Scientific Debates in Space Science: Discoveries in the Early Space Era (Springer Praxis Books) 3031415973, 9783031415975

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Warren David Cummings Louis J. Lanzerotti

Scientific Debates in Space Science Discoveries in the Early Space Era

Springer Praxis Books

Astronomy and Planetary Sciences Series Editors Martin A. Barstow, Department of Physics & Astronomy, University of Leicester, Leicester, UK Ian Robson, UK Astronomy Technology Centre, Royal Observatory, Edinburgh, UK Steven N. Shore, Dipartimento di Fisica “Enrico Fermi”, Università di Pisa, PISA, Pisa, Italy Derek Ward-Thompson, Jeremiah Horrocks Institute of Maths, Physics and Astronomy, Preston, UK

Textbooks and monographs published in this series from 2013 onward are written for advanced undergraduate students in astronomy and the planetary sciences and advanced amateur astronomers. The editors insist on good readability and encourage new approaches to teaching astronomy and planetary sciences. Books published before 2013 serve a spectrum of readership: Some are at advanced amateur to advanced undergraduate level. Others are targeted at PhD students and researchers. Topics covered in the series include • • • • • • •

Astronomical telescopes and instrumentation Astronomical techniques, software and data Astrophysics, Astrochemistry, Astrobiology Solar system science (excluding the Earth sciences proper) and exoplanets Stellar physics and black hole astrophysics Galactic astronomy Extragalactic astronomy and cosmology

The books are well illustrated with line diagrams and photographs throughout, with targeted use of colour for scientific interpretation and understanding. Many feature worked examples or problems and solutions.

Warren David Cummings • Louis J. Lanzerotti

Scientific Debates in Space Science Discoveries in the Early Space Era

Warren David Cummings Retired from Universities Space Research Association Washington, DC, USA

Louis J. Lanzerotti Center for Solar-Terrestrial Research New Jersey Institute of Technology Newark, NJ, USA

Springer Praxis Books ISSN 2366-0090 (electronic) ISSN 2366-0082 Astronomy and Planetary Sciences ISBN 978-3-031-41597-5 ISBN 978-3-031-41598-2 (eBook) https://doi.org/10.1007/978-3-031-41598-2 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2023, corrected publication 2024 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Book cover figure: John Oertel, Green Pepper Media, LLC This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland Paper in this product is recyclable.

We dedicate this book to our wives, Mrs. Sue Johnston Cummings and Dr. Mary Yvonne Dewolf Lanzerotti.

Foreword

With the establishment of the National Aeronautics and Space Administration (NASA), the United States made a bid for a leadership role in space exploration and development. This was, in particular, a response to the Soviet Union’s early successes in space, including the launch of the first artificial satellite, Sputnik, in 1957. This establishment of NASA also came on the heels of the success of Explorer 1, on January 31, 1958. Explorer 1 was a significant achievement for the United States and marked the beginning of the country’s active participation in the Space Race—and also in the peaceful exploration of space. Most importantly, Explorer 1 whetted the appetite of a science community that could not wait to break away from Earth and start exploring space, from the near-Earth environment to the beginning of the Universe. The 1958 Space Act (Section 102c) establishes the objectives for NASA. The Act starts with the most foundational and aspirational objective, “the expansion of human knowledge of phenomena in the atmosphere and space.” This remains the first objective of the agency to this day. This book on the early scientific debates chronicles some of the early missions that followed. Yet, this approach is timely today, as NASA’s science program has become a key element of many disciplines, from quantum effects and fundamental biology in space to gaining a deeper understanding of the Big Bang and the formation of stars and galaxies that ultimately constitute our own history. Space observations are also foundational to our understanding of our own planet, its place in the solar system, and its changes due to natural and human impacts. Scientific debates were and remain an essential part of the scientific process and have value in multiple ways. On the one hand, they are key to advancing scientific knowledge, because they allow researchers to challenge and refine each other’s ideas and theories, leading to a better understanding of the natural world. Some of these discussions can turn personal at times, but such challenges are not only interesting but are also essential. Furthermore, scientific debates promote critical thinking by requiring researchers to evaluate and analyze evidence and arguments from different perspectives. Scientific debates can lead to collaborations between researchers with vii

