 2006

3:54

Meteorites

(a)

253

(b)

FIGURE 1 Fusion crusts: (a) Noblesville H chondrite; (b) Lafayette Martian meteorite. Noblesville, which fell on 31 August 1991, has nearly complete fusion crust but exposed surface at lower right next to the 1-cm cube shows a genomict (H6 in H4) breccia. (Photo courtesy of NASA Johnson Space Center.) Lafayette exhibits very delicate, redeposited droplets on its sides, indicating an orientation with its top pointing Earthward late in atmospheric traversal. (Photo courtesy of the Smithsonian Institution.)

University by O. C. Farrington (a prominent meteoriticist), the chemistry professor on whose desk it was found thought it a terrestrial glacial artifact. Who actually recovered this martian meteorite is a mystery. Meteorites derive from asteroids and, less commonly, from larger parent bodies: 18 individual samples represent-

TABLE 2

ing 31 separate falls (all but 5 from Antarctica) come from Earth’s Moon; and 32 others (6 from Antarctica) almost certainly are from Mars [see Mars: Surface and Interior; The Moon]. Some interplanetary dust particles may also come from these sources, and/or comets. Meteorites are rocks and therefore polymineralic (Table 2), with each of

Common Meteoritic or Cited Minerals

Mineral

Formula

Mineral

Formula

Mineral

Anorthite Clinopyroxene Chromite Cohenite Cristobalite Diamond Diopside Enstatite Epsomite Fayalite Feldspar solid solution albite (Ab) anorthite (An) orthoclase (Or) Ferrosilite Forsterite Gehlenite Graphite

CaAl2 Si2 O8 (Ca,Mg,Fe)SiO3 FeCr2 O4 (Fe,Ni)3 C SiO2 C CaMgSi2 O6 MgSiO3 MgSO4 ·7H2 O Fa2 SiO4

Hibonite Ilmenite Kamacite Lonsdaleite Magnetite Melilite solid solution ˚ a˚ kermanite (Ak) gehlenite (Ge) Oldhamite Olivine Olivine solid solution fayalite (Fa) forsterite (Fo) Orthopyroxene Pentlandite Plagioclase albite (Ab) anorthite (An)

CaAl12 O19 FeTiO3 α-(Fe,Ni) C Fe3 O4

Pyroxene solid solution enstatite (En) ferrosilite (Fs) wollastonite (Wo) Schreibersite Serpentine (chlorite) Spinel Spinel solid solution spinel hercynite chromite Taenite Tridymite Troilite Whitlockite

NaAlSi3 O8 CaAl2 Si2 O8 KAlSi3 O8 FeSiO3 Mg2 SiO4 Ca2 Al2 SiO7 C

Ca2 MgSi2 O7 Ca2 Al2 SiO7 CaS (Mg,Fe)2 SiO4 Fe2 SiO4 Mg2 SiO4 (Mg,Fe)SiO3 (Fe,Ni)9 S8 NaAl2 Si2 O8 CaAl2 Si2 O8

Formula

MgSiO3 FeSiO3 CaSiO3 (Fe,Ni)3 P (Mg,Fe)6 Si4 O10 (OH)8 MgAl2 O4 MgAl2 O4 FeAl2 O4 FeCr2 O4 γ -(Fe,Ni) SiO2 FeS Ca3 (PO4 )2

CLBE001-ESS2E

November 14, 2006

3:54

254 Encyclopedia of the Solar System

FIGURE 2 From nebula to meteorite: genetic processes and the corresponding age determinable for each process. Nuclides of nearly all elements were formed by nuclear reactions in interiors of large stars, which then ejected them in very energetic supernova events. Ejected nebular gas and dust subsequently nucleated, condensed, and accreted into primitive bodies. Source bodies for most meteorites were heated, causing solid-state metamorphism or, at higher temperatures, differentiation involving separation of solids, liquids, and gases. As a body evolved, it suffered numerous impacts, and, if atmosphere-free, its surface was irradiated by solar and galactic particles that embedded in the skins of small grains and/or caused nuclear reactions. Larger impacts ejected fragments that orbited the Sun. Subsequently, orbital changes caused by large-body gravitational attraction placed meteoroids into Earth-crossing orbits allowing their landing and immediate recovery (as a fall) or later (as a find). Each process can alter elemental and/or isotopic contents. Which of these processes affected a given meteorite and the time elapsed since it occurred are definable.

(from compressional, nonadiabatic heat) after decompression. Residual temperatures as high as 1250◦ C, have been recorded in stony meteorites and correspond to pressures >57 GPa or 570,000 atm (570,000 times the Earth’s sea level pressure). Significantly higher temperatures (pressures) would vaporize matter, so there is a limit to the shock-induced ejection velocity of survivable meteoroids (i.e., Mars’ escape velocity, 5.4 km/s). In very special scenarios, ejecta can be accelerated by impact-jetting—especially during oblique impacts—thus acquiring a velocity higher than expected from the degree of shock-loading. At least some martian meteorites, the 7 nakhlites, are not heavily shocked and may signal this unusual case. In general, however, a parent body much larger than Mars is unlikely to provide meteorites to Earth. The overwhelming majority of meteorites, those of asteroidal origin, seemingly sample a few hundred dominant asteroids, not the thousands known. These may include the near-Earth asteroids (NEA) already in Earth-crossing orapproaching orbits, ejected from Kirkwood Gap regions by chaotic motion and gravitational effects of Jupiter [see Main-Belt Asteroids and Near-Earth Objects]. As discussed later, some types of meteorites and asteroids can be linked. The nine meteorite falls whose orbits were determined photographically seem NEA-like (Fig. 3). Some evidence suggests that co-orbital streams of meteorites and/or asteroids exist—perhaps arising from meteoroids’ gentle

the hundred or so known meteoritic minerals generally having some chemical compositional range, reflecting its formation and/or subsequent alteration processes. Important episodes during meteorite genesis are in Fig. 2.

1.2 From Parent Body to Earth To arrive on Earth, a meteoroid (meteorite-to-be) must be excavated and removed from the gravitational field of its parent body by an impact. This impact can generate shortlived but intense shocks, which provides the impulse necessary for the meteoroid to exceed the parent body’s escape velocity. In general, the higher the shock pressure acting upon matter, the higher its ejection velocity and temperature, both the shock temperature derived from passage of the pressure wave and the postshock residual temperature

FIGURE 3 Orbits determined from overlapping camera coverage for nine recovered chondrite falls: Pr-Pribram (H5, 7 Apr. 1959); LC—Lost City (H5, 3 Jan. 1970); In—Innisfree (L5, 5 Feb. 1977); Pe—Peekskill (H6, 9 Oct. 1992); TL—Tagish Lake (C, 18 Jan. 2000); Mo—Moravka (H5-6, 6 May 2000); Ne—Neuschwanstein (EL6, 6 Apr. 2002); PF—Park Forest (L5, 26 Mar. 2003); Vb—Villalbeto de la Pena ˜ (L6, 4 Jan. 2004). The orbits shown are projections onto the ecliptic plane (orbits of the terrestrial planets and Jupiter in color are included with γ , the vernal equinox). Pribram and Neuschwanstein had identical orbits, but are of different chondritic types.

CLBE001-ESS2E

November 14, 2006

3:54

Meteorites

FIGURE 4 Concentrations of cosmic ray–produced radioactive and stable nuclides during cosmic ray exposure and after the meteorite’s fall to Earth.

disruption in space—but this is very controversial. Evidence from temperature-sensitive components indicates that, in their orbits about the Sun, some meteorites have perihelia within 0.5 AU resulting in detectable solar heating. Some meteorites contain regolithic material bombarded by very energetic particles. Once material is ejected from its parent body by an impact until it falls on Earth, meter-sized meteoroids are irradiated by cosmic rays (mainly protons) of solar or galactic origin. Solar cosmic rays have a power– law energy distribution with the particle flux increasing rapidly with decreasing energy: most solar particles have energies 11 km/s. (Obviously, distinguishing a large meteoroid from a small asteroid is arbitrary.) Such explosive, craterforming impacts can do considerable damage. The 1-kmdiameter Meteor Crater (Fig. 5b) in northern Arizona, which formed 50,000 years (i.e., 50 ka) ago by the impact of a 25- to 86-m meteoroid, yielded fragments now surviving as Canyon Diablo iron meteorites. At least 40 terrestrial craters exhibit features believed to be produced only by intensive explosive impact of a large meteoroid (e.g., as in the 1908 event at Tunguska in Siberia) or perhaps even a comet nucleus. Another 269 features on Earth may be of impact origin. One expert classed 130 of them as definite impact craters. The 180-km-diameter Chicxulub feature in Yucatan, Mexico, is suspected as the impact site of a 10-km meteoroid. By consensus, this impact generated the climatic consequences responsible for the extinction of ∼60% of then-known species of biota—including dinosaurs—ending the Cretaceous period and beginning the Tertiary, 65 Ma ago (the K-T event). Other, less well-established events are suggested as having caused extinctions at other times. Some meteorites have struck man-made objects. The Peekskill stone meteorite (H6), with a recovered mass of 12.4 kg, ended its journey on the trunk of a car (Fig. 5d). Its descent in 1992 was videotaped over a five-state area of the eastern United States by many at Friday evening high school football games (Fig. 5c), yielding a well-determined orbit. Two authenticated reports of humans hit by meteorite falls exist. The first involved a 3.9 kg (H4) stone (the larger

CLBE001-ESS2E

November 14, 2006

3:54

256 Encyclopedia of the Solar System

(a)

(b)

(c)

(d)

FIGURE 5 Large meteoroids: (a) fireball of 80-m object (estimated mass, 1 Mt) on 10 August 1972 moving left to right (see arrow) over Grand Teton National Park that apparently skipped out of the atmosphere. (Photo by Dennis Milon.) (b) The 1-km-diameter Meteor Crater in Arizona formed by the explosive impact of the Canyon Diablo IA octahedrite meteoroid about 50 ka ago. (Photo by Allan E. Morton.) (c) From the videotape record of the Peekskill meteoroid during its atmospheric traverse on 9 October 1992. During fragmentation episodes such as this one (over Washington, D.C.), large amounts of material fell, but nothing was recovered. (d) Landing site of Peekskill chondrite in the right rear of an automobile. (Photo by Peter Brown, University of Western Ontario.)

of two fragments), which, after passing through her roof in Sylacauga, Alabama in 1954 struck a recumbent woman’s thigh, badly bruising her. The second involved a 3.6-g piece of the Mbale (Uganda) L6 meteorite shower of 1992, which bounced off a banana tree’s leaves and hit a boy on the head. Chinese records from 616 to 1915 claim numerous human and animal casualties, including many killed, by meteorite falls. Unauthenticated reports of human injuries or human deaths exist: One undocumented report tells of a dog being killed by a piece of the 40-kg Nakhla meteorite shower of 1911 near Alexandria, Egypt. This, incidentally, is one of the 32 martian meteorites. Despite the small number of casualties to date, the probability of dying in a meteoroid impact exceeds that of being killed in an airplane crash.

This arises because the impact of a large meteoroid, small asteroid, or comet nucleus is capable of causing devastating loss, indeed the total extinction of life. Such impacts seem rare. Meteorites may impact anywhere on Earth, and, as of November 2004, the numbers of known falls and isolated, non-desert-cluster finds are 1046 and 1840, respectively (cf. Table 1). For these, it can readily be established whether meteorite fragments found nearby are from the same meteoroid; however, such linkages are difficult for the numerous meteorite pieces found clustered in hot or cold (Antarctic) deserts since 1969. So far, starting in 1969, but mainly since 1976, Antarctica has yielded over 31,000 fragments [16,500 collected by JARE (Japane´se` Antarctic Research

CLBE001-ESS2E

November 14, 2006

3:54

Meteorites

257

the more common meteorites in these other collections, has not yet been addressed.

2. Meteorite Classification 2.1 General

FIGURE 6 Cross section of Antarctic ice sheet and subice topography: meteorites fall (1), are collected by the ice sheet and buried (i.e., preserved), transported, and concentrated near a barrier to the ice sheet (2), and are exposed by strong South Polar winds that ablate the stagnant ice (3). [Reprinted from “Workshop on Antarctic Glaciology and Meteorites,” C. Bull and M. E. Lipschutz (eds.), LPI Tech. Rept. 82-03. Copyright 1982 with kind permission from the Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, TX 77058-1113.)

Expedition) in Queen Maud Land; 13,907 by ANSMET (Antarctic Search for Meteorites), the US-led team upstream of the Trans-Antarctic Mountain Range; and >677 by a European consortium, which is now an Italian-led effort]. Hot desert-clusters in Australia, North Africa (mainly Algeria and Libya), China, and the United States have yielded >5000 more to date. (These discoveries are possible in these areas because dark meteorites can be readily distinguished from the local, light-colored terrestrial rocks, “meteorwrongs.”) The 14-million-km2 ancient Antarctic ice sheet is a meteorite trove because of the continent’s unique topography and its effect on ice motion, which promotes the meteorites’ collection, preservation, transportation and concentration (Fig. 6). Assuming four fragments per meteoroid, Antarctic meteorites recovered thus far correspond to about 7500 different impact events; no one has estimated the number of fragments produced in a hot-desert meteorite fall. Desert meteorites are named for the nearest topographic feature, usually abbreviated by a one- or threeletter code, and number: the first two digits of Antarctic meteorites denote the expedition year. To complicate matters, expeditions have taken two paths in characterizing their meteorite recoveries. ANSMET chooses to characterize each fragment by type. Other expeditions scan their collection to identify meteorites of rare type, which are of intrinsic interest for more complete study (see Table 1). The “pairing” of even these samples, let alone

Meteorites, like all solar system matter, ultimately derive from primitive materials that condensed and accreted from the gas- and dust-containing presolar disk. Most primitive materials were altered by postaccretionary processes—as in lunar, terrestrial, and martian samples—but some survived essentially intact, as specific chondrites or inclusions in them. Some primitive materials are recognizable unambiguously (albeit with considerable effort), usually from isotopic abundance peculiarities; others are conjectured as unaltered primary materials. Postaccretionary processes produced obvious characteristics that permit classification of the thousands of known meteorites into a much smaller number of types. Many classification criteria contain genetic implications, which we now summarize. At the coarsest level, we class meteorites as irons, stones, or stony-irons from their predominant constituent (Figs. 7a and 8): each can then be classified by a scheme with genetic implications (Fig. 7b). Stones include the numerous, more-or-less primitive chondrites (Table 1; Figs. 8a and 8b) and the achondrites (Fig. 8d) of igneous origin. Irons (Fig. 8e), stony-irons (Fig. 8c), and achondrites are differentiated meteorites, presumably formed from melted chondritic precursors by secondary processes in parent bodies (Fig. 2). During melting, physical (and chemical) separation occurred, with high-density iron sinking to form pools or a core below the lower density achondritic parent magma. Ultimately, these liquids crystallized as parents of the differentiated meteorites, the irons forming parent body cores or, perhaps, dispersed “raisins” within their parent. Stony-iron meteorites are taken to represent metal-silicate interface regions. Pallasites (Fig. 8c), which have large (centimeter-sized) rounded olivines embedded in well-crystallized metal, resemble an “equilibrium” assemblage that may have solidified within a few years but that cooled slowly at iron meteorite formation-rates, a few degrees per million years (Ma). Mesosiderite structures suggest more rapid and violent metal and silicate mixing, possibly by impacts. During differentiation, siderophilic elements are easily reduced to metal; they follow metallic iron geochemically and are extracted into metallic melts. Such elements (e.g., Ga, Ge, Ni, or Ir) are thus depleted in silicates and enriched in metal to concentrations well above those in precursor chondrites. Conversely, magmas become enriched in lithophilic elements—like rare earth elements (REE), Ca, Cr, Al, or Mg—above chondritic levels: concentrations of such elements approach zero in metallic iron.

CLBE001-ESS2E

November 14, 2006

3:54

258 Encyclopedia of the Solar System

(a)

(b)

FIGURE 7 Meteorite classifications: (a) the most common classes and some chemical-petrologic classification criteria (in left-hand boxes, red denotes iron-nickle metal, orange indicates FeS and white signifies silicates); (b) genetic associations involving meteorites.

CLBE001-ESS2E

November 14, 2006

3:54

Meteorites

FIGURE 8 Common meteorite types (approximate longest dimension in cm): (a) Whitman, H5 (6 cm); (b) Allende; C3V (8 cm)—note 1-cm chondrule in center; (c) Springwater pallasite (18 cm); (d) Sioux County eucrite (8 cm); (e) Sanderson IIIB medium octahedrite (13 cm)—note large FeS inclusions.

259

CLBE001-ESS2E

November 14, 2006

3:54

260 Encyclopedia of the Solar System During substantial heating, noble gases and other atmophile elements—like carbon and nitrogen—are vaporized and lost from metallic or siliceous regions. Chalcophilic elements that form sulfides like troilite (Table 2) include Se, Te, Tl, or Bi. Chalcophiles and a few siderophiles and lithophiles are also often quite easily mobilized (i.e., vaporized from condensed states of matter) so that they may be enriched in sulfides in the parent body or lost from it. Concentrations of these elements in specific meteorites then depend in part on the fractionation histories of their parents and are markers of heating.

2.2 Characteristics of Specific Classes It is obvious, even to the naked eye, that most iron meteorites consist of large metallic iron crystals, which are usually single-crystal, bcc α-Fe (kamacite) lamellae 0.2– 50 mm thick with decimeter to meter lengths (Fig. 8e). These relatively wide Ni-poor lamellae are bounded by thin, Ni-rich fcc γ -Fe (taenite). The solid-state nucleation and diffusive growth process by which kamacite grew at slow cooling rates from taenite previously nucleated from melt is quite well understood. The 1-atm Fe–Ni phase diagram and measurement of Ni-partitioning between kamacite and taenite permits cooling rate estimation between ∼900 and 400◦ C. These typically are a few degrees or so per Ma, depending on the iron meteorite group, consistent with formation in objects of asteroidal size. The Ni concentration in the melt determines the temperature of incipient crystallization, and this, in turn, establishes kamacite orientation in the final meteorite. These orientations are revealed in iron meteorites by brief etching (with nitric acid in alcohol) of highly polished cut surfaces: Baron Alois von Widmanst¨atten discovered this in the 18th century, and the etched structure is called the “Widmanst¨atten pattern.” (An Englishman, G. Thomsen, independently discovered this, but his contribution was unrecognized.) Meteorites containing 3.3 mm) and yield the very coarsest Widmanst¨atten pattern, while those highest in Ni are composed of very thin (16% Ni nucleate kamacite at such low temperatures that large single crystals could not form over the 4.57 billion years (Ga) of solar system history: they lack a Widmanst¨atten pattern and are called Ni-rich ataxites (i.e., without structure). The Ni-poor ataxites are hexahedrites or octahedrites that were reheated either in massive impacts or artificially after they fell on Earth. As noted earlier, when primitive parent bodies differentiated, siderophilic elements were extracted into molten

FIGURE 9 Contents of Ni and Ga in iron meteorites. (Some larger chemical groups are indicated by Roman numerals and letters.)

metal. During melt crystallization, fractionation or separation of siderophiles could occur. About 50 years ago, Ga and Ge contents of iron meteorites were found to be quantized, not continuous: They could then be used to classify irons into groups denoted as I to IV. Originally, these Ga–Ge groups, which correlate well with Ni content and the Widmanst¨atten pattern, were thought to sample core materials from a very few parent bodies. Subsequent studies of many additional meteorites and some additional elements, especially Ni and Ir, modified this view. At present, the chemical groups (Fig. 9) suggest that iron meteorites sample perhaps 100 parent bodies, although many, if not most, irons derive from but 5 parents (Fig. 9) represented by the IAB, IIAB, IIIABCD, IVA, and IVB irons. (The earlier Roman numeral notation for Ga–Ge groups was retained to semiquantitatively indicate the meteorite’s Ga or Ge content. However, a letter suffix was added to indicate whether siderophiles fractionated from each other.) In addition to the major minerals (kamacite, taenite, and mixtures of them), minor amounts of other minerals like troilite, and graphite may be present. Also, silicates or other oxygen-containing inclusions exist in some iron meteorites. In most cases, chondrites contain spherical millimeterto centimeter-sized chondrules or their fragments. These chondrules were silicates that melted rapidly at temperatures near 1600◦ C and cooled rapidly at some ∼1000◦ C/h early in the solar system’s history; others cooled more slowly at 10–100◦ C/h. Rapid heating and cooling are relatively easy to do in the laboratory but are difficult on a larger, solar system–sized scale. Yet, large volumes of chondrules must

CLBE001-ESS2E

November 14, 2006

3:54

Meteorites

have existed in the solar system because chondrites are numerous (Table 1). Chondrites (and many achondrites) date back to the solar system’s formation—indeed, they provide chronometers for it (see Sections 6.4 and 6.5)— and represent accumulated primary nebular condensate and accretionary products. A portion of this condensate formed from the hot nebula as millimeter-sized Ca- and Al-rich inclusions (CAI), mineral aggregates predicted as vapor-deposition products by thermodynamic calculations. These CAI, found mainly in chondrites rich in carbonaceous (organic) material, exhibit many isotopic anomalies and contain atoms with distinct nucleosynthetic histories. Other inclusions (like SiC and extremely fine diamond) represent relict presolar material. Other condensates formed at much lower temperatures. Some—perhaps even many— CAI may be refractory residues, not condensates. Although most chondrites contain the same minerals, the proportions of these and their compositions differ in the 6 or so principal chondritic chemical groups. The primary bases for chondrite classification involve proportions of iron as metal and silicate (in which oxidized iron—expressed as FeO—may be present), and total iron (from Fe, FeO, and FeS) content (Fig. 7a). The last (Fig. 10) defines meteorites with high and low total iron (H and L, respectively) or low total iron and low metal (LL). Numbers of H, L, and LL chondrites are so large (Table 1) that these are called

TABLE 3

261

FIGURE 10 Silicon-normalized contents of Fe as metal and in FeS (ordinate) vs. Fe in ferromagnesian silicates (abscissa) in various chondritic groups. (Each diagonal defines constant total iron content.)

the ordinary chondrites. Obviously, chondrite compositions (typically, as in Table 3, with elements apportioned by chemical form) are not continuous but, rather, quantized. Table 3 lists major element ratios diagnostic of specific chondritic

Average Chemical Compositions and Elemental Ratios of Carbonaceous and Ordinary Chondrites and Eucrites

Speciesa

C1

SiO2 TiO2 Al2 O3 Cr2 O3 Fe2 O3 FeO MnO MgO CaO Na2 O K2 O P 2 O5 H 2 O+ H 2 O− Fe0 Ni Co FeS C S (elem)

22.69 0.07 1.70 0.32 13.55 4.63 0.21 15.87 1.36 0.76 0.06 0.22 10.80 6.10

C2M

C3V

H

L

LL

EUC

28.97 34.00 36.60 39.72 40.60 48.56 0.13 0.16 0.12 0.12 0.13 0.74 2.17 3.22 2.14 2.25 2.24 12.45 0.43 0.50 0.52 0.53 0.54 0.36

22.14 26.83 10.30 14.46 17.39 19.07 0.25 0.19 0.31 0.34 0.35 0.45 19.88 24.58 23.26 24.73 25.22 7.12 1.89 2.62 1.74 1.85 1.92 10.33 0.43 0.49 0.86 0.95 0.95 0.29 0.06 0.05 0.09 0.11 0.10 0.03 0.24 0.25 0.27 0.22 0.22 0.05 8.73 0.15 0.32 0.37 0.51 0.30 1.67 0.10 0.12 0.09 0.20 0.08 0.14 0.16 15.98 7.03 2.44 0.13 0.29 1.74 1.24 1.07 0.01 0.01 0.08 0.06 0.05 0.00 9.08 5.76 4.05 5.43 5.76 5.79 0.14 2.80 1.82 0.43 0.11 0.12 0.22 0.00 0.10

Speciesa

NiO CoO NiS

C1

C2M

1.33 0.08

1.71 0.08

C3V

H

L

LL

EUC

99.99 27.45

99.99 21.93

99.92 19.63

100.07 15.04

1.72

CoS SO3 CO2

5.63 1.50

1.59 0.78

0.08

Total Fe

98.86 18.85

99.82 21.64

99.84 23.60

Ca/Al 1.08 1.18 1.10 1.11 1.12 1.16 Mg/Si 0.90 0.89 0.93 0.82 0.80 0.80 Al/Si 0.085 0.085 0.107 0.066 0.064 0.062 Ca/Si 0.092 0.100 0.118 0.073 0.071 0.072 CaTi/Si 0.004 0.006 0.006 0.004 0.004 0.004 Fe/Si 1.78 1.60 1.48 1.60 1.18 1.03 Fe/Ni 18.12 16.15 16.85 15.84 17.73 18.64 Fe0 /Ni 9.21 5.67 2.29 Fe0 /Fe 0.58 0.32 0.12

1.12 0.19 0.29 0.325 0.0019 0.66

a Fe includes all iron in the meteorite whether existing in metal (Fe0 ), FeS, or in silicates as Fe2+ (FeO) or Fe3+ (Fe2 O3 ). The symbol H2 O− indicates loosely bound (adsorbed?) water removable by heating to 110◦ C: H2 O+ indicates chemically bound .water that can be lost only above 110◦ C. (Data courtesy of Dr. E. Jarosewich, Smithsonian Institution.)

CLBE001-ESS2E

November 14, 2006

3:54

262 Encyclopedia of the Solar System groups. The total iron in some enstatite (E) chondrites exceeds that in the H group of ordinary chondrites, denoting them as EH chondrites; the EL chondrite designation is self-evident. Achondrites, formed at high-temperatures, contain essentially no metal or sulfide and are enriched in refractory lithophiles (cf. Table 3), which, with their constituent minerals, allow classification into specific groups (Fig. 7a). Most groups are named for a specific prototypical meteorite; others—howardites, eucrites, and diogenites (HED meteorites)—were named nonsystematically. At least 10 achondrite groups can be distinguished from their oxidized iron and calcium contents (FeO and CaO). Some apparently were associated in the same parent body but derive from different regions: the HED and the SNC (Shergottites–Nakhlites–Chassigny) associations. The HED meteorites are thought to come from 4 Vesta, and/or other V class asteroids produced from it. The consensus that the 32 SNC meteorites come from Mars is so strong, that these are often called martian meteorites, not SNCs.

2.3 Oxygen Isotopics and Interpretation Meteorites “map” the solar system by isotopic composition of oxygen (Fig. 11), a major element in all but the irons. Because its high chemical reactivity causes oxygen to form numerous compounds, it exists in many meteoritic minerals, even in silicate inclusions in iron meteorites. In standard references, such as the Chart of the Nuclides, the terrestrial composition of its three stable (i.e., nonradioactive) isotopes is given as 99.756% 16 O, 0.039% 17 O, and 0.205% 18 O. In fact, any physical or chemical reaction alters its isotopic composition slightly by mass-fractionation. Since the mass difference between 16 O and 18 O is twice that existing between 16 O and 17 O, a mass-dependent reaction (e.g., physical changes and most chemical reactions) increases or decreases the 18 O/16 O ratio by some amount and will alter the 17 O/16 O ratio in the same direction, but by half as much. Accordingly, in a plot of 17 O/16 O vs. 18 O/16 O or units derived from these ratios (i.e., δ 17 O and δ 18 O; cf. Fig. 11 caption), all mass-fractionated samples derived by chemical or physical processes from an oxygen reservoir with a fixed initial isotopic composition will lie along a line of slope ∼1/2. Data from terrestrial samples define the Terrestrial Fractionation Line (TFL) in Fig. 11, whose axes are like those described earlier, but normalized to a terrestrial reference material, Standard Mean Ocean Water (SMOW). Not only do all terrestrial data lie along the TFL line, but so too do the oxygen isotopic compositions of lunar samples, which occupy a small part of it. The single Earth–Moon line (defined by data covering the solid line’s full length) suggests that both bodies sampled a common oxygen isotopic reservoir, thus supporting the idea that the Moon’s matter spun

FIGURE 11 Relation between oxygen isotopic compositions in whole-rock and separated mineral samples from the Earth, Moon, and various meteorite classes. Units, δ 17 O (‰) and δ 18 O (‰), are those used by mass spectrometrists and are, in effect, 17 O/16 O and 18 O/16 O ratios, respectively. Both δ 17 O (‰) and 18 δ O (‰) are referenced to SMOW. Oxygen isotopic compositions for carbonaceous chondrites are much more variable than for other meteorite classes (dashed box in the upper part expanded in the lower one).

off during the massive impact of a Mars-sized projectile with a proto-Earth (see relevant chapters). One important feature of Fig. 11 is that many chondrite and achondrite groups defined by major element composition and mineralogy (e.g., Figs. 7a and 7b) occupy their own regions in oxygen isotope space. These data suggest that at least eight major chondritic groups (H, L, LL, CH, CI, CM, CR, and E) and a minor one (R), acapulcoites and brachinites, the two achondrite associations (SNC and HED), ureilites (U) and the silicate inclusions in group IAB iron meteorites derive from different “batches” of nebular material. The HED region also includes data for most pallasites and many mesosiderites suggesting derivation from a common parent body. Extension of the HED region by a line with slope 1/2 passes through the isotopic region of

CLBE001-ESS2E

November 14, 2006

3:54

Meteorites

the oxygen-containing silicate inclusions from IIIAB irons, suggesting that they, too, may be related to the HED association. Perhaps these irons come from deeper in the HED parent body, but this would imply more complete disruption than V-class asteroids (e.g., 4 Vesta) exhibit. Even though oxygen isotopic compositions of the rare angrites and brachinites resemble those of the HED association, differences in other properties weaken the connection. Other possible links indicating common nebular reservoirs (based upon limited oxygen isotopic data) are silicate inclusions in IIE irons with H chondrites, silicates in IVA irons with L or LL chondrites, aubrites with E chondrites, winonaites (primitive meteorites modified at high-temperatures) with silicates from IAB and IIICD irons, and the very rare, highly-metamorphosed—even melted—primitive acapulcoites and lodranites. One interpretation of Fig. 11 is that the solar system was isotopically inhomogeneous because each batch of nebular matter seems to have its characteristic oxygen isotopic composition. Isotopic homogenization of gases is more facile than is chemical homogenization so that the isotopic inhomogeneity demonstrated by Fig. 11 implies that the solar system condensed and accreted from a chemically inhomogeneous presolar nebula (Fig. 2). The other important feature to be noted from Fig. 11 is the “carbonaceous chondrite anhydrous minerals line,” with slope near 1. A feature distinguishing C1 and C2 chondrites (Section 2.4.4.1) from all others (cf. Fig. 7b) is evidence for preterrestrial aqueous alteration or hydrolysis of some phases in them. (Evidence for hydrous alteration of minerals is also observed in some unequilibrated ordinary chondrites.) Anhydrous minerals (including CAI) in carbonaceous chondrites were seemingly never exposed to water so that these chondrites are regarded as a mixture of materials with different histories. As seen from Fig. 11, oxygen isotopic compositions of anhydrous minerals in CM, CV, and CO chondrites are consistent with a line defined by CAI whose slope cannot reflect the mass-fractionation process indicated by a slope 1/2 line like TFL. Instead, the anhydrous minerals line seems to represent a mixture of two end members (batches of nebular material), which, at the 16 O-rich (i.e., low 17 O, 18 O) end lie at or beyond the CO region. Ureilite oxygen isotopic compositions lie on an anhydrous minerals line near CM, suggesting a link. These achondrites contain carbon (as graphite-diamond mixtures) in amounts intermediate to those of CV or CO chondrites and CM. Ureilite data do not indicate formation by differentiation of material with uniform oxygen isotopic composition. Rather, ureilite formation may reflect carbonaceous chondrite-like components mixed in various proportions. As originally interpreted, the anhydrous minerals line represented a mixture of nebular material containing pure 16 O with others higher in 17 O and 18 O. If so, the former reflected a unique nucleosynthetic history, perhaps ma-

263

terial condensed from an expanding, He- and C-burning supernova shell. Subsequently, photochemical reactions of molecular oxygen with a given isotopic composition were shown to yield oxygen molecules with isotopic composition defining a slope 1 line as in Fig. 11. Which process—nebular or photochemical—produced the trends in Fig. 11 is unknown. Even so, Fig. 11 still serves to link meteorites or groups of them produced from one batch of solar system matter. Moreover, the position of any sample(s) could reflect some combination of the mass-fractionated and mixing (slope 1) lines. For example, primary matter that ultimately yielded L chondrites (or any ordinary chondrite group) and HED meteorites could have had a single initial composition, subsequently massfractionated and/or mixed or reacted photochemically to produce meteorite groups with very different oxygen isotopic compositions. However, suitable meteorites with intermediate oxygen isotopic compositions are unknown.

2.4 Chondrites The available data suggest that heat sources for melting primitive bodies (presumably compositionally chondritic) that formed differentiated meteorites were within rather than external to parent bodies. Important sources no doubt include radioactive heating from radionuclides—both extant (40 K, 232 Th, 235 U, and 238 U) and extinct (e.g., 26 Al)— which were more abundant in the early solar system, and impact heating. Calculations show that 26 Al was important in heating small (a few kilometers) primitive parents; other heat sources were effective in differentiating larger ones. Electrical inductive heating driven by dense plasma outflow along strong magnetic lines of force associated with the very early, pre-main-sequence (T-Tauri stage) Sun is possible but not proven. 2.4.1 PETROGRAPHIC PROPERTIES

Major element and/or oxygen isotope data demonstrate that differences between parent materials of chondrites of the various chemical groups (e.g., H, CM or EH) are of primary nebular—preaccretionary—origin. Parent body differentiation, on the other hand is secondary (postaccretionary). Such heating does not necessarily melt the entire parent body, and it is thus reasonable to expect an intermediate region between the primitive surface and the molten differentiated interior. Properties of many chondrites support this expectation and suggest that solid-state alteration of primary chondritic parent material (similar to type 3 chondrites) occurred during secondary heating. Eight characteristics observed during petrographic study of optically thin sections (Fig. 12) serve to estimate the degree of thermal metamorphism experienced by a chondrite and to categorize it into the major 3–6 types (Table 4). The absence of chondrules and the presence of abnormally large

CLBE001-ESS2E

November 14, 2006

3:54

264 Encyclopedia of the Solar System

(a)

(b)

(c)

FIGURE 12 Petrographic (2.5-mm-wide) thin sections in polarized transmitted light. Partial large chondrules are obvious in the H3 chondrite Sharps (a) but barely recognizable in the H6 chondrite Kernouve (b); Nakhla (c) is of martian origin. (Photos courtesy of Dr. Robert Hutchison, Natural History Museum, London.)

CLBE001-ESS2E

November 14, 2006

3:54

Meteorites

(>100 μm) feldspar characterize very rare type 7. These pigeonholes approximate a chondritic thermal metamorphic continuum. Petrographic properties (with bulk carbon and water contents) suggest increasing aqueous alteration of type 3 material into types 2 and 1. Two of these characteristics are illustrated in Fig. 12: the opaque matrix and distinct chondrules of the type 3 chondrite Sharps (Fig. 12a) should be contrasted with the recrystallized matrix and poorly defined chondrules of extensively metamorphosed (type 6) Kernouve (Fig. 12b). Chemically, Fe2+ contents of the ferromagnesian silicates— olivine and pyroxene (Table 2)—are almost completely random in a chondrite like Sharps and quite uniform in

TABLE 4

265

one like Kernouve. Chondrites of higher numerical types could acquire their petrographic characteristics (Table 4) by extended thermal metamorphism of a more primitive (i.e., lower type) chondrite of the same chemical group. Temperature ranges estimated for formation of types 3–7 are 300–600, 600–700, 700–750, 750–950, and >950◦ C, respectively. The petrography of achondrites, like the martian meteorite Nakhla, clearly indicates igneous processes in parent bodies at temperatures 1000◦ C. The resultant melting and differentiation erased all textural characteristics of the presumed chondritic precursor (Fig. 12c) so its nature can only be inferred.

Definitions of Chondrite Petrographic Typesa Petrographic Types

Uniform

1

2

(i) Homogeneity of olivine and pyroxene compositions

—

>5% mean deviations

(ii) Structural state of — low-Ca pyroxene

3

4

5

>5% mean deviations to uniform

Uniform

Monoclinic

Predominantly monoclinic >20%

13 GPa. Somehow, parent bodies of the stonyirons and half of the iron meteorites were disrupted, and the meteoroids were excavated from appreciable depth without subjecting them to major shock-loading. More puzzling is the fact that silicate portions of mesosiderites contain much shocked material. Apparently, these stony-irons formed by intrusion of shock-loaded silicate into or onto preexisting, generally unshocked metal, possibly after excavation from parent body interiors.

CLBE001-ESS2E

November 14, 2006

3:54

Meteorites

269

3. Meteorites of Asteroidal Origin and their Parent Bodies 3.1 The Meteorite–Asteroid Connection Two links have already been noted that suggest or imply an asteroidal origin for most meteorites. These are: 1. Photographically determined orbits for seven ordinary chondrites, one unique carbonaceous one and an EL6 (Fig. 3). 2. Mineralogic evidence indicating origin of most meteorites in asteroidal-sized objects. (Some chondrites could come from much smaller primary objects.) This evidence includes iron meteorite cooling rates (implying formation depths of asteroidal dimensions), the presence of minerals (e.g., tridymite), and phase relations (e.g., the Widmanst¨atten pattern) indicative of low-pressure (1 GPa) origin, and the absence of any mineral indicating high lithostatic (generated by the rocky overburden)—rather than shock-pressures. Another property linking meteorites and certain asteroidal types, spectral reflectance, is a research area of strong current interest. The reflectivity (albedo)-wavelength variation for an asteroid, involving white (solar) incident light, can characterize its mineralogy and mineral chemistry somewhat [see Main-Belt Asteroids]. To uncover possible links, asteroidal spectral reflectance can be compared with possible meteoritic candidates, both as-recovered or treated in the laboratory to simulate effects of extraterrestrial processes (Fig. 13). The best matches exist between the HED association and rare V-class asteroids (4 Vesta and its smaller progeny), iron meteorites and numerous M-class asteroids; CI and CM chondrites thermally metamorphosed at temperatures up to 900◦ C with the very numerous C-class and apparently related B-, F-, and G-class asteroids; aubrites with the somewhat unusual E-class asteroids; pallasites with a few of the very abundant, and diverse S-class—which constitute a plurality of all classified asteroids—and/or rare A-type asteroids; and ordinary chondrites with the very rare Q-type asteroids, which are near-Earth asteroids, or 6 Hebe, an inner Belt object belonging to the S(IV) subclass of S asteroids. A typical good news/bad news situation results. The good news is that specific meteorite types are similar to (derive from) surface regions of identifiable asteroid types. The bad news is that relative frequencies with which meteorites of a given type and asteroids of a supposedly similar type are encountered do not agree. Specifically, there is the ordinary chondrite-S asteroid paradox (cf. Table 1): Why are there so few asteroidal candidates for the very numerous ordinary chondrites and so few olivine-dominated stony-irons from the very numerous S asteroids? One obvious answer is that “space weathering” (energetic dust impingement on a meteoroid surface causing metal reduction and dispersion)

FIGURE 13 Spectral reflectances of the Coopertown IIIE coarse octahedrite, Juvinas eucrite and V-class asteroid 4 Vesta, and Vigarano C3V chondrite and G-class asteroid 1 Ceres. The albedo scale for all but Coopertown is on the left: Coopertown’s is at right. Solid and dashed lines delineate meteorite and asteroid spectra, respectively. (Courtesy of Dr. Lucy-Ann McFadden, University of Maryland.)

could mask ordinary chondrite-like interiors. Another, is that Earth collects a biased meteorite sampling compared with the asteroid population, in either near-Earth space or in the Asteroid Belt. This may also account for the near absence of meteorites from the numerous D or P asteroids, located at >3 AU from the Sun. Alternatively, ejecta from such asteroids might not survive atmospheric passage because D and P surface materials are inferred to be very organic-rich and, presumably, very friable. Tagish Lake, the only meteorite that spectrally resembles a D-class asteroid, contains organic globules and is extremely friable.

3.2 Sampling Bias The contemporary flux of meteorites is biased and unrepresentative of the meteoroid population in near-Earth space, let alone in the Asteroid Belt, so generalizations about parent body formation and evolution from studies of meteorites falling today may be incomplete. The contemporary flux of meteorites includes not only observed falls during mainly the past 200 years, but also by non-desert-cluster finds (i.e., omitting the many from hot and cold deserts). Most finds contain metallic iron, which should be readily oxidized on Earth, but they are surprisingly resistant to destructive oxidation, even in temperate climates. The smaller iron–nickel grains of chondrite finds are more readily oxidized.

CLBE001-ESS2E

November 14, 2006

3:54

270 Encyclopedia of the Solar System Meteoritic terrestrial ages are generally based upon decay of cosmogenic radionuclides (Fig. 4). Non-desertcluster finds have been on Earth for up to ∼20 ka, but the oldest one actually dated is the Tamarugal IIIA octahedrite that has a terrestrial age of 3.6 Ma. As is discussed in Section 6.1, terrestrial ages for meteorite cluster finds from hot deserts usually range up to 50 ka: many Antarctic meteorites are much older. A few dozen fossil meteorites found in Ordovician seabed layers in several Swedish quarries have ∼480 Ma terrestrial ages (Section 6.4). The oldest Antarctic meteorite is Lazarev, an Antarctic octahedrite that is not part of any established iron– meteorite chemical group. Its terrestrial age is 5 Ma. Although an Antarctic chondrite has a terrestrial age of ∼2 Ma, the more typical terrestrial ages for these are in the 0.1- to 1-Ma range (averaging 0.3 Ma for the population from the Allan Hills, Victoria Land; Section 6.1.1). Conceivably, the meteorite population landing on Earth during that time window could have differed from the contemporary one. The number of iron and stony-iron observed falls is comparable with those from Victoria Land (Table 1); however, Antarctic achondrites and chondrites are more numerous. Additional differences exist in the details. For example, samples from Victoria Land have, on average, smaller masses than do those from more contemporary falls; small samples are readily detected in Antarctica. Meteorites of rare types—like achondrites—are easily recognized, even in hand-specimen, and pieces can be readily paired with others of the same fall. Hence, in the Victoria Land (ANSMET) population, the numbers of different Antarctic achondrites are reliable (Table 1). When small populations are compared, the results are always suspect. We note that the number of aubrites and howardites are comparable but the number of ureilites and lunar meteorites are larger in the Victoria Land population. A difference may exist for C1 chondrites, but they are typically friable and might not survive pulverization in the Antarctic ice sheet. At face value, Antarctic ordinary chondrites seem very numerous, but their pairing uncertainties are particularly serious. Ordinary chondrites differ only subtly from each other—even as falls—so the apparent excess of Antarctic LL chondrites is clouded. Numerous studies of Antarctic meteorites reveal many preterrestrial genetic differences between them and falls, but detailed interpretations of these differences remain controversial. The 16,500 fragments collected from Queen Maud Land, Antarctica, by Japanese meteorite recovery teams include quite a few fragments of rare or unique meteorite types. These include 6 different lunar meteorites (9 fragments), 4 martian meteorites (6 fragments), 6 thermally metamorphosed (open-system) C1–C3 chondrites, and a unique C1M or C2I chondrite. In general, Queen Maud Land samples have terrestrial ages of up to 0.3 Ma, averaging 0.1 Ma (i.e., intermediate between those of contemporary falls and Antarctic samples from Victoria Land) and are

of smaller mass, on average, than even those from Victoria Land. The Queen Maud Land population is less well characterized than the Victoria Land population, so, except for lunar and martian meteorites, we do not list any of them in Table 1.

4. Meteorites from Larger Bodies During the early Apollo program, NASA decided to quarantine lunar samples and the astronauts that brought them to prevent contamination of Earth by some hypothetical “Andromeda Strain.” This quarantine cost much and proved ineffective. Years before Apollo 11 (in 1969), E. Anders argued that because lunar escape velocity was so low (2.38 km/s) and shock-induced ejecta velocities were so high, lunar samples must already be on Earth to contaminate us (if they were going to). To eliminate this unnecessary expenditure, he offered to eat the first gram of lunar sample brought by Apollo 11. His offer was not accepted: quarantine ended with Apollo 12, and the first meteorite recognized as lunar (ALH A81005) was found in Antarctica in 1982. Yamato 791197 was recovered in 1979 but its lunar origin was not recognized then. Today, 31 lunar meteorites are known as are 32 martian meteorites (Table 1) and NASA plans an expensive quarantine to protect humankind from another hypothetical “Andromeda Strain” if and when they bring martian samples to Earth. One author of this chapter (MEL) offers to eat the first gram of that sample to demonstrate that an expenditure of between $10 million and $1 billion ($1,000,000,000) for quarantine is unwarranted. These two are the only likely large-body sources for meteorites. Other solar system bodies have escape velocities comparable to that of Mars’ (e.g., Mercury, Pluto, and the satellites of giant planets like Jupiter or Saturn), but their distances from Earth and their proximities to much larger objects with greater gravitational attraction makes Earthcapture of ejecta from them virtually impossible. Thus, we need only consider the Moon or Mars as meteorite sources. (For additional information, see the websites listed in the footnote to Table 1).

4.1 Lunar Meteorites The minerals, textures, chemical compositions, and isotope ratios of these 31 individuals (each between 2 g and 1.8 kg) are similar to those of samples brought to Earth by the Apollo and Luna missions [see The Moon] and unlike those of terrestrial rocks or martian and other meteorites (Fig. 14). Only their fusion crust differentiates them from Apollo and Luna samples. Most are regolith, fragmental, or melt breccias from the lunar highlands: 7 are Mare basalts, 3 of which include regolith breccias or cumulate clasts. Their cosmic ray exposure ages range up to 10 Ma

CLBE001-ESS2E

November 14, 2006

3:54

Meteorites

FIGURE 14 Lithophile element concentrations (K vs. La) in ordinary and Cl chondrites and samples of evolved bodies: lunar samples from various Apollo missions and lunar meteorites; terrestrial rocks; and martian meteorites. Data for HED achondrites parallel and lie between the Earth and Moon lines.

but, in principle, could be much less; they probably originated from ∼20 impacts forming lunar explosion craters that are a few kilometers in diameter, and possibly as small as 0.5 km. Lunar meteorites total ∼11.2 kg, much less than the 382 kg of Apollo and Luna material, but they provide very important lunar information. Because Apollo and Luna landing sites (all Nearside) were chosen for safety reasons or as geologically interesting but unrepresentative, their regional sampling of the Moon is biased. Lunar meteorites represent random (but unknown) impact sites. Indeed, when compared with lunar spectral reflectance data from the Clementine spacecraft, the distribution of FeO contents, KREEP-associated U and Th contents, and, indeed, the highlands nature of lunar meteorites themselves parallel the overall lunar character. One meteorite, NWA 773, samples a Mare basalt region unlike any provided by the Apollo or Luna missions. Much will doubtless be learned about the Moon and its history from these lunar meteorites and others, yet discovered.

271

lized from parent melts ≤1.3 Ga ago (the youngest, 170 Ma ago). This alone suggests a large parent because only a planetary body could retain interior temperatures sufficient to maintain igneous melts that recently. Asteroid-sized objects could have been differentiated early but would have cooled rapidly, crystallizing igneous rocks 4.5 Ga ago. That is the age of ALH 84001, which must be a rare survivor of early martian differentiation. It is, of course, linked by oxygen isotopic composition to the other 31 rocks in the SNC portion of Fig. 11. They are linked to Mars specifically by gases (e.g., 20 Ne, 36 Ar, 40 Ar, 84 Kr, 131 Xe, N2 , CO2 ) in shockformed glass in EET A79001, the only meteorite showing a contact between two igneous regions. Contents of these gases in EET A79001 match those in the martian atmosphere measured in 1976 by the Viking landers. The Martian atmosphere apparently lost more light gases than did the Earth. Because martian escape velocity is higher than that of the Moon, impacts intense enough to propel Martian meteorites Earthward must be greater, requiring larger explosion craters, 10–100 km in diameter. From cosmic ray histories, the 32 martian meteorites apparently derive from 6–8 events. All solidified near, but below, the martian surface; none were surface samples irradiated by cosmic rays or heavily weathered so that Fe2+ is present in them rather than the red Fe3+ of martian surface samples. Martian sedimentary rocks and soil may be too friable to survive impactejection. Considering evidence for water flow on Mars’ surface, Martian meteorites are surprisingly dry, and evidence for desiccated salts is slight. Curiously, low initial (radiogenic) Sr and Nd isotopic data indicate that parent magmas in the martian mantle were depleted in heat-producing radionuclides (Fig. 14) relative to chondrites (Section 6.4). Shergottites are also depleted in light REE. Not surprisingly, we cannot specify sites from which martian meteorites derive. From crystallization ages, ALH 84001 apparently originated in the old (heavily cratered) southern highlands, while the other 31 come from the young (less-cratered) volcanic northern plains areas.

5. Chemical and Isotopic Constituents of Meteorites Earlier, we summarized meteorite compositions and genetic processes as necessary to understand general meteoritic properties. Here, we focus upon these topics in greater detail.

4.2 Martian Meteorites

5.1 Noble Gases

The 32 martian meteorites are unusual igneous meteorites (of five different types), and all but ALH 84001 crystal-

The chemical inertness of noble gases allows their ready separation from all other chemical elements. Thus, gas mass

CLBE001-ESS2E

November 14, 2006

3:54

272 Encyclopedia of the Solar System spectrometers can determine very small noble gas concentrations in a meteorite and, in addition, measure the isotopic composition. Most analyses are carried out on meteorite samples of H6, consistent with loss at progressively higher metamorphic temperatures in stratified parent(s). As discussed in Section 6.4, chronometric data also are consistent with this theory for H chondrite parent(s). Such a model cannot be established for the L chondrites because late shock (evident in the petrographic properties of many of them) affected other thermometric characteristics, thus obscuring earlier histories. In addition to the petrographic evidence, strongly shocked L4–L6 chondrites exhibit loss of some noble gases, highly mobile elements and siderophiles, and lithophile enrichments. Mean contents of Ag, Te, Zn, Cd, Bi, Tl, and In decrease in L4–6 chondrites with increasing shock-loading (and, therefore, residual temperature) estimated from petrographic shock indicators. Trace element contents of H chondrites do not vary with shock. In unshocked chondrites, volatile contents are significantly lower in H than in L chondrites, suggesting that L chondrite parent material formed from the nebula at lower temperatures than did H. Apparently, nebular temperatures during H chondrite parent material formation were so high (∼700 K) that only a very small complement of volatile trace elements could condense. Hence, essentially none was present to be lost later at high, shock-induced residual temperatures. The H chondrite regolith breccias, like Noblesville (Fig. 1a), differ from “normal” H chondrites in that the dark, gas-rich portions of the breccias are quite rich in volatile trace elements, sometimes exceeding C1 levels. These volatiles, distributed very heterogeneously in the dark matrix, were apparently not implanted by the solar wind but rather occur in black clasts. These black clasts represent either volatile-rich nebular condensate or a sink for material degassed from the parent body interior. During exposure on the asteroidal surface, these dark clasts and light ones (containing “normal” levels of volatiles) were apparently gardened by repeated impacts, ultimately forming the regolith breccia matrix. Less is known about equilibrated LL chondrites: They may have a unique thermal history or one like that of H or L chondrites. In contrast to ordinary chondrites, volatile trace elements in carbonaceous chondrites are very homogeneously distributed. These elements are unfractionated from each

other in almost all carbonaceous chondrites, implying that their parent material incorporated greater or lesser amounts of C1-like matter during accretion. The proportions define a continuum from 100% C1 down to about 20% in C5 or C6. As in enstatite chondrites, volatile-rich samples have higher proportions of more siderophile trace elements. These trends accord with oxygen isotope data, implying a continuum of formation conditions for parent materials of carbonaceous chondrites. Contents of mobile trace elements and noble gases, and the petrography of 15 C1–C3 chondrites (14 from Antarctica and 1 from a hot desert) provide unambiguous evidence for open-system thermal metamorphism in their parent bodies. These properties permit a semiquantitative metamorphic temperature in the 400–900◦ C range to be estimated for each of the 15. Each was dehydrated during metamorphism and none (including the 14 Antarctic chondrites) was rehydrated during terrestrial residence. As noted in Section 3.1, spectral reflection properties of these 15 thermally metamorphosed carbonaceous chondrites (and none of the more numerous “normal” ones) link them to C, G, B, and F asteroids. Petrographic properties of C1–C6 chondrites were established during nebular condensation and accretion. If C4–C6 or CK chondrites experienced thermal metamorphism, it occurred under closedsystem conditions. Enstatite chondrites present a special problem because nonvolatile siderophiles in them define high (EH) and low (EL) groups established during primary nebular and accretion. Prior to discoveries of desert meteorites, volatile element contents in E3,4 chondrites were known to be orders of magnitude higher than in E6. Whether this difference reflected primary or secondary processes was unclear since E3–E5 chondrites were EH and E6 were EL. Fortunately, Antarctic collections include previously unknown EL3 chondrites among others, and new data show that EL3 and EH3–EH4 chondrites contain comparable levels of the most volatile elements. These data suggest source regions of E3 and E4 chondrites, whether EL or EH, essentially reflect primary nebular condensation and/or accretion. Volatiles in E5 and especially E6 (whether EH or EL) are greatly depleted from E3 and E4 levels in a manner suggesting open-system loss during thermal metamorphism of their primitive parent(s). Data for these elements suggest further that enstatite achondrites derived from E6 chondrite-like material that previously experienced FeS– Fe eutectic loss (formation temperature, 980◦ C). Oxygen isotopic data for all enstatite meteorites (i.e., chondrites and achondrites) are similar (Fig. 11) with δ 18 O increasing systematically in E3–E6, independent of their being EH or EL. The oxygen isotopic compositions in EL chondrites lie along the terrestrial fractionation line (Fig. 11), but the distribution in EH chondrites falls along a line of slope 0.66, neither purely mass-dependent

CLBE001-ESS2E

November 14, 2006

3:54

Meteorites

6. Meteorite Chronometry How old are meteorites? An “age” is a time interval between two events marked by specific chronometers. An accurate chronometer must involve a mechanism operating on a predictable, but not necessarily constant rate. The “clock” starts by an event beginning the time interval and its end must be clearly and sharply recorded. Chronometers used in modern geo- and cosmochronology usually involve longlived, naturally occurring radioactive isotopes such as the Uisotopes, 87 Rb, or 40 K. Radioactive decay allows calculation of an age if the concentrations of both parent and daughter nuclide are known, the time interval beginning is defined, and the system is not disturbed (i.e., it is a “closed system”) during the time interval. Some meteorite ages involve production of particular stable or radioactive nuclides, or decay of the latter. Typically, the chronometer half-life should be comparable with the time interval being measured. Meteorites yield a variety of ages, each reflecting a specific episode in its history. Some of these are shown in Fig. 2: the end of nucleosynthesis in a star, the first formation of solids in the solar system, melt crystallization in parent bodies, excavation of meteoroids from these bodies, and the meteorite’s fall to Earth. Other events, like volcanism or metamorphism on parent objects can be established as can formation intervals (based on extinct radionuclides) mea-

suring the time between the last production of new nucleosynthetic material and mineral formation in early solar system materials. CRE ages date the exposure of a meteoroid as a small body (10 Ma, whereas all meteoritic materials possessing isochrons span ∼55 Ma. Apparently, the only systematic variation of the I–Xe formation interval with chondritic petrographic type involves E chondrites: EH chondrite parent material formed earlier than did EL. Clearly, while the nuclide 129 I was still alive (i.e., during or shortly after nucleosynthesis), primitive nebular matter condensed and evolved into essentially the materials that we now receive as meteorites. The conclusion is supported by other isotopic and charged-particle track evidence (see Sections 4.1, 4.2, 5.3, and 5.4). As we have seen from the foregoing summary, the meteoritic record can be read best in an interdisciplinary light. Results of one type of study—say, trace element chemical analysis—provide insight to another—orbital dynamics, for example. Early experience gained from meteorite studies, provided guidance for proper handling, preservation, and analysis of Apollo lunar samples. Studies of these samples, in turn, led to the development of extremely sensitive techniques now being used to analyze meteorites and microgram-sized interplanetary dust particles of probable cometary origin collected in Antarctica (Fig. 21) and just successfully brought to Earth by the Genesis spacecraft, despite its hard landing. Undoubtedly, this experience will prove invaluable as samples from other planets, their satellites, and small solar system bodies are brought to Earth for study. Previous studies of meteorites have provided an enormous amount of knowledge about the solar system, and there is no indication that the scientific growth curve in this area is beginning to level off. Indeed, work on the present version began late in 2004, and we were amazed to see how much had been learned about meteorites since 1998 when

CLBE001-ESS2E

November 14, 2006

3:54

282 Encyclopedia of the Solar System Further Reading

FIGURE 21 A French Southern and Antarctic Territories stamp illustrating a micrometeorite or cosmic dust particle (left) collected by melting Antarctic ice cores, the coring drill being at the right. Representations of meteor trails (of cometary origin) and a fireball are at the top.

the first edition of this Encyclopedia was published. Predictions about future developments are very hazardous, but we can expect future surprises, probably from desert meteorites, which seem to include so many peculiar objects. As has been said in another connection, those who work with meteorites don’t pray for miracles, they absolutely rely on them.

Buchwald, V. F. (1975). “Handbook of Iron Meteorites.” Univ. California Press, Berkeley. Dick, S. J. (1998). “Life on Other Worlds: The 20th-Century Extraterrestrial Life Debate.” Cambridge Univ. Press, Cambridge, England. Grady, M. M. (2000). “Catalogue of Meteorites,” 5th edition. Cambridge Univ. Press, Cambridge, England. Hewins, R. H., Jones, R. H., and Scott, E. R. D. (eds.) (1996). “Chondrules and the Protoplanetary Disk.” Cambridge Univ. Press, Cambridge, England. Hutchison, R. (2004). “Meteorites, A Petrologic, Chemical and Isotopic Synthesis.” Cambridge Univ. Press, Cambridge, England. Kerridge, J. F., and Matthews, M. S. (eds.) (1988). “Meteorites and the Early Solar System.” Univ. Arizona Press, Tucson. Papike, J. J. (ed.) (1998). “Planetary Materials.” Mineralogical Society of America, Washington, D.C. Porcelli, D. P., Ballentine, C. J., and Wieler, R. (eds.) (2002). “Noble Gases in Geochemistry and Cosmochemistry.” Mineralogical Society of America and Geochemical Society, Washington, D.C. Taylor, J., and Martel, L. (1996–present). Planetary Science Research Discoveries (PSRD). http://www.psrd.hawaii.edu Wooten, H. A. (2004). The 125 reported interstellar and circumstellar molecules. National Radio Astronomy Observatory. http://www.cv.nrao.edu/∼awootten/allmols.html

CLBE001-ESS2E

November 11, 2006

100-C

25-C

50-C

75-C

23:4

C+M

50-C+M

C+Y

50-C+Y

M+Y

50-M+Y

100-M

25-M

50-M

75-M

100-Y

25-Y

50-Y

75-Y

100-K

25-K

25-19-19

50-K

50-40-40

75-K

75-64-64

Near-Earth Objects

Lucy A. McFadden University of Maryland

Richard P. Binzel Massachusetts Institute of Technology

CHAPTER

1. Introduction 2. Significance 3. Origins

1. Introduction Near-earth objects (NEOs) reside in the vicinity of Earth near 1.0 AU (the mean distance between Earth and the Sun). Any object, such as an asteroid or comet, orbiting the Sun with a perihelion, q < 1.3 AU, well inside the orbit of Mars, is defined as an NEO. Aphelia, Q, of NEOs generally lie within a sphere of radius 5.2 AU, defined by Jupiter’s orbit. Among this broad group are four subgroups: Amors, Apollos, Atens, and interior Earth objects (IEOs). Comets, releasing gas and dust with q < 1.3 would be referred to as near-Earth comets (NECs) if they posed an impact threat to Earth. Amors approach but don’t cross the orbit of Earth. They have a semimajor axis, a > 1.0 AU, and perihelion 1.017 ≤ q = 1.3 AU, between the aphelion of Earth’s orbit and inside the perihelion of Mars (Fig. 1a). Those that actually cross Earth’s orbit, Apollos, have a > 1.0 AU and q = 1.017 AU, Earth’s aphelion distance (Fig. 1b). Atens have a ≥ 1.0 AU and q > 0.983 AU, Earth’s perihelion distance. An object with both a and q < 0.983 AU, is an IEO. The Amor asteroid, 433 Eros, was the first NEO discovered in 1898, by D. Witt of Berlin, Germany, using a photographic plate to record its position. It is also one of the largest NEOs, being 33 km in its longest dimension, with two other axes of 10.2 × 10.2 km diameter. 1862 Apollo, the first Earth-crossing asteroid, and 1221 Amor, the namesake

4. Population 5. Physical Properties 6. In Situ Studies

14

7. Impact Hazards Bibliography

of that group, were both discovered in 1932. It wasn’t until 44 years later that 2062 Aten, the first of the group orbiting within Earth’s orbit, was discovered by Eleanor Helin, still using photographic plates for the search. 1998 DK36 was the first IEO discovered in 1998. As the dynamical evolution of asteroids and their role in probably causing biological extinction events on the Earth was recognized in the 1980s, dedicated searches for NEOs resulted in increased discovery rates. Due to both increased sky coverage and availability of sensitive digital detectors, the known NEOs number >4100 at this writing, compared to 85 known in early 1980 (Fig. 2). About 25 of the NEOs found since the 1990s are binary objects orbiting around a common center of mass; 15% of all NEOs are estimated to be binaries. 1862 Apollo, an asteroid between 1.2 and 1.5 km in diameter, was reported to be a binary in 2005. Most of the near-Earth objects originated in the Main Asteroid Belt, located between Mars and Jupiter, although some of them probably evolved into their current orbits from the reservoir of short-period comets extending beyond Jupiter and into the outer solar system. The range of composition and physical characteristics of asteroid-like near-Earth objects spans those found among the Main Belt, though 15% of them probably are derived from cometary reservoirs. The Near-Earth Asteroid Rendezvous (NEAR) mission was the first designed to orbit an asteroid. 433 Eros was

C 2007 by Academic Press. All rights of reproduction in any form reserved. Encyclopedia of the Solar System 2e 

283

CLBE001-ESS2E

November 11, 2006

23:4

284 Encyclopedia of the Solar System Apollo

Aten

Amor

Semimajor Axis < 1.0 AU Perihelion < 1.02 AU Earth Crossing

Semimajor Axis < 1.0 AU Perihelion < 1.0167 AU Earth Crossing

1.02 AU < Perihelion < 1.3 AU

Inner Earth Objects (IEOs) Apohelion < 0.983 AU Always inside Earth’s orbit (aka Apohele)

Type Apollo Aten Amor Aphohel e

FIGURE 1 (a) Amors approach Earth but do not cross its orbit. (b) Apollo orbits cross that of Earth.

Population 62% of known 6% of known 32% of known Unknown number

its target, and the spacecraft remained in orbit from 2000 to 2001, ending its mission with a controlled descent and successfully becoming the first spacecraft to land on an asteroid. NEAR accomplished the first detailed in situ measurements of an asteroid’s surface morphology, mineralogy, chemistry, internal state, and magnetic properties. The Japanese-led Hayabusa mission was launched on May 9, 2003, on a 4 year mission to investigate asteroid 25143 Itokawa and to demonstrate the technology necessary to return samples to Earth. The spacecraft went into orbit around Itokawa in September 2005, performing remote sensing measurements for 3 months. The shape and surface morphology of this small near-Earth object is unlike any seen before. In November 2005, there were two scheduled touchdowns in which some surface material may have been collected. The return capsule is scheduled for a June

2010 return to Earth and will hopefully contain some surface material collected from Itokawa.

FIGURE 2 Cumulative total of discovered near-Earth objects versus time. Large NEOs are defined as those with an absolute magnitude (H) of 18 or brighter. (Data compiled by Alan Chamberlin, NASA/JPL.)

2.2 Hazard Assessment

2. Significance 2.1 Remnants of the Early Solar System From a scientific point of view, near-Earth objects are studied for the same reason as comets and main-belt asteroids: They are remnants of the early solar system (Fig. 3). As such, they contain information that has been lost in the planets through large-scale, planetary processes such as accretion, tectonism, volcanism, and metamorphism. Knowledge of the asteroids and comets as less processed material from the early solar nebula, studied together with direct samples in the form of meteorites, is critical to piecing together a scenario for the formation of the solar system. [See The Origin of the Solar System.] Most near-Earth objects are asteroid-like in their nature, being derived from the Main Belt. This region is a dividing point in the solar system, where the planets that formed closer to the Sun, the terrestrial planets, are dominated by rocky, lithophile material. Beyond the Asteroid Belt, the planets are composed predominately of nebula gases. Perhaps 10–20% of all near-Earth objects originated elsewhere in the solar system, such as the cometary reservoirs lying at great distances from the Sun, beyond the gaseous planets. Knowing about material from these reservoirs reveals information about both chemical and physical processes that were active in the outer regions of the solar system, both in the near and distant past. The objective of scientific study of the near-Earth objects is to determine which of them might be derived from which regions of asteroidal and cometary reservoirs.

Although chips, hand-sized rocks, and large boulders, all called meteorites, are continually landing on Earth [See

CLBE001-ESS2E

November 11, 2006

23:4

Near-Earth Objects

285

FIGURE 3 Clearing Out the Solar Nebula: The First Planetesimels. Painting by William K. Hartmann, reprinted with permission.

Meteorites], and astronomers find house-sized objects occasionally passing between Earth and the Moon, knowledge of the near-Earth objects, their locations, and physical and chemical characteristics is needed to inventory and assess their hazard potential to the Earth. Disastrous impacts by asteroids and comets have been the popular subject of Hollywood movies, books, newspaper articles, and television shows. The recognition that a giant asteroid or comet perhaps 10 km across most likely caused the extinction of the dinosaurs in a geological episode known as the Cretaceous–Tertiary Event has highlighted the potential for destruction should an energetic collision occur again (Fig. 4). Furthermore, as scientists analyze the energy involved in collisions, they realize that the impacts are tremendous and larger than anything created by human activities (e.g., nuclear weapons) or naturally occurring phenomena on Earth (e.g., volcanoes, earthquakes, or tsunamis).

Scientists ponder the results of computer simulations that consider the interactions of colliding objects with various Earth systems both natural and civilized. Coupled with these computer simulations is the very real phenomenon of the collision of comet Shoemaker–Levy 9 with Jupiter, which was observed worldwide through telescopes in 1994. The possibly devastating hazard posed to Earth if hit by a high-energy asteroid or comet is now well recognized by scientists and policy makers. One of the objectives of NASA’s Deep Impact mission, which sent an impactor spacecraft to collide with comet 9P/Tempel 1 in July 2005, was to study a comet nucleus and its interior and to assess the hazard to Earth of impact by a comet. When that analysis is complete, additional basic knowledge of comets will be available to assess what would happen should a new comet be found on a collision course with Earth. The most hazardous cometary impact would be one with a large orbital velocity

CLBE001-ESS2E

November 11, 2006

23:4

286 Encyclopedia of the Solar System

FIGURE 4 “Dinosaur’s Demise.” (Painting by Don Davis. Reprinted with permission.)

relative to Earth’s. [See Planetary Impacts; Cometary Dynamics.]

2.3 Exploration Destinations and Resource Potential NEOs come closer to Earth than any other planetary bodies. With low orbital inclinations and small semimajor axes, they are accessible targets for spacecraft. As humans extend their activities beyond low Earth orbit, relatively nearby destinations are attractive as training venues for missions to Mars. Considering the very long-term future in space one realizes that launching materials from Earth is expensive. As civilization moves beyond Earth, knowledge of materials in space is critical to their efficient use in situ. It is probably more economical to use space resources than transporting material from Earth (Fig. 5).

called accretion. Asteroids are planetesimals that were prevented from growing to the size of the major planets by pervasive eroding forces that counteract accretion, the net effect being to keep the asteroids relatively small. The formation of Jupiter was a major force in interfering with the growth of a larger planet between Mars and Jupiter at ∼2.8 AU. The details of the main-belt formation are not well known because it formed early in the history of the solar system, ∼4.5 billion years ago. Since the earliest formation times, gravitational interactions between planets and small objects (asteroids and comets) have resulted in perturbations of their orbits. These perturbations result in

3. Origins In the widely accepted scenario of the formation of the solar system, gas and dust collapse into a disk-shaped nebula from which planetesimals and eventually planets form. Planets grow after seeding conditions begin and molecules and dust grains form aggregates, which then form clumps that continue growing into objects large enough to be called planetesimals. This process starts with dust and ice grains about 1 mm in diameter. They behave at first as discrete particles sweeping up smaller grains as they grow. Both electromagnetic and gravitational forces come into play to overcome the destructive forces of erosion from particle collisions. Planet growth is gravitationally controlled and is

FIGURE 5 Painting showing the beginning of a mission to an Earth-approaching asteroid (Denise Watt, NASA).

CLBE001-ESS2E

November 11, 2006

23:4

Near-Earth Objects

the orbit, over time, evolving into one crossing a planet’s orbit, the subject of this chapter.

3.1 Relationship to Main Belt Asteroids Early asteroid studies in the 1940s revealed a range of colors (see Section 5). Techniques to study both reflected and emitted electromagnetic radiation from the asteroids were developed and used to derive information about their mineral and chemical composition. In the late 1970s, two scientists, Jonathan Gradie and Edward Tedesco, recognized that there is a relationship between the apparent composition of the asteroids and their distance from the Sun. This finding represented observational support for a model predicted by another astronomer, John Lewis, in which the solar nebula was in a state of chemical equilibrium when it formed. Asteroid composition changes as a function of temperature, and hence distance from the Sun. Therefore, one does not expect all asteroids to have the same composition. Furthermore, the exact nature of asteroidal material holds clues to the temperature and location where the material formed. This information is valuable as scientists piece together the scenario leading to the formation of our solar system and look for evidence of the existence of other solar systems. Studies of the composition of near-Earth objects led to the conclusion that NEO composition spans the range found among the Main Asteroid Belt, thus establishing that many or most of the NEOs are derived from the main belt. Follow-on research has confirmed these findings and identified the proportion that is derived from comets as ∼15%. Furthermore, physical information derived from NEOs can be reasonably considered to apply to Main Belt Asteroids. Statistical analysis of the evolution of many asteroid orbits over the age of the solar system indicates that the lifetime of an Earth-crossing body against gravitational perturbations is relatively short, on the order of 10 million years or less. Within this time frame, the bodies will either collide with a planet or be dynamically ejected from the solar system. This time interval applies to the average of the entire population and does not refer to the exact lifetime of any particular asteroid. It turns out that the orbital evolution of a specific asteroid or comet cannot actually be determined very far into the future or the past owing to the difficulty of knowing the exact starting conditions and accurately predicting frequent close approaches between the NEO and the planets. [See Solar System Dynamics: Regular and Chaotic Motion.]

3.2 Relationship to Meteorites Exploring the relationship between NEOs and meteorites is motivated by the possibility of making a very rich connection between the geochemical, isotopic, and structural informa-

287

tion on meteorites available from laboratory studies and the near-Earth objects. Meteorites fall to Earth frequently, but most often land unnoticed in the oceans or in remote areas. In January 2000, an exceptionally bright bolide was seen by eyewitnesses in the Yukon, Northern British Columbia, parts of Alaska, and the Canadian Northwest Territories. Nearly 10 kg of precious samples were recovered from the surface of frozen Tagish Lake. Using eyewitness reports and the bolide’s detection by military satellites, the orbit of the impacting body was traced back to the Asteroid Belt (Fig. 6). Prior to striking the Earth, the body is estimated to have been about 5 m across with a mass of 150 metric tons. [See Meteorites.] The determination of meteorite orbits serves as a constraint on the mechanisms that result in meteoroid delivery to Earth. Numerical computer simulations reveal regions of the Asteroid Belt that act as “escape hatches” for delivering material to the terrestrial planets zone. One such region corresponds to a Kirkwood gap, located where an asteroid’s orbital period is shorter than Jupiter’s by the ratio of two small integers, such as 3:1, 5:2, or 2:1. Any asteroid or debris that migrates into this gap finds Jupiter to be especially effective in increasing its orbital eccentricity. As the orbit becomes increasingly elongated, it can intersect the orbit of the Earth. In the 1980s, work by Jim Williams, Jack Wisdom, and others illuminated the importance and efficiency of resonances in the Asteroid Belt and their role in supplying meteorites.

3.3 Relationship to Comets Comets are predominantly icy and dusty objects that come from the outer reaches of the solar system. Their orbital periods are long, their orbital eccentricities are high, and they may have large or small orbital inclinations. What is their re¨ lationship to near-Earth objects? In the 1950s, Ernst Opik concluded that comets must be a partial source of nearEarth objects because he could not produce the number of observed meteorites from the Asteroid Belt alone via his cal¨ culations. Building on Opik’s work, George Wetherill predicted that 20% of the near-Earth object population consists of extinct cometary nuclei. Some now find evidence that the fraction of comets is smaller, closer to 15%. The hypothesis that NEOs derive from comets continues to merit consideration as knowledge of comets and asteroids increases and simulations of the dynamical evolution of interacting small bodies under the gravitational influences of the planets continues to develop. [See Cometary Dynamics; Physics and Chemistry of Comets.] Are there hints that any particular near-Earth object that looks like an asteroid was once a comet? If an object sometimes has a tail like a comet and sometimes looks just like an asteroid (no coma or tail), which is it: asteroid or comet? There is both dynamical and physical evidence that addresses this question.

CLBE001-ESS2E

November 11, 2006

23:4

288 Encyclopedia of the Solar System FIGURE 6 Orbit of Tagish Lake meteorite with other recovered meteorite orbits. (Credit: AAAS Science 13 October 2000, Vol. 290.)

3.3.1 TISSERAND PARAMETER

The first clue that an asteroid-like object may be a comet in disguise comes from its orbit. Examining orbital elements, asteroids and comets separate out readily when plotting orbital eccentricity versus semimajor axis (Fig. 7). Another way to characterize an orbit is to calculate its Tisserand parameter from the equation: T = aJ /a + 2[(a/aJ )(1 − e 2 )]1/2 cos i In this equation, a and aJ refer to the semimajor axis values for the object and Jupiter. The parameters i and e are the inclination and eccentricity of the object’s orbit. The Tisserand parameter is useful because it is a constant even if the comet’s orbit is perturbed by Jupiter. Also it helps describe whether an object is in an orbit that is strongly controlled by Jupiter or not. Most objects that display the characteristics of comets have a value T < 3, while most objects that are asteroid-like have T > 3. The value of T = 3 is represented by the solid line in Fig. 7. Objects with T < 3 are excellent candidates for being comets in disguise – they do

not currently display any telltale coma or tail because they are at present dormant or inactive.