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different areas of expertise, which can stimulate new and innovative research projects, including new and innovative tests of theories. But the most important benefits to scientific debates are those that are open and transparent. Such debates can help build trust in science by demonstrating that researchers are willing to acknowledge and address any uncertainties or controversies in their findings. Only by subjecting scientific findings to rigorous debate and scrutiny can researchers ensure that their results are accurate, reliable, and reproducible. Cummings and Lanzerotti take the reader on a journey through these early debates. As long-time leaders in science, both have been avid observers of these debates firsthand and, in some areas, have actively contributed observations that helped settle some of the discussions. Cummings was a long-term executive of the nonprofit organization called the Universities Space Research Association (USRA). USRA was motivated by then NASA Administrator James Webb and the National Academy of Sciences to be focused on building a science community ready for the opportunities and challenges that came from the beginning of the space age. Cummings has proven himself as a careful chronicler of the science community’s history during that time till today. Lanzerotti worked at the famed Bell Laboratories and with science missions ranging over decades. As an editor of multiple journals in space science, he also had a front-row seat to the serpentine road that so many of the debates took before a key experimental result finally settled the disagreement. The first debate I became aware of was in a church in Switzerland in the early 1970s, where I participated with my family. The pastor on the pulpit loudly pointed out that Professor Gold from Cornell had predicted a large depth of dust on the surface of the Moon, too deep for the Apollo lunar lander to land safely. During the most memorable TV show of the late 1960s—I am too young to remember, and our family never owned a TV—it was clear that Gold’s prediction about the lunar lander was in fact incorrect. The pastor was quick to point out that this implied that the Moon was young, inconsistent with the prevailing theories. I never forgot this sermon, and what I learned when, as a young astrophysicist, we examined Gold’s prediction, the age of the impact craters on the Moon, and the age of the entire solar system. Gold’s prediction was not stupid, or proof of a young Moon, but at the heart of some of the most exciting research about the most visible celestial body in the sky at night, and with it our own history. Being on the losing side of a scientific debate is not a sign of weakness but the very result of the scientific process. It may be difficult to understand that from the pulpit of a Swiss mountain village in the early 1970s, but the entire science community relies on those who dare to stand up for an opinion that remains unpopular. The scientific discourse is not a democratic process. It may well be that 99% of all scientists agree with a given explanation, and a new thought arises, often in a young scientist, and over time the entire community is proven wrong. Einstein’s papers in 1905 about special relativity is one such case, and —to a certain extent— also Eugene Parker’s 1958 paper on the solar wind as discussed in this book. The controversy surrounding the paper had negative consequences for Parker’s career. He only got his foundational paper published because the journal editor

Foreword

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Professor Subrahmanyan Chandrasekhar, his University of Chicago colleague, decided to set aside reports from referees, one of which recommended that Parker go into the library and learn more. When I joined NASA in 2016, one of the priorities was to celebrate Parker’s achievements in this and many other areas of plasma astrophysics, by naming the Solar Probe mission after him. Parker was the only person ever to see a mission launch with his name on it, and his excitement was discernable by everyone who was present when he was shown the first results of “his mission”: the Parker Solar Probe, exploring for the first time the variable solar wind near its source, the solar corona. This book is written by two senior members of the community, but I hope that many scientists starting their career in space science will read it. And I hope that these young scientists get three lessons from the book by Cummings and Lanzerotti. First, they will gain an appreciation for those who came before them, and the intellectual risks they took to get to the state of knowledge we are at today. Yes, it took debates between multiple opinions to gain deep insights and trust in the solution ultimately winning the day. Secondly, I hope that what they see in these stories will motivate early career researchers to “turn over rocks by the wayside,” as Charles Townes, who won the Nobel prize for his contributions that led to the laser, wrote in his book How the Laser Happened. He points out how so many scientists follow leaders and walk down the same path as others do. Great and promising research, he argues, comes from focusing on things others have missed (“rocks by the wayside”) and asking deep questions, even though they are neither popular and perhaps even wrong. Finally, I hope that early career scientists recognize that courage to challenge the status quo is at the heart of progress, and it is much better to be wrong than to be irrelevant when it comes to science.

Thomas Zurbuchen, former Associate Administrator for NASA’s Science Mission Directorate. Credit Thomas Zurbuchen, courtesy of NASA

ETH Zurich|Space, Zurich, Switzerland NASA Science Mission Directorate, Washington, DC, USA April 2023

Thomas H. Zurbuchen

Preface

Understanding of the physical and biological universe through scientific inquiry has long occupied human intelligence. The very nature of human scientific inquiry has always resulted in hypotheses and theories, often beginning with speculations, to explain phenomena observed. In recent centuries, the settlement of scientific disputes about theories and speculations has been through careful observations and measurements, often by persons dedicated to such inquiry. The opening of opportunities through rocketry for observations and measurements above Earth’s atmosphere resulted in a revolution in the seeking of understanding of many physical processes and phenomena that could only be previously speculated about or that were not even thought of. This book is about several of the important scientific debates that helped to develop space science in the early space era and led to significant new understandings of physical phenomena. Each chapter in the book focuses on a specific debate, some of which were more heated than others and led to prolonged controversies. While there is logic in the presentation of the chapters, from near-Earth outward to the universe, the chapters can be read independently, in any order. Most of the debates covered were prompted by scientific speculations on the part of one or more of the protagonists. The book is not a comprehensive history of space science, but it does recount several of the important turning points in its development. Nor does the book contain accounts of all the significant scientific debates that have occurred in the field since the advent of Earth-circling satellites in 1957–1958. No two space scientists are likely to come up with exactly the same list. The reader will get the benefit (or disadvantage) of our choice of topics. Our choice of chapter topics has been influenced by our own experiences as space scientists during the past 60 years. One of us (WDC) has published research related to Earth’s magnetosphere, taught college physics, and served for more than 30 years as Executive Director of the Universities Space Research Association (USRA) which has 117 research universities as its members. The other author (LJL) has been involved in a dozen space missions, either as a principal investigator or co-principal investigator, and was also involved through his primary employer in applications of space research findings to commercial interests. He has served on a xi

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number of space-focused advisory boards, including the NASA Advisory Council and as Chair of the Space Studies Board of the National Academies of Sciences, Engineering, and Medicine. He recently completed a 6-year term on the Board of Trustees of USRA. Through these professional experiences, we have come to know virtually all the protagonists that are featured in this book. A significant number of them had associations and involvements at various times with USRA, as might be expected, and USRA facilitated several important meetings. In some cases, we have witnessed at professional meetings heated exchanges associated with the debates that we describe. We augmented our personal knowledge by correspondences with some of the colleagues of deceased protagonists and by conducting an extensive search of the literature so that we could accurately reproduce the arguments of the principals, often quoting from their research publications. We decided to write about the debates that seemed important for the development of space science and for which we had some knowledge of the underlying subject matter. Though not by design, it turned out that our choice of topics included debates in three of the major sub-disciplines of space science, i.e., the science areas that are known as heliophysics (the study of the plasma and magnetic field flowing from the Sun, its impact on planetary bodies, and its interaction with the adjacent plasma and magnetic field of our galaxy), planetary science (including the science related to comets, asteroids, the Moon, and the moons of other planets), and space astronomy (astronomical and astrophysical studies that make use of space-based observations). We acknowledge with gratitude the help of our colleagues and other professionals, including university archivists, in the United States and Europe, in providing us with photos of many of the scientists involved in the development of space science. Finally, we are grateful for the encouragement and support of our sponsoring institutions—USRA for WDC and the New Jersey Institute of Technology for LJL. We are also indebted to the comments and reviews of our colleagues in space science, but we take responsibility for the conclusions drawn in the book and any possible, though inadvertent, misrepresentations contained in it. Washington, DC, USA Newark, NJ, USA

David Cummings Louis J. Lanzerotti

Contents

1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 The Beginning of Space Science . . . . . . . . . . . . . . . . . . . . . . 1.2 Eddington and Scientific Speculation . . . . . . . . . . . . . . . . . . . 1.3 The Eddington-Jeans Debate . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Eddington’s Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . .

1 1 4 6 8 10

2

Solar Wind or Solar Breeze? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Background: Evidence of Solar-Terrestrial Links . . . . . . . . . . . . 2.2.1 Magnetic Storms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Comets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 The Theories of Parker and Chamberlain . . . . . . . . . . . . . . . . . 2.3.1 Parker’s Solution: The Solar Wind . . . . . . . . . . . . . . . 2.3.2 Chamberlain’s Evaporation Model . . . . . . . . . . . . . . . . 2.3.3 Parker’s Response to Chamberlain . . . . . . . . . . . . . . . 2.3.4 Chamberlain’s Response to Parker: The Solar Breeze . . 2.4 The Direct Measurements of the Solar Wind . . . . . . . . . . . . . . . 2.4.1 Luna Missions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2 Venera 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.3 Explorer 10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.4 Mariner 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Eddington’s Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Continuing Understanding . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11 11 11 12 18 21 21 23 23 25 27 27 28 28 29 30 31 33

3

Open Versus Closed Magnetosphere . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Some Precursors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Dungey’s Speculation of an Open Magnetosphere . . . . . . . . . . . 3.3.1 Giovanelli’s Research . . . . . . . . . . . . . . . . . . . . . . . . .