3.3.2 DYNAMICAL AND PHYSICAL EVIDENCE FOR EXTINCT COMETS

A powerful way to investigate the mystery of how many extinct comets reside in the near-Earth object population is to explore both dynamical factors and physical measurements to identify possible candidates. For example, numerical simulations of the orbits of short-period comets can reveal how likely it is that gravitational interactions with Jupiter and the other planets can send them into the near-Earth object population. In these simulations, many thousands of hypothetical comets, each with slightly different initial orbits can be tracked for millions of years to see how they are tossed around chaotically by the gravitational tugs and pulls of the planets. In the same way, thousands of different starting places for main-belt asteroid orbits can be modeled to reveal the effectiveness of resonances for sending asteroids into near-Earth space. Alessandro Morbidelli, William Bottke,

CLBE001-ESS2E

November 11, 2006

23:4

Near-Earth Objects

289

exhibit the low albedo and color characteristics seen for comet nuclei. From the surveys searching for NEOs, correcting for the fact that for any given size, dark objects will be more difficult to detect than bright ones, about 30% of the all NEOs reside in T < 3 orbits. If one half of these are comet-like in their physical characteristics, this suggests up to 15% of all NEOs are extinct comet candidates. Other researchers find a smaller percentage of 5–15% derived from simulations of orbital dynamics. Until 2001 only upper limits on cometary activity were derived for the extinct cometary candidates. Object 2001 OG108 has an orbital period of 50 years and inclination almost perpendicular to the ecliptic plane, similar to that of Comet Halley. Upon its discovery, there was no detectable coma. At a distance of 1.4 AU, the object became active as it passed through the inner solar system. Its bare nucleus has the characteristics of cometary nuclei, and when close to the Sun, it outgases like a comet.

3.4 Meteor Shower Associations

FIGURE 7 Tisserand parameter. The solid line represents the Tisserand parameter with a value of 3. (Graph provided by Jeff Bytof, NASA/JPL.)

and co-workers have done extensive computer calculations to assess the relative effectiveness of these dynamical processes. Their calculations suggest that, when considering NEOs of all sizes, about 15–20% of all NEOs have their origins as comets. Nearly all of these are currently inactive, showing no evidence of a coma or a tail. They are comets disguised as asteroids. Spacecraft and telescopic measurements of known comets reveal what characteristics to look for when trying to determine if a given asteroid-like NEO is a comet in disguise. For example, the inactive surface regions of comets Halley, Borrelly, Wild 2, and Tempel 1 are very dark (low albedo) and have gray to reddish colors. Some other comets go through periods of very low activity, allowing astronomers to clearly see and measure the albedos and colors of the nucleus. All of these measurements consistently show low albedos (reflecting only about 4% or less of the incoming light) and gray or reddish colors. When observed in reflected sunlight, these objects exhibit featureless spectra with no absorption bands due to olivine or pyroxene (mineral types) on their surfaces. Knowing the dynamical signature (Tisserand parameter, T < 3), low albedo and gray/red color, allows asteroid-like NEOs to be identified as extinct comet candidates. A survey of nearly 50 NEOs residing in orbits having T < 3 conducted by one author (RPB) shows about one half of them

The near-Earth objects 2101 Adonis and 2201 Oljato have orbits similar to those of meteor showers. Adonis is very difficult to observe and not much is known about it. Oljato, also a difficult target for telescopes, has intrigued scientists since it was first observed in 1979. The jury is still out on whether or not this asteroid is an extinct comet, but the evidence now seems to suggest that it is asteroidal in its origin. One thing is certain: The object is not normal even when considered as an asteroid. In 1983, Fred Whipple recognized the orbital elements of an asteroid found by an Earth-orbiting infrared telescope to be essentially the same as the Geminid meteor shower, which occurs in mid-December. [See Infrared Views of the Solar System from Space]. There is little doubt that this asteroid, now named 3200 Phaethon, is the parent body of the Geminid meteors. But is Phaethon an extinct cometary nucleus? The supposition is yes, according to one line of thought based on similarities of orbital inclinations and the location of perihelion (longitude of perihelion relative to the ecliptic) of asteroids and comets compared to meteor showers. Its reflectance spectrum (see Section 5) is unlike other comet nuclei, however. There are currently nine NEOs that have orbital elements that, over the past 5000 years, may be associated with the path of existing meteor showers.

3.5 Dynamical History Dynamicists have simulated the pathways that objects might take from unstable regions of the Asteroid Belt using computations of dynamical forces acting in the solar system. In some cases, fragments from asteroid collisions may be violently cast into these regions of instability. However, a softer touch may play an even bigger role. Constant

CLBE001-ESS2E

November 11, 2006

23:4

290 Encyclopedia of the Solar System warming by the Sun causes asteroids of all sizes to reradiate their heat back into space. Because the asteroids are rotating, the reradiation does not occur in the same direction as the incoming sunlight, resulting in a small force acting on the asteroid. This force acts as a very gentle push on the asteroid, which over many millions of years can cause the asteroid to slowly drift inward or outward from its original main-belt location. This is called Yarkovsky drift and is especially effective on small objects; it may be particularly important for supplying meteoroids to Earth. Cast away fragments or drifting bodies that enter regions where resonances with Jupiter’s orbit are particularly strong, such as the 3:1 Kirkwood gap, find that small changes in the semimajor axis can result in large, exponential changes in other orbital elements, in particular eccentricity, changing the orbit significantly on a short timescale. Thus, the effects of chaotic regions are more than the sum of small changes in motion over long periods of time. These regions of chaotic motion are associated with resonances with both Jupiter and Saturn (Fig. 8). The two gas giant planets are believed to play a significant role in directing meteoroids to Earth, and presumably also many of the near-Earth objects. Other objects evolve from Jupiter-family comets or Halley-type short-period comets. Life in the Jupiter family is not long-lived, as Jupiter imparts changes to the orbits on timescales of 104 –106 years. Leaving Jupiter’s gravitational sphere of influence, the soon-to-be near-Earth objects may

FIGURE 8 Dynamical resonances are regions where gravitational interactions either deplete or protect asteroids from changes in their orbit. (From Jim Williams, NASA/JPL.)

sometimes be perturbed by Mars and other terrestrial planets and also affected by the influences of nongravitational forces, such as volatile outgassing or splitting of the cometary nucleus. These phenomena also contribute to orbital changes that result in planet-crossing orbits.

4. Population 4.1 Search Programs and Techniques Organized, telescopic search programs for near-Earth objects operate worldwide. The search programs supported by the National Aeronautics and Space Administration (NASA) include the Lincoln Near-Earth Asteroid Research (LINEAR) program, the Near-Earth Asteroid Tracking (NEAT) system, Lowell Observatory’s Near-Earth Object Search (LONEOS), the Catalina Sky Survey, and Spacewatch, the last two operated by independent teams at the University of Arizona. International efforts and interests are also strong at Japan’s National Space Development Agency (NASDA) and a joint venture among the Department of Astronomy of the University of Asiago, the Astronomical Observatory of Padua in Italy, and the DLR Institute of Space Sensor Technology and Planetary Exploration in BerlinAdlershof, Germany. Though the objectives of these programs are all similar, to inventory the objects in the vicinity of Earth, each has its own design and approach. In the past, when astronomers imaged the sky with photographic plates, it was an eye-straining process to compare them and determine if something moved. Search programs today employ digital imaging devices known as charge-coupled devices or CCDs that cover large areas of the sky in a single exposure. Typically a given area of sky is imaged and reimaged 3–5 times at intervals of 10 minutes to an hour. With digital images, fast computers can compare the images, identify and subtract all of the “uninteresting” objects that remain fixed, leaving behind the tracks of a moving asteroid or comet. By rapidly repeating this process for many patches of sky throughout a night, nearly the whole sky can be scanned in the course of about 2 weeks. Increasingly rapid and increasingly sensitive search systems are expected to come on line by the end of the decade. When a near-Earth object is first discovered, astronomers initially trace only a short piece of its orbit as measured over a few hours or even over a few weeks. With each new NEO discovery, astronomers wish to assess whether the object poses any immediate or future impact threat. Orbit calculations for most objects can be made reliably for many decades into the future, but of course if only a tiny part of the orbit has been observed, the extrapolation into the future becomes increasingly uncertain. Sometimes that extrapolation shows that the Earth itself resides within the overall uncertainty region for an NEO’s future position. If the cross section of the Earth occupies 1/10,000th of this

CLBE001-ESS2E

November 11, 2006

23:4

Near-Earth Objects

291

FIGURE 9 The Torino scale was designed for the press and public to assess the hazard of a discovered NEO.

space, then there is a 1 in 10,000 chance of an impact with the Earth. Even though headlines may proclaim the end of the world, statistically speaking, the odds are actually 10,000 to 1 in our favor that continued observations refining the orbit will show a collision is ultimately ruled out. Thus, daily activities should continue unchanged. Working with many colleagues, one of us (RPB) has developed the 10 point Torino scale (Fig. 9) to help the media and the public assess whether any NEO discovery merits public concern or response. Indeed, continued observations have ruled out any substantial threat from all previous headline makers. There

are currently two objects with a rating of 1 meriting careful monitoring, according to the Near Earth Object Program posting at http://neo.jpl.nasa.gov. The value of the searches is to change our knowledge from probably being safe to being highly certain about any threat from impacts for many generations.

4.2 How Many? It is difficult to quote the definitive size of the near-Earth object population. Search programs are constantly adding to

CLBE001-ESS2E

November 11, 2006

23:4

292 Encyclopedia of the Solar System the inventory, but there are inherent limitations in search techniques. Consider setting out to count the number of near-Earth objects. First, one can only look for them at night. At any one time, one can only search half the sky. Then there are limitations in how much sky one can cover in one night, controlled by the telescope field of view and the recording instrumentation. The realities of weather and equipment performance further hinder the search. The combination of these factors represents an estimate of what fraction of an expected population has been found for a range of size and brightness. To date, search programs have found more than 4100 near-Earth objects of all sizes. The biggest objects appear brightest and are most easily found. Searchers know of 30 NEOs as large as 5 km across and believe all objects this size and larger have been found. Around 300 objects have been cataloged that are larger than 2 km, and NEO catalogs are nearly complete at this size. Catalogs are known to be incomplete for objects smaller than 2 km, but by knowing how much area of the sky has been searched and how sensitive these searches have been, it is possible to estimate how many objects are left to find. A recent Ph.D. thesis by J. Scott Stuart carefully analyzed the search statistics from the LINEAR program, taking into account the different colors and reflectivities (albedos) that are typical for NEOs. Based on Stuart’s work, the best estimate is that there are about 1100 total NEOs larger than 1 km in diameter and up to 85,000 NEOs larger than 100 m (Fig. 10). When considering impact hazards on Earth, most scientists consider 1 km as the size large enough for an impact to present a global threat to human survival. Thus, current search efforts have as their most immediate goal to find all objects larger than 1 km. The good news is that more

than 870 of all cataloged NEOs are estimated to be 1 km or larger and thus astronomers are 80% toward completing the most immediate goal, and that may be reached in just a few more years. In the process, many smaller objects are found, and these begin to help bring completeness to all sizes. Searchers have a long way to go to complete the survey of all 85,000 objects that may be larger than 100 m; these may be capable of Tunguska-like (or somewhat greater) amounts of damage. Completing the surveys down to these sizes will require new, large, specialized telescopes with huge CCD arrays to scan the skies more frequently and with greater sensitivity. Another possibility would be to conduct the search using small telescopes in space.

5. Physical Properties The first physical measurement after the position of a nearEarth object is established is its brightness measured on the astronomical magnitude scale. The changing cross section of an object as viewed from Earth affects its brightness and with time reflects the shape and rotation rate of the object. Analysis of this changing brightness, accounting for the observational geometry, results in constraints on its shape and the determination of its rotation rate and orientation in space. From analyses of reflected sunlight off main-belt asteroid surfaces at different wavelengths, NEO colors are classified into different taxonomic types. [See Main-Belt Asteroids.] Further analysis can determine surface mineralogy, and, from that, constraints on the temperatures at which these objects formed can be made. The Near-Earth Asteroid Rendezvous mission studied the physical and chemical properties of asteroid 433 Eros from orbit and at the spacecraft’s landing site. From its shape and surface morphology, astronomers deduced information about its global structure. An X-ray and gamma ray spectrometer provided information about its surface chemistry. See Section 6 for details.

5.1 Brightness

FIGURE 10 diameter.

Estimated number of NEOs as a function of

The standard asteroid photometric magnitude system compensates for the distance and phase angle at which the object is observed. The magnitude scales by the inverse square law. As the distance from both the Sun and the observer increases, the brightness decreases by a factor equal to the inverse square of those distances. Scattering properties of the surface are expressed in the phase function, which is compensated for by extrapolating the magnitude to 0◦ phase. For comparison purposes, a magnitude measurement is converted to an absolute scale, H, which is defined as the brightness of an object at a distance of 1.0 AU from both the Earth and Sun, and viewed at 0◦ phase angle. The measured slope of brightness changes with phase, G, has

CLBE001-ESS2E

November 11, 2006

23:4

Near-Earth Objects

been measured for some of the brighter near-Earth objects. Large phase coefficients indicate a very rough surface with significant effects due to shadowing, such that the magnitude changes significantly with changing phase angle. Low values of G indicate either a very dark surface, where the impact of shadows is not significant against a dark surface, or that few scattering centers exist and hence there is minimal shadowing. When observations are made over a range of phase angles, fits to theoretical models with multiple variables can be made. Combined with other observational techniques (e.g., radar, polarimetry, lidar), constraints on the physical characteristics of the surface regolith can be made.

5.2 Configuration Lightcurves are measurements of brightness as a function of time (Fig. 11). If the object is perfectly spherical such that its cross section does not change with time, there will be no variation, and the lightcurve would be flat. There are no such objects known, although there are lightcurves with very small amplitudes (not commonly found among nearEarth asteroids). Lightcurves of NEOs often show two or more maxima and minima, often with inflections embedded within. The triaxial ellipsoid shape of each NEO can be modeled using observations. Inflections in the lightcurves represent changes in the object’s cross section that reflect either the large-scale shape or albedo variations across the surface or both. Radar measurements are also analyzed to produce images that reveal the shape of asteroids. Coded wave packets transmitted from Earth to an asteroid reflect back and are

FIGURE 11 Lightcurve for Amor asteroid 3908 Nyx indicating its irregular shape. (Courtesy of Petr Pravec, Astronomical Institute, Academy of Sciences of the Czech Republic.)

293

received as a radar echo. The bandwidth of the echo power spectrum is proportional to the cross section of the asteroid presented to Earth and normal to the line of sight at the time of interaction with the surface, convolved with Doppler shifts in the returned signals caused by the object’s rotation. The signal can be built up as the asteroid rotates, producing an image that represents its shape. For those objects that have approached Earth at close enough range to employ this technique, such as 4769 Castalia, 4179 Toutatis, 1627 Ivar, 1620 Geographos, and 433 Eros, the results show shapes varying from slightly noncircular to very irregular. [See Planetary Radar.] Knowledge of the objects’ shapes provides clues to the collisional history of this population. If all objects were spherical, astronomers would believe them to have formed from a viscous and rotating material that was not disturbed since formation. The fact that many near-Earth objects are irregularly shaped implies that they are products of collisions that have knocked off significant chunks of material from a larger body. Images of 433 Eros (Fig. 12) show it described as an ellipse measuring 33 × 10.2 × 10.2 km. Its shape is irregular and controlled by large impact craters.

FIGURE 12 Asteroid 433 Eros’s eastern and western hemispheres. Two mosaics created from 6 images when the NEAR spacecraft was orbiting 355 km (220 mi) above the surface. Smallest detail is 35 m (120 ft) across. The large depression on the top image is Himeros (10 km across). In the bottom image, the 5.3 km crater Psyche is prominent. Bright exposures can be seen on interior walls of craters. (Credit: NASA/JHU/APL.)

CLBE001-ESS2E

November 11, 2006

23:4

294 Encyclopedia of the Solar System Some near-Earth objects’ shapes have been interpreted as being two bodies stuck together and are referred to as a contact binary. This interpretation is intriguing because it leads to speculation that the two components were brought together in a low-velocity collision and just stuck together instead of one or both being destroyed. An alternative interpretation is that the asteroid is so irregularly shaped that it appears to be two pieces, but really is continuous. Such a situation would imply a history of collisional fragmentation that kept the main body of the asteroid intact, albeit severely altering its shape, but not disrupting it totally. Measurements at different aspect angles are required to truly confirm the interpretation that some objects are contact binaries. About 16% of near-Earth objects larger than 200 m in diameter may be contact binary systems according to estimates.

5.3 Rotation Rates Of 32 measured near-Earth objects with an average diameter of 3 km, the mean rotation rate is 4.94 ± 0.54 rev/day, whereas a sample of the same number of comparably sized, main-belt asteroids has a mean rotation rate of 4.30 ± 0.46 rev/day. Because the standard deviation of these means overlaps, no statistical significance is placed on these differences. The mean rotation rate of comets is larger than the mean of the NEOs. Comets rotate on average more slowly than NEOs. The implications of different rotation rates for the history of the object are discussed elsewhere. [See Asteroids.] Because of their proximity to Earth, NEOs are the smallest objects in space for which we can measure their rotational properties. In some cases, the rotation rates for NEOs smaller than about 150 m are 100 rev/day or faster (i.e., they have rotation periods of just a few minutes). These objects are likely relatively strong and intact rock fragments. Larger objects that spin substantially slower, may be less strong “rubble piles” composed of individual fragments or fractured rock held together only by gravity. A rubble pile must spin at a rate slower than once every 2.2 hours, or else it will fly apart. Thus, near-Earth objects give us insights into the likely range of internal structures occurring within small bodies in our solar system.

5.4 Size For an object illuminated by the Sun alone, the sum of the reflected and emitted (thermal) radiation from the object (assuming no internal energy sources or sinks) is equal to the total incident solar radiation upon it. Knowing where the object is, in terms of its distance from the Sun and the output of the Sun, the amount of incident energy on the object’s surface can be calculated. By measuring the reflected and reemitted (thermal) components of radiation, and with some rudimentary knowledge of the nature of the body’s surface materials determined from spectral measurements,

one can estimate its albedo and determine its diameter. The two parameters, diameter and albedo, are derived in tandem, with the requirement that the sum of reflected and emitted components is equal to the incident solar flux. This can be expressed mathematically as π R2 (F/r 2 )(1 − A) = 4π R2 εσ T 4 In this equation, R is the object’s mean radius and F is the solar flux, a constant. The distance from the Sun is r, and A is a term called the bolometric Bond albedo. The emissivity of the asteroid, ε, is assumed to be 1, and the parameter σ is the Stefan–Boltzmann constant. The temperature, T , is derived from the radiated flux from the asteroid measured in the thermal infrared spectral region. One can then solve for the bolometric Bond albedo, A, which is the integrated reflected light at all wavelengths. Albedo and diameter are calculated based on measurements of visible and infrared flux. Another method of estimating the size of small asteroids is from their measured brightness and an assumed albedo. This method is referred to as a photometric diameter. It is used when no thermal measurements and only visual magnitudes are available. The diameter is given by the equation log d = 3.1295 − 0.5 log pH − 0.2 Hv where pH , the geometric albedo, is assumed, and Hv is the magnitude defined by the International Astronomical Union magnitude system for asteroids in the V, or visual bandpass. Unfortunately, the range of asteroid albedos is large, from only a few percent up to 50% or more, producing considerable uncertainty in the photometric diameters. However, the taxonomic type of the asteroid (see below), determined from brightness measurements at several different wavelengths, can be used to narrow the range of probable albedos. Notice that an object with a lower albedo, reflecting the same amount of light, will be significantly larger than a high-albedo object. For example, a 15th magnitude object (on the bright end of any NEO) with an albedo of 0.15, an average, “bright” asteroid, would have a diameter of 3.4 km, whereas an asteroid with a 0.06 albedo, at the high end of the range of dark asteroids, would be 1.6 [5.4/3.4 = 1.588] times as large at 5.4 km. Keep in mind that the plot showing the frequency of near-Earth objects as a function of brightness and size (Fig. 10) provides only an estimate of the size and frequency of objects and, except at the large end of the magnitude scale, is an extrapolation and estimate of the size of the complete population.

5.5 Mass The mass of binary asteroids can be determined from Kepler’s third law, P 2 /a 3 = 4π 2 /G(Mp + ms ), where P is the period of revolution, a is the semimajor axis, both observed quantities. G is the universal gravitational constant,

CLBE001-ESS2E

November 11, 2006

23:4

Near-Earth Objects

and Mp and ms are the mass of the primary and secondary and are solved for as a sum. Using this expression, the mass of the binary near-Earth object 2000 DP107, for example, is calculated to be 4.6 ± 0.5 × 1011 kg, a little more than 1/1000th the mass of all living matter on Earth, estimated at 3.6 × 1014 kg. At least 25 other NEOs are known to be binaries. From Doppler and range measurements of the NEAR–Shoemaker spacecraft, the mass of Eros was measured to be 6.687 ± 0.003 × 1015 kg. For comparison, the Moon’s mass is more than 10 million times greater at 7.348 × 1022 kg. While the range of measured NEO masses spans four orders of magnitude, their total mass is small compared to the solar system’s total planetary mass.

5.6 Color and Taxonomy Since the early part of the 20th century, astronomers have recognized that small bodies come in different colors. As observational techniques evolved and the ability to investigate them improved, the number of observable characteristics increased. Sorting objects into meaningful groups is the process of classification or taxonomy. Asteroid taxonomy developed in response to advances in observing techniques and new technology in the field of stellar photometric astronomy. Current taxonomy is based on the application of statistical clustering techniques to the parameters of color and albedo. The intention of the classification scheme is to reflect the compositional variations and thus their origin and evolution. Astronomers are constantly attempting to test and refine the asteroid taxonomy by employing new statistical methods and extending the number of meaningful parameters that are included in the classification process, while eliminating meaningless or redundant parameters. Today, the alphabet soup of asteroid taxonomy extends to about 12 letters with subtypes numbering up to 26. The taxonomy too has evolved, and one has to be aware of which system is being used and what the exact definitions are. Bobby Bus presented a taxonomy in 1999 that has 26 classes. [See Asteroids.] Near-Earth objects have representatives from all taxonomic types except one, indicating that many locations in the Asteroid Belt feed the near-Earth population. Ninety percent of NEOs fall in the S-, Q-, C-, and X-complexes (a complex is a grouping of taxa from different instrument types and different taxonomies combined into a general category that can encompass all available observations). Two thirds of NEOs are bright and members of the S- (40%) or Q- (25%) complexes. When considering the observed ratio of dark objects to bright, there are almost four times as many bright objects observed compared to dark ones in the NEO population. However, darker objects are more difficult to discover and measure. Accounting for this discovery bias against darker objects is especially important when estimating how many extinct comets may be present in the near-Earth object population (Section 3.2).

295

5.7 Mineralogy By measuring the percentage of reflected sunlight from the surface of an object, it is possible to constrain its surface mineralogy. This technique was pioneered by Tom McCord and his students and colleagues in the 1970s. In 2006, spectral reflectance measurements of over 200 NEOs were available. The inventory is still growing. Astronomers find that 65% of near-Earth objects contain two strong absorption bands, one in the ultraviolet with a band centered below 0.35 μm and the other in the near infrared near 1 μm. Sometimes a second near-infrared band is observed at a wavelength of 2 μm. Other objects do not have prominent absorption bands: They are found to be featureless and either flat or sloped. Most often these featureless objects also have a low albedo. Figure 13 shows spectral reflectance measurements of some near-Earth objects. Three spectra have prominent ultraviolet and near-infrared

FIGURE 13 Spectral reflectance measurements of four NEOs. The range of spectra reflects the range of surface characteristics including mineralogy and particle sizes of the surface material.

CLBE001-ESS2E

November 11, 2006

23:4

296 Encyclopedia of the Solar System absorption bands that are common in silicate minerals. The broad band at 1 μm of asteroid 5641 McCleese is diagnostic of a mineral called olivine, which consists of silicon oxide tetrahedra bound in eightfold symmetry by magnesium, calcium, and iron cations. Subtle differences in the position of the center of the band constrain the chemistry of the olivine, which can accommodate a range of magnesium and iron in its mineralogical structure. The presence of a second absorption near 2 μm indicates that a second silicate, pyroxene, is present. The spectrum of 433 Eros contains both olivine and two types of pyroxene. Detailed spectral analysis and modeling suggest the presence of an additional component that may be a glassy material, or possibly vapor-deposited coatings of nanometer size iron grains. They are inferred because the brightness of the spectrum is lower than mixtures of only crystalline silicates. These mineral constituents are present in ordinary chondrite meteorites; the deviation from ordinary chondritic composition and the processes controlling that have been studied and ascribed to space weathering and/or partial melting. The spectrum of asteroid 3908 Nyx (Fig. 13) is dominated by pyroxene and has the same spectral characteristics as the basaltic achondrite meteorites. This asteroid may have traveled to the near-Earth region of space over the age of the solar system and may be a fragment of the large main-belt asteroid, 4 Vesta. [See Main-Belt Asteroids.] The lower spectrum in Fig. 13 is characteristic of a subgroup of C-types, labeled B. There is no UV absorption and not much of an infrared absorption. Interpretation of this spectrum is uncertain. This asteroid, 3200 Phaethon, is a candidate for an extinct comet, though its albedo (9–11%) is higher than most comets observed to date (∼4%). Mineralogical studies of near-Earth objects show that they are not all alike. Nor are they alike in the Main Asteroid Belt. The range of variations in mineral composition reflects that seen in the Main Asteroid Belt, indicating that NEOs are mostly derived from the Main Belt. None of the NEOs are compositionally similar to any of the major planets because they do not share any of the spectral reflectance characteristics of the major planets or the Moon. NEOs with low albedo, featureless spectra with higher IR reflectance relative to the UV, might be extinct cometary nuclei.

6. In Situ Studies 6.1 NEAR The Near-Earth Asteroid Rendezvous spacecraft was launched from Cape Canaveral, Florida, on February 16, 1996, on a 3 year journey to asteroid 433 Eros. NEAR orbited Eros for 1 year in 2000–2001, training its 6 scientific instruments on the asteroid’s surface. It provided the first

detailed characterization of a NEO’s chemical and physical properties. The objective was to study Eros’ relationship to meteorites, the nature of its surface and collisional history as well as aspects of its interior state and structure. The spacecraft carried a complement of instruments covering the electromagnetic spectrum. The magnetometer measured no magnetic field down to its detection limit of 1–2 nano-Teslas (Earth’s magnetic field measures 50,000 nano-Teslas). A possible explanation for this unexpected result is that magnetic material within Eros is randomly oriented to the point of canceling all fields. If this is the case, then there has been no heating of the asteroid to the point of producing any preferred orientation of any magnetic material. Orbital imaging of Eros revealed an irregularly shaped body dominated at the global scale by both convex and concave forms, including a 10 km diameter depression named Himeros, and a 5.3 km bowl-shaped crater named Psyche (Fig. 12). At scales of 1 km to 100 m, (Fig. 14) there are grooves and ridge patterns superimposed on a heavily cratered surface, mostly covered by overlapping craters. At the 4◦ .

sample, it turned out that this is not true. It has been shown that more distant objects should have been discovered by now, unless either (1) the Kuiper Belt population steeply decays in number beyond 48–50 AU or (2) the maximal size of the objects beyond this limit is much smaller than that in the observed Kuiper Belt. For various reasons, astronomers tend to favor hypothesis (1): the existence of a physical outer edge of the Kuiper Belt. An important issue is to understand which of the orbital properties discussed earlier is due to the dynamical processes that are still occurring in the Kuiper Belt or not. For instance, do the eccentricities and the inclinations slowly grow due to some dynamical phenomenon? Are the low eccentricity objects beyond 48 AU unstable? If these are the cases, then the existence of large eccentricities and inclinations, as well as the outer edge of the Kuiper Belt could be simply explained. In the opposite case, these properties—like the existence of the extended scattered disk—reveal that the solar system was different in the past. Dynamical astronomers have studied in great detail the dynamics beyond Neptune, using numerical simulations and semianalytic models. Figures 4 and 5 show maps of the dynamical lifetime of trans-Neptunian bodies on a wide range of initial semimajor axes, eccentricities, and inclinations. These maps have been computed numerically, by simulating the evolution of thousands of massless particles under the gravitational perturbations of the giant planets. The latter have been assumed to be initially on their current orbits. Each particle was followed until it suffered a close encounter with Neptune. Objects encountering Neptune, would then evolve in the scattered disk for a time of order ∼108 years, until they are transported by planetary encounters into the inner planets region, or are ejected to the Oort cloud or to interstellar space. This issue is described in more detail in Section 6. In Fig. 4, the colored strips indicate the length of time required for a particle to encounter Neptune as a function of its initial semimajor axis and eccentricity. The initial inclination of the particles was set equal to 1◦ . Strips that are colored yellow represent objects that survive for the length of the simulation, 4 × 109 years, the approximate age of the solar system. As can be seen in the figure, the Kuiper Belt can be expected to have a complex structure, although the general trends are readily explained. Objects with perihelion distances less than ∼35 AU (shown as a red curve) are unstable, unless they are near, and presumably librating about, a mean-motion resonance with Neptune (Section 2). Indeed, the results in Fig. 4 show that many of the Neptunian mean-motion resonances (shown in blue) are stable for the age of the solar system. Objects with semimajor axes between 40 and 42 AU are unstable. This is presumably due to the presence of three overlapping secular resonances that occur in this region of the solar system: two with Neptune and one with Uranus.

CLBE001-ESS2E

November 8, 2006

14:19

Kuiper Belt: Dynamics

595

FIGURE 4 The dynamical lifetime for small particles in the Kuiper Belt derived from 4 billion year integrations by M. Duncan, H. Levison, and M. Budd. Each particle is represented by a narrow vertical strip of color, the center of which is located at the particle’s initial eccentricity and semimajor axis (initial orbital inclination for all objects was 1◦ ). The color of each strip represents the dynamical lifetime of the particle. Strips colored yellow represent objects that survive for the length of the integration, 4 × 109 years. Dark regions are particularly unstable on these timescales. For reference, the locations of the important Neptune mean-motion resonances are shown in blue and two curves of constant perihelion distance, q, are shown in red. The orbital distribution of the real objects is also plotted. Big dots correspond to objects with i < 4◦ , and small dots to objects with larger inclination. Remember that the dynamical lifetime map has been computed assuming i = 1◦ .

Indeed, secular resonances appear to play a critical role in ejecting particles from this region of the Kuiper Belt. This can be better seen in Fig. 5, which is an equivalent map, but plotted relative to the initial semimajor axis and inclination for particles with initial eccentricity of 0.01. Also shown are the locations of the Neptune longitude of perihelion secular resonance (in red) and the Neptune longitude of the ascending node secular resonance (in yellow). It is important to note that much of the clearing of the Kuiper Belt occurs where these two resonances overlap. This includes the low inclination region between 40 and 42 AU, which is indeed depleted of bodies (compare with Fig. 2). The Neptune mean-motion resonances are also shown (in green). It is interesting to compare the numerical results to the current best orbital elements of the known Kuiper Belt objects. This comparison is also made in Fig. 4, where the observed objects with good orbital determination are overplotted with green dots. Big dots refer to bodies with I < 4◦ , consistent with the low inclination at which the sta-

bility map has been computed. Small dots refer to objects with larger inclination and are plotted only for completeness. The conclusion is that most observed objects (with the exception of scattered disk bodies) are associated with stable zones. Their orbits do not significantly change over the age of the solar system. Thus, their current excited eccentricities and inclination cannot be obtained from primordial circular and coplanar orbits in the framework of the current planetary system orbital configuration. Likewise, the region beyond the 1:2 mean-motion resonance with Neptune is totally stable. Thus, the absence of bodies beyond 48 AU cannot be explained by current dynamical instabilities. Therefore, it is evident that the orbital structure of the Kuiper Belt has been sculpted by mechanisms that are no longer at work, but presumably were active when the solar system formed. The main goal of dynamical astronomers interested in the Kuiper Belt is to uncover these mechanisms and from them deduce, as far as possible, how the solar system formed and early evolved.

CLBE001-ESS2E

November 8, 2006

14:19

596 Encyclopedia of the Solar System FIGURE 5 The dynamical lifetime for test particles with initial eccentricity of 0.01 derived from 1 billion year integrations by M. Duncan, H. Levison, and M. Budd. This plot is similar to that of Fig. 4 except that coordinates are semimajor axis and inclination (instead of semimajor axis and eccentricity) and a different color table was used for the solid bars. In addition, the red and yellow curves show the locations of Neptune longitude of perihelion secular resonances (v8 ) and the Neptune longitude of the ascending node secular resonances (v18 ), respectively. The green lines show the location of the important Neptune mean motion resonances.

4. Correlations Between Physical and Orbital Properties The existence of two distinct classical Kuiper Belt populations, called the hot (i > 4◦ ) and cold (i < 4◦ ) classical populations, could be caused in one of two general manners. Either a subset of an initially dynamically cold population was excited, leading to the creation of the hot classical population, or the populations are truly distinct and formed separately. One manner in which we can attempt to determine which of these scenarios is more likely is to examine the physical properties of the two classical populations. If the objects in the hot and cold populations are physically different, it is less likely that they were initially part of the same population. The first suggestion of a physical difference between the hot and the cold classical objects came from the observation that the intrinsically brightest classical belt objects (those with lowest absolute magnitudes) are preferentially found with high inclination. Figure 6 shows the distribution of the classical objects in an inclination vs. absolute magnitude diagram. As one sees, for an absolute magnitude H > 5.5, there is a given proportion between the number of objects discovered in the cold and the hot populations respectively. This ratio is completely different for H < 5.5, where cold population objects are almost absent. All the biggest classical objects, such as, for instance, 50000 Quaoar, 20000 Varuna, 19521 Chaos, 28978 Ixion, 2005 FY9 , and 2003 EL61 have inclinations larger than 5◦ . Their median inclination is 12◦ . It has been argued that this is a result

of an observational bias because the brightest objects have been discovered in wide field surveys not confined around the ecliptic, which are thus more likely to find large inclination objects than the deep ecliptic surveys that detected the fainter bodies. However, a recent survey for bright objects, which covered ∼70% of the ecliptic, found many hot classical objects but few cold classical objects, confirming that the effect illustrated in Fig. 6 is real. The second possible physical difference between hot and cold classical Kuiper Belt objects is their colors. With the name “color” astronomers generically refer to the slope of the spectrum of the light reflected by a trans-Neptunian object at visible wavelengths, relative to that of the light emitted by the Sun. “Red” objects reflect more at long than at short wavelengths, while “gray” objects have a more or less uniform reflectance. Colors relate in a poorly understood manner to objects’ surface composition. It has been shown and repeatedly confirmed that, for the classical belt, the inclination, and possibly the perihelion distance, is correlated with color. In essence, the low inclination classical objects tend to be redder than higher inclination objects. More interestingly, colors naturally divide into distinct red and gray populations at precisely the location of the divide between the inclinations of the hot and cold classical objects. These populations differ at a 99.9% confidence level. Interestingly, the cold classical population also differs in color from the Plutinos and the scattered objects at the 99.8 and 99.9% confidence level, respectively, while the hot classical population appears identical in color to these other populations. The possibility remains, however, that the colors of the objects, rather than being markers of different populations,

CLBE001-ESS2E

November 8, 2006

14:19

Kuiper Belt: Dynamics

597

FIGURE 6 The inclination of the classical Kuiper Belt objects as a function of their absolute magnitude. The horizontal dashed line at i = 4◦ separates the cold from the hot population. The vertical dotted line is plotted at H = 5.5. The distribution on the left side of the dotted line is clearly different from that on the right-hand side. The largest classical objects are all in the hot population.

are actually caused by the different inclinations. For example, it has been suggested that the higher average impact velocities of the high inclination objects could cause large-scale resurfacing by fresh water ice and carbonaceous materials, which could be gray in color. However, a similar color-inclination trend should be observed also among the plutinos and the scattered disk objects, which is not the case. A careful analysis shows that there is no clear correlation between average impact velocity and color. In summary, the significant color and size differences between the hot and cold classical objects imply that these two populations are physically different in addition to being dynamically distinct.

5. Size Distribution of the Trans-Neptunian Population and Total Mass As briefly described in Section 1, the disk out of which the planetary system accreted was created as a result of the Sun shedding angular momentum as it formed. As the Sun condensed from a molecular cloud, it left behind a disk of material (mostly gas with a little bit of dust) that contained a small fraction of the total mass but most of the angular momentum of the system. It is believed that the initial solid objects in the protoplanetary disk were pebble-sized, of the order of centimeters in size. These objects formed larger objects through a process of accretion to form asteroids and comets, which in turn accreted to form planets (or the cores of the giant planets which then accreted gas directly from the solar nebula). Understanding this process is one of the main goals of astronomy today.

There are few clues in our planetary system about this process. We know that the planets formed, and we know how big they are. Unfortunately, the planets have been so altered by internal and external processes that they preserve almost no record of their formation process. Luckily, we also have the Asteroid Belt, the Kuiper Belt, and the scattered disk. These structures contain the best clues to the planet formation process because they are regions where the process started, but for some reason, did not run to completion (i.e., a large planet). Thus, the size distribution of objects in these regions may show us how the processes progressed with time and (hopefully) what stopped them. The Kuiper Belt and the scattered disk are perhaps the best places to learn about the accretion process. Because the size of the object is not a quantity that can be easily measured (one needs to make hypotheses on the intrinsic reflectivity of the objects, or albedo), and the absolute magnitude is readily obtained from the observations, astronomers generally prefer the absolute magnitude distribution, instead of the size distribution. The magnitude distribution is usually given in the form Log N(< H) ∝ H a , a > 0 where N is the cumulative number of objects brighter than absolute magnitude H. The slope of this distribution, a, contains important clues about the physical strengths, masses, and orbits of the objects involved in the accretion process. For example, there are two extremes to the accretion process. If two large, strong objects collide at low velocities, then the amount of kinetic energy in the collision is

CLBE001-ESS2E

November 8, 2006

14:19

598 Encyclopedia of the Solar System FIGURE 7 The cumulative magnitude distribution of the cold population (red) and of the hot population and scattered disk (green) according to a recent analysis by G. Bernstein and collaborators. A turnover of the magnitude distribution is detected around H ∼ 10. The slope of the magnitude distribution is very uncertain beyond this limit.

small compared to the amount of energy holding the objects together. In this case, the objects merge to form a larger object. If two small, weak objects collide at high velocities, then the energy in the collision overpowers the gravitational and material binding energies. In this case, the objects break apart, forming a large number of much smaller objects. In realistic models of the Kuiper Belt with a range of sizes and velocities, we expect small objects to fragment and large objects to grow. This produces a size distribution with a ∼ 0.4−0.5 at small sizes and a much steeper slope at large sizes where accretion is important. Statistics of discoveries of Kuiper Belt objects (Fig. 7) suggest that the absolute magnitude distribution is indeed very steep for H < 9 (approximately equivalent to a diameter D > 100 km), with a ∼ 0.6−0.7, and then turns over toward a significantly shallower slope. Interestingly, the hot and the cold classical population seems to have two different values of a in the steep part. More precisely, the hot population and the scattered disk have a shallower magnitude distribution than the cold population (Fig. 7). This is consistent with the fact that the largest bodies are all in the hot population, and yet the hot and cold populations and the scattered disk contain roughly the same number of bodies bigger than 100 km. The value of a in the shallow part of the magnitude distribution beyond H ∼ 10 is very uncertain. Only few surveys with the most powerful telescopes could probe this region, but they have discovered very few objects. The results are therefore affected by small number statistics. It is possible that a < 0.5 in some magnitude range. In fact, in the

Asteroid Belt, the magnitude distribution is wavy, and the canonical values of 0.4–0.5 of a is only a mean value. It is possible that the magnitude distribution in the Kuiper Belt is wavy as well, and that the range 10 < H < 14 corresponds to the very shallow part of one of these waves. It is possible to integrate under the magnitude distribution shown in Fig. 7 in order to estimate the total mass in the Kuiper Belt between 30 and 50 AU. Such an integration with limits between R = 1 km and 1200 km (the approximate radius of Pluto) and assuming a density of 1 g cm−3 , shows that the total mass is a few hundredths of an Earth mass. Given the uncertainties, it is possible that the mass is of order of 0.1 M⊕ , but not significantly larger. As with many scientific endeavors, the discovery of new information tends to raise more questions than it answers. Such is the case with the preceding mass estimate. Edgeworth’s and Kuiper’s original arguments for the existence of the Kuiper Belt were based on the idea that it seemed unlikely that the disk of planetesimals that formed the planets would have abruptly ended at the current location of the outermost known planet. An extrapolation into the Kuiper Belt (between 30 and 50 AU) of the current surface density of nonvolatile material in the outer planets region predicts that there should originally have been about 30 M⊕ of material there. However, as stated previously, our best estimate is over 200 times less than that figure! Edgeworth’s and Kuiper’s argument is not the only indication that the mass of the primordial Kuiper Belt had to be significantly larger in the past. Models of collisional accretion show that it is not possible for objects with radii

CLBE001-ESS2E

November 8, 2006

14:19

Kuiper Belt: Dynamics

greater than about 30 km to form in the current Kuiper Belt, at least by pairwise accretion, over the age of the solar system. The current surface density of solid material is too low to accrete bodies larger than this size. However, the models show that objects the size of 1992 QB1 could have grown in a more massive Kuiper Belt (provided that, as already said in Section 3, the mean orbital eccentricities of the accreting objects were much smaller than the current ones). A Kuiper Belt of at least several Earth masses is required in order for 100 km sized objects to have formed. The same applies even more strongly to the accretion of Pluto and Charon. For those two bodies to have grown to their current sizes in the trans-Neptunian region, there must have originally been a far more massive Kuiper Belt. A massive and dynamically cold primordial Kuiper Belt is also required by the models that attempt to explain the formation of the observed numerous binary Kuiper Belt objects. Therefore, the general formation picture of an initial massive Kuiper Belt appears secure, and understanding the ultimate fate of the 99% (or 99.9%) of the initial Kuiper Belt mass that appears to be no longer in the Kuiper Belt is a crucial step in reconstructing the history of the outer solar system.

6. Ecliptic Comets As described in Section 1, the current renaissance in Kuiper Belt research was prompted by the suggestion that the Jupiter-family comets originated there. We now know that there are mainly two populations of small bodies beyond Neptune: the Kuiper Belt and the scattered disk. Which one is the dominant source of these comets? To answer this question, we need to examine a few considerations on the origin of the scattered disk. We have seen in Section 3 that the bodies in the scattered disk have intrinsically unstable orbits. The close encounters with Neptune move them in semimajor axis, until they either evolve into the region with a < 30 AU or reach the Oort cloud at the frontier of the solar system. In both of these cases, the bodies are removed from the scattered disk. Despite this possibility of dynamical removal, we still observe scattered disk bodies today. How can this be? There are a priori two possibilities. The first one is that the scattered disk population is sustained in a sort of steady state by the bodies escaping from the Kuiper Belt. This means that on a timescale comparable to that for the dynamical removal of scattered disk bodies, new bodies enter the scattered disk from the Kuiper Belt. For example, a similar situation occurs for the population of near-Earth asteroids (NEAs). NEA dynamical lifetimes are only of a few million years because they intersect the orbits of the terrestrial planets. Nevertheless, the population remains roughly constant because new asteroids enter the NEA population

599

from the Main Asteroid Belt at the same rate at which old NEAs are eliminated. The second possibility is that the scattered disk that we see today is only what remains of a much more numerous population that has been decaying in number since planetary formation. Numerical simulations show that roughly 1% of the scattered disk bodies can survive in the scattered disk for the age of the solar system. Thus, the primordial scattered disk population should have been about 100 times more numerous. Which of these possibilities is true? In the first case, we would expect that the Kuiper Belt is much more populated than the scattered disk. For instance, the Asteroid Belt contains about 1000 times more objects than the NEA population, at comparable sizes. However, observations indicate that the scattered disk and the Kuiper Belt contain roughly the same number of objects. Thus, the second possibility has to be true. Scattered disk objects most likely formed in the vicinity of the current positions of Uranus and Neptune. When these planets grew massive, they scattered them away from their neighborhoods. In this way, a massive scattered disk of about 10 M⊕ formed. What we see today is just the last vestige of that primordial population, which is still decaying in number. The fact that the scattered disk is not sustained in steady state by the Kuiper Belt, but it is still decaying, implies that the scattered disk provides more objects to the giant planet region (a < 30 AU) than it receives from the Kuiper Belt. Thus, the outflow from the scattered disk is more important than the outflow from the Kuiper Belt. This implies that the scattered disk, not the Kuiper Belt, is the dominant source of Jupiter family comets. A significant amount of research has gone into understanding the dynamical behavior of objects that penetrate into the a < 30 AU region from the scattered disk. These studies show that the encounters with the planets spread them throughout the planetary system. These objects are usually called ecliptic comets, even if at large distances from the Sun they typically do not show any cometary activity. The distribution of these objects as predicted by numerical integrations is shown in Fig. 8. The ecliptic comets that get close to the Sun become active. When their semi-major axis is smaller than that of Jupiter, they are called Jupiter-family comets. It is somewhat surprising that about a third of the objects leaving the scattered disk in the simulations spend at least some of their time as Jupiter-family comets. The Jupiter-family comets that we see today are, in majority, small, R 15◦ , large eccentricities, e > 0.3, and large semimajor axes, a > 45 AU. Centaur objects are on outer planet crossing orbits with q > 5.2 AU and a < 30.1 AU. Relatively recent gravitational interactions between SDOs and Neptune, and to a lesser extent between classical KBOs and Neptune, result in Centaur objects. Because Centaur objects cross the orbits of the outer planets, they are dynamically unstable and have mean lifetimes of ∼106 years. As mentioned above, some Centaurs evolve into Jupiter-family comets, others are ejected from the Solar System, and yet others impact the giant planets. In addition, some Jupiter-family comets evolve back into Centaurs.

the Sun has V = −26.74 and the faintest star visible in the sky with the unaided eye has V ∼ 6.

5.2 Luminosity Function There are many more faint KBOs than bright KBOs. Figure 4 comes from KBO discoveries made by a number of surveys, and shows the number of KBOs per unit magnitude per square degree on the sky near the ecliptic plane as a function of brightness (R-band magnitude), a luminosity function. Surveys find ∼100 KBOs with 27 < R < 28, ∼2 KBOs with 23 < R < 24, and only ∼0.001 KBOs with 19 < R < 20, all per square degree of sky. For reference, the full Moon occupies ∼ one-quarter of a square degree of sky and the Sun has R = −27.10.

5.3 Absolute Magnitude The apparent magnitude of a KBO or Centaur depends on its heliocentric distance, r, and geocentric distance, , in AU. For example, a KBO receding from the Sun and Earth will become fainter and its apparent magnitude will become larger in value. The absolute magnitude, H, of a KBO is a way to compare the intrinsic brightness of one KBO with another KBO and it does not depend on distance. The absolute magnitude of the same KBO receding from the Sun and Earth will not change. The absolute magnitude of a KBO is the brightness it would have if it were located at a distance of 1 AU from the Sun and 1 AU from the Earth, and had a Sun-KBO-Earth (phase) angle, α, of 0◦ . The relation

5. Brightness 5.1 Apparent Magnitude The first physical property measured for a KBO is typically its brightness. A KBO is brightest in visible light (4000– ˚ by virtue of the sunlight it reflects toward the Earth. 8000 A) It is possible to isolate the brightness of a KBO in a particular bandpass by placing a colored glass filter in front of a CCD camera at the focal plane of a telescope. For example, a blue, green, or red filter in front of a CCD camera makes it possible to measure the brightness of blue, green, ˚ V or red light from a KBO,i.e., its B (λcenter = 4500 A), ˚ ˚ (λcenter = 5500 A), or R (λcenter = 6500 A) magnitudes. Table 2 lists V magnitudes of the brightest KBOs. At the other extreme of brightness, Gary Bernstein used the Hubble Space Telescope (HST) to discover and measure the brightness of the faintest known KBO, V ∼ 28. The Centaur in Figure 3, 1994 TA, has V = 24.31 ± 0.05. For comparison,

FIGURE 4 Number of KBOs per unit magnitude interval per square degree of sky vs. R-band magnitude. There are many more faint KBOs than bright KBOs. This is typical of small body populations in the Solar System that have been collisionally processed. (Courtesy of Gary Bernstein)

CLBE001-ESS2E

November 11, 2006

22:40

Kuiper Belt Objects: Physical Studies

between absolute magnitude, Hv , and apparent magnitude, V, is given by Hv = V − 5 log(r) + 2.5 log [(1 − G)1 (α) + G2 (α)] , where the last term of the equation is an empirical phase function that describes how Hv of an object varies with phase angle. G = 0.15 and 1 and 2 given by  i (α) = exp −Ai



1 tan α 2

 Bi 

where i = 1 and 2, A1 = 3.33, B1 = 0.63, A2 = 1.87, and B2 = 1.22 seem most appropriate for KBOs. Hv values for discovered KBOs and Centaurs range from about −1 to 15. Table 2 lists KBOs with the brightest Hv values.

6. Diameter Size is among the most fundamental physical properties of an astronomical object, yet we are only beginning to get accurate diameter measurements for KBOs and Centaurs. The most direct way to measure the diameter of a KBO, D (in km), is to measure its angular diameter, θ (in arc sec), and geocentric distance,  (in AU). Geometry gives D = 727θ. Unfortunately, KBOs and Centaurs have sufficiently small values for D and sufficiently large values for  that the resulting values for θ are too small for measurement even by the HST. Michael Brown pushed the HST to its limits and measured θ = 0.0343 ± 0.0014 arc sec for the KBO Eris, which was at a geocentric distance of 96.4 AU at the time of their observations. They found a diameter of 2400 ± 100 km for Eris, making it slightly larger than Pluto, D = 2302 km. For KBOs and Centaurs with θ too small for measurement, it is possible to estimate their diameters from their brightness, pD2 = 9x 1016r 2 2 100.4(m−V ) , where, as before, r is the heliocentric distance in AU and  is the geocentric distance in AU, m is the V-band brightness of the Sun (−26.74), V is the brightness of the KBO, p is the albedo of the object, and  = [(1 − G)1 (α) + G2 (α)] . Since Jupiter-family comets come from Centaurs and the Kuiper Belt, most KBO diameter estimates assume an

609

albedo similar to albedo measurements for a handful of Jupiter-family comets, i.e., p = 0.04. Diameter estimates from V magnitudes for about 100 objects range between D = 25 km for 2003 BH91 to D = 2400 km for Eris. KBO and Centaur object diameters on the scale of Figure 1 range from the tiniest specks to Pluto. The assumption of a comet-like albedo, although reasonable, is dangerous because Jupiter-family comets come much closer to the Sun than KBOs and Centaur objects. The frequent close proximity of short-period comets to the Sun results in the sublimation of H2 O ice and produces surfaces largely covered by a dark, refractory-rich, lag deposit. The surfaces of Jupiter-family comets may have chemical and physical properties quite different from the surfaces of Centaurs and KBOs. Charon has a relatively large albedo of 0.37. If we assume that a KBO has p = 0.04, but it is actually has p = 0.4, we will estimate a diameter that is more than three times too large. Measurements of albedos are essential for accurate measurements of KBO and Centaur object diameters.

7. Albedo By measuring the brightness of sunlight reflected from a KBO at visible wavelengths and the brightness of heat emitted by the same KBO at thermal infrared wavelengths, it is possible to disentangle albedo from diameter, and thereby measure separate values for both quantities. The Spitzer Space Telescope, an infrared telescope in orbit about the Sun, is enabling John Stansberry of the University of Arizona, Dale Cruikshank of NASA’s Ames Research Center, William Grundy of Lowell Observatory, and John Spencer of Southwest Research Institute to observe much fainter levels of heat from KBOs and Centaurs than is possible with telescopes on the Earth. As a result of their work, we have accurate diameters and albedos for more than a dozen KBOs (Table 2).

8. Brightness Variation KBOs and Centaurs may have weak internal constitutions (i.e., rubble pile type interiors) due to fracturing by past impacts between objects. In other words, it is possible that KBOs and Centaurs are nearly strengthless bodies, held together primarily by their own self-gravity. If so, then some objects may deform from spheres into triaxial ellipsoids with axes a > b > c as a result of their rotation. The rotation of an ellipsoid can result in periodic variation of its projected area on the sky and hence a periodic variation of the sunlight it reflects and its brightness (Fig. 5). Monitoring such a brightness variation can result in a wealth of physical data about the object (e.g., its period of rotation, shape, and perhaps even its density and porosity).

CLBE001-ESS2E

November 11, 2006

22:40

610 Encyclopedia of the Solar System

TABLE 2 Name

Triton Eris Pluto

KBO Magnitudes, Albedos, and Diameters1 Number

Prov Des

136199 134340 136472

2003 UB313

136108 90377 90482 50000 55637 55565 90568 20000 28978 38628 47171 15874 15789 15875 29981

2003 EL61 2003 VB12 2004 DW 2002 LM60 2002 UX25 2002 AW197 2004 GV9 2000WR106 2001 KX76 2000 EB173 1999 TC36 1996 TL66 1993 SC 1996 TP66 1999 TD10

2005 FY9

Charon Sedna Orcus Quaoar

Varuna Ixion Huya

V2

H3v

p4v

D5

13.5 18.7 14.0 17.0 15.9 17.5 21.1 19.3 19.2 19.9 20.2 19.8 20.1 19.9 19.5 19.6 20.9 22.4 21.1 21.1

−1.2 −1.1 −0.7 0.1

75 >70 61 70–90 37 55–75 >8.5 27 12 10 12 15 14 19 6.6 7.9 >1.8 3.5 1.1 5.3

2707 1.7. Surprisingly, there are no Centaurs with 1.3 < B-R < 1.7. (b) All 21 objects of a sample of classical KBOs with q > 40 AU, e < 0.1 and e < 10◦ are all red (B-R > 1.5). (c) A sample of 20 SDOs are mostly gray (B-R < 1.5). The mechanisms responsible for these correlations between color and orbital properties are not well understood yet.

CLBE001-ESS2E

November 11, 2006

22:40

Kuiper Belt Objects: Physical Studies

615

9.1.4 REASONS FOR COLOR PATTERNS

What could cause these color signatures? One possibility is the radiation-reddening and impact-graying mechanism discussed earlier in Section 1. However, such a mechanism should result in a uniform distribution of B-R colors for Centaurs and not two clusters of B-R colors. In addition, gray impact craters and their ejecta blankets would be randomly distributed on the surface so that one hemisphere might have more than another, resulting in measurable color changes as the object rotates. However, repeated and random measurements of individual rotating KBOs and Centaurs give the same B-R color. Also, extensive observations of Pholus suggest that it has a highly homogeneous surface color. Figures 10a and 10b show the R-band brightness and B-band brightness of Pholus as a function of a single rotation phase taking 9.980 hr. Figure 10c is the difference of 10a from 10b, yielding the B-R color across the entire surface of Pholus as it makes one rotation about its axis. The solid horizontal line is the average of the points. The dashed lines are plus or minus one standard deviation, σ = 0.04. Any variation in the B-R surface color of Pholus must be smaller than 0.04 magnitude (4%). Again, there is no evidence of gray impact craters on a radiation-reddened surface. Another possibility is that the colors of KBOs are the remaining signature of a temperature-induced, primordial composition gradient. The small, rocky terrestrial planets (Mercury, Venus, Earth, and Mars) close to the Sun and the giant, hydrogen-rich gas giant planets (Jupiter, Saturn, Uranus, and Neptune) farther away from the Sun are the result of such a gradient. In the inner Solar System, temperatures were so high that only metal and rock forming elements could condense from the nebular gas to form small, rocky, and metal-rich solids. At and beyond the orbit of Jupiter, the hydrogen-dominated nebular gas was cold enough for the H2 O to condense out. We may be seeing a similar effect on the colors of KBOs and Centaurs. We now suspect KBOs did not all form at about the same distance from the Sun. Perhaps the red classical KBOs formed farther out in the nebula where it was cold enough to hang on to their CH4 ice reddening agent. Perhaps the gray KBOs formed closer to the Sun and were not able to hang on to their CH4 ice reddening agent. Additional work is necessary to figure out whether the radiation-reddening and collisional-graying mechanism, the temperature-gradient mechanism, or some other mechanism is responsible for the colors of KBOs and Centaurs.

9.2 Spectroscopy There are only a handful of KBOs and Centaurs that are known to exhibit ice absorption bands in their spectra. H2 O-ice bands are seen in the spectra of Charon, 19308 (1996 TO66 ), Varuna, Quaoar, Orcus, Pholus, and Chariklo. CH4-ice bands are seen in the spectra of Pluto, Neptune’

FIGURE 10 Homogeneous B-R surface color of Pholus. (a) R-band magnitude vs. rotation phase. The x-axis spans a time interval of 9.980 hr. (b) B-band magnitude vs. rotational phase. (c) Difference between above two panels yield B-R color vs. rotational phase. The solid line is the average of the 94 points. The dashed lines are plus or minus one standard deviation, σ , of 0.04 magnitude. Any variation in the surface color of Pholus as it completes one rotation on its axis must be less than 0.04 magnitude (4%). Pholus exhibits a homogeneous surface color.

CLBE001-ESS2E

November 11, 2006

22:40

616 Encyclopedia of the Solar System

FIGURE 11 Near-infrared reflection spectrum of Quaoar (black) compared to a spectrum of H2 O ice (red). The broad absorption bands near 1.5 μm and 2.0 μm reveal the presence of H2 O ice on the surface of Quaoar. The narrow absorption band near 1.65 μm indicates the presence of crystalline H2 O ice and is not present in amorphous ice. (Courtesy of David Jewitt and Jane Luu)

satellite Trition, which may be a captured KBO, Eris, and 136472 (2005 FY9 ). Perhaps one of the most intriguing spectroscopic results comes from David Jewitt and Jane Luu’s observations of Quaoar. Specifically, they find not only the H2 O ice bands at 1.5 and 2.0 μm, but they also find another H2 O band at 1.65 μm (Figure 11). The later band suggests the surprising result that the H2 O-ice has a crystalline rather than an amorphous structure. The H2 O molecules of crystalline ice have a periodic structure whereas the H2 O molecules of amorphous ice do not. Crystalline H2 O on Quaoar is a surprise because Quaoar’s maximum surface temperature is only ∼50◦ K. At such a low temperature, it is difficult for the H2 O molecules to arrange themselves into a coordinated structure of a crystal lattice; somewhere around 100◦ K, amorphous ice arranges itself into an ordered crystalline lattice. In other words, the 1.65-μm band suggests that the H2 O-ice on Quaoar was somehow heated to temperatures above 100◦ K. An intriguing possibility for the source of the “warm” H2 O on Quaoar is NH3 -H2 O volcanism. Long ago, longlived radioactive elements heated the interior of Quaoar, and that heat may still be propagating through its interior. The heat may have been sufficient to create a melt of H2 O and NH3 . The lower density melt may have percolated upward, perhaps forming fluid-filled cracks all the way or nearly all the way to the surface in the surrounding, higher density icy-rock mixture. Eventually, the cooling “lava” containing crystalline H2 O ice and crystalline am-

FIGURE 12 Near-infrared spectrum of Quaoar compared to near-infrared spectra of Pluto and Charon. The spectra of Quaoar and Charon are similar in that they exhibit three strong H2 O-ice absorption bands at 1.5 μm, 1.65 μm, and 2.0 μm, but no CH4 -ice bands. The spectrum of Pluto exhibits strong CH4 ice bands. (Courtesy of David Jewitt and Jane Luu)

monium hydrate might become exposed by occasional impacts on Quaoar’s surface. What makes this mechanism even more intriguing is that Jewitt and Luu claim there is evidence for an ammonia hydrate band in their spectra of Quaoar. Ammonia-water volcanism as the source of the crystalline H2 O ice is highly speculative. Some other mechanism, not requiring a warm interior and volcanoes, may explain the presence of the crystalline H2 O ice on Quaoar. Figure 12 illustrates that Quaoar has a spectrum similar to Charon, but quite different from Pluto. Quaoar and Charon exhibit the 1.5- and 2.0-μm H2 O-ice bands as well as the 1.65-μm crystalline band, but none of the strong CH4 ice bands seen on Pluto. Note that Quaoar has the 1.65-μm band despite having a larger semimajor axis, a = 43.6 AU, than Pluto, a = 39.8 AU. Another intriguing spectroscopic result comes from Javier Licandro’s observations of 136472 (2005 FY9 ). He finds that CH4 -ice bands in the spectra of 136472 (2005 FY9 ) are much deeper than the CH4 -ice bands in the spectra

CLBE001-ESS2E

November 11, 2006

22:40

Kuiper Belt Objects: Physical Studies

617

10.1 System Mass Two KBOs of a binary pair revolve about their common center of mass. However, it is far more convenient to observe the position of the fainter of the two components as it makes a complete revolution about the brighter component on the plane of the sky, i.e., to observe the apparent relative orbit. Figure 14 illustrates the apparent relative orbit of 1998 WW31 . The true orbit of the KBO binary system will not happen to lie exactly in the plane of the sky. Hence, the apparent relative orbit is merely a projection of the true relative orbit onto the plane of the sky. Techniques exist to determine the inclination of the true orbit relative to the plane of the sky. Once the period of revolution, P, and the semimajor axis, a, of the true relative orbit are known, it is possible to use Kepler’s Third Law to calculate the combined mass of the binary system, FIGURE 13 Optical spectrum of 136472 (2005 FY9 ) (black line) and a Hapke model of pure CH4 -ice (red line). The CH4 absorption bands of 136472 (2005 FY9 ) are blue shifted by 3.25 ± 2.25 A˚ relative to the pure CH4 model indicating the presence of another molecular ice, possibly N2 , CO, or Ar.

of Pluto, implying that the abundance of CH4 on the surface of 136472 (2005 FY9 ) could be higher than on the surface of Pluto. This author finds the CH4 -ice bands in his spectrum and Javier Licandro’s spectrum of 136472 (2005 FY9 ) are blueshifted by 3.25 A˚ relative to the positions of pure CH4 ice bands (Figure 13). Such a shift suggests the presence of another ice component on the surface of 136472 (2005 FY9 ), possibly N2 -ice, CO-ice, or Ar. In addition, Licandro finds CH4 -ice bands blueshifted in a spectrum of Eris. It is odd that some KBOs exhibit strong CH4 bands and others exhibit strong H2 0 bands. Pluto and Charon are part of the same system, yet they exhibit very different spectra. Perhaps the difference is due to Pluto’s size, it may have experienced some form of methane ice volcanism. In the end, we may find only the largest KBOs exhibit CH4 -ice bands. Eris, Pluto, and possibly 2005 FY9 are the three largest KBOs and they all exhibit CH4 -ice bands.

10. KBO Binaries In 2001, Christian Veillet announced the discovery of two components to the KBO 1998 WW31 . Over the next few years, Keith Noll used the superior imaging resolution of HST to observe 122 KBOs for additional binaries. His survey was sensitive to binaries with separations ≥0.15 arc sec and a magnitude difference between components ≤1 magnitude. Noll discovered six more binaries. Currently, 22 KBO binaries are known (Table 4).

m1 + m2 =

4π 2 a 3 . GP2

From the HST observations in Figure 14, Veillet and Noll found that 1998 WW31 has a true relative orbit with a semimajor axis of 22,300 km, an eccentricity of 0.8, and a period of revolution of the fainter component about the brighter component of 574 days. The 1998 WW31 system has a combined mass of 2.7 × 1018 kg, much smaller than the Pluto-Charon system combined mass of 1.46 × 1022 kg. Table 4 lists the true relative orbital properties and combined masses for the better studied binary systems.

10.2 Mutual Events Between 1985 and 1990, Pluto’s orbital motion about the Sun caused the Pluto-Charon orbital plane to sweep through the line of sight to the Earth. As a result, mutual eclipses (also known as mutual events) occurred every 3.2 days (half of Charon’s orbital period). Because of the mutual events, observers were able to accurately measure diameters of 2302 ± 12 km and 1186 ± 26 km for Pluto and Charon, and with the total mass of the binary, they were able to derive an average density for the system of 1.95 ± 0.10 g cm−3 . A key objective of current binary KBO work is to discover as many binaries as possible and to determine their orbits sufficiently well to predict when the onset of mutual events will occur. By observing KBO mutual events, we will obtain radii and density measurements that only a spacecraft encounter could improve upon. At present, no KBO binary orbit (other than Pluto and Charon) is known well enough to predict the onset of a mutual event with confidence.

10.3 Origin of KBO Binaries Two of the most unusual features of KBO binaries, compared to main belt asteroid and near-Earth asteroid binaries,

CLBE001-ESS2E

November 11, 2006

22:40

618 Encyclopedia of the Solar System

TABLE 4

Binary KBOs1

Name

Resonant Pluto/Charon

Number

Prov Des

134340 47171 26308

1999 TC36 1998 SM165

134860 88611 80806 79360 66652 58534

2005 EO304 2003 UN284 2003 QY90 2001 QW322 2000 CQ114 2000 CF105 1999 OJ4 1998 WW31 2000 OJ67 2001 QT297 2000 CM105 1997 CS29 1999 RZ253 1997 CQ29

136199 136108 82075 48639

2001 QC298 2003 UB313 2003 EL61 2000 YW134 1995 TL8

a2

e3

Period4

Mass5

19,636 7,640 11,310

0.0076

6.38722 50.4 130

14,710 13.9 6.78

22,300

0.82

574

2.7

27,300

0.240

825

2.3

4,660 8,010

0.46 0.45

46.263 312

3.7 0.42

19.2

10.8

49.12

4,200

Classical

Scattered Eris

3,690 49,500

0.05

1

Courtesy Keith Noll. Semimajor axis in km. 3 Eccentricity. 4 Period in days. 5 Mass in units of 1018 kg. 2

FIGURE 14 Binary KBO. The apparent orbit of the fainter component of 1998 WW31 relative to the brighter component on the plane of the sky. (Courtesy of Christian Veillet, Keith Noll, and NASA)

are the wide separation and similar diameter of each pair of components. These unusual features make it unlikely that collisions between two KBOs created each binary system, as in the case of the Earth and the Moon. Similarly, it isn’t likely that one KBO gravitationally captured another KBO to form a binary system. A mechanism put forth by Stuart Weidenschilling suggests that it is possible to create a loosely bound KBO binary by collision and capture in the presence of a third body. His mechanism requires many more KBOs than are seen today; perhaps such a mechanism operated long ago in a more densely populated Kuiper Belt (see the next section). Peter Goldreich put forth a mechanism wherein capture takes place during a close encounter as a result of the dynamical friction with the many surrounding small bodies. Each of these mechanisms produces its signature on the population of binaries we see today. For example, Weidenschilling’s mechanism favors the production

CLBE001-ESS2E

November 11, 2006

22:40

Kuiper Belt Objects: Physical Studies

of wide binary pairs, and Goldreich’s mechanism favors the production of closer pairs. Only the discovery of many more binaries will allow us to determine whether either of these mechanisms or some other mechanism is responsible for the formation of KBO binaries.

11. Mass of the Kuiper Belt What’s the mass of the entire Kuiper Belt? Gary Bernstein combined his HST survey for the faintest KBOs with ground-based telescope surveys for brighter KBOs, and assumed KBOs have an albedo of 0.04 and a density of 1 g cm−3 , to estimate a Kuiper Belt mass of ∼3 percent of the Earth’s mass, or about 14 times the mass of Pluto. A major source of uncertainty in his mass estimate is the uncertainty in the albedos and densities of KBOs. It appears that the Kuiper Belt did not always have a mass of ∼3 percent of the Earth’s mass. Specifically, the present number of KBOs per AU3 is too small to grow KBOs larger than ∼100 km in diameter by accretion in less time than the age of the Solar System. Since 1000-km sized KBOs exist, it is likely that the Kuiper Belt initially had many more KBOs per AU3 than today. Calculations by Alan Stern suggest that the initial Kuiper Belt probably had a mass ten times the mass of the Earth, and as Neptune grew to a fraction of its present size, it stirred KBOs from their initial circular orbits to more eccentric orbits, resulting in frequent disruptive, rather than accretive collisions especially between KBOs smaller than 40 to 60 km in diameter. These collisions probably eroded the Kuper belt mass down to its current value.

12. New Horizons Because astronomers can discover and then measure the physical properties of many KBOs, their work is important because it gives us a global view of the Kuiper Belt and context for in situ spacecraft measurements. In January of 2006, NASA’s New Horizons spacecraft departed Earth on a journey that will culminate in the first flyby of the Pluto-Charon system in 2015, and hopefully the first flyby of a KBO sometime before 2020. The $500 million spacecraft weighs only 416 kg (917 lb) and has four instrument packages: (1) a CCD camera, (2) an ultraviolet, optical, and near-infrared imaging-spectrometer, (3) a charged particle detector, and (4) a radio telescope. These instrument packages will provide in-depth observations impossible with telescopes on and near the Earth. For example, if New Horizons comes within a few thousand kilometers to a KBO, it could image the surface of the KBO with a resolution of 25 m pixel−1 . For comparison, HST can only image a KBO at 42 AU with a resolution of about 1200 km pixel−1 .

619

What kind of surface might the spacecraft image? If New Horizons visits a small KBO, perhaps it will image a surface with numerous craters, suggestive of an ancient surface bombarded by other small bodies (KBOs and comets) over the age of the Solar System? On the other hand, if New Horizons visits a large KBO, perhaps it will see few craters on the surface, suggestive of some process erasing older craters. Perhaps the images of a large KBO will show long linear features in an icy crust, and some roughly round basins that appear flooded by liquids from the interior, much like the Voyager spacecraft images of Triton. Perhaps the spacecraft will catch a geyser erupting, and shooting a plume of gas and ice above the surface. There are some problems concerning a New Horizon’s flyby of a KBO. The spacecraft trajectory is fixed since first it will fly by Pluto. In addition, the spacecraft has a limited fuel supply for adjusting its trajectory after the Pluto encounter. At present, none of the almost 1000 currently-known KBOs are close to the spacecraft’s trajectory. A flyby of a KBO by New Horizons depends on discovering a candidate close to the spacecraft’s trajectory. Perhaps New Horizons will have enough fuel to visit one of the smaller (50 km diameter) and more common KBOs. The chances for the spacecraft visiting one of the larger (1000 km diameter) and rarer KBOs appear slim at the moment.

13. Future Work It is likely that future work on the physical properties of KBOs and Centaurs will be driven by future state-of-theart observatories. The 6-m James Webb Space Telescope (JWST) near the L2 point will be able to obtain images and spectra of very large numbers of KBOs and Centaurs from 0.6 to 27 μm. It should be possible to measure diameters, albedos, surface colors, and optical and infrared spectra for many more objects than possible today. A large increase in the number of objects with physical property measurements will make it possible to look for statistically significant correlations between many more physical properties than possible with today’s telescopes, and thereby better constrain the important formation and evolution mechanisms in the outer Solar System. Large ground-based telescopes of the future will likely play a big role in the field too. For example, the Giant Magellan Telescope (GMT), a configuration of six off-axis 8.4m mirror segments around a central on-axis segment that is equivalent to a filled aperture 21.4 meters in diameter, and the Thirty Meter Telescope (TMT), a configuration of more than 700 hexagonal-shaped mirror segments that is equivalent to a filled aperture 30 meters in diameter, will make it possible to obtain higher signal precision optical and infrared spectra than possible with current 10 meter telescopes. Better spectra and models will make it possible to map surface concentration of ices (e.g., the CH4 /N2

CLBE001-ESS2E

November 11, 2006

22:40

620 Encyclopedia of the Solar System concentration) as a function of depth and as a function of rotational phase over the surfaces of numerous objects. Such measurements will provide a wealth of data for constraining cosmochemistry models of the outer Solar System. Finally, the Atacama Large Millimeter Array (ALMA), a configuration of about sixty-four 12-meter antennas located at an elevation of 16,400 feet in Chile, may reveal extra-solar Kuiper Belts for comparison with our Kuiper Belt. ALMA may provide density and temperature profiles as well as chemical measurements through the detection of spectral lines in

the belts. ALMA may initiate a new field of study, comparative Edgeworth-Kuiper Belt object ology, i.e., comparative EKO-logy.

Bibliography Davies, J. (2001). Beyond Pluto: Exploring the Outer Limits of the Solar System. Cambridge University Press. Jewitt, D. (2005). Kuiper Belt. http://www.ifa.hawaii.edu/ faculty/jewitt/kb.html

CLBE001-ESS2E

November 9, 2006

100-C

25-C

50-C

75-C

16:13

C+M

50-C+M

C+Y

50-C+Y

M+Y

50-M+Y

100-M

25-M

50-M

75-M

100-Y

25-Y

50-Y

75-Y

100-K

25-K

25-19-19

50-K

50-40-40

75-K

75-64-64

Solar System Dust

¨ Eberhard Grun Max-Planck-Institut fur ¨ Kernphysik Heidelberg, Germany Hawaii Institute of Geophysics and Planetology Honolulu, Hawaii

CHAPTER

1. Introduction 2. Observations

S

olar system dust is finely divided particulate matter that exists between the planets. Sources of this dust are larger meteoroids, comets, asteroids, the planets, and their satellites and rings; there is interstellar dust sweeping through the solar system. These cosmic dust particles are also often called micrometeoroids and range in size from assemblages of a few molecules to tenth- millimeter-sized grains, above which size they are called meteoroids. Because of their small sizes, forces additional to solar and planetary gravity affect their trajectories. Radiation pressure and the interactions with ubiquitous magnetic fields disperse dust particles in space away from their sources. In this way, micrometeoroids become messengers of their parent bodies in distant regions of the solar system. Because of their small sizes, a tablespoon of finely dispersed micrometersized dust grains scatter about 10 million times more light than a single meteoroid of the same mass. Therefore, a tiny amount of dust becomes recognizable, while the parent body from which it derived may remain undetected.