37 37 37 39 39 xiii

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3.3.2 The Contributions of Hoyle . . . . . . . . . . . . . . . . . . . . 3.3.3 The Contributions of Sweet and Parker . . . . . . . . . . . . 3.4 Dungey’s 1961 Paper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Petschek’s Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 The Controversies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.1 Parker’s Reservations . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.2 Dessler’s Objections . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 The Evidence and Reconsiderations . . . . . . . . . . . . . . . . . . . . . 3.7.1 Dessler’s Vacuum Merging Model . . . . . . . . . . . . . . . 3.7.2 Analyses of Fairfield, Arnoldy, Russell and McPherron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.3 The ISEE Evidence . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.4 The Evidence from Polar and Cluster . . . . . . . . . . . . . . 3.7.5 The MMS Evidence . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 Eddington’s Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9 Continuing Understanding . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

40 43 44 46 48 48 49 53 53

Influx of Small Comets into Earth’s Atmosphere . . . . . . . . . . . . . . 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 The Beginning of the Controversy . . . . . . . . . . . . . . . . . . . . . . 4.2.1 The Decision of a Journal Editor . . . . . . . . . . . . . . . . . 4.2.2 The Initial Papers by Frank and His Colleagues . . . . . . 4.2.3 A Deluge of Comments Following the Opening Papers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Editorial Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Dessler’s Article . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Paul Feldman’s Article . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Initial Attempts by Others to Find Small Comets . . . . . . . . . . . . 4.4.1 The Viking Sweden Satellite Images . . . . . . . . . . . . . . 4.4.2 Cragin’s Comments on the Claims That Atmospheric Holes Are Seen in the Viking Data . . . . . . . . . . . . . . . 4.4.3 The University of Arizona Spacewatch Camera at Kitt Peak . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Publication of The Big Splash . . . . . . . . . . . . . . . . . . . . . . . . . 4.6 The Review Papers of Dessler and of Frank . . . . . . . . . . . . . . . 4.7 Polar Satellite Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.1 Frank and Sigwarth Analyses of Polar Data . . . . . . . . . 4.7.2 The Washington Post Article . . . . . . . . . . . . . . . . . . . . 4.7.3 Analyses of Polar Data by Parks and Colleagues . . . . . 4.7.4 Computer Simulations of Polar Pixel Responses by the Berkeley Group . . . . . . . . . . . . . . . . . . . . . . . . 4.7.5 The Response of Frank and Sigwarth . . . . . . . . . . . . . . 4.8 Search for Small Comets Using Radar . . . . . . . . . . . . . . . . . . . 4.9 Orbital Analysis Critique of Harris . . . . . . . . . . . . . . . . . . . . . . 4.10 The Iowa Optical Search for Small Comets . . . . . . . . . . . . . . . .

67 67 67 67 68

54 55 58 59 61 63 63

70 70 70 70 73 73 74 75 76 77 79 80 83 83 88 90 93 95 96

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4.11 The Tragic Aftermath . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 4.12 Eddington’s Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 5

Origin of the Moon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Pre Space-Age Speculations on the Origin of the Moon . . . . . . . 5.2.1 Co-accretion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2 Fission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.3 Capture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 The Initial Speculation of a Giant Impact . . . . . . . . . . . . . . . . . 5.4 The Apollo Explorations of the Moon . . . . . . . . . . . . . . . . . . . 5.4.1 The Formation of the Lunar Science Institute . . . . . . . . 5.4.2 Key Results of the Apollo Explorations . . . . . . . . . . . . 5.5 The Various Theories of the Origin of the Moon . . . . . . . . . . . . 5.5.1 Urey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.2 Ringwood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.3 O’Keefe and Wise . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6 The Giant Impact Speculation . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.1 Hartmann and Davis . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.2 Cameron and Ward . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.3 Clayton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.4 Safronov and Wetherill . . . . . . . . . . . . . . . . . . . . . . . . 5.6.5 The Kona Conference . . . . . . . . . . . . . . . . . . . . . . . . . 5.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8 Eddington’s Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.9 Continuing Understanding . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