1. Introduction One of the earliest known phenomena caused by solar system dust is the zodiacal light. Zodiacal light is a prominent light phenomenon that is visible to the human eye in the morning and evening sky in nonpolluted areas (Fig. 1). Already in 1683, Giovanni Domenico Cassini presented the correct explanation of this phenomenon: It is sunlight

3. Dynamics and Evolution 4. Future Studies

34

Bibliography

scattered by dust particles orbiting the Sun. The relation to other “dusty” interplanetary phenomena, like comets, was soon suspected. Comets shed large amounts of dust, visible as dust tails, during their passage through the inner solar system. The genetic relation between meteors and comets was already known in the 19th century. Meteoroids became the link between interplanetary dust and the larger objects: meteorites, asteroids, and comets. Cosmic dust can have different appearances in different regions of the solar system. It consists not only of refractory rocky or metallic material as in stony and iron meteorites, but also of carbonaceous material; dust in the outer solar system can even be ice particles. Individual dust particles in interplanetary space have much shorter lifetimes than the age of the solar system. Several dynamic effects disperse the material in space and in size (generally going from bigger to smaller particles). Therefore, interplanetary dust must have contemporary sources, namely, bigger objects like meteoroids, comets, and asteroids in interplanetary space but also planetary satellites and rings. In addition there are dust particles immersed in the local interstellar cloud through which the solar system currently passes that penetrate the planetary system. Dust is often a synonym for dirt, which is annoying and difficult to quantify. This is also true for interplanetary dust. Astronomers who want to observe extra–solar system objects have to fight separating the foreground scattered light from the zodiacal light. Theoreticians who want to model

C 2007 by Academic Press. All rights of reproduction in any form reserved. Encyclopedia of the Solar System 2e 

621

CLBE001-ESS2E

November 9, 2006

16:13

622 Encyclopedia of the Solar System

FIGURE 1 A wedge of interplanetary dust. The dusk twilight sky (pink) toward the northwest shows zodiacal light (blue), framed by the Pleiades (upper left), Comet Hale–Bopp (upper right), and Mercury in Aries (left of center above horizon). (Courtesy M. Fulle.)

interplanetary dust have the difficulty of representing these particles by simplified models, for example, a spherical particle of uniform composition and optical properties of a pure material. True interplanetary dust particles can be very different from these simple models (Fig. 2). Another practical aspect of dust is its danger to technical systems. A serious concern of the first spaceflights was the hazard from meteoroid impacts. Among the first instruments flown in space were simple dust detectors, many of which were unreliable devices that responded not only to impacts but also to mechanical, thermal, or electrical interference. A dust belt around Earth was initially suggested, which was dismissed only years later when instruments had developed enough to suppress this noise by several orders of magnitude. Modern dust detectors are able to reliably measure dust impact rates from a single impact per month up to a thousand impacts per second. In the early days of spaceflight, measures were taken to protect spacecraft against the heavy bombardment by meteoroids. The bumper shield concept found its ultimate verification in the European Space Agency’s Giotto mission to comet Halley. This spacecraft was designed to survive impacts of particles of up to 1 g mass at an impact speed of 70 km/s. These grains carry energies comparable to cannon balls that are 1000 times more massive. Heavy metal armor was not possible because spacecraft are notoriously

lightweight. The Giotto bumper shield combined a 1-mmthick aluminum sheet positioned 23 cm in front of a 7-cmthick lightweight composite rear shield. A dust particle that struck the thin front sheet was completely vaporized. The vapor cloud then expanded into the empty space between the two sheets and struck the rear shield, where its energy was absorbed by being distributed over a large area. In this way, the 2.7-m2 front surface of the spacecraft was effectively protected by armor that weighed only 50 kg. Only recently has the dust hazard become important again, because of man-made space debris in Earth orbit. Each piece of equipment carried into space becomes, after disruption by an explosion due to malfunctioning batteries or fuel systems or by an impact, the source of small projectiles, which endanger other satellites. Some estimates indicate that, in 50 years, the continuous increase in man-made space activity will lead to a runaway effect that will make the near-Earth space environment unhabitable to humans and equipment. However, we are not concerned with this aspect of interplanetary dust; rather, the topic of this chapter is interplanetary dust as an exciting object of astrophysical research. Through its wide distribution over the solar system, cosmic dust can tell stories about its parents (comets, asteroids, even interstellar matter) that otherwise are not easily accessible. This view, however, requires that dust particles be traced back to their origins. To do this, we must understand their dynamics. Dust particles not only follow the gravitational pull of the Sun and the planets but also feel the interplanetary magnetic field and the electromagnetic radiation that fills the solar system. In addition, they interact with the solar wind and with other dust particles that they encounter in space, generally at high speeds. These collisions lead to erosion or to disruption of both particles, thus generating many smaller particles. The dynamics of interplanetary dust cannot be described solely in terms of position and velocity; their size or mass must also be considered.

2. Observations Different methods are available to study cosmic dust (Fig. 3). They are distinguished by the size or mass range of particles that can be studied. The earliest methods were ground-based zodiacal light and meteor observations. Fifty years ago, radar observations of meteor trails became available. With the onset of spaceflight, in situ detection by space instrumentation provided new information on small interplanetary dust particles. Among the first reliable instruments were simple penetration detectors; modern impact ionization detectors allow not only the detection but also the chemical analysis of micrometeoroids. Deep space probes have identified micrometeoroids in interplanetary space from 0.3 to 18 AU from the Sun. Natural (e.g., lunar samples) and artificial surfaces exposed to micrometeoroid

CLBE001-ESS2E

November 9, 2006

16:13

Solar System Dust

623

FIGURE 2 Interplanetary dust particles collected in the stratosphere by NASA’s cosmic dust program. Three grains are of chondritic composition and of various degrees of compactness, and there is one Fe–S–Ni sphere (lower right). The widths of the photographs are 15 μm (first and third photos, clockwise from upper left) and 30 μm (second and fourth photos). (Courtesy of NASA.)

impacts have been returned from space and analyzed. Highflying aircraft have collected from the stratosphere dust that was identified as extraterrestrial material and that was analyzed by the most advanced microanalytic tools. Modern space-based infrared observatories now allow the observation of the thermal emission from interplanetary dust in the outer solar system.

2.1 Meteors

FIGURE 3 Comparison of meteoroid sizes and masses covered by different observational methods.

Looking up at the clear night sky, one can record about 10 faint meteors (or shooting stars in colloquial language) per hour. Once in a while, a brighter streak or trail of light or “fireball” will appear. Around the year 1800, the extraterrestrial nature of meteors was established when triangulation was used to deduce their height and speed. This technique is still used in modern meteor research by employing specifically equipped cameras and telescopes. About 50 years ago, radar techniques were also developed to observe faint meteor trains even during daylight. Visible meteors result when centimeter-sized meteoroids enter the Earth’s atmosphere at a speed greater than 10 km/s. At this speed, the energy of motion, which is

CLBE001-ESS2E

November 9, 2006

16:13

624 Encyclopedia of the Solar System

TABLE 1

Major Meteor Showers, Date of Shower Maximum, Radiant in Celestial Coordinates, Geocentric Speed (km/s), Maximum Hourly Rate, Parent Objectsa Radiantb

Name

Date

RA

DEC

Speed

Rate

Quadrantids April Lyrids Eta Aquarids June Lyrids S. Delta Aquarids Alpha Capricornids S. Iota Aquarids N. Delta Aquarids Perseids N. Iota Aquarids Aurigids Giacobinids Orionids Taurids Taurids Leonids Geminids Ursids

Jan. 3 Apr. 22 May 3 June 16 July 29 July 30 Aug. 5 Aug. 12 Aug. 12 Aug. 20 Sept. 1 Oct. 9 Oct. 21 Nov. 3 Nov. 13 Nov. 17 Dec. 14 Dec. 22

230 271 336 278 333 307 333 339 46 327 84 262 95 51 58 152 112 217

+49 +34 −2 +35 −17 −10 −15 −5 +57 −6 +42 +54 +16 +14 +22 +22 +33 +76

42 48 66 31 41 23 34 42 59 31 66 20 66 27 29 71 34 33

140 10 30 10 30 30 15 20 400 (1993) 15 30 10 30 10 10 3,000 (1966) 70 20

Parent Objectc

Comet1861 I Thatcher P/Halley

P/Honda-Mrkos-Pajdusakova

P/Swift-Tuttle Comet1911 II Kiess P/Giacobini-Zinner P/Halley P/Encke P/Encke P/Tempel-Tuttle Phaeton P/Tuttle

a

After A. F. Cook (1973), In “Evolutionary and Physical Properties of Meteoroids” (C. L. Hemenway, P. M. Millman, and A. F. Cook, eds.), NASA SP-319, 183–191. b RA, right ascension, and DEC, declination, in degrees. c If known, short-period comets are indicated by P/.

converted to heat, is sufficient to totally vaporize the meteoroid. During the deceleration of the meteoroid in the atmosphere at about 100 km altitude, the meteoroid will heat up and atoms from its outer surface will be ablated until it is completely evaporated. A luminous train several kilometers in length follows the meteoroid. It is this ionized and luminous atmospheric gas and material from the meteoroid that is visible and that scatters radar signals. From triangulation of the meteor train by ground stations (several cameras or a radar station), the preatmospheric meteoroid orbit is obtained with high accuracy. During the atmospheric entry of objects larger than several tens of kilograms or about 10 cm in diameter, a surface layer of several centimeters in thickness will burn away, and the object will be decelerated. That which reaches Earth’s surface is called a meteorite. Meteorites of 1 kg to several tons are sufficiently decelerated and fall on Earth with the interior little altered by atmospheric entry. These meteorites are the source of our earliest knowledge about extraterrestrial material. [See Meteorites; Near-Earth Objects; Planetary Impacts.] Much of the ablated material from a meteor will condense again into small droplets, which will cool down and form cosmic spherules that subsequently rain down to Earth. These cosmic spherules can be found and identi-

fied in abundance in deep-sea sediments and on the large ice masses of Greenland, the Arctic, and Antarctica. An average of 40 tons of extraterrestrial material per day in the form of fine dust falls onto the surface of Earth. At certain times, meteor showers can be observed at a rate that is a hundred (and more) times higher than the average sporadic meteor rate (Table 1). Figure 4 shows several meteors in a photograph of the night sky taken on 17 November 1966. The visible rate was about one meteor per second. Because all of these meteoroids travel on parallel trajectories, to an observer they seem to arrive from a common point in the sky (the radiant), which in this case lies in the constellation Leo. Therefore, this meteor shower is called the Leonid shower. The explanation for the yearly occurrence of meteor showers is that all meteoroids in one stream closely follow a common elliptic orbit around the Sun but are spread out all along the orbit. Each year when the Earth crosses this orbit on the same day, some meteoroids of the stream hit the atmosphere and cause the shower. Many meteor streams have orbits similar to those of known comets (cf. Table 1). It is a generally accepted view that meteor streams are derived from comets. Millimeterto centimeter-sized particles that are emitted from comets at low speeds (m/s) are not visible in the normal comet tail

CLBE001-ESS2E

November 9, 2006

16:13

Solar System Dust

625

2.2 Interplanetary Dust Particles

FIGURE 4 An unusually strong meteor shower (Leonid) was observed on 17 November 1966. The meteor trails seem to radiate from the constellation Leo.

but form so-called comet trails along a short segment of the comet’s orbit. Their different speeds will slowly spread the particles out over the full orbit. Infrared observations by the Infrared Astronomical Satellite (IRAS) identified many such trails connected to short-period comets. Gravitational interactions with planets and collisions with other cosmic dust particles will scatter meteoroids out of the stream, and they will become part of the sporadic background cloud of meteoroids. The fact that some meteor showers display strong variations of their intensities indicates that they are young streams that are still concentrated in a small segment of the parent’s orbit. The parent comet of the Leonids, the periodic Comet Tempel–Tuttle, has the same periodicity of 33.3 years. The parent object of one of the strongest yearly meteor showers, the Geminids, is 3200 Phaeton, which had been previously classified as an asteroid because it shows no cometary activity. However, its association with a meteor stream indicates that it is an inactive, dead comet that at some time in the past emitted large quantities of meteoroids. [See Physics and Chemistry of Comets; Cometary Dynamics; Near-Earth Objects.] Fewer than one out of ten thousand radar meteors has been identified to be caused by interstellar meteoroids that pass through the solar system on a hyperbolic orbit. Their heliocentric speed is significantly higher than the solar system escape speed, confirming that they are of truly interstellar origin. The radius of these interstellar meteoroids is about 20 μm. These particles have been found to arrive generally from southern ecliptic latitudes with enhanced fluxes from discrete sources.

There is another “window” through which extraterrestrial material reaches the surface in a more or less undisturbed state. Small interplanetary dust particles (IDPs) of a few to 50 μm in diameter are decelerated in the tenuous atmosphere above 100 km. At this height, the deceleration is so gentle that the grains will not reach the temperature of substantial evaporation (T ∼ 800◦ C), especially, since these small particles have a high surface area-to-mass ratio that enables them to effectively radiate away excessive heat. These dust particles subsequently sediment through the atmosphere and become accessible to collection and scientific examination. The abbreviation IDP (or “Brownlee particle” after Don Brownlee, who first reliably identified their extraterrestrial nature) is often used for such extraterrestrial particles that are collected in Earth’s atmosphere. Early attempts to collect IDPs by rockets above about 60 km were not successful because of the very low influx of micrometeoroids into the atmosphere and the short residence times of IDPs at these altitudes. More successful were airplane collections in the stratosphere at or above 20 km. At this height, the concentration of 10-μm-diameter particles is about 106 times higher than in space and terrestrial contamination of this sized particles is still low. Only micrometer- and submicrometer-sized terrestrial particles (e.g., from volcanic eruptions) can reach these altitudes in significant amounts. Another type of interference is caused by man-made contamination: About 90% of all collected particles in the 3- to 8-μm size range are aluminum oxide spheres, which are products of solid rocket fuel exhaust. Because of this overwhelming contamination problem for small particles, the lower size limit of IDPs collected by airplanes is a few micrometers in diameter. Since 1981, IDP collection by airplanes has been routinely performed by NASA using high-flying aircraft, which can cruise at 20 km for many hours. On its wings it carries dust collectors that sweep huge amounts of air. Dust particles stick to the collector surface that is coated with silicone oil. After several hours of exposure, the collector is retracted into a sealed storage container and returned to the laboratory. There, all particles are removed from the collector plate, the silicone oil is washed off, and the particles are preliminarily examined and catalogued. Individual IDPs can be ordered for further scientific investigation. A wide variety of microanalytic tools is used to examine and analyze IDPs. Scanning electron microscopes (SEM) can image atomic lattice layer structures. Focused ion beams in combination with a SEM are used for sample preparation and secondary ion mass spectrometers (SIMS) can measure the distribution of individual elements and isotopes at submicrometer resolution, deriving important information on the mineralogy of the samples. According to their elemental composition, IDPs come in three major types: chondritic, 60% (cf. Table 2);

CLBE001-ESS2E

November 9, 2006

16:13

626 Encyclopedia of the Solar System

TABLE 2 Element

Mg Si Fe S Al Ca Ni Cr Mn Cl K Ti Co Zn Cu Ge Se Ga Br

Average Elemental Composition (All Major and Selected Minor and Trace Elements) of Several Chondritic IDPs Is Compared with C1 Chondrite Compositiona C1

IDP

Variation

Tc

1,071,000 1,000,000 900,000 515,000 84,900 61,100 49,300 13,500 9,550 5,240 3,770 2,400 2,250 1,260 522 119 62 38 12

0.9 1.2 1 0.8 1.4 0.4 1.3 1.1 1.1 3.6 2.2 1.5 1.9 1.4 2.8 2.3 2.2 2.9 34

0.6–1.1 0.8–1.7 1 0.6–1.1 0.8–2.3 0.3–0.6 1.0–1.7 0.9–1.4 0.8–1.6 2.8–4.6 2.0–2.5 1.3–1.7 1.2–2.9 1.1–1.8 1.9–4.2 1.6–3.4 1.6–3.0 2.1–3.9 23–50

1067 1311 1336 648 1650 1518 1354 1277 1190 863 1000 1549 1351 660 1037 825 684 918 690

The IDP abundances are normalized to iron (Fe) and to C1. C1 abundance is normalized to Si = 1,000,000 condensation temperatures Tc (◦ C). From E. K. Jessberger et al. (1992), Earth Planet. Sci. Lett. 112, 91.

a

iron–sulfur–nickel, 30%; and mafic silicates (iron– magnesium–rich silicates, i.e., olivine and pyroxene), 10%. Most chondritic IDPs are porous aggregates, but some smooth chondritic particles are found as well. Chondritic aggregates may contain varying amounts of carbonaceous material of unspecified composition. Table 2 shows a significant enrichment in volatile (low condensation temperature) elements when compared to C1 chondrites. This observation is being used to support the argument that these particles consist of some very primitive solar system material that had never seen temperatures above about 500◦ C, as is the case for some cometary material. This and compositional similarity with comets argue for a genetic relation between comets and IDPs. A remarkable feature of IDPs is their large variability in isotopic composition. Extreme isotopic anomalies have been found in some IDPs. Under typical solar system conditions, only fractions of a percent of isotopic variations can occur. These huge isotopic variations indicate that some grains are not homogenized with other solar system material but have preserved much of their presolar character. Submicrometer-sized grains known as GEMS (glass with embedded metal and sulfides) are major constituents of the chondritic porous class of IDPs. Several GEMS with nonsolar oxygen isotopic compositions were identified, confirming that at least some are indeed presolar grains. These amorphous interstellar silicates are considered one of the fundamental building blocks of the solar system.

2.3 Zodiacal Light The wedge-shaped appearance of the zodiacal light (see Fig. 1) demonstrates its concentration in the ecliptic plane. For an observer on Earth, the zodiacal light extends in the ecliptic all the way around to the antisolar direction, however, at strongly reduced intensities. In the direction opposite to the Sun, this light forms a hazy area of a few degrees in dimension known as the gegenschein, or counterglow. If seen from outside the solar system, the zodiacal dust cloud would have a flattened, lenticular shape that extends along the ecliptic plane about seven times farther from the Sun than perpendicular to the ecliptic plane. The brightness of zodiacal light is the result of light scattered by a huge number of particles in the direction of observation. The observed zodiacal brightness is a mean value, averaged over all sizes, compositions, and structures of particles along the line of sight. Zodiacal light brightness can be traced clearly into the solar corona. However, most of this dust is foreground dust close to the observer because of a favorable scattering function. Nevertheless, the vicinity of the Sun is of considerable interest for zodiacal light measurements because it is expected that close to the Sun the temperature of the dust rises, and the dust particle starts to sublimate, first the more volatile components and closer to the Sun even the refractory ones. Inside about four solar radii distance, dust should completely sublimate. Some observers have found a sharp edge of a dust-free zone at four

CLBE001-ESS2E

November 9, 2006

16:13

Solar System Dust

solar radii; others have not seen a sharp edge. Perhaps the inner edge of the zodiacal cloud may change in time. The large-scale distribution of the zodiacal dust cloud is obtained from zodiacal light measurements onboard interplanetary spacecraft spanning a distance ranging from 0.3 to approx. 3 AU from the Sun. Even though the intensity decreases over this distance by a factor 150, the spatial density of dust needs only to decrease by a factor 15. The radial dependence of the number density is slightly steeper than an inverse distance dependence. From zodiacal light measurements, a slight inclination of about 3◦ of the symmetry plane of zodiacal light with respect to the ecliptic plane has been determined. At visible wavelengths, the spectrum of the zodiacal light closely follows the spectrum of the Sun. A slight reddening (i.e., the ratio of red and blue intensities is larger for zodiacal light than for the Sun) indicates that the majority of particles are larger than the mean visible wavelength of 0.54 μm. In fact, most of the zodiacal light is scattered by 10to 100-μm-sized particles. Therefore, the dust seen by zodiacal light is only a subset of the interplanetary dust cloud. Submicrometer- and micrometer-sized particles, as well as millimeter and bigger particles, are not well represented by the zodiacal light at optical wavelengths. Above about 1 μm in wavelength, the intensities in the solar spectrum rapidly decrease. The zodiacal light spectrum follows this decrease until about 5 μm, above which the thermal emission of the dust particles prevails. Because of the low albedo (fraction of incident sunlight reflected back and scattered in all directions is smaller than 10%) of interplanetary dust particles, most visible radiation (>90%) is absorbed and emitted at infrared wavelengths. The maximum of the thermal infrared emission from the zodiacal dust cloud lies between 10 and 20 μm. From the thermal emission observed by the IRAS and Cosmic Background Explorer (COBE) satellites, an average dust temperature at 1 AU distance from the Sun between 0◦ C and 20◦ C has been derived. Some spatial structure has been observed at thermal infrared wavelengths. Asteroid bands mark several asteroids families as significant sources of solar system dust just as comet trails identify dust emitted from individual comets. Optical and infrared observations of other extraterrestrial dusty phenomena have also provided important insights into the zodiacal complex. Cometary and asteroidal dust is considered to be an important source of the zodiacal cloud. The study of circumplanetary dust and rings has stimulated much research in the dynamics of dust clouds. Interstellar dust is believed to be the ultimate source of all refractory material in the solar system. Circumstellar dust clouds like the one around β-Pictoris are “zodiacal clouds” of their own right. The study of which may eventually give information on extra solar planetary systems. [See Infrared Views of the Solar System from Space; Planetary Rings; Extra-Solar Planets.]

627

FIGURE 5 Microcraters on the glassy surface of a lunar sample. Bright spallation zones surround circular central pits.

2.4 Lunar Microcraters and the Near-Earth Dust Environment The size distribution of interplanetary dust particles is represented by the lunar microcrater record. Microcraters on lunar rocks have been found ranging from 0.02 μm to millimeters in diameter (Fig. 5). Laboratory simulations of high-velocity impacts on lunar-like materials have been performed to calibrate crater sizes with projectile sizes and impact speeds. Submicrometer- to centimeter-sized projectiles have been used with speeds above several kilometers per second. The typical impact speed of interplanetary meteoroids on the Moon is about 20 km/s. For the low-mass particles, electrostatic dust accelerators that reach projectile speeds of up to 100 km/s were used. The high-mass projectiles were accelerated with light-gas guns, which reached speeds up to about 10 km/s. For the intermediate mass range, plasma drag accelerators reached impact speeds of 20 km/s. The crater diameter to projectile diameter ratio varies from 2 for the smallest microcrater to about 10 for centimeter-sized projectiles. The difficulty in deriving the impact rate from a crater count on the Moon is that the degree to which rocks shield other rocks and thus the exposure time of any surface is generally unknown. Therefore, the crater size or meteoroid distribution has to be normalized with the help of an impact rate or meteoroid flux measurement obtained by other means. In situ detectors or recent analyses of impact plates that were exposed on NASA’s Long Duration Exposure Facility to the meteoroid flux for several years provided this flux calibration (Fig. 6). Flux of the smallest particles dominates, and the mass flux of meteoroids peaks at 10−5 g. The total mass density of interplanetary dust at 1 AU is 10−16 g/m3 and the total mass of the zodiacal cloud inside Earth’s orbit is between 1016 and 1017 kg, which corresponds

CLBE001-ESS2E

November 9, 2006

16:13

628 Encyclopedia of the Solar System 2.5 In Situ Dust Measurements

FIGURE 6 Cumulative flux of interplanetary meteoroids on a spinning flat plate at 1 AU distance from the Sun. The solid line has been derived from lunar microcrater statistics, and it is compared with satellite and spaceprobe measurements.

to the mass of a single object (comet or asteroid) of about 20 km in diameter. In low Earth orbit the meteoroid flux is about a factor of two higher than in deep space because of the Earth’s gravitational concentration. However, micrometersized natural meteoroids are outnumbered (by a factor of three) by man-made space debris. Craters produced by space debris particles are identified by chemical analyses of residues in the craters. Residues have been found from space materials and signs of human activities in space, such as paint flakes, plastics, aluminum, titanium, and human excretion.

TABLE 3

Complementary to ground-based and astronomical dust observations are in situ observations by dust impact detectors on board interplanetary spacecraft. In situ measurements have been performed in interplanetary space between 0.3 and 18 AU heliocentric distance (Table 3). Two types of impact detectors were mainly used for interplanetary dust measurements: penetration detectors and impact ionization detectors. Penetration detectors record the mechanical destruction from a dust particle’s impact, for example, a 25- or 50-μm-thick steel film has a detection threshold of 10−9 or 10−8 g (approx. 10 or 20 μm radius) at a typical impact speed of 20 km/s. At lower impact speeds the minimum detectable particle mass is bigger and vice versa. A more sensitive penetration detector is the PVDF (PolyVinylidine Fluoride) film. PVDF is a polarized material (i.e., all dipolar molecules in the material are aligned so that they are pointing in the same direction). When a dust particle impacts the film, it excavates some polarized material. This depolarization generates an electric signal, which is then detected. The pulse height of the signal is a function of the mass and speed of the dust particle. A typical measurement range is from 10−13 to 10−9 g (1–10 μm radius). The most sensitive dust detectors are impact ionization detectors. Figure 7 shows a photo of the dust detector flown on the Cassini spacecraft. The detector has an aperture of 0.1 m2 and is based on the impact ionization effect: A dust particle that enters the detector and hits the hemispherical target in the back at speeds above 1 km/s will produce an impact crater and part or all of the projectile’s material will vaporize. Because of the high temperature at the impact site some electrons are stripped off atoms and molecules and generate a vapor that is partially ionized. These ions and electrons are separated in an electric field within the detector and collected by electrodes. Coincident electric

In Situ Dust Detectors in Interplanetary Space: Distance of Operation, Mass Sensitivity, and Sensitive Area.

Spacecraft

Year of Launch

Distances (AU)

Mass Threshold (g)

Area (m2 )

Pioneer 8 Pioneer 9 HEOS 2 Pioneer 10 Pioneer 11 Helios 1/2 Galileo Hiten Ulysses Cassini Nozomi

1967 1968 1972 1972 1973 1974/76 1989 1990 1990 1997 1998

0.97–1.09 0.75–0.99 1 1–18 1–10 0.3–1 0.7–5.3 1 1–5.4 0.7–10 1–1.5

2 × 10−13 2 × 10−13 2 × 10−16 2 × 10−9 10−8 10−14 10−15 10−15 10−15 2 × 10−16 10−15

0.0094 0.0074 0.01 0.26 0.26 0.012 0.1 0.01 0.1 0.1 0.01

CLBE001-ESS2E

November 9, 2006

16:13

Solar System Dust

629

front of the target pick up any electric charge of dust particles. Measurements of the electric charge on interplanetary dust particles have been accomplished for the first time by the Cassini detector. Dust detectors incorporating a mass spectrometer have been flown on the Helios spacecraft, the Giotto and VEGA missions to Comet Halley, the Stardust mission to Comet Wild 2, and the Cassini mission to Saturn. Electrostatic dust accelerators are used to calibrate these detectors with micrometer- and submicrometer-sized projectiles at impact speeds of up to about 100 km/s. 2.5.1 INTERPLANETARY DUST

FIGURE 7 The Cassini cosmic dust analyzer consists of two types of dust detectors—the high rate detector (HRD) and the dust analyzer (DA). The cylindrical DA (upper center) has a diameter of 43 cm. The bottom of the sensor contains the hemispherical impact target; in the center are charge-collecting electrodes and the multiplier for measurement of the mass impact spectrum. Two entrance grids sense the electric charge of incoming dust grains. The detector records impacts of submicrometer- and micrometer-sized dust particles above an impact speed of 1 km/s. HRD consists of two circular film detectors that record impacts of micrometer-sized dust particles at a rate of 10,000 per second. The detectors are carried by the electronics box that is mounted on top of a turntable bolted to the spacecraft.

pulses on these electrodes signal the impact of a highvelocity dust particle. The strength and the wave form of the signal are measures of the mass and speed of the impacting particle. The small central part of the Cassini detector is a time-of-flight mass spectrometer: A high electric field between the target and a grid 3 mm in front of the target accelerates the ions at high speed. During the flight between the grid and the ion collector, ions of different masses separate and arrive at different times at the multiplier. The lightest ions arrive first and the heavier ones appear later. In this way, a mass spectrum that represents the elementary composition of the dust grain is measured. Entrance grids in

The radial profile of the dust flux in the inner solar system between 1 and 0.3 AU from the Sun has been determined by the Helios 1 and 2 space probes. Three dynamically different interplanetary dust populations have been identified in the inner solar system. First, particles that orbit the Sun in low-eccentricity orbits had already been detected by the Pioneer 8/9 and HEOS 2 dust experiments. They relate to particles originating in the asteroid belt and spiraling under the Poynting–Robertson effect toward the Sun. Second, there are particles on highly eccentric orbits that have, in addition, large semimajor axes and that derive from shortperiod comets. Third, the Pioneer 8/9 dust experiments detected a significant flux of small particles, which were called β-meteoroids, from approximately the solar direction. Existence of these particles was recently confirmed by measurements with the Japanese Hiten satellite. Recently, the Galileo and Ulysses spacecraft carried dust detectors through interplanetary space between the orbits of Venus and Jupiter and above the ecliptic plane. Swingbys of Venus and Earth (two times) were necessary to give the heavy Galileo spacecraft (mass of 2700 kg) the necessary boost to bring it to Jupiter within 6 years of flight time, where it became the first man-made satellite of this giant planet. The Ulysses spacecraft, being much lighter (mass of 375 kg), made the trajectory to Jupiter within 1.5 years. In a swing-by of Jupiter, the Ulysses spacecraft was brought into an orbit almost perpendicular to the ecliptic plane that carried it under the South Pole, through the ecliptic plane, and over the North Pole of the Sun. Interplanetary dust measurements were obtained by the Galileo spacecraft in the ecliptic plane between Venus’s orbit and the Asteroid Belt. The dust impact rate was generally higher closer to the Sun than it was farther away. After all planetary flybys, the spacecraft moved away from the Sun. At these times, the impact rate was more than an order of magnitude higher than before the flyby when the spacecraft moved toward the Sun. This observation is explained by the fact that interplanetary dust inside the asteroid belt orbits the Sun on low-inclination (8 × 10−10 kg (about 10 μm in size) in the outer solar system measured by the Pioneer 10 penetration detector. At 18 AU from the Sun, the instrument quit operation. The measurements are in agreement with a model of constant spatial dust density in the outer planetary system. [From D. H. Humes (1980), J. Geophys. Res. 85, 5841–5852.]

spacecraft moves in the same direction (outward) than in the opposite case when the spacecraft moves inward. The spatial dust density follows roughly an inverse radial distance dependence. Close passages of the asteroids Gaspra and Ida did not exhibit increased dust impact rates. In the outer solar system, the dust detectors on board Pioneers 10/11, Galileo, Ulysses, and recently Cassini measured the flux of interplanetary dust particles. The flux of micrometer-sized particles decreased from 1 AU going outward. No sign of a flux enhancement in the Asteroid Belt was detected. Outside Jupiter’s orbit, Pioneer 10 recorded a flat flux profile (Fig. 8), which indicates a constant spatial density of micrometer-sized dust in the outer solar system. This observation has been interpreted to be due to the combined input of dust from the Kuiper Belt and comets like Halley and Schwassmann-Wachmann 1. In interplanetary space, the highest dust fluxes have been observed near comets. So far, four comets were visited by spacecraft that carried dust detectors: Comets P/GiacobiniZinner, P/Halley, P/Grigg-Skjellerup, and Wild 2. Specially optimized dust analyzers have been used to study Comet Halley’s dust. Chemical analyses showed that, in addition to the expected dust particles consisting of silicates, a large fraction of cometary dust consists of carbonaceous materials. Extreme isotopic anomalies have been found to exist in some of these particles. Similar compositions are expected for interplanetary dust. [See Physics and Chemistry of Comets; Meteorites.)

FIGURE 9 Dust impact rate observed by the Ulysses dust detector during the 400 days around the closest approach to Jupiter (CA, 8 February 1992). At the beginning and end of the period shown, Ulysses was 240 million km (1.6 AU) from Jupiter, while at CA the distance was only 450,000 km. Except for the flux peak at CA, when bigger particles were detected, the peaks at other times consisted of submicrometer-sized dust particles.

2.5.2 PLANETARY DUST STREAMS

Inside a distance of about 3 AU from Jupiter, both Ulysses and Galileo spacecraft detected unexpected swarms of submicrometer-sized dust particles arriving from the direction of Jupiter. Figure 9 shows the strongly time-variable dust flux observed by Ulysses during its flyby of Jupiter. About one month after its closest approach to Jupiter, Ulysses encountered the most intense dust burst at about 40 million km from Jupiter. For about 10 hours, the impact rate of submicrometer-sized particles increased by a factor 1000 above the background rate. The similarity of the impact signals and the sensor-pointing directions indicated that the particles in the burst were moving in collimated streams at speeds of several 100 km/s. Even stronger and longer lasting dust streams were observed in 1995 by the Galileo dust detector during its approach to Jupiter. Dust measurements inside the jovian magnetosphere showed a modulation of the small particle impact rate with a period of 10 hours, which is the rotation period of Jupiter and its magnetic field. Positively charged dust particles in the 10-nm size range coupled to the magnetic field and are thrown out of Jupiter’s magnetosphere in the form of a warped dust sheet. Sources of these dust particles are the volcanoes on Jupiter’s moon Io and to a smaller extent Jupiter’s ring. During Cassini’s flyby of Jupiter, this phenomenon was also observed, and mass spectra of the particles were obtained. Both sodium chloride and sulfurous components were identified in the mass spectra, which is consistent with spectral measurements of Io’s volcano-induced environment. At Saturn, Cassini observed dust streams emanating from this system as well. In this case, Saturn’s dense A ring

CLBE001-ESS2E

November 9, 2006

16:13

Solar System Dust

631

and the extended E ring have been identified as sources. The ejection mechanism is very similar to that acting at Jupiter. Freshly generated nanometer-sized dust grains get charged and—if the charge is positive—thrown out by Saturn’s magnetic field. In some parts of the magnetosphere, dust particles become negatively charged; these particles remain bound to the magnetic field and stay in the vicinity of Saturn. The stream particles primarily consist of silicate materials that imply that the particles are the contamination of icy ring material rather than the ice particles themselves. [See Planetary Rings.]

2.5.3 INTERSTELLAR DUST IN THE HELIOSPHERE

The solar system is currently passing through a region of low-density, weakly ionized interstellar material in our galaxy, which shows a larger abundance of heavy refractory elements in the gas phase such as iron, magnesium, and silicon than in cold dense interstellar clouds. Interstellar dust is part of the interstellar medium, although it has not been directly observed by astronomical means in the tenuous local interstellar cloud. Interstellar dust is formed as stardust in the cool atmospheres of giant stars and in nova and supernova explosions. More than a decade ago, interstellar dust was positively identified inside the planetary system. At the distance of Jupiter, the dust detector on board the Ulysses spacecraft detected impacts predominantly from a direction that was opposite to the expected impact direction of interplanetary dust grains. The impact velocities exceeded the local solar system escape velocity, even if radiation pressure effects were considered. The motion of interstellar grains through the solar system is parallel to the flow of neutral interstellar hydrogen and helium gas, both traveling at a speed of 26 km/s with respect to the Sun. The interstellar dust flow persisted at higher latitudes above the ecliptic plane, even over the poles of the Sun, whereas interplanetary dust is strongly concentrated toward the ecliptic plane (Fig. 10). Since that time, Ulysses monitored the stream of interstellar dust grains through the solar system at higher latitudes. It was found that the flux of small interstellar grains showed some variation with the period of the solar cycle, which indicates a coupling of the flux to the solar wind magnetic field. Interstellar dust has initially been identified outside 3 AU out to Jupiter’s distance. However, refined analyses showed that both Cassini and Galileo recorded several hundred interstellar grains in the region between 0.7 and 3 AU from the Sun. Even in the Helios dust data interstellar grains were identified down to 0.3 AU distance from the Sun. The radii of clearly identified interstellar grains range from 0.1 μm to above 1 μm with a maximum at about 0.3 μm. Even bigger interstellar particles have been reliably

FIGURE 10 Ulysses dust impact rate observed around the time of its ecliptic plane crossing (ECL). ECL occurred on 12 March 1995 at a distance of 1.3 AU from the Sun. The boxes indicate the mean impact rates and their uncertainties. The top scales give the spacecraft latitude. Model calculations of the impact rate during Ulysses’ south to north traverse through the ecliptic plane are shown by the lines. Contributions from interplanetary dust on bound orbits and interstellar dust on hyperbolic trajectories and the sum of both are displayed. From these measurements, it is concluded that interstellar dust is not depleted to a distance of 1.3 AU from the Sun.

identified by their hyperbolic speeds in radar meteor observations. The flow direction of these bigger particles varies over a much wider angular range than that of small (submicrometer-sized) grains observed by spacecraft. The deficiency of small grain masses ( 1 and a is taken negative. The aphelion distances (furthest from the Sun) are finite only for circular and elliptical orbits. The inclination is the angle between the orbit plane and the ecliptic (i.e., the orbit plane of Earth). Dust particles in interplanetary space move on very different orbits, and several classes of orbits with similar characteristics have been identified. One class of meteoroids moves on orbits that are similar to those of asteroids, which peak in the Asteroid Belt. Another class of orbits that represents the majority of zodiacal light particles has a strong concentration toward the Sun. Both orbit populations have low to intermediate eccentricities (0 < e < 0.6) and low inclinations (i < 40◦ ). These asteroidal and zodiacal core populations satisfactorily describe meteors, the lunar crater size distribution, and a major portion of zodiacal light observations. Also, spacecraft measurements inside 2 AU are well represented by the core population. [See Main-Belt Asteroids.]

3.2 Radiation Pressure and the Poynting–Robertson Effect Electromagnetic radiation from the Sun (most intensity is in the visible wavelength range at λmax = 0.5 μm) being absorbed, scattered, or diffracted by any particulate exerts pressure on this particle. Because solar radiation is directed outward from the Sun, radiation pressure is also directed away from the Sun. Thus, gravitational attraction is reduced by the radiation pressure force. Both radiation pressure and gravitational forces have an inverse square

FIGURE 11 Ratio β of the radiation pressure force over solar gravity as a function of particle radius. Values are given for particles made of astronomical silicates (from Gustafson et al., 2001) in various shapes: sphere (solid curve), long cylinders (dashes), and flat plates (dots).

dependence on the distance from the Sun. Radiation pressure depends on the cross section of the particle and gravity on the mass; therefore, for the same particle, the ratio β of radiation pressure, FR , over gravitational force, FG , is constant everywhere in the solar system and depends only on particle properties: β = FR /FG ∼ Qpr /sρ, where Qpr is the efficiency factor for radiation pressure, s is the particle radius, and ρ is its density. Figure 11 shows the dependence of β on the particle size for different shapes. For big particles (s  λmax ), radiation pressure force is proportional to the geometric cross section giving rise to the 1/s-dependence of β. At particle sizes comparable to the wavelength of sunlight s ≈ λmax , β-values peak and decline for smaller particles as their interaction with light decreases. A consequence of the radiation pressure force is that particles with β > 1 are not attracted by the Sun but rather are repelled by it. If such particles are generated in interplanetary space either by a collision or by release from a comet, they are expelled from the solar system on hyperbolic orbits. But even particles with β values smaller than 1 will leave the solar system on hyperbolic orbits if their speed at formation is high enough so that the reduced solar attraction can no longer keep the particle on a bound orbit. If a particle that is released from a parent body moving on a circular orbit has β > 1/2, then it will leave on a hyperbolic orbit. These particles are termed beta-meteoroids. Because of the finite speed of light (c ≈ 300,000 km/s) radiation pressure does not act perfectly radial but has an aberration in the direction of motion of the particle around the Sun. Thus, a small component (approximately proportional to v/c, where v is the speed of the particle) of the radiation pressure force always acts against the orbital motion

CLBE001-ESS2E

November 9, 2006

16:13

Solar System Dust

reducing its orbital energy. This effect is called Poynting– Robertson effect. As a consequence of this drag force, the particle is decelerated. This deceleration is largest at its perihelion distance where both the light pressure and the velocity peak. Consequently, the eccentricity (aphelion distance) is reduced, and the orbit is circularized. Subsequently, the particle spirals toward the Sun, where it finally sublimates. The lifetime τPR of a particle on a circular orbit that spirals slowly to the Sun is given by τPR = 7 × 105 ρsr 2 /Qpr , where τPR is in years, r is given in AU, and all other quantities are in SI units. Even a centimeter-sized (s = 0.01 m), stony (ρ = 3000 kg/m3 , Qpr ≈ 1) particle requires only 21 million years to spiral to the Sun if it is not destroyed by an earlier collision. This example shows that all interplanetary dust had to be recently generated; no dust particles remain from the times of the formation of the solar system. The dust we find today had to be stored in bigger objects (asteroids and comets), which have sufficient lifetimes. The effect of solar wind impingement on particulates is similar to radiation pressure and Poynting–Robertson effect. Although direct particle pressure can be neglected with respect to radiation pressure, solar wind drag is about 30% of Poynting–Robertson drag. Particle orbits that evolve under Poynting–Robertson drag will eventually cross the orbits of the inner planets and, thereby, will be affected by planetary gravitation. During the orbit evolution of particles, resonances with planetary orbits may occur even if the orbit periods of the particle and the planet are not the same but form a simple integer ratio. This effect is largest for big particles, the orbits of which evolve slower and which spend more time near the resonance position. Density enhancements of interplanetary dust have been found (i.e., the Earth resonant ring was identified in IRAS data and later confirmed by data from the Cosmic Background Explorer satellite COBE). Dust near other stars will also evolve under Poynting– Robertson effect and form a dust disk around this star. Such a disk has been found around many stars (e.g., β-Pictoris). There is an ongoing search in this disk for resonance enhancements that would indicate planets around this star [See Extra-Solar Planets.]

3.3 Collisions Mutual high-speed (v > 1 km/s) collisions among dust particles lead to grain destruction and generation of fragments. By these effects, dust grains are modified or destroyed, and many new fragment particles are generated in interplanetary space. From impact studies in stony material, we know that, at a typical collision speed of 10 km/s, an impact crater is formed on the surface of the target particle if it is more than 50,000 times more massive than the projectile. This mass ratio is strongly speed- and material-dependent. A typical impact crater in brittle stony material (Fig. 5) consists of a central hemispherical pit surrounded by a shallow

633

FIGURE 12 Schematics of meteoroid collisions in space. If the projectile is very small compared to the target particle, only a crater is formed in the bigger one. If the projectile exceeds a certain size limit, the bigger particle is also shattered into many fragments. The transition from one type to another is abrupt.

spallation zone. The largest ejecta particle (from the spallation zone) can be many times bigger than the projectile; however, it is emitted at a very low speed on the order of meters per second. The total mass ejected from an impact crater at an impact speed of 10 km/s is about 500 times the projectile mass. However, if the target particle is smaller than the stated limit, the target will be catastrophically destroyed. The material of both colliding particles will be transformed into a huge number of fragment particles (Fig. 12). Thus, catastrophic collisions are a very effective process for generating small particles in interplanetary space. It has been found that interplanetary particles bigger than about 0.1 mm in diameter will be destroyed by a catastrophic collision rather than transported to the Sun by Poynting–Robertson drag.

3.4 Charging of Dust and Interaction with the Interplanetary Magnetic Field Any meteoroid in interplanetary space will be electrically charged, and several competing charging processes determine the actual charge of a meteoroid (Fig. 13). Irradiation by solar ultraviolet (UV) light frees photoelectrons, which leave the grain. Electrons and ions are collected from the ambient solar wind plasma. Energetic ions and electrons then cause the emission of secondary electrons. Whether electrons or ions can reach or leave the grain depends on their energy and on the polarity and electrical potential of the grain. Because of the predominance of the photoelectric effect in interplanetary space, meteoroids are mostly charged positively at a potential of a few volts. Only at times

CLBE001-ESS2E

November 9, 2006

16:13

634 Encyclopedia of the Solar System

FIGURE 13 Charging processes of meteoroids in interplanetary space. UV radiation releases photoelectrons, electrons, and ions from the solar wind plasma, and they are collected; the impact of energetic particle radiation releases secondary electrons.

of very high solar wind densities does the electron flux to the particle dominate and the particle gets charged negatively. The final charging state is reached when all currents to and from the meteoroid cancel. The timescale for charging is seconds to hours depending on the size of the particle; small particles charge slower. Electric charges on dust particles in interplanetary space have been measured by the Cassini Cosmic Dust Analyzer. These measurements indicate a dust potential of +5 V. In the dense plasma of the inner Saturnian magnetosphere, dust particles at −2 V potential have been found. The outward-streaming (away from the Sun) solar wind carries a magnetic field away from the Sun. Due to the rotation of the Sun (at a period of 25.7 days), magnetic field lines are drawn in a spiral, like water from a lawn sprinkler. The polarity of the magnetic field can be positive or negative depending on the polarity at the base of the field line in the solar corona, which varies spatially and temporally. For an observer or a meteoroid in interplanetary space, the magnetic field sweeps outward at the speed of the solar wind (400 to 600 km/s). [See The Sun.] In the magnetic reference frame, the meteoroid moves inward at about the same speed because its orbital speed is comparatively small. The Lorentz force on a charged dust particle near the ecliptic plane is mostly either upward or downward depending on the polarity of the magnetic field. Near the ecliptic plane, the polarity of the magnetic field changes at periods (days to weeks) that are much faster than the orbital period of an interplanetary dust particle, and the net effect of the Lorentz force on micrometer-sized particles is small. Only secular effects on the bigger zodiacal particles are expected to occur, which could have an effect on the symmetry plane of the zodiacal cloud close to the Sun. For nanometer-sized particles, like the ones that have been found in the dust streams, the Lorentz force dominates all other forces, and

as a result the particles gyrate about the magnetic field lines and are eventually convected with the solar wind out of the solar system. The overall polarity of the solar magnetic field changes with the solar cycle of 11 years. For one solar cycle, positive magnetic polarity prevails away from the ecliptic in the northern hemisphere and negative polarity in the southern hemisphere. Submicrometer-sized interstellar particles that enter the solar system are deflected either toward the ecliptic plane or away from it depending on the overall polarity of the magnetic field. Interstellar particles entering the heliosphere from one direction at a speed of 26 km/s need about 20 years (two solar cycles) to get close to the Sun. Therefore, trajectories of small interstellar grains (0.1 μm in radius) are strongly diverted: In some regions of space, their density is strongly increased; in others, they are depleted. At the time of the initial Ulysses and Galileo measurements (1992 to 1996), the overall solar magnetic field had changed to the unfavorable configuration; therefore, only big (micrometer-sized) interstellar particles reached the positions of Ulysses and Galileo. By 2003, the magnetic field had changed to the focusing configuration and the interstellar dust flux had recovered. [See The Solar Wind.]

3.5 Evolution of Dust in Interplanetary Space Forces acting on interplanetary particles are compared in Table 4. The force from solar gravity depends on the mass of the particle; therefore, it depends on the size as FG ∼ s 3 . Radiation pressure depends on the cross section of the particle, hence FR ∼ s 2 . The electric charge on a dust grain depends on the size directly, as does the Lorentz force FL ∼ s. Therefore, these latter forces become more dominating at smaller dust sizes. At a size comparable to the wavelength of visible light (s ∼ 0.5 μm), radiation pressure is dominating gravity, and below that size the Lorentz force dominates the particles’ dynamics. Though gravity is attractive to the Sun, radiation pressure is repulsive. The net effect of solar wind interactions on small particles is that they are convected out of the solar system. Besides energy-conserving forces, there are also dissipative forces: the Poynting–Robertson effect and the ion drag from the solar wind. They cause a loss of orbital energy and force particles to slowly spiral to the Sun, where they eventually evaporate. These atoms and molecules become ionized and are flushed out of the solar system by the solar wind. Figure 14 shows the flow of meteoritic matter through the solar system as a function of the meteoroid size. There is a constant input of mass from comets and asteroids. From the intensity enhancement of zodiacal light toward the Sun, it was deduced that, inside 1 AU, significant amounts of mass have to be injected by short-period comets into the zodiacal cloud. While comets shed their debris over a large

CLBE001-ESS2E

November 9, 2006

16:13

Solar System Dust

TABLE 4 s (μm)

0.01 0.1 1 10 100 a

635

Comparison of Various Forces Acting on Dust Particles of Size s Under Typical Interplanetary Conditions at 1 AU Distance from the Suna FG (N)

FR (N)

FL (N)

FPR (N)

FID (N)

9 × 10−23 9 × 10−20 9 × 10−17 9 × 10−14 9 × 10−11

1.4 × 10−21 1.4 × 10−19 1.4 × 10−17 1.4 × 10−15 1.4 × 10−13

1.5 × 10−20 1.5 × 10−19 1.5 × 10−18 1.5 × 10−17 1.5 × 10−16

1.4 × 10−25 1.4 × 10−23 1.4 × 10−21 1.4 × 10−19 1.4 × 10−17

4 × 10−26 4 × 10−24 4 × 10−22 4 × 10−20 4 × 10−18

Notes: Dominating forces are in bold. Subscripts refer to gravity, radiation pressure, Lorentz force, Poynting-Robertson drag, and ion drag.

range of heliocentric distances but preferentially close to the Sun, asteroid debris is mostly generated in the Asteroid Belt, between 2 and 4 AU from the Sun. Collisions dominate the fate of big particles and are a constant source of smaller fragments. Meteoroids in the range of 1 to 100 μm

are dragged by the Poynting–Robertson drag to the Sun. Smaller fragments are driven out of the solar system by radiation pressure and Lorentz force. Estimates of the mass loss from the zodiacal cloud inside 1 AU give the following numbers. About 10 tons per second are lost by collisions from the big (meteor-sized) particle population. A similar amount (on the average) has to be replenished by cometary and asteroidal debris. Nine tons per second of the collisional fragments are lost as small particles to interstellar space, and the remainder of 1 ton per second is carried by the Poynting–Robertson effect toward the Sun, evaporates, and eventually becomes part of the solar wind. Interstellar dust transiting the solar system becomes increasingly important farther away from the Sun. At 3 AU from the Sun, the interstellar dust flux seems to already dominate the flux of submicrometer- and micrometer-sized interplanetary meteoroids.

4. Future Studies

FIGURE 14 Mass flow of meteoric matter through the solar system. Most of the interplanetary dust is produced by collisions of large meteoroids, which represent a reservoir continually being replenished by disintegration of comets or asteroids. Most of it is blown out of the solar system as submicrometer-sized grains. The remainder is lost by evaporation after being driven close to the Sun by the Poynting–Robertson effect. In addition to the flow of interplanetary matter shown, there is a flow of interstellar grains through the planetary system.

New techniques will generate new insights. These techniques will include innovative observational methods, new space missions to unexplored territory, and new experimental and theoretical methods to study the processes affecting solar system dust. Questions to address are: the composition (elemental, molecular, and isotopic) and spatial distribution of interplanetary dust; the quantitative understanding of effects or processes affecting dust in interplanetary space; and the quantitative determination of the contributions from different sources (asteroids, comets, planetary environs, and interstellar dust). Analyses of brightness measurements at infrared wavelengths up to 200 μm by the COBE satellite result in refined models of the distribution of dust mostly outside 1 AU. Spectrally resolved observations of asteroids, comets, and zodiacal dust by the infrared space observatories (ISO and Spitzer) show the genetic relation between these larger bodies and interplanetary dust. Improved observations of the inner zodiacal light and the edge of the dust-free zone around the Sun will provide some clues to the composition of zodiacal dust. Optical and infrared observations of

CLBE001-ESS2E

November 9, 2006

16:13

636 Encyclopedia of the Solar System extrasolar systems will bring new insights to zodiacal clouds around other stars. Interplanetary space missions presently under way that carry dust detectors are the Ulysses and Cassini missions. Ulysses has probed the space above the poles of the Sun and outside 1 AU and continues its study of the interplanetary dust cloud at times of high solar activity. Galileo and Cassini had become the first man-made satellites of Jupiter and Saturn, respectively, and are studying their dust environments. The detailed study of cometary and interstellar dust is the goal of NASA’s Stardust mission, which returned samples of dust from Comet Wild 2 in early 2006. The Japanese Hyabusa mission collected dust from Asteroid Itokawa and is on its return to Earth. The European Space Agency’s Rosetta mission will follow Comet Churyumov Gerasimenko through its perihelion and investigate its release of dust to interplanetary space. Dust particles, like photons, are born at remote sites in space and time, and carry from there information that may not be accessible to direct investigation. From knowledge of the dust particles’ birthplace and the particles’ bulk properties, we can learn about the remote environment out of which the particles were formed. This approach is called dust astronomy and is carried out by means of dust telescopes on dust observatories in space. Targets for dust telescopes are dust from the local interstellar medium, cometary, asteroidal dust, and space debris. Dust particles’ trajectories are determined by the measurements of the electric charge signals that are induced when the charged grains fly through charge-sensitive grid systems. Modern in situ dust detectors are capable of providing mass, speed, and physical and chemical information of dust grains in space. A dust telescope can, therefore, be considered as a combination of detectors for dust particle trajectories along with detectors for physical and chemical analysis of dust particles. Both dust trajectory sensors and large-area dust analyzers have been developed recently and await their use in space.

In near-Earth space, ambitious new techniques will be applied to collect meteoritic material that is not accessible by other methods. High-speed meteoroid catchers, which permit the determination of the trajectory as well as the recovery of material for analysis in ground laboratories, are under development. A cosmic dust collector is currently being flown on the International Space Station (ISS). Laboratory studies are instrumental in improving our understanding of planetary and interplanetary processes in which dust plays a major role. The study of dust–plasma interactions is a new and expanding field that is attracting considerable attention. New phenomena are expected to occur when plasma is loaded with large amounts of dust. Processes of this type are suspected to play a significant role in cometary environments, in planetary rings, and in protoplanetary disks.

Bibliography Green, S. F., Williams, I. P., McDonnell, J. A. M., and McBride, N., eds. (2002). “Dust in the Solar System and Other Planetary Systems.” Pergamon, Amsterdam. Grun, ¨ E., Gustafson, B. A. S., Dermott, S., and Fechtig, H., eds. (2001). “Interplanetary Dust.” Springer, Heidelberg. Gustafson, B. A. S., Greenberg, J. M., Kolokolova, L., Xu, Y., Stognienko, R. (2001) Interactions with electromagnetic radiation: Theory and laboratory simulations. In “Interplanetary Dust” (E. Grun, ¨ B. A. S. Gustafson, S. F. Dermott, H. Fechtig, eds.), Springer, Heidelberg. Gustafson, B. A. S., and Hanner, M. S., eds. (1996). “Physics, Chemistry, and Dynamics of Interplanetary Dust,” Conference Series Vol. 104. Astronomical Society of the Pacific, San Francisco. Leinert, C., and Grun, ¨ E. (1990) In “Physics of the Inner Heliosphere” (R. Schwenn and E. Marsch, eds.), pp. 207–275. SpringerVerlag, Berlin. Levasseur-Regourd, A. C., and Hasegawa, H., eds. (1991). “Origin and Evolution of Interplanetary Dust.” Kluwer, Dordrecht.

CLBE001-ESS2E

November 17, 2006

100-C

25-C

50-C

75-C

22:4

C+M

50-C+M

C+Y

50-C+Y

M+Y

50-M+Y

100-M

25-M

50-M

75-M

100-Y

25-Y

50-Y

75-Y

100-K

25-K

25-19-19

50-K

50-40-40

75-K

75-64-64

X-Rays in the Solar System Anil Bhardwaj Space Physics Laboratory Vikram Sarabhai Space Centre Trivandrum, India

Carey M. Lisse Applied Physics Laboratory Johns Hopkins University Laurel, Maryland

1. Introduction 2. Earth 3. The Moon 4. Venus 5. Mars

1. Introduction The usually defined range of X-ray photons spans ∼0.1–100 keV. Photons in the lower (20 keV X-rays. Nevertheless, these early omnidirectional measurements of X-rays revealed detailed information of temporal structures from slowly varying bay events to fast pulsations and microburst. The PIXIE instrument aboard POLAR is the first X-ray detector that provides true two-dimensional global X-ray image at energies >3 keV. In Fig. 2 two images taken by PIXIE in two different energy bands. The auroral Xray zone can be clearly seen. Data from the PIXIE camera have shown that the X-ray bremsstrahlung intensity statistically peaks at midnight, is significant in the morning sector, and has a minimum in the early dusk sector. During solar substorms X-ray imaging shows that the energetic electron precipitation brightens up in the midnight sector and has a prolonged and delayed maximum in the morning sector due to the scattering of magnetic-drifting electrons and shows an evolution significantly different than viewing in the UV emissions. During the onset/expansion phase of a typical substorm the electron energy deposition power is about 60–90 GW, which produces 10–30 MW of bremsstrahlung X-rays. By combining the results of PIXIE with the UV imager aboard POLAR, it has been possible to derive the energy distribution of precipitating electrons in the 0.1–100 keV range with a time resolution of about 5 min (see Fig. 2). Because these energy spectra cover the entire energy range important for

the electrodynamics of the ionosphere, important parameters like Hall and Pedersen conductivity and Joule heating can be determined on a global scale with larger certainties than parameterized models can do. Electron energy deposition estimated from global X-ray imaging also give valuable information on how the constituents of the upper atmosphere, like NOx , is modified by energetic electron precipitation. Limb scans of the nighttime Earth at low- to mid-latitude by the X-ray astronomy satellite HEAO-1 in 1977, in the energy range 0.15–3 keV, showed clear evidence of the Kα lines for nitrogen and oxygen sitting on top of the bremsstrahlung spectrum. Recently, the High-Resolution Camera (HRC-I) aboard the Chandra X-ray Observatory imaged the northern auroral regions of the Earth in the 0.1- to 10-keV X-ray range at 10 epochs (each ∼20 min duration) between December 2003 and April 2004. These first soft X-ray observations of Earth’s aurora (see Fig. 3) showed that it is highly variable (intense arcs, multiple arcs, diffuse patches, at times absent). Also, one of the observations showed an isolated blob of emission near the expected cusp location. Modeling of the observed soft X-ray emissions suggests that it is a combination of bremsstrahlung and characteristic K-shell line emissions of nitrogen and oxygen in the atmosphere produced by electrons. In the soft X-ray energy range of 0.1–2 keV, these line emissions are ∼5 times more intense than the X-ray bremsstrahlung.

CLBE001-ESS2E

November 17, 2006

22:4

640 Encyclopedia of the Solar System FIGURE 3 Earth’s aurora. Four X-rays images (shown on the same brightness scale) of the north polar regions of Earth obtained by Chandra HRC-I on different days (marked at the top of each image), showing large variability in soft (0.1–10.0 keV) X-ray emissions from Earth’s aurora. The bright arcs in these Chandra images show low-energy X-rays generated during auroral activity. The images—seen here superimposed on a simulated image of Earth—are from an approximately 20-minute scan during which Chandra was pointed at a fixed point in the sky while the Earth’s motion carried the auroral region through the field of view. Distance from the North Pole to the black circle is 3,340 km. (From Bhardwaj et al., 2006, J. Atmos. Sol-Terr. Phys., and http://chandra.harvard.edu/press/ 05 releases/press 122805.html.)

2.2 Nonauroral Emissions

3. The Moon

The nonauroral X-ray background above 2 keV from the Earth is almost completely negligible except for brief periods during major solar flares. However, at energies below 2 keV, soft X-rays from the sunlit Earth’s atmosphere have been observed even during quiet (nonflaring) Sun conditions. The two primary mechanisms for the production of X-rays from the sunlit atmosphere are: (1) Thomson (coherent) scattering of solar X-rays from the electrons in the atomic and molecular constituents of the atmosphere, and (2) the absorption of incident solar X-rays followed by the resonance fluorescence emission of characteristic K lines of nitrogen, oxygen, and argon. During flares, solar X-rays light up the sunlit side of the Earth by Thomson and fluorescent scattering; the X-ray brightness can be comparable to that of a moderate aurora. Around 1994, the Compton Gamma Ray Observatory (CGRO) satellite detected a new type of X-ray source from the Earth. These are very short-lived (1 ms) X-ray and γ -ray bursts (∼25 keV to 1 MeV) from the atmosphere above thunderstorms, whose occurrence is also supported by the more recent Reuvan Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observations. It has been suggested that these emissions are bremsstrahlung from upward-propagating, relativistic (MeV) electrons generated in a runaway electron discharge process above thunderclouds by the transient electric field following a positive cloud-to-ground lightning event.

X-Ray emissions from the Earth’s nearest planetary body, the Moon, have been studied in two ways: close up from lunar orbiters (e.g., Apollo 15 and 16, Clementine, and SMART-1), and more distantly from Earth-orbiting X-ray astronomy telescopes (e.g., ROSAT and Chandra). Lunar X-rays result mainly from fluorescence of sunlight by the surface, in addition to a low level of scattered solar radiation and a very low level of bremsstrahlung from solar wind electrons impacting the surface. Thus, X-ray fluorescence studies provide an excellent way to determine the elemental composition of the lunar surface by remote sensing, since at X-ray wavelengths the optical properties of the surface are dominated by its elemental abundances. Elemental abundance maps produced by the X-ray spectrometers on the Apollo 15 and 16 orbiters were limited to the equatorial regions but succeeded in finding geochemically interesting variations in the relative abundances of Al, Mg, and Si. Although the energy resolution of the Apollo proportional counters was low, important results were obtained, such as the enhancement of Al/Si in the lunar highlands relative to the mare. Recently, the D-CIXS instrument on SMART-1 has obtained abundances of Al, Si, Fe, and even Ca at 50-km resolution from a 300-km altitude orbit about the Moon. Upcoming missions planned for launch in 2007–2008 by Japan (SELENE), India (Chandrayaan-1), and China (Chang’e) will each carry X-ray spectrometers to obtain further improved maps of the Moon’s elemental

CLBE001-ESS2E

November 17, 2006

22:4

composition, at ∼20-to 50-km resolution from ∼100- to 200-km altitude polar orbits. Early observations from Earth orbit were made using the ROSAT. A marginal detection by the Advanced Satellite for Cosmology and Astrophysics (ASCA) is also reported. Figure 4a shows the ROSAT images of the Moon, the right image is data from a lunar occultation of the bright Xray source GX5-1. The power of the reflected and fluoresced X-rays observed by ROSAT in the 0.1- to 2-keV range coming from the sunlit surface was determined to be only 73 kW. The faint but distinct lunar night side emissions (100 times less bright than the day side emissions) were until recently a matter of controversy. Earlier suggestions had the night side X-rays produced by bremsstrahlung of solar wind electrons of several hundred eV impacting the night side of the Moon on its evening (leading) hemisphere. However, this was before the GX5-1 data were acquired, which clearly show lunar night side X-rays from the early morning (trailing) hemisphere as well. A new, much better and accepted explanation is that the heavy ions in the solar wind charge exchange with geocoronal and interstellar H atoms that lie between the Earth and Moon resulting in foreground X-ray emissions between ROSAT and the Moon’s dark side. This was confirmed by Chandra ACIS observations in 2001 (see Fig. 4c). The July 2001 Chandra observations also provide the first remote measurements that clearly resolve discrete Kshell fluorescence lines of O, Mg, Al, and Si on the sunlit side of the Moon (see Fig. 4b). The observed O–K line photons correspond to a flux of 3.8 × 10−5 photons/s/cm2 /arcmin2 (3.2 × 10−14 erg/s/cm2 /arcmin2 ). The Mg–K, Al–K, and Si–K lines each had roughly 10% as many counts and 3% as much flux as O–K line, but statistics were inadequate to draw any conclusions regarding differences in element abundance ratios between highlands and maria. More recent Chandra observations of the Moon used the photon counting, high spatial resolution HRC-I imager to look for albedo variations due to elemental composition differences between highlands and maria. The observed albedo contrast was noticeable, but very slight, making remote elemental mapping difficult.

direct imaging with ACIS-I. This combination yielded data of high spatial, spectral, and temporal resolution. Venus was clearly detected as a half-lit crescent, exhibiting considerable brightening on the sunward limb (Fig. 5); the LETG/ACIS-S data showed that the spectrum was dominated by O–Kα and C–Kα emission, and both instruments indicated temporal variability of the X-ray flux. An average luminosity of 55 MW was found, which agreed well with the theoretical predictions for scattered solar X-rays. In addition to the C–Kα and O–Kα emission at 0.28 and 0.53 keV, respectively, the LETG/ACIS-S spectrum also showed evidence for N–Kα emission at 0.40 keV. An additional emission line was indicated at 0.29 keV, which might be the signature of the C 1s → π ∗ transition in CO2 . The observational results are consistent with fluorescent scattering of solar X-rays by the majority species in the Venusian atmosphere, and no evidence of the 30 times weaker charge exchange interactions was found. Simulations showed that fluorescent scattering of solar X-rays is most efficient in the Venusian upper atmosphere at heights of ∼120 km, where an optical depth of one is reached for incident X-rays with energy 0.2–0.9 keV. The appearance of Venus is different in optical light and X-rays. The reason for this is that the optical light is reflected from clouds at a height of 50–70 km, while scattering of X-rays takes place at higher regions extending into the tenuous, optically thin parts of the thermosphere and exosphere. As a result, the Venusian sun-lit hemisphere appears surrounded by an almost transparent luminous shell in X-rays, and Venus looks brightest at the limb because more luminous material is there. Because X-ray brightening depends sensitively on the density and chemical composition of the Venusian atmosphere, its precise measurement will provide direct information about the atmospheric structure in the thermosphere and exosphere. This opens up the possibility of using X-ray observations for monitoring the properties of these regions that are difficult to investigate by other means, as well as their response to solar activity. In 2007, Chandra will reobserve Venus during its best window for 2 years, while the MESSENGER spacecraft, flying by on its way to Mercury, and the Venus Express spacecraft in Venusian orbit probe the temperature, density, pressure, and composition of the Venusian atmosphere.

4. Venus The first X-ray observation of Venus was obtained by Chandra in January 2001. It was expected that Venus would be an X-ray source due to two processes: (1) charge exchange interactions between highly charged ions in the solar wind and the Venusian atmosphere and (2) scattering of solar X-rays in the Venusian atmosphere. The predicted X-ray luminosities were ∼0.1–1.5 MW for the first process, and ∼35 MW for the second one, with an uncertainty factor of about two. The Chandra observation of 2001 consisted of two parts: grating spectroscopy with LETG/ACIS-S and

5. Mars The first X-rays from Mars were detected on 4 July 2001 with the ACIS-I detector onboard Chandra. In the Chandra observations, Mars showed up as an almost fully illuminated disk (Fig. 6). An indication of limb brightening on the sunward side, accompanied by some fading on the opposite side, was observed. The observed morphology and X-ray luminosity of ∼4 MW, about 10 times less than at Venus,

641

CLBE001-ESS2E

November 17, 2006

22:4

642 Encyclopedia of the Solar System (a)

(b)

FIGURE 4 The Moon. (a) ROSAT soft X-ray (0.1–2 keV) images of the Moon at first (left side) and last (right side) quarter. The day side lunar emissions are thought to be primarily reflected and fluoresced sunlight, while the faint night side emissions are foreground due to charge exchange of solar wind heavy ions with H atoms in Earth’s exosphere. The brightness scale in R assumes an average effective area of 100 cm2 for the ROSAT PSPC over the lunar spectrum. [From Bhardwaj et al., 2002, ESA-SP-514, 215–226.] (b) Chandra spectrum of the bright side of the Moon. The green dotted curve is the detector background. K-shell fluorescence lines from O, Mg, Al, and Si are shifted up by 50 eV from their true values because of residual optical leak effects. Features at 2.2, 7.5, and 9.7 keV are intrinsic to the detector. [From Wargelin et al., 2004, Astrophys. J., 607, 596–610.] (c) Observed and background-subtracted spectra from the September 2001 Chandra observation of the dark side of the Moon, with 29-eV binning. Left panel is from the higher-QE but lower-resolution ACIS S3 CCD; right panel shows the higher resolution ACIS front-illuminated (FI) CCDs. Oxygen emission from charge exchange is clearly seen in both spectra, and energy resolution in the FI chips is sufficient that O Lyman α is largely resolved from O Kα. High-n H-like O Lyman lines are also apparent in the FI spectrum, along with what is likely Mg Kα around 1340 eV. (From Wargelin et al., 2004, Astrophys. J., 607, 596–610.)

CLBE001-ESS2E

November 17, 2006

22:4

FIGURE 5 Venus. (a) First X-ray image of Venus, obtained with Chandra ACIS-I on 13 January 2001. The X-rays result mainly from fluorescent scattering of solar X-rays on C and O in the upper Venus atmosphere, at heights of 120–140 km. In contrast to the Moon, the X-ray image of Venus shows evidence for brightening on the sunward limb. This is caused by the scattering that takes place on an atmosphere and not on a solid surface. [From Dennerl et al., 2002, Astron. Astrophys., 386, 319]. (b) Expected LETG spectrum of Venus on the ACIS-S array. Energy and wavelength scales are given along the dispersion direction. Images of Venus are drawn at the position of the C, N, and O fluorescence lines, with the correct size and orientation. The dashed rectangle indicates the section of the observed spectrum shown below. (c) Observed spectrum of Venus, smoothed with a Gaussian function with σ = 20 . The two bright crescents symmetric to the center are images in the line of the O-Kα fluorescent emission, while the elongated enhancement at left is at the position of the C-Kα fluorescent emission line. The Sun is at bottom. (d) Spectral scan along the region outlined above. Scales ˚ The observed C, N, and O fluorescent emission lines are enclosed by dashed lines; the width of these intervals are given in keV and A. matches the size of the Venus crescent (22.8 ). (From Dennerl et al., 2002, Astron. Astrophys., 386, 319.)

643

CLBE001-ESS2E

November 17, 2006

22:4

644 Encyclopedia of the Solar System

FIGURE 6 Mars. (a) First X-ray image of Mars, obtained with Chandra ACIS-I. The X-rays result mainly from fluorescent scattering of solar X-rays on C and O in the upper Mars atmosphere, at heights of 110–130 km, similar to Venus. The X-ray glow of the Martian exosphere is too faint to be directly visible in this image. (From Dennerl, 2002, Astronomy and Astrophysics, 394, 1119–1128.) (b) Spatial distribution of the photons around Mars in the soft (E = 0.2–1.5 keV) and hard (E = 1.5–10.0 keV) energy range, in terms of surface brightness along radial rings around Mars, separately for the day side (offset along projected solar direction >0) and the night side (offset 2 keV) energy component present in the spectrum of Jupiter’s aurora; they found it to be variable on timescales of days. The observed spectrum and flux, at times, appears consistent with that predicted from bremsstrahlung of energetic electrons precipitating from the magnetosphere, but at energies greater than 2 keV (at lower energies bremsstrahlung still fall short by an order of magnitude). The variability suggests a link to changes in the energy distribution of the precipitating magnetospheric electrons and may be related to the solar activity at the time of observation.

6.2 Nonauroral (Disk) Emission The existence of low-latitude “disk” X-ray emission from Jupiter was first recognized in ROSAT observations made in 1994. These X-rays were initially thought to be the result of precipitation of energetic S and O ions from Jupiter’s inner radiation belts into the planet’s atmosphere. Later, as for the inner planets, it was suggested that elastic scattering of solar X-rays by atmospheric neutrals (H2 ) and fluorescent scattering of carbon K-shell X-rays from CH4 molecules located below the jovian homopause was the source of the disk X-rays. A general decrease in the overall X-ray brightness of Jupiter observed by ROSAT over the years 1994–1996 was found to be coincident with a similar decay in solar activity index (solar 10.7 cm flux). A similar trend is seen in the data obtained by Chandra in 2000 and 2003; Jupiter disk was about 50% dimmer in 2003 compared to that in 2000, which is consistent with variation in the solar activity index. First direct evidence for temporal correlation between jovian disk X-rays and solar X-rays is provided by XMM-Newton observations of Jupiter in November 2003, which demonstrated that day-to-day variation in disk Xrays of Jupiter are synchronized with variation in the solar X-ray flux, including a solar flare that has a matching feature in the jovian disk X-ray light curve. Chandra observations of December 2000 and February 2003 also support this association between light curves of solar and planetary X-rays. However, there is an indication of higher X-ray counts from regions of low surface magnetic field in the Chandra data, suggesting the presence of some particle precipitation. The higher spatial resolution observation by Chandra has shown that nonauroral disk X-rays is relatively more spatially uniform than the auroral X-rays (Fig. 8). Unlike the ∼40 ± 20-min quasi-periodic oscillations seen in auroral X-ray emission, the disk emission does not show any systematic pulsations. There is a clear difference between the X-ray spectra from the disk and auroral region on Jupiter; the disk spectrum peaks at higher energies (0.7– 0.8 keV)

than the aurorae (0.5–0.6 keV) and lacks the high–energy component (above ∼ 3 keV) present in the latter (see Fig. 8).

7. Galilean Satellites The jovian Chandra observations on 25–26 November 1999 and 18 December 2000 discovered X-ray emission from the Galilean satellites (Fig. 9). These satellites are very faint when observed from Earth orbit (by Chandra), and the detections of Io and Europa, although statistically very significant, were based on ∼10 photons each! The energies of the detected X-ray events ranged between 300 and 1890 eV and appeared to show a clustering between 500 and 700 eV, suggestive of oxygen K-shell fluorescent emission. The estimated power of the X-ray emission was 2 MW for Io and 3 MW for Europa. There were also indications of Xray emission from Ganymede. X-Ray emission from Callisto seems likely at levels not too far below the CXO sensitivity limit because the magnetospheric heavy ion fluxes are an order of magnitude lower than at Ganymede and Europa, respectively. The most plausible emission mechanism is inner (K shell) ionization of the surface (and incoming magnetospheric) atoms followed by prompt X-ray emission. Oxygen should be the dominant emitting atom either in an SiOx (silicate) or SOx (sulfur oxides) surface (Io) or on an icy one (the outer Galilean satellites). It is also the most common heavy ion in the jovian magnetosphere. The extremely tenuous atmospheres of the satellites are transparent to X-ray photons with these energies, as well as to much of the energy range of the incoming ions. However, oxygen absorption in the soft X-ray is strong enough that the X-rays must originate within the top 10 micrometers of the surface in order to escape. Simple estimates suggest that excitation by incoming ions dominates over electrons and that the X-ray flux produced is within a factor of 3 of the measured flux. The detection of X-ray emission from the Galilean satellites thus provides a direct measure of the interactions of the magnetosphere of Jupiter with the satellite surfaces. An intriguing possibility is placement of an imaging X-ray spectrometer on board a mission to the Jupiter system. If such an instrument was in orbit around a Galilean satellite (e.g., Europa or Ganymede), even though it would be immersed in a fierce radiation environment, it would be able to map the elemental abundances of the surface for elements from C through Fe.

8. Io Plasma Torus The Io Plasma Torus (IPT) is known to emit at extreme ultraviolet (EUV) energies and below, but it was a surprise when Chandra discovered that it was also a soft X-ray source. The 1999 jovian Chandra observations

CLBE001-ESS2E

November 17, 2006

22:4

X-Rays in the Solar System

649

FIGURE 9 Galilean Moons. Chandra X-ray images of Io and Europa (0.25 keV < E < 2.0 keV) from November, 1999 observations. The images have been smoothed by a two-dimensional gaussian with σ = 2.46 arcsec (5 detector pixels). The axes are labeled in arcsec (1 arcsec ∼= 2995 km) and the scale bar is in units of smoothed counts per image pixel (0.492 by 0.492 arcsec). The solid circle shows the size of the satellite (the radii of Io and Europa are 1821 km and 1560 km, respectively), and the dotted circle the size of the detect cell. [from Elsner et al., Astrophysical Journal, 572, 1077–1082, 2002].

detected a faint diffuse source of soft X-rays from the region of the IPT. The 2000 Chandra image, obtained with the HRC-I camera (Fig. 10), exhibited a dawn-to-dusk asymmetry similar to that seen in the EUV. Figure 10 shows the background-subtracted Chandra/ACIS-S IPT spectrum for 25–26 November 1999. This spectrum shows evidence for line emission centered on 574 eV (very near a strong O VII line), together with a very steep continuum spectrum at the softest X-ray energies. Although formed from the same source, the spectrum is different than from the jovian aurora because the energies, charge states, and velocities of the ions in the torus are much lower—the bulk ions have not yet been highly accelerated. There could be contributions from other charge states because current plasma torus models consist mostly of ions with low charge states, consistent with photoionization and ion-neutral charge exchange in a low-density plasma and neutral gas environment. The 250–1000 eV energy flux at the telescope aperture was 2.4 × 10−14 erg cm−2 s−1 , corresponding to a luminosity of 0.12 GW. Although bremsstrahlung from nonthermal electrons might account for a significant fraction of the continuum X-rays, the physical origin of the observed IPT X-ray emission is not yet fully understood. The 2003 jovian Chandra observations also detected X-ray emission from the IPT, although at a fainter level than in 1999 or 2000. The morphology exhibited the familiar dawn-to-dusk asymmetry.