107 107 108 109 111 113 114 116 116 117 118 118 119 119 121 121 122 123 124 126 128 131 133 134

6

Lunar Dust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Background on the Nature of the Lunar Surface . . . . . . . . . . . . 6.2.1 The Majority View: Lunar Volcanoes . . . . . . . . . . . . . 6.2.2 The Minority View: Impact Craters . . . . . . . . . . . . . . . 6.3 Gold’s Speculation and the Ensuing Controversy . . . . . . . . . . . 6.3.1 Gold’s 1955 Paper . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.2 Urey’s Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.3 “More Treacherous than Quicksand” . . . . . . . . . . . . . . 6.3.4 Whipple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.5 Kuiper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.6 The Space Race . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.7 Gold’s 1962 Paper . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.8 Interpretation of the Ranger Photos . . . . . . . . . . . . . . . 6.3.9 The First Soft Landings on the Moon . . . . . . . . . . . . . 6.3.10 The Apollo Explorations . . . . . . . . . . . . . . . . . . . . . . .

137 137 138 138 139 139 139 141 141 143 144 145 145 146 150 152

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6.4

Assessments of Gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.1 Wilhelms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.2 Taylor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.3 Hapke . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Eddington’s Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6 Continuing Understanding . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

155 156 156 156 158 159 159

Did the Chicxulub Impact Cause the Cretaceous Extinctions? . . . . 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 The Speculation of Luis and Walter Alvarez and Their Colleagues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 The Controversy over the Speculation . . . . . . . . . . . . . . . . . . . 7.3.1 The First Snowbird Conference . . . . . . . . . . . . . . . . . . 7.3.2 The Search for Evidence of the Crater . . . . . . . . . . . . . 7.3.3 The Second Snowbird Conference . . . . . . . . . . . . . . . . 7.3.4 The “Discovery” of the Chicxulub Crater . . . . . . . . . . . 7.4 Continuation of the Controversy . . . . . . . . . . . . . . . . . . . . . . . 7.5 Conclusions and Reflections . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6 Eddington’s Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7 Continuing Understanding . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

163 163

Size of the Solar System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 The Early Speculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.1 Leverett Davis Jr. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.2 The Termination Shock: Francis Clauser . . . . . . . . . . . 8.2.3 Revised Estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.4 Neutral Hydrogen and Charge Exchange . . . . . . . . . . . 8.2.5 Additional Speculations on the Effect of Galactic Cosmic Rays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 To the Outer Planets—and Beyond: Pre-voyager Speculations . . 8.3.1 Dessler’s Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.2 Jovian Radio Emissions . . . . . . . . . . . . . . . . . . . . . . . 8.3.3 Estimates Based on Cosmic Ray Modulation . . . . . . . . 8.3.4 Solar Wind and Neutral Hydrogen Densities . . . . . . . . 8.3.5 Evidence of the Penetration of Interstellar Neutral Hydrogen Deep into the Heliosphere: OGO 5 Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.6 The Experts are Polled on the Distance to the Termination Shock . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.7 Heliosphere Radio Emissions . . . . . . . . . . . . . . . . . . .

164 165 165 169 173 174 178 179 180 181 183 187 187 187 187 189 190 192 195 196 198 199 199 202

204 206 206

Contents

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8.4

Direct Measurements of the Termination Shock and the Heliopause . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4.1 Pioneers 10 and 11 . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4.2 Crossing of the Termination Shock by Voyagers 1 and 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4.3 Crossing of the Heliopause by Voyagers 1 and 2 . . . . . 8.5 Eddington’s Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.1 Estimates Based on Pressure Balance Models . . . . . . . . 8.5.2 Estimates Based on the Observations of the Orientation of Comet Tails . . . . . . . . . . . . . . . . . . . . . 8.5.3 Estimates Based on Lyman-α Measurements . . . . . . . . 8.5.4 Estimate Based on Jupiter Radio Emissions . . . . . . . . . 8.5.5 Estimates Based on Cosmic-Ray Gradients . . . . . . . . . 8.5.6 Estimates Based on Measurements in the Far Heliosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6 Continuing Understanding . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