9. Saturn The production of X-rays at Saturn was expected because, like the Earth and Jupiter, Saturn was known to possess a magnetosphere and energetic electrons and ions particles within it; however, early attempts to detect X-ray emission from Saturn with Einstein in December 1979 and with ROSAT in April 1992 were negative and marginal, respectively. Saturnian X-rays were unambiguously observed by XMM-Newton in October 2002 and by the Chandra X-ray Observatory in April 2003. In January 2004, Saturn was again observed by the Chandra ACIS-S in two exposures, one on 20 January and other on 26–27 January, with each observation lasting for about one full Saturn rotation. The Xray power emitted from Saturn’s disk is roughly one-fourth of that from Jupiter’s disk, which is consistent with Saturn being twice as far as Jupiter from Sun and Earth. The January 2004 Chandra observation showed (Fig. 11) that X-rays from Saturn are highly variable—a factor of 2 to 4 variability in brightness over 1 week. These observations also revealed X-rays from Saturn’s south polar cap on January 20 (see Fig. 11, left panel), which are not evident in the January 26 observation (see Fig. 11, right panel) and in earlier Chandra observations. X-rays from the south polar cap region were present only in the 0.7–1.4 keV energy band, in contrast with Jupiter’s X-ray aurora for which the

CLBE001-ESS2E

November 17, 2006

22:4

650 Encyclopedia of the Solar System FIGURE 10 Plasma Torus. (a) Chandra/HRC-I image of the IPT (2000 December 18). The image has been smoothed by a two-dimensional Gaussian with σ = 7.38 (56 HRC-I pixels). The axes are labeled in units of Jupiter’s radius, RJ , and the scale bar is in units of smoothed counts per image pixel. The paths traces by Io, Europa, and Ganymede are marked on the image. Callisto is off the image to the dawn side, although the satellite did fall within the full microchannel plate field of view. The regions bounded by rectangles were used to determine background. The regions bounded by dashed circles or solid ellipses were defined as source regions. (b) Chandra/ACIS-S spectrum for the Io Plasma Torus from November 1999. The solid line presents a model fit for the sum of a power-law spectrum and a Gaussian line, while the dashed line represents just a pure power law spectrum. The line is consistent with K-shell flurorescent emission from oxygen ions. [From Elsner et al., Astrophysical Journal, 572, 1077–1082, 2002].]

emission is mostly in the bands 0.3–0.4 keV and 0.6–0.7 keV. Because of this, it is likely that the X-ray emission from the south polar cap is unlikely to be auroral in nature, and more likely that they are an extension of the disk X-ray emission of Saturn. Any emission from the north polar cap region was blocked by Saturn’s rings. As is the case for Jupiter’s disk, X-ray emission from Saturn seems likely to be due to the scattering of the incident solar X-ray flux. An X-ray flare has been detected from the nonauroral disk of Saturn during the Chandra observation on 20 January 2004. Taking light travel time into account, this X-ray flare from Saturn coincided with an M6class flare emanating from a sunspot that was clearly visible from both Saturn and Earth. Moreover, the lightcurve for the X-rays from Saturn was very similar to that of the solar X-ray flux. This was the first direct evidence suggesting that Saturn’s disk X-ray emission is principally controlled by processes happening on the Sun. Further, a good correla-

tion has been observed between Saturn X-rays and F10.7 solar activity index: suggesting a solar connection. The spectrum of X-rays from Saturn’s disk is very similar to that from Jupiter’s disk. Saturn’s disk spectrum measured on 20 January 2004 is quite similar to that measured on 14–15 April 2003 in the 0.3–0.6 keV range. However, at energies 0.6–1.2 keV, the former is stronger by a factor of 2 to 4. This is probably due to the nature of the M6-class solar X-ray flare on 20 January, with a corresponding hardening of the solar X-ray flux driving Saturn’s X-ray emission.

10. Rings of Saturn The rings of Saturn, known to be made of mostly water (H2 O) ice, are one of the most fascinating objects in our solar system. Recently, the discovery of X-rays from the rings

CLBE001-ESS2E

November 17, 2006

22:4

X-Rays in the Solar System

651

FIGURE 11 Saturn. (a) Chandra ACIS X-ray 0.24–2.0 keV images of Saturn on January 20 and 26, 2004. Each continuous observation lasted for about one full Saturn rotation. The horizontal and vertical axes are in units of Saturn’s equatorial radius. The white scale bar in the upper left of each panel represents 10?. The two images, taken a week apart and shown on the same color scale, indicate substantial variability in Saturn’s X-ray emission. [from Bhardwaj et al. Astrophysical Journal Letters, 624, L121-L124 2005]. (b) Disk X-ray spectrum of Saturn (red curve) and Jupiter (blue curve). Values for Saturn spectrum are plotted after multiplying by a factor of 5. [from Bhardwaj, Advances in Geosciences, vol.3, 215–230, 2006].

of Saturn was made from the Chandra ACIS-S observations of the Saturnian system conducted in January 2004 and April 2003. X-rays from the rings are dominated by emission in a narrow (∼130 eV wide) energy band of 0.49– 0.62 keV (Fig. 12). This band is centered on the oxygen Kα fluorescence line at 0.53 keV, suggesting that fluorescent scattering of solar X-rays from oxygen atoms in the surface of H2 O icy ring material is the likely source mechanism for ring X-rays. The X-ray power emitted by the rings in the 0.49–0.62 keV band on 20 January 2004 is 84 MW, which is about one-third of that emitted from the Saturn disk in the 0.24- to 2.0-keV band. The projected rings have about half the surface area of the Saturn disk, consistent with this ratio. During 14–15 April 2003, the X-ray power emitted by the rings in the 0.49- to 0.62-keV band is about 70 MW. Figure 12 shows the X-ray image of the Saturnian system in January 2004 in the 0.49- to 0.62-keV band, the energy range where X-rays from the rings are unambiguously detected. The observations of January 2004 also suggested that, similar to Saturn’s X-ray emission, the ring X-rays are highly variable—a factor of 2–3 variability in brightness over

1 week. There is an apparent asymmetry in X-ray emission from the east (morning) and west (evening) ansae of the rings (see Fig. 12a). However, when the Chandra ACIS-S data set of January 2004 and April 2003 is combined, the evidence for asymmetry is not that strong.

11. Comets The discovery of high-energy X-ray emission in 1996 from C/1996 B2 (Hyakutake) created a new class of X-ray– emitting objects. Observations since 1996 have shown that the very soft (E < 1 keV) emission is due to an interaction between the solar wind and the comet’s atmosphere, and that X-ray emission is a fundamental property of comets. Theoretical and observational work has demonstrated that charge exchange collision of highly charged heavy solar wind ions with cometary neutral species is the best explanation for the emission. The X-rays are extremely easy to detect because the neutral atmosphere of a comet is large and extended and gravitationally unbound, intercepting a

CLBE001-ESS2E

November 17, 2006

22:4

652 Encyclopedia of the Solar System (a)

Jan 20, 2004

Jan 26, 2004

1. RS

1.

0

RS

−1.

0 −1.

−2. −1. 0 1. 2. RS

0

−2. −1. 0 1. 2. RS

0.01 0.02 0.03 Brightness (R)

(b)

large amount of solar wind ions as they stream away from the Sun. The observed characteristics of the emission can be organized into the following four categories: (1) spatial morphology, (2) total X-ray luminosity, (3) temporal variation, and (4) energy spectrum. Any physical mechanism that purports to explain cometary X-ray emission must account for all of these characteristics. X-Ray and EUV images of C/1996 B2 (Hyakutake) made by the ROSAT and EUVE satellites look very similar (Fig. 13). Except for images of C/1990 N1 and C/Hale-Bopp 1995 O1, all EUV and X-ray images of comets have exhibited similar spatial morphologies. The emission is largely confined to the sunward side of the cometary coma; almost no emission is found in the extended tails of dust or

FIGURE 12 Saturn’s Rings. (a) Chandra ACIS X-ray images of the Saturnian system in the 0.49–0.62 keV band on 2004 January 20 and 26–27. The X-ray emission from the rings is clearly present in these restricted energy band images; the emission from the planet is relatively weak in this band (see Fig. 11(a) for an X-ray image of the Saturnian system in the 0.24–2.0 keV band). (b) Background-subtracted Chandra ACIS-S3–observed X-ray energy spectrum for Saturn’s rings in the 0.2–2.0 keV range on 2004 January 20 and 26–27. The cluster of X-ray photons in the ∼0.49–0.62 keV band suggests the presence of the oxygen Kα line emission at 0.53 keV in the X-ray emission from the rings. The inset shows a Gaussian fit (peak energy = 0.55 keV, σ = 140 eV), shown by the dashed line, to the ACIS-observed rings’ spectrum on January 20. Each spectral point (filled circle with error bar) represents ≥10 measured events. The spectral fitting suggests that X-ray emissions from the rings are predominantly oxygen Kα photons. [from Bhardwaj et al., Astrophys. J. Lett., 627, L73-L76, 2005].

plasma. The peak X-ray brightness gradually decreases with increasing cometocentric distance r with a dependence of about r −1 . The brightness merges with the soft X-ray background emission at distances that exceed 104 km for weakly active comets, and can exceed 106 km for the most luminous comets. The region of peak emission is crescent-shaped with a brightness peak displaced towards the Sun from the nucleus. The distance of this peak from the nucleus appears to increase with increasing values of Q (total gas production rate); for Hyakutake, it was located at rpeak ∼ 2 × 104 km. The observed X-ray luminosity, Lx , of C/1996 B2 (Hyakutake) was 4 × 1015 ergs s−1 for an aperture radius at the comet of 1.2 × 105 km. (Note that the photometric luminosity depends on the energy bandpass and the observational

CLBE001-ESS2E

November 17, 2006

22:4

X-Rays in the Solar System

aperture at the comet. The quoted value assumes a ROSAT photon emission rate of PX ∼ 1025 s−1 (0.1–0.6 keV), in comparison to the EUVE estimate of PEUV ∼ 7.5 × 1025 s−1 (0.07–0.18 keV and 120,000-km aperture). A positive correlation between optical and X-ray luminosities was demonstrated using observations of several comets having similar gas (QH2 O )-to-dust (Afρ) emission rate ratios. Lx correlates more strongly with the gas production rate Qgas than it does with Lopt ∼ Qdust ∼ Afρ. Particularly dusty comets, like Hale–Bopp, appear to have less X-ray emission than would be expected from their overall optical luminosity Lopt . The peak X-ray surface brightness decreases with increasing heliocentric distance r, independent of Q, although the total luminosity appears roughly independent of r. The maximum soft X-ray luminosity observed for a comet to date is ∼2 × 1016 erg s−1 for C/Levy at 0.2–0.5 keV. Photometric lightcurves of the X-ray and EUV emission typically show a long-term baseline level with superimposed impulsive spikes of a few hours’ duration, and maximum amplitude 3 to 4 times that of the baseline emission level. Figure 13 demonstrates the strong correlation found between the time histories of the solar wind proton flux (a proxy for the solar wind minor ion flux), the solar wind magnetic field intensity, and a comet’s X-ray emission, for the case of comet 2P/Encke 19997. Comparison of the ROSAT and EUVE luminosity of C/1996 B2 (Hyakutake) with time histories of the solar wind proton flux, oxygen ion flux, and solar X-ray flux, showed a strongest correlation between the cometary emission and the solar wind oxygen ion flux, a good correlation between the comet’s emission and the solar wind proton flux, but no correlation between the cometary emission and the solar X-ray flux. Until 2001, all published cometary X-ray spectra had very low spectral energy resolution (E/E ∼ 1 at 300–600 eV), and the best spectra were those obtained by ROSAT for C/1990 K1 (Levy) and by BeppoSAX for comet C/ 1995 O1 (Hale–Bopp). These observations were capable of showing that the spectrum was very soft (characteristic thermal bremsstrahlung temperature kT ∼ 0.23 ± 0.04 keV) with intensity increasing toward lower energy in the 0.01- to 0.60keV energy range and established upper limits to the contribution of the flux from K-shell resonance fluorescence of carbon at 0.28 keV and oxygen at 0.53 keV. However, even in these “best” spectra, continuum emission (such as that produced by the thermal bremsstrahlung mechanism) could not be distinguished from a multiline spectrum, such as would result from the SWCX mechanism. Nondetections of comets C/Hyakutake, C/Tabur, C/Hale–Bopp, and 55P/Tempel–Tuttle using the XTE PCA (2–30 keV) and ASCA SIS (0.6–4 keV) imaging spectrometers were consistent with an extremely soft spectrum. Higher resolution spectra of cometary X-ray emission have now appeared in the literature. The Chandra X-ray Observatory (CXO) measured soft X-ray spectra

653

from comet C/1999 S4 (LINEAR) over an energy range of 0.2–0.8 keV, and with a full width half maximum energy resolution of E = 0.11 keV (Fig. 13). The spectrum is dominated by line emission from C+4 , C+5 , O+6 , and O+7 excited ions, not by continuum. A spectrum of comet C/1999 T1 (McNaught–Hartley) showed similar line emission features, with a somewhat higher ratio of OVII to OVIII emission, and emission due to Ne+9 . A new spectrum of comet 2P/Encke shows a very different ratio of line emission in the C+4 , C+5 , O+6 , and O+7 lines, due to the collisionally thin nature of the low activity coma, and the unusual postshock charge state of the solar wind at the time of observation. Line emission is also found in XMM-Newton spectra of comet C/1999 T1 (McNaught–Hartley) and, more recently, in CXO spectra of C/2001 WM1 (Lincoln NearEast Asteroid Research, Linear) and C/2002 Ikeya–Zhang. An XMM-Newton spectrum of C/2001 WM1 (LINEAR) shows characteristic SWCX X-ray signatures in unprecedented detail. From other work, there are suggestions of charge exchange line emission from other species than C+4 /C+5 , O+6 /O+7 , and Ne+9 . A reanalysis of archival EUVE Deep Survey spectrometer spectra suggests EUV line emission features from comet C/1996 B2 (Hyakutake) due to O+ , O+5 , O+4 , C+4 , O+6 , C+5 , He+ , and Ne+7 . It has been suggested that emission lines are attributable to Mg and Si in C/McNaught–Hartley, and He+2 in C/Hale–Bopp, although these remain unconfirmed and controversial due to the sensitivity of the results on the details of the instrumental background subtraction. Hints of possible emission due to N+6 at 425 eV contributing to a reduced 380/450 eV ratio were found in Chandra observations of 2P/Encke in 2003. Numerical simulations of the solar wind interaction with Hyakutake including SWCX have been used to generate Xray images. A global magneto-hydrodynamic (MHD) model and a hydrodynamic model were used to predict solar wind speeds and densities in addition to the X-ray emission around a comet. The simulated X-ray images are similar to the observed images. Recent work has shown that by determining the location of the emission maximum in the collisionally thick case, the neutral gas production rate can be determined in 5 comets observed by ROSAT and XMMNewton. On comet WM1, the position of the cometary bow shock has been determined using the location of rapid changes in the first and second derivatives of the flux with distance from the nucleus. It is not clear that the emission pattern always follows the plasma structures. New work suggests that the crescentshaped, sunward offset morphology is found only for comets with coma dense enough to be in the collisionally thick regime—for low activity comets, the emission will be maximal wherever the coma has its maximum density, typically at the nucleus. This may explain the unusual emission

CLBE001-ESS2E

November 17, 2006

22:4

654 Encyclopedia of the Solar System

CLBE001-ESS2E

November 17, 2006

22:4

X-Rays in the Solar System

morphologies seen in comets like d’Arrest 1997 and 2P/ Encke 2003. Up until now, the temporal variation of the solar wind dominated the observed behavior on all but the longest timescales of weeks to months. A “new” form of temporal variation has recently been demonstrated in the Chandra observations of comet 2P/Encke 2003, wherein the observed X-ray emission is modulated at the 11.1-hour period of the nucleus rotation. Rotational modulation of the signal should be possible only in collisionally thin (to SWCX) comae with weak cometary activity, where a change in the coma neutral gas density can directly affect power density of cometary X-ray. Driven by the solar wind, cometary X-rays provide an observable link between the solar corona, where the solar wind originates, and the solar wind where the comet resides. Once we have understood the SWCX mechanism’s behavior in cometary comae in sufficient detail, we will be able to use comets as probes to measure the solar wind throughout the heliosphere. This will be especially useful in monitoring the solar wind in places hard to reach with spacecraft—such as over the solar poles, at large distances above and below the ecliptic plane, and at heliocentric distances greater than a few AU. For example, about one-third of the observed soft X-ray emission is found in the 530- to 700-eV oxygen O+7 and O+6 lines; observing photons of this energy will allow studies of the oxygen ion charge ratio of the solar wind, which is predicted to vary significantly between the slow and fast solar winds at low and high solar latitudes, respectively.

655

12. Asteroids X-Rays from asteroids have been studied by experiments on two in situ missions, the X-ray/gamma-ray spectrometer (XGRS) on the Near Earth Asteroid Rendezvous (NEAR)–Shoemaker mission to asteroid 433 Eros, and the X-ray spectrometer (XRS) on the Hayabusa mission to asteroid 25143 Itokawa. The only attempt to detect X-rays from an asteroid was a 10-ks distant, remote observation by Chandra on 11 December 2001 of 1998 WT24, but it was unsuccessful. The results of the in situ observations show Xray emission due to fluorescence and scattering of incident solar X-rays, similar to the emission seen from the surface of the airless Moon. In fact, the best measurements were obtained during a strong solar flare, when the incident solar X-rays were highly amplified. As for the Moon, X-ray spectroscopy of resonantly scattered solar X-rays can be used to map the elemental composition of the surface. NEAR–Shoemaker entered Eros orbit on 14 February 2000 and completed a 1-year long mission around it. Eros at 33 × 13 × 13 km in size is the second largest near-Earth asteroid, and its “day” is 5.27 hours long. Eros exhibits a heavily cratered surface with one side dominated by a huge, scallop-rimmed gouge; a conspicuous sharp, raised rimmed crater occupies the other side. The XRS part of the XGRS detected X-rays in the 1- to 10-keV energy range to determine the major elemental composition of Eros’ surface. The XRS observed the asteroid in low orbit ( 8 bar) are 3–4 times solar, while radio spectra (Fig. 6)  show a subsolar abundance of NH3 gas at pressures P < 2 bar. The apparent decrease in the NH3 abundance at higher altitudes may be caused by dynamical processes, but the jury is still out on this. Radio images of Jupiter clearly show bright zonal bands across the disk (Fig. 7a), which coincide with the brown belts seen at visible wavelengths. These bands have a higher brightness temperature, likely due to a lower opacity in the belts relative to the zonal regions, so deeper warmer layers are probed in the belts. This phenomenon is suggestive of gas rising up in the zones; when the temperature drops below ∼140 K, ammonia gas condenses out. In the belt regions, the air, now depleted in ammonia gas (i.e., dry air), descends. This general picture agrees with that suggested from analyses of visible and infrared data. Note, though, that the radio data probe the gas from which the clouds condense, while visible and infrared data are sensitive primarily to the cloud particles. Thus, the base level of the clouds is determined through radio observations, whereas the cloud tops are probed at optical and infrared wavelengths.

CLBE001-ESS2E

November 10, 2006

18:0

The Solar System at Radio Wavelengths

701

FIGURE 6 (a) Microwave spectrum of Jupiter, with superposed models for a solar composition atmosphere (dashed line), and one in which the altitude profiles for the condensable gases were based upon the Galileo probe data (solid line). (b) Microwave spectrum of Uranus, with superposed models for a solar composition atmosphere (dashed line), and one in which H2 S and H2 O gases are enhanced by a factor ∼10 above solar values. In these models, ammonia gas is significantly depleted at higher altitudes in Uranus atmosphere through formation of NH4 SH, so that deeper warmer levels are probed. (c) Cloud structure in Jupiter’s atmosphere as calculated assuming thermochemical equilibrium. The altitude profile of ammonia gas, based on Galileo and ground-based radio data, is superposed. (d) Cloud structure in Uranus’s atmosphere as calculated assuming thermochemical equilibrium and CH4 , H2 O and H2 S abundances 30 times solar. The altitude profiles for H2 S and NH3 gas are indicated. (I. de Pater and J. J. Lissauer, forthcoming, “Planetary Sciences,” Rev. Ed., Cambridge Univ. Press.)

CLBE001-ESS2E

November 10, 2006

18:0

702 Encyclopedia of the Solar System (a)

(b)

FIGURE 7 (a) A radio photo of Jupiter at a wavelength of 2 cm, integrated over 6–7 hours, so any longitudinal structure is smeared out. The data were obtained with the VLA on 25 January 1996. The angular resolution is 1.4,” which was 0.044 Jupiter radii at the time of the observations. (I. de Pater et al., 2001, Comparison of Galileo probe data with ground-based radio measurements, Icarus 149, 66–78.) (b) A comparison of the North Equatorial Belt of Jupiter at an infrared wavelength of 5 μm (IRTF, NASA’s Infrared Telescope Facility) and radio wavelength of 2 cm (VLA). The latter image is constructed from the same data as displayed in Fig. 7a, but using novel new data reduction techniques. (R. J. Sault et al., 2004, Longitude-resolved imaging of Jupiter at λ = 2 cm, Icarus 168, 336–343.)

In recent years, an algorithm has been developed to construct longitude-resolved images of Jupiter, and these maps reveal, for the first time, hot spots at radio wavelengths that are strikingly similar to those seen in the infrared. An example of Jupiter’s North Equatorial Belt is shown in Fig. 7b. At radio wavelengths, the hot spots indicate a relative absence of NH3 gas, whereas they suggest a lack of cloud particles in the infrared. Models show that ammonia must be de-

pleted down to pressure levels of ∼5 bar in the hot spots, the approximate altitude of the water cloud. 2.5.3 SATURN

Images of Saturn’s microwave emission at different viewing geometries are shown in Fig. 8. The planet itself is visible through its thermal emission. The emission from the FIGURE 8 Radio photographs of Saturn at 2 and 3.6 cm, at different viewing aspects of the planet. (a) 3.6 cm, 1990; (b) 2 cm, 1994; (c) 2 cm, 1998; (d) 3.6 cm, 2002. (Dunn, D.E., I. de Pater, and L.A. Molnar, 2006. Examining the wake structure in Saturn’s rings from microwave observations over varying ring opening angles and wave lengths. (Icarus, in press.)

CLBE001-ESS2E

November 10, 2006

18:0

The Solar System at Radio Wavelengths

planet’s rings is dominated by Saturn’s thermal radiation reflected off the ring particles. Only a small fraction of the radiation at centimeter wavelengths is thermal emission from the rings themselves. Like on Jupiter, radio spectra of the atmospheric emission can be interpreted in terms of its ammonia abundance and local variations therein with altitude and latitude. The ammonia and hydrogen sulfide abundances on Saturn are likely ∼3 times more enhanced than on Jupiter. The latitudinal structure on Saturn’s disk, presumably caused by latitudinal variations in microwave opacity, changes considerably over time. The classical A, B and C rings, with the Cassini Division, are clearly visible on Fig. 8. The inner B ring is brightest, with a brightness temperature of ∼10 K. At 1–3 mm the temperature rises to ∼20–25 K. In front of the planet, the rings block out part of Saturn’s radio emission, resulting in an absorption feature. From this feature one can determine the optical depth of the rings, which is approximately 1 in the B ring. The West (right) ring ansa is usually somewhat brighter than the East side, which has been attributed to the presence of gravitational ‘wakes’, which are 10–100 m sized density enhancements behind large ring particles which, because of Keplerian shear, travel at an angle to the big particle’s orbit. Similar asymmetries have been seen in the A ring in front of the planet. A combination of radio and radar data show that the ring particles have sizes from ∼1 cm up to ∼5–10 m, where the number of particles, N, at a given size, R, varies approximately as N ∼ R−3 . Such a particle size distribution would be expected from a collisionally evolved population of particles. 2.5.4 URANUS AND NEPTUNE

Radio spectra of Uranus and Neptune (Fig. 6b) suggest an overall depletion of ammonia gas in their upper atmospheres, by roughly 2 orders of magnitude compared to the solar nitrogen value. This apparent depletion is likely caused by a nearly complete removal of NH3 gas in the upper atmosphere through the formation of NH4 SH. This is possible if H2 S is considerably (factor of >5) enhanced above solar S. Radio models predict enhancements by a factor of ∼10 on Uranus and ∼30 on Neptune. Good fits to Uranus’ spectrum are obtained if NH3 is close to the solar N abundance in Uranus’ deep atmosphere. However, ammonia gas must be depleted in Neptune’s atmosphere to match radio spectra. Nitrogen on Neptune may therefore be present in the form of both N2 and NH3 , rather than only in the form of ammonia gas. An alternative idea that is advocated by some researchers is based on a large uptake of ammonia in the icy giant’s ionic oceans, deep in their interiors. Uranus is unique among the planets in having its rotation axis closely aligned with the plane in which the planet orbits the Sun. With its orbital period of 84 years, the seasons

703

on Uranus last 21 years. During the Voyager encounter, in 1986, Uranus’ south pole was facing the Sun (and us). Since that time, this pole is slowly moving out of sight, while the north pole is coming into view. Uranus brightness temperature has been monitored since 1966. A pronounced increase in brightness temperature was noticed when the south pole came into view, followed by a decrease when the pole moved away again (Fig. 6b). These measurements suggest that Uranus’ south pole is considerably warmer than the equatorial region, a theory later confirmed by radio images from the VLA. Figure 9 on the following page shows one such image taken in the summer of 2003, along with an image at near-infrared wavelengths (1.6 μm) taken with the adaptive optics system on the Keck telescope. The VLA image shows that the south pole is brightest. It also shows enhanced brightness in the far-north (to the right on the image). At near-infrared wavelengths, Uranus is visible in reflected sunlight. The bright regions are clouds at high (upper troposphere) altitudes. The bright band around the south pole is at the lower edge of the VLA-bright south polar region. Air in this band may rise up, with condensables forming clouds, and descend over the pole. At radio wavelengths, this dry air allows us to probe deeper warmer layers in Uranus’ atmosphere. On Neptune we also see the poles (at least the visible south pole) to be the hottest region on the planet, indicative of a similarity in atmospheric dynamics between the two ice giants.

2.6 Major Satellites and Small Bodies 2.6.1 GALILEAN SATELLITES

Radio spectra of the Galilean satellites are diverse. The brightness temperature at infrared wavelengths can be related directly to the satellite’s albedo, and hence Callisto, with its relatively low albedo (A = 0.13) is warmer than Io and Europa. The brightness temperature at radio wavelengths is determined by the physical temperature and radio emissivity, e, of the subsurface, e = 1 − a, with a the radar geometric albedo. The observed brightness temperatures for Ganymede and, in particular, Europa are well below the physical temperature of the subsurface layers. This measurement is consistent with the high radar albedo for these objects: a = 0.33 for Ganymede and 0.65 for Europa. These high albedos and consequently low emissivities and radio brightness temperatures are likely caused by coherent backscattering in fractured ice. Since the detection of an ionosphere around Io by the Pioneer 10 spacecraft in 1973, this satellite is known to posses a tenuous atmosphere. The first detection of a global atmosphere was obtained in 1990, where a rotational line of sulfur dioxide (SO2 ) gas was measured at 222 GHz. Io is the only object with an atmosphere dominated by SO2 gas, the origin of which can ultimately be attributed to the satellite’s volcanism.

CLBE001-ESS2E

November 10, 2006

18:0

704 Encyclopedia of the Solar System (a)

(b)

FIGURE 9 (a) VLA image of Uranus at 2 cm wavelength taken in the summer of 2003. Note the hot (red) poles. (M. D. Hofstadter and B. J. Butler, 2003, Seasonal change in the deep atmosphere of Uranus. Icarus 165, 168–180.) (b) Infrared (1.6 μm) image of Uranus taken with the Keck adaptive optics system in October 2003. The polar collar around the south pole (left in figure) lines up with the edge of the hot pole seen at radio wavelengths. Several cloud features are visible in the infrared image, and the thin line near the right is Uranus’ ring system. Hammel, H. B., I. de Pater, S. Gibband, G. W. Lockwood, and K. Rages, 2005. Uranus in 2003: Zonal winds, banded structures, and discrete features. Icarus, 175, 534–545.

Part of the gas is of direct volcanic origin, and part is driven by subliming SO2 frost, which itself is a product of volcanic eruptions. Several SO2 , as well as SO, lines have now been observed, which have been used to derive Io’s atmospheric structure. The surface pressure is of the order of a few, perhaps up to 40 nbar, covering 5–20% of the surface, and the atmosphere may be relatively hot, 500– 600 K at 40 km altitude on the trailing, and 250–300 K on the leading hemisphere.

2.6.2 TITAN

Of all solar system bodies, Titan’s atmosphere is most similar to that of Earth, being dominated by nitrogen gas and with a surface pressure 1.5 times that on Earth. Methane gas, with an abundance of a few percent, has a profound effect on the atmosphere. [See Titan.] Photolysis and subsequent chemical reactions lead to the formation of hydrocarbons and nitriles. Because CO and the nitriles HCN, HC3 N (cyanoacetylene), and CH3 CN (acetonitrile) have several transitions at (sub)millimeter wavelengths (Fig. 10), radio observations can be used to constrain the vertical distributions of these species. As expected from photochemical models, their abundances increase with altitude and are highest in the stratosphere.

Disk-resolved spectra, such as obtained with the Submillimeter Array (SMA) and the IRAM Plateau de Bure Interferometer, also contain information on the zonal wind profile. Although 12 μm spectroscopic measurements had already suggested the winds to be prograde at ∼100–300 km altitude, the radio data confirmed the direction of the winds and reported more precise values for the wind speeds in the upper stratosphere (160 ± 60 m/s at ∼200–400 km altitude), and lower mesosphere (60 ± 20 m/s at ∼350– 550 km). At lower altitudes, the winds were determined via the Doppler Wind Experiment on the Huygens probe, when it went down through Titan’s atmosphere. The radio signal from the probe (communication to the Cassini orbiter) was recorded by the very long baseline interferometry (VLBI) network. Winds in Titan’s atmosphere affected the horizontal velocity of the probe during its descent, which was measured by ground-based radio telescopes through a shift in the probe’s transmitted frequency (Doppler shift). These measurements revealed weak prograde winds near the surface, rising to ∼100 m/s at 100–150 km altitude, with a substantial drop (down to a few m/s at most) near 60–80 km altitude. The isotopic carbon and nitrogen ratios were first determined from ground-based radio data, and subsequently confirmed/improved by instruments on board the Cassini spacecraft and Huygens probe. The 12 C/13 C isotope ratio

CLBE001-ESS2E

November 10, 2006

18:0

The Solar System at Radio Wavelengths

705

warm regions are composed of an organic sludge, or perhaps more solid water ice (higher dielectric constant). 2.6.3 ASTEROIDS AND TRANS-NEPTUNIAN OBJECTS

FIGURE 10 A spectrum of Titan’s 12–11 transition of CH3 CN, taken in December 1999 with the IRAM 30-m telescope in Spain. The upper panel shows the spectrum at 1 MHz resolution; the lower panel shows it at 78 kHz. (A. T. Marten et al., 2002, New millimeter heterodyne observations of Titan: Vertical distributions of nitriles HCN, HC3 N, CH3 CN, and the isotopic ratio 15 N/14 N in its atmosphere, Icarus 158, 532–544.)

on Titan is very similar to that on Earth (89), while 14 N/15 N was measured to be several times less than the terrestrial value of 272. This has been explained by a large loss of Titan’s primitive atmosphere over time, which would lead to an isotopic fractionation in nitrogen. In contrast, the similarto-Earth value in 12 C/13 C hints at a continuous or periodic replenishment of methane gas into Titan’s atmosphere, such as could happen, for example, through cryovolcanism or a methane cycle akin to the hydrology cycle on Earth. Radiometry maps of Titan obtained with the Cassini spacecraft can be used with radar and infrared measurements to better constrain the surface composition and compactness. Observations show variations in brightness temperature up to ∼10 K, which are more or less anticorrelated with infrared brightness (i.e., the infrared/optically bright areas have a low radio brightness temperature). The Cassini radar team suggests the optically bright, radio-cold areas perhaps are composed of fractured or porous ice (as on Europa and Ganymede), and the optically dark, radio-

In analogy with the terrestrial planets, a comparison of multiwavelength radio data of small airless bodies with thermophysical models provides information on the (sub)surface properties of the material, as composition and compactness. Radio spectra of several main-belt asteroids suggest that these bodies are typically overlain by a layer of fluffy (highly porous) dust a few centimeters thick, as on the Moon and Mercury. It has been challenging to observe trans-Neptunian (TNO) or Kuiper Belt objects (KBO) at radio wavelengths, including Pluto, due to their small angular extent and low surface temperature. Much progress has been made in the past decades, however. For an object in radiative equilibrium, with an albedo of ∼0.6, the surface temperature should be approximately 50 K, consistent with the 53–59 K temperatures for Pluto as measured by IRAS at 60 and 100 μm. Since at radio wavelengths one probes approximately 10 wavelengths deep into the surface, a brightness temperature of ∼40 K is expected. Observations of Pluto with the 30-m IRAM telescope at millimeter wavelengths revealed brightness temperatures closer to 30 K, indicative of a low radio emissivity (e ≈ 0.6–0.7), similar to that seen on Ganymede. Such a low emissivity can be reconciled with a surface composed of icy grains, and hence relatively high porosity. Radio measurements of KBOs have been used to determine the size and albedo of several of the largest objects (Quaoar, Ixion, Varuna, 2002 AW197), in concert with optical measurements and the so-called Standard Thermal Model (STM) to interpret the data. Although one has to be aware of the assumptions made in the STM, which can lead to over- or underestimates of the size and albedo, such measurements are usually our only means to get a reasonable size estimate for these objects.

2.7 Comets Radio observations of comets provide information that complements studies at other wavelengths. Continuum measurements are sensitive to the thermal emission from a cometary nucleus and of large dust grains in its coma, while spectroscopic observations provide information on the “parent” molecules in a comet’s coma. Upper limits to the radio continuum emission of a few comet nuclei suggest that the temperature gradient in the nucleus may be very steep, or, alternatively, that the emission is substantially suppressed by subsurface scattering. Quasi-continuum spectra reveal a spectral slope that is steeper than that of blackbody thermal emission, yet smaller than that expected from Rayleigh scattering from small

CLBE001-ESS2E

November 10, 2006

18:0

706 Encyclopedia of the Solar System (a)

(b)

FIGURE 11 Contour plots of comet Halley, November 13–16, 1985. The image is taken at the peak flux density of the line (0.0 km/s in the reference frame of the comet). The left side shows a low-resolution image (3 ), and the right side shows a high-resolution image (1 ), after the data for both dates were combined. Contour levels for the low-resolution image are 4.9, 7.8, 10.8, 13.7, 16.7, and 18.6 mJy/beam. For the high-resolution image, they are 4.4, 4.4, 6.0, 7.7, 9.3, and 10.4 mJy/beam. Dashed contours indicate negative values. The beam size, a linear scale, the direction of motion, and the direction to the Sun are indicated in the figures. The cross indicates the position of the nucleus at the time of the observations. (I. de Pater et al., 1986, The brightness distribution of OH around comet Halley, Astrophys. J. Lett. 304, L33–L36.)

particles. These data thus hint at the presence of grains with sizes in the (sub)millimeter range. The most significant advances in cometary radio research have been obtained from spectroscopic studies. The cometary nucleus consists primarily of water ice, which sublimates off the surface when the comet approaches the Sun. After about a day, H2 O dissociates into OH and H. Since the early 1970s, the 18 cm OH line has been observed and monitored in many comets. Such observations are important because they provide indirect information on the production rate, and time variability therein, of water, a molecule that remains difficult to observe on a routine basis. The OH line is sometimes seen in emission, and at other times in absorption against the galactic background. The OH emission is maser emission (i.e., stimulated emission from molecules in which the population of the various energy levels is inverted, so that the higher energy level is overpopulated compared to the lower energy level). This population inversion is caused by absorption of solar photons at ultraviolet (UV) wavelengths. However, this excitation process depends on the comet’s velocity with respect to the Sun (heliocentric velocity), the so-called Swings effect. If the heliocentric velocity is such that solar Fraunhofer (absorption) lines are Doppler shifted into the OH excitation frequency, the molecule is not excited. In that case, OH will absorb 18 cm photons from the galactic background and be seen in absorption against the galactic background. If the line is excited, background radiation at the same wavelength (18 cm) will trigger its deexcitation, and the line is

seen in emission (maser or stimulated emission). With radio interferometers, the OH emission can be imaged. Such images have, for example, revealed the so-called quenching region directly, a region around the nucleus where collisions between particles thermalize the energy levels of OH molecules, so they no longer produce maser emission (Fig. 11). One of the strengths of radio astronomy is the detection of “parent” molecules in a cometary coma, molecules that evolve directly from its icy surface. Such observations are crucial for our understanding of a comet’s composition, and, indirectly, on the conditions in the early solar nebula from which our planetary system formed because cometary nuclei have presumably not been altered by excessive heating or high pressures. A growing number of molecular species have been detected at radio wavelengths. Figure 12 shows the time evolution of observed production rates for a large number of gases, subliming from comet Hale–Bopp (C/1995 O2). Only the most volatile materials sublime at heliocentric distances r > 5 AU, while OH (from  H2 O) becomes dominant at r < 3 AU. Whereas most molecules sublime directly off the cometary nucleus, some gases, such as carbon monoxide and formaldehyde, are also released from dust grains in a comet’s coma. With the advent of new powerful (sub)millimeter arrays, much improved images of the spatial distribution of parent molecules in a cometary coma can be obtained. Figure 13 shows a composite of the formaldehyde emission from comet Hale–Bopp, as observed with the

CLBE001-ESS2E

November 10, 2006

18:0

The Solar System at Radio Wavelengths

707

extreme high spatial (down to 5 m at the comet’s surface) and spectral resolution. These measurements will provide unprecedented information on the outgassing of the comet as a function of heliocentric distance.

3. Nonthermal Radiation

FIGURE 12 Time evolution of the observed production rates of comet C/Hale–Bopp as a function of heliocentric distance, with superposed fitted power laws (dashed lines). (N. Biver et al., 1999, Post-perihelion observations of the distant gaseous activity of comet C/1995 O1 (Hale–Bopp) with the Swedish–ESO Submillimeter Telescope (SEST), Asteroids, Comets and Meteors.)

ARO 12 m telescope and the Berkeley–Illinois–Maryland Association (BIMA) array. Transitions at several frequencies are shown, as well as a contour map from BIMA at 72.8 GHz (in bold) superposed on the ARO 225.7 GHz image. These observations show that formaldehyde indeed originates both from the nucleus and in the coma, where the comasource appears dominated by a single fragment in this case. ESA’s Rosetta spacecraft, currently on its way to comet 67P/Churyumov–Gerasimenko, carries a microwave instrument, MIRO, with receivers centered at 190 and 562 GHz. Upon rendezvous at a heliocentric distance of 3.5 AU, Rosetta will move with the comet down to perihelion near 1.3 AU. MIRO is one of the instruments that will observe the comet during this time. It has broadband channels on both receivers to measure near-surface temperatures and temperature gradients in the comet’s nucleus. Particularly exciting is the high spectral resolution spectrometer connected to the 562 GHz receiver, which will measure several major volatile species (H2 O, CO, CH3 OH, and NH3 ) at

Nonthermal planetary radio emissions are usually produced by electrons spiraling around magnetic field lines. Until the era of spacecraft missions, we had only received nonthermal radio emissions from the planet Jupiter, and these were usually limited to frequencies >10 MHz, since radia tion at lower frequencies is blocked by Earth’s ionosphere. Strong radio bursts at frequencies below 40 MHz were attributed to emission via the cyclotron maser instability in which auroral electrons with energies of a few to several keV power the emission, while radiation at frequencies >100  MHz was interpreted as synchrotron radiation, emitted by high energy (MeV range) electrons trapped in Jupiter’s radiation belts, a region in Jupiter’s magnetic field analogous to the Earth’s Van Allen belts. Like Earth, the magnetic fields of the four giant planets resemble to first approximation that of a dipole magnetic field. Despite several searches, no positive detections of nonthermal radio emissions from any of the other three giant planets were made until the Voyager spacecraft approached these objects. Now we know that all four giant planets as well as Earth are strong radio sources at low frequencies (kilometric wavelengths). Jupiter’s moon Ganymede is also a source of nonthermal radio emissions. The strongest planetary radio emissions usually originate near the auroral regions and are intimately related to auroral processes. A graph of the average normalized spectra of the auroral radio emissions from the four giant planets and Earth is displayed in Fig. 14. All data are adjusted to a distance of 1 AU. Jupiter is the strongest low-frequency radio source, followed by Saturn, Earth, Uranus, and Neptune. In Sections 3.3–3.7, we discuss the emissions from each planet.

3.1 Low-Frequency Emissions 3.1.1 CYCLOTRON MASER EMISSIONS

Radio emission at frequencies of a few kHz to 40 MHz (for Jupiter) is usually attributed to electron cyclotron maser radiation, emitted by keV (nonrelativistic) electrons in the auroral regions of a planet’s magnetic field. The radiation is emitted at the frequency that electrons spiral around the local magnetic field lines (the cyclotron or Larmor frequency): νL =

qB 2πme c

(6)

CLBE001-ESS2E

November 10, 2006

18:0

708 Encyclopedia of the Solar System

FIGURE 13 Images and spectra of H2 CO in comet C/1995 O1 (Hale–Bopp) taken with the BIMA array and ARO 12 m telescope on different days and in different transitions, as indicated in each panel. Panel b shows a contour map from BIMA at 72.8 GHz (in bold; from spectrum in panel a), superposed on the ARO 225.7 GHz image (from spectrum in panel c). The synthesized beam for BIMA is shown in the lower left, and that of ARO appears in the lower right. (S. N. Milam et al., Formaldehyde in comets C/1995 O1 (Hale–Bopp), C/2002 T7 (LINEAR), and C/2001 Q4 (NEAT): Investigating the cometary origin of H2 CO, Astrophys. J . 649, 1169–1177.)

with q the elemental charge, B the magnetic field strength, me the electron mass, and c the speed of light. Propagation of the radiation depends on the interaction of the radiation with the local plasma, or charged particle population. The oscillation of these particles, as caused by the electromagnetic properties of the plasma, leads to a complex interaction between the propagating radiation (the electromagnetic waves) and the local plasma. For example, the radiation can escape its region of origin only if the local cyclotron frequency is larger than the electron plasma frequency:  νe =

4π Ne q 2 me

1/2 (7)

with Ne the electron density. Hence, the plasma frequency is the frequency at which electrons oscillate about their equilibrium positions in the absence of a magnetic field. This similarly sets the limit for propagation through Earth’s ionosphere at ∼10 MHz. If the local cyclotron frequency is less than the electron plasma frequency, the waves are locally trapped and amplified, until it reaches a region from where it can escape. The cyclotron maser instability also

requires a large ratio of ν L /ν e . The auroral regions in planetary magnetospheres are characterized by such conditions. The mode of propagation (or polarization) of auroral radio emissions is in the so-called extraordinary (X) sense, and the polarization (direction of the electric vector of the radiation) depends upon the direction of the magnetic field. The emission is right-handed circularly polarized (RH) if the field at the source is directed toward the observer and left-handed circularly polarized (LH) if the field points away from the observer.1 Cyclotron radiation is emitted in a dipole pattern, where the lobes are bent in the forward direction. The resulting emission is like a hollow cone pattern, as displayed in Fig. 15. The radiation intensity is zero along the axis of the 1

Circular polarization is in the RH sense when the electric vector of the radiation in a plane perpendicular to the magnetic field direction rotates in the same sense as a RH screw advancing in the direction of propagation. Thus, rotation is counterclockwise when propagation is toward and viewed by the observer. RH polarization is defined as positive; LH, as negative. In some cases, the radio emissions propagate in the ordinary (O) magneto-ionic mode. In this mode the polarization is reversed. The theory of the cyclotron maser instability does admit the possibility of emission in the ordinary mode. However, it is less common.

CLBE001-ESS2E

November 10, 2006

18:0

The Solar System at Radio Wavelengths

FIGURE 14 A comparison of the peak flux density spectrum of the kilometric continuum radio emissions of the four giant planets and Earth. All emissions were scaled such that the planets appear to be at a distance of 1 AU. Jovian emissions shown include quasi-periodic bursts (QP), nonthermal continuum (NTC), broadband and narrowband kilometric radiation (bKOM, nKOM), hectometric radiation (HOM), decametric radiation (DAM), and decimetric radiation (DIM). Saturn’s kilometric radiation is designated SKR, and its electrostatic discharge emissions are labeled SED. Terrestrial auroral kilometric radiation is designated AKR. UKR and NKR refer to kilometric radiation from Uranus and Neptune, respectively. Uranus’ electrostatic discharges are labeled UED. (Adapted from P. Zarka and W. S. Kurth, 2005, Radio wave emission from the outer planets before Cassini, Space Sci. Rev. 116, 371–397.)

cone, in the direction of the particle’s parallel motion, and reaches a maximum at an angle . Theoretical calculations show that is very close to 90◦ . Observed opening angles, however, can be much smaller, down to ∼50◦ , which has been attributed to refraction of the electromagnetic waves as they depart from the source region. The cyclotron maser instability derives energy from a few keV electrons, which have distribution functions with a positive slope in the direction perpendicular to the magnetic field. Recent observations in the source of Earth’s auroral kilometric radiation reveal “horseshoe”-shaped electron distributions that provide a highly efficient (of order 1%) source of free energy for the generation of the radio waves. This distribution is thought to be the result of parallel electric fields in the auroral acceleration region, the loss of small pitch-angle electrons to the planetary atmosphere,

709

FIGURE 15 Radiation patterns in a magnetic field. Indicated are the hollow cone pattern caused by cyclotron (dipole) radiation from nonrelativistic electrons in the auroral zone. The electrons move outward along the planet’s magnetic field lines. The hollow cone opening half-angle is given by . At low magnetic latitudes, in the Van Allen belts, the filled radiation cone of a relativistic electron is indicated. The angle between the particle’s direction of motion and the magnetic field, commonly referred to as the particle’s pitch angle, α, is indicated on the sketch. The emission is radiated into a narrow cone with a half width of 1/γ . (I. de Pater and J. J. Lissauer, 2001, “Planetary Sciences,” Cambridge Univ. Press.)

and trapping of reflected electrons. Radio emissions generated in planetary magnetospheres by this mechanism often display a bewildering array of structure on a frequencytime spectrogram including narrowband tones that rise or fall in frequency, sharp cutoffs, and more continuum-like emissions. While it is generally accepted that emissions that rise or fall in frequency are related to tiny sources moving down or up the magnetic field line (hence, to regions with higher or lower cyclotron frequencies), there is no generally accepted theoretical explanation for the fine structure.

3.1.2 OTHER TYPES OF LOW-FREQUENCY RADIO EMISSIONS

While the radio emissions generated by the cyclotron maser instability are, by far, the most intense in any planetary magnetosphere, other types of radio emissions do occur that are of interest. Perhaps the most ubiquitous of these is the so-called nonthermal continuum radiation that arises from the conversion of wave energy in electrostatic waves near the source plasma frequency to radio waves, usually propagating in the ordinary mode. There are arguments for both linear and nonlinear conversion mechanisms. The term “continuum” was originally assigned to this class of

CLBE001-ESS2E

November 10, 2006

18:0

710 Encyclopedia of the Solar System emissions because they can be generated at very low frequencies and can be trapped in low-density cavities in the outer portions of the magnetosphere when the surrounding solar wind density is higher. The mixture of multiple sources at different frequencies and multiple reflections off the moving walls of the magnetosphere tend to homogenize the spectrum. However, at higher frequencies, these emissions are often created as narrowband emissions from narrowband electrostatic bands at the upper hybrid resonance frequency on density gradients in the inner magnetosphere and can propagate directly away from the source, yielding a complex narrowband spectrum. These emissions were first discovered at Earth and have been found at all of the magnetized planets. Furthermore, emissions of this nature are also produced by Ganymede’s magnetosphere. Another type of planetary radio emission is closely related to a common solar emission mechanism, the conversion of Langmuir waves to radio emissions at either the plasma frequency or its harmonic. The Langmuir waves are common features of the solar wind upstream of a planetary bow shock, which arises from the interaction of the supersonic flow of solar wind plasma past the planets. This mechanism is a nonlinear mechanism involving three waves: the Langmuir wave, the radio wave, and either a low-frequency wave in the case of emission near the plasma frequency or a second Langmuir wave in the case of harmonic emission. The resulting emissions are weak, narrowband emissions. 3.1.3 ATMOSPHERIC LIGHTNING

Radio emissions from planets are sometimes associated with atmospheric lightning. The lightning discharge, in addition to producing the visible flash, also produces broad, impulsive radio emissions. If the spectrum of this impulse extends above the ionospheric plasma frequency and if absorption in the atmosphere is not too great, a remote observer can detect the high-frequency end of the spectrum. The “interference” detected with an AM radio on Earth during a thunderstorm is the same phenomenon.

3.2 Synchrotron Radiation Synchrotron radiation is emitted by relativistic electrons gyrating around magnetic field lines. In essence, this emission consists of photons emitted by the acceleration of electrons as they execute their helical trajectories about magnetic field lines. The emission is strongly beamed in the forward direction (see Fig. 15) within a cone 1/γ : 1 = γ

1−

v2 c2

(8)

with v the particle’s velocity and c the speed of light. The relativistic beaming factor γ = 2E, with E the energy in

MeV. The radiation is emitted over a wide range of frequencies, but shows a maximum at 0.29 ν c , with the critical frequency, ν c , in MHz: νc = 16.08E 2 B

(9)

where the energy E is in MeV and the field strength B is in Gauss. The emission is polarized, where the direction of the electric vector depends on the direction of the local magnetic field. Jupiter is the only planet for which this type of emission has been observed. It has been mapped by ground-based radio telescopes and by Cassini to provide some of the most comprehensive, though indirect, information about Jupiter’s intense radiation belts.

3.3 Earth The terrestrial version of the cyclotron maser emission, commonly referred to as auroral kilometric radiation (AKR), has been studied both at close range and larger distances by many Earth-orbiting satellites. The radiation is very intense; the total power is 107 W, sometimes up to 109 W. The intensity is highly correlated with geomagnetic substorms, thus it is indirectly modulated by the solar wind. It originates in the night side auroral regions and in the day side polar cusps at low altitudes and high frequencies and spreads to higher altitudes and lower frequencies. Typical frequencies are between 100 and 600 kHz. Since AKR is generated by auroral electrons, it can be used as a proxy for auroral activity. And, since numerous in situ studies of the terrestrial auroral electron populations and the resulting radio emissions have been carried out, we can apply our understanding of this emission process to similar emissions at other planets where in situ studies have not yet been carried out. Earth is also the source of nonthermal continuum radiation. Below the solar wind plasma frequency this radiation is trapped within the magnetosphere. The spectrum is relatively smooth down to the local plasma frequency, typically in the range of a few kHz, where the emission cuts off at the ordinary mode cutoff. A few observations of this emission also show an extraordinary mode cutoff. Hence, the emission is either generated in both polarizations, or some of the initially dominant ordinary mode is converted into the extraordinary mode via reflections or other interactions with the magnetospheric medium. Above the solar wind plasma frequency, typically at a few tens of kHz, the “continuum” radiation spectrum exhibits a plethora of narrowband emissions; some of these extend well into the range of a few hundred kHz. While not as important as the auroral radio emissions from an energetics point of view, the low-frequency limit of the continuum radiation at the plasma frequency provides an accurate measure of the plasma density, an often difficult

CLBE001-ESS2E

November 10, 2006

18:0

The Solar System at Radio Wavelengths

711

FIGURE 16 Time variability in Jupiter’s radio emission. Panel (a) shows the radio intensity at a wavelength of 13 cm between the years 1963 and 1998. (Courtesy M. J. Klein.) Panels (b) and (c) show Jupiter’s radio intensity at 11–13 and 21 cm, respectively, during 1994 up to the summer of 1995. The impact of comet D/Shoemaker–Levy 9 with Jupiter occurred in July of 1994 (indicated by the vertical dashed lines). (I. de Pater and J. J. Lissauer, 2001, “Planetary Sciences,” Cambridge Univ. Press.)

measurement for a plasma instrument because of spacecraft charging effects.

3.4 Jupiter’s Synchrotron Radiation Jupiter is the only planet from which we receive synchrotron radiation. The variation in total intensity and polarization characteristics during one jovian rotation (the so-called beaming curves) indicate that Jupiter’s magnetic field is approximately dipolar in shape, offset from the planet by roughly one tenth of a planetary radius toward a longitude of 140◦ , and inclined by ∼10◦ with respect to the rotation axis. Most electrons are confined to the magnetic equatorial plane. The magnetic north pole is in the northern hemisphere, tipped toward a longitude of 200◦ . The total flux density of the planet varies significantly over time (Fig. 16). These variations seem to be correlated with solar wind parameters, in particular the solar wind ram pressure, suggesting that the solar wind may influence the supply and/or loss of electrons into Jupiter’s inner magnetosphere. In addition to variations in the total flux density, the radio spectrum changes as well (Fig. 18). An image of Jupiter’s synchrotron radiation obtained with the VLA in 1994 is shown in Fig. 17a. This image was obtained at a wavelength of 20 cm and has a spatial

resolution of ∼ 6 or 0.3 R J . Since Jupiter’s synchrotron radiation is optically thin, one can use tomography to extract the 3-dimensional distribution of the radio emissivity from data obtained over a full jovian rotation. The example in Fig. 17b shows that most of the synchrotron radiation is concentrated near the magnetic equator, which, due to the higher order moments in Jupiter’s field, is warped like a potato chip. The secondary emission regions, apparent at high latitudes in Fig. 17a, show up as rings of emission north and south of the main ring. These emissions are produced by electrons at their mirror points and reveal the presence of a rather large number of electrons bouncing up and down field lines that thread the magnetic equator at ∼2.5 jovian radii. This emission may be “directed” by the moon Amalthea. A fraction of the electrons near Amalthea’s orbit undergoes a change in their direction of motion, caused perhaps by interactions with low-frequency plasma waves near Amalthea (such plasma noise was detected by the Galileo spacecraft when it crossed Amalthea’s orbit), and through interactions with dust in Jupiter’s rings, while regular synchrotron radiation losses also lead to small changes in an electron’s direction of motion. Figure 18 shows radio spectra of Jupiter’s synchrotron radiation from 74 MHz up to >20 GHz. These spectra show  that the electrons in Jupiter’s radiation belts do not follow

CLBE001-ESS2E

November 10, 2006

18:0

712 Encyclopedia of the Solar System (a)

(b)

FIGURE 17 (a) Radio photograph of Jupiter’s decimetric emission at a wavelength of 20 cm, and a central meridian longitude of λcml ∼ 312◦ . Magnetic field lines at equatorial distances of 1.5 and 2.5 Jupiter radii are superposed. Field lines are shown every 15◦ , between λcml −90◦ and λcml + 90◦ . The image was taken with the VLA in June 1994. The resolution is 0.3 Jupiter radii, roughly the size of the high latitude emission regions. [I. de Pater et al., 1997, Synchrotron evidence for Amalthea’s influence on Jupiter’s electron radiation belt, J. Geoph. Res., 102 (A10), 22,043–22,064; Copyright 1997 American Geophysical Union. Reproduced/modified by permission of American Geophysical Union.] (b) Three-dimensional reconstruction of Jupiter’s nonthermal radio emissivity, from VLA data taken in June 1994, as seen from Earth at λcml = 140◦ (DE = −3◦ ). The planet is added as a black sphere in this visualization. (I. de Pater and R. J. Sault, 1998, An intercomparison of 3-D reconstruction techniques using data and models of Jupiter’s synchrotron radiation. J. Geophys. Res. Planets 103 (E9), 19,973–19,984; Copyright 1998 American Geophysical Union. Reproduced/modified by permission of American Geophysical Union.)

FIGURE 18 Jupiter’s radio spectrum as measured in September 1998 and June 1994. Superposed are various model calculations. (Adapted from I. de Pater et al., 2003, Jupiter’s radio spectrum from 74 MHz up to 8 GHz. Icarus 163, 434–448, and I. de Pater and D. E. Dunn, 2003, VLA Observations of Jupiter’s synchrotron radiation at 15 and 22 GHz, Icarus 163, 449–455.

a simple N(E) ∞ E −a power law. Well outside the synchrotron radiation region, beyond Io’s orbit at 6 jovian radii, the electron energy spectrum appears to follow a double power law, N(E) ∞ E −0.5 (1 + E/100)−3 , consistent with in situ measurements by the Pioneer spacecraft. Processes as radial diffusion, pitch angle scattering, synchrotron radiation losses, and absorption by moons and rings change the electron spectrum. The radio spectra superposed on the data were derived from such models. Early in the 20th century (∼1930), Jupiter captured a comet, now known as comet D/Shoemaker–Levy 9. During a close encounter with the planet, this comet was ripped apart by Jupiter’s strong tidal force into over 20 pieces. These comet fragments, all in orbit about Jupiter, were discovered by the Shoemaker–Levy comet hunting team in May 1993. About a year later, from July 16 to 22 (1994), all comet fragments hit Jupiter. These events were widely observed, at wavelengths across the entire electromagnetic spectrum. At infrared wavelength these impacts were incredibly bright, while at optical wavelengths the impact sites were visible as dark spots with even the smallest telescopes. This collision also triggered large temporary changes in Jupiter’s synchrotron radiation. The total flux density increased by ∼20% (Fig. 16), the radio spectrum hardened, and the spatial brightness distribution changed considerably (Fig. 19a, b). These changes were brought about by a

CLBE001-ESS2E

November 10, 2006

18:0

The Solar System at Radio Wavelengths

FIGURE 19 Real and synthetic false color images at a wavelength of 20 cm (1.5 GHz) of Jupiter following the impacts of comet D/Shoemaker–Levy 9 with the planet. (a and b) Observations of the synchroptron radiation before (June 1994) and after several impacts (19 July 1994), respectively. (c) Theoretical emission based on a model of the ambient relativistic electron distribution within a multipole magnetic field configuration. (d) Theoretical synchrotron radiation after an enhancement in the radial diffusion coefficient by a factor of a few million. (e) Enhancement in the theoretical synchrotron radiation, as produced from just shock acceleration. (f) Theoretical synchrotron radiation using the shock model and radial diffusion combined. (S. H. Brecht et al., 2001, Modification of the jovian radiation belts by Shoemaker–Levy 9: An Explanation of the data, Icarus 151, 25–38.)

complex interaction of the radiating particles with shocks and electromagnetic waves induced in the magnetosphere by the series of cometary impacts. Results from models simulating the effects are shown in Figs. 19c–f.

3.5 Jupiter at Low Frequencies Jupiter has the most complex low-frequency radio spectrum of all the planets. Examples of most of these are shown in Fig. 20 and are discussed in this section.

713

auroral regions: 40 MHz for RH emissions translates into ∼14 Gauss in the north polar region, and 20 MHz for LH into ∼7 Gauss in the south. The dynamic spectra in the frequency–time domain are extremely complex, but well ordered. On time scales of minutes, the emission displays a series of arcs, like open or closed parentheses (Fig. 20). Within one storm, the arcs are all oriented the same way. The emissions have been interpreted as coherent cyclotron emissions. The satellite Io appears to modulate some of the emissions: Both the intensity and the probability of the occurrence of bursts increase when Io is at certain locations in its orbit with respect to Jupiter and the observer. The non-Io emission originates near Jupiter’s aurora, and is produced by electrons that travel along magnetic field lines from the middle-to-outer magnetosphere toward Jupiter’s ionosphere. Particles that enter the atmosphere are “lost.” These may locally excite atoms and molecules through collisions, which upon deexcitation are visible as aurora at UV and IR wavelengths. Other electrons are reflected back along the field lines, and produce DAM, where their motion along the field line is reflected in the form of arcs in the radio emission (i.e., a drift with frequency). The Io-dependent emissions are produced at or near the footprints of the magnetic flux tube passing through Io (similar, but much weaker, emissions originate along the flux tubes passing through Ganymede, and perhaps Callisto). Hectometric (HOM) emissions are, in many ways, indistinguishable from DAM except that they are found at lower frequencies, from a few hundred kHz to a few MHz, with a local maximum near 1 MHz. The source region of HOM must be further from Jupiter than the DAM source. Otherwise, like DAM, HOM is predominantly emitted in the extraordinary mode and is likely generated by the cyclotron maser instability. Because the dipole moment of Jupiter is tilted by some 10◦ from the rotational axis, most jovian radio emissions exhibit a strong rotational modulation. Given that Jupiter is a gas giant, this modulation is thought to be the best indicator of the rotation of the deep interior of the planet. The rotation period of the interior is important, for example, because this provides a rotating coordinate system against which the atmospheric winds can be measured. Because these radio observations have been recorded over many decades of time, analysis of these data lead to an extremely accurate determination of Jupiter’s rotation period, 9h 55m 29s .6854.

3.5.1 DECAMETRIC AND HECTOMETRIC RADIO EMISSIONS

3.5.2 KILOMETRIC RADIO EMISSIONS

From the ground, Jupiter’s decametric (DAM) emission, confined to frequencies below 40 MHz, has routinely been observed since its discovery in the early 1950s, occasionally down to frequencies of 4 MHz. The upper-frequency cutoff is determined by the local magnetic field strength in the

Between a few kHz up to 1 MHz various spacecraft detected both broadband (bKOM) and narrowband (nKOM) kilometric radiation from Jupiter (Fig. 20). The lower frequency cutoff for bKOM, ∼20 kHz (sometimes down to ∼5 kHz) is likely set by propagation of the radiation through the Io

CLBE001-ESS2E

November 10, 2006

18:0

714 Encyclopedia of the Solar System FIGURE 20 A representative dynamic spectrum of several of Jupiter’s low-frequency radio emissions. The color bar is used to relate the color to the intensity of the emission. The emission is plotted as a function of frequency (along the y-axis) and time (along the x -axis). (A. Lecacheux, 2001, Radio Observations During the Cassini Flyby of Jupiter, in Planetary Radio Emissions V, edited by H. O. Rucker, M. L. Kaiser, and Y. Leblanc, Austrian Academy of Sciences Press, Vienna, pp. 1–13.)

plasma torus. The source of these emissions is at high magnetic latitudes and appears fixed in local time. The forward lobe near the north magnetic pole is of opposite polarization than a “back lobe” of the same source. The nKOM emissions last longer (up to a few hours) than bKOM, are confined to a smaller frequency range, 50–180 kHz, and show a smooth rise and fall in intensity. The recurrence period for nKOM events suggests the source lags behind Jupiter’s rotation by 3–5%, which was the first indication that this emission, in contrast to any other low-frequency emissions, is produced by distinct sources near the outer edge of the plasma torus. Galileo and Ulysses studies have shown that these emissions occur as a part of an apparently global magnetospheric dynamic event. There is a sudden onset of these emissions, they are visible for a few to several planetary rotations, and finally, they fade away. 3.5.3 VERY LOW FREQUENCY EMISSIONS

The Voyager spacecraft detected continuum radiation in Jupiter’s magnetosphere at frequencies below 20 kHz, both in its escaping and trapped form. As discussed in Section 3.1, radiation can be trapped inside the magnetic cavity if it cannot propagate through the high plasma density magnetosheath. This trapped emission has been observed from a few hundred Hz up to ∼5 kHz. Occasionally, it has been detected up to 25 kHz, suggesting a compression of the mag-

netosphere caused by an increased solar wind ram pressure. Outside the magnetosphere the lower frequency cutoff of the freely propagating radiation corresponds to the plasma frequency in the magnetosheath and appears to be well correlated with the solar wind ram pressure. This escaping component is characterized by a complex narrowband spectrum, attributed to a linear or nonlinear conversion of electrostatic waves near the plasma frequency into freely propagating electromagnetic emissions. The linear mechanism favors ordinary mode radiation, but the trapped emission appears to be a mix of both ordinary and extraordinary radiation, perhaps from the multiple reflections off high density regions in the magnetosphere and at the magnetopause. The quasi-periodic (QP), or jovian type III emissions (in analogy to solar type III bursts, because of their similar dispersive spectral shape) often occur at intervals of 15 and 40 min as observed by Ulysses, but neither Galileo nor Cassini found particularly dominant periodicities at these or other intervals (see Fig. 20). The emission likely originates near the poles. Simultaneous measurements by the Galileo and Cassini spacecraft, both in the solar wind but at different locations, observed similar QP characteristics, suggestive of a strobe light pattern rather than a search light rotating with the planet. Within the magnetosphere, the QP bursts can then appear as enhancements of the continuum emission. At the magnetosheath, the lower frequency components of the bursts are dispersed by the

CLBE001-ESS2E

November 10, 2006

18:0

The Solar System at Radio Wavelengths

higher density plasma, which produces the characteristic type III spectral shape. The 40-minute QP bursts were correlated with energetic (∼1 MeV) electrons observed by Ulysses. Chandra detected similar periods in X-rays from the auroral region, although not directly correlated with QP bursts themselves. Such observations suggest that the QP bursts are related to an important particle acceleration process, but the details of the relationship and the details of the process remain elusive.

715

a mystery, although it may be indirect evidence of higher order moments in Saturn’s magnetic field. Even more mysterious, however, is that the SKR modulation period measured by Ulysses and Cassini varies by 1% or more (several minutes) on timescales of a few years or less. Clearly, this change in period cannot represent a change in the planet’s rotation itself, but there is no commonly accepted explanation. 3.6.2 VERY LOW FREQUENCY EMISSIONS

Saturn’s nonthermal radio spectrum consists of several components, as displayed in Fig. 21, and discussed in the following section.

While the spacecraft was within Saturn’s magnetosphere, it detected low-level continuum radiation (trapped radiation) at frequencies below 2–3 kHz (VLF, very Low Frequency). At higher frequencies, the emission can escape and appears to be concentrated in narrow frequency bands. It is believed that both the “trapped” and narrowband radio emissions are generated by the same mechanism, that is, mode conversion from electrostatic waves near the upper hybrid resonance frequency. However, the source location has not been determined. In particular, one source that has been suggested is related to Saturn’s icy moons. During the passage of the Cassini spacecraft through the inner region of the Saturnian system on July 1, 2004, the Radio and Plasma Wave Science (RPWS) instrument detected many narrowband emissions in a plasma density minimum over the A and B rings. These have been shown to be propagating in the z-mode, at least partially. It is not clear how these narrowband emissions are related, if at all, to those measured well beyond the planet.

3.6.1 SATURN KILOMETRIC RADIO EMISSIONS

3.6.3 SATURN ELECTROSTATIC DISCHARGES

Saturn’s kilometric radiation (SKR) is characterized by a broad band of emission, 100% circularly polarized, covering the frequency range from 20 kHz up to several hundred kHz. When displayed in the frequency–time domain, it is sometimes organized in arc-like structures, reminiscent of Jupiter’s DAM arcs (see Fig. 21a). Cassini has revealed some fine structure characteristic of cyclotron maser emissions (Fig. 21b). As on Earth, the SKR source appears to be fixed at high latitudes primarily in the local morning to noon sector, but it also appears at other local times. The SKR intensity is strongly correlated with the solar wind ram pressure, perhaps suggesting a continuous transfer of the solar wind into Saturn’s low-altitude polar cusps. In fact, a detailed comparison between high-resolution Hubble Space Telescope (HST) images of Saturn’s aurora with SKR suggests a strong correlation between the intensity of UV auroral spots and SKR. Even though the emission is highly variable over time, a clear periodicity at 10h 39m 24s ± 7s was derived from the Voyager data, which was adopted as the planet’s rotation period. Because the emission is tied to Saturn’s magnetic field, which is axisymmetric, the cause of the modulation remains

Saturn electrostatic discharges (SEDs) are strong, impulsive events, which last for a few tens of milliseconds from a few hundred kHz to the upper frequency limit of the Voyager planetary radio astronomy experiment (40.2 MHz), and are also detected by the Cassini spacecraft. Structure in individual bursts can be seen down to the Voyager time resolution limit of 140 μs, which suggests a source size less than 40 km. During the Voyager era, episodes of SED emissions occurred approximately every 10h 10m , distinctly different from the periodicity in SKR. In contrast to SKR, the SED source is fixed relative to the planet-observer line. The emissions are likely electrostatic discharge events as a counterpart of lightning flashes in Saturn’s atmosphere. Some SED episodes have been linked directly to cloud systems observed in Saturn’s atmosphere by the Cassini spacecraft. Cassini, however, has found SEDs to be much less common, generally speaking, than Voyager. Cassini can go months without seeing the discharges. Perhaps it may be a seasonal effect or related to the extent of ring shadowing on the atmosphere (or ionosphere, if propagation is an issue). Cassini should continue to observe through similar seasonal and ring

3.5.4 GANYMEDE

Jupiter’s satellite Ganymede has its own magnetosphere embedded within Jupiter’s magnetic field. It presents a rich plasma wave spectrum, similar to that expected from a planetary magnetosphere. It also is the source of nonthermal narrowband radio emissions at 15–50 kHz, very similar to the escaping continuum emissions from Jupiter. The more intense cyclotron maser emission, seen from the auroral regions of all giant planets and Earth, is absent, however. This is almost certainly because the electron plasma frequency is greater than the cyclotron frequency; hence, the cyclotron maser instability does not operate.

3.6 Saturn

CLBE001-ESS2E

November 10, 2006

18:0

716 Encyclopedia of the Solar System (a)

FIGURE 21 (a) Dynamic spectra of Saturn’s SKR emission. This illustrates a dramatic intensification of the SKR in response to an interplanetary shock that passed Cassini at about 20:30 on June 8, 2004. (b) A high temporal and spectral resolution record of SKR obtained by Cassini. This spectrogram illustrates the complex structure and variations in the SKR spectrum, which is also typical of cyclotron maser emissions at Jupiter and Earth. (After Kurth, et al., 2005, High Spectral and Temporal Resolution Observations of Saturn Kilometric Radiation. Geophys. Res. Lett. 32, L20S07, doi:1029/2005 GL022648.)

(b)

shadowing conditions to the Voyager era late in Cassini’s orbital tour, so such speculation can be tested.

3.7 Uranus and Neptune Like Saturn’s radio emissions, both smooth and bursty components are apparent in the radio emissions from Uranus

and Neptune, and these emissions probably originate in the southern auroral regions of the planets. Note, though, that the magnetic fields of these planets are inclined by large angles (47◦ for Uranus, 59◦ for Neptune) with respect to their rotational axes, and hence the auroral regions are not near the rotation poles. The periodicity of the emissions leads to the determination of the rotation periods of both

CLBE001-ESS2E

November 10, 2006

18:0

The Solar System at Radio Wavelengths

717

planets, 17.24 ± 0.01 hours for Uranus and 16.11 ± 0.02 hours for Neptune. The upper bound to the frequency of the emissions is determined by (and indicative of) the planets’ surface magnetic field strength. From Uranus, we have also received impulsive bursts, similar to the SED events of Saturn, which are referred to as UED or Uranus electrostatic discharge events. They were fewer in number and less intensive than the SEDs. If these emissions are caused by lightning, the lower frequency cutoff suggests peak ionospheric electron densities on the day side of ∼ 6 × 105 cm−3 . In addition to the broadband emissions, both planets also emit trapped continuum and narrowband radiation.

Io’s volcanic plumes, Titan’s hydrocarbon chemistry, and “proto-Jupiters” in nearby stellar systems. We expect, besides simple detection experiments, to actually carry out scientific research in these areas, such as to determine the mass and chemical composition of protoplanets. SKA will improve maps at centimeter wavelengths by orders of magnitude; it will enable mapping thermal emissions from giant planets in minutes of time and obtain maps of Jupiter’s synchrotron emission at many wavelengths quasisimultaneously. At lower frequencies, below 40 MHz, arrays such as LOFAR will allow, for the first time, mapping of Jupiter’s decametric emissions, and pinpoint its sources with high accuracy.