208 208 209 211 214 215 216 216 217 218 219 219 219 221

Sources of Gamma-Ray Bursts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 The Discovery of Gamma–Ray Bursts . . . . . . . . . . . . . . . . . . . 9.3 The Burst and Transient Source Experiment (BATSE) . . . . . . . . 9.4 The Debate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.1 The Source of Soft Gamma-Ray Repeaters . . . . . . . . . . 9.5 Eddington’s Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.6 Continuing Understanding . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.6.1 The Resolution of the Question of the Distance Scale for Gamma-Ray Bursts . . . . . . . . . . . . . . . . . . . 9.6.2 The Physical Mechanism(s) for GRBs . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

236 238 239

Reflections on Space Science Research . . . . . . . . . . . . . . . . . . . . . 10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Other Important Space-Related Discoveries . . . . . . . . . . . . . . 10.2.1 Discovery of the Earth’s Radiation Belts . . . . . . . . . . 10.2.2 Tidal Heating of Planetary Moons . . . . . . . . . . . . . . . 10.3 Eddington’s Guidelines: Conclusions . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . .

241 241 242 242 247 249 251

Correction to: Scientific Debates in Space Science . . . . . . . . . . . . . . . . .

C1

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10

225 225 226 227 230 234 235 236

Author Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253 Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257

About the Authors

David Cummings was the first PhD graduate from the Space Science Department at Rice University. Upon graduation from Rice, he conducted research on hydromagnetic waves in what is now the Earth, Planetary, and Space Sciences Department of the University of California, Los Angeles (UCLA). Following his time at UCLA, he served as Chair of the Physics Department at Grambling College (later Grambling State University) for 7 years. He then became the Executive Director of the Universities Space Research Association, serving in this capacity for more than 30 years.

Louis J. Lanzerotti spent his research career at Bell Laboratories and the New Jersey Institute of Technology in experimental space plasma physics, both space-based and ground-based. He was also active in experiments in studies of the effects of Earth’s space environment on technological systems, especially in telecommunications. He chaired a number of advisory committees related to space research. He was elected to the National Academy of Engineering and the International Academy of Astronautics.

xix

Chapter 1

Introduction

1.1

The Beginning of Space Science

Until the advent of rocketry, hot air balloons were the only methods of exploration above Earth’s surface. In fact, personally taking instruments in 1911–1913 up to and above 5 km in risky balloon flights led Austrian physicist Victor F. Hess (1883–1964) to the discovery of cosmic rays and to the award of the Nobel Prize in 1936 (Hess, 1936). Hess developed highly sensitive electroscope instrumentation for his research. His balloon measurements showed importantly that the discharges of the electroscopes were the same under nighttime and daytime conditions, results contributing to the conclusion of radiation beyond Earth. Hess completed his career as a professor at Fordham University in New York, having immigrated to the United States in 1938. The subsequent study of cosmic rays, their nature and charge states, was a major area of physics research in many nations around the world. Expeditions were made to numerous mountain peaks. Increasingly improved instrumentation was installed at these locations for longer term studies of cosmic rays at higher altitudes than could be achieved by short term balloon flights (e.g., LePrince-Ringuet, 1950). A very famous scientific debate occurred in the 1930s involving cosmic rays. The debate was centered around two Nobel Prize laureates, Arthur H. Compton (1892–1962) and Robert A. Millikan (1868–1953), and was related to the charge of the cosmic rays that were being measured on mountain tops and at sea level. Compton’s argument for charged particles won the debate over Millikan’s neutral particles (photons) as a result of research by several groups making careful measurements as a function of latitude, using Earth’s magnetic field as the “spectrometer” (e.g., Lemaitre & Vallarta, 1933). However, as was shown by further studies, Compton’s particles were not negatively charged (he suggested electrons) but positively charged. Indeed, cosmic rays and their nature were among the strong motivations for researchers to ultimately pursue space science above Earth’s atmosphere. © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 W. D. Cummings, L. J. Lanzerotti, Scientific Debates in Space Science, Astronomy and Planetary Sciences, https://doi.org/10.1007/978-3-031-41598-2_1

1

2

1

Introduction

Fig. 1.1 Sergei Vernov. Reproduced from Baker and Panasyuk (2017), with permission of American Association of Physics Teachers, courtesy of Daniel Baker, LASP, University of Colorado