4. Future of Ground-Based Radio Astronomy for Solar System Research

Bibliography

This chapter highlighted the value of radio observations for planetary atmospheres (composition, dynamics), surface composition and structure, comets (parent molecules, source of material, outgassing), and magnetospheres (magnetic field configurations, particle distributions). Momentarily, many exciting projects are not quite doable with existing telescopes. The prospects for the future, however, when new large arrays come on-line, are spectacular. Planetary science may be advanced in significant ways with these arrays. At millimeter wavelengths, the BIMA and Owens Valley Radio Observatory (OVRO) arrays are combined (and expanded) into the Combined Array for Research in Millimeter-wave Astronomy (CARMA), located at Cedar Flat in eastern California, at ∼8000 ft altitude. The Atacama Large Millimeter Array (ALMA) is being built in Chili, jointly by the United States, Canada, Europe, and Chili. The Smithsonian Submillimeter Array (SMA) is already in existence, and has produced interesting scientific results; in this chapter we highlighted some of its results on Titan. At longer wavelengths, the Allan Telescope Array (ATA), operating at ∼0.5–∼10 GHz, is being built in California by the SETI institute and UC Berkeley, with funding from Paul Allan. Several low-frequency arrays are either under construction (the Low Frequency Array LOFAR in the Netherlands) or being planned, while the ultimate Square Kilometer Array (SKA) is under discussion in many countries. These new arrays open up a wealth of potential observations for planetary research, in all areas. For example, several millimeter telescopes observed the apparition of comet Hale–Bopp, with fantastic results, as described in Section 2.7. ALMA will enable detection of hundreds of asteroids, “bare” cometary nuclei, emissions from molecular “jets” from comets at high spatial and time resolution,

Berge, G. L., and Gulkis, S. (1976). Earth based radio observations of Jupiter: Millimeter to meter wavelengths. In “Jupiter” (T. Gehrels, ed.), pp. 621–692, Univ. Arizona Press, Tucson. Butler, B. J., Campbell, D. B., de Pater, I., and Gary, D. E. (2004). Solar system science with SKA. New Astronomy Reviews, 48 (11–12), 1511–1535. Carr, T. D., Desch, M. D., and Alexander, J. K. (1983). Phenomenology of magnetospheric radio emissions. In “Physics of the Jovian Magnetosphere” (A. J. Dessler, ed.), pp. 226–284. Cambridge Univ. Press, Cambridge, United Kingdom. Crovisier, J., and Schloerb, F. P. (1991). The study of comets at radio wavelengths. In “Comets in the Post-Halley Era” (R. L. Newburn and J. Rahe, eds.); a book as a result from an international meeting on Comets in the Post-Halley Era, Bamberg, April 24–28, 1989, 149–174. de Pater, I., Schulz, M., and Brecht, S. H. (1997). Synchrotron evidence for Amalthea’s influence on Jupiter’s electron radiation belt. J. Geoph. Res. 102 (A10), 22,043–22,064. de Pater, I., Butler, B., Green, D. A., Strom, R., Millan, R., Klein, M. J., Bird, M. K., Funke, O., Neidhofer, J., Maddalena, R., Sault, R. J., Kesteven, M., Smits, D. P., and Hunstead, R. (2003). Jupiter’s radio spectrum from 74 MHz up to 8 GHz. Icarus 163, 434–448. Desch, M. D., Kaiser, M. L., Zarka, P., Lecacheux, A., LeBlanc, Y., Aubier, M., and Ortega-Molina, A. (1991). Uranus as a radio source. In “Uranus” (J. T. Bergstrahl, A. D. Miner, and M. S. Matthews, eds.), pp. 894–925. Univ. Arizona Press, Tucson. Gulkis, S., and de Pater, I. (2002). Radio astronomy, planetary. In “Encyclopedia of Physical Science and Technology,” vol., 13, 3rd Ed., pp. 687–712. Academic Press. Harrington, J., de Pater, I., Brecht, S. H., Deming, D., Meadows, V. S., Zahnle, K., and Nicholson, P. D. (2004). Lessons from Shoemaker–Levy 9 about Jupiter and planetary impacts. In “Jupiter: Planet, Satellites & Magnetosphere” (F. Bagenal, T. E. Dowling, and W. McKinnon, eds.), pp. 158–184. Cambridge Univ. Press, Cambridge, United Kingdom. Kaiser, M. L., Desch, M. D., Kurth, W. S., Lecacheux, A., Genova, F., Pederson, B. M., and Evans, D. R. (1984). Saturn as

CLBE001-ESS2E

November 10, 2006

18:0

718 Encyclopedia of the Solar System a radio source. In “Saturn” (T. Gehrels and M.S. Matthews, eds.), pp. 378 –415. Univ. Arizona Press, Tucson. Kraus, J. D. (1986). “Radio Astronomy.” Cygnus Quasar Books, Powell, Ohio. Kurth, W. S., Hospodarsky, G. B., Gurnett, D. A., Cecconi, B., Louarn, P., Lecacheux, A., Zarka, P., Rucker, H. O., Boudjada, M., and Kaiser, M. L. (2005). High Spectral and Temporal Resolution Observations of Saturn Kilometric Radiation. Geophys. Res. Lett., 32, L20S07, doi:10.1029/2005GL022648. Perley, R. A., Schwab, F. R., and Bridle, A. H. (1989). Synthesis imaging in radio astronomy, NRAO Workshop no. 21, Astronomical Society of the Pacific.

Thompson, A. R., Moran, J. M., and Swenson, Jr., G. W. (1986). “Interferometry and Synthesis in Radio Astronomy.” John Wiley and Sons, New York. Zarka, P. (1998). Auroral radio emissions at the outer planets: Observations and theories. J. Geophys. Res. 103, 20, 159–20, 194. Zarka, P., and Kurth, W. S. (2005). Radio wave emission from the outer planets before Cassini. Space Sci. Rev. 116, 371–397. Zarka, P., Pederson, B. M., Lecacheux, A., Kaiser, M. L., Desch, M. D., Farrell, W. M., and Kurth, W. S. (1995). Radio emissions from Neptune. In “Neptune” (D. Cruikshank, ed.), pp. 341–388. Univ. Arizona Press, Tucson.

CLBE001-ESS2E

November 11, 2006

100-C

25-C

50-C

75-C

4:20

C+M

50-C+M

C+Y

50-C+Y

M+Y

50-M+Y

100-M

25-M

50-M

75-M

100-Y

25-Y

50-Y

75-Y

100-K

25-K

25-19-19

50-K

50-40-40

75-K

75-64-64

New Generation Ground-Based Optical/ Infrared Telescopes Alan T. Tokunaga Robert Jedicke Institute for Astronomy University of Hawaii Honolulu, Hawaii

CHAPTER

T

39

1. Introduction 2. Advances in the Construction of Large Telescopes and in Image Quality

4. Advances in Adaptive Optics 5. Sky Survey Telescopes 6. Concluding Remarks

3. Advances with Detector Arrays

Bibliography

he telescope is a crucial tool for astronomers. This chapter gives an overview of the recent advances in ground-based telescope construction and instrumentation for visible and infrared wavelengths, which have spurred extraordinary advances in our understanding of the solar system. Although space-based observatories such as the Hubble Space Telescope and the Spitzer Space Telescope have also immensely enriched our understanding of the solar system we live in, the results from space observatories are discussed elsewhere in this encyclopedia. Astronomers strive to build ever-larger telescopes in order to collect as much light as possible. While cosmologists need the large collecting area of telescopes to study the distant universe, solar system astronomers need the large collecting area to study both nearby small objects and faint objects at the limits of our solar system, and to exploit the high angular resolution they provide. We discuss future telescope projects that promise to make further discoveries possible in the next few decades and offer the prospect of studying solar systems other than our own. Advances in instrumentation have in equal measure revolutionized the way astronomy is done. We discuss two major advances in this chapter: the advent of the large-format solid-state detector for visible and infrared wavelengths and the development of adaptive optics. The development of large-format arrays has led to ambitious digital sky surveys. These surveys allow searches for

objects that may collide with Earth and are leading to a fundamental understanding of the early history of our solar system. The development of adaptive optics is reaching maturity and is allowing routine observations to be made at the diffraction-limit at the largest telescopes in the world. Thus the limitation on image sharpness imposed by the atmosphere since the invention of the telescope is now removed with adaptive optics.

1. Introduction The telescope has played a critical role in planetary science from the moment of its use by Galileo in 1608. The observations that he made of the craters on our Moon and the moons of Jupiter were the first astronomical discoveries made with a telescope. The development of larger refracting and reflecting telescopes led to the seminal discoveries of the rings of Saturn, asteroids, the outer planets Uranus and Neptune, new satellites of Mars and the outer planets, and Pluto by 1930. Although spacecraft missions have revolutionized our understanding of the solar system (of which there are many examples in this encyclopedia), ground-based telescopes continue to play a very important role in making new discoveries, and this is the focus of this chapter. The discovery

C 2007 by Academic Press. All rights of reproduction in any form reserved. Encyclopedia of the Solar System 2e 

719

CLBE001-ESS2E

November 11, 2006

4:20

720 Encyclopedia of the Solar System of the first Kuiper Belt Object (KBO) was made in 1992 on the University of Hawaii 2.2-m telescope. Tremendous advances have been made in detecting KBOs since then: presently over 900 KBOs have been discovered. Using several of the largest telescopes in the world, it was recently found that the largest KBO known, 2003 UB313 , has methane ice on its surface and a moon (Fig. 1). This finding has challenged our definition of what is considered to be a planet in our solar system. Another recent result was the discovery of comets among the main-belt asteroids. The most recent of these, asteroid 118401 was discovered by the 8-m Gemini-North telescope. Two other comets in the main belt were detected previously by other astronomers, and many more such comets are now thought to exist in the asteroid main belt. If this is confirmed then such comets were likely the main source of water delivered to the Earth during its formation. A final example is the Near-Earth Object (NEO) designated 2004 MN4 , which was discovered with the University of Arizona’s 2.3m telescope. For a short time at the end of December 2004, this NEO had the highest probability of any yet found for colliding with Earth (Fig. 3). These discoveries demonstrate the importance of ground-based astronomy, and they will no doubt provide the scientific motivation for future missions. Solar system astronomers typically use telescopes built for other fields of astronomy. However, during the 1970s, NASA constructed ground-based telescopes to support its planetary missions. NASA funded the construction of the 2.7-m McDonald telescope, the University of Hawaii 2.2m telescope, and the 3.0-m NASA Infrared Telescope Facility (IRTF) to provide mission support, but currently only the IRTF continues to be funded by NASA for that purpose. NASA also provides funding for searches for NEOs as part of a Congressional directive. Telescopes are designed to collect and focus starlight onto a detector. While conceptually simple, ground-based observers have to contend with limitations imposed by physics, the atmosphere, and technology. First, the collecting area of a telescope is limited in size. The largest optical telescope in the world presently has an equivalent collecting area of an 11.8-m diameter mirror. Although larger telescopes could be built, there are serious technical and financial difficulties to overcome. Larger telescopes not only allow more light to be collected and put onto the detector, they also allow sharper images to be obtained at the diffraction limit of the telescope. Second, the atmosphere limits observations to specific observing “windows” where the atmosphere is transparent, and the wavelength range 25 μm to 350 μm is largely inaccessible to ground-based observers because of water absorption bands. Third, for infrared observations, the thermal emission of the atmosphere at wavelengths longer than 2.5 μm greatly reduces the sensitivity of observations. To overcome the problems of atmospheric absorption and ther-

mal emission, it is necessary to go to high-mountain sites such as Mauna Kea in Hawaii and Atacama in Chile, or to use balloons, aircraft, or spacecraft. Fourth, atmospheric seeing typically limits the sharpness of images to 0.25– 0.5 arcseconds at the best high-altitude sites. To achieve

FIGURE 1 (a) Image of KBO UB313 obtained with the 10-m Keck II telescope with a laser guide star adaptive optics system. With a diameter estimated to be about 2400 km, it is the largest KBO known and is slightly larger than Pluto. It was recently named Eris. This image shows that UB313 has a satellite, as does Pluto. (b) A near-infrared spectrum of UB313 and Pluto. The spectrum of Pluto was obtained with the 8-m Gemini North telescope. Both objects have methane ice on their surface (methane ice absorption marked with arrows), thus strengthening the idea that there is a common origin for these objects. (Courtesy of M. Brown and C. Trujillo.)

CLBE001-ESS2E

November 11, 2006

4:20

New Generation Ground-Based Optical/Infrared Telescopes

721

FIGURE 2 Images of known comets in the asteroid main belt taken with the University of Hawaii 2.2-meter telescope. These objects are known as the main-belt comets and are a fundamentally new class of comets. The fuzzy appearance of these comets are due to reflected light from dust particles that are ejected by a volatile material, most likely sublimating water ice. (Courtesy of H. Hsieh and D. Jewitt.)

diffraction-limited imaging, one must employ special techniques that actively reduce it many times per second. One such technique, called adaptive optics, is discussed later in Section 4. Very large and low-noise visible and infrared detector arrays have been developed in the past decade, and this advance has been as significant as improvement of telescope construction in providing greater observing capability. An important capability of large-format detector arrays has been to allow large sky surveys to be undertaken. The key objectives of these sky surveys are to detect asteroids that may present an impact hazard to Earth and to complete the reconnaissance of KBOs. The major challenges of these survey projects are obtaining large enough detector arrays to provide the field-of-view required, and analyzing and storing the tremendous amounts of data that they generate. In this chapter, we discuss very large telescopes that have been developed in the past 15 years to maximize collecting area, optimize image quality, and achieve diffractionlimited imaging with techniques to reduce the atmospheric turbulence. We also discuss sky survey telescopes that take advantage of the large-format detectors for the detection of solar system objects.

FIGURE 3 (a) Image of the asteroid 99942 Apophis. When it was discovered during its last close approach to the Earth in 2004, it had a significant probability of striking the Earth in the future. Subsequent observations show that it will pass within 5.6 Earth radii of the Earth in 2029 (see panel b). However, the future trajectory of the asteroid cannot be predicted well and the asteroid will have to be carefully monitored with ground-based telescopes. The diameter of the asteroid is about 250 m. Close passages by an asteroid of this size are estimated to occur about once in 1300 years. (Courtesy of R. Tucker, D. Tholen, and F. Bernardi.)

CLBE001-ESS2E

November 11, 2006

4:20

722 Encyclopedia of the Solar System 2. Advances in the Construction of Large Telescopes and in Image Quality The Hale 5.1-m telescope went into operation in 1949. It represented the culmination of continual telescope design improvements since the invention of the reflecting telescope by Newton in 1668. The basic approach was to scale up and improve design approaches that were used previously. Figure 4 shows the increase in telescope aperture with time. After the completion of the Hale telescope, astronomers recognized that building larger telescopes would require completely new approaches. Simple scaling of the classical techniques would lead to primary mirrors that would be too massive and an observatory (including the dome enclosure) that would be too costly to build. Since the 1990s, a number of ground-breaking approaches have been tried, and the barrier imposed by classical telescope design has been broken. Table 1 shows a list of telescopes with apertures greater than 5 meters. Some of the telescopes listed in Table 1 are still under development. Major technical advances that have led to the development of large telescopes include:

FIGURE 4 Increase in telescope area with time. Only the area of the largest telescopes at each time period is shown, so this indicates the envelope of maximum telescope area as a function of year. The time for the telescope area to double is about 26 years from the invention of the telescope in 1608 to the current year. However the doubling time has decreased from about 1900 to the present. The solid line shows a doubling of telescope aperture about every 19 years. The next jump in aperture size is likely to be in the range of 20–50 meters. For comparison the square symbol shows a 30-m class telescope in the year 2020, and this indicates an even shorter doubling time. The increase in telescope area is due to advances in telescope construction technology and the willingness of society to bear the costs. How much longer can this increase in telescope area continue on the ground? (See Racine 2004, Pub. Astron. Soc. Pacific, vol. 116, p. 77) for data on the growth of telescope aperture with time.)

(1) Advances in computer-controlled hardware allows correction for flexure of the primary mirror. This has permitted thinner mirrors to be used, reducing the mass of the mirror and the total mass of the telescope. For example, the mass of the ESO Very Large Telescope 8.2-m primary mirror is 23 tons with an aspect ratio (mirror diameter to mirror thickness ratio) of 46. This is a very thin mirror compared with the 5.1-m Hale telescope, which has a weight of 14.5 tons and an aspect ratio of 9. (2) Altitude-azimuth (alt-az) mounts reduce the size of the required telescope enclosure. An 8-m alt-az telescope can fit into the same size enclosure as a 4-m equatorial telescope. An alt-az telescope requires computercontrolled pointing and tracking on two axes (whereas the traditional mount requires tracking on only a single axis). The Hale telescope is the largest equatorial telescope ever built. All larger and more recent telescopes use alt-az mountings. Figure 5 illustrates the basic types of telescope mounts, and Figure 6 shows examples of the equatorial and alt-az mounts. (3) Advances in mirror casting and computercontrolled mirror polishing allow the production of larger primary mirrors with shorter focal lengths. A shorter focal length allows the telescope structure to be smaller, thus lowering the weight and cost of the telescope. It also greatly reduces the cost of the dome enclosure. The stateof-the-art in short focal length primary mirrors are those with a focal length to diameter ratio (f/no) of 1.14 installed in the Large Binocular Telescope. This can be compared to the Hale telescope primary mirror that has an f/no of 3.3. The smaller telescope structure with reduced mass requires less time to reach thermal equilibrium, and its lower mass makes it easier to move. This is extremely important in achieving the best image quality and to efficiently reposition in the telescope. (4) Advances in reducing dome seeing led to significant improvement in image quality. Dome seeing is caused by temperature differences within the dome, especially differences between the mirror and the surrounding air. To reduce dome seeing, it is necessary to flush the dome with outside air at night, refrigerate it during the daytime, and cool the primary mirror to about 0.5◦ C below the ambient air temperature. Dome seeing is so important that large telescope projects use wind tunnel experiments to determine what type of dome design to employ. Careful attention to dome design is critical in eliminating dome seeing and achieving the very best seeing at the observatory site. Figure 6b shows an innovative approach to providing dome flushing by providing slits in the dome. (5) Advances in telescope construction have led to novel methods of reducing the cost of building extremely large telescopes. For example, the 10-m Keck telescopes have segmented mirrors to make up the primary mirror (Fig. 6c). Although this technique had been used to build radio telescopes, the difficulty of making the segments and

CLBE001-ESS2E

TABLE 1

Telescopes with Apertures Greater than 5 Meters

(4) Location

11.8 10.0 10.0 10.0 9.2 9.2 8.2 8.2 8.2 8.2 8.2 8.0 8.0 6.5 6.5 6.5 6.0 6.0 5.1

Large Binocular Telescope (LBT) Keck I Keck II Gran Telescopio Canarias (GTC) Hobby-Eberley Telescope Southern African Large Telescope (SALT) Subaru Very Large Telescope (VLT) UT1 Antu Very Large Telescope (VLT) UT2 Kueyen Very Large Telescope (VLT) UT3 Melipal Very Large Telescope (VLT) UT4 Yepun Gemini North Gemini South MMT Conversion Magellan I - Walter Baade Magellan II - Landon Clay Large Zenith Telescope (LZT) Bol’shoi Teleskop Azimultal’nyi (BTA) Hale

Mt. Graham, Arizona Mauna Kea, Hawaii Mauna Kea, Hawaii La Palma, Canary Islands Mt. Fowlkes, Texas Sutherland South Africa Mauna Kea, Hawaii Cerro Paranal, Chile Cerro Paranal, Chile Cerro Paranal, Chile Cerro Paranal, Chile Mauna Kea, Hawaii Cerro Pachon, Chile Mt. Hopkins, Arizona Cerro Manqui, Chile Cerro Manqui, Chile Vancouver, Canada Mt. Pastukhova, Russia Mt. Palomar, California

(2006) 1993 1996 (2007) 1997 2005 1999 1998 1999 2000 2000 1998 2000 1999 2000 2002 2005 1977 1949

1.14 1.75 1.75 1.65 1.4 1.4 1.8 1.75 1.75 1.75 1.75 1.8 1.8 1.25 1.25 1.25 1.5 4 3.3

Honeycomb Segmented Segmented Segmented Segmented Segmented Meniscus Meniscus Meniscus Meniscus Meniscus Meniscus Meniscus Honeycomb Honeycomb Honeycomb Liquid Hg Solid Honeycomb

9.4 133 133 125 200 200 41 46 46 46 46 40 40 9 9 9 n/a 6 8

(9) Mounting Type

(10) Ref.

Alt-Az Alt-Az Alt-Az Alt-Az Azimuth only Azimuth only Alt-Az Alt-Az Alt-Az Alt-Az Alt-Az Alt-Az Alt-Az Alt-Az Alt-Az Alt-Az Fixed Alt-Az Equatorial

1 2 2 3 4 5 6 7 7 7 7 8 8 9 10 10 11 12 13

References (1) http:// lbto.org/, (2) http:// http://www.keckobservatory.org//, (3) http://www.gtc.iac.es/, (4) http://www.as.utexas.edu/mcdonald/het/het.html, (5) http://www.salt.ac.za/, (6) http://www.naoj.org/, (7) http://www.eso.org/, (8) http://www.gemini.edu/, (9) http://www.mmto.org/, (10) http://www.ociw.edu/magellan/magellan.html, (11) http://www.astro.ubc.ca/LMT/, (12) http://www.sao.ru/Doc-en/index.html, (13) http://astro.caltech.edu/observatories/palomar/ This table is adapted from J.M. Hill’s web site: http://abell.as.arizona.edu/∼hill/list/bigtel99.htm. Column (1). The aperture is the diameter of the primary that can collect light. Unless specified, the number given is the diameter of a circular aperture. The LBT consists of two 8.4-m mirrors that are on a single mount and the light from both mirrors are combined to form a single image. The Keck, HET, and SALT telescopes have primary mirrors that are made from hexagonal segments. The primary mirror has a hexagonal shape and the largest and smallest widths of the hexagon are given. Column (2). This is the diameter of the equivalent circular aperture equal to the total light collecting area of the telescope. For the HET and SALT telescopes this is the maximum equivalent circular aperture that is accepted by the prime focus optics. The LBT, Keck, and VLT observatories can combine light from the mirrors for use as an interferometer. This mode of observations is not considered in this table for the purpose of determining the equivalent circular aperture. Column (5). Year that science operations started. Parentheses denote year science operations expected. Column (6). Primary mirror f/no, which is equal to the focal length of the telescope divided by the mirror diameter. Column (7). Honeycomb: Primary mirror that is lightened with a honeycomb structure in the back. Segmented: Primary mirror is made out of hexagonal segments. Meniscus: Single thin concave mirror. Liquid Hg: Liquid mercury mirror. Parabolic shape is obtained by spinning the mirror. Solid: Thick mirror with no light-weighting. Column (8). The aspect ratio is the primary mirror diameter divided by the mirror (or segment) thickness. Column (9). The azimuth only and fixed telescope mounts conduct observations by tracking object in the focal plane of the telescope. For such telescopes the telescope is fixed but the instrumentation tracks the object.

4:20

723

2 × 8.4 11 × 9.4 Hexagon 11 × 9.4 Hexagon 11 × 9.4 Hexagon 11 × 10 Hexagon 11 × 10 Hexagon 8.2 8.2 8.2 8.2 8.2 8.0 8.0 6.5 6.5 6.5 6.0 6.0 5.1

(3) Telescope Name

(8) (5) (6) Mirror Date of primary (7) Aspect Operation f/no Mirror Type Ratio

November 11, 2006

(1) Aperture (m)

(2) Circular Aperture Equivalent (m)

CLBE001-ESS2E

November 11, 2006

4:20

724 Encyclopedia of the Solar System

FIGURE 5 Schematic of different telescope mounts: (a) equatorial, (b) alt-az, (c) azimuth-only, (d) fixed. The Hale 5.1-m telescope was the last large telescope to be built with an equatorial mount. The equatorial mount has one axis aligned to the rotation axis of the Earth. (Note: there are many types of equatorial mounts. The Hale telescope uses a type known as the horseshoe equatorial mount.) All fully steerable large telescopes utilize the alt-az mount, such as the Keck, Gemini, VLT, and Subaru telescopes (see Table 1). In the alt-az mount, the azimuth axis points to the zenith with a perpendicular altitude axis. Two large telescopes built specially for spectroscopy use the azimuth-only mount—the Hobby-Eberly and the South African Large Telescope. The telescope moves only in azimuth and is fixed in declination. The only large telescope to date that uses a fixed mount (the telescope points only to the zenith) is the Large Zenith Telescope, and it uses a liquid mercury mirror.

FIGURE 6a Hale 5.1-m telescope. The last large telescope to be built in the “classical style” with an equatorial mount, a culmination of about 280 years of development of the reflecting telescope.  2005 Gigapxl Project C

the high-precision alignment at visible wavelengths presented formidable obstacles. Fortunately, the problems of fabricating segmented mirrors and aligning them were solved. The hexagonal mirror segments have a thickness of 75 mm, and so the aspect ratio of the 10-m primary is 133 and the total weight of the glass required is 14.4 tons, about the same weight as the 5-m Hale telescope. Another novel approach uses two 8.4-m primary mirrors on a single structure as in the Large Binocular Telescope (Fig. 6d). A third approach involves building a telescope with a fixed vertical elevation. Stars move past the prime focus and are tracked for a limited time. This approach has limitations but is much less expensive to build. Two projects (the HobbyEberly Telescope and the South African Large Telescope) have adopted this design to achieve 9-m class telescopes at about 15–20% of the cost of an equivalent alt-az telescope. An even less expensive approach is to simply stare at the zenith with a liquid mercury mirror as demonstrated by the Large Zenith Telescope.

Large telescopes generally employ one of three different types of primary mirror fabrication. These are (1) Segmented mirrors. Each segment is figured appropriately and all segments are aligned so as to act as a single mirror. (2) Thin meniscus mirror using low expansion glass. Such mirrors are made as thin as possible to be light weight and to have a short thermal time constant (thus coming into equilibrium with the atmospheric temperature quickly). (3) Thick honeycomb mirror using borosilicate glass. The advantage of using borosilicate glass instead of low expansion glass is that the former is much cheaper. The disadvantage of borosilicate glass is that the mirror temperature needs to be controlled more carefully. All of these types of primary mirror fabrication approaches have been proven successful. Column (7) in Table 1 shows the type of mirror used. All large telescopes use active optics to control the shape of the primary mirror. Active optics is the slow adjustment of a mirror to correct aberrations in the image. These

CLBE001-ESS2E

November 11, 2006

4:20

New Generation Ground-Based Optical/Infrared Telescopes

FIGURE 6b 8-m Gemini South telescope. Instruments are mounted on the back of the telescope. These instruments are on the telescope all of the time so that instrument changes can be made very quickly. The dome has vents to allow flushing of the dome by the night air. This allows the telescope and dome to quickly reach equilibrium with the air temperature. (Courtesy of Gemini Observatory/AURA)

adjustments are not fast enough to correct for the atmospheric turbulence but they can correct for flexure in the telescope structure and for temperature changes (which will cause the telescope structure to expand and contract). The process for doing this is illustrated in Figure 7. A star is required for the active optics system to be able to compute the deformations on the primary mirror that are needed to correct the image. Although Figure 7 illustrates the case for a single mirror, a similar approach is employed for correcting the surface figure of a segmented primary mirror, although the details are quite different. Efforts to escape the harmful effects of the Earth’s atmosphere have led to telescopic observations using balloons, aircraft, and rockets. Although we do not discuss space observatories in this article, we note here that a major program undertaken by NASA and the German Aerospace Center (DLR) is to fly a 2.5-meter telescope in the stratosphere using a Boeing 747SP aircraft. At this high altitude it will be possible to observe throughout the 25 μm to 350 μm wavelength range that is inaccessible from the ground. This facility will provide long-term access to a critical wavelength range that otherwise would only be exploited infrequently with spacecraft. We do not know what ultimately will be the largest ground-based telescope to be built (see Fig. 4). The

725

FIGURE 6c 10-m Keck telescope. This image shows one of the two Keck telescopes. The primary mirror consists of 36 hexagonal segments that are aligned to optical precision. The instruments are located on a platform on two sides of the telescope facing the declination bearings. Light from the two telescopes can be combined to provide angular resolution equivalent to an 85 m telescope. (Courtesy R. Wainscoat.)

FIGURE 6d Large Binocular Telescope consisting of two 8.4-m primary mirrors. First light with a single mirror took place in in 2005 and the second mirror was installed in 2006. The light-gathering power of the two primary mirrors combined is equivalent to a 11.8-m telescope. Both mirrors are on a single structure and the light from both mirrors is combined for imaging, spectroscopy, and interferometry. The combined light from the two mirrors will have the angular resolution of a 22.8 m telescope when the LBT is used as an interferometer. (Courtesy of the Large Binocular Telescope Observatory)

CLBE001-ESS2E

November 11, 2006

4:20

726 Encyclopedia of the Solar System not to designs such as the Hobby-Eberly Telescope or the Large Zenith Telescope. The drive to build ever-larger telescopes is motivated by the need to collect as much light as possible and thereby increase the signal-to-noise (S/N) ratio of observations. One can derive that for a diffraction-limited telescope and a detector that is background-limited, the S/N in a given integration time is proportional to: S/N ≈ (A ∗ η/ε)0.5 /(FWHM),

FIGURE 7 Schematic of an active optics system. Starlight from the telescope is sent to a beamsplitter that simultaneously sends light to the focus and to a wavefront sensor. The computer analyses the output of the wavefront sensor and sends control signals to the primary and secondary mirrors to correct any aberrations in the image. (Courtesy of C. Barbieri.)

limitations arise from the need to be diffraction limited, the difficulty of building a suitable enclosure, and the cost. To be competitive with space observatories, all large telescopes must work at the diffraction limit using adaptive optics. But the need to be diffraction limited will ultimately cause adaptive optics systems to be too complex on an extremely large telescope. An enclosure is necessary to keep the disturbance by wind to acceptable levels, and the cost to build and operate the telescope will be enormous. At some point, it may be more cost effective to go into space, where gravity and the weather are not factors driving the design. This has been estimated to be at approximately 70-m in diameter. This argument applies to fully steerable telescopes,

(1)

where A is the area of the telescope, η is the total transmission of the optics and the detector quantum efficiency, ε is the background emission, and FWHM is the full width at half maximum of a stellar image. η takes into account all of the light losses that occurs from the reflection of the mirrors and transmission losses of lenses as light propagates from the telescope to the detector. In order to minimize these losses it is necessary to utilize high reflection coatings on mirrors and lenses as well as to minimize the number of lenses. The detector quantum efficiency is the fraction of light that is absorbed by the detector material. This is near the theoretical maximum of 1.0 at visual wavelengths and about 0.8–0.9 for the 1–15 μm wavelength range. The background emission, ε, arises from the sky emission lines at visual wavelengths and thermal background from the telescope and sky at wavelengths longer than 2 μm. To reduce the thermal emission from the telescope, it is necessary to have the highest reflectivity mirrors available and to reduce or eliminate the thermal emission from the secondary mirror. The latter is often accomplished by forming an image of the secondary within the instrument and then blocking it with a cooled metal plate. Then the infrared detector will only sense the thermal emission from the sky and the object being observed. After maximizing η and reducing ε as much as possible, one can only increase the telescope area and reduce the FWHM to further increase the S/N. Reducing the image FWHM requires decreasing the dome seeing to the absolute minimum, building on sites that have good atmospheric seeing, and working at the diffraction-limit of the telescope. Astronomical sites in Hawaii, Chile, and La Palma are prime locations for large telescopes due to the good seeing they offer as well as having good weather conditions. Figure 8 shows the advances in image quality that have been achieved. The development of adaptive optics has led to the ability to work at the diffraction limit in the near-infrared and to achieve improvements in S/N given by equation 1. Adaptive optics is discussed in Section 4. The advances in constructing large telescopes coupled with reducing dome seeing and adaptive optics have provided the means for studying the surfaces of some KBOs and larger planetary satellites (see Fig. 1). Ground-based telescopes provide the discoveries that pose new questions

CLBE001-ESS2E

November 11, 2006

4:20

New Generation Ground-Based Optical/Infrared Telescopes

727

FIGURE 8 Improvement in angular resolution at optical wavelengths. The development of adaptive optics has permitted diffraction-limited observations from ground-based observatories since 1990, largely eliminating the effects of the atmosphere. The dashed line shows the theoretical diffraction-limited resolution for the telescope. The solid line shows the seeing limit imposed by the atmosphere. Improvements were obtained by going to very good seeing sites. The resolution of the Hubble Space Telescope is shown, (From P. Bely, 2003.)

and motivation for future planetary missions. This is likely to continue in the coming decades as the push to build everlarger telescopes continues. Several groups in the US are proposing the next leap in technology to a telescope in the 20–30-m class, and the engineering studies have started. One proposal is the Thirty-Meter Telescope, an international consortium consisting of research groups in the US and Canada (http://www.tmt.org/). This project proposes to build a telescope similar in concept to the Keck telescopes that will have over 700 hexagonal segments composing the primary mirror. As the name implies, the collecting area is equivalent to a circular mirror 30 m in diameter. The other project is the Giant Magellan Telescope, which is supported by a group of public and private institutions in the US (http://www.gmto.org/). This telescope concept consists of seven 8.4-m mirrors to create a single telescope with the collecting area equivalent to a 21.4-m circular mirror. The European Southern Observatory is also considering an even larger telescope concept (see http://www.eso.org/ projects/owl/). Thus it seems inevitable that a ground-based telescope larger than 10 m will be built.

3. Advances with Detector Arrays Initial observations with telescopes were conducted solely with the human eye (still much recommended for the nonprofessional), but the advantages of using photographic plates to record and archive observations of the sky were quickly exploited beginning in the 1850s. Photographic plates were eventually supplemented with electronic devices like the photomultiplier tube, which amplified the signal from stars by about one million. At infrared wave-

lengths, there were specialized detectors that employed bolometers, photovoltaic devices, and photoconductive devices. However, photographic plates were a necessity for recording high-resolution images of large areas of sky and recording spectra with a wide wavelength range. Images recorded by photographic plates depend on the chemical reaction that is induced by a photon of light. Although the efficiency of the photographic plate in converting a photon to an image is only a few percent, it allows quantitative measurements to be made on the brightness of stars and the strength of spectral lines. Most importantly, the information is archived on the photographic plate for future use. This was absolutely necessary for the development of astrophysics. The next technological revolution came with the invention of the charge-coupled device (CCD) in 1973. CCDs are composed of millions of picture elements, or pixels. Each pixel is a single detector and is capable of converting photons to electrons. The accumulated electrons can then be sent to an amplifier to be “read out” and recorded by a computer. CCD technology is employed in digital cameras, and just as digital photography is gradually replacing photography, a similar transformation has taken place in astronomy. The impact of the CCD on astronomy was immediately apparent after its first use. CCDs have two major advantages over the photographic plate: the capability to directly record photons with an efficiency of 80–90% and to store data electronically. The stored data can then be processed with a computer. Until recently, the main deficiency of the CCD relative to the photographic plate was the relatively small amount of sky that could be covered. However, the recent development of very large CCD mosaics now permits larger areas of sky to be covered by a CCD than by a photographic

CLBE001-ESS2E

November 11, 2006

4:20

728 Encyclopedia of the Solar System

FIGURE 9 Large CCD mosaic installed in MegaCam, a prime focus camera at the Canada-France-Hawaii. This mosaic consists of 40 CCDs, each with 9.5 million pixels. In total the camera has 380 million pixels, the largest mosaic CCD currently in use. This camera is capable of generating 100 billion bytes (100 gigabytes) per night. Larger mosaic cameras are being planned. Each telescope of the Pan-STARRS survey telescope will have a 1.4-Gigapixel camera and the Large Synoptic Survey Telescope will have a single 3.2-Gigapixel camera. (Courtesy of CFHT)

plate. The rapid development of computing power and disk storage has made it practical to use large CCD mosaics. While astronomers have worked hard to develop CCD technology that is optimized for astronomy, they are fortunate that the consumer market has driven the development of the necessary computing power and storage. Figure 9 shows an example of a state-of-the-art large format CCD.

There has been a similar revolution in the development of infrared arrays. The first infrared arrays for astronomy were used in the early 1980s. While initially very modest in size (32 × 32 pixels), infrared arrays now typically contain a million pixels. There are several significant differences between CCDs and infrared arrays. One is that a CCD has a single readout amplifier, while an infrared array has one readout amplifier per pixel. The electrons in a CCD are transferred to a single readout amplifier (hence the origin of the term “charge transfer”). Only a single readout amplifier is needed since the readout electronics and the detector material are made out of the same semiconductor material. In an infrared array, the detector material and the readout amplifier have to be made out of different materials, so each pixel must have a separate amplifier. A second difference is that the infrared arrays must be cooled to much lower temperatures. CCDs can operate effectively at about −30 to −40◦ C. Infrared arrays must be cooled to liquid nitrogen (−196◦ C) or liquid helium (−269◦ C) temperatures. We show in Figure 10 an example of Saturn imaged at a wavelength of 18 micrometers. At these wavelengths, we are observing the thermal emission (heat) from the planet. Thus temperatures can be measured in the atmosphere of Saturn and for the dust particles in the rings. The development of large-format CCDs and infrared arrays has enabled astronomers to undertake large-scale digital sky surveys at visible and infrared wavelengths, just as the use of large photographic plates enabled the first deep sky surveys over 50 years ago.

4. Advances in Adaptive Optics Adaptive optics (AO) is a technique that removes the atmospheric disturbance and allows a telescope to achieve

FIGURE 10 Image of Saturn and its rings obtained in 2004 with the 10-m Keck I telescope at a wavelength of 17.6 micrometers. This is a false color image, where higher signal levels are shown lighter. At these wavelengths we are seeing the heat radiated by the atmosphere and rings of Saturn. The South pole has an elevated temperature (–182 C) compared to its surrounding. This is likely due to the fact that the South pole has been illuminated by the sun for the past 15 years. (Courtesy of G. Orton, JPL).

CLBE001-ESS2E

November 11, 2006

4:20

New Generation Ground-Based Optical/Infrared Telescopes

729

altitude of 90 km). This provides a point source that acts as an artificial star for the AO system. Figure 14 shows a laser guide star being used at the Keck Observatory. This laser guide star system was used to detect the satellite of the largest KBO known (see Fig. 1). With AO we can look forward to the exploration of other solar systems. Figure 15 shows a faint object next to a brighter object that is thought to have a mass 5 times that of Jupiter—a planet. This is one of the first planetary-mass objects to be imaged. Most planets are found by detecting radial velocity variations in the star they are orbiting. About 160 planets have already been detected by the radial velocity method and there is a possibility to detect Earth-mass planets around nearby low-mass stars. We can expect future planetary systems to be discovered, and thus to be able to study the physical characteristics of other solar systems for the first time. The study of extrasolar planets is a key science area for all large telescopes.

FIGURE 11 Simplified diagram of an AO system. Light from the telescope is collimated and sent to an adaptive or deformable mirror. If there were no atmospheric turbulence, the wavefront of the light would be perfectly straight and parallel. The light is then reflected to a beamsplitter, where part of the light is reflected to the wavefront sensor. The wavefront sensor measures the distortion of the wavefront and sends a correction signal to the adaptive mirror. The adaptive mirror is capable of changing its shape to remove the deformations in the light wave caused by the atmospheric turbulence. In this way the light with a corrected wavefront reaches the high-resolution camera, where a diffraction-limited image is formed. (Courtesy of C. Max)

diffraction-limited imaging from the ground. This is critical in achieving the maximum S/N given in equation (1). The basic idea of AO is to first measure the amount of atmospheric disturbance, then correct for it before the light reaches the camera. A schematic of how this can be done is shown in Figure 11. The effect of using AO is dramatic. It is like taking the telescope into space. An impressive example of how AO can improve image quality is shown in Figure 12. AO has been essential for detecting binary asteroids. With it over 60 systems have been found, and the first triple system was recently found as shown in Figure 13. AO requires a star or another object bright enough to use for rapidly and accurately measuring the incoming wavefront. If the object of interest is not bright enough, then it is necessary to use a nearby bright star. This limits the sky coverage, since not every region of the sky will have a bright enough star nearby. If there is no nearby bright star, then it is necessary to use a laser guide star. A laser is pointed in the same direction as the telescope and is used to excite a thin layer of sodium atoms in the Earth’s ionosphere (at an

5. Sky Survey Telescopes Although large telescope projects tend to get a lot of attention, recently there has been a corresponding quantum jump in the construction of visible and infrared survey telescopes. This has been made possible by the availability of large-format CCD and infrared arrays. In addition, the discovery of the Kuiper Belt has led to fundamental advances in our understanding of how our solar system formed. There is a great need to continue the survey of the Kuiper Belt because detailed knowledge of the size and orbit distributions of these objects will allow us to test theories of the orbital migration of the outer planets (Jupiter, Saturn, Uranus, Neptune), the origin of the short-period comets, and the cause of the late heavy bombardment of the inner solar system. There is also an increased awareness that it is important to identify asteroids and comets that could collide with Earth (see Fig. 3). In 1998 the Congress of the United States directed NASA to identify within 10 years at least 90% of NEOs larger than 1 km that may collide with Earth. There are a number of scientific benefits that arise from the NEO surveys, including determining the origin of NEOs, identifying interesting NEOs that could be visited by spacecraft, improving our knowledge of the numbers and sizes of the asteroids in the main asteroid belt, and the discovery of new comets. The reason that the discovery of all NEOS larger than 1 km is important is because if such an object collides with Earth the consequences will be catastrophic. If it is possible to predict that there will be a collision, it may be possible to divert the asteroid so that it misses Earth. The earlier such a prediction can be made, the more likely it is that the diversion is possible. This is a case in which there is a practical use for astronomy, and it is very fitting.

CLBE001-ESS2E

November 11, 2006

4:20

730 Encyclopedia of the Solar System

FIGURE 12 Images of Uranus with and without adaptive optics. This is a striking demonstration of the effectiveness of adaptive optics in removing atmospheric turbulence. One can also see that the signal-to-noise is greatly enhanced because light is concentrated into a diffraction-limited image with adaptive optics, thus greatly increasing the ability to detect faint spots and cloud structure. At a wavelength of 1.6 micrometers, we are seeing reflected light from low-altitude clouds while at 2.2 micrometers the high-altitude clouds are revealed. The planet is much darker at 2.2 micrometers due to absorption of methane gas in the atmosphere. This allows a much longer exposure and for the rings to be seen clearly. The point-like cloud features at 2.2 micrometers show that in certain places turbulence is very strong and is pushing material from lower altitudes into the stratosphere. (Courtesy of H. B. Hammel, I. de Pater, and the W. M. Keck Observatory.)

A number of programs are underway in the US and other countries that meet or exceed the requirements set by Congress. Table 2 shows a partial list of sky survey programs that are currently in progress or planned. Current productivity of various programs is shown in Figure 16, which shows all NEOs discovered irrespective of size. While the NASA directive is aimed at identifying NEOs larger than 1 km diameter, many NEOs smaller than 1 km are also discovered due to the sensitivity of the search programs and because small objects that come very close to Earth may be bright enough to be detected. A recent NEO, 2005 WX, approached to within 1.3 million km of the Earth and had an estimated diameter of only 10 m! The number of known NEOS has been increasing due to the larger number of funded survey programs and advances in detector arrays that have allowed much larger areas of sky

to be covered in a single exposure. The number of NEOs discovered as a function of time is shown in Figure 16. Note that while the total number of asteroids discovered is still increasing at a rapid rate, the number of new asteroids larger than 1 km discovered each year is decreasing. This is a result of the fact that the remaining unknown NEAs are intrinsically more difficult to detect. Their size and orbit distribution is different from the known population due to observational selection effects in the population of known objects. It is likely that existing survey programs (see Table 2) will just miss the goal of discovering at least 90% of all near-Earth asteroids larger than 1 km by 2008 as mandated by Congress. However, when the next generation surveys (see Table 2) come online within the next decade they will quickly complete the inventory of NEAs larger than 1 km.

CLBE001-ESS2E

November 11, 2006

4:20

New Generation Ground-Based Optical/Infrared Telescopes

FIGURE 13 Image of the asteroid 87 Sylvia showing its two satellites. This image was taken with the European Southern Observatory 8-m Very Large Telescope at 2.2 micrometers with an adaptive optics system. The cross marks the location of the asteroid and the scale bar shown is 0.25 arcseconds. The diameter of 87 Sylvia is about 280 km, and the diameters of the satellites are about 7 and 14 km. The orbits of the satellites were measured in order to determine a density of about 1.2 grams/cm3 for 87 Sylvia—only 20% higher than the density of water. Thus 87 Sylvia is likely to have a rubble pile internal structure with 20-60% of its volume being empty. (Courtesy of F. Marchis.)

731

FIGURE 15 Infrared image of 2M 1207 (a brown dwarf and planet binary system) obtained with one of the 8.2-m VLT telescopes. The brown dwarf (white) is 100 times brighter than the planet (red) and both are emitting heat left over from their formation. Their masses are estimated to be 25 and 5 Jupiter masses. In this image the infrared colors at wavelengths 3.8, 2.2, and 1.6 microns are portrayed as red, green, and blue, respectively. The separation of the objects in the sky is 0.78 arcseconds and this corresponds to a physical separation of 55 AU. (Courtesy Gael Chauvin / ESO).

FIGURE 14 Sodium laser guide star in use at Keck II. The laser operates at a wavelength of 5890 Angstroms (0.589 micrometers), and the laser light is propagated through a smaller telescope attached to the Keck telescope. It excites sodium atoms in a layer in the Earth’s atmosphere at an altitude of 90 km. The sodium atoms emit light at the same wavelength as the laser and this is viewed as an artificial star by the telescope. (This is a long exposure photograph. The laser guide star is barely visible with the naked eye from this angle. The lights of the island of Hawaii are below the clouds. (Courtesy of Jean-Charles Cuillandre.)

CLBE001-ESS2E November 11, 2006

TABLE 2

Summary of Sky Survey Telescopes

732

CSS – Mt. Lemmon CSS – Catalina Schmidt CSS – Siding Spring Uppsala Schmidt LINEAR LONEOS (Schmidt) LONEOS (USNO) NEAT (Palomar) NEAT (MSSS) NEAT (Schmidt) Spacewatch (Mosaic) Spacewatch (1.8 m) Pan-STARRS (Hawaii) Discovery Channel Telescope (Lowell) Large Synoptic Survey Telescope

Status

Aperture (m)

f/no

Field-of-view (degree2 )

Magnitude limit

operational operational operational operational operational in development operational operational in development operational operational in development in development proposed

1.5 0.68 0.5 2 × 1.0 0.44 1.3 1.2 1.2 1.2 0.93 1.82 4 × 1.8 4.0 6.9

2.0 1.9 3.5 2.2 1.9 2.4 1.5 3.0 2.5 3.0 2.7 4 2.2 1.25

1.3 8 4.2 2.0 8.3 1.3 9.5 2.3 9.4 2.9 0.32 3.0 3.1 7.0

21 19.5 19.5 19.4 19.3 21.4 22.5 19.7 ∼20.0 21.5 22.5 24.0 21.8 24.0

Speed (degree2 per hour)

20 150 75 1200 106 15 85 40.5 50 160 8.9 700 110 2500

Ref.

1 1 1 2 3 3 4 4 4 5 5 6 7 8

References: (1) Catalina Sky Survey, http://www.lpl.arizona.edu/css/, (2) Lincoln Near Earth Asteroid Research, http://www.ll.mit.edu/LINEAR/, (3) Lowell Observatory Near-Earth-Object Search, http://asteroid.lowell.edu/asteroid/loneos/loneos1.html, (4) Near-Earth Asteroid Tracking, http://neat.jpl.nasa.gov/, (5) http://spacewatch.lpl.arizona.edu/, (6) Panoramic Survey Telescope & Rapid Response System, http://pan-starrs.ifa.hawaii.edu/public/, (7) http://www.lowell.edu/DCT/, (8) http://www.lsst.org/ 1. Field-of-view is the area of sky covered in a single exposure. 2. Magnitude limit is the faintest star recorded at visible wavelengths. 3. Speed is the rate at which observations can be carried out. One can see that of the operational facilities, LINEAR covers the most sky per hour (1200 degree2 /hour) but the faintest stars it can observe at this speed is 19.4 mag. The Spacewatch (1.8 m) telescope can observe stars that are 3 magnitudes fainter but at a speed of only 8.9 degree2 /hour).

4:20

Survey

CLBE001-ESS2E

November 11, 2006

4:20

New Generation Ground-Based Optical/Infrared Telescopes

733

FIGURE 16 Cumulative discoveries of near-Earth asteroids. The total number of large near-Earth asteroids (larger than 1 km) is increasing at a slower rate since most of the easy-to-detect NEOs have already been discovered. The remaining unknown NEOs are on orbits that are intrinsically more difficult to detect and therefore require a longer time to discover. (From NEO.)(Courtesy of Alan Chamberlin.)

There are three major ground-based sky surveys currently under development or study (see Table 2). The Discovery Channel Telescope is a 4.2-m telescope that is under construction near Flagstaff in Northern Arizona and should be operational by 2009. Another survey telescope that is under development is Pan-STARRS, which consists of four 1.8-m telescopes (with a combined aperture approximately equivalent to a 3.6-m telescope) to perform rapid wide-field surveying of the entire sky on a weekly basis. It is hoped that the full system will be operational by 2010, but a prototype single telescope unit will be operational on Haleakala on Maui by the end of 2007. The proposed Large Synoptic Survey Telescope is currently under engineering and design study and is envisioned to be a monolithic 8.4-m wide-field telescope (with a collecting area equal to a 6.7-m telescope). With its large diameter and fast focal ratio it should be capable of reaching 24th magnitude in single 10-s exposures. Due to their extreme depth and wide-field coverage each of these surveys should reach 99% completion for NEOs larger than 1 km diameter within two years of beginning operation.

6. Concluding Remarks Space does not allow coverage of all of the relevant subjects related to the vibrant topics of novel telescope construction, optical fabrication techniques, advances in mirror figure control, adaptive optics, and detector improvements at visible and infrared wavelengths. The topics covered in this chapter can only hint at the tremendous advances that have

taken place in recent years and that carry on unabated. Since the invention of the refractive and reflective telescopes by Galileo and Newton, the construction of ground-based telescopes continues to challenge the very best minds in physics and engineering. At the present time there are strong scientific drivers to build larger telescopes in the 20–50 m range. It seems only a matter of time before such extremely large telescopes are built. Solar system astronomy is driven by the need to have large telescopes in order to study very faint objects in the Kuiper Belt and very faint NEOs that may present a hazard to Earth. It is also necessary to have the highest spatial resolution possible by working at the diffraction limit of large telescopes. This will enable researchers to study the surface and atmospheric features of the outer planets, dwarf planets, and their satellites. Large telescopes also allow the study of exo-planets, and thus bring about a merging of studies of our solar system with those around distant stars. Another driver of solar system astronomy is to detect and characterize NEOs that may present an impact hazard to the Earth. Numerous sky survey programs are underway to detect at least 90% of all NEOs larger than 1 km, and there is a push at the present time to expand this program to detect at least 90% of all NEOs larger than 140 m. These survey programs will play a significant role in greatly expanding our knowledge of the building blocks of our solar system— the asteroidal and cometary bodies from the inner to the outer reaches of the solar system. These studies are likely to profoundly affect understanding of the formation of our solar system and life itself.

CLBE001-ESS2E

November 11, 2006

4:20

734 Encyclopedia of the Solar System We anticipate continuing growth in telescope and instrument development for at least another generation. It is indeed a period great innovation—a renaissance in telescope building and instrumentation—that we are fortunate to be able to witness and participate in.

Bibliography Bely, P.Y. (ed.) (2003). The Design and Construction of Large Optical Telescopes. Springer-Verlag, New York. Kitchin, C.R. (2003). Telescopes and Techniques. SpringerVerlag, London.

McLean, I. (1997). Electronic Imaging in Astronomy. John Wiley & Sons, Chichester. Tyson, R.K. (2000). Introduction to Adaptive Optics. Soc. Of Photo-Optical Instrumentation Eng., Bellingham. NEO web site: http://neo.jpl.nasa.gov/programs/discovery .html. Racine, R. 2004, “The Historical Growth of Telescope Aperture”, Pub. Astron. Soc. Pacific, vol. 116, p. 77. Zirker, J.B. (2005). An Acre of Glass: A History and Forecast of the Telescope. The Johns Hopkins University Press, Baltimore.

CLBE001-ESS2E

November 8, 2006

100-C

25-C

50-C

75-C

2:31

C+M

50-C+M

C+Y

50-C+Y

M+Y

50-M+Y

100-M

25-M

50-M

75-M

100-Y

25-Y

50-Y

75-Y

100-K

25-K

25-19-19

50-K

50-40-40

75-K

75-64-64

Planetary Radar

Steven J. Ostro Jet Propulsion Laboratory California Institute of Technology Pasadena, California

CHAPTER

1. Introduction 2. Techniques and Instrumentation 3. Radar Measurements and Target Properties

P

lanetary radar astronomy is the study of solar system entities (the Moon, asteroids, and comets, as well as the major planets and their satellites and ring systems) by transmitting a radio signal toward the target and then receiving and analyzing the echo. This field of research has primarily involved observations with Earth-based radar telescopes, but it also includes certain experiments with the transmitter and/or the receiver onboard a spacecraft orbiting or passing near a planetary object. However, radar studies of Earth’s surface, atmosphere, or ionosphere from spacecraft, aircraft, or the ground are not considered part of planetary radar astronomy. Radar studies of the Sun involve such distinctly individual methodologies and physical considerations that solar radar astronomy is considered a field separate from planetary radar astronomy.

1. Introduction 1.1 Scientific Context Planetary radar astronomy is a field of science at the intersection of planetology, radio astronomy, and radar engineering. A radar telescope is a radio telescope equipped with a high-power radio transmitter and specialized electronic instrumentation designed to link transmitter, receiver, data acquisition, and telescope-pointing components together in an integrated radar system. The principles underlying

40

4. Prospects for Planetary Radar Bibliography

operation of this system are not fundamentally very different from those involved in radars used, for example, in marine and aircraft navigation, measurement of automobile speeds, and satellite surveillance. However, planetary radars must detect echoes from targets at interplanetary distances (∼105 –109 km) and therefore are the largest and most powerful radar systems in existence. The advantages of radar observations in astronomy stem from the high degree of control exercised by the observer on the transmitted signal used to illuminate the target. Whereas virtually every other astronomical technique relies on passive measurement of reflected sunlight or naturally emitted radiation, the radar astronomer controls all the properties of the illumination, including its intensity, direction, polarization, and time/frequency structure. The properties of the transmitted waveform are selected to achieve particular scientific objectives. By comparing the properties of the echo to the very well known properties of the transmission, some of the target’s properties can be deduced. Hence, the observer is intimately involved in an active astronomical observation and, in a very real sense, performs a controlled laboratory experiment on the planetary target. Radar delay–Doppler and interferometric techniques can spatially resolve a target whose angular extent is dwarfed by the antenna’s beamwidth (that is, its diffraction-limited angular resolution), thereby bestowing a considerable advantage on radar over optical techniques in the study of asteroids, which appear like “point sources” through

C 2007 by Academic Press. All rights of reproduction in any form reserved. Encyclopedia of the Solar System 2e 

735

CLBE001-ESS2E

November 8, 2006

2:31

736 Encyclopedia of the Solar System ground-based optical telescopes. Furthermore, by virtue of the centimeter-to-meter wavelengths employed, radar is sensitive to scales of surface structure many orders of magnitude larger than those probed in visible or infrared regions of the spectrum. Radar is also unique in its ability to “see through” the dense clouds that enshroud Venus and Titan and the glowing gaseous coma that conceals the nucleus of a comet. Because of its unique capabilities, radar astronomy has made notable contributions to planetary exploration for four decades.

1.2 History Radar technology was developed rapidly to meet military needs during World War II. In 1946, soon after the war’s conclusion, groups in the United States and Hungary obtained echoes from the Moon, giving birth to planetary radar astronomy. These early postwar efforts were motivated primarily by interest in electromagnetic propagation through the ionosphere and the possibility of using the Moon as a “relay” for radio communication. During the next two decades, the need for ballistic missile warning systems prompted enormous improvements in radar technology. This period also saw rapid growth in radio astronomy and the construction of huge radio telescopes. In 1957, the Soviet Union launched Sputnik and with it the space age, and in 1958, with the formation by the U.S. Congress of the National Aeronautics and Space Administration (NASA), a great deal of scientific attention turned to the Moon and to planetary exploration in general. During the ensuing years, exhaustive radar investigations of the Moon were conducted at wavelengths from 0.9 to 20 m, and the results generated theories of radar scattering from natural surfaces that still see wide application. By 1963, improvements in the sensitivity of planetary radars in both the United States and the U.S.S.R. had permitted the initial detections of echoes from the terrestrial planets (Venus, Mercury, and Mars). During this period, radar investigations provided the first accurate determinations of the rotations of Venus and Mercury and the earliest indications for the extreme geologic diversity of Mars. Radar images of Venus have revealed small portions of that planet’s surface at increasingly fine resolution since the late 1960s, and in 1979 the Pioneer Venus Spacecraft Radar Experiment gave us our first look at Venus’ global distributions of topography, radar reflectivity, and surface slopes. During the 1980s, maps having sparse coverage but resolution down to ∼1 km were obtained from the Soviet Venera 15 and 16 orbiters and from ground-based observations with improved systems. In the early 1990s, the Magellan spacecraft radar revealed most of the planet’s surface with unprecedented clarity (∼100-m resolution), revealing a rich assortment of volcanic, tectonic, and impact features. The first echoes from a near-Earth asteroid (1566 Icarus) were detected in 1968; it would be nearly another decade

before the first radar detection of a main belt asteroid (1 Ceres in 1977), to be followed in 1980 by the first detection of echoes from a comet (Encke). During 1972 and 1973, detection of 13-cm-wavelength radar echoes from Saturn’s rings shattered prevailing notions that typical ring particles were 0.1–1.0 mm in size—the fact that decimeter-scale radio waves are backscattered efficiently requires that a large fraction of the particles be larger than a centimeter. Observations by the Voyager spacecraft confirmed this fact and further suggested that particle sizes extend to at least 10 m. In the mid-1970s, echoes from Jupiter’s Galilean satellites Europa, Ganymede, and Callisto revealed that the manner in which these icy moons backscatter circularly polarized waves is extraordinarily strange, and totally outside the realm of previous radar experience. We now understand that those echoes were due to high-order multiple scattering from within the top few decameters of the satellites’ regoliths, which are orders of magnitude more transparent to radio waves than rocky regoliths. The late 1980s saw the initial detections of Phobos and Titan, the accurate measurement of Io’s radar properties, the discovery of large-particle clouds accompanying comets, dual-polarization mapping of Mars and the icy Galilean satellites, and radar imaging of asteroids that presaged the diversity of these objects’ shapes and rotations. During the 1990s, the novel use of instrumentation and waveforms yielded the first full-disk radar images of the terrestrial planets, revealing the startling presence of radarbright polar anomalies on Mercury as well as Mars. Similarities between the polarization and albedo signatures of these features and those of the icy Galilean satellites argue persuasively that Mercury’s polar anomalies are deposits of water ice in the floors of craters that are perpetually shaded from sunlight by Mercury’s low obliquity. On the other hand, conjectures about radar-detectable lunar ice deposits have not been substantiated by radar imaging and topographic mapping. In 1992, the first time-delay-resolved (“ranging”) measurements to Ganymede and Callisto were carried out, and delay–Doppler images of the closely approaching asteroid 4179 Toutatis revealed it to be in a very slow, nonprincipal-axis spin state and provided the first geologically detailed pictures of an Earth-crossing asteroid. The 1990s also saw the first intercontinental radar observations and the beginning of planetary radar experiments in Germany, Japan, and Spain. The Arecibo telescope was upgraded in the mid-1990s, and with the resultant order-of-magnitude improvement in its sensitivity (along with significant improvements in Goldstone hardware and software), a new era of radar contributions to planetary science had begun. As of July 2006, radar had detected 12 comets and 194 near-Earth asteroids, as well as 112 main-belt asteroids (Table 1). Radar’s unique capabilities for trajectory refinement and physical characterization give it a natural role in predicting and preventing collisions with small bodies. During the past few years, radar has discovered the existence

CLBE001-ESS2E

November 8, 2006

TABLE 1

2:31

Radar-Detected Planetary Targetsa

Moon Mercury Venus Mars Mars satellite: Jupiter satellites: Saturn satellites: Saturn’s rings

Phobos Io, Europa, Ganymede, Callisto Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, Iapetus, Phoebe

Comets: C/IRAS–Araki–Alcock C/1996 B2 Hyakutake C/2004 Q2 Machholz Catalina (P/2005 JQ5) 73P/Schwassmann–Wachmann 3 (B,C) 2P/Encke C/1998 K5 LINEAR 26P/Grigg–Skjellerup C/Sugano–Saigusa–Fujikawa 1P/Halley C/2002 O6 Swan C/2001 LINEAR A2-B

(nucleus and coma) (nucleus and coma) (nucleus and coma) (nucleus and coma) (nucleus and coma) (nucleus) (nucleus) (nucleus) (nucleus) (coma) (coma) (coma)

Main-belt asteroids: 1 Ceres 2 Pallas 3 Juno 4 Vesta 5 Astraea 6 Hebe 7 Iris 8 Flora 9 Metis 12 Victoria 13 Egeria 15 Eunomia 16 Psyche 18 Melpomene 19 Fortuna 20 Massalia 21 Lutetia 22 Kalliope 23 Thalia 25 Phocaea 27 Euterpe 28 Bellona 31 Euphrosyne 33 Polyhymnia 36 Atalante 38 Leda 41 Daphne 46 Hestia 49 Pales

50 Virginia 53 Kalypso 54 Alexandra 56 Melete 59 Elpis 60 Echo 66 Maja 71 Niobe 78 Diana 80 Sappho 83 Beatrix 84 Klio 85 Io 88 Thisbe 91 Aegina 97 Klotho 101 Helena 105 Artemis 109 Felicitas 111 Ate 114 Kassandra 127 Johanna 128 Nemesis 129 Antigone 135 Hertha 137 Meliboea 139 Juewa 140 Siwa 141 Lumen

144 Vibilia 145 Adeona 164 Eva 165 Loreley 182 Elsa 192 Nausikaa 194 Prokne 198 Ampella 211 Isolda 212 Medea 216 Kleopatra 220 Stephania 224 Oceana 225 Henrietta 230 Athamantis 247 Eukrate 253 Mathilde 266 Aline 270 Anahita 288 Glauke 313 Chaldea 324 Bamberga 325 Heidelberga 335 Roberta 336 Lacadiera 354 Eleonora 356 Liguria 363 Padua 377 Campania

Near-Earth asteroids: 433 Eros 1580 Betulia 1620 Geographos 1627 Ivar 1685 Toro 1862 Apollo

1036 Ganymed 7482 (1994 PC1) 7753 (1988 XB) 7822 (1991 CS) 7889 (1994 LX) 8014 (1990 MF)

1566 Icarus 89136 (2001 US16) 99942 Apophis 100085 (1992 UY4) 101955 (1999 RQ36) 1990 OS

393 Lampetia 405 Thia 407 Arachne 429 Lotis 434 Hungaria 444 Gyptis 455 Bruchsalia 463 Lola 476 Hedwig 488 Kreusa 505 Cava 524 Fidelio 532 Herculina 554 Peraga 622 Esther 654 Zelinda 690 Wratislavia 694 Ekard 704 Interamnia 711 Boliviana 785 Zwetana 796 Sarita 914 Palisana 963 Bezovec 1139 Atami

(Continued )

737

CLBE001-ESS2E

TABLE 1

November 8, 2006

Radar-Detected Planetary Targetsa (Continued )

1866 Sisyphus 1915 Quetzalcoatl 1917 Cuyo 1981 Midas 2062 Aten 2063 Bacchus 2100 Ra-Shalom 2101 Adonis 2201 Oljato 3103 Eger (1982 BB) 3199 Nefertiti 3757 (1982 XB) 3908 Nyx 4034 (1986 PA) 4183 Cuno 4179 Toutatis 4197 (1982 TA) 4486 Mithra 4544 Xanthus 4660 Nereus 4769 Castalia 4953 (1990 MU) 5189 (1990 UQ) 5381 Sekhmet 5604 (1992 FE) 5660 (1974 MA) 6037 (1988 EG) 6239 Minos 6178 (1986 DA) 6489 Golevka 7025 (1993 QA) 7335 (1989 JA) 7341 (1991 VK) 2001 CP36 2001 EB18 2001 EC16 2001 FR85 2001 GQ2 2001 JV1 2001 KZ66 2001 SE286 2001 SG276 2001 SP263 2001 UP 2001 WM15 2001 XX4 2001 YE4 2001 YP3 2002 AA29 2002 AL14 2002 AV 2002 AY1 2002 BG25 2002 BM26 2002 CE26 2002 CQ11 2002 FC 2002 FD6 2002 HK12 2002 HW a

2:31

8201 (1994 AH2) 9856 (1991 EE) 10115 (1992 SK) 11066 Sigurd 12711 (1991 BB) 13651 (1997 BR) 14827 Hypnos 16834 (1997 WU22) 17511 (1992 QN) 22753 (1998 WT) 22771 (1999 CU3) 23187 (2000 PN9) 25143 Itokawa 26663 (2000 XK47) 29075 (1950 DA) 30825 1990 TG1 33342 (1998 WT24) 35396 (1997 XF11) 37655 Illapa 38071 (1999 GU3) 52387 (1993 OM7) 52760 (1998 ML14) 53319 (1999 JM8) 54509 (2000 PH5) 65803 Didymos 65909 (1998 FH12) 66063 (1998 RO1) 66391 (1999 KW4) 68950 (2002 QF15) 69230 Hermes 85182 (1991 AQ) 85774 (1998 UT18) 85938 (1999 DJ4) 2002 KK8 2002 NY40 2002 SR41 2002 SY50 2002 TD60 2002 TS69 2002 TZ66 2002 VE68 2003 CY18 2003 EP4 2003 GY 2003 HN16 2003 HM 2003 KP2 2003 MS2 2003 QB30 2003 RU11 2003 SR84 2003 SS84 2003 TH2 2003 TL4 2003 UC20 2003 YT1 2004 AD 2004 DC 2004 FY31 2004 HX53

Updated lists of radar-detected asteroids and comets are available at http://echo.jpl.nasa.gov.

738

1991 BN 1994 XD 1996 JG 1998 BY7 1998 KY26 1998 ST27 1999 FN19 1999 FN53 1999 LF6 1999 MN 1999 NW2 1999 RR28 1999 TN13 1999 TY2 2000 BD19 2000 CE59 2000 DP107 2000 ED14 2000 EE104 2000 EH26 2000 EW70 2000 GD2 2000 JS66 2000 LF3 2000 QW7 2000 RD53 2000 UG11 2000 UK11 2000 YA 2000 YF29 2001 AV43 2001 BE10 2001 BF10 2004 JA27 2004 RF84 2004 RQ10 2004 VB 2004 VG64 2004 WG1 2004 XP14 2005 AB 2005 CR37 2005 ED318 2005 EU2 2005 FA 2005 HB4 2005 JE46 2005 OE3 2005 TD49 2005 TF49 2005 TU50 2005 WA1 2005 WC1 2005 WK56 2005 XA 2006 GY2

CLBE001-ESS2E

November 8, 2006

2:31

Planetary Radar

of binary systems and extremely rapid rotators among the near-Earth asteroids; detected the nongravitational, thermal-recoil “Yarkovsky” acceleration of a near-Earth asteroid and used the measurement to estimate the asteroid’s mass; and discovered the dumbbell shape and metallic composition of a large main-belt asteroid. Four-station radar-interferometry-assisted selection of the landing sites for the Mars Exploration Rovers, and a novel two-station “radar speckle displacement” technique has produced ultraprecise measurements of Mercury’s spin state that should constrain the nature of the core. As the Cassini spacecraft approached the Saturn system, Arecibo echoes revealed surfaces on Titan with the radar signature expected for areas of liquid hydrocarbons. At this writing, Cassini’s RADAR instrument is well into its multiyear reconnaissance of Titan and eight other Saturnian satellites, returning the first clear pictures of an utterly strange, geologically young world.

2. Techniques and Instrumentation 2.1 Echo Detectability How close must a planetary target be for its radar echo to be detectable? For a given transmitted power PT and antenna gain G, the power flux a distance R from the radar will be PT G/4π R2 . We define the target’s radar cross section, σ , as 4π times the backscattered power per unit of solid angle per unit of flux incident at the target. Then, letting λ be the radar wavelength and defining the antenna’s effective aperture as A = Gλ2 /4π , we have the received power PR = PT G Aσ/(4π )2 R4

where System Factor ∼P T A2 /λ3/2 TS ∼PT G 2 λ5/2 /TS

(3)

Target Factor ∼ ηD3/2 P 1/2 /R4

(4)

and

The inverse-fourth-power dependence of SNR on target distance is a severe limitation in ground-based observations, but it can be overcome by constructing very powerful radar systems.

2.2 Radar Systems The world has two active planetary radar facilities: the Arecibo Observatory (part of the NSF’s National Astronomy and Ionosphere Center) in Puerto Rico and NASA’s Goldstone Solar System Radar in California. Radar wavelengths are 13 cm and 70 cm for Arecibo and 3.5 cm and 13 cm for Goldstone. With each instrument, enormously more sensitivity is achievable with the shorter wavelength. The upgraded Arecibo telescope has twice the range and can see three times the volume of Goldstone, whereas Goldstone can see twice as much sky as Arecibo and can track targets at least three times longer. Figure 1 shows the relative

(1)

This power might be much less than the receiver noise power, PN = kTS  f , where k is Boltzmann’s constant, TS is the receiver system temperature, and  f is the frequency resolution of the data. However, the mean level of PN constitutes a background that can be determined and removed, so PR will be detectable as long as it is at least several times larger than the standard deviation of the random fluctuations in PN . These fluctuations can be shown to have a distribution that, for usual values of  f and the integration time t, is nearly Gaussian with standard deviation PN = PN /( f t)1/2 . The highest signal-to-noise ratio, or SNR = PR /PN , will be achieved for a frequency resolution equal to the effective bandwidth of the echo. As discussed in the following, that bandwidth is proportional to D/λP , where D is the target’s diameter and P is the target’s rotation period, so let us assume that  f ∼ D/λP . By writing σ = ηπ D2 /4, where the radar albedo η is a measure of the target’s radar reflectivity, we arrive at the following expression for the echo’s signal-to-noise ratio: SNR ∼ (System Factor) (Target Factor)(t)1/2

739

(2)

FIGURE 1 Sensitivities of planetary radar systems. Curves plot the single-date, signal-to-noise ratio of echoes from a typical 1-km asteroid at a distance of 0.1 AU for the upgraded Arecibo telescope (A), Goldstone (G), and bistatic configurations using those instruments and the Very Large Array (VLA) or the Greenbank Telescope (GBT).

CLBE001-ESS2E

November 8, 2006

2:31

740 Encyclopedia of the Solar System FIGURE 2 The Arecibo telescope in Puerto Rico. The triangular platform suspended above the 305-m primary reflector supports the azimuth and elevation structures that let the Gregorian feed inside the 26-m radome or the line feed point up to 20◦ off the zenith. The S-band (2380-MHz, 13-cm) transmitter and front-end receiver are inside the radome. (Courtesy of the NAIC—Arecibo Observatory, a facility of the NSF.)

sensitivities of planetary radar systems as a function of target declination. The Arecibo telescope (Fig. 2) consists of a 305 m diameter, fixed reflector whose surface is a 51-m-deep section of a 265-m-radius sphere. Movable feeds designed to correct for spherical aberration are suspended from a triangular platform 137 m above the reflector and can be aimed toward various positions on the reflector, enabling the telescope to point within about 20◦ of the overhead direction (declination 18.3◦ N). Components of the 1990s upgrade included a megawatt transmitter, a ground screen to reduce noise generated by radiation from the ground, and replacement of most of the old single-frequency line feeds with a Gregorian reflector system (named after the 17th-century mathematician James Gregory) that employs 22-m secondary and 8-m tertiary subreflectors enclosed inside a 26-m radome. The Goldstone main antenna, DSS-14 (DSS stands for Deep Space Station), is part of NASA’s Deep Space Network, which is run by the Jet Propulsion Laboratory (JPL). It is a fully steerable, 70-m, parabolic reflector (Fig. 3). Bistatic experiments using DSS-14 transmissions and reception of echoes at DSS-13, a 34-m antenna 22 km away, have been conducted on several very close targets. Bistatic observations between Arecibo and Goldstone, or using transmission from Arecibo or Goldstone and reception at the 100-m Greenbank Telescope (GBT) in West Virginia, have proven advantageous for the Moon, the inner planets, outer planet satellites, and nearby asteroids and comets. Figure 4 is a simplified block diagram of a planetary radar system. A waveguide switch, a movable subreflector, or a moveable mirror system is used to place the antenna in a transmitting or receiving configuration. The heart of the transmitter is one or two klystron vacuum-tube amplifiers. In these tubes, electrons accelerated by a potential drop of some 60 kV are magnetically focused as they enter the first of five or six cavities. In this first cavity, an oscillating elec-

FIGURE 3 The 70-m Goldstone Solar System Radar main antenna, DSS-14, in California. The 3.5-cm planetary radar transmitter and front-end receivers are inside the lowest cone near the focus of the antenna, which is fully steerable.

tric field at a certain radio frequency (RF, e.g., 2380 MHz for Arecibo or 8560 MHz) modulates the electrons’ velocities and hence their density and energy flux. Subsequent resonant cavities enhance this velocity bunching (they

CLBE001-ESS2E

November 8, 2006

2:31

Planetary Radar

741

FIGURE 4 Block diagram of a planetary radar system. RF LO and IF LO denote radio frequency and intermediate frequency local oscillators, and ADC denotes analog-to-digital converter.

constitute what is called a cascade amplifier), and about half of the input DC power is converted to RF power and sent out through a waveguide to the antenna feed system and radiated toward the target. The other half of the input power is waste heat and must be transported away from the klystron by cooling water. The impact of the electrons on the collector anode generates dangerous X-rays that must be contained by heavy metal shielding surrounding the tube, a requirement that further boosts the weight, complexity, and hence cost of a high-power transmitter. In most single-antenna observations, one transmits for a duration near the roundtrip propagation time to the target (i.e., until the echo from the beginning of the transmission is about to arrive) and then receives for a similar duration. In the “front end” of the receiving system, the echo signal is amplified by a cooled, low-noise amplifier and converted from RF down to intermediate frequencies (IF, e.g., 30 MHz), for which transmission line losses are small and passed from the proximity of the antenna feed to a remote control room containing additional stages of signalprocessing equipment, computers, and digital recorders.

The signal is filtered, amplified, and converted to frequencies low enough for analog voltage samples to be digitized and recorded. The frequency down-conversion can be done in several stages using analog devices called superheterodyne mixers, but in recent years it has become possible to do this digitally, at increasingly higher frequencies. The nature of the final processing prior to recording of data on a hard disk or magnetic tape depends on the nature of the radar experiment and particularly on the time/frequency structure of the transmitted waveform. Each year, systems for reducing and displaying echoes in “real time” and techniques for processing recorded data are becoming more ambitious as computers get faster.

2.3 Echo Time Delay and Doppler Frequency The time between transmission of a radar signal and reception of the echo is called the echo’s roundtrip time delay, τ , and is of order 2R/c, where c is the speed of light, by definition equal to 299,792,458 m sec−1 . Because planetary targets are not points, even an infinitesimally short transmitted

CLBE001-ESS2E

November 8, 2006

2:31

742 Encyclopedia of the Solar System pulse would be dispersed in time delay, and the total extent τTARGET of the distribution σ (τ ) of echo power (in units of radar cross section) would be D/c for a sphere of diameter D and in general depends on the target’s size and shape. The translational motion of the target with respect to the radar introduces a Doppler shift ν in the frequency of the transmission. Both the time delay and the Doppler shift of the echo can be predicted in advance from the target’s ephemeris, which is calculated using the geodetic position of the radar and the orbital elements of Earth and the target. The predicted Doppler shift can be removed electronically by continuously tuning the local oscillator used, for example, for RF-to-IF frequency conversion (see Fig. 4). Sometimes it is convenient to “remove the Doppler on the uplink” by modulating the transmission so that echoes return at a fixed frequency. The predicted Doppler (i.e., the predicted rate of change of the delay) must be accurate enough to avoid smearing out the echo in delay, and this requirement places stringent demands on the quality of the observing ephemeris. Time and frequency measurements are critical because the delay/Doppler distribution of echo power is the source of fine spatial resolution and also can be used to refine the target’s orbit. Reliable, precise time/frequency measurements are made possible by high-speed data acquisition systems and stable, accurate clocks and frequency standards. Because different parts of the rotating target will have different velocities relative to the radar, the echo will be dispersed in Doppler frequency as well as in time delay. The basic strategy of any radar experiment always involves measurement of some characteristic(s) of the function σ (τ , ν), perhaps as a function of time and perhaps using more than one combination of transmitted and received polarizations. Ideally, one would like to obtain σ (τ , ν) with very fine resolution, sampling that function within intervals whose dimensions τ × ν are minute compared to the echo dispersions τTARGET and νTARGET . Figure 5 shows the geometry of delay-resolution cells and Doppler-resolution cells for a spherical target and sketches their relation to σ (τ ) and σ (ν).

2.4 Radar Waveforms In the simplest radar experiment, the transmitted signal is a highly monochromatic, unmodulated, continuous wave (cw) signal. Analysis of the received signal comprises Fourier transformation of a series of time samples and yields an estimate of the echo power spectrum σ (ν), but it contains no information about the distance to the target or σ (τ ). To avoid aliasing, the sampling rate must be at least as large as the bandwidth of the low-pass filter (see Fig. 4) and usually is comparable to or larger than the echo’s intrinsic dispersion νTARGET from Doppler broadening. Fast Fourier transform (FFT) algorithms greatly speed the calculation of discrete spectra from time series and are ubiquitous in

FIGURE 5 Time-delay and Doppler-frequency resolution of the radar echo from a rotating sphere.

radar astronomy. In a single FFT operation, a string of N time samples taken at intervals of t seconds is transformed into a string of N spectral elements with frequency resolution ν = 1/(Nt). To obtain delay resolution, one must apply some sort of time modulation to the transmitted waveform. For example, a short-duration pulse of cw signal lasting 1 μs would provide delay resolution of 150 m. However, the echo would have to compete with the noise power in a bandwidth of order 1 MHz (i.e., the reciprocal of 1 μs), so the echo power from many consecutive pulses would probably have to be summed to yield a detection. One would not want these pulses to be too close together, however, or there would be more than one pulse incident on the target at once, and interpretation of echoes would be insufferably ambiguous. Thus, one arranges the pulse repetition period tPRP to exceed the target’s intrinsic delay dispersion τTARGET , ensuring that the echo will consist of successive, nonoverlapping “replicas” of σ (τ ) separated from each other by tPRP . To generate this “pulsed cw” waveform, the transmitter is switched on and off while the frequency synthesizer (see Fig. 4) maintains phase coherence from pulse to pulse. Then Fourier transformation of time samples taken at the same position within each of N successive replicas of σ (τ ) yields the power spectrum of echo from a certain delay resolution cell on the target. This spectrum has an unaliased bandwidth of 1/tPRP and a frequency resolution of 1/(NtPRP ). Repeating this process for a different position within each

CLBE001-ESS2E

November 8, 2006

2:31

Planetary Radar replica of σ (τ ) yields the power spectrum for echo from a different delay resolution cell, and in this manner one obtains the delay–Doppler image σ (τ , ν). In practice, instead of pulsing the transmitter, one usually codes a cw signal with a sequence of 180◦ phase reversals and cross-correlates the echo with a representation of the code (e.g., using the decoder in Fig. 4), thereby synthesizing a pulse train with the desired values of t and tPRP . With this approach, one optimizes SNR because it is much cheaper to transmit the same average power continuously than to pulse the transmitter. Most modern ground-based radar as-

TABLE 2

tronomy observations employ cw or repetitive, phase-coded cw waveforms. A limitation of coherent-pulsed or repetitive, binaryphase-coded cw waveforms follows from combining the requirement that there never be more than one echo received from the target at any instant (i.e., that tPRP > τTARGET ) with the antialiasing frequency requirement that the rate (1/tPRP ) at which echo from a given delay resolution cell is sampled be no less than the target bandwidth νTARGET . Therefore, a target must satisfy τ TARGET νTARGET < 1 or it is “overspread” (Table 2) and cannot be investigated

Characteristics of Selected Planetary Radar Targetsa Maximum Dispersionsc

Minimum Echo Delayb (min)

Radar Cross Section (km2 )

Radar Albedo, ηOC

Circular polarization ratio, μC

Delay (ms)

Doppler (Hz)

Product

Moon Mercury Venus Mars Phobos 1 Ceres 2 Pallas 12 Victoria 16 Psyche 216 Kleopatra 324 Bamberga 1685 Toro 1682 Apollo 2100 Ra-Shalom 2101 Adonis 4179 Toutatis 4769 Castalia 6178 1986DA

0.04 9.1 4.5 6.2 6.2 26 25 15 28 20 13 2.3 0.9 3.0 1.5 0.4 0.6 3.4

6.6 × 105 1.1 × 106 1.3 × 107 2.9 × 106 22 2.7 × 104 1.7 × 104 2.3 × 103 1.4 × 104 7.1 × 103 2.9 × 103 1.7 0.2 1.1 0.02 1.0 0.2 2.4

0.07 0.06 0.11 0.08 0.06 0.05 0.08 0.22 0.31 0.44 0.06 0.1 0.1 0.2 20◦ . OC echo spectra obtained from asteroid 4 Vesta and Jupiter’s satellite Io have similar shapes, but these objects’ substantial polarization ratios (μC ∼ 0.3 and 0.5, respectively) suggest that small-scale roughness is at least partially responsible for the diffuse echoes. Circular polarization ratios between 0.5 and

FIGURE 11 Structure on the lunar surface near the Apollo 17 landing site. Most of the surface is smooth and gently undulating at scales much larger than a centimeter. This smooth component of the surface is responsible for the predominantly quasi-specular character of the Moon’s radar echo at λ >> 1 cm. Wavelength-scale structure produces a diffuse contribution to the echo. Wavelength-sized rocks are much more abundant at λ ∼ 4 cm than at λ ∼ 10 m (the scale of the boulder being inspected by astronaut H. Schmitt), and hence diffuse echo is more substantial at shorter wavelengths.

CLBE001-ESS2E

November 8, 2006

2:31

Planetary Radar

751

FIGURE 12 This lava flow near Sunset Crater in Arizona is an example of an extremely rough surface at decimeter scales and is similar to terrestrial flows yielding large circular polarization ratios at decimeter wavelengths.

1.0 have been measured for several asteroids (see Table 2) and parts of Mars and Venus, implying extreme decimeterscale roughness, perhaps analogous to terrestrial lava flows (Fig. 12). Physical interpretations of the diffusely scattered echo employ information about albedo, scattering law, and polarization to constrain the size distributions, spatial densities, and electrical properties of wavelength-scale rocks near the surface, occasionally using the same theory of multiple light scattering applied to radiative transfer problems in other astrophysical contexts.

3.7 Jupiter’s Icy Galilean Satellites Europa, Ganymede, and Callisto have extraordinary 3.5and 13-cm radar properties. Their reflectivities are enormous compared with those of the Moon and inner planets (see Table 2); Europa is the extreme example (Fig. 10), with an OC radar albedo (1.0) as high as that of a metal sphere. Since the radar and visual albedos and estimates of fractional water frost coverage increase by satellite in the order Callisto–Ganymede–Europa, the presence of water ice has long been understood to be somehow responsible for the unusually high reflectivities even though ice is less radarreflective than silicates. In spite of the satellites’ smooth appearances in Voyager and Galileo high-resolution images, a diffuse scattering process and hence a high degree of nearsurface structure at centimeter to meter scales is indicated by broad spectral shapes and large linear polarization ratios (μL ∼ 0.5). The most peculiar aspect of the satellites’ echoes is their circular polarization ratios, which exceed unity. That is, in contrast to the situation with other planetary targets, the scattering largely preserves the handedness, or helicity, of the transmission. Mean values of μC for Europa,

FIGURE 13 Radar properties of Europa, Ganymede, and Callisto compared to those of some other targets. The icy Galilean satellites’ total-power radar albedos do not depend on wavelength between 3.5 and 13 cm, but plummet at 70 cm. Solid symbols shaped like Greenland indicate properties of that island’s percolation zone at 5.6 and 68 cm. The domain of most of the bright polar features on Mars and Mercury is sketched.

Ganymede, and Callisto are about 1.5, 1.4, and 1.2, respectively. Wavelength dependence is negligible from 3.5 to 13 cm, but dramatic from 13 to 70 cm ( Fig. 13). Significant polarization and/or albedo features are present in the echo spectra and in a few cases correspond to geologic features in Voyager and Galileo images. The icy satellites’ echoes are due not to external surface reflections but to subsurface “volume” scattering. The high radar transparency of ice compared with that of silicates permits deeper radar sounding, longer photon path lengths, and higher-order scattering from regolith heterogeneities; radar is seeing Europa, Ganymede, and Callisto in a manner that the Moon has never been seen. The satellites’ radar behavior apparently involves the coherent backscatter effect, which accompanies any multiple-scattering process; occurs for particles of any size, shape, and refractive index; and was first discovered in laboratory studies of the scattering of electrons and of light. Coherent backscatter yields strong echoes and μC > 1 because the incident, circularly polarized wave’s direction is randomized before its helicity is randomized and also before its power is absorbed and because photons traveling along identical paths but in opposite

CLBE001-ESS2E

November 8, 2006

2:31

752 Encyclopedia of the Solar System directions interfere constructively. The vector-wave theory of coherent backscatter accounts for the unusual radar signatures in terms of high-order, multiple anisotropic scattering from within the upper few decameters of the regoliths, which the radar sees as an extremely low-loss, disordered random medium. Inter- and intrasatellite albedo variations probably are due to variation in ice purity. As sketched in Fig. 13, there are similarities between the icy Galilean satellites’ radar properties and those of the radar-bright polar caps on Mars, features inside perpetually shadowed craters at the poles of Mercury (see Section 3.9), and the percolation zone in the Greenland ice sheet. However, the subsurface configuration in the Greenland zone, where the scattering heterogeneities are “ice pipes” produced by seasonal melting and refreezing, are unlikely to resemble those on the satellites. Therefore, unique models of subsurface structure cannot be deduced from the radar signatures of any of these terrains.

3.8 Radar Mapping of Spherical Targets The term “radar image” usually refers to a measured distribution of echo power in delay, Doppler, and/or up to two angular coordinates. The term “radar map” usually refers to a display in suitable target-centered coordinates of the residuals with respect to a model that parameterizes the target’s size, shape, rotation, average scattering properties, and possibly its motion with respect to the delay–Doppler ephemerides. Knowledge of the dimensions of the Moon and inner planets has long permitted conversion of radar images to maps of these targets. For small asteroids, the primary use of images is to constrain the target’s shape (see Section 3.12). As illustrated in Fig. 5, intersections between constantdelay contours and constant-Doppler contours on a sphere constitute a “two-to-one” transformation from the target’s surface to delay–Doppler space. For any point in the northern hemisphere, there is a conjugate point in the southern hemisphere at the same delay and Doppler. Therefore, the source of echo in any delay–Doppler resolution cell can be located only to within a twofold ambiguity. This north– south ambiguity can be avoided completely if the radar beamwidth (∼2 arcmin for Arecibo at 13 cm or Goldstone at 3.5 cm) is comparable to or smaller than the target’s apparent angular radius, as in the case of observations of the Moon (angular radius ∼ 15 arcmin). Similarly, no such ambiguity arises in the case of side-looking radar observations from spacecraft (e.g., Magellan or Cassini) for which the geometry of delay–Doppler surface contours differs somewhat from that in Fig. 5. For ground-based observations of Venus and Mercury, whose angular radii never exceed a few tens of arcseconds, the separation of conjugate points is achievable by either offsetting the pointing to place a null of the illumination pattern on the undesired hemisphere or interferometrically, using two receiving antennas, as follows.

The echo waveform received at either antenna from one conjugate point will be highly correlated with the echo waveform received at the other antenna from the same conjugate point. However, echo waveforms from the two conjugate points will be largely uncorrelated with each other, no matter where they are received. Thus, echoes from two conjugate points can, in principle, be distinguished by crosscorrelating echoes received at the two antennas with themselves and with each other, and performing algebraic manipulations on long time averages of the cross product and the two self products. The echo waveform from a single conjugate point will experience slightly different delays in reaching the two antennas, so there will be a phase difference between the two received signals, and this phase difference will depend only on the geometrical positions of the antennas and the target. This geometry will change as the Earth rotates, but it will change very slowly and in a predictable manner. The antennas are best positioned so contours of constant phase difference on the target disk are as orthogonal as possible to the constant-Doppler contours, which connect conjugate points. Phase difference hence becomes a measure of north–south position, and echoes from conjugate points can be distinguished on the basis of their phase relation. The total number of “fringes,” or cycles of phase shift, spanned by the disk of a planet with diameter D and a distance R from the radar is approximately (D/R)(bPROJ /λ), where bPROJ is the projection of the interferometer baseline normal to the mean line of sight. For example, Arecibo interferometry linked the main antenna to a 30.5-m antenna about 11 km farther north. It placed about seven fringes on Venus, quite adequate for separation of the north–south ambiguity. The Goldstone main antenna has been linked to smaller antennas to perform multielement interferometry, which permit one to solve so precisely for the north–south location of a given conjugate region that one can obtain the region’s elevation relative to the mean planetary radius. Goldstone interferometry of the Moon’s polar regions has produced both topographic maps and backscatter maps (Fig. 14) using somewhat more advanced radar techniques. In constructing a radar map, the unambiguous delay– Doppler distribution of echo power is transformed to planetocentric coordinates, and a model is fit to the data, using a maximum-likelihood or weighted-least-squares estimator. The model contains parameters for quasi-specular and diffuse scattering as well as prior information about the target’s dimensions and spin vector. For Venus, effects of the dense atmosphere on radar wave propagation must also be modeled. Residuals between the data and the best-fit model constitute a radar reflectivity map of the planet (e.g., Fig. 15). Reflectivity variations can be caused by many different physical phenomena, and their proper interpretation demands due attention to the radar wavelength, echo polarization, viewing geometry, prior knowledge about surface properties, and the nature of the target’s mean scattering

CLBE001-ESS2E

November 8, 2006

2:31

Planetary Radar

753

FIGURE 15 Arecibo 13-cm OC delay–Doppler image of Venus. In the middle of the right half of the image is the bright, 1200-km-wide Alpha Regio, a complex of intersecting ridges. Just south of Alpha is the 300-km-diameter circular feature Eve. The three prominent craters in the middle of the left half of the image are seen close-up in Fig. 17d. Courtesy of D. B. Campbell.

behavior. For example, subsurface scattering of an incident circularly polarized signal results in a linearly polarized component in the radar echo due to the differing transmission coefficients at a smooth surface boundary for the horizontally and vertically polarized components of the incident wave. A linearly polarized component in 70-cm echoes from certain topographic features on Venus has been attributed to subsurface echoes from a mantled substrate or from buried rocks.

3.9 Radar Evidence for Ice Deposits at Mercury’s Poles

FIGURE 14 Radar backscatter and digital elevation models of regions on the Moon from Goldstone 3.5-cm, OC, tristatic and quadristatic interferometry. (a) The south polar region. The radar results establish that the interiors of many of the Moon’s polar craters are in permanent shadow from solar illumination. (Reprinted with permission from J. M. Margot, D. B. Campbell, R. F. Jurgens, and M. A. Slade, 1999, Science 284, 1658–1660, copyright 1999 American Association for the Advancement of Science.) (b) The crater Tycho. (J. L. Margot, D. B. Campbell, R. F. Jurgens, and M. A. Slade, 1999, J. Geophys. Res. 104, E5, 11875–11882.)

The first full-disk (Goldstone–VLA) radar portraits of Mercury surprisingly showed bright polar features with μC > 1, and subsequent delay–Doppler imagery from Arecibo established that the anomalous echoes originate from interiors of craters that are perpetually shaded from sunlight because of Mercury’s near-zero obliquity (see Fig. 16). The angle between the orbital planes of Mercury and Earth is 7◦ , so portions of the permanently shadowed regions are visible to Earth-based radars. At each pole, bright radar features correlate exactly with craters seen in Mariner 10 images; numerous features also lie in the hemisphere not imaged by that spacecraft. Similarities between the radar scattering properties of the Mars and Mercury polar anomalies and those of the icy Galilean satellites (see Section 3.7) support the inference that the radar anomalies are deposits of water ice. Temperatures below 120 K in the permanent shadows are expected and are low enough for ice to be stable against sublimation for billions of years. Temperatures several tens of kelvins lower may exist inside high-latitude craters and perhaps

CLBE001-ESS2E

November 8, 2006

2:31

754 Encyclopedia of the Solar System legacy includes pervasive volcanic planes, thousands of tiny shield volcanoes, monumental edifices, sinuous lava flow channels, pyroclastic deposits, and pancake-like domes. The superposition of volcanic signatures and elaborate, complex tectonic forms records a history of episodic crustal deformation. The paucity of impact craters smaller than 25 km and the lack of any as small as a few kilometers attests to the protective effect of the dense atmosphere. The multilobed, asymmetrical appearance of many large craters presumably results from atmospheric breakup of projectiles before impact. Atmospheric entrainment and transport of ejecta are evident in very elongated ejecta blankets. Numerous craters are surrounded by radar-dark zones, perhaps the outcome of atmospheric pressure-wave pulverization and elevation of surface material that upon resettling deposited a tenuous and hence unreflective “impact regolith.” Figure 17 shows examples of Magellan radar images.

3.11 The Radar Heterogeneity of Mars FIGURE 16 Arecibo 13-cm, SC radar image of the north polar region of Mercury. The resolution is 1.5 km, and the image is 395 km wide. The bright features are thought to be ice deposits on permanently shadowed crater floors. (Harmon, J. K., Perillat, P. J., and Slade, M. A., 2001, Icarus 149, 1–15.)

also beneath at least 10 cm of visually bright regolith. Detection of the north polar features at 70-cm wavelength indicates that the deposits may be at least several meters thick. Plausible sources of water on Mercury include comet impacts and outgassing from the interior. It has been noted that most water vapor near the surface is photodissociated, but that some molecules will random-walk to polar cold traps. Ices of other volatiles, including CO2 , NH3 , HCN, and SO2 , might also be present. There are perpetually shadowed craters at the Moon’s poles (Fig. 14), but no convincing radar evidence has been found for ice there. If ice exists on the Moon, it is likely to have low concentrations in the soil.

3.10 Venus Revealed by Magellan The Magellan spacecraft entered Venus orbit in August 1990 and during the next two years explored the planet with a single scientific instrument operating as a 13-cm radar imager, altimeter, and thermal radiometer. Magellan’s imaging resolution (∼100 m) and altimetric resolution (5 to 100 m) improved upon the best previous spacecraft and groundbased measurements by an order of magnitude, and did so with nearly global coverage. Venus’ surface contains a plethora of diverse tectonic and impact features, but its formation and evolution have clearly been dominated by widespread volcanism, whose

Ground-based investigations of Mars have achieved more global coverage than those of the other terrestrial targets because the motion in longitude of the subradar point on Mars (whose rotation period is only 24.6 hours) is rapid compared to that on the Moon, Venus, or Mercury, and because the geometry of Mars’s orbit and spin vector permits subradar tracks throughout the Martian tropics. The existing body of Mars radar data reveals extraordinary diversity in the degree of small-scale roughness as well as in the rms slope of smooth surface elements. Slopes on Mars have rms values from less than 0.5◦ to more than 10◦ . Radar slope estimates, polarization-ratio estimates, and/or multistation interferometric images have been used in selection of Viking Lander, Mars Pathfinder, and Mars Exploration Rover landing sites (Fig. 18). Diffuse scattering from Mars is much more substantial than for the other quasi-specular targets, and often accounts for most of the echo power; therefore, the average near-surface abundance of centimeter-to-meter-scale rocks is much greater on Mars than on the Moon, Mercury, or Venus. Features in Mars SC spectra first revealed the existence of regions of extremely small-scale roughness, and the trajectory of these features’ Doppler positions versus rotation phase suggested that their primary sources are the Tharsis and Elysium volcanic regions. The best terrestrial analog for this extremely rough terrain might be young lava flows. Goldstone–VLA images of Mars at longitudes that cover the Tharsis volcanic region confirmed that this area is the predominant source of strong SC echoes and that localized features are associated with individual volcanoes. A 2000-km-long band with an extremely low albedo cuts across Tharsis; the radar darkness of this “Stealth” feature probably arises from an under-dense, unconsolidated blanket of pyroclastic deposits ∼1 m deep.

CLBE001-ESS2E

November 8, 2006

2:31

Planetary Radar

A

755

B

FIGURE 17 Magellan 13-cm, SL radar maps of Venus: (a) Northern-hemisphere projection of mosaics. The North Pole is at the center of the image, with 0◦ and 90◦ E. longitudes at the 12 and 9 o’clock positions. Gaps use Pioneer Venus data or interpolations. The bright, porkchop-shaped feature is Maxwell Montes, a tectonically produced mountain range first seen in ground-based images. (b) 120-m-resolution map of Cleopatra, a double-ringed impact basin on the eastern slopes of Maxwell Montes. The diameter of the outer ring is about 100 km. (c) Image of a 350-km wide portion of the Atla region of Venus’ southern hemisphere showing several types of volcanic features criss-crossed by numerous superimposed, and hence more recent, surface fractures. Various flower-shaped patterns formed from linear fissures or lava flows emanate from circular pits. (d) Mosaic of part of Lavinia showing three large craters, with diameters ranging from 37 to 50 km, that were discovered in Arecibo images (Fig. 16). (e) Pancake-like, ∼25-km-diameter, volcanic domes located southeast of Alpha Regio. (NASA/JPL.)

3.12 Asteroids Radar has been established as the most powerful postdiscovery, Earth-based technique for determining the physical properties and orbits of asteroids. As of mid 2006, echoes from 112 main belt asteroids (MBAs) and 194 near-Earth asteroids (NEAs), including 109 Potentially hazardous asteroids (PHAs; see discussion that follows) have provided a wealth of new information about these objects’ sizes, shapes, spin vectors, and surface characteristics such as decimeter-scale roughness, topographic relief, regolith porosity, and metal concentration. 3.12.1 DISK-INTEGRATED PROPERTIES

The low polarization ratios and broad spectral shapes of some of the largest MBAs (e.g., 1 Ceres and 2 Pallas) reveal surfaces that are smoother than that of the Moon at decimeter scales but much rougher at some much larger scale. For some asteroids in the 200-km-diameter range (including 7 Iris, 9 Metis, and 654 Zelinda), brightness spikes within narrow ranges of the rotation phase suggest large, flat regions.

There is a 10-fold variation in asteroid radar albedos, implying substantial variations in these objects’ surface porosities or metal concentrations, or both. The lowest MBA albedo estimate, 0.04 for Ceres, indicates a lower surface bulk density than that on the Moon. The highest MBA albedo estimates, 0.31 for 16 Psyche and 0.44 for Kleopatra, are consistent with metal concentrations near unity and lunar porosities. These objects might be the collisionally exposed interiors of differentiated asteroids and by far the largest pieces of refined metal in the solar system. The radar albedo of the 2-km NEA 6178 (1986DA), 0.58, strongly suggests that it is a regolith-free metallic fragment, presumably derived from the interior of a much larger object that melted, differentiated, cooled, and subsequently was disrupted in a catastrophic collision. 1986 DA might be (or have been a part of) the parent body of some iron meteorites. At the other extreme, the range for 1986 JK’s radar albedo (0.005 to 0.07) suggests a surface bulk density within a factor of 2 of 0.9 g cm−3 . Similarly, the distribution of NEA circular polarization ratios runs from near zero to near unity. The highest values, for 2101 Adonis, 1992QN, 3103 Eger, 3980 1980PA, 2000 EE104, and 2004

CLBE001-ESS2E

November 8, 2006

2:31

756 Encyclopedia of the Solar System

C

D

E

FIGURE 17 (Continued)

XP14, indicate extreme near-surface structural complexity, but we cannot distinguish between multiple scattering from subsurface heterogeneities (see Section 3.7) and single scattering from complex structure on the surface. 3.12.2 IMAGING AND SHAPE RECONSTRUCTION

During the past decade, delay–Doppler imaging of asteroids has produced spatial resolution as fine as a decameter. The images generally can be “north–south” ambiguous, that is, they constitute a two-to-one (or even many-to-one) mapping from the surface to the image. However, if the radar is not in the target’s equatorial plane, then each surface point has a unique delay–Doppler trajectory as the target rotates. Hence images that provide adequate orientational coverage can be inverted, and in principle one can reconstruct the target’s three-dimensional shape as well as its spin state. The first asteroid radar data set suitable for reconstruction of the target’s shape was a 2.5-hour sequence

of 64 delay–Doppler images of 4769 Castalia, obtained 2 weeks after its August 1989 discovery. The images, which were taken at a subradar latitude of about 35◦ , show a bimodal distribution of echo power over the full range of sampled rotation phases, and least-squares estimation of Castalia’s three-dimensional shape reveals it to consist of two kilometer-sized lobes in contact. Castalia was the first of several “contact binaries” revealed by radar. If the radar view is equatorial, unique reconstruction of the asteroid’s three-dimensional shape is ruled out, but a sequence of images that thoroughly samples rotation phase can allow unambiguous reconstruction of the asteroid’s pole-on silhouette. For example, observations of 1620 Geographos yield several hundred images with ∼100-m resolution. The pole-on silhouette’s extreme dimensions are in a ratio, 2.76 + 0.21, that establishes Geographos as the most elongated solar system object imaged so far (see Fig. 7b). Delay–Doppler imaging of 4179 Toutatis in 1992 and 1996 achieved resolutions as fine as 125 ns (19 m in range)

CLBE001-ESS2E

November 8, 2006

2:31

Planetary Radar

757

FIGURE 18 Mars Lander sites and estimates of 3.5-cm rms slope and circular polarization ratio. The width across the front of the image is about 3 m in the right column and about 2 m in the left column. (NASA/JPL, courtesy of A. F. Haldemann.)

and 8.3 mHz (0.15 mm s−1 in radial velocity), placing thousands of pixels on the asteroid. This data set provided physical and dynamical information that was unprecedented for an Earth-crossing object. Extraction of the information in this imaging data set required inversion with a much more comprehensive physical model than in the analysis of Castalia images; free parameters included the asteroid’s

shape and inertia matrix, initial conditions for the asteroid’s spin and orientation, the radar scattering properties of the surface, and the delay–Doppler trajectory of the center of mass (see Fig. 19). Toutatis has complex linear features as well as circular crater-like structures down to the several-decameter resolution limit. The features suggest a complex interior

CLBE001-ESS2E

November 8, 2006

2:31

758 Encyclopedia of the Solar System FIGURE 19 High-resolution Goldstone 3.5-cm, OC, delay–Doppler images from each of three observation dates in 1996 and the corresponding plane-of-sky (POS) appearance of the radar model. The crosshairs are 5 km long and centered on Toutatis’ center of mass (COM). In the radar images, time delay (range) increases from top to bottom, and Doppler frequency (radial velocity) increases from left to right. The model is rendered with a Lambertian scattering law, with the viewer co-located with the illumination source. The crosshairs are aligned north–south and east–west on the plane of the sky. In each POS frame, the arrow radiating from the COM shows the POS projection of the instantaneous spin vector.

configuration involving monolithic fragments with various sizes and shapes, presumably due to collisions in various energy regimes. Toutatis might be an impact-sculpted, single, coherent body, or it might consist of two separate objects that came together in a gentle collision; the difference in the two lobes’ gravitational slopes supports the latter idea (Fig. 20). Toutatis is rotating in a long-axis non-principal-axis (NPA) spin state (see Fig. 21) characterized by periods of 5.4 days (rotation about the long axis) and 7.4 days (average for long-axis precession about the angular momentum vector). The asteroid’s principal moments of inertia are in ratios within 1% of 3.22 and 3.09, and the inertia matrix is indistinguishable from that of a homogeneous body. Such information has yet to be determined for any other small body except the NEAR-Shoemaker target 433 Eros and probably is impossible to acquire in a fast spacecraft flyby. Images of another NPA rotator, 53319 (1999 JM8), reveal an asymmetric, irregularly shaped, 7-km object (Fig. 22). The asteroid’s rotation has a dominant 7-day period and is not far from uniform rotation. 1999 JM8 has pronounced topographic relief, prominent facets several kilometers in extent, and numerous crater-like features between ∼100 m and 1.5 km in diameter. Radar images of 6489 Golevka (1991 JX) reveal a halfkilometer object whose shape is extraordinarily angular, with flat sides, sharp edges and corners, and peculiar concavities. Extremely large gravitational slopes in some areas of the radar-derived model indicate the presence of exposed, solid, monolithic rock (Fig. 20). This asteroid, the

first sub-kilometer object studied in this much detail, probably is a monolithic collision fragment rather than a rubble pile. Golevka was the target of the first intercontinental radar observations, in June 1995, when Goldstone provided a transmission and echoes were received by the Russian 70-m Evpatoria antenna and also by the Japanese 34-m Kashima antenna. The asteroid’s name is made from leading letters of those antennas’ names. Radar has revealed numerous NEAs to have nearly circular pole-on silhouettes [e.g., 1999 RQ36, 7822 (1991 CS), 2100 Ra-Shalom, 1998 ML14 and 1998 FH12]. 1998 ML14 has isolated, several-hundred-meter protrusions on one side, while 1999 RQ36 has no noticeable features anywhere. At the opposite extreme, several NEAs, including 22771 (1999 CU3) and 2003 MS2 have elongated shapes with curious irregularities. Ironically, the dogbone shape of the 235-km-long main-belt object Kleopatra is the most exotic yet discerned by radar (Fig. 20). Asteroids with visual absolute magnitude HV > 21 (diameters 0.2 km or less) constitute about one-fourth of radardetected NEAs; the smallest are comparable in size to boulders seen on the surface of Eros. Most of them have rotation periods no longer than an hour and in some cases only a few minutes, but at least two, 2001 EC16 and 2004 XP14, are very slow rotators. 3.12.3 BINARY SYSTEMS

Radar obtained the first undeniable evidence for NEA binary systems and has now imaged 20 of them. Current detection statistics, including evidence from optical

CLBE001-ESS2E

November 8, 2006

2:31

Planetary Radar

759

FIGURE 20 Radar-derived shapes of (from top to bottom) main-belt asteroid 216 Kleopatra (maximum model dimension 217 km) and the near-Earth asteroids 4179 Toutatis (4.60 km) and 6489 Golevka (0.685 km), color-coded for gravitational slope (degrees), defined as the acute angle a plumb line would make with the local surface normal. Uniform internal density is assumed.

lightcurves, suggest that between 10 and 20% of PHAs are binary systems. For 2000 DP107, with an 800-m primary and a 300-m secondary, the orbital period of 1.767 days and orbital semimajor axis of 2620 + 160 m yield a bulk density of 1.7 + 1.1 g cm−3 for the primary. For 66391 (1999 KW4), very highSNR, high-resolution delay–Doppler images characterized

the components and their dynamics in detail (Fig. 23). The resemblance of the primary to a canonical oblate spheroid is striking. For 1998 ST27, the orbital period is several days, the semimajor axis could be as large as 7 km, and the rotation period of the secondary is more than an order of magnitude shorter than its orbital period, the first such case among binary NEAs. 1998 ST27 and the somewhat similar

CLBE001-ESS2E

November 8, 2006

2:31

760 Encyclopedia of the Solar System FIGURE 21 Spin state of asteroid 4179 Toutatis derived from radar. The axes with no arrow tips are the asteroid’s principal axes of inertia and the vertical arrow is its angular momentum vector. The direction of the spin vector (yellow arrow) relative to the principal axes is a (5.41-day) periodic function. A flashlamp attached to the short axis of inertia and flashed every 15 minutes for 20 days would trace out the intricate path indicated by the small spheres stacked end to end; the path never repeats. Toutatis’ spin state differs radically from those of the vast majority of solar system bodies that have been studied, which are in principal-axis spin states. For those objects, the spin vector and angular momentum vector point in the same direction, and the flashlamp’s path would be a circle.

binaries 1990 OS and 2003 YT1 may be relatively young systems.

FIGURE 22 Arecibo 13-cm, OC radar image of 53319 (1999 JM8). Radar illumination is from the top. The vertical resolution is 15 m. The horizontal resolution depends on the asteroid’s NPA spin state, which is not yet known. [From Benner, L. A. M., et al., 2002, Meteoritics Planet. Sci. 37, 779–792.)

3.12.4 COLLISION PREDICTION AND PREVENTION

The NEA collision hazard has emerged as a primary driving issue in asteroid science. Radar provides very precise astrometric positions and leads to more accurate trajectory predictions than optical data alone. On average, radar has added a third of a millennium to the window of accurate future predictions of PHA close Earth approaches. When radar astrometry is excluded from single-apparition PHA radar + optical orbit solutions, some 40% cannot have their next close approach predicted within the adopted confidence level using only the single apparition of optical data. The net effect of radar for multiapparition cases is to improve the orbit’s accuracy. For example, integrations of the radar-refined orbit of 29075 (1950 DA) revealed that in 2880 there could be a potentially hazardous approach that had not been indicated in the half-century arc of preradar optical data. The dominant source of uncertainty in the collision probability involves the Yarkovsky effect, which is the nongravitational “recoil” acceleration of a rotating object due to its anisotropic thermal emission of absorbed sunlight, and which depends on the asteroid’s size, shape, mass, rotation, and optical and thermal characteristics.

CLBE001-ESS2E

November 8, 2006

2:31

Planetary Radar

761

FIGURE 23 Several-hour delay–Doppler time exposures of radar echoes from binary asteroid 66391 (1999 KW4). Distance from Earth increases toward the bottom, and speed from Earth increases toward the left. The motion of the secondary (smaller) component about the primary component is clockwise. Gaps in the trail are due to breaks in the data-taking. The primary appears much wider than the secondary because it is a few times bigger and is rotating much faster. Although the components have the same speeds along the radar line of sight and the same distances from the radar where their echoes overlap, their positions in space are never the same. The components orbit a common center of mass, and each component’s average distance from that point is inversely proportional to its mass. The motion of the relatively massive primary is much less obvious than the motion of the secondary, but it can be seen in the double appearance of the primary’s top edge in the two time exposures that follow the secondary from in front of the primary to behind it. These Goldstone (8560-MHz, 3.5-cm, OC) images have overall extents of 37.5 μs by 67 Hz (5.6 km by 1.2 m s−1 ). (NASA/JPL.)

It was suggested that radar-refined orbits with sufficiently long astrometric time bases could permit direct detection of nongravitational acceleration of NEAs due to the Yarkovsky effect. The first such detection was achieved via radar ranging to Golevka. That experiment, which constitutes the first estimation of the mass (and, using the previously derived radar shape model, the density) of a small solitary asteroid using ground-based observations.

In late 1985, radar observations of comet Halley, which was much more active than IRAS–Araki–Alcock, yielded echoes with a substantial broadband component presumed to be from a large-particle swarm, but no narrowband component, a negative result consistent with the hypothesis that the surface of the nucleus has an extremely low bulk density. In 1996, Goldstone obtained 3.5-cm echoes from the nucleus and coma of comet Hyakutake (C/1996 B2). The coma-to-nucleus ratio of radar cross section is about 12 for Hyakutake versus about 0.3 for IAA.

3.13 Comets Because a cometary coma is nearly transparent at radio wavelengths, radar is much more capable of unambiguous detection of a cometary nucleus than are visible-wavelength and infrared methods, and radar observations of several comets (see Table 1) have provided useful constraints on nuclear dimensions. The radar signature of one particular comet (IRAS–Araki–Alcock, which came within 0.03 AU of Earth in May 1983) altered our concepts of the physical nature of these intriguing objects. Echoes obtained at both Arecibo (Fig. 24) and Goldstone have a narrowband component from the nucleus as well as a much weaker broadband component from large particles ejected mostly from the sunlit side of the nucleus. Models of the echoes suggest that the nucleus is very rough on scales larger than a meter, that its maximum overall dimension is within a factor of 2 of 10 km, and that its spin period is 2–3 days. The particles are probably several centimeters in size and account for a significant fraction of the particulate mass loss from the nucleus. Most of them appear to be distributed within ∼1000 km of the nucleus, that is, in the volume filled by particles ejected at several meters per second over a few days. The typical particle lifetime may have been this short, or the particle ejection rate may have been highly variable.

3.14 The Saturn System and First Cassini Results 3.14.1 RINGS

The only radar-detected ring system is quite unlike other planetary targets in terms of both the experimental techniques employed and the physical considerations involved. For example, the relation between ring-plane location and delay–Doppler coordinates for a system of particles traveling in Keplerian orbits is different from the geometry portrayed in Fig. 5. The rings are grossly overspread (see Table 2), requiring the use of frequency-stepped waveforms in delay–Doppler imaging. Radar determinations of the rings’ backscattering properties complement results of the Voyager spacecraft radio occultation experiment (which measured the rings’ forward scattering efficiency at identical wavelengths) in constraining the size and spatial distributions of ring particles. The rings’ circular polarization ratio is ∼1.0 at 3.5 cm and ∼0.5 at 13 cm, more or less independent of the inclination angle δ between the ring plane and the line of sight. Whereas multiple scattering between particles might cause some of the depolarization, the lack of strong dependence of μC on δ suggests that the particles are intrinsically rougher at

CLBE001-ESS2E

November 8, 2006

2:31

762 Encyclopedia of the Solar System FIGURE 24 Arecibo OC and SC echo spectra obtained for comet IRAS–Araki–Alcock, truncated at 2% of the maximum OC amplitude. The narrowband echo from the nucleus is flanked by broadband echo from large (= 1 cm) particles in a 1000-km-radius cloud surrounding the nucleus. (From Harmon, J. K., Campbell, D. B., Hine, A. A., Shapiro, I. I., and Marsden, B. G., 1989, Astrophys. J. 338, 1071–1093.)

the scale of the smaller wavelength. Delay–Doppler resolution of ring echoes indicates that the portions of the ring system that are brightest optically (the A and B rings) also return most of the radar echoes. The C ring has a very low radar reflectivity, presumably because of either a low particle density in that region or compositions or particle sizes that lead to inefficient scattering. Recent 13-cm images show a pronounced azimuthal asymmetry in the reflectivity of the A ring. The analogous phenomenon at visual wavelengths is ascribed to gravitational “wakes” generated by individual large ring particles or arising from internal instabilities, which are distorted by Keplerian shear into elongated structures trailing at angles of 70◦ from the radial direction. The strength of the radar asymmetry may be due to strongly forward-scattering meter-size ice particles and the resultant sensitivity to optical depth variations.

near-IR albedo are also responsible for the variation in the radar cross section. A specular component is present for about 75% of the sub-Earth locations. The most specular echoes (e.g., Fig 25), which are subradar glints that must come from extremely smooth surfaces, have properties consistent with those expected for irregularly shaped 50-km or larger bodies of liquid hydrocarbons. Titan is the primary target of the Cassini mission. Cassini’s radar instrument, a 13.8-GHz (2.2-cm)

3.14.2 TITAN

Titan’s thick, hazy atmosphere poses challenges to visiblewavelength and near-infrared imaging of its surface. Voyager and ground-based data indicate a surface temperature and pressure of 94 K and 1.5 bar and show that the atmosphere is mostly N2 with traces of hydrocarbons and nitriles. Thermodynamic considerations imply a near-surface reservoir of liquid hydrocarbons. Arecibo 13-cm echoes show most of the power to be diffusely scattered, and the longitude dependence of the radar albedo mimics the dependence of the disk-integrated near-IR albedos, indicating that whatever properties of the surface—roughness, composition, etc.—that are responsible for the variation in the

FIGURE 25 Arecibo 13-cm OC radar echo spectrum ot Titan at 1.0-Hz resolution for sub-Earth longitude of 80◦ . A fit of a composite model with Hagfors and Cosine terms gives an rms slope of 0.2◦ and a reflection coefficient of 0.023. Titan’s echo bandwidth is 325 Hz. (From Campbell, D. B., Black, G. J., Carter, L. M., and Ostro, S. J., 2003, Science 302, 431–434.)

CLBE001-ESS2E

November 8, 2006

2:31

Planetary Radar

763

FIGURE 26 Cassini RADAR (2.2-cm, SL) images of Titan. The bright, rough region on the left side of the image seems to be topographically high terrain cut by channels and bays. The boundary of the bright (rough) region and the dark (smooth) region appears to be a shoreline. The patterns in the dark area indicate that it may once have been flooded. The image is 175 × 330 km and is centered at 66 S, 356 W. (NASA/JPL.)

synthetic-aperture radar (SAR) imager, altimeter, scatterometer, and radiometer, will operate during around half of the 44 Titan flybys, covering about a fifth of the surface with imaging resolution of 2 km or finer. Scatterometry observations will cover most of the surface, albeit at resolution no finer than tens of kilometers, and a limited number of short altimetry tracks will give regional topographic information. At this writing, Cassini has completed its first 6 Titan SAR flybys (Fig. 26), revealing a surface with low relief and an Earth-like variety of surface features providing evidence for fluvial/pluvial, cryovolcanic, Aeolian, impact, and probably tectonic modification processes. Diverse styles of channels are seen; some suggest that precipitated liquids are collected and transported hundreds of kilometers, others indicate a cryovolcanic origin, and some may be spring-fed. Circular features apparently include cryovolcanic vents as well as a surprisingly small number of impact craters. Regions of dune-like forms that run for hundreds of kilometers establish that particulate matter is available and that there are winds that can transport them. The radar-bright, continentsized landform Xanadu is revealed in the radar images to be a landmass of Appalachian-sized mountains and valleys cut by channels and marked with craters and dark patches. Such patches, seen in numerous images, are tentatively identified as hydrocarbon lakes or organic sludge.

the order Enceladus/Tethys, Rhea, Dione, Hyperion, Iapetus, and Phoebe. This sequence most likely corresponds to increasing contamination of near-surface water ice, which is intrinsically very transparent at radio wavelengths. Plausible candidates for contaminants include ammonia, silicates, metallic oxides, and polar organics. There is correlation of our targets’ radar and optical albedos, probably due to variations in the concentration of optically dark contaminants in near-surface water ice and the resulting variable attenuation of the high-order multiple scattering responsible for high radar albedos. Iapetus’ 2.2-cm radar albedo is dramatically higher on the optically bright trailing side than the optically dark leading side, whereas 13-cm results show hardly any hemispheric asymmetry and give a mean radar reflectivity several times lower than the reflectivity measured at 2.2 cm. These Iapetus results are understandable if ammonia is much less abundant on both sides within the upper one to several decimeters than at greater depths, and if the leading side’s optically dark contaminant is present to depths of at least one to several decimeters. A combination of ion erosion and micrometeoroid gardening may have depleted ammonia from the surfaces of Saturn’s icy satellites. Given the hypersensitivity of water ice’s absorption length to ammonia concentration, an increase in ammonia with depth could allow efficient 2.2-cm scattering from within the top one to several decimeters while attenuating 13-cm echoes, which would require a 6-fold thicker scattering layer.

3.14.3 ICY SATELLITES

Cassini radar and radiometric observations of Saturn’s icy satellites yield properties that apparently are dominated by subsurface volume scattering and are similar to those of the icy Galilean satellites. Average radar albedos decrease in

4. Prospects for Planetary Radar There is growing interest in the possibility of a subsurface ocean on Europa and in the feasibility of using an orbiting

CLBE001-ESS2E

November 8, 2006

2:31

764 Encyclopedia of the Solar System radar sounder to probe many kilometers below that object’s fractured crust using meter- to several-decameter wavelengths. Anather possibility is to use radar reflection tomography to construct a three-dimensional image of the interior of an asteroid or comet. Meanwhile, the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on the European Space Agency’s Mars Express spacecraft has probed the depths of Mars’ north and south polar deposits, and the Shallow Subsurface Radar (SHARAD) is about to start searching for subsurface water on Mars from NASA’s Mars Reconnaissance Orbiter. Reconnaissance of near-Earth asteroids will occupy ground-based radar astronomy indefinitely. Most of the optically discoverable NEAs traverse the detectability windows of Arecibo and/or Goldstone at least once every few decades, and efforts are under way to increase the NEA discovery rate by more than an order of magnitude. The power of radar observations for orbit refinement and physical characterization motivates radar observations of newly discovered NEAs whenever possible. Eventually the initial radar detection of a new NEA could become an almost daily opportunity.

Bibliography Black, G. J., Campbell, D. B., and Nicholson, P. D. (2001). Icy Galilean satellites: Modeling radar reflectivities as a coherent backscatter effect. Icarus 151, 167–180. Butrica, A. J. (1996). “To See the Unseen: A History of Plane-

tary Radar Astronomy,” NASA History Series No. SP-4218. NASA, Houston. Harmon, J. K., Nolan, M. C., Ostro, S. J., and Campbell, D. B. (2004). Radar studies of comet nuclei and grain comae. In “Comets II” (M. Festou, U. Keller, and H. Weaver, eds.), pp. 265–279. Univ. Arizona Press, Tucson. Magri, C., Ostro, S. J., Rosema, K. D., Thomas, M. L., Mitchell, D. L., Campbell, D. B., Chandler, J. F., Shapiro, I. I., Giorgini, J. D., and Yeomans, D. K. (1999). Mainbelt asteroids: Results of Arecibo and Goldstone radar observations of 37 objects during 1980–1995. Icarus 140, 379–407. Ostro, S. J. (1993). Planetary radar astronomy. Rev. Modern Physics 65, 1235–1279. Ostro, S. J., and Giorgini, J. D. (2004). The role of radar in predicting and preventing asteroid and comet collisions with Earth. In “Mitigation of Hazardous Comets and Asteroids” (M. J. S. Belton, D. K. Yeomans, and T. H. Morgan, eds.), pp. 38–65. Cambridge Univ. Press, Cambridge, England. Ostro, S. J., Hudson, R. S., Benner, L. A. M., Giorgini, J. D., Magri, C., Margot, J.-L., and Nolan, M. C. (2002). Asteroid radar astronomy. In “Asteroids III” (W. Bottke, A. Cellino, P. Paolicchi, and R. P. Binzel, eds.), pp. 151–168. Univ. Arizona Press, Tucson. Pettengill, G. H., Ford, P. G., Johnson, W. T. K., Raney, R. K., and Soderblom, L. A. (1991). Magellan: Radar performance and data products. Science 252, 260–265. Shapiro, I. I., Chandler, J. F., Campbell, D. B., Hine, A. A., and Stacy, N. J. S. (1990). The spin vector of Venus. Astron. J. 100, 1363–1368. Tyler, G. L., Ford, P. G., Campbell, D. B., Elachi, C., Pettengill, G. H., and Simpson, R. A. (1991). Magellan: Electrical and physical properties of Venus’ surface. Science 252, 265–270.

CLBE001-ESS2E

November 8, 2006

100-C

25-C

50-C

75-C

2:37

C+M

50-C+M

C+Y

50-C+Y

M+Y

50-M+Y

100-M

25-M

50-M

75-M

100-Y

25-Y

50-Y

75-Y

100-K

25-K

25-19-19

50-K

50-40-40

75-K

75-64-64

Remote Chemical Sensing Using Nuclear Spectroscopy Thomas H. Prettyman Los Alamos National Laboratory, Los Alamos, New Mexico

CHAPTER

1. Introduction 2. Origin of gamma rays and neutrons 3. Instrumentation

41

5. Science 6. Future Prospects Bibliography

4. Missions

1. Introduction Nuclear spectroscopy techniques are used to determine the elemental composition of planetary surfaces and atmospheres. Radiation, including gamma rays and neutrons, is produced steadily by cosmic ray bombardment of the surfaces and atmospheres of planetary bodies and by the decay of radionuclides within the solid surface. The leakage flux of gamma rays and neutrons contains information about the abundance of major elements, selected trace elements, and light elements such as H and C. Gamma rays and neutrons can be measured from high altitudes (less than a planetary radius), enabling global mapping of elemental composition by an orbiting spacecraft. Radiation that escapes into space originates from shallow depths (1 ppm). To illustrate a typical gamma ray leakage spectrum, a Monte Carlo simulation of the lunar gamma ray leakage current induced by galactic cosmic ray protons is shown in Fig. 5. The composition of the surface was assumed to be the mean soil composition from the Apollo 11 landing site. Contributions from nonelastic reactions and capture are plotted separately. A background component associated primarily

CLBE001-ESS2E

November 8, 2006

2:37

Remote Chemical Sensing Using Nuclear Spectroscopy

771

FIGURE 5 The current of gamma rays leaking away from the Moon for a composition representative of the Apollo 11 landing site.

with the decay of pions is also shown. The peaks correspond to gamma rays that escape into space without interacting with the surface material. The peaks are superimposed on a continuum, which results from the scattering of gamma rays in the surface. The total number of gamma rays escaping the surface per incident cosmic ray proton was 2.7, which is within the range of values for the number of neutrons escaping the martian surface, presented in Section 2.1. Gamma ray peaks associated with neutron interactions with major elements are labeled with the target element in Fig. 5. The intensity (or area) of each peak is proportional to the product of the abundance of the target element and the number density of neutrons slowing down in the medium. Specifically, the measured intensity (I) of a gamma ray peak with energy E for a selected reaction can be modeled as the product of three terms: I ∝ f yR, where f accounts for attenuation of gamma rays by intervening surface materials and the variation of detection efficiency with gamma ray energy; y is the number of gamma rays of energy E produced per reaction; and R = ϕ Nσ is the reaction rate, the product of the neutron flux, cross section, and number density of the target element. Because gamma rays are produced by neutron interactions, the absolute number density or, equivalently, the weight fraction of the target element cannot be determined unless the neutron flux is known. Thus, neutron spectroscopy plays an important role in the analysis of gamma ray data. Relative abundances can be determined without knowledge of the magnitude of the neutron flux. For example, the ratio of Fe to Si abundances can be determined from the ratio of the intensities of the prominent Fe doublet (at 7.65 MeV and 7.63 MeV) the Si gamma ray at 4.93 MeV. Because the magnitude of the attenuation of gamma rays by surface materials depends on gamma ray energy and the distribution of gamma ray production with depth, models

of the depth profile of the neutron flux are needed in order to analyze gamma ray data. For homogeneous surfaces, accurate results can be obtained for absolute and relative abundances; however, surfaces with strong stratigraphic variations present a difficult challenge for analyzing nuclear spectroscopy data. Compositional layering of major elements on a submeter scale is widespread on Mars as shown, for example, by the Spirit and Opportunity rovers [see Mars Site Geology and Geochemistry]. In some cases, geophysical assumptions can be made that simplify the analysis and allow quantitative results to be obtained; however, it is often the case that insufficient information is available. In these cases, it is sometimes possible to establish bounds on composition that are useful for geochemical analysis. Development of accurate algorithms for determining elemental abundances, absolute or relative, requires careful synthesis of nuclear physics with constraints from geology, geophysics, and geochemistry.

3. Detection of Gamma Rays and Neutrons In this section, a simple model of the counting rate observed by orbiting neutron and gamma ray spectrometers is presented along with an overview of radiation detection concepts for planetary science applications.

3.1 Counting Rate Models The flux of radiation reaching an orbiting spectrometer varies in proportion to the solid angle subtended by the planet at the detector, which depends on orbital altitude. The fractional solid angle of a spherical body is given by (h) = 1 −



1 − R2 /(R + h)2 ,

(1)

CLBE001-ESS2E

November 8, 2006

2:37

772 Encyclopedia of the Solar System where h is the orbital altitude and R is the radius. The fractional solid angle varies from 1 at the surface (for h = 0) to 0 far away from the planet. For galactic cosmic ray interactions, the flux of gamma rays or neutrons at the orbiting spectrometer is approximately φ(h) = 1/4J (h),

(2)

where  is the flux of galactic cosmic ray protons far from the planet (about 4 protons/cm2 /s, depending on the solar cycle and location within the heliosphere), and J is the leakage current. Because alpha particles and heavier nuclei of galactic origin contribute to neutron and gamma ray production, Eq. 2 must be multiplied by a factor, approximately 1.4, in order to estimate the total leakage flux. Eq. 2 can be used, for example, to calculate the flux of neutrons incident on the Mars Odyssey neutron spectrometer. The orbital altitude for Mars Odyssey is 400 km, and the volumetric mean radius of Mars is 3390 km. The fractional solid angle, given by Eq. 1, is 0.55. The total leakage current for a surface consisting of thick CO2 ice, representative of the polar seasonal caps during winter, was J = 5 (from Section 2.2). Consequently, from Eq. 2, the total flux of neutrons at Odyssey’s orbit from thick CO2 deposits is approximately 4 neutrons per cm2 per s. For a surface that is 100% water, which is representative of the north polar residual cap, J was 1, and the total flux at orbital altitude is expected to be 0.8 neutrons per cm2 per s. Radiation detectors, such as the gamma ray and neutron spectrometers on Mars Odyssey, count particle interactions and bin them into energy or pulse-height spectra, for example, with units of counts per s per unit energy. For both gamma rays and neutrons, the net counting rate (with units of counts per s) for selected peaks in the spectrum is needed in order to determine elemental abundances. The flux of particles (gamma rays or neutrons) incident on a spectrometer can be converted to counting rate (C), given the intrinsic efficiency (ε) and projected area (A) of the spectrometer in the direction of the incident particles: C = ϕ(h) ε A.

(3)

The intrinsic efficiency is the probability that an incident particle will interact with the spectrometer to produce an event that is counted. Because particles can pass through the spectrometer without interacting, the intrinsic efficiency is always less than or equal to 1. For example, ε A is on the order of 10 cm2 for the Mars Odyssey epithermal neutron detector, which has a maximum projected area of about 100 cm2 . The efficiency-area product (ε A) varies with the energy and angle of incidence of the particles. So, the value for ε A used in Eq. 3 must be appropriately averaged over neutron energy and direction. One of the main sources of uncertainty in measured counting rates is statistical fluctuations due to the random

nature of the production, transport, and detection of radiation. While a detailed discussion of error-propagation is beyond the scope of this article, the most important result is given here: The statistical uncertainty (precision) in the √ counting rate is given by σ = C/t, where t is the measurement time and C is the mean counting rate. For example, to achieve a precision of (1% σ/C = 0.01) when C = 10 counts per s, which is typical of the epithermal and thermal counting rates measured by the Mars Odyssey neutron spectrometer, a counting time of 1000 s is required. Longer counting times are needed when background contributions are subtracted, for example, to determine counting rates for peaks in gamma ray and neutron spectra. Uncertainties in the counting rate due to random fluctuations propagate to the uncertainties in elemental abundance and other parameters determined in the analysis of spectroscopy data. Long counting times are desired to minimize statistical contributions. Alternatively, improved precision can be achieved by increasing the counting rate, which can be accomplished through instrument design, by maximizing, and/or by making measurements at low altitude.

3.2 Gamma Ray and Neutron Detection Radiation spectrometers measure ionization produced by the interaction of particles within a sensitive volume. Gamma ray interactions produce swift primary electrons that cause ionization as they slow down in the sensitive volume. Neutrons undergo reactions that produce energetic ions and gamma rays. The recoil proton from neutron elastic scattering with hydrogen can produce measurable ionization. The charge liberated by these interactions can be measured using a wide variety of techniques, two of which are illustrated here. Semiconductor radiation detectors typically consist of a semiconductor dielectric material sandwiched between two electrodes. An electric field is established in the dielectric by applying high voltage across the electrodes. Gamma ray interactions produce free electron-hole pairs which drift in opposite directions in the electric field. As they drift, they induce charge on the electrodes, which is measured using a charge-sensitive preamplifier. The amplitude of the charge pulse, or pulse-height, is proportional to the energy deposited by the gamma ray. Consequently, a histogram of pulse heights, known as a pulse-height spectrum, measured for many interactions provides information about the energy distribution of the incident gamma rays. For example, a diagram of a high-purity germanium (HPGe) detector is shown in Fig. 6a along with a photograph of an HPGe crystal in Fig. 6b. The closed-end coaxial geometry is designed to minimize trapping of carriers as they drift to the electrodes. To minimize noise due to leakage current, the HPGe must be operated at very low temperatures. The requirement for cooling adds to the mass and complexity of the design for space applications.

CLBE001-ESS2E

November 8, 2006

2:37

Remote Chemical Sensing Using Nuclear Spectroscopy

773

FIGURE 6 (a) Schematic diagram of a coaxial HPGe spectrometer and gamma ray interactions; (b) photograph of a HPGe cystal; (c) diagram of a scintillation-based spectrometer with neutron interactions; and (d) assembly diagram for a boron-loaded plastic scintillator for a flight experiment, including the mechanical structure (including packaging designed to withstand the vibrational environment during launch). (Part b courtesy of AMETEK, Advanced Measurement Technology, Inc., ORTEC Product Line, 801 South Illinois Avenue, Oak Ridge, TN 37830).

A hypothetical gamma ray interaction is superimposed on the diagram in Fig. 6a. Gamma rays undergo three types of interactions: pair production, Compton scattering, and the photoelectric effect. High-energy gamma rays (greater than 1.022 MeV) can undergo pair production, in which the gamma ray disappears and an electron-positron pair is produced. The kinetic energy of the electron and positron is absorbed by the medium. When the positron is annihilated by an electron, two, back-to-back (511 keV) gamma rays are produced, which can undergo additional interactions. In Compton scattering, a portion of the energy of the gamma ray is transferred to an electron. The energy lost by the gamma ray depends on the scattering angle. At low energies, the gamma ray can be absorbed by an electron via the

photoelectric effect. All of these interactions vary strongly with the atomic number (Z) and density of the detector material. High Z, high density and a large sensitive volume is desired to maximize the probability that all of the energy of the incident gamma ray is absorbed in the detector. A pulse height spectrum for a large volume (slightly larger than the crystal flown on Mars Odyssey), coaxial HPGe detector is shown in Fig. 7. The gamma rays were produced by moderated neutrons, with an energy distribution similar to the lunar leakage spectrum, incident on an Fe slab. Well-defined peaks corresponding to neutron capture and inelastic scattering with Fe appear in the spectrum. For example, the doublet labeled Fe(1) corresponds to gamma

CLBE001-ESS2E

November 8, 2006

2:37

774 Encyclopedia of the Solar System

FIGURE 7 Gamma ray spectra acquired by HPGe (black) and BGO (red) spectrometers. To improve visualization, the spectrum for BGO has been multiplied by 100. The source was moderated neutrons, with energy distribution similar to a planetary leakage spectrum, incident on an iron slab. Gamma rays from natural radioactivity in the environment are also visible (from K at 1461 keV and Th at 2615 keV). A gamma ray at 2223 keV from neutron capture with H (from polyethylene in the moderator) is a prominent feature in the HPGe and BGO spectra. Major gamma rays from neutron interactions with Fe that are resolved by the HPGe spectrometer are labeled: (1) 7646- and 7631-keV doublet from neutron capture; (2) their single escape peaks; (3) 6019- and 5921-keV gamma rays from neutron capture; (4) their single escape peaks; and (5) 846.7 keV gamma ray from neutron inelastic scattering. (HPGe spectrum courtesy of S. Garner, J. Shergur, and D. Mercer of Los Alamos National Laboratory).

rays (7646- and 7631-keV) produced by neutron capture with Fe. The peaks labeled Fe(2) are shifted 511 keV lower in energy and correspond to the escape of one of the 511 keV gamma rays produced by pair production in the spectrometer. The continuum that underlies the peaks is caused by external Compton scattering and the escape of gamma rays that scattered in the spectrometer. Gamma rays from neutron capture with H and the radioactive decay of K and Th are also visible. Scintillators provide an alternative method of detecting ionizing radiation, which can be used for gamma ray and neutron spectroscopy. Scintillators consist of a transparent material that emits detectable light when ionized. The light is measured by a photomultiplier tube or photodiode, which is optically coupled to the scintillator. The amount of light produced and the amplitude of the corresponding charge pulse from the photomultiplier tube and pulse processing circuit is proportional to the energy deposited by the radiation interaction.

A diagram of a boron-loaded, plastic scintillation detector is shown in Fig. 6c along with an assembly diagram of flight sensor (Fig. 6d). Thermal and epithermal neutrons are detected by the 10 B(n,αγ )7 Li reaction. The recoiling reaction products (alpha particle and 7 Li ion) produce ionization equivalent to a 93 keV electron, which makes a well-defined peak in the pulse height spectrum. The area of the peak depends on the flux of incident thermal and epithermal neutrons. Thermal neutrons can be filtered out by wrapping the scintillator in a Cd foil, which strongly absorbs neutrons with energies below about 0.5 eV. Thus, the combination of a bare and Cd-covered scintillator can be used to separately measure contributions from thermal and epithermal neutrons. Above about 500 keV, light is produced by recoiling protons from neutron elastic scattering with hydrogen in the scintillator. Fast neutrons (greater than about 500 keV) can be detected by a prompt pulse from proton recoils followed a short time later by a second pulse, corresponding to neutron capture of the moderated neutron by

CLBE001-ESS2E

November 8, 2006

2:37

Remote Chemical Sensing Using Nuclear Spectroscopy 10

B. This characteristic, double-pulse time signature can be used to identify, and separately measure, fast neutron events. Scintillators are also used routinely for gamma ray spectroscopy. For example, a pulse height spectrum acquired by a bismuth germanate (BGO) scintillator is shown in Fig. 7. The source was exactly the same as measured by the HPGe spectrometer, and the two spectra share similar peak features. Note, however, that the peaks measured by BGO are considerably broader than those measured by HPGe. The width of the peaks is caused by statistical variations in the number of scintillation photons produced in the BGO. Similar dispersion occurs for charge carriers (electrons and holes) in the HPGe crystal; however, the effect is far less pronounced. The pulse height resolution as measured by the full-width-at-half-maximum (FWHM) of the gamma ray peaks is much worse for the BGO than the HPGe. The ability of the HPGe technology to resolve individual peaks is coveted by the planetary spectroscopist; however, the added cost and complexity of HPGe relative to scintillation technology has made scintillators competitive for some missions. Other technologies that have been flown for gamma ray and neutron detection include 3 He ionization chambers (for thermal and epithermal neutron detection on Lunar Prospector) and various scintillators, including Tl-doped NaI on NEAR and Apollo and Tl-doped CsI on Phobos. The Dawn mission will fly a new compound semiconductor technology (CdZnTe), which has significantly improved pulse height resolution relative to BGO and, in contrast to HPGe, can be operated at ambient temperatures.

3.3 Spatial Resolution The spatial resolution that can be achieved by a spectrometer depends on the angular distribution of radiation emitted from the surface, the angular response of the spectrometer, and the altitude of the orbit. The angular response of most spectrometers is roughly isotropic or weakly dependent on incident direction. Consequently, the spectrometer is sensitive to radiation emitted from locations from underneath the spectrometer all the way out to the limb. Due to their increased area, off-nadir regions contribute more to the counting rate than regions directly beneath the spacecraft. When the spectrometer passes over a point source of radiation on the surface, the counting rate as a function of distance along the orbital path has an approximately Gaussian shape, with the peak occurring when the spacecraft passes over the source. Consequently, the ability of the spectrometer to resolve spatial regions with different compositions depends on the FWHM of the Gaussian, which as a rule of thumb is approximately 1.5 times the orbital altitude. For example, the lowest orbital altitude of Lunar Prospector was 30 km for which the spatial resolution was 45 km or 1.5◦ of arc length. For Mars Odyssey, the orbital altitude

775

was 400 km, and the spatial resolution was approximately 600 km or 10◦ of arc length. The broad spatial response of gamma ray and neutron spectrometers must be considered in the analysis and interpretation of data, especially where comparisons to highresolution data (for example, from optical spectroscopy) are concerned. It may be possible to increase the resolution of a spectrometer by the addition of a collimator, which would add mass to the instrument and also reduce the precision of the measurements. Alternatively, spatial deconvolution and instrument modeling techniques can sometimes be employed to study regions that are smaller in scale than the spatial resolution of the spectrometer.

4. Missions Since the dawn of space flight, nuclear spectroscopy has been used for a wide variety of applications, from astrophysics to solar astronomy. Orbital planetary science missions with gamma ray and/or neutron spectrometers on the payload are listed in Table 1. While nuclear spectroscopy was used on earlier missions to the Moon, Mars, and the surface of Venus, the first major success was the Apollo Gamma Ray Experiment, which flew on the Apollo 15 and 16 missions, providing global context for lunar samples. Phobos II traveled to Mars and provided a glimpse of the regional composition of the western hemisphere, which includes Tharsis and Valles Marineris. Due to the small size of Eros and high orbital altitudes, the gamma ray spectrometer on NEAR provided little useful information about Eros until the NEAR landed on the asteroid. Once on the surface, the NEAR gamma ray spectrometer acquired data with sufficient precision to determine the abundance of O, Mg, Si, Fe, and K. NEAR also had an x-ray spectrometer that provided complementary information about surface elemental composition. The first intended use of neutron spectroscopy for global mapping was on Mars Observer, which was lost before reaching Mars. Lunar Prospector was the first mission to combine gamma ray and neutron spectroscopy to provide accurate, high-precision global composition maps of a planetary body. The missions that followed Lunar Prospector, including 2001 Mars Odyssey and MESSENGER, a mission to the planet Mercury, also included neutron and gamma ray spectrometers on the payload. Dawn, a mission to the main asteroid belt, and Selene, a lunar mission, represent the future of orbital planetary spectroscopy. Both are in preparation for launch in the 2006–2007 timeframe.

5. Science Lunar Prospector and Mars Odyssey acquired highprecision gamma ray and neutron data sets for the Moon and

CLBE001-ESS2E November 8, 2006 2:37

TABLE 1

Summary of orbital planetary science and exploration missions with gamma ray and/or neutron spectrometers. Missions prior to Apollo, including Luna and Ranger, are not listed.

Mission

Country/ Program

Launch date(s)

Status

776

Planet or minor body Orbit

Mapping durationa

Gamma ray spectrometer

Neutron spectrometer

10.5 days (Apollo 15 and 16 combined) 2 orbits analyzed

NaI(Tl) with plastic anticoincidence shield CsI(Tl)

None

Maps of major and radioactive elements, including Fe, Th, and Ti.

None

1 Mars yearb

HPGe, passively cooled

Boron-loaded plastic scintillators

Abundances for O, Si, Fe, K and Th in two equatorial regions in the western hemisphere Global maps major elements and water-equivelent hydrogen (Objectives not achieved) Abundances for O, Mg, Si, Fe, and K

Apollo 15 and 16 U.S.

26-Jul-1971 Completed 16-Apr-1972

Moon

Equatorial orbit covering 20% of the lunar surface

Phobos IIc

U.S.S.R.

12-July-1988

Lost during Phobos encounter

Mars and Phobos

Mars Observer

25-Sep-1992 U.S., NASA Mars Exploration Program

Lost prior to orbital insertion

Mars

Elliptical, equatorial orbit, 900 km periapsis, 80,000 km apoapsis 400 km altitude circular polar mapping orbitb

Near Earth Asteroid Rendezvous (NEAR) Lunar Prospector

U.S., NASA Discovery Program

17-Feb-1996

Completed mission

Eros

U.S., NASA Discovery Program

6-Jan-1998

Completed mission by planned impact in a south polar crater

Moon

Results or Objectivesb

None Useful data acquired 7 days on the NaI(Tl) with surface BGO anticofollowing incidence successful landing shield on Eros 3 BGO with High and low altitude 300 days at Discovery of enhanced He gas boron-loaded high circular polar water-equivalent proportional plastic antialtitude; mapping orbits hydrogen associated with counters and coincidence 220 days at (100 km and 30 polar cold traps; global boron-loaded shield low altitude km, respectively) maps of major and plastic radioactive elements scintillator

Mars

400 km altitude circular polar mapping orbit

MESSENGER

U.S., NASA Discovery Program

3-Aug-2004

Cruise phase

Mercury

Dawn

U.S., NASA Discovery Program

Summer, 2007 Perparing for launche

Vesta and Ceres

Elliptical polar orbit with periapsis at 200 km altitude, 60◦ N latitude, 15,000 km apoapsisb Survey, high, and low 6 months at altitude circular each polar mapping asteroidb orbitsb

Selene

Japan

2007, TBD

Moon

Preparing for launche

Boron-loaded plastic scintillators (NS); Stilbene and 3 He tubes (HENDd )

Distribution of water-equivalent hydrogen and high-latitude stratigraphy; seasonal variations in CO2 ice and noncondensable gasses; and global maps of major and radioactive elements 1 year starting HPGe, actively 6 Li-loaded glass Maps of elemental composition in the in 2011b cooled and northern hemisphere; boron-loaded search for polar hydrogen plastic deposits scintillators

100 km circular polar 1 year mapping orbit

6

Li-loaded glass Global maps of major and radioactive elements and and ice constituents (H and boron-loaded C) plastic scintillators HPGe, actively None Abundance of major and cooled radioactive elements.

CdZnTe and BGO

Refers to the time periods during which gamma ray and/or neutron data were acquired. Objectives are listed for Mars Observer, MESSENGER, Dawn, and Selene. c Neutron and gamma ray spectrometers were flown on Phobos I, which was launched on 7-July-1988; however, Phobos I was lost during the cruise phase of the mission. The Mars 4 and 5 missions (U.S.S.R., 1973) flew identical sodium iodide gamma ray spectrometers. A few gamma ray spectra were acquired by Mars 5 while in an elliptical orbit around Mars (apoapsis 32,560 km, periapsis 1760 km, inclination 35◦ to the equator). d The high energy neutron detector (HEND) was provided by the Russian Federation. e Future missions that have advanced past the planning stage are listed. b

2:37

Completed primary mission; extended mission underway

November 8, 2006

7-Apr-2001 U.S., NASA Mars Exploration Program

CLBE001-ESS2E

777 a

HPGe, Over 2 passively Mars years cooled completed, Extended mission in progress

2001 Mars Odyssey

CLBE001-ESS2E

November 8, 2006

2:37

778 Encyclopedia of the Solar System Mars, respectively. Highlights of the science carried out on these missions are presented along with a description of their instrumentation. The Moon and Mars are very different, both in their origin and composition. With the possible exception of polar water ice, the Moon is bone dry and has no atmosphere. The lunar surface has been extensively modified by cratering and basaltic volcanism. Mars has a tenuous atmosphere, extensive water ice deposits, and seasonal CO2 caps. Volcanic, aqueous, and eolian processes have continued to shape the surface of Mars long past the primordial formation of the crust. The differences between these two bodies will provide the reader with insights into the wealth of information provided by nuclear spectroscopy and the challenges faced in the analysis of the data. For Lunar Prospector, emphasis is placed on the combined analysis of neutron and gamma ray data to determine the abundance of major and trace radioactive elements. For Mars Odyssey, results from the neutron spectrometer for global water abundance and the seasonal caps are presented.

5.1 Lunar Prospector Lunar Prospector was a spin-stabilized spacecraft, with the spin axis perpendicular to the plane of the ecliptic. The instruments were deployed on booms to minimize

backgrounds from the spacecraft (Fig. 8a). The payload included a large-volume BGO gamma ray spectrometer (GRS), which was surrounded by a boron-loaded plastic anticoincidence shield (Fig. 8b). The shield served two purposes: (1) to suppress the Compton continuum caused by gamma rays escaping the BGO crystal and to reject energetic particle events; and (2) to measure the spectrum of fast neutrons from the lunar surface using the double-pulse technique described in Section 3.2. Sn- and Cd-covered 3 He gas proportional counters were used to detect and separately measure thermal and epithermal neutrons. Gamma ray and neutron spectroscopy data were acquired for long periods of time (Table 1), providing full coverage of the Moon at 100- and 30-km altitude. The data were analyzed to determine global maps of surface elemental composition. The resulting abundance data were mapped on different spatial scales, depending on the precision of the data and the altitude of the spectrometer. Results of the analysis were submitted to the NASA Planetary Data System and include the following data sets: the abundance of hydrogen from neutron spectroscopy (0.5◦ equal angle map); the average atomic mass from fast neutron spectroscopy (2◦ equal area maps); the abundance of major oxides, including MgO, Al2 O3 , SiO2 , CaO, TiO2 , and FeO, and trace incompatible elements K, Th, and U

FIGURE 8 (a) Annotated artist’s conception of Lunar Prospector; (b) Cross sectional view of the gamma ray and fast neutron spectrometer; (c) annotated artist’s conception of 2001 Mars Odyssey; (d) engineering drawing of the Neutron Spectrometer on Odyssey cut-away to show the boron loaded plastic scintillators. A schematic diagram of the arrangement of scintillators and their orientation relative to spacecraft motion and nadir is also shown. (Parts a and c courtesy of NASA.)

CLBE001-ESS2E

November 8, 2006

2:37

Remote Chemical Sensing Using Nuclear Spectroscopy

779

FIGURE 9 Lunar Prospector gamma ray spectrum for a 20◦ equal-area pixel in the western mare is compared to the fitted spectrum and elemental spectral components (see text for details).

(2◦ -, 5◦ -, and 20◦ -equal area maps) using a combination of gamma ray and neutron spectroscopy; and the abundance of the rare-earth elements (Gd + Sm) from neutron spectroscopy (0.5◦ equal angle map). Perhaps the most significant result of Lunar Prospector was the discovery of enhanced hydrogen at the poles in association with craters in permanent shadow, which are thought to be cold traps for water ice. If present, water ice could be an important resource for manned exploration. Consequently, the polar cold traps are a prime target for future missions. Geochemical results from the analysis of neutron and gamma ray spectra fully reveal the dichotomy in the composition of the Moon, with a near side that is enriched in incompatible elements and mafic minerals and a thick far-side crust primarily consisting of plagioclase feldspar. Global geochemical trends observed by Lunar Prospector are not significantly different from trends observed in the sample and meteoritic data; however, there are some notable discrepancies that point to the existence of unique lithologies that are not well represented by the lunar sample data. Interpretation and analysis of the data is ongoing with emphasis on regional studies. For example, the impact that formed the South Pole Aitken (SPA) basin could have excavated into the mantle. Analysis of the composition of

the basin floor may reveal information about the bulk composition of the mantle and lower crust. The analysis of major and radioactive elements was carried out using a combination of gamma ray and neutron spectroscopy data. A typical gamma ray spectrum is shown in Fig. 9 for a 20◦ equal-area pixel in the western mare. Two intense, well-resolved peaks, labeled in Fig. 9, were analyzed to determine the abundance of Fe and Th. In addition, a spectral unmixing algorithm similar to those used to analyze spectral reflectance data was developed to simultaneously determine the abundance of all major and radioactive elements from the gamma ray spectrum. Lunar gamma ray spectra can be modeled as a linear mixture of elemental spectral shapes. The magnitude of the spectral components must be adjusted to account for the nonlinear coupling of gamma ray production to the neutron number density (for neutron capture reactions) and the flux of fast neutrons (for nonelastic reactions). Once the adjustment is made, a linear least squares problem can be solved to determine elemental weight fractions. Fitted elemental spectral shapes are shown, for example, in Fig. 9. Abundance maps for selected elements determined by Lunar Prospector are shown in Fig. 10. To provide context for the elemental abundance maps, a map of topography determined by Clementine, superimposed on a shaded

CLBE001-ESS2E

November 8, 2006

2:37

780 Encyclopedia of the Solar System

FIGURE 10 Orthographic projections of the lunar near and far sides: (a) Elevation; (b–d) abundance (weight fraction) of selected elements. The map data are superimposed on a shaded relief image. (FeO data courtesy NASA Planetary Data System ; image courtesy United States Geological Survey.)

relief image, is shown in Fig. 10a. The far side includes the feldspathic highlands and the SPA basin. The near side consists of major basins, including Procellarum and Imbrium, which contain mare basalts. The mare basalts are rich in Fe, with the highest concentrations occurring in

western Procellarum (Fig. 10b). The low abundance of FeO in the highlands, which are rich in plagioclase feldspar, reflects a significant lunar geochemical trend in which mafic silicate minerals are displaced by plagioclase, which is Fe-poor.