Interest in scientific exploration above Earth’s atmosphere continued and expanded beyond cosmic-ray research. Following the second World War, in 1946, the United States established the Upper Atmosphere Rocket Research Panel. Known originally as the V-2 Panel, it was formed to oversee experiments that could be conducted using the V-2 rockets brought to the country after the war. The original Panel was composed of nine individuals from academia, industry, and government. Over the years of its meetings and activities, the Panel advised on sub-orbital rocketbased experiments that were conducted in studies that included the upper atmosphere, solar radiation at multiple wavelengths, and X-ray astronomy. The tenth meeting of the Panel, chaired by James A. Van Allen (1914–2006), was held in January 1956 (Van Allen, 1956). The meeting was devoted exclusively to detailed proposals for the use of scientific satellites orbiting Earth that would be launched by rockets. As Van Allen writes in his Preface to the volume: . . .the principal purpose of the book will have been served if it brings the potential value of artificial satellites to the attention of the scientific community at large and stimulates broad professional participation in the great and continuing undertaking of extending human knowledge of our physical environment by every conceivable means. (Van Allen, 1956: page v)

The motivations for such a significant gathering for examining experiments that could be carried out on satellites can largely be attributed to the initiation of the International Geophysical Year (IGY), 1957–1958. This large international undertaking originated in a dinner meeting of several leading scientists in 1950 in Van Allen’s home in Maryland (Van Allen, 1983). In the course of the planning for the IGY, both the United States and the Union of Soviet Socialist Republics announced plans that their participation would include the launch of satellites. Space science thus originated in the IGY and is commonly dated to 4 October 1957. On that date the Soviet Union launched the first artificial satellite (sputnik in Russian) to orbit Earth. Sputnik 1 carried no scientific instrumentation, but soon after there would be Soviet sputniks that did. In addition to the dog Laika, Sputnik 2, launched on 3 November 1957, carried instrumentation (two gas discharge counters) by Sergei N. Vernov (1910–1982) (Fig. 1.1) and his colleagues to measure cosmic rays above the atmosphere (Vernov & Chudakov, 1960). Transmissions of data from Vernov’s instrumentation that recorded radiation at the high-altitude

1.1

The Beginning of Space Science

3

Fig. 1.2 The success of Explorer 1. From left to right—William Pickering, James Van Allen, and Werner Von Braun. Credit NASA

orbital locations where the radiation occurred were beyond the range where signals could be received by stations within the Soviet Union. Sputnik 2 was at low altitudes, largely below Earth’s radiation belts, when its transmissions over Moscow could be received. The signals of Sputnik 2 were heard all over the world, but because of secrecy concerns the Soviets had not provided information internationally so that they could be decoded. Recordings of transmissions from the third Soviet satellite (Sputnik 3, launched 15 May 1958) that were acquired by several nations around the world and from the Soviet IGY Antarctic expedition were sent to Moscow, were analyzed, and showed huge increases in radiation measured at high altitudes (Van Allen, 1956: Fig. 4). The launch on 31 January 1958 of the U.S. Explorer 1 was nearly 3 months after the launch of Sputnik 2 but 3½ months before the launch of Sputnik 3. Van Allen’s Geiger tube instrument on Explorer 1 was thus the first to measure and provide data on Earth’s trapped radiation. (Chapter 10 contains additional discussions of Van Allen’s discovery.) Van Allen’s announcement in the auditorium of the United States National Academy of Sciences on 1 May 1958, presented the very momentous first space science measurements above the atmosphere, though the attention of the American public was perhaps more focused on the success of the launch of Explorer 1, following a number of prior failed launch attempts (Fig. 1.2). Because of the dependence of space exploration on the technology of rocketry, we define “space science” as science that depends in a broad sense on space vehicles for observations and measurements. It is a definition that we believe accords with the thinking of scientists in the field, most of whom receive their research encouragement and support from their nations’ space agencies.