CLBE001-ESS2E

November 8, 2006

2:37

Remote Chemical Sensing Using Nuclear Spectroscopy

FIGURE 10

781

(Continued )

A large portion of the western near side is enriched in radioactive elements such as Th (Fig. 10c). K, Th, and U are incompatible with major lunar minerals and were likely concentrated in the residual melt during lunar differentiation. Consequently, their distribution on the surface and with depth has important implications to lunar evolution. The association of high Th concentrations with the mare suggests that heating by radioactive elements may have significantly influenced lunar thermal evolution and mare volcanism.

The distribution of TiO2 is shown in Fig. 10d as a 5◦ equal area map. The low spatial resolution of the TiO2 map compared to FeO and Th is a consequence of the relatively low intensity of the Ti gamma rays and their position in the gamma ray spectrum near strong peaks from O and Fe (Fig. 9). The abundance of TiO2 can be used to classify mare basalts. Strong spatial variations in the abundance of TiO2 , for example, indicate that different source regions and processes were involved in creating the basalts

CLBE001-ESS2E

November 8, 2006

2:37

782 Encyclopedia of the Solar System that comprise the mare. The highest concentrations of TiO2 are found in Tranquillitatis as shown in Fig. 10d; however, high concentrations are also found in western Procellarum. The abundances of Fe and Ti observed in western Procellarum suggest that this region has a unique composition that is not well represented by the lunar samples.

5.2 Mars Odyssey As of this writing, 2001 Mars Odyssey is in an extended mission having successfully completed over two Mars years of mapping (each Mars year is 687 days). Odyssey is in a circular polar mapping orbit around Mars at an altitude of approximately 400 km (Table 1). The nuclear spectroscopy payload consists of a GRS, a neutron spectrometer (NS), and a Russian-supplied high energy neutron detector (HEND). Gamma ray and neutron spectroscopy data acquired by Mars Odyssey provide constraints on geochemistry, the water cycle, climate history, and atmospheric processes, including atmospheric dynamics and atmosphere-surface interactions [see Mars Atmosphere: History and Surface Interaction]. Since the discovery of abundant subsurface waterequivalent hydrogen (WEH) at high latitudes, Odyssey’s gamma ray and neutron spectrometers have continued to provide a wealth of new information about Mars, including the global distribution of near-surface WEH, the elemental composition of the surface, seasonal variations in the composition of the atmosphere at high latitudes, and the column abundance of CO2 ice in the seasonal caps. This information has contributed to our understanding of the recent history of Mars: The climate is driven strongly by short-term variations in orbital parameters, principally the obliquity, and the surface distribution of surface water-ice is controlled by atmosphere-surface interactions. The discovery of anomalously large amounts of WEH at low latitudes, where water ice is not stable, has stirred considerable debate about the mineral composition of the surface and climate change. The GRS on Odyssey is boom-mounted, passively cooled, HPGe spectrometer, similar in design to the instrument flown on Mars Observer (Fig. 8c). The NS is a deck-mounted instrument that consists of a boron-loaded plastic block (roughly 10 cm on a side), which has been diagonally segmented into four prisms and read out by separate photomultiplier tubes (Fig. 8d). The orientation of the spacecraft is constant such that one of the prisms faces nadir (P1), one faces zenith (P3), one faces in the direction of spacecraft motion (P2), and one faces opposite the spacecraft motion (P4). P1 is covered with a Cd foil that prevents thermal neutrons from entering the prism. Consequently, P1 is sensitive to epithermal and fast neutrons originating from the surface and atmosphere. Neutrons with energy less than the gravitational binding energy of Mars, approximately 0.13 eV, corresponding to an escape speed of about 5000 m/s, travel on parabolic trajecto-

ries and return to Mars unless they decay by beta emission. The mean lifetime of a neutron is approximately 900 s. The most probable energy for neutrons in thermal equilibrium with the surface of Mars (for the mean martian temperature of 210 K) is 0.018 eV, which corresponds to a neutron speed of 1860 m/s. Consequently, a significant portion of the thermal neutron population travels on ballistic trajectories and are incident on the spectrometer from above and below. Neutrons that leave the atmosphere with energies less than about 0.014 eV, just below the most probable energy, cannot reach the 400 km orbital altitude of Odyssey. Consequently, gravitational binding has a significant effect on the flux and energy distribution of neutrons at Odyssey’s orbital altitude, and, in contrast to the simplified discussion in Section 3.1, gravitational effects must be accounted for in models of the flux and instrument response. To separate thermal and epithermal neutrons, the NS makes use of the orbital speed of the spacecraft, which is approximately 3400 m/s, the same speed as a 0.05 eV neutron. Neutrons below the speed of the spacecraft (most of the thermal neutron population) can’t catch up to P4. So, P4 is primarily sensitive to epithermal neutrons. In contrast, P2 “rams” into thermal neutrons that arrive at the orbital altitude ahead of the spacecraft. P2 has roughly the same sensitivity as P4 for epithermal neutrons. Consequently, the thermal flux is given by the difference between the counting rates for P2 and P4. Thermal, epithermal, and fast neutrons are sensitive to surface and atmospheric parameters, including the abundance and stratigraphy of hydrogen in the surface, the presence of strong neutron absorbers such as Cl and Fe in the Martian rocks and soil, the presence of CO2 ice on the surface, the column abundance of the atmosphere, and the enrichment and depletion of noncondensable gasses, N2 and Ar, as CO2 is cycled through the seasonal caps (Table 2). The effect of these parameters on the neutron counting rate can be explored using a simple physical model of the surface and atmosphere as described in Section 2.2 (Fig. 4a). Models of the counting rate are then used to develop algorithms to determine parameters from observations. For example, the variation of thermal, epithermal, and fast neutron counting rates as a function of water abundance in a homogeneous surface is shown in Fig. 11a. Epithermal and fast neutrons are sensitive to hydrogen (as described in Section 2.2) and their counting rates decrease monotonically with water abundance. Both are insensitive to the abundance of elements in the surface other than hydrogen, as illustrated in Fig. 11a by changing the abundance of Cl, which is a strong thermal neutron absorber. In contrast, thermal neutrons are sensitive to variations in major-element composition and relatively insensitive to hydrogen when the abundance of WEH is less than about 10%. Epithermal neutrons are a good choice for determining the WEH abundance because of their high counting rate and relative insensitivity to other parameters. Measured

CLBE001-ESS2E

November 8, 2006

2:37

Remote Chemical Sensing Using Nuclear Spectroscopy

TABLE 2

783

Sensitivity of neutron energy ranges to Mars surface and atmospheric parameters Major interactions

CO2 -free surface parameters∗

Atmospheric/seasonal parameters

>0.2 MeV

Inelastic scattering, elastic scattering

Epithermal

0.5 eV (Cd cutoff) to 0.2 MeV

Elastic scattering

WEH abundance and stratigraphy, Average atomic mass WEH abundance and stratigraphy

Thermal

1 and p = q(1 + e), where q is again the pericentric separation. For all orbits, the three orientation angles i, , and ω are defined as in the elliptical case.

3. Planetary Perturbations and the Orbits of Small Bodies Gravity is not restricted to interactions between the Sun and the planets or individual planets and their satellites, but rather all bodies feel the gravitational force of one another. Within the solar system, one body typically produces the dominant force on any given body, and the resultant motion can be thought of as a Keplerian orbit about a primary, subject to small perturbations by other bodies. In this section some important examples of the effects of these perturbations on the orbital motion are considered. Classically, much of the discussion of the evolution of orbits in the solar system used perturbation theory as its foundation. Essentially, the method involves writing the equations of motion as the sum of a part that describes the independent Keplerian motion of the bodies about the Sun plus a part (called the disturbing function) that contains terms due to the pairwise interactions among the planets and minor bodies and the indirect terms associated with the back-reaction of the planets on the Sun. In general, one can then expand the disturbing function in terms of the small parameters of the problem (such as the ratio of the planetary masses to the solar mass, the eccentricities and inclinations, etc.), as well as the other orbital elements of the

CLBE001-ESS2E

November 11, 2006

22:43

Solar System Dynamics: Regular and Chaotic Motion

bodies, including the mean longitudes (i.e., the location of the bodies in their orbits), and attempt to solve the resulting equations for the time-dependence of the orbital elements.

3.1 Perturbed Keplerian Motion and Resonances Although perturbations on a body’s orbit are often small, they cannot always be ignored. They must be included in short-term calculations if high accuracy is required, for example, for predicting stellar occultations or targeting spacecraft. Most long-term perturbations are periodic in nature, their directions oscillating with the relative longitudes of the bodies or with some more complicated function of the bodies’ orbital elements. Small perturbations can produce large effects if the forcing frequency is commensurate or nearly commensurate with the natural frequency of oscillation of the responding elements. Under such circumstances, perturbations add coherently, and the effects of many small tugs can build up over time to create a large-amplitude, long-period response. This is an example of resonance forcing, which occurs in a wide range of physical systems. An elementary example of resonance forcing is given by the simple one-dimensional harmonic oscillator, for which the equation of motion is m

d2 x + m 2 x = Fo cos ϕt. dt 2

(19)

In Eq. (19), m is the mass of the oscillating particle, Fo is the amplitude of the driving force,  is the natural frequency of the oscillator, and ϕ is the forcing or resonance frequency. The solution to Eq. (19) is x = x o cos ϕt + Acos t + Bsint,

(20a)

791

Nearly exact orbital commensurabilities exist at many places in the solar system. Io orbits Jupiter twice as frequently as Europa does, which in turn orbits Jupiter twice as frequently as Ganymede does. Conjunctions (at which the bodies have the same longitude) always occur at the same position of Io’s orbit (its perijove). How can such commensurabilities exist? After all, the probability of randomly picking a rational from the real number line is 0, and the number of small integer ratios is infinitely smaller still! The answer lies in the fact that orbital resonances may be held in place as stable locks, which result from nonlinear effects not represented in the foregoing simple mathematical example. For example, differential tidal recession (see Section 7.5) brings moons into resonance, and nonlinear interactions among the moons can keep them there. Other examples of resonance locks include the Hilda asteroids, the Trojan asteroids, Neptune–Pluto, and the pairs of moons about Saturn, Mimas–Tethys and Enceladus– Dione. Resonant perturbation can also force material into highly eccentric orbits that may lead to collisions with other bodies; this is believed to be the dominant mechanism for clearing the Kirkwood gaps in the asteroid belt (see Section 5.1). Spiral density waves can result from resonant perturbations of a self-gravitating particle disk by an orbiting satellite. Density waves are seen at many resonances in Saturn’s rings; they explain most of the structure seen in Saturn’s A ring. The vertical analog of density waves, bending waves, are caused by resonant perturbations perpendicular to the ring plane due to a satellite in an orbit that is inclined to the ring. Spiral bending waves excited by the moons Mimas and Titan have been seen in Saturn’s rings. In the next few subsections these manifestations of resonance effects that do not explicitly involve chaos are discussed. Chaotic motion produced by resonant forcing is discussed later in the chapter.

where xo ≡

Fo , m( 2 − ϕ 2 )

(20b)

and A and B are constants determined by the initial conditions. Note that if ϕ ≈ , a large-amplitude, long-period response can occur even if Fo is small. Moreover, if ϕ = , this solution to Eq. (19) is invalid. In this case the solution is given by x=

Fo tsin t + Acos t + Bsin t. 2m

(21)

The t in front of the first term at the right-hand side of Eq. (21) leads to secular growth. Often this linear growth is moderated by the effects of nonlinear terms that are not included in the simple example provided here. However, some perturbations have a secular component.

3.2 Examples of Resonances: Lagrangian Points, and Tadpole and Horseshoe Orbits Many features of the orbits considered in this section can be understood by examining an idealized system in which two massive (but typically of unequal mass) bodies move on circular orbits about their common center of mass. If a third body is introduced that is much less massive than either of the first two, its motion can be followed by assuming that its gravitational force has no effect on the orbits of the other bodies. By considering the motion in a frame co-rotating with the massive pair (so that the pair remain fixed on a line that can be taken to be the x axis), Lagrange found that there are five points where particles placed at rest would feel no net force in the rotating frame. Three of the so-called Lagrange points (L1 , L2 , and L3 ) lie along a line joining the two masses m1 and m2 . The other two Lagrange points

CLBE001-ESS2E

November 11, 2006

22:43

792 Encyclopedia of the Solar System (L4 and L5 ) form equilateral triangles with the two massive bodies. Particles displaced slightly from the first three Lagrangian points will continue to move away and hence these locations are unstable. The triangular Lagrangian points are potential energy maxima, which are stable for sufficiently large primary to secondary mass ratio due to the Coriolis force. Provided that the most massive body has at least 27 times the mass of the secondary (which is the case for all known examples in the solar system larger than the Pluto– Charon system), the Lagrangian points L4 and L5 are stable points. Thus, a particle at L4 or L5 that is perturbed slightly will start to “orbit” these points in the rotating coordinate system. Lagrangian points L4 and L5 are important in the solar system. For example, the Trojan asteroids in Jupiter’s Lagrangian points and both Neptune and Mars confine their own Trojans. There are also small moons in the triangular Lagrangian points of Tethys and Dione, in the Saturnian system. The L4 and L5 points in the Earth–Moon system have been suggested as possible locations for space stations.

3.2.1 HORSESHOE AND TADPOLE ORBITS

Consider a moon on a circular orbit about a planet. Figure 3 shows some important dynamical features in the frame corotating with the moon. All five Lagrangian points are indicated in the picture. A particle just interior to the moon’s orbit has a higher angular velocity than the moon in the stationary frame, and thus moves with respect to the moon in the direction of corotation. A particle just outside the moon’s orbit has a smaller angular velocity, and moves away from the moon in the opposite direction. When the outer particle approaches the moon, the particle is slowed down (loses angular momentum) and, provided the initial difference in semimajor axis is not too large, the particle drops to an orbit lower than that of the moon. The particle then recedes in the forward direction. Similarly, the particle at the lower orbit is accelerated as it catches up with the moon, resulting in an outward motion toward the higher, slower orbit. Orbits like these encircle the L3 , L4 , and L5 points and are called horseshoe orbits. Saturn’s small moons Janus and Epimetheus execute just such a dance, changing orbits every 4 years. Since the Lagrangian points L4 and L5 are stable, material can librate about these points individually: such orbits are called tadpole orbits. The tadpole libration width at L4 and L5 is roughly equal to (m/M)1/2r, and the horseshoe width is (m/M)1/3r, where M is the mass of the planet, m the mass of the satellite, and r the distance between the two objects. For a planet of Saturn’s mass, M = 5.7 × 1029 g, and a typical small moon of mass m = 1020 g (e.g., an object with a 30-km radius, with density of ∼1 g/cm3 ), at a distance of 2.5 Saturnian radii, the tadpole libration half-width is about 3 km and the horseshoe half-width about 60 km.

FIGURE 3 Diagram showing the five Lagrangian equilibrium points (denoted by crosses) and three representative orbits near these points for the circular restricted three-body problem. In this example, the secondary’s mass is 0.001 times the total mass. The coordinate frame has its origin at the barycenter and corotates with the pair of bodies, thereby keeping the primary (large solid circle) and secondary (small solid circle) fixed on the x axis. Tadpole orbits remain near one or the other of the L4 and L5 points. An example is shown near the L4 point on the diagram. Horseshoe orbits enclose all three of L3 , L4 , and L5 but do not reach L1 or L2 . The outermost orbit on the diagram illustrates this behavior. There is a critical curve dividing tadpole and horseshoe orbits that encloses L4 and L5 and passes through L3 . A horseshoe orbit near this dividing line is shown as the dashed curve in the diagram.

3.2.2 HILL SPHERE

The approximate limit to a planet’s gravitational dominance is given by the extent of its Hill sphere, 

m RH = 3(M + m)

1/3 a,

(22)

where m is the mass of the planet and M is the Sun’s mass. A test body located at the boundary of a planet’s Hill sphere is subjected to a gravitational force from the planet comparable to the tidal difference between the force of the Sun on the planet and that on the test body. The Hill sphere essentially stretches out to the L1 point and is roughly the limit of the Roche lobe (maximum extent of an object held together by gravity alone) of a body with m  M. Planetocentric orbits that are stable over long periods of time are those well within the boundary of a planet’s Hill sphere; all known natural satellites lie in this region. The trajectories

CLBE001-ESS2E

November 11, 2006

22:43

Solar System Dynamics: Regular and Chaotic Motion

VIII Pasiphae

z (Rj)

400 200 0

600

−200 −400 300

400 200 200

100

793

FIGURE 4 The orbit of J VIII Pasiphae, a distant retrograde satellite of Jupiter, is shown as seen in a nonrotating coordinate system with Jupiter at the origin (open circle). The satellite was integrated as a massless test particle in the context of the circular restricted three-body problem for approximately 38 years. The unit of distance is Jupiter’s radius, RJ . During the course of this integration, the distance to Jupiter varied from 122 to 548RJ . Note how the large solar perturbations produce significant deviations from a Keplerian orbit. [Figure reprinted with permission from Jose Alvarellos (1996). “Orbital Stability of Distant Satellites of Jovian Planets,” M.Sc. thesis, San Jose State University.]

0 0

−100

−200

y (Rj)

−200 −300

−400

−400

of the outermost planetary satellites, which lie closest to the boundary of the Hill sphere, show large variations in planetocentric orbital paths (Fig. 4). Stable heliocentric orbits are those that are always well outside the Hill sphere of any planet.

3.3 Examples of Resonances: Ring Particles and Shepherding In the discussions in Section 2, the gravitational force produced by a spherically symmetric body was described. In this section the effects of deviations from spherical symmetry must be included when computing the force. This is most conveniently done by introducing the gravitational potential (r), which is defined such that the acceleration d 2 r/dt2 of a particle in the gravitational field is 2

2

d r/dt = ∇.

(23)

In empty space, the Newtonian gravitational potential (r) always satisfies Laplace’s equation ∇ 2  = 0.

(24)

Most planets are very nearly axisymmetric, with the major departure from sphericity being due to a rotationally induced equatorial bulge. Thus, the gravitational potential can be expanded in terms of Legendre polynomials instead of the complete spherical harmonic expansion, which would be required for the potential of a body of arbitrary shape:   ∞  Gm (r, φ, θ) = − J n Pn (cosθ )(R/r)n . 1− r n=2

(25)

x (Rj)

This equation uses standard spherical coordinates, so that θ is the angle between the planet’s symmetry axis and the vector to the particle. The terms Pn (cos θ) are the Legendre polynomials, and J n are the gravitational moments determined by the planet’s mass distribution. If the planet’s mass distribution is symmetrical about the planet’s equator, the J n are zero for odd n. For large bodies, J 2 is generally substantially larger than the other gravitational moments. Consider a particle in Saturn’s rings, which revolves around the planet on a circular orbit in the equatorial plane (θ = 90◦ ) at a distance r from the center of the planet. The centripetal force must be provided by the radial component of the planet’s gravitational force [see Eq. (9)], so the particle’s angular velocity n satisfies 

∂ rn (r) = ∂r



2

θ=90◦

.

(26)

If the particle suffers an infinitesimal displacement from its circular orbit, it will oscillate freely in the horizontal and vertical directions about the reference circular orbit with radial (epicyclic) frequency κ(r) and vertical frequency μ(r), respectively, given by κ 2 (r) = r −3  μ2 (r) =

d [(r 2 n)2 ], dr ∂ 2 ∂z2

(27)

 .

(28)

z=0

From Eqs. (24)–(28), the following relation is found between the three frequencies for a particle in the equatorial

CLBE001-ESS2E

November 11, 2006

22:43

794 Encyclopedia of the Solar System plane: μ2 = 2n2 − κ 2 .

(29)

For a perfectly spherically symmetric planet, μ = κ = n. Since Saturn and the other ringed planets are oblate, μ is slightly larger and κ is slightly smaller than the orbital frequency n. Using Eqs. (24)–(29), one can show that the orbital and epicyclic frequencies can be written as

n

κ

2

2

  2  4 R R 3 GM 15 = 3 1 + J2 − J4 r 2 r 8 r   6 R 35 + J6 + ··· , 16 r   2  4 3 R R GM 45 J4 = 3 1 − J2 + r 2 r 8 r   6 R 175 J6 − + ··· , 16 r

(30)

(31)

(32)

Thus, the oblateness of a planet causes apsides of particle orbits in and near the equatorial plane to precess in the direction of the orbit and lines of nodes of nearly equatorial orbits to regress. Resonances occur where the radial (or vertical) frequency of the ring particles is equal to the frequency of a component of a satellite’s horizontal (or vertical) forcing, as sensed in the rotating frame of the particle. In this case the resonating particle is always near the same phase in its radial (vertical) oscillation when it experiences a particular phase of the satellite’s forcing. This situation enables continued coherent “kicks” from the satellite to build up the particle’s radial (vertical) motion, and significant forced oscillations may thus result. The location and strengths of resonances with any given moon can be determined by decomposing the gravitational potential of the moon into its Fourier components. The disturbance frequency, ω, can be written as the sum of integer multiples of the satellite’s angular, vertical, and radial frequencies: ω = j ns + kμs + κs ,

ω − j n(rL ) = ±κ(rL ).

(33)

(34)

It will undergo vertical resonance if its radial position rv , satisfies ω − j n(rL ) = ±μ(rv ).



 2  4 9 GM 75 R R 2 μ = 3 1 + J2 − J4 r 2 r 8 r   6 245 R + + ··· . J6 16 r

where the azimuthal symmetry number, j , is a nonnegative integer, and k and are integers, with k being even for horizontal forcing and odd for vertical forcing. The subscript s refers to the satellite. A particle placed at distance r = rL will undergo horizontal (Lindblad) resonance if rL satisfies

(35)

When Eq. (34) is valid for the lower (upper) sign, rL is referred to as the inner (outer) Lindblad or horizontal resonance. The distance rv is called an inner (outer) vertical resonance if Eq. (35) is valid for the lower (upper) sign. Since all of Saturn’s large satellites orbit the planet well outside the main ring system, the satellite’s angular frequency ns is less than the angular frequency of the particle, and inner resonances are more important than outer ones. When m = 1, the approximation μ ≈ n ≈ κ may be used to obtain the ratio n(rL,v ) j +k+ = . ns j −1

(36)

The notation ( j + k + )/( j − 1) or ( j + k + ):( j − 1) is commonly used to identify a given resonance. The strength of the forcing by the satellite depends, to lowest order, on the satellite’s eccentricity, e, and inclination, i, as e | | [sin i]|k| . The strongest horizontal resonances have k = = 0, and are of the form j :( j − 1). The strongest vertical resonances have k = 1, = 0, and are of the form ( j + 1):( j − 1). The location and strengths of such orbital resonances can be calculated from known satellite masses and orbital parameters and Saturn’s gravity field. Most strong resonances in the Saturnian system lie in the outer A ring near the orbits of the moons responsible for them. If n = μ = κ, the locations of the horizontal and vertical resonances would consider: rL = rv . Since, owing to Saturn’s oblateness, μ > n > κ, the positions rL and rv do not coincide: rv < rL . A detailed discussion of spiral density waves, spiral bending waves, and gaps at resonances produced by moons is presented elsewhere in this encyclopedia. [See PLANETARY RINGS.]

4. Chaotic Motion 4.1 Concepts of Chaos In the nineteenth century, Henri Poincare´ studied the mathematics of the circular restricted three-body problem. In this problem, one mass (the secondary) moves in a fixed, circular orbit about a central mass (the primary), while

CLBE001-ESS2E

November 11, 2006

22:43

Solar System Dynamics: Regular and Chaotic Motion

a test massless particle moves under the gravitational effect of both masses but does not perturb their orbits. From this work, Poincare´ realized that despite the simplicity of the equations of motion, some solutions to the problem exhibit complicated behavior. Poincare’s ´ work in celestial mechanics provided the framework for the modern theory of nonlinear dynamics and ultimately led to a deeper understanding of the phenomenon of chaos, whereby dynamical systems described by simple equations can give rise to unpredictable behavior. The whole question of whether or not a given system is stable to sufficiently small perturbations is the basis of the Kolmogorov-Arnol’d-Moser (KAM) theory, which has its origins in the work of Poincare. ´ One characteristic of chaotic motion is that small changes in the starting conditions can produce vastly different final outcomes. Since all measurements of positions and velocities of objects in the solar system have finite accuracy, relatively small uncertainties in the initial state of the system can lead to large errors in the final state, for initial conditions that lie in chaotic regions in phase space. This is an example of what has become known as the “butterfly effect,” first mentioned in the context of chaotic weather systems. It has been suggested that under the right conditions, a small atmospheric disturbance (such as the flapping of a butterfly’s wings) in one part of the world could ultimately lead to a hurricane in another part of the world. The changes in an orbit that reveal it to be chaotic may occur very rapidly, for example during a close approach to the planet, or may take place over millions or even billions of years. Although there have been a number of significant mathematical advances in the study of nonlinear dynamics since Poincare’s ´ time, the digital computer has proven to be the most important tool in investigating chaotic motion in the solar system. This is particularly true in studies of the gravitational interaction of all the planets, where there are few analytical results.

4.2 The Three-body Problem as a Paradigm The characteristics of chaotic motion are common to a wide variety of dynamical systems. In the context of the solar system, the general properties are best described by considering the planar circular restricted three-body problem, consisting of a massless test particle and two bodies of masses m1 and m2 moving in circular orbits about their common center of mass at constant separation with all bodies moving in the same plane. The test particle is attracted to each mass under the influence of the inverse square law of force given in Eq. (5). In Eq. (16), a is the constant separation of the two masses, and n = 2π /p is their constant angular velocity about the center of mass. Using x and y as components of the position vector of the test particle referred to the center of mass of the system (Fig. 5), the equations of motion of the particle in a reference frame

y

795

P

r1

r2 r

m1

m2 O

x

FIGURE 5 The rotating coordinate system used in the circular restricted three-body problem. The masses are at a fixed distance from one another and this is taken to be the unit of length. The position and velocity vectors of the test particle (at point P ) are referred to the center of mass of the system at O.

rotating at angular velocity n are  x + μ2 x − μ1 x¨ − 2n y˙ − n2 x = −G m1 , − m 2 r13 r23  y¨ + 2n x˙ − n2 y = −G

m1 m2 + 3 3 r1 r2

(37)

y,

(38)

where μ1 = m1 a/(m1 + m2 ), and μ2 = m2 a/(m1 + m2 ) are constants and r12 = (x + μ2 )2 + y2 ,

(39)

r22 = (x − μ1 )2 + y2 ,

(40)

where r1 and r2 are the distances of the test particle from the masses m1 and m2 , respectively. These two second-order, coupled, nonlinear differential equations can be solved numerically provided the initial position (x 0 , y0 ) and velocity (x˙ 0 , y˙ 0 ) of the particle are known. Therefore the system is deterministic and at any given time the orbital elements of the particle (such as its semimajor axis and eccentricity) can be calculated from its initial position and velocity. The test particle is constrained by the existence of a constant of the motion called the Jacobi constant, C, given by  C = n2 (x 2 + y2 ) + 2G

m1 m2 + r1 r2

− x˙ 2 − y˙ 2 .

(41)

The values of (x 0 , y0 ) and (x˙ 0 , y˙ 0 ) fix the value of C for the system, and this value is preserved for all subsequent motion. At any instant the particle is at some position on the two-dimensional (x , y) plane. However, since the actual orbit is also determined by the components of the velocity (x˙ , y˙ ), the particle can also be thought of as being at a particular position in a four-dimensional (x , y, x˙ , y˙ ) phase space. Note that the use of four dimensions rather than the customary two is simply a means of representing the position and the velocity of the particle at a particular instant in time;

CLBE001-ESS2E

November 11, 2006

22:43

796 Encyclopedia of the Solar System the particle’s motion is always restricted to the x − y plane. The existence of the Jacobi constant implies that the particle is not free to wander over the entire 4-D phase space, but rather that its motion is restricted to the 3-D “surface” defined by Eq. (41). This has an important consequence for studying the evolution of orbits in the problem. The usual method is to solve the equations of motion, convert x , y, x˙ , and y˙ into orbital elements such as semimajor axis, eccentricity, longitude of periapse, and mean longitude, and then plot the variation of these quantities as a function of time. However, another method is to produce a surface of section, also called a Poincare´ map. This makes use of the fact that the orbit is always subject to Eq. (41), where C is determined by the initial position and velocity. Therefore if any three of the four quantities x , y, x˙ , and y˙ are known, the fourth can always be determined by solving Eq. (41). One common surface of section that can be obtained for the planar circular restricted three-body problem is a plot of values of x and x˙ whenever y = 0 and y˙ is positive. The actual value of y˙ can always be determined uniquely from Eq. (41), and so the two-dimensional (x , x˙ ) plot implicitly contains all the information about the particle’s location in the four-dimensional phase space. Although surfaces of section make it more difficult to study the evolution of the orbital elements, they have the advantage of revealing the characteristic motion of the particle (regular or chaotic) and a number of orbits can be displayed on the same diagram. As an illustration of the different types of orbits that can arise, the results of integrating a number of orbits using a mass m2 /(ml + m2 ) = 10−3 and Jacobi constant C = 3.07 are described next. In each case, the particle was started with the initial longitude of periapse 0 = 0 and initial mean longitude λ0 = 0. This corresponds to x˙ = 0 and y = 0. Since the chosen mass ratio is comparable to that of the Sun-Jupiter system, and Jupiter’s eccentricity is small, this

will be used as a good approximation to the motion of fictitious asteroids moving around the Sun under the effect of gravitational perturbations from Jupiter. The asteroid is assumed to be moving in the same plane as Jupiter’s orbit. 4.2.1 REGULAR ORBITS

The first asteroid has starting values x = 0.55, y = 0, x˙ = 0, with y˙ = 0.9290 determined from the solution of Eq. (41). Here a set of dimensionless coordinates are used in which n = 1, G = 1, and m1 + m2 = 1. In these units, the orbit of m2 is a circle at distance a = 1 with uniform speed v = 1. The corresponding initial values of the heliocentric semimajor axis and eccentricity are a0 = 0.6944 and e0 = 0.2065. Since the semimajor axis of Jupiter’s orbit is 5.202 AU, this value of a0 would correspond to an asteroid at 3.612 AU. Figure 6 shows the evolution of e as a function of time. The plot shows a regular behavior with the eccentricity varying from 0.206 to 0.248 over the course of the integration. In fact, an asteroid at this location would be close to an orbit–orbit resonance with Jupiter, where the ratio of the orbital period of the asteroid, T , to Jupiter’s period, TJ , is close to a rational number. From Kepler’s third law of planetary motion, T 2 ∝ a 3 . In this case, T /TJ = (a/aJ )3/2 = 0.564 ≈ 4/7 and the asteroid orbit is close to a 7:4 resonance with Jupiter. Figure 7 shows the variation of the semimajor axis of the asteroid, a, over the same time interval as shown in Fig. 6. Although the changes in a are correlated with those in e, they are smaller in amplitude and a appears to oscillate about the location of the exact resonance at a = (4/7)2/3 ≈ 0.689. An asteroid in resonance experiences enhanced gravitational perturbations from Jupiter, which can cause regular variations in its orbital elements. The extent of these variations depends on the asteroid’s location within the resonance, which is, in turn, is determined by the starting conditions.

FIGURE 6 The eccentricity as a function of time for an object moving in a regular orbit near the 7:4 resonance with Jupiter. The plot was obtained by solving the circular restricted three-body problem numerically using initial values of 0.6944 and 0.2065 for the semimajor axis and eccentricity, respectively. The corresponding position and velocity in the rotating frame were x 0 = 0.55, y0 = 0, x˙ = 0, and y˙ = 0.9290.

0.25

Eccentricity

0.24 0.23 0.22 0.21 0.2 0

50

100

150

200

Time (Jupiter periods)

250

300

CLBE001-ESS2E

November 11, 2006

22:43

Solar System Dynamics: Regular and Chaotic Motion

FIGURE 7 The semimajor axis as a function of time for an object using the same starting conditions as in Fig. 6. The units of the semimajor axis are such that Jupiter’s semimajor axis (5.202 AU) is taken to be unity.

0.695 Semimajor axis

797

0.69 0.685 0.68 0.675 50

100

150

200

250

300

Time (Jupiter periods)

The equations of motion can be integrated with the same starting conditions to generate a surface of section by plotting the values of x and x˙ whenever y = 0 with y˙ > 0 (Fig. 8). The pattern of three distorted curves or “islands” that emerges is a characteristic of resonant motion when displayed in such plots. If a resonance is of the form ( p + q): p, where p and q are integers, then q is said to be the order of the resonance. The number of islands seen in a surface of section plot of a given resonant trajectory is equal to q. In this case, p = 4, q = 3 and three islands are visible. The center of each island would correspond to a starting condition that placed the asteroid at exact resonance where the variation in e and a would be minimal. Such points are said to be fixed points of the Poincare´ map. If the starting location was moved farther away from the center, the subsequent variations in e and a would get larger, until eventually some starting values would lead to trajectories that were not in resonant motion. 0.4

0.2

. x

0

-0.2

-0.4 0.5

0.6

0.7

0.8

0.9

x

FIGURE 8 A surface of section plot for the same (regular) orbit shown in Figs. 6 and 7. The 2000 points were generated by plotting the values of x and x˙ whenever y = 0 with positive y˙ . The three “islands” in the plot are due to the third-order 7:4 resonance.

4.2.2 CHAOTIC ORBITS

Figures 9 and 10 show the plots of e and a as a function of time for an asteroid orbit with starting values x 0 = 0.56, y0 = 0, x˙ 0 = 0, and y˙ determined from Eq. (41) with C = 3.07. The corresponding orbital elements are a0 = 0.6984 and e0 = 0.1967. These values are only slightly different from those used earlier, indeed the initial behavior of the plots is quite similar to that seen in Figs. 6 and 7. However, subsequent variations in e and a are strikingly different. The eccentricity varies from 0.188 to 0.328 in an irregular manner, and the value of a is not always close to the value associated with exact resonance. This is an example of a chaotic trajectory where the variations in the orbital elements have no obvious periodic or quasi-periodic structure. The anticorrelation of a and e can be explained in terms of the Jacobi constant. The identification of this orbit as chaotic becomes apparent from a study of its surface of section (Fig. 11). Clearly, this orbit covers a much larger region of phase space than the previous example. Furthermore, the orbit does not lie on a smooth curve, but is beginning to fill an area of the phase space. The points also help to define a number of empty regions, three of which are clearly associated with the 7:4 resonance seen in the regular trajectory. There is also a tendency for the points to “stick” near the edges of the islands; this gives the impression of regular motion for short periods of time. Chaotic orbits have the additional characteristic that they are sensitively dependent on initial conditions. This is illustrated in Fig. 12, where the variation in e as a function of time is shown for two trajectories; the first corresponds to Fig. 9 (where x 0 = 0.56) and the second has x 0 = 0.56001. The initial value of y˙ was chosen so that the same value of C was obtained. Although both trajectories show comparable initial variations in e, after 60 Jupiter periods it is clear that the orbits have drifted apart. Such a divergence would not occur for nearby orbits in a regular part of the phase space.

CLBE001-ESS2E

November 11, 2006

22:43

798 Encyclopedia of the Solar System FIGURE 9 The eccentricity as a function of time for an object moving in a chaotic orbit started just outside the 7:4 resonance with Jupiter. The plot was obtained by solving the circular restricted three-body problem numerically using initial values of 0.6984 and 0.1967 for the semimajor axis and eccentricity, respectively. The corresponding position and velocity in the rotating frame were x 0 = 0.56, y0 = 0, x˙ 0 = 0, and y˙ = 0.8998.

Eccentricity

0.32

0.28

0.24

0.2

50

100

150

200

250

300

Time (Jupiter periods)

The rate of divergence of nearby trajectories in such numerical experiments can be quantified by monitoring the evolution of two orbits that are started close together. In a dynamical system such as the three-body problem, there are a number of quantities called the Lyapunov characteristic exponents. A measurement of the local divergence of nearby trajectories leads to an estimate of the largest of these exponents, and this can be used to determine whether or not the system is chaotic. If two orbits are separated in phase space by a distance d0 at time t0 , and d is their separation at time t, then the orbit is chaotic if d = d0 exp γ (t − t0 ),

of a numerical integration by writing ln(d/d0 ) t→∞ t − t0

γ = lim

and monitoring the behavior of γ with time. A plot of γ as a function of time on a log–log scale reveals a striking difference between regular and chaotic trajectories. For regular orbits, d ≈ d0 and a log–log plot has a slope of –1. However, if the orbit is chaotic, then γ tends to a constant non-zero value. This method may not always work because γ is defined only in the limit as t → ∞ and sometimes chaotic orbits may give the appearance of being regular orbits for long periods of time by sticking close to the edges of the islands. If the nearby trajectory drifts too far from the original one, then γ is no longer a measure of the local divergence of the orbits. To overcome this problem, it helps to rescale the separation of the nearby trajectory at fixed intervals. Figure 13 shows log γ as a function of log t calculated using this

(42)

where γ is a positive quantity equal to the maximum Lyapunov characteristic exponent. However, in practice the Lyapunov characteristic exponents can only be derived analytically for a few idealized systems. For practical problems in the solar system, γ can be estimated from the results

FIGURE 10 The semimajor axis as a function of time for an object using the same starting conditions as in Fig. 9. The units of the semimajor axis are such that Jupiter’s semimajor axis (5.202 AU) is taken to be unity.

0.7

Semimajor axis

(43)

0.68

0.66

0.64 50

100

150

200

Time (Jupiter periods)

250

300

CLBE001-ESS2E

November 11, 2006

22:43

Solar System Dynamics: Regular and Chaotic Motion

FIGURE 11 A surface of section plot for the same chaotic orbit as shown in Figs. 9 and 10. The 2000 points were generated by plotting the values of x and x˙ whenever y = 0 with positive y˙ . The points are distributed over a much wider region of the (x , x˙ ) plane than the points for the regular orbit shown in Fig. 8, and they help to define the edges of the regular regions associated with the 7:4 and other resonances.

0.4

0.2

. x

799

0

-0.2

-0.4 0.5

0.6

0.7

0.8

0.9

x

FIGURE 12 The variation in the eccentricity for two chaotic orbits started close to one another. One plot is part of Fig. 9 using the chaotic orbit started with x 0 = 0.56, and the other is for an orbit with x 0 = 0.56001. Although the divergence of the two orbits is exponential, the effect becomes noticeable only after 60 Jupiter periods.

0.28

Eccentricity

0.26 x0 = 0.56001

0.24

x0

0.22

0.2 52

54

56

58

60

62

64

Time (Jupiter periods)

FIGURE 13 The evolution of the quantity γ [defined in Eq. (43)] as a function of time (in Jupiter periods) for a regular (x 0 = 0.55) and chaotic (x 0 = 0.56) orbit. In this log–log plot, the regular orbit shows a characteristic slope of −1 with no indication of log γ tending toward a finite value. However, in the case of the chaotic orbit, log γ tends to a limiting value close to −0.77.

-0.5

chaotic orbit

Log

-1.0

-1.5 regular orbit -2.0

-2.5 2.2

2.4

2.6 Log t

2.8

3.0

3.2

CLBE001-ESS2E

November 11, 2006

22:43

800 Encyclopedia of the Solar System method for the regular and chaotic orbits described here. This leads to an estimate of γ = 10−0.77 (Jupiter periods)−1 for the maximum Lyapunov characteristic exponent of the chaotic orbit. The corresponding Lyapunov time is given by 1/γ , or in this case ∼6 Jupiter periods. This indicates that for this starting condition the chaotic nature of the orbit quickly becomes apparent. It is important to realize that a chaotic orbit is not necessarily unbounded. The maximum Lyapunov characteristic exponent concerns local divergence and provides no information about the global stability of the trajectory. The phrase “wandering on a leash” is an apt description of objects on bounded chaotic orbits—the motion is contained but yet chaotic at the same time. Another consideration is that numerical explorations of chaotic systems have many pitfalls both in how the physical system is modeled and whether or not the model provides an accurate portrayal of the real system.

tances from Jupiter. For the case μ2 = 0.001 and C > 3.04, it is impossible for their orbits to intersect, although the perturbations can still be significant. Close inspection of the separatrices in Figs. 14 and 15 reveals that they consist of chaotic regions with regular regions on either side. As the value of the Jacobi constant decreases, the extent of the chaotic separatrices increases until the regular curves separating adjacent resonances are broken down and neighboring chaotic regions begin to merge. This can be thought of as the overlap of adjacent resonances giving rise to chaotic motion. It is this process that permits chaotic orbits to explore regions of the phase space that are inaccessible to the regular orbits. In the context of the Sun–Jupiter–asteroid problem, this observation implies that asteroids in certain orbits are capable of large excursions in their orbital elements.

5. Orbital Evolution of Minor Bodies 4.2.3 LOCATION OF REGULAR AND CHAOTIC REGIONS

The extent of the chaotic regions of the phase space of a dynamical system can depend on a number of factors. In the case of the circular restricted three-body problem, the critical quantities are the values of the Jacobi constant and the mass ratio μ2 . In Figs. 14 and 15, ten trajectories are shown for each of two different values of the Jacobi constant. In the first case (Fig. 14), the value is C = 3.07 (the same as the value used in Figs. 8 and 11), whereas in Fig. 15 it is C = 3.13. It is clear that the extent of the chaos is reduced in Fig. 15. The value of C in the circular restricted problem determines how close the asteroid can get to Jupiter. Larger values of C correspond to orbits with greater minimum dis-

1

5.1 Asteroids With more than 130,000 accurately determined orbits and one major perturber (the planet Jupiter), the asteroids provide a natural laboratory in which to study the consequences of regular and chaotic motion. Using suitable approximations, asteroid motion can be studied analytically in some special cases. However, it is frequently necessary to resort to numerical integration. [See Main-Belt Asteroids.] Investigations have shown that a number of asteroids have orbits that result in close approaches to planets. Of particular interest are asteroids such as 433 Eros, 1033 Ganymed, and 4179 Toutatis, because they are on orbits

5:2

C = 3.07 0.5

. x

5:3

0

5:2

2:1

3:2 5:3

-0.5

5:2

-1 0.2

0.4

0.6 x

0.8

FIGURE 14 Representative surface of section plots for x 0 = 0.25, 0.29, 0.3, 0.45, 0.475, 0.5, 0.55, 0.56, 0.6, and 0.8 with = 0, y0 = 0, and Jacobi constant C = 3.07. Each trajectory was followed for a minimum of 500 crossing points. The plot uses the points shown in Figs. 8 and 11 (although the scales are different), as well as points from other regular and chaotic orbits. The major resonances are identified.

CLBE001-ESS2E

November 11, 2006

22:43

Solar System Dynamics: Regular and Chaotic Motion

1 C = 3.13

5:2

0.5 . x

9:4

9:4

0

5:2

2:1

801

FIGURE 15 Representative surface of section plots for x 0 = 0.262, 0.3, 0.34, 0.35, 0.38, 0.42, 0.52, 0.54, 0.7, and 0.78 with x˙ 0 = 0, y0 = 0, and Jacobi constant C = 3.13. Each trajectory was followed for a minimum of 500 crossing points. It is clear from a comparison with Fig. 14 that the phase space is more regular; chaotic orbits still exist for this value of C,but they are more difficult to find. The major resonances are identified.

9:4

-0.5 5:2

-1 0.2

0.4

0.6

0.8

x

the orbit of Jupiter. The objects in the orbit of Jupiter are the Trojan asteroids (Section 3.2), which are located ∼ 60◦ ahead of and behind Jupiter. The cleared region near Jupiter’s orbit can be understood in terms of chaotic motion due to the overlap of adjacent resonances. In the context of the Sun–Jupiter–asteroid restricted three-body problem, the perturber (Jupiter) has

1.26 Semi-major axis (AU)

that bring them close to Earth. One of the most striking examples of the butterfly effect (see Section 4.1) in the context of orbital evolution is the orbit of asteroid 2060 Chiron, which has a perihelion inside Saturn’s orbit and an aphelion close to Uranus’s orbit. Numerical integrations based on the best available orbital elements show that it is impossible to determine Chiron’s past or future orbit with any degree of certainty since it frequently suffers close approaches to Saturn and Uranus. In such circumstances, the outcome is strongly dependent on the initial conditions as well as the accuracy of the numerical method. These are the characteristic signs of a chaotic orbit. By integrating several orbits with initial conditions close to the nominal values, it is possible to carry out a statistical analysis of the orbital evolution. Studies suggest that there is a 1 in 8 chance that Saturn will eject Chiron from the solar system on a hyperbolic orbit, while there is a 7 in 8 chance that it will evolve toward the inner solar system and come under strong perturbations from Jupiter. Telescopic observations of a faint coma surrounding Chiron imply that it is a comet rather than an asteroid; perhaps its future orbit will resemble that of a short-period comet of the Jupiter family. Numerical studies of the orbital evolution of planetcrossing asteroids under the effects of perturbations from all the planets have shown a remarkable complexity of motion for some objects. For example, the Earth-crossing asteroid 1620 Geographos gets trapped temporarily in a number of resonances with Earth in the course of its chaotic evolution (Fig. 16). A histogram of the number distribution of asteroid orbits in semimajor axis (Fig. 17) shows that apart from a clustering of asteroids near Jupiter’s semimajor axis at 5.2 AU, there is an absence of objects within 0.75 AU of

1.24

13:18 8:11 11:15

11:15 14:19 1.22

1.20 -100,000

0

100,000

Time (years)

FIGURE 16 A plot of the semimajor axis of the near-Earth asteroid 1620 Geographos over a backward and forward integration of 100,000 years starting in 1986. Under perturbations from the planets, Geographos moves in a chaotic orbit and gets temporarily trapped in a number of high-order, orbit–orbit resonances (indicated in the diagram) with Earth. The data are taken from a numerical study of planet-crossing asteroids undertaken by A. Milani and coworkers. (Courtesy of Academic Press.)

CLBE001-ESS2E

November 11, 2006

22:43

802 Encyclopedia of the Solar System

Number of Asteroids

5000

3:1 5:2

2:1

3:2

1:1

4000 3000 2000 1000

2

4 3 Semimajor Axis (AU)

5

FIGURE 17 A histogram of the distribution of the numbered asteroids with semimajor axis together with the locations of the major jovian resonances. Most objects lie in the main belt between 2.0 and 3.3 AU, where the outer edge is defined by the location of the 2:1 resonance with Jupiter. As well as gaps (the Kirkwood gaps) at the 3:1, 5:2, 2:1, and other resonances in the main belt, there are small concentrations of asteroids at the 3:2 and 1:1 resonances (the Hilda and Trojan groups, respectively).

an infinite sequence of first-order resonances that lie closer together as its semimajor axis is approached. For example, the 2:1, 3:2, 4:3, and 5:4 resonances with Jupiter lie at 3.3, 4.0, 4.3, and 4.5 AU, respectively. Since each ( p + 1): p resonance (where p is a positive integer) has a finite width in semimajor axis that is almost independent of p, adjacent resonances will always overlap for some value of p greater than a critical value, pcrit . This value is given by  pcrit ≈ 0.51

m m+M

−2/7 (44)

where, in this case, m is the mass of Jupiter and M is the mass of the Sun. This equation can be used to predict that resonance overlap and chaotic motion should occur for p values greater than 4; this corresponds to a semimajor axis near 4.5 AU. Therefore chaos may have played a significant role in the depletion of the outer asteroid belt. The histogram in Fig. 17 also shows a number of regions in the main belt where there are few asteroids. The gaps at 2.5 and 3.3 AU were first detected in 1867 by Daniel Kirkwood using a total sample of fewer than 100 asteroids; these are now known as the Kirkwood gaps. Their locations coincide with prominent Jovian resonances (indicated in Fig. 17), and this led to the hypothesis that they were created by the gravitational effect of Jupiter on asteroids that had orbited at these semimajor axes. The exact removal mechanism was unclear until the 1980s, when several numerical and analytical studies showed that the central regions of these resonances contained large chaotic zones. The Kirkwood gaps cannot be understood using the model of the circular restricted three-body problem described in Section 4.2. The eccentricity of Jupiter’s orbit, although small (0.048), plays a crucial role in producing the large chaotic zones that help to determine the orbital evolution of asteroids. On timescales of several hundreds of thousands of years, the mutual perturbations of the planets act to change their orbital elements and Jupiter’s eccentricity can vary from 0.025 to 0.061. An asteroid in the chaotic zone at the 3:1 resonance would undergo large, essentially unpredictable changes in its orbital elements. In particular, the eccentricity of the asteroid could become large enough for it to cross the orbit of Mars. This is illustrated in Fig. 18 for a fictitious asteroid with an initial eccentricity of 0.15 moving in a chaotic region of the phase space at the 3:1 resonance. Although the asteroid can have periods of relatively low eccentricity, there are large deviations and

FIGURE 18 The chaotic evolution of the eccentricity of a fictitious asteroid at the 3:1 resonance with Jupiter. The orbit was integrated using an algebraic mapping technique developed by J. Wisdom. The line close to e = 0.3 denotes the value of the asteroid’s eccentricity, above which it would cross the orbit of Mars. It is believed that the 3:1 Kirkwood gap was created when asteroids in chaotic zones at the 3:1 resonance reached high eccentricities and were removed by direct encounters with Mars, Earth, or Venus.

Mars crossing

Eccentricity

0.3

0.2

0.1 0

2000

4000

6000

Time (Jupiter periods)

8000

10000

CLBE001-ESS2E

November 11, 2006

22:43

Solar System Dynamics: Regular and Chaotic Motion

orbit of asteroid e = 0.15

orbit of Mars

orbit of asteroid e = 0.33

Sun

Sun orbit of Mars

e can reach values in excess of 0.3. Allowing for the fact that the eccentricity of Mars’s orbit can reach 0.14, this implies that there will be times when the orbits could intersect (Fig. 19). In this case, the asteroid orbit would be unstable, since it is likely to either impact the surface of Mars or suffer a close approach that would drastically alter its semimajor axis. Although Jupiter provides the perturbations, it is Mars, Earth or Venus that ultimately removes the asteroids from the 3:1 resonance. Figure 20 shows the excellent correspondence between the distribution of asteroids close to the 3:1 resonance and the maximum extent of the chaotic region determined from numerical experiments. The situation is less clear for other resonances, although there is good evidence for large chaotic zones at the 2:1 and 5:2 resonances. In the outer part of the main belt, large changes in eccentricity will cause the asteroid to cross the orbit of Jupiter before it gets close to Mars. There may also

FIGURE 19 The effect of an increase in the orbital eccentricity of an asteroid at the 3:1 Jovian resonance on the closest approach between the asteroid and Mars. For e = 0.15, the orbits do not cross. However, for e = 0.33, a typical maximum value for asteroids in chaotic orbits, there is a clear intersection of the orbits, and the asteroid could have a close encounter with Mars.

be perturbing effects from other planets. In fact, it is now known that secular resonances have an important role to play in the clearing of the Kirkwood gaps, including the one at the 3:1 resonance. Once again, chaos is involved. Studies of asteroid motion at the 3:2 Jovian resonance indicate that the motion is regular, at least for low values of the eccentricity. This may help to explain why there is a local concentration of asteroids (the Hilda group) at this resonance, whereas others are associated with an absence of material. Since the dynamical structure of the asteroid belt has been determined by the perturbative effects of nearby planets, it seems likely that the original population was much larger and more widely dispersed. Therefore, the current distribution of asteroids may represent objects that are either recent collision products or that have survived in relatively stable orbits over the age of the solar system.

FIGURE 20 The eccentricity and semimajor axes of asteroids in the vicinity of the 3:1 jovian resonance; the Kirkwood gap is centered close to 2.5 AU. The two curves denote the maximum extent of the chaotic zone determined from numerical experiments, and there is excellent agreement between these lines and the edges of the 3:1 gap.

0.4

0.3

Eccentricity

803

0.2

0.1

0.0 2.45

2.50 Semimajor Axis (AU)

2.55

CLBE001-ESS2E

November 11, 2006

22:43

804 Encyclopedia of the Solar System 5.2 Meteorites Most meteorites are thought to be the fragments of material produced from collisions in the asteroid belt, and the reflectance properties of certain meteorites are known to be similar to those of common types of asteroids. Since most collisions take place in the asteroid belt, the fragments have to evolve into Earth-crossing orbits before they can hit Earth and be collected as samples. An estimate of the time taken for a given meteorite to reach Earth after the collisional event that produced it can be obtained from a measure of its cosmic ray exposure age. Prior to the collisions, the fragment may have been well below the surface of a much larger body, and as such it would have been shielded from all but the most energetic cosmic rays. However, after a collision the exposed fragment would be subjected to cosmic ray bombardment in interplanetary space. A detailed analysis of meteorite samples allows these exposure ages to be measured. In the case of one common class of meteorites called the ordinary chondrites, the cosmic ray exposure ages are typically less than 20 million years and the samples show little evidence of having been exposed to high pressure, or “shocking.” Prior to the application of chaos theory to the origin of the Kirkwood gaps, there was no plausible mechanism that could explain delivery to Earth within the exposure age constraints and without shocking. However, small increments in the velocity of the fragments as a result of the initial collision could easily cause them to enter a chaotic zone near a given resonance. Numerical integrations of such orbits near the 3:1 resonance showed that it was possible for them to achieve eccentricities large enough for them to cross the orbit of Earth. This result complemented previous research that had established that this part of the asteroid belt was a source region for the ordinary chondrites. Another effect that must be considered to obtain agreement between theory and observations is the Yarkovski effect which is discussed below. [See Meteorites.]

5.3 Comets Typical cometary orbits have large eccentricities and therefore planet-crossing trajectories are commonplace. Many comets are thought to originate in the Oort cloud at several tens of thousands of AU from the Sun; another reservoir of comets, known as the Kuiper belt, exists just beyond the orbit of Neptune. Those that have been detected from Earth are classified as either long period (most of which have made single apparitions and have periods >200 yr) or Halley-type (with orbital periods of 20 – 200 yr) or Jupiter-family, which have orbital periods < 20 yr. All comets with orbital periods of less than ∼103 yr have experienced a close approach to Jupiter or one of the other giant planets. By their very nature, the orbits of comets are chaotic, since the outcome

of any planetary encounter will be sensitively dependent on the initial conditions. Studies of the orbital evolution of the short-period comet P/Lexell highlight the possible effects of close approaches. A numerical integration has shown that prior to 1767 it was a short-period comet with a semimajor axis of 4.4 AU and an eccentricity of 0.35. In 1767 and 1779, it suffered close approaches to Jupiter. The first encounter placed it on a trajectory which brought it into the inner solar system and close (0.0146 AU) to the Earth, leading to its discovery and its only apparition in 1770, whereas the second was at a distance of ∼3 Jovian radii. This changed its semimajor axis to 45 AU with an eccentricity of 0.88. A more recent example is the orbital history of comet Shoemaker-Levy 9 prior to its spectacular collision with Jupiter in 1994. Orbit computations suggest that the comet was first captured by Jupiter at some time during a 9-year interval centered on 1929. Prior to its capture, it is likely that it was orbiting in the outer part of the asteroid belt close to the 3:2 resonance with Jupiter or between Jupiter and Saturn close to the 2:3 resonance with Jupiter. However, the chaotic nature of its orbit means that it is impossible to derive a more accurate history unless prediscovery images of the comet are obtained. [See Physics and Chemistry of Comets; Cometary Dynamics.]

5.4 Small Satellites and Rings Chaos is also involved in the dynamics of a satellite embedded in a planetary ring system. The processes differ from those discussed in Section 3.1, A because there is a near-continuous supply of ring material and direct scattering by the perturber is now important. In this case, the key quantity is the Hill’s sphere of the satellite. Ring particles on near-circular orbits passing close to the satellite exhibit chaotic behavior due to the significant perturbations they receive at close approach. This causes them to collide with surrounding ring material, thereby forming a gap. Studies have shown that for small satellites, the expression for the width of the cleared gap is 

m2 W ≈ 0.44 m1

2/7 a

(45)

where m2 and a are the mass and semimajor axis of the satellite and m1 is the mass of the planet. Thus, an icy satellite with a radius of 10 km and a density of 1 g cm−3 orbiting in Saturn’s A ring at a radial distance of 135,000 km would create a gap approximately 140 km wide. Since such a gap is wider than the satellite that creates it, this provides an indirect method for the detection of small satellites in ring systems. There are two prominent gaps in Saturn’s A ring: the ∼35-km-wide Keeler gap at 135,800 km

CLBE001-ESS2E

November 11, 2006

22:43

Solar System Dynamics: Regular and Chaotic Motion

and the 320-km-wide Encke gap at 133,600 km. The predicted radii of the icy satellites required to produce these gaps are ∼2.5 and ∼24 km, respectively. In 1991, an analysis of Voyager images by M. Showalter revealed a small satellite, Pan, with a radius of ∼10 km orbiting in the Encke gap. In 2005, the moon Daphnis of radius ∼3–4 km was discovered in the Keeler gap by the Cassini spacecraft. Voyager 2 images of the dust rings of Uranus show pronounced gaps at certain locations. Although most of the proposed shepherding satellites needed to maintain the narrow rings have yet to be discovered, these gaps may provide indirect evidence of their orbital locations.

805

In the early nineteenth century, Pierre Simon de Laplace claimed that he had demonstrated the long-term stability of the solar system using the results of his secular perturbation theory. Although the actual planetary system violates some of the assumed conditions (e.g., Jupiter and Saturn are close to a 5:2 resonance), the Laplace–Lagrange theory can be modified to account for some of these effects. However, such analytical approaches always involve the neglect of potentially important interactions between planets. The problem becomes even more difficult when the possibility of near-resonances between some of the secular periods of the system is considered. However, nowadays it is always possible to carry out numerical investigations of long-term stability.

6. Long Term Stability of Planetary Orbits 6.2 Stability of the Solar System

6.1 The N-Body Problem The entire solar system can be approximated by a system of nine planets orbiting the Sun. (Tiny Pluto has been included in most studies of this problem to date, because it was classified as a planet until 2006. But Pluto does not substantially perturb the motions of the eight larger planets.) In a center of mass frame, the vector equation of motion for planet i moving under the Newtonian gravitational effect of the Sun and the remaining 8 planets is given by r¨ = G

9  j =0

mj

r j − ri ( j = i), ri3j

(46)

where ri and mi are the position vector and mass of planet i(i = 1, 2, . . . , 9), respectively, ri j ≡ r j − ri , and the subscript 0 refers to the Sun. These are the equations of the N-body problem for the case where N = 10, and although they have a surprisingly simple form, they have no general, analytical solution. However, as in the case of the three-body problem, it is possible to tackle this problem mathematically by making some simplifying assumptions. Provided the eccentricities and inclinations of the N bodies are small and there are no resonant interactions between the planets, it is possible to derive an analytical solution that describes the evolution of all the eccentricities, inclinations, perihelia, and nodes of the planets. This solution, called Laplace–Lagrange secular perturbation theory, gives no positional information about the planets, yet it demonstrates that there are long-period variations in the planetary orbital elements that arise from mutual perturbations. The secular periods involved are typically tens or hundreds of thousands of years, and the evolving system always exhibits a regular behavior. In the case of Earth’s orbit, such periods may be correlated with climatic change, and large variations in the eccentricity of Mars are thought to have had important consequences for its climate.

Numerical integrations show that the orbits of the planets are chaotic, although there is no indication of gross instability in their motion provided that the integrations are restricted to durations of 5 billion years (the age of the solar system). The eight planets as well as dwarf planet Pluto remain more or less in their current orbits with small, nearly periodic variations in their eccentricities and inclinations; close approaches never seem to occur. Pluto’s orbit is chaotic, partly as a result of its 3:2 resonance with the planet Neptune, although the perturbing effects of other planets are also important. Despite the fact that the timescale for exponential divergence of nearby trajectories (the inverse of the Lyapunov exponent) is about 20 million years, no study has shown evidence for Pluto leaving the resonance. Chaos has also been observed in the motion of the eight planets, and it appears that the solar system as a whole is chaotic with a timescale for exponential divergence of 4 or 5 million years, although different integrations give different results. However, the effect is most apparent in the orbits of the inner planets. Though there appear to be no dramatic consequences of this chaos, it does mean that the use of the deterministic equations of celestial mechanics to predict the future positions of the planets will always be limited by the accuracy with which their orbits can be measured. For example, some results suggest that if the position of Earth along its orbit is uncertain by 1 cm today, then the exponential propagation of errors that is characteristic of chaotic motion implies that knowledge of Earth’s orbital position 200 million years in the future is not possible. The solar system appears to be “stable” in the sense that all numerical integrations show that the planets remain close to their current orbits for timescales of billions of years. Therefore the planetary system appears to be another example of bounded chaos, where the motion is chaotic but always takes place within certain limits. Although an analytical proof of this numerical result and a detailed understanding of how the chaos has arisen have yet to be achieved, the

CLBE001-ESS2E

November 11, 2006

22:43

806 Encyclopedia of the Solar System solar system seems to be chaotic yet stable. When the planetary orbits are integrated forward for timescales for several billion years using the averaged equations of motion, it is found that there is a very small but finite probability that the orbit of Mercury can become unstable and intersect the orbit of Venus. Many challenges remain in understanding how structural stability of planetary systems in the presence of transient and intermittent chaos can be maintained, and this subject remains a rich field for dynamical exploration.

and the Sun’s gravity: β≡

Fgeff =

The foregoing sections describe the gravitational interactions between the Sun, planets, and moons. Solar radiation has been ignored, but this is an important force for small particles in the solar system. Three effects can be distinguished: (1) the radiation pressure, which pushes particles primarily outward from the Sun (micron-sized dust); (2) the Poynting–Robertson drag, which causes centimeter-sized particles to spiral inward toward the Sun; and (3) the Yarkovski effect, which changes the orbits of meter- to kilometer-sized objects owing to uneven temperature distributions at their surfaces. The latter two effects are relativistic and thus quite weak at solar system velocities, but they can nonetheless be significant as they can lead to secular changes in orbital angular momentum and energy. Each of these effects is discussed in the next three subsections and then the effect of gas drag is examined. In the final subsection the influence of tidal interactions is discussed; this effect (in contrast to the other dissipative effects described in this section) is most important for larger bodies such as moons and planets. [See Solar System Dust.]

7.1 Radiation Force (Micron-Sized Particles) The Sun’s radiation exerts a force, Fr , on all other bodies of the solar system. The magnitude of this force is LA Qpr , 4π cr 2

(47)

where A is the particle’s geometric cross section, L is the solar luminosity, c is the speed of light, r is the heliocentric distance, and Qpr is the radiation pressure coefficient, which is equal to unity for a perfectly absorbing particle and is of order unity unless the particle is small compared to the wavelength of the radiation. The parameter β is defined as the ratio between the forces due to the radiation pressure

(48)

where the radius, R, and the density, ρ, of the particle are in c.g.s. units. Note that β is independent of heliocentric distance and that the solar radiation force is important only for micron- and submicron-sized particles. Using the parameter β, a more general expression for the effective gravitational attraction can be written:

7. Dissipative Forces and the Orbits of Small Bodies

Fr =

Qpr Fr = 5.7 × 10−5 , Fg ρR

−(1 − β)GmM , r2

(49)

that is, the small particles “see” a Sun of mass (1 − β)M. It is clear that small particles with β > 1 are in sum repelled by the Sun, and thus quickly escape the solar system, unless they are gravitationally bound to one of the planets. Dust which is released from bodies traveling on circular orbits at the Keplerian velocity is ejected from the solar system if β > 0.5. The importance of solar radiation pressure can be seen, for example, in comets. Cometary dust is pushed in the antisolar direction by the Sun’s radiation pressure. The dust tails are curved because the particles’ velocity decreases as they move farther from the Sun, due to conservation of angular momentum. [See Cometary Dynamics; Physics and Chemistry of Comets.]

7.2 Poynting–Robertson Drag (Centimeter-Sized Grains) A small particle in orbit around the Sun absorbs solar radiation and reradiates the energy isotropically in its own frame. The particle thereby preferentially radiates (and loses momentum) in the forward direction in the inertial frame of the Sun. This leads to a decrease in the particle’s energy and angular momentum and causes dust in bound orbits to spiral sunward. This effect is called the Poynting–Robertson drag. The net force on a rapidly rotating dust grain is given by Frad

LQpr A ≈ 4π cr 2



2vr 1− c



 vθ ˆ rˆ − θ . c

(50)

The first term in Eq. (50) is that due to radiation pressure and the second and third terms (those involving the velocity of the particle) represent the Poynting–Robertson drag. From this discussion, it is clear that small-sized dust grains in the interplanetary medium are removed: (sub)micron sized grains are blown out of the solar system, whereas larger particles spiral inward toward the Sun.

CLBE001-ESS2E

November 11, 2006

22:43

Solar System Dynamics: Regular and Chaotic Motion

Typical decay times (in years) for circular orbits are given by τP−R ≈ 400

r2 , β

(51)

with the distance r in AU. Particles that produce the bulk of the zodiacal light (at infrared and visible wavelengths) are between 20 and 200 μm, so their lifetimes at Earth orbit are on the order of 105 yr, which is much less than the age of the solar system. Sources for the dust grains are comets as well as the asteroid belt, where numerous collisions occur between countless small asteroids.

Consider a rotating body heated by the Sun. Because of thermal inertia, the afternoon hemisphere is typically warmer than the morning hemisphere, by an amount T  T . Let us assume that the temperature of the morning hemisphere is T − T /2, and that of the evening hemisphere T + T /2. The radiation reaction upon a surface element dA, normal to its surface, is dF = 2σ T 4 dA/3c. For a spherical particle of radius R, the Yarkovski force in the orbit plane due to the excess emission on the evening side is FY =

σ T 4 T 8 π R2 cos ψ, 3 c T

In the laboratory, gas drag slows solid objects down until their positions remain fixed relative to the gas. In the planetary dynamics case, the situation is more complicated. For example, a body on a circular orbit about a planet loses mechanical energy as a result of drag with a static atmosphere, but this energy loss leads to a decrease in semimajor axis of the orbit, which implies that the body actually speeds up! Other, more intuitive effects of gas drag are the damping of eccentricities and, in the case where there is a preferred plane in which the gas density is the greatest, the damping of inclinations relative to this plane. Objects whose dimensions are larger than the mean free path of the gas molecules experience Stokes’ drag, FD = −

7.3 Yarkovski Effect (Meter-Sized Objects)

(52)

where σ is the Stefan–Boltzmann constant and ψ is the particle’s obliquity, that is, the angle between its rotation axis and orbit pole. The reaction force is positive for an object that rotates in the prograde direction, 0 < ψ < 90◦ , and negative for an object with retrograde rotation, 90◦ < ψ < 180◦ . In the latter case, the force enhances the Poynting– Robertson drag. The Yarkovski force is important for bodies ranging in size from meters to several kilometers. Asymmetric outgassing from comets produces a nongravitational force similar in form to the Yarkovski force. [See Cometary Dynamics.]

7.4 Gas Drag Although interplanetary space generally can be considered an excellent vacuum, there are certain situations in planetary dynamics where interactions with gas can significantly alter the motion of solid particles. Two prominent examples of this process are planetesimal interactions with the gaseous component of the protoplanetary disk during the formation of the solar system and orbital decay of ring particles as a result of drag caused by extended planetary atmospheres.

807

CD Aρv 2 , 2

(53)

where v is the relative velocity of the gas and the body, ρ is the gas density, A is the projected surface area of the body, and CD is a dimensionless drag coefficient, which is of order unity unless the Reynolds number is very small. Smaller bodies are subject to Epstein drag, FD = −Aρvv 

(54)

where v  is the mean thermal velocity of the gas. Note that as the drag force is proportional to surface area and the gravitational force is proportional to volume (for constant particle density), gas drag is usually most important for the dynamics of small bodies. The gaseous component of the protoplanetary disk in the early solar system is believed to have been partially supported against the gravity of the Sun by a negative pressure gradient in the radial direction. Thus, less centripetal force was required to complete the balance, and consequently the gas orbited less rapidly than the Keplerian velocity. The “effective gravity” felt by the gas is geff = −

GMS dP − (1/ρ) . 2 r dr

(55)

To maintain a circular orbit, the effective gravity must be balanced by centripetal acceleration, rn2 . For estimated protoplanetary disk parameters, the gas rotated ∼0.5% slower than the Keplerian speed. Large particles moving at (nearly) the Keplerian speed thus encountered a headwind, which removed part of their angular momentum and caused them to spiral inward toward the Sun. Inward drift was greatest for mid-sized particles, which have large ratios of surface area to mass yet still orbit with nearly Keplerian velocities. The effect diminishes for very small particles, which are so strongly coupled to the gas that the headwind they encounter is very slow. Peak rates of inward drift occur for particles that collide

CLBE001-ESS2E

November 11, 2006

22:43

808 Encyclopedia of the Solar System with roughly their own mass of gas in one orbital period. Meter-sized bodies in the inner solar nebula drift inward at a rate of up to 106 km/yr! Thus, the material that survives to form the planets must complete the transition from centimeter to kilometer size rather quickly, unless it is confined to a thin dust-dominated subdisk in which the gas is dragged along at essentially the Keplerian velocity. Drag induced by a planetary atmosphere is even more effective for a given density, as atmospheres are almost entirely pressure supported, so the relative velocity between the gas and particles is high. As atmospheric densities drop rapidly with height, particles decay slowly at first, but as they reach lower altitudes, their decay can become very rapid. Gas drag is the principal cause of orbital decay of artificial satellites in low Earth orbit.

7.5 Tidal Interactions and Planetary Satellites Tidal forces are important to many aspects of the structure and evolution of planetary bodies: 1. On short timescales, temporal variations in tides (as seen in the frame rotating with the body under consideration) cause stresses that can move fluids with respect to more rigid parts of the planet (e.g., the familiar ocean tides) and even cause seismic disturbances (though the evidence that the Moon causes some earthquakes is weak and disputable, it is clear that the tides raised by Earth are a major cause of moonquakes). 2. On long timescales, tides cause changes in the orbital and spin properties of planets and moons. Tides also determine the equilibrium shape of a body located near any massive body; note that many materials that behave as solids on human timescales are effectively fluids on very long geological timescales (e.g., Earth’s mantle). The gravitational attraction of the Moon and Earth on each other causes tidal bulges that rise in a direction close to the line joining the centers of the two bodies. Particles on the nearside of the body experience gravitational forces from the other body that exceed the centrifugal force of the mutual orbit, whereas particles on the far side experience gravitational forces that are less than the centripetal forces needed for motion in a circle. It is the gradient of the gravitational force across the body that gives rise to the double tidal bulge. The Moon spins once per orbit, so that the same face of the Moon always points toward Earth and the Moon is always elongated in that direction. Earth, however, rotates much faster than the Earth–Moon orbital period. Thus, different parts of Earth point toward the Moon and are tidally stretched. If the Earth was perfectly fluid, the tidal bulges would respond immediately to the varying force, but the finite response time of Earth’s figure causes the tidal bulge to lag behind, at the point on Earth where the Moon was overhead slightly earlier. Since Earth rotates faster than

the Moon orbits, this “tidal lag” on Earth leads the position of the Moon in inertial space. As a result, the tidal bulge of Earth accelerates the Moon in its orbit. This causes the Moon to slowly spiral outward. The Moon slows down Earth’s rotation by pulling back on the tidal bulge, so the angular momentum in the system is conserved. This same phenomenon has caused most, if not all, major moons to be in synchronous rotation: the rotation and orbital periods of these bodies are equal. In the case of the Pluto–Charon system, the entire system is locked in a synchronous rotation and revolution of 6.4 days. Satellites in retrograde orbits (e.g., Triton) or satellites whose orbital periods are less than the planet’s rotation period (e.g., Phobos) spiral inward toward the planet as a result of tidal forces. Mercury orbits the Sun in 88 days and rotates around its axis in 59 days, a 3:2 spin–orbit resonance. Hence, at every perihelion one of two locations is pointed at the Sun: the subsolar longitude is either 0◦ or 180◦ . This configuration is stable because Mercury has both a large orbital eccentricity and a significant permanent deformation that is aligned with the solar direction at perihelion. Indeed, at 0◦ longitude there is a large impact crater, Caloris Planitia, which may be the cause of the permanent deformation. 3. Under special circumstances, strong tides can have significant effects on the physical structure of bodies. Generally, the strongest tidal forces felt by solar system bodies (other than Sun-grazing or planet-grazing comets) are those caused by planets on their closest satellites. Near a planet, tides are so strong that they rip a fluid (or weakly aggregated solid) body apart. In such a region, large moons are unstable, and even small moons, which could be held together by material strength, are unable to accrete because of tides. The boundary of this region is known as Roche’s limit. Inside Roche’s limit, solid material remains in the form of small bodies and rings are found instead of large moons. The closer a moon is to a planet, the stronger is the tidal force to which it is subjected. Let us consider Roche’s limit for a spherical satellite in synchronous rotation at a distance r from a planet. This is the distance at which a loose particle on an equatorial subplanet point just remains gravitationally bound to the satellite. At the center of the satellite of mass m and radius Rs , a particle would be in equilibrium and so GM = n2r, r2

(56)

where M( m) is the mass of the planet. However, at the equator, the particle will experience (i) an excess gravitational or centrifugal force due to the planet, (ii) a centrifugal force due to rotation, and (iii) a gravitational force due to the satellite. If the equatorial particle is just in equilibrium, these forces will balance and −

d dr



GM r2

Rs + n2r =

Gm . Rs2

(57)

CLBE001-ESS2E

November 11, 2006

22:43

Solar System Dynamics: Regular and Chaotic Motion

In this case, Roche’s limit rRoche is given by  rRoche = 3

1/3

ρplanet ρs

1/3 Rplanet ,

(58)

with ρ planet and ρ s are the densities for the planet and satellite, respectively, and Rplanet is the planetary radius. When a fluid moon is considered and flattening of the object due to the tidal distortion is taken into account, the correct result for a liquid moon (no internal strength) is 

rRoche

ρplanet = 2.456 ρs

1/3 Rplanet .

(59)

Most bodies have significant internal strength, which allows bodies with sizes ≤∼100 km to be stable somewhat inside Roche’s limit. Mars’s satellite Phobos is well inside Roche’s limit; it is subjected to a tidal force equivalent to that in Saturn’s B ring. 4. Internal stresses caused by variations in tides on a body in an eccentric orbit or not rotating synchronously with its orbital period can result in significant tidal heating of some bodies, most notably in Jupiter’s moon Io. If no other forces were present, this would lead to a decay of Io’s orbital eccentricity. By analogy to the Earth–Moon system, the tide raised on Jupiter by Io will cause Io to spiral outward and its orbital eccentricity to decrease. However, there exists a 2:1 mean-motion resonant lock between Io and Europa. Io passes on some of the orbital energy and angular momentum that it receives from Jupiter to Europa, and Io’s eccentricity is increased as a result of this transfer. This forced eccentricity maintains a high tidal dissipation rate and large internal heating in Io, which displays itself in the form of active volcanism. [See IO]

7.6 Tidal Evolution and Resonances Objects in prograde orbits that lie outside the synchronous orbit can evolve outward at different rates, so there may have been occasions in the past when pairs of satellites evolved toward an orbit–orbit resonance. The outcome of such a resonant encounter depends on the direction from which the resonance is approached. For example, capture into resonance is possible only if the satellites are approaching one another. If the satellites are receding, then capture is not possible, but the resonance passage can lead to an increase in the eccentricity and inclination. In certain circumstances it is possible to study the process using a simple mathematical model. However, this model breaks down near the chaotic separatrices of resonances and in regions of resonance overlap. It is likely that the major satellites of Jupiter, Saturn, and Uranus have undergone significant tidal evolution and

809

that the numerous resonances in the Jovian and Saturnian systems are a result of resonant capture. The absence of orbit–orbit resonances among the major moons in the Uranian system is thought to be related to the fact that the oblateness of Uranus is significantly less than that of Jupiter or Saturn. In these circumstances, there can be large chaotic regions associated with resonances and stable capture may be impossible. However, temporary capture into some resonances can produce large changes in eccentricity or inclination. For example, the Uranian satellite Miranda has an anomalously large inclination of 4◦ , which is thought to be the result of a chaotic passage through the 3:1 resonance with Umbriel at some time in its orbital history. Under tidal forces, a satellite’s eccentricity is reduced on a shorter timescale than its inclination, and Miranda’s current inclination agrees with estimates derived from a chaotic evolution. [See Planetary Satellites.]

8. Chaotic Rotation 8.1 Spin–Orbit Resonance One of the dissipative effects of the tide raised on a natural satellite by a planet is to cause the satellite to evolve toward a state of synchronous rotation, where the rotational period of the satellite is approximately equal to its orbital period. Such a state is one example of a spin–orbit resonance, where the ratio of the spin period to the orbital period is close to a rational number. The time needed for a near-spherical satellite to achieve this state depends on its mass and orbital distance from the planet. Small, distant satellites take a longer time to evolve into the synchronous state than do large satellites that orbit close to the planet. Observations by spacecraft and ground-based instruments suggest that most regular satellites are in the synchronous spin state, in agreement with theoretical predictions. The lowest energy state of a satellite in synchronous rotation has the moon’s longest axis pointing in the approximate direction of the planet–satellite line. Let θ denote the angle between the long axis and the planet–satellite line in the planar case of a rotating satellite (Fig. 21). The variation of θ with time can be described by equating the time variation of the rotational angular momentum with the restoring torque. The resulting differential equation is ω2 θ¨ + 03 sin 2(θ − f ) = 0, 2r

(60)

where ω0 is a function of the principal moments of inertia of the satellite, r is the radial distance of the satellite from the planet, and f is the true anomaly (or angular position) of the satellite in its orbit. The radius is an implicit function of time and is related to the true anomaly by the

CLBE001-ESS2E

November 11, 2006

22:43

810 Encyclopedia of the Solar System

orbit of satellite

satellite r

f periapse planet

FIGURE 21 The geometry used to define the orientation of a satellite in orbit about a planet. The planet–satellite line makes an angle f (the true anomaly) with a reference line, which is taken to be the periapse direction of the satellite’s orbit. The orientation angle, θ , of the satellite is the angle between its long axis and the reference direction.

equation r=

a(1 − e 2 ) , 1 + ecos f

(61)

where a and e are the constant semimajor axis and the eccentricity of the satellite’s orbit, respectively, and the orbit is taken to be fixed in space. Equation (60) defines a deterministic system where the initial values of θ and θ˙ determine the subsequent rotation of the satellite. Since θ and θ˙ define a unique spin position ˙ once every of the satellite, a surface of section plot of (θ, θ) orbital period, say at every periapse passage, produces a picture of the phase space. Figure 22 shows the resulting surface of section plots for a number of starting conditions using e = 0.1 and ω0 = 0.2. The chosen values of ω0 and e are larger than those that are typical for natural satellites, but they serve to illustrate the structure of the surface of section; large values of e are unusual since tidal forces also act to damp eccentricity. The surface of section shows large, regular regions surrounding narrow islands associated with the 1:2, 1:1, 3:2, 2:1, and 5:2 spin–orbit resonances at θ˙ = 0.5, 1, 1.5, 2, and 2.5, respectively. The largest island is associated with the strong 1:1 resonance and, although other spin states are possible, most regular satellites, including Earth’s Moon, are observed to be in this state. Note the presence of diffuse collections of points associated with small chaotic regions at the separatrices of the resonances. These are particularly obvious at the 1:1 spin–orbit state at θ = π /2, θ˙ = 1. Although this is a completely different dynamical system compared to the circular restricted three-body problem, there are distinct similarities in the types of behavior visible in Fig. 22 and parts of Figs. 14 and 15.

FIGURE 22 Representative surface of section plots of the orientation angle, θ , and its time derivative, θ˙ , obtained from the numerical solution of Eq. (59) using e = 0.1 and ω0 = 0.2. The values of θ and θ˙ were obtained at every periapse passage of the satellite. Four starting conditions were integrated for each of the 1:2, 1:1, 3:2, 2:1, and 5:2 spin–orbit resonances in order to illustrate motion inside, at the separatrix, and on either side of each resonance. The thickest “island” is associated with the strong 1:1 spin–orbit state θ = 1, whereas the thinnest is associated with the weak 5:2 resonance at θ = 2.5.

In the case of near-spherical objects, it is possible to investigate the dynamics of spin–orbit coupling using analytical techniques. The sizes of the islands shown in Fig. 22 can be estimated by expanding the second term in Eq. (60) and isolating the terms that will dominate at each resonance. Using such a method, each resonance can be treated in isolation and the gravitational effects of nearby resonances can be neglected. However, if a satellite is distinctly nonspherical, ω0 can be large and this approximation is no longer valid. In such cases it is necessary to investigate the motion of the satellite using numerical techniques.

8.2 Hyperion Hyperion is a satellite of Saturn that has an unusual shape (Fig. 23). It has a mean radius of 135 km, an orbital eccentricity of 0.1, a semimajor axis of 24.55 Saturn radii, and a corresponding orbital period of 21.3 days. Such a small object at this distance from Saturn has a large tidal despinning timescale, but the unusual shape implies an estimated value of ω0 = 0.89. The surface of section for a single trajectory is shown in Fig. 24 using the same scale as Fig. 22. It is clear that there is a large chaotic zone that encompasses most of the spin–orbit resonances. The islands associated with the synchronous and other resonances survive but in a much reduced form. Although this calculation assumes that Hyperion’s spin axis remains perpendicular to its orbital plane, studies have shown that the satellite should also be undergoing

CLBE001-ESS2E

November 11, 2006

22:43

Solar System Dynamics: Regular and Chaotic Motion

811

a

b

FIGURE 24 A single surface of section plot of the orientation angle, θ, and its time derivative, θ˙ , obtained from the numerical solution of Eq. (10) using the values e = 0.1 and ω0 = 0.89, which are appropriate for Hyperion. The points cover a much larger region of the phase space than any of those shown in Fig. 22, and although there are some remaining islands of stability, most of the phase space is chaotic.

for periodicities in plots of the brightness of the object as a function of time (the lightcurve of the object). The results of one such study for Hyperion are shown in Fig. 25. Since there is no recognizable periodicity, the lightcurve is consistent with that of an object undergoing chaotic rotation. Hyperion is the first natural satellite that has been observed to have a chaotic spin state, and results from Cassini images confirm this result. Observations and numerical studies of Hyperion’s rotation in three dimensions have shown that its

FIGURE 23 Two Cassini images of the Saturnian satellite Hyperion show the unusual shape of the satellite, which is one cause of its chaotic rotation. Panel (a) is a true color image, while panel (b) uses false color and has better resolution because it was obtained at closer range. [Courtesy of NASA/JPL/Space Science Institute.]

a tumbling motion, such that its axis of rotation is not fixed in space. Voyager observations of Hyperion indicated a spin period of 13 days, which suggested that the satellite was not in synchronous rotation. However, the standard techniques that are used to determine the period are not applicable if it varies on a timescale that is short compared with the timespan of the observations. In principle, the rotational period can be deduced from ground-based observations by looking

Mean Opposition Magnitude

13.6

13.8

14.0

14.2 0

20

40

60

Time (days)

FIGURE 25 Ground-based observations by J. Klavetter of Hyperion’s lightcurve obtained over 13 weeks (4.5 orbital periods) in 1987. The fact that there is no obvious curve through the data points is convincing evidence that the rotation of Hyperion is chaotic. (Courtesy of the American Astronomical Society.)

CLBE001-ESS2E

November 11, 2006

22:43

812 Encyclopedia of the Solar System spin axis does not point in a fixed direction. Therefore the satellite also undergoes a tumbling motion in addition to its chaotic rotation. The dynamics of Hyperion’s motion is complicated by the fact that it is in a 4:3 orbit–orbit resonance with the larger Saturnian satellite Titan. Although tides act to decrease the eccentricities of satellite orbits, Hyperion’s eccentricity is maintained at 0.104 by means of the resonance. Titan effectively forces Hyperion to have this large value of e and so the apparently regular orbital motion inside the resonance results, in part, in the extent of the chaos in its rotational motion. [See Planetary Satellites.]

8.3 Other Satellites Although there is no evidence that other natural satellites are undergoing chaotic rotation at the present time, it is possible that several irregularly shaped regular satellites did experience chaotic rotation at some time in their histories. In particular, since satellites have to cross chaotic separatrices before capture into synchronous rotation can occur, they must have experienced some episode of chaotic rotation. This may also have occurred if the satellite suffered a large impact that affected its rotation. Such episodes could have induced significant internal heating and resurfacing events in some satellites. The Martian moon Phobos and the Uranian moon Miranda have been mentioned as possible candidates for this process. If this happened early in the history of the solar system, then the evidence may well have been obliterated by subsequent cratering events. [See Planetary Satellites.]

8.4 Chaotic Obliquity The fact that a planet is not a perfect sphere means that it experiences additional perturbing effects due to the gravitational forces exerted by its satellites and the Sun, and these can cause long-term evolution in its obliquity (the angle between the planet’s equator and its orbit plane). Numerical investigations have shown that chaotic changes in obliquity are particularly common in the inner solar system. For example, it is now known that the stabilizing effect of the Moon results in a variation of ± 1.3◦ in Earth’s obliquity around a mean value of 23.3◦ . Without the Moon, Earth’s obliquity would undergo large, chaotic variations. In the case of Mars there is no stabilizing factor and the obliquity varies chaotically from 0◦ to 60◦ on a timescale of 50 million years. Therefore an understanding of the longterm changes in a planet’s climate can be achieved only by an appreciation of the role of chaos in its dynamical evolution.

9. Epilog It is clear that nonlinear dynamics has provided us with a deeper understanding of the dynamical processes that have helped to shape the solar system. Chaotic motion is a natural consequence of even the simplest systems of three or more interacting bodies. The realization that chaos has played a fundamental role in the dynamical evolution of the solar system came about because of contemporary and complementary advances in mathematical techniques and digital computers. This coincided with an explosion in our knowledge of the solar system and its major and minor members. Understanding how a random system of planets, satellites, ring and dust particles, asteroids, and comets interacts and evolves under a variety of chaotic processes and timescales, ultimately means that this knowledge can be used to trace the history and predict the fate of other planetary systems.

Bibliography Burns, J. A. (1987). The motion of interplanetary dust. In “The Evolution of the Small Bodies of the Solar System,” (Fulchignoni, M. and Kresak, L., eds.) pp. 252–275. Soc. Italiana di Fisica, Bologna, Italy. Danby, J. M. A. (1992). “Fundamentals of Celestial Mechanics.” Willmann–Bell, Richmond, Virginia. Diacu, F., and Holmes, P. (1996). “Celestial Encounters. The Origins of Chaos and Stability.” Princeton Univ. Press, Princeton, NJ. Duncan, M., and Quinn, T. (1993). The long-term dynamical evolution of the solar system. Annu. Rev. Astron. Astrophys. 31, 265–295. Dvorak, R., and Henrard, J. (eds.) (1988). “Long Term Evolution of Planetary Systems.” Kluwer, Dordrecht, Holland. Ferraz-Mello, S. (ed.) (1992). “Chaos, Resonance and Collective Dynamical Phenomena in the Solar System.” Kluwer, Dordrecht, Holland. Lichtenberg, A. J., and Lieberman, M. A. (1992). “Regular and Chaotic Dynamics,” volume 38 of Applied Mathematical Sciences. Springer-Verlag, New York, second edition. Lissauer, J. J. (1993). Planet formation. Annu. Rev. Astron. Astrophys. 31, 129–174. Morbidelli, A. (2002). “Modern Celestial Mechanics.” Taylor & Francis, London. Murray, C. D., and Dermott, S. F. (1999). “Solar System Dynamics.” Cambridge University Press, Cambridge. Peale, S. J. (1976). Orbital resonances in the solar system. Annu. Rev. Astron. Astrophys. 14, 215–246. Peterson, I. (1993). “Newton’s Clock. Chaos in the Solar System.” W. H. Freeman, New York. Roy, A. E., and Steves, B. A. (eds.) (1995). “From Newton to Chaos: Modern Techniques for Understanding and Coping with Chaos in N-Body Dynamical Systems.” Plenum, New York.

CLBE001-ESS2E

November 10, 2006

100-C

25-C

50-C

75-C

18:1

C+M

50-C+M

C+Y

50-C+Y

M+Y

50-M+Y

100-M

25-M

50-M

75-M

100-Y

25-Y

50-Y

75-Y

100-K

25-K

25-19-19

50-K

50-40-40

75-K

75-64-64

Planetary Impacts

Richard A.F. Grieve Natural Resources Canada Ottawa, Canada

Mark J. Cintala∗ NASA Johnson Space Center Houston, Texas

Roald Tagle

CHAPTER

43

Humboldt University Berlin, Germany

1. Impact Craters 2. Impact Processes 3. Impacts and Planetary Evolution

P

lanetary impacts have occurred throughout the history of the solar system. Small bodies, such as asteroids and comets, can have their orbits disturbed by gravitational forces, which results in their having a finite probability of colliding with another body or planet. Indeed, the collision of small bodies to form larger bodies was the fundamental process of planetary formation which, in its final stages, involved impacts between planetesimal-sized objects. As the solar system stabilized, the impact rate decreased but was still sufficient as late as ∼4.0 billion years ago to produce impact basins with diameters measured in hundreds to thousands of kilometers. As a result, impacts were a major geologic process in early planetary evolution and served to characterize the early upper crusts and surfaces of planetary bodies. Although impacts producing craters 100–200 km in diameter are relatively rare in more recent geologic time, they still occur on timescales of approximately 100 million years. One such event on the Earth marks the boundary between the Cretaceous and Tertiary geologic periods and resulted in the mass extinction of approximately 75% of the species living on Earth 65 million years ago. The 180 km diameter Chicxulub impact crater in the Yucatan, Mexico, is now known to be the site of this global-extinction impact.

4. Planetary Impactors Bibliography

1. Impact Craters 1.1 Crater Shape On bodies that have no atmosphere, such as the Moon, even the smallest pieces of interplanetary material can produce impact craters down to micrometer-sized cavities on individual mineral grains. On larger bodies, atmosphereinduced breakup and deceleration serve to slow smaller impacting objects. On the Earth, for example, impacting bodies with masses below 104 g can lose up to 90% of their velocity during atmospheric penetration, and the resultant impact pit is only slightly larger than the projectile itself. Atmospheric effects on larger masses, however, are less severe, and the body impacts with relatively undiminished velocity, producing a crater that is considerably larger than the impacting body. The processes accompanying such events are rooted in the physics of impact, with the differences in response among the various planets largely being due to differences in the properties of the planetary bodies (e.g., surface gravity, atmospheric density, and target composition and strength). The basic shape of virtually all impact craters is

∗

The views expressed by the author are his own and do not represent the views of NASA or any NASA employee. C 2007 by Academic Press. All rights of reproduction in any form reserved. Encyclopedia of the Solar System 2e 

813

CLBE001-ESS2E

November 10, 2006

18:1

814 Encyclopedia of the Solar System

FIGURE 2 Schematic cross section of a simple crater, based on terrestrial observations. D is diameter and da and dt are the depths of the apparent and true crater, respectively. See text for details.

FIGURE 1 Approximately 1 km diameter, relatively young simple martian crater. Large blocks, ejected late in the cratering process, can be seen on the ejecta near the rim. The ejecta can be differentiated into continuous ejecta and discontinuous ejecta, which appear as separate fingers and braids (Mars Global Surveyor).

a depression with an upraised rim. With increasing diameter, impact craters become proportionately shallower and develop more complicated rims and floors, including the appearance of central topographic peaks and interior rings. There are three major subdivisions in shape: simple craters, complex craters, and impact basins Simple impact structures have the form of a bowl-shaped depression with an upraised rim (Fig. 1). An overturned flap of ejected target materials exists on the rim, and the exposed rim, walls, and floor define the apparent crater. Observations at terrestrial impact craters reveal that a lens of brecciated target material, roughly parabolic in cross section, exists beneath the floor of this apparent crater (Fig. 2). This breccia lens is a mixture of different target materials, with fractured blocks set in a finer-grained matrix. These are allochthonous materials, having been moved into their present position by the cratering process. Beneath the breccia lens, relatively in-place, or parautochthonous, fractured target materials define the walls and floor of what is known as the true crater (Fig. 2). In the case of terrestrial simple craters, the depth to the base of the breccia lens (i.e., the base of the true crater) is roughly twice the depth to the top of the breccia lens (i.e., the floor of the apparent crater).

With increasing diameter, simple craters display signs of wall and rim collapse, as they evolve into complex craters. The diameter at which this transition takes place varies between planetary bodies and is, to a first approximation, an inverse function of planetary gravity. Other variables, such as target strength, and possibly projectile type, and impact angle and velocity, play a role and the transition actually occurs over a small range in diameter. For example, the transition between simple and complex craters occurs in the 15–25 km diameter range on the Moon. The effect of target strength is most readily apparent on Earth, where complex craters can occur at diameters as small as 2 km in sedimentary target rocks, but do not occur until diameters of 4 km, or greater, in stronger, crystalline target rocks. Complex craters are highly modified structures. A typical complex crater is characterized by a central topographic peak or peaks, a broad, flat floor, and a terraced, inwardly slumped rim area (Fig. 3). Observations at terrestrial complex craters show that the flat floor consists of a sheet of impact melt rock and/or polymict breccia (Fig. 4). The central region is structurally complex and, in large part, occupied by the central peak, which is the topographic manifestation of a much broader and extensive volume of uplifted rocks that occur beneath the center of complex craters (Fig. 4). With increasing diameter, a fragmentary ring of interior peaks appears, marking the beginning of the morphologic transition from craters to basins. While a single interior ring is required to define a basin, they can be subdivided further into central-peak basins, with both a peak and ring; peak ring basins (Fig. 5), with a single ring; and multiring basins, with two or more interior rings (Fig. 6). The transition from central-peak basins to peak-ring basins to multiring basins also represents a sequence with increasing diameter. As with the simple to complex crater transition, there is a small amount of overlap in basin shape near transition diameters.

CLBE001-ESS2E

November 10, 2006

18:1

Planetary Impacts

815

FIGURE 4 Schematic cross section of a complex crater, based on terrestrial observations. Notation is as in Fig. 2, with SU corresponding to the structural uplift and Dcp , to the diameter of the central uplift area. Note the preservation of the upper beds (different shades of gray) in the outer portion of the crater floor, indicating excavation was limited to the central area. See text for details.

Thus, at increasing distance from the crater, the final ejecta blanket on the ground includes increasing amounts of local materials. Secondary crater fields, resulting from the impact of larger, coherent blocks and clods of ejecta, surround fresh craters and are particularly evident on bodies with no or thin atmospheres, such as the Moon, Mercury, and Mars. They are often associated with typically bright

FIGURE 3 Complex central peak crater in the Isidis basin on Mars, with the terraced walls of the crater rim stepping down to a flat floor and a central peak. Also evident are the external rays of continuous (linear) and discontinuous (braided) ejecta on the surrounding terrain. (Mars Global Surveyor).

Ejected target material surrounds impact craters and can be subdivided into continuous and discontinuous ejecta facies (Figs. 1 and 3). The continuous deposits are those closest to the crater, being thickest at the rim crest. In the case of simple craters, the net effect of the ejection process is to invert the stratigraphy at the rim. As the distance from the crater rim increases, the ejecta are emplaced at higher velocities and, therefore, land with higher kinetic energies, resulting in the mixing of ejecta with local surface material.

FIGURE 5 The 50 km diameter peak ring basin Barton on Venus, with a discontinuous peak ring. Barton is close to the lower limit of the diameter where peak rings appear in impact craters on Venus and has a discontinuous peak ring (Magellan).

CLBE001-ESS2E

November 10, 2006

18:1

816 Encyclopedia of the Solar System

FIGURE 6 With a diameter of ∼900 km in diameter, as defined by the outer ring, the Cordillera mountains, Orientale the youngest and best-preserved multiring basin on the Moon (Lunar Orbiter).

or high-albedo “rays” that define an overall radial pattern to the primary crater. Two principal processes have been suggested to explain the rays. The first is a compositional effect, where the ejecta are chemically different from the material on which it is deposited. While this most often results in rays that are brighter than the surrounding material, the reverse can also occur. The second effect is a consequence of “maturity” due to prolonged exposure to “space weathering” agents like radiation and micrometeoroid bombardment on surface materials. [See Main-Belt Asteroids.] Fresher material excavated by an impact and deposited in the rays is generally brighter than the more mature material of the deposition surface. Many martian craters display examples of apparently fluidized ejecta (Fig. 7). They have been called “fluidized– ejecta,” “rampart,” or “pedestal” craters, where their ejecta deposits indicate emplacement as a ground-hugging flow. Most hypotheses on the origin of these features invoke the presence of ground ice (or water), which, upon heating by impact, is incorporated into the ejecta in either liquid or vapor form. This, then, provides lubrication for the mobilized material. On Venus, impact craters more than 15–20 km in diameter exhibit central peaks and/or peak rings (Fig. 8) and appear, for the most part, to be similar to complex craters and

FIGURE 7 This 7.5 km diameter martian central peak crater is close to the transition diameter to complex craters and has a small central peak and simple terraced walls. Ejecta can be discriminated into a fluidized material, which extends farthest and has lobate margins, overlain by a second type of ejecta, which does not extend as far and displays radial linear features (Mars Global Surveyor).

basins on the other terrestrial planets. Many of the craters smaller than 15 km, however, have rugged, multiple floors or occur as crater clusters. This is attributed to the effects of the dense atmosphere of Venus (surface pressure of ∼90 bar), which effectively crushes and breaks up smaller impacting bodies, so that they result in clusters of relatively shallow craters. Also due to atmospheric effects, there is a deficit in the number of expected craters with diameters up to 35 km, and there are no craters smaller than 3 km in diameter on Venus.

CLBE001-ESS2E

November 10, 2006

18:1

Planetary Impacts

817

FIGURE 8 Complex venusian central peak crater Aurelia, 32 km in diameter, which exhibits terraced walls, a flat floor, central peaks and long-running lobate flows, particularly in the lower right. Its ejecta pattern is asymmetric, indicating an oblique impact. The crater and the ejecta are also partially surrounded by terrain with a radar dark halo (Magellan).

FIGURE 9 The Valhalla multiring basin on Callisto. The overall structure may be as large as 4000 km in diameter, but only the central bright area is believed to be formed directly by impact. The surrounding, multiple scarps were likely formed in response to the subsurface flow of material back toward the initial crater, due to the relatively low internal strength of Callisto (Voyager).

In many cases, craters on Venus have ejecta deposits that are visible out to greater distances than expected from simple ballistic emplacement, and the distal deposits are clearly lobate (Fig. 8). These deposits likely owe their origin to entrainment effects of the dense atmosphere and/or the high proportion of impact melt that would be produced on a relatively high-gravity, high–surface temperature planet such as Venus. Another unusual feature on Venus is radardark zones surrounding some craters that can extend three to four crater diameters from the crater center (Fig. 8). They are believed to be due to the modification of surface roughness by the atmospheric shock wave produced by the impacting body. Small crater clusters have dark halos and dark circular areas where no central crater form has been observed. In these latter cases, the impacting body did not survive atmospheric passage, but the accompanying atmospheric shock wave had sufficient energy to interact with the surface to create a dark, radar-smooth area. [See Venus: Surface and Interior.] The situation is somewhat analogous to the 1908 Tunguska event, when a relatively small body exploded over Siberia at an altitude of ∼10 km, and the resultant atmospheric pressure wave leveled some 2000 km2 of forest. Remarkable ring structures occur on the Galilean satellites of Jupiter, Callisto, and Ganymede. The largest is the 4000-km feature Valhalla on Callisto (Fig. 9), which consists of a bright central area up to 800 km in diameter,

surrounded by a darker terrain with bright ridges 20–30 km apart. This zone is about 300 km wide and gives way to an outer zone with graben or rift-like features 50–100 km apart. These (very) multiring basins are generally considered to be of impact origin, but with the actual impact crater confined to the central area. The exterior rings are believed to be formed as a result of the original crater puncturing the outer, strong shell, or lithosphere, of these bodies. This permitted the weaker, underlying layer, the asthenosphere, to flow toward the crater, setting up stresses that led to fracturing and the formation of circumscribing scarps and graben. On Callisto and Ganymede, there is also a unique class of impact craters that no longer have an obvious crater form but appear as bright, or high-albedo, spots on the surfaces of these bodies. These are known as palimpsests and are believed to have begun as complex craters but have had their topography relaxed by the slow, viscous creep of the target’s icy crust over time. Palimpsests are old impact features and may have been formed when the icy satellites were young and relatively warm, with a thin crust possibly incapable of retaining significant topography. Other anomalous crater forms are developed on Ganymede and Callisto. On these icy satellites, most craters larger than 25 km have a central pit or central dome (Fig. 10), rather than a central peak. Pit and dome craters are shallower than other craters of comparable size, and

CLBE001-ESS2E

November 10, 2006

18:1

818 Encyclopedia of the Solar System impact velocities result in greater melting of the target, or to changes in the mechanical behavior of the crust and its response to impact with time. Interpretations of the origin of the various anomalous crater forms on the icy satellites, however, are generally not well constrained.

1.2 Crater Dimensions

FIGURE 10 Complex crater Har, 50 km in diameter, on Callisto, with a central dome in place of a central peak. The origin of the central mound is some form of response to the weak icy nature of the target material. A smaller (20 km) and younger central peak complex crater with a central peak, Tindr, occurs on the western rim of Har (Galileo).

it has been suggested that the pits are due to the formation of slushy or fluid material by impact melting and the domes are due to uplift of the centers of the craters as a result of layers in the crust with different mechanical properties. The fact that some craters on these icy bodies are anomalous has been ascribed to a velocity effect, as higher

TABLE 1

The depth–diameter relations for craters on the terrestrial or silicate planets are given in Table 1. (Relations are in the form d = aDb , where d is apparent depth, D is rimcrest diameter, and units are in kilometers.). Other relations involving parameters such as rim height, rim width, central peak diameter, and central peak height can be found in the literature. Due to the abundant detailed imagery and low rate of crater-modifying process, such as erosion, the bestdefined morphometric relations for fresh impact craters are from the Moon. Simple craters have similar apparent depth–diameter relationships on all the terrestrial planets (Table 1). At first glance, terrestrial craters appear to be shallower than their planetary counterparts. Compared to the other terrestrial planets, erosion is most severe on Earth, and crater rims are rapidly affected by erosion. Few terrestrial craters have well-preserved rims, and it is common to measure terrestrial crater depths with respect to the ground surface, which is known and is assumed to erode more slowly. In the case of other planetary bodies, depths are measured most often by the shadow that the rim casts on the crater floor. That is, the topographic measure is a relative one between the rim crest and the floor. Thus, the measurements of depth for Earth and for other planetary bodies are not exactly the same. For the very few cases in which the rim is well preserved

Apparent Depth-Diameter Relations for Craters on the Terrestrial Planets

Planetary Body

Exponent (b)

Coefficient (a)

Gravity (cm −2 )

Simple Craters

Moon Mars Mercury Earth

1.010 1.019 0.995 1.06

0.196 0.204 0.199 0.13

162 372 378 981

Complex Central Peak Craters

Moon Mars Mercury Venus Earth Sedimentary Crystalline

0.301 0.25 0.415 0.30

1.044 0.53 0.492 0.40

162 372 378 891

0.12 0.15

0.30 0.43

981 981

CLBE001-ESS2E

November 10, 2006

18:1

Planetary Impacts

in terrestrial craters, depths from the top of the rim to the crater floor are comparable to those of similar-sized simple craters on the other terrestrial planets. Unlike simple craters, the depths of complex craters with respect to their diameters do vary between the terrestrial planets (Table 1). While the sense of variation is that increasing planetary gravity shallows final crater depths, this is not a strict relationship. For example, martian complex craters are shallower than equivalent-sized mercurian craters (Table 1), even though the surface gravities of the two planets are very similar. This is probably a function of differences between target materials, with the trapped volatiles and relatively abundant sedimentary deposits making Mars’ surface, in general, a weaker target. Mars has also evidence of wind and water processes, which will reduce crater-related topography by erosion and sedimentary infilling. The secondary effect of target strength is also well illustrated by the observation that terrestrial complex craters in sedimentary targets are shallower than those in crystalline targets (Table 1). Data from the Galileo mission indicates that depth– diameter relationships for craters on the icy satellites Callisto, Europa, and Ganymede have the same general trends as those on the rocky terrestrial planets. Interestingly, the depth–diameter relationship for simple craters is equivalent to that on the terrestrial planets. Although the surface gravities of these icy satellites is only 13–14% of that of the Earth, the transition diameter to complex crater forms occurs at ∼3 km, similar to that on the Earth. This may be a reflection of the extreme differences in material properties between icy and rocky worlds. There are also inflections and changes in the slopes of the depth–diameter relationships for the complex craters, with a progressive reduction in absolute depth at diameters larger than the inflection diameter. These anomalous characteristics of the depth– diameter relationship have been attributed to changes in the physical behavior of the crust with depth and the presence of subsurface oceans. [See Europa; Ganymede and Callisto.]

2. Impact Processes The extremely brief timescales and extremely high energies, velocities, pressures, and temperatures that accompany impact are not encountered, as a group, in other geologic processes and make studying impact processes inherently difficult. Small-scale impacts can be produced in the laboratory by firing projectiles at high velocity (generally below about 8 km s−1 ) at various targets. Some insights can also be gained from observations of high-energy, including nuclear explosions. Most recently, “hydrocode” numerical models have been used to simulate impact crater formation. The planetary impact record also provides constraints on the process. The terrestrial record is an important source of

819

ground-truth data, especially with regard to the subsurface nature and spatial relations at impact craters, and the effects of impact on rocks. When an interplanetary body impacts a planetary surface, it transfers about half of its kinetic energy to the target. The kinetic energy of such interplanetary bodies is extremely high, with the mean impact velocity on the terrestrial planets for asteroidal bodies ranging from ∼12 km s−1 for Mars to over ∼25 km s−1 for Mercury. The impact velocity of comets is even higher. Long-period comets (those with orbital periods greater than 200 years) have an average impact velocity with Earth of ∼55 km s−1 , whereas short-period comets have a somewhat lower average impact velocity. [See Cometary Dynamics.]

2.1 Crater Formation On impact, a shock wave propagates back into the impacting body and also into the target. The latter shock wave compresses and heats the target, while accelerating the target material (Fig. 11). The direction of this acceleration is perpendicular to the shock front, which is roughly hemispherical, so material is accelerated downward and outward. Because a state of stress cannot be maintained at a free surface, such as the original ground surface or the edges and rear of the impacting body, a series of secondary release or “rarefaction” waves are generated, which bring the shock-compressed materials back to ambient pressure. As the rarefaction wave interacts with the target material, it alters the direction of the material set in motion by the shock wave, changing some of the outward and downward motions in the relatively near-surface materials to outward and upward, leading to the ejection of material and the growth of a cavity. Directly below the impacting body, however, the two wave fronts are more nearly parallel, and material is still driven downward (Fig. 11). These motions define the cratering flow-field and a cavity grows by a combination of upward ejection and downward displacement of target materials. This “transient cavity” reaches its maximum depth before its maximum radial dimensions, but it is usually depicted in illustrations at its maximum growth in all directions (Fig. 11). At this point, it is parabolic in cross section and, at least for the terrestrial case, has a depth-to-diameter ratio of about 1 to 3. As simple craters throughout the solar system appear to have similar depth–diameter ratios, the 1:3 ratio for the transient cavity can probably be treated as universal. An asteroidal body of density 3 g cm−3 impacting crystalline target rocks at 25 km s−1 will generate initial shock velocities in the target faster than 20 km s−1 , with corresponding velocities over 10 km s−1 for the materials set in motion by the shock wave. The rarefaction wave has an initial velocity similar to that of the shock wave but, because the target materials are compressed by the shock, the rarefaction has a smaller distance to cover to overtake

CLBE001-ESS2E

November 10, 2006

18:1

820 Encyclopedia of the Solar System

FIGURE 11 Schematic illustration of the formation of a simple crater (Figs. 1 and 2). (a) On impact, the shock wave, indicated by the roughly hemispherical solid lines of shock pressure, propagates into the target rocks. Closer to the point of impact, the combination of the motions imparted by the shock and rarefaction waves has opened up a growing cavity through excavation and displacement of the target rocks. Melted and vaporized material is driven down into this expanding transient cavity. Ultimately, target rocks set in motion by the cratering flow-field will follow the paths outlined by the solid lines with arrows. (b) Close to the end of formation of the transient cavity formed by the cratering flow-field, with melted and shocked target rocks that are moving up the walls on their way to being ejected. (c) The unstable transient cavity walls collapse downward and inward, carrying the lining of melt and shocked target rocks into the cavity and mix them together with the wall rocks to form a breccia deposit. The collapse of the cavity walls also enlarges slightly the diameter of the final crater. (d) Final form of a simple crater with an interior breccia lens. (After Melosh, 1989.)

the moving material and alter its direction of movement. Transient-cavity growth is an extremely rapid event. For example, the formation of a 2.5 km diameter transient cavity will take only about 10 seconds on Earth. The cratering process is sometimes divided into stages: initial contact and compression, excavation, and modification. In reality, however, it is a continuum with different volumes of the target undergoing different stages of the cratering process at the same time (Fig. 11). As the excavation stage draws to a close, the direction of movement of target material changes from outward to inward, as the unstable transient cavity collapses to a final topographic form more in equilibrium with gravity. This is the modification stage, with collapse ranging from landslides on the cavity walls of the smallest simple craters to complete collapse and modification of the transient cavity, involving the uplift of the center and collapse of the rim area to form central peaks and terraced, structural rims in larger complex craters. The interior breccia lens of a typical simple crater is the result of this collapse. As the cratering flow comes to an end, the fractured and over-steepened cavity walls become unstable and collapse inward, carrying with them a lining of shocked and melted debris (Fig. 11). The inward-collapsing

walls undergo more fracturing and mixing, eventually coming to rest as the bowl-shaped breccia lens of mixed unshocked and shocked target materials that partially fill simple craters (Fig. 11). The collapse of the walls increases the rim diameter, such that the final crater diameter is about 20% larger than that of the transient cavity. This is offset by the shallowing of the cavity accompanying production of the breccia lens, with the final apparent crater being about half the depth of the original transient cavity (Fig. 11). The collapse process is rapid and probably takes place on timescales comparable to those of transient-cavity formation. Much of our understanding of complex-crater formation comes from observations at terrestrial craters, where it has been possible to trace the movement of beds to show that central peaks are the result of the uplift of rocks from depth (Fig. 4). Shocked target rocks, analogous to those found in the floors of terrestrial simple craters, constitute the central peak at the centers of complex structures, with the central structure representing the uplifted floor of the original transient cavity. The amount of uplift determined from terrestrial data corresponds to a value of approximately one tenth of the final rim-crest diameter. Further observations at terrestrial complex craters indicate excavation is also limited

CLBE001-ESS2E

November 10, 2006

18:1

Planetary Impacts

to the central area and that the transient cavity diameter was about 50–65% of the diameter of the final crater. Radially beyond this, original near-surface units are preserved in the down-dropped annular floor. The rim area is a series of fault terraces, progressively stepping down to the floor (Fig. 3). Although models for the formation of complex craters are less constrained than those of simple craters, there is a general consensus that, in their initial stages, complex craters were not unlike simple craters. At complex craters, however, the downward displacements in the transient cavity floor observed in simple craters are not locked in and the cavity floor rebounds upward (Fig. 12). As the maximum depth of the transient cavity is reached before the cavity’s maximum diameter, it is likely that this rebound and reversal of the flow-field in the center of a complex crater occurs while the diameter of the transient cavity is still growing by excavation (Fig. 12). With the upward movement of material in the transient cavity’s floor, the entire rim area of the transient cavity collapses downward and inward (Fig. 12), greatly enlarging the crater’s diameter compared to that of the transient cavity. There have been a number of reconstructions of large lunar craters, in which the terraces are restored to their original, pre-impact positions, resulting in estimated transient cavity diameters of about 60% of the final rim-crest diameter. It is clear that

821

uplift and collapse, during the modification stage at complex craters, is extremely rapid and that the target materials behave as if they were very weak. A number of mechanisms, including “thermal softening” and “acoustic fluidization,” by which strong vibrations cause the rock debris to behave as a fluid, have been suggested as mechanisms to produce the required weakening of the target materials. There is less of a consensus on the formation of rings within impact basins. The most popular hypothesis for central peak basins is that the rings represent uplifted material in excess of what can be accommodated in a central peak (Fig.12). This may explain the occurrence of both peaks and rings in central peak basins but offers little explanation for the absence of peaks and the occurrence of only rings in peak ring and multiring basins. A number of analogies have been drawn with the formation of “craters” in liquids and semiconsolidated materials such as muds, where the initial uplifted peak of material has no strength and collapses completely, sometimes oscillating up and down several times. At some time in the formation of ringed basins, however, the target rocks must regain their strength, so as to preserve the interior rings. An alternative explanation is that the uplift process proceeds, as in central peak craters, but the uplifted material in the very center is essentially fluid due to impact melting. In large impact events, the depth of impact melting may reach and even exceed the depth of the transient cavity floor. When the transient cavity is uplifted in such events, the central, melted part has no strength and, therefore, cannot form a positive topographic feature, such as a central peak. Only rings from the unmelted portion of the uplifted transient cavity floor can form some distance out from the center (Fig. 5).

2.2 Changes in the Target Rocks

FIGURE 12 Schematic illustration of the formation of complex crater forms: (a) central peak crater (Figs. 3 and 4) and (b) peak ring basin (Fig. 5). Initial excavation and displacement by the cratering flow-field are similar to that of a simple crater (Fig. 11). The downward displacement of the target rocks is permanent, but not locked in, and the floor of the transient cavity is uplifted, even as the transient cavity diameter continues to grow in diameter. As the floor rises, the rim of the transient cavity collapses downward and inward to create a final rim that is a structural set of faulted terraces, considerably enlarging the final rim diameter. Excavation of target material is limited to the central area, and the extensive modification of the transient cavity leads to a final crater with a flat floor and topographically uplifted target material in the center. In the case of the peak ring basin (b), the uplifted material is in excess of what can be accommodated in a central peak and it collapses to form a peak ring. (After Melosh, 1989.)

The target rocks are initially highly compressed by the passage of the shock wave, transformed into high-density phases, and then rapidly decompressed by the rarefaction wave. As a result, they do not recover fully to their preshock state but are of slightly lower density, with the nature of their constituent minerals changed. The collective term for these shock-induced changes in minerals and rocks is shock metamorphism. Shock metamorphic effects are found naturally in many lunar samples and meteorites and at terrestrial impact craters. They have also been produced in nuclear explosions and in the laboratory, through shockrecovery experiments. No other geologic process is capable of producing the extremely high transient pressures and temperatures required for shock metamorphism, and it is diagnostic of impact. Metamorphism of rocks normally occurs in planetary bodies as a consequence of thermal and tectonic events originating within the planet. The maximum pressures and temperatures recorded in surface rocks by such metamorphic events in planetary crusts are generally on the order of

CLBE001-ESS2E

November 10, 2006

18:1

822 Encyclopedia of the Solar System 1 GPa (10 kb) and 1000◦ C. During shock metamorphism, materials deform along their “Hugoniot curves,” which describe the locus of pressure–volume states achieved by the material while under shock compression. Shock metamorphic effects do not appear until the material has exceeded its “Hugoniot elastic limit (HEL),” which is on the order of 5–10 GPa for most geologic materials. This is the pressure–volume point beyond which the shocked material no longer deforms elastically and permanent changes are recorded on recovery from shock compression. The peak pressures generated on impact control the upper limit of shock metamorphism. These vary with the type of impacting body and target material but are principally a function of impact velocity, reaching into the hundreds to thousands of GPa. For example, the peak pressure generated when a stony asteroidal body impacts crystalline rock at 15 km s−1 is over 300 GPa, not much less than the pressure at the center of the Earth (∼390 GPa). Shock metamorphism is also characterized by strain rates that are orders of magnitude higher than those produced by internal geologic

processes. For example, the duration of regional metamorphism associated with tectonism on Earth is generally considered to be in the millions of years. In contrast, the peak strains associated with the formation of a crater 20 km in diameter are attained in less than a second. 2.2.1 SOLID EFFECTS

At pressures below the HEL, minerals and rocks respond to shock with brittle deformation, which is manifested as fracturing, shattering, and brecciation. Such features are generally not readily distinguished from those produced by endogenic geologic processes, such as tectonism. There is, however, a unique, brittle, shock-metamorphic effect, which results in the development of unusual, striated, and horse-tailed conical fractures, known as shatter cones (Fig. 13). Shatter cones are best developed at relatively low shock pressures (5–10 GPa) and in fine-grained, structurally homogeneous rocks, such as carbonates, quartzites, and basalts.

FIGURE 13 Some shock metamorphic effects at terrestrial impact craters. (a) Shatter cones in basalt at the Slate Islands structure, Canada. (b) Photomicrograph of planar deformation features (e.g., in the left grain, thin parallel lines tending upwards to the right) in quartz from the Mistastin structure, Canada. Width of field of view is 0.5 mm, crossed polars. (c) Hand samples of target rocks from the Wanapitei structure, Canada, that are beginning to melt to form mixed mineral glasses and to vesiculate or froth. (d) Outcrop of coherent impact melt rock 80 m high, with columnar cooling joints, at the Mistastin structure, Canada.

CLBE001-ESS2E

November 10, 2006

18:1

Planetary Impacts

Apart from shatter cones, all other diagnostic shock effects are microscopic in character. The most obvious are planar deformation features and diaplectic glasses. Planar deformation features are intensely deformed, are a few micrometers wide, and are arranged in parallel sets (Fig. 13). They are best known from the common silicate minerals, quartz and feldspar, for which shock-recovery experiments has calibrated the onset shock pressures for particular crystal orientations. They develop initially at ∼10 GPa and continue to 20–30 GPa. The increasing effects of shock pressure are mirrored by changes in X-ray characteristics, indicative of the increasing breakdown of the internal crystal structure of individual minerals to smaller and smaller domains. By shock pressures of ∼30–40 GPa, quartz and feldspar are converted to diaplectic (from the Greek, “to strike”) glass. These are solid-state glasses, with no evidence of flow, that exhibit the same outline as the original crystal. For this reason, they are sometimes referred to as thetamorphic (from the Greek, “same shape”) glasses. The variety produced from plagioclase is known as maskelynite and was originally discovered in the Shergotty meteorite in 1872. The thermodynamics of shock processes are highly irreversible, so the pressure–volume work that is done during shock compression is not fully recovered upon decompression. This residual work is manifested as waste heat and, as a result, shock pressures of 40–50 GPa are sufficient to initiate melting in some minerals (Fig. 13). For example, feldspar grains show incipient melting and flow at shock pressures of ∼45 GPa. Melting tends initially to be mineral specific, favoring mineral phases with the highest compressibilities and to be concentrated at grain boundaries, where pressures and temperatures are enhanced by reverberations of the shock wave. As a result, highly localized melts of mixed mineral compositions can arise. The effects of shock reverberations on melting are most obvious when comparing the pressures required to melt particulate materials, such as those that make up the lunar regolith [see The Moon], and solid rock of similar composition. Shock recovery experiments indicate that intergranular melts can occur at pressures as low as 30 GPa in particulate basaltic material, compared to 45 GPa necessary to melt solid basalt. Most minerals undergo transitions to dense, highpressure phases during shock compression. Little is known, however, about the mineralogy of the high-pressure phases, as they generally revert to their low-pressure forms during decompression. Nevertheless, metastable high-pressure phases are sometime preserved, as either high-pressure polymorphs of preexisting low-pressure phases or highpressure assemblages due to mineral breakdown. Some known high-pressure phases, such as diamond from carbon or stishovite from quartz (SiO2 ), form during shock compression. Others, such as coesite (SiO2 ), form by reversion of such minerals during pressure release. Several high-pressure phases that have been noted in shocked me-

823

teorites, however, are relatively rare at terrestrial craters. This may be due to post-shock thermal effects, which are sufficiently prolonged at a large impact crater to inhibit preservation of metastable phases. 2.2.2 MELTING

The waste heat trapped in shocked rocks is sufficient to result in whole-rock melting above shock pressures of ∼60 GPa. Thus, relatively close to the impact point, a volume of the target rocks is melted and can even be vaporized (Figs. 11 and 12). Ultimately, these liquids cool to form impact melt rocks. These occur as glassy bodies in ejecta and breccias, as dikes in the crater floor, as pools and lenses within the breccia lenses of simple craters (Figs. 2 and 11), and as annular sheets surrounding the central structures and lining the floors of complex craters and basins (Figs. 4, 12, and 13). Some terrestrial impact melt rocks were initially misidentified as having a volcanic origin. In general, however, impact melt rocks are compositionally distinct from volcanic rocks. They have compositions determined by a mixture of the compositions of the target rocks, in contrast to volcanic rocks that have compositions determined by internal partial melting of more mafic and refractory progenitors within the planetary body’s mantle or crust. Impact melt rocks can also contain shocked and unshocked fragments of rocks and minerals. During the cratering event, as the melt is driven down into the expanding transient cavity (Figs. 11 and 12), it overtakes and incorporates less-shocked materials such as clasts, ranging in size from small grains to large blocks. Impact melt rocks that cool quickly generally contain large fractions of clasts, while those that cool more slowly show evidence of melting and resorption of the clastic debris, which is possible because impact melts are initially a superheated mixture of liquid melt and vapor. This is another characteristic that sets impact melt rocks apart from volcanic rocks, which are generally erupted at their melting temperature and no higher.

3. Impacts and Planetary Evolution As the impact flux has varied through geologic time, so has the potential for impact to act as an evolutionary agent. The ancient highland crust of the Moon records almost the complete record of cratering since its formation. Crater counts combined with isotopic ages on returned lunar samples have established an estimate of the cratering rate on the Moon and its variation with time. Terrestrial data have been used to extend knowledge of the cratering rate, at least in the Earth–Moon system, to more recent geologic time. The lunar data are generally interpreted as indicating an exponential decrease in the rate until ∼4.0 billion years (Ga) ago, a slower decline for an additional billion years, and a relatively constant rate, within a factor of two, since ∼3.0 Ga

CLBE001-ESS2E

November 10, 2006

18:1

824 Encyclopedia of the Solar System ago. The actual rate before ∼4.0 Ga ago is imprecisely known, as there is the question of whether the ancient lunar highlands reflect all of the craters that were produced (i.e., a production population) or only those that have not been obliterated by subsequent impacts (i.e., an equilibrium population). Thus, it is possible that the oldest lunar surfaces give only a minimum estimate of the ancient cratering rate. Similarly, there is some question as to whether the largest recorded events, represented by the major multiring basins on the Moon, occurred over the relatively short time period of 4.2–3.8 Ga ago (the “called lunar cataclysm”) or were spread more evenly with time. [See The Moon.]

3.1 Impact Origin of Earth’s Moon The impacts of the greatest magnitude dominate the cumulative effects of the much more abundant smaller impacts in terms of affecting planetary evolution. In the case of Earth, this would be the massive impact that likely produced the Moon. Earth is unique among the terrestrial planets in having a large satellite and the origin of the Moon has always presented a problem. The suggestion that the Moon formed from a massive impact with Earth was originally proposed some 30 years ago, but, with the development of complex numerical calculations and more efficient computers, it has been possible more recently to model such an event. Most models involve the oblique impact of a Mars-sized object with the proto-Earth, which produces an Earth-orbiting disk of impact-produced vapor, consisting mostly of mantle material from Earth and the impacting body. This disk, depleted in volatiles and enriched in refractory elements, would cool, condense, and accrete to form the Moon. [See The Moon.] In the computer simulations, very little material from the iron core of the impacting body goes into the accretionary disk, accounting for the low iron and, ultimately, the small core of the Moon. In addition to the formation of the Moon, the effects of such a massive impact on the earliest Earth itself would have been extremely severe, leading to massive remelting of Earth and loss of any existing atmosphere.

3.2 Early Crustal Evolution Following planetary formation, the subsequent high rate of bombardment by the remaining “tail” of accretionary debris is recorded on the Moon and the other terrestrial planets and the icy satellites of the outer solar system that have preserved some portion of their earliest crust. Due to the age of its early crust, the relatively large number of space missions, and the availability of samples, the Moon is the source of most interpretations of the effects of such an early, high flux. In the case of the Moon, a minimum of 6000 craters with diameters greater than 20 km are believed to have been formed during this early period. In addition, ∼45 impacts produced basins, ranging in diameter from Bailly at 300 km, through the South Pole–Aitken Basin at 2600 km,

to the putative Procellarum Basin at 3500 km, the existence of which is still debated. The results of the Apollo missions demonstrate clearly the dominance of impact in the nature of the samples from the lunar highlands. Over 90% of the returned samples from the highlands are impact rock units, with 30–50% of the hand-sized samples being impact melt rocks. The dominance of impact as a process for change is also reflected in the age of the lunar highland samples. The bulk of the near-surface rocks, which are impact products, are in the range of 3.8–4.0 Ga old. Only a few pristine, igneous rocks from the early lunar crust, with ages >3.9 Ga, occur in the Apollo collection. Computer simulations indicate that the cumulative thickness of materials ejected from major craters in the lunar highlands is 2–10 km. Beneath this, the crust is believed to be brecciated and fractured by impacts to a depth of 20–25 km. The large multiring basins define the major topographic features of the Moon. For example, the topography associated with the Orientale Basin (Fig. 6), the youngest multiring basin at ∼3.8 Ga and, therefore, the basin with the least topographic relaxation, is over 8 km, somewhat less than Mt. Everest at ∼9 km. The impact energies released in the formation of impact basins in the 1000 km size range are on the order of 1027 –1028 J, one to ten million times the present annual output of internal energy of Earth. The volume of crust melted in a basin-forming event of this size is on the order of a 1 × 106 km3 . Although the majority of crater ejecta is generally confined to within ∼2.5 diameters of the source crater, this still represents essentially hemispheric redistribution of materials in the case of an Orientale-sized impact on the Moon. Following formation, these impact basins localized subsequent endogenic geologic activity in the form of tectonism and volcanism. A consequence of such a large impact is the uplift of originally deep-seated isotherms and the subsequent tectonic evolution of the basin, and its immediate environs is then a function of the gradual loss of this thermal anomaly, which could take as long as a billion years to dissipate completely. Cooling leads to stresses, crustal fracturing, and basin subsidence. In addition to thermal subsidence, the basins may be loaded by later mare volcanism, leading to further subsidence and stress. All the terrestrial planets experienced the formation of large impact basins early in their histories. Neither Earth nor Venus, however, retains any record of this massive bombardment, so the cumulative effect of such a bombardment on the Earth is unknown. Basin-sized impacts will have also affected any existing atmosphere, hydrosphere, and potential biosphere. For example, the impact on the early Earth of a body in the 500 km size range, similar to the present day asteroids Pallas and Vesta, would be sufficient to evaporate the world’s present oceans, if only 25% of the impact energy were used in vaporizing the water. Such an event would have effectively sterilized the surface of Earth. The planet would have been enveloped by an atmosphere of hot rock and water vapor that would radiate heat downward

CLBE001-ESS2E

November 10, 2006

18:1

Planetary Impacts

onto the surface, with an effective temperature of a few thousand degrees. It would take thousands of years for the water-saturated atmosphere to rain out and reform the oceans. Models of impact’s potential to frustrate early development of life on Earth indicate that life could have survived in a deep marine setting at 4.2–4.0 Ga, but smaller impacts would continue to make the surface inhospitable until ∼4.0–3.8 Ga.

3.3 Biosphere Evolution Evidence from the Earth–Moon system suggests that the cratering rate had essentially stabilized to something approaching a constant value by 3.0 Ga. Although major basinforming impacts were no longer occurring, there were still occasional impacts resulting in craters in the size range of a few hundred kilometers. The terrestrial record contains remnants of the Sudbury, Canada, and Vredefort, South Africa, structures, which have estimated original crater diameters of ∼250 km and ∼300 km, respectively, and ages of ∼2 Ga. Events of this size are unlikely to have caused significant long-term changes in the solid geosphere, but they likely affected the biosphere of Earth. In addition to these actual Precambrian impact craters, a number of anomalous spherule beds with ages ranging from ∼2.0 to 3.5 Ga. have been discovered relatively recently in Australia and South Africa. Geochemical and physical evidence (shocked quartz) indicate an impact origin for some of these beds; at present, however, their source craters are unknown. If, as indicated, one of these spherule beds in Australia is temporally correlated to one in South Africa, its spatial extent would be in excess of 32,000 km2 . At present, the only case of a direct physical and chemical link between a large impact event and changes in the biostratigraphic record is at the “Cretaceous–Tertiary boundary,“ which occurred ∼65 million years (Ma) ago. The worldwide physical evidence for impact includes: shock-produced, microscopic planar deformation features in quartz and other minerals; the occurrence of stishovite (a high-pressure polymorph of quartz) and impact diamonds; high-temperature minerals believed to be vapor condensates; and various, generally altered, impact-melt spherules. The chemical evidence consists primarily of a geochemical anomaly, indicative of an admixture of meteoritic material. In undisturbed North American sections, which were laid down in swamps and pools on land, the boundary consists of two units: a lower one, linked to ballistic ejecta, and an upper one, linked to atmospheric dispersal in the impact fireball and subsequent fallout over a period of time. This fireball layer occurs worldwide, but the ejecta horizon is known only in North America. The Cretaceous–Tertiary boundary marks a mass extinction in the biostratigraphic record of the Earth. Originally, it was thought that dust in the atmosphere from the impact led to global darkening, the cessation of photosynthesis, and cooling. Other potential killing mechanisms have

825

been suggested. Soot, for example, has also been identified in boundary deposits, and its origin has been ascribed to globally dispersed wildfires. Soot in the atmosphere may have enhanced or even overwhelmed the effects produced by global dust clouds. Recently, increasing emphasis has been placed on understanding the effects of vaporized and melted ejecta on the atmosphere. Models of the thermal radiation produced by the ballistic reentry of ejecta condensed from the vapor and melt plume of the impact indicate the occurrence of a thermal-radiation pulse on Earth’s surface. The pattern of survival of land animals 65 Ma ago is in general agreement with the concept that this intense thermal pulse was the first global blow to the biosphere. Although the record in the Cretaceous–Tertiary boundary deposits is consistent with the occurrence of a major impact, it is clear that many of the details of the potential killing mechanism(s) and the associated mass extinction are not fully known. The “killer crater” has been identified as the ∼180 km diameter structure, known as Chicxulub, buried under ∼1 km of sediments on the Yucatan peninsula, Mexico. Variations in the concentration and size of shocked quartz grains and the thickness of the boundary deposits, particularly the ejecta layer, point toward a source crater in Central America. Shocked minerals have been found in deposits both interior and exterior to the structure, as have impact melt rocks, with an isotopic age of 65 Ma. Chicxulub may hold the clue to potential extinction mechanisms. The target rocks include beds of anhydrite (CaSO4 ), and model calculations for the Chicxulub impact indicate that the SO2 released would have sent anywhere between 30 billion and 300 billion tons of sulfuric acid into the atmosphere, depending on the exact impact conditions. Studies have shown that the lowering of temperatures following large volcanic eruptions is mainly due to sulfuric-acid aerosols. Models, using both the upper and lower estimates of the mass of sulfuric acid created by the Chicxulub impact, lead to a calculated drop in global temperature of several degrees Celsius. The sulfuric acid would eventually return to Earth as acid rain, which would cause the acidification of the upper ocean and potentially lead to marine extinctions. In addition, impact heating of nitrogen and oxygen in the atmosphere would produce NOx gases that would affect the ozone layer and, thus, the amount of ultraviolet radiation reaching the Earth’s surface. Like the sulfur-bearing aerosols, these gases would react with water in the atmosphere to form nitric acid, which would result in additional acid rains. The frequency of Chicxulub-size events on Earth is on the order of one every ∼100 Ma. Smaller, but still significant, impacts occur on shorter timescales and could affect the terrestrial climate and biosphere to varying degrees. Some model calculations suggest that dust injected into the atmosphere from the formation of impact craters as small as 20 km could produce global light reductions and temperature disruptions. Such impacts occur on Earth with a frequency of approximately two or three every million years

CLBE001-ESS2E

November 10, 2006

18:1

826 Encyclopedia of the Solar System but are not likely to have a serious affect upon the biosphere. The most fragile component of the present environment, however, is human civilization, which is highly dependent on an organized and technologically complex infrastructure for its survival. Though we seldom think of civilization in terms of millions of years, there is little doubt that if civilization lasts long enough, it could suffer severely or even be destroyed by an impact event. Impacts can occur on historical timescales. For example, the Tunguska event in Russia in 1908 was due to the atmospheric explosion of a relatively small body at an altitude of ∼10 km. The energy released, based on that required to produce the observed seismic disturbances, has been estimated as being equivalent to the explosion of ∼10 megatons of TNT. Although the air blast resulted in the devastation of ∼2000 km2 of Siberian forest, there was no loss of human life. Events such as Tunguska occur on timescales of a thousand of years. Fortunately, 70% of the Earth’s surface is ocean and most of the land surface is not densely populated.

Nevertheless, under conditions of rapid protection from weathering processes, it may be possible to find other types of impactor remnants associated with larger and older impact structures. This may be the case for a carbonaceous chondrite discovered at the Cretaceous–Tertiary boundary in a sedimentary core from the Pacific Ocean and inferred to be a small fragment of the impactor responsible for the Chicxulub structure. There are two other terrestrial cases where the physical presence of impactor-derived fragments has been inferred in larger impacts: East Clearwater, Canada (D = 22 km) and Morokweng, South Africa (D = 70 km). In both cases, however, the possible impactor materials have been reprocessed by their residence in impact melt rocks. The melt rocks at these craters have the highest known chemical admixture of impactor material of all terrestrial impact melt rocks (see later). Perhaps surprisingly, although there is no appreciable weathering on the Moon, few impactor fragments have been reported from the Apollo collection of lunar samples, although on the basis of geochemistry the lunar regolith is believed to contain a few percent of meteoritic material.

4. Planetary Impactors Apart from inferences from the compositions of asteroids, comets, and meteorites, the specific identification of actual impacting bodies is limited to occasional evidence from samples in or near craters on the Earth and Moon. For the majority of the ∼170 impact craters so far identified on the Earth, however, the impactor types are either unknown or the identification is uncertain. The case for the Moon is no better. There are two methods used to determine projectile types: the physical identification of impactor fragments associated with a crater and identification of geochemical traces of an impactor component within impact melt rocks.

4.1 Physical Identification of Impactors Although there is a widespread belief that the impactor is completely vaporized in large-scale impacts, this is not supported by numerical modeling. For example, at impact angles of ∼45◦ or lower and velocities of 20 km s−1 , less than 50% of the impactor’s mass vaporizes and the remaining fraction “survives” the impact, as melt or solid, and is deposited within or down range of the crater. Unfortunately, impactor fragments are rarely found associated with terrestrial impact craters. Any exposed remnants of the impactor are strongly affected by weathering processes and are normally destroyed after a few thousand years. As a result, virtually all impactor fragments have been found in the vicinity of very young terrestrial impact craters. Due to the size–frequency relation for impacts, these craters are also relatively small (50 AU, large eccentricity, and perihelion distance large enough to avoid destabilizing encounters with Neptune. The apparent similarity with the orbits of objects in the scattered disk suggests that the latter extended further in perihelion distance in the past, due to a different

CLBE001-ESS2E

November 10, 2006

23:56

Glossary

orbital architecture of the planets or of the environment of the solar system. The most prominent members of the extended scattered disk population are 2000 CR105 and 90377 Sedna. External comet A returning comet with a semimajor axis greater than ∼34.2 AU. External comets have Tisserand parameters with respect to Jupiter less than 2. Also known as a long-period comet. Extrasolar planet than the Sun. Feldspar

A planetary companion to a star other

A common group of aluminum silicate minerals.

Filaments Near-horizontal magnetic field lines on the Sun suspended above magnetic inversion lines that are filled with cool and dense chromospheric mass, seen on the solar disk. Flares A magnetic instability in the solar corona that impulsively releases large energies that go into heating of coronal and chromospheric plasma, as well as into acceleration of high-energy particles. A flare is usually accompanied by impulsive emission in gamma rays, hard x-rays, soft x-rays, EUV, and radio emission. Fluctus (pl., flucti) field.

Term meaning (on Io) a volcanic flow

Fluorescence Photons emitted immediately after electron decay. The electron had been elevated to a higher energy state by external stimulation of their parent atoms, ions, and molecules. In planetary atmospheres, the external stimulation is usually sunlight or electrons. Flux density Power per unit area and per unit frequency interval received from an object. The units of flux density are Janskies: 1 Jy = 10−26 W m−2 Hz−1 . Flux transfer event A localized spatial region in which magnetic reconnection links the solar wind magnetic field to a planetary magnetic field producing a configuration that transports flux from the day side to the night side of the planet. Flux The flux of particles (denoted ϕ with units of cm−2 s−1 ) given by the product of the speed of the neutrons, v (cm/s), and the number density (particles per cm3 ). Fractionation Separation of elements or isotopes based on their masses or chemistry. Frequency–time spectrogram A graph of the emission intensity as a function of frequency and time. Usually the intensity is shown on a gray scale ranging from black to white or one of several color schemes, with frequency plotted along the vertical y-axis and time along the horizontal x -axis. Galactic cosmic rays Energetic particles, including photons, electrons, protons, and heavy ions, that originate outside the heliosphere.

925

the de-excitation of residual nuclei produced by nuclear reactions. Gas chromatography A chemical technique for separating gas mixtures, in which the gas is passed through a long column containing a fixed absorbent phase that separates the gas into its component parts. Gas drag Drag force experienced by a solid object when it moves through a surrounding gas. Gaussian year The orbital period of a massless particle in a circular orbit with a semimajor axis of 1 AU, equal to 365.256898326 . . . days. Formally, the Gaussian year is defined as 2 p/k, where k is the Gaussian gravitational constant, 0.01720209895. Geochemistry The study of the chemical components of the lithosphere of the Earth and other planets, chemical processes and reactions that produce and modify rocks and soils, and the cycles of matter and energy that transport chemical components in space and time. Geodesy The measurement and representation of Earth’s topography, its gravitational field and geodynamic phenomena (e.g., polar motion, tides, and crustal motion) in 3-dimensional, time-varying space. Geomagnetic activity Disturbances in the magnetized plasma of a magnetosphere associated with fluctuations of the surface field, auroral activity, reconfiguration and changing flows within the magnetosphere, strong ionospheric currents, and particle precipitation into the ionosphere. Geomagnetic storm The response of the Earth to the arrival of an interplanetary medium disturbance, usually associated with a CME. Geomagnetism The Earth’s magnetic field, which is approximately a magnetic dipole, with the magnetic poles offset from the corresponding geographic poles by approximately 11.3◦ , and extending several tens of thousands of kilometers into space. Geometric albedo Ratio of the brightness at a phase angle of zero degrees (full illumination) compared with a diffuse, perfectly reflecting disk of the same size and under the same illumination conditions. Geomorphology Science of landscape analysis. Geomorphic investigations deal with the processes and timescales of landscape formation and degradation. Geospace The Earth’s magnetosphere and upper atmosphere, including the ionosphere. Graben A long, usually linear fault trough (valley) produced by subsidence between two inward dipping boundary faults. It is the result of extensional stresses in a body’s upper crust.

Galilean satellites The four major satellites of Jupiter: Io, Europa, Ganymede and Callistio, discovered by Galileo in 1610.

Granite Light-colored intrusive rock containing more than 50% silica. On Earth, continents are largely granite and other high silica rocks.

Gamma ray A high energy quantum of electromagnetic radiation (photon) emitted by nuclear transitions. Gamma rays originate from nuclear processes, such as radioactive decay and

Gravitational focusing The tendency of an object’s trajectory to curve toward a massive body due to gravitational attraction.

CLBE001-ESS2E

November 10, 2006

23:56

926 Encyclopedia of the Solar System Gravitational instability Spontaneous collapse of a portion of a protoplanetary disk due to mutual gravitational attraction. This can refer to either the solid or gaseous component of the disk. Greenhouse effect Heating of a planetary surface above the temperature that it would have been in the absence of an atmosphere. The atmosphere transmits solar radiation in the visible, but impedes the escape of thermal infrared energy (usually due to absorbing clouds), thus creating the increased temperature. Gyro radius Radius of the orbit of a charged particle gyrating in a magnetic field. Gyrofrequency The frequency of the circular motion of a charged particle perpendicular to a magnetic field. Habitable zone The region of space around a star in which a geologically active, rocky planet can maintain liquid water on its surface. Hadley circulation A major component of atmospheric circulation driven directly by latitude-averaged heat sources and sinks. Warm air rises in regions near the equator, flows poleward at higher altitudes, and loses heat in the colder, higher latitude regions. The cooler, denser air then descends and has a flow component near the surface back toward the low-latitude heat source, which completes a circulation cell. The near-surface and high-altitude branches of the flow have eastward (“trade wind”) and westward components, respectively, arising from Coriolis forces. When the heat source is located on the equator, the Hadley circulation tends to be symmetric about the equator, but the Hadley circulation is asymmetric about the equator if the heat source is located off the equator, as occurs during solstice seasons on Earth and Mars. Halley-type comet A returning comet with a semimajor axis less than ∼34.2 AU. Halley-type comets have Tisserand parameters with respect to Jupiter less than 2. Heat flow Heat emitted (or received) at the surface of a body that is ultimately radiated to (or absorbed from) space. Heavy elements In astrophysics, all elements other than hydrogen and helium. Heliocentric

A Sun-centered coordinate system.

Heliopause Interface between the heliosphere and the interstellar plasma; the outer boundary of the heliosphere. Heliosphere The cavity carved in the interstellar plasma by the solar wind, containing the solar system and plasma and magnetic field of solar origin;

must consume other life forms to obtain the organic products necessary for life; e.g., animals, fungi, most bacteria. Hill sphere Region around a secondary in which the secondary’s gravity is more influential for the motion of a particle about the secondary than is the tidal influence of the primary. Hilly and lineated terrain The broken-up surface of Mercury at the antipode of the Caloris impact basin. Homopause Level in an atmosphere, above the stratosphere, at which gases cease being uniformly mixed and separate by diffusion, with the lighter elements diffusing upward. Horseshoe orbits Librating orbits encircling the L3 , L4 , and L5 Lagrangian points in the circular restricted three-body problem. These orbits appear to be shaped like horseshoes in the frame rotating with the mean motion of the system. Hot Jupiter An extrasolar gas giant planet at a very small orbital separation of 0.03–0.05 AU from its host star and with an orbital period of a few days. The proximity of the discovered hot Jupiters to their host stars is probably a result of inward orbital migration. Hot poles The alternating perihelion subsolar points on Mercury at the 0◦ and 180◦ meridians. Hot spots Regions of enhanced thermal emission on Io, a marker of volcanic activity. The term does not imply a particular eruption mechanism. Hugoniot elastic limit Stress at which a rock or mineral’s response to shock changes from elastic to plastic. Stresses above the Hugoniot elastic limit cause the rock or mineral to deform plastically. Hydrated A mineral in which water molecules or hydroxyl radicals are attached to the crystalline structure. Hydrodynamic escape A limiting case of atmospheric escape that occurs when the escape rate is so rapid that the atmosphere at high altitudes reaches an outward velocity comparable to the speed of sound. This occurs if the thermal energy of the gas molecules becomes comparable to the gravitational binding energy. Hydrodynamic escape allows the upper atmosphere of a planet to escape wholesale, as opposed to the usually slower processes of Jeans-type thermal leakage or solar wind ion pickup. Hydrogen cloud The huge cloud of atomic hydrogen surrounding most comets. The hydrogen cloud is produced by the dissociation of water and the hydroxyl molecule (OH). Hydrostatic equation Relationship that says pressure is equal to the weight of gas or liquid above the level of interest.

Heliospheric current sheet The surface in interplanetary space separating solar wind flows of opposite magnetic polarity; the interplanetary extension of the solar magnetic equator.

Hyperbolic orbit Unbound orbit in which the object escapes the gravitational attraction of the central body: examples are orbits of beta meteoroids and interstellar grains.

Heliospheric magnetic field Remnant of the solar magnetic field dragged into interplanetary space by the solar wind.

Hypsometry Geodetic observations of terrain elevations with respect to sea level.

Heterotrophy Literally, other-feeding; the condition of an organism that is not able to obtain nutrients by synthesizing nonorganic materials from the environment, and that therefore

Ice dwarf The term given to the planetesimals believed to have been created in large numbers during the formation of the giant planets and later scattered to the Oort cloud or ejected

CLBE001-ESS2E

November 10, 2006

23:56

Glossary

from the solar system by close encounters with the forming giant planets. Pluto and Triton are thought to be among the largest remnants of this population. Ice Mixture of water, ammonia, methane, and other volatile compounds in the interiors of jovian planets, not literally in the form of condensed “ice.” IDP Interplanetary dust particle, collected by aircraft in the stratosphere. Impact melt Melt of target rocks resulting from the waste heat generated in an impact event. When solidified, it can be either glassy or crystalline and contain clasts of rock and mineral debris from unmelted portions of the target. Inclination The angle between the plane of the orbit of a planet, comet, or asteroid and the ecliptic plane, or between a satellite’s orbit plane and the equatorial plane of its primary. Inclination takes on values between 0◦ and 180◦ . Insolation The flux of sunlight at all wavelengths falling on a body. For the Earth this amounts to a flux of 1.368× 106 ergs cm−2 s−1 . Integral of the motion Any function of the position and velocity coordinates of an object that remains constant with time along all orbits. In the circular restricted three-body problem, the Jacobi constant is an integral of the motion. The Jacobi constant can be approximated by the Tisserand parameter. Intercrater plains The oldest plains on Mercury that occur in the highlands and formed during the period of late heavy bombardment. Intrusion Geological structure of igneous material that forces its way into an existing formation. Invariable plane The plane passing through the center of mass of the solar system, which is perpendicular to its total angular momentum vector. The invariable plane is inclined 0.5◦ to the orbital plane of Jupiter and 1.6◦ to the ecliptic. Ionopause The surface separating ionospheric plasma and the solar wind in the vicinity of an unmagnetized planet. Ionosphere Outer portion of an atmosphere where charged particles are abundant. Isolation mass The mass of a planetary embryo if it sweeps up all the accessible solid material in its vicinity. Jacobi constant An integral of the motion in the circular restricted three-body problem. It is proportional to the total orbital energy of the small body in a reference frame rotating with the two massive bodies. Jeans escape The process by which fast (energetic or hot) molecules of an atmosphere escape into space. The energy distribution of a gas at a given temperature has a hot tail—a few atoms moving faster than the rest. If, at an altitude where collisions between molecules are rare, the molecules in the hot tail move faster than the local escape velocity, they can escape to space. This process is fastest for hot atmospheres of light gases (hydrogen, helium) on bodies with low gravity.

927

Jets The observed, collimated emission of gas and dust that occurs in restricted areas on the surface of a cometary nucleus. Jets are usually active on the sunlit side of the nucleus. Joule heating Heating that occurs when a current flows through a resistive medium. In the high atmospheres of the giant planets, it may be an important process in heating the atmosphere to high temperature as currents of charged particles driven by magnetospheric electric fields collide with the neutral atmosphere atoms, which provide resistance. Jovian planet A planet like Jupiter, which is composed mostly of hydrogen, with helium and other gases, but possibly with a silicate/iron core. Also called a gaseous or a giant planet. The jovian planets are Jupiter, Saturn, Uranus, and Neptune. Jupiter-family comet An ecliptic comet with a Tisserand parameter between 2 and 3. It is typically on a low to moderate inclination orbit, with a semimajor axis less than that of Jupiter’s orbit. Most Jupiter-family comets are in orbits that cross or closely approach Jupiter’s orbit. K or kelvin Unit of absolute temperature. The freezing and boiling points of water are 273.16 K and 373.16 K, respectively. Keplerian orbit The path that a body would follow if it were subject only to the gravitational attraction of its primary, e.g. a planet orbiting the Sun, a satellite orbiting a planet. Keplerian velocity The speed with which a solid body moves on a circular orbit about a larger body. Kepler’s laws Three rules that describe the unperturbed motion of planets about the Sun (and of moons about planets): (1) Planets move on elliptical paths with the Sun at one focus. (2) An imaginary line from the Sun to a planet sweeps out area at a constant rate. (3) The square of a planet’s orbital period varies as the cube of the semimajor axis of its orbit. Kirkwood gaps Zones in the asteroid belt that have been depleted of objects due to mean-motion orbital resonances with Jupiter. Klystron Vacuum-tube amplifier used in planetary radar transmitters. Kozai resonance A resonance where an object’s nodal precession rate is equal in magnitude and direction to its periapse precession rate. Objects within a Kozai resonance undergo oscillations in eccentricity and inclination that are out of phase (i.e., when one increases, the other decreases). Kozai resonances affect the motion of Pluto and some comets and asteroids in the solar system. Kuiper belt Generally used to refer to the population of trans-Neptunian bodies, i.e., those with semimajor axes >30 AU. In a more detailed classification, which partitions the trans-Neptunian population into the Kuiper belt, the scattered disk and the extended scattered disk, the name “Kuiper belt” is associated with a collection of bodies on essentially stable, low inclination, low eccentricity orbits. Almost all Kuiper belt objects discovered so far have semimajor axes