4

1

Introduction

Scientific progress in the new era of space exploration was made possible by the development of ever more capable rockets and other space technologies. At first, the part of space around the Earth where charged particle behavior is influenced by Earth’s magnetic field began to be studied. More powerful rockets allowed the exploration of more distant parts of space, including the particles and magnetic fields flowing from the Sun. Other planetary bodies in the solar system could be studied. Eventually rocket technology advanced to the point where humans could explore the Moon, setting foot on Earth’s companion for the first time on 20 July 1969. Satellite platforms with telescopes allowed the study of objects beyond our solar system without the interference of Earth’s atmosphere. From the early years of space science until now, practically every successful space flight mission has resulted in new discoveries. These new results were sometimes preceded by new theories that predicted them, but more commonly they were followed by new theories and models, and these new ideas often resulted in vigorous scientific debates. This book is about some of the important scientific debates that helped to develop space science and led to significant new understandings of the constituents and processes taking place beyond Earth’s atmosphere. Each of the eight subsequent chapters in the book focuses on a specific debate, some of which were more heated than others and led to prolonged controversies. The final chapter outlines two major early discoveries that were not accompanied by much debate. Most of the debates in the next eight chapters were prompted by scientific speculations on the part of one or more of the protagonists. Our choice of topics includes debates in each of the major sub-disciplines of space science, i.e., the science areas that are known as heliophysics (the study of the plasma and magnetic field flowing from the Sun, its impact on planetary bodies, and its interaction with the adjacent plasma and magnetic fields of our galaxy), planetary science (including the science related to comets, asteroids, our Moon, and the moons of other planets), and space astronomy (astronomical and astrophysical studies that make use of space-based observations).

1.2

Eddington and Scientific Speculation

As noted above in the case of the charge of cosmic rays, scientific speculation and debate were not new to science at the time of the beginning of space science. In 1920, while still in the midst of a great scientific debate on the age of the sun, English astronomer Arthur Stanley Eddington (1882–1944) (Fig. 1.3) gave a lecture in which he commented on scientific speculation as follows: If we are not content with the dull accumulation of experimental facts, if we make any deductions or generalizations, if we seek for any theory to guide us, some degree of speculation cannot be avoided. Some will prefer to take the interpretation which seems to be most immediately indicated and at once adopt that as an hypothesis; others will rather seek to explore and classify the widest possibilities which are not definitely inconsistent with the facts. Either choice has its dangers: the first may be too narrow a view and lead progress

1.2

Eddington and Scientific Speculation

5

Fig. 1.3 Arthur Eddington. Credit George Grantham Bain Collection, Library of Congress Prints and Photographs Division Washington, D.C. [LC-B2–6358-11]

into a cul-de-sac; the second may be so broad that it is useless as a guide and diverges indefinitely from experimental knowledge. (Eddington, 1920:356)

Eddington was highly respected as an astronomer and mathematician. He is especially known among astronomers for the “Eddington Limit,” the natural limit on the luminosity of stars. He is also well known for the expeditions that he organized and led to use the solar eclipse of 1919 to test Albert Einstein’s theory of General Relativity, specifically to test whether the mass of the Sun bent the light rays coming from distant stars in accordance with Einstein’s theory. Great Britain was at war with Germany at the time of the planning for the expedition, and some of Eddington’s colleagues questioned the wisdom of perhaps proving correct the work of a German scientist. Eddington was a Quaker and a pacifist, however, and he wanted to lessen the isolation of German scientists. Eddington claimed that the results of his eclipse observations verified Einstein’s theory, though some then and even now questioned his interpretation of the data. Einstein’s theory of General Relativity has since been verified in other ways many times. Matthew Stanley gives an interesting and thorough discussion of the views of Eddington in his book Practical Mystic: Religion, Science and A. S. Eddington (2007). From his perspectives as an astronomer, Eddington developed ideas about the kinds of speculation and model development that are productive and the kinds that are not, and we will refer to Eddington’s criteria as we examine the topics in subsequent chapters of this book.

6

1.3

1 Introduction

The Eddington-Jeans Debate

The underlying scientific question that prompted the debate between Eddington and the English astronomer and physicist James H. Jeans (1877–1946) was the age of the Sun. In 1862, Scottish physicist and engineer Sir William Thomson (later Lord Kelvin) (1824–1907) published a paper (Thomson, 1862) in which he estimated the lifetime of the Sun as about 20 million years, within a probability range between 10 and 100 million years. He based his estimate on a theory of “contraction” originally developed by physicist and physician Hermann L. F. von Helmholtz (1821–1894) (Helmholtz, 1856). In the Kelvin-Helmholtz contraction theory, the Sun obtained a calculable amount of energy from the falling together of its constituent material during its formation. The kinetic energy of the infalling material would be converted to thermal energy as particles collided with other particles. The temperature and pressure in the newly formed Sun would increase and the thermal energy would be radiated away over time at a known rate, taken to be the currently measured rate of radiative energy loss from the Sun. The original amount of energy divided by the rate of energy loss would give an estimate of the Sun’s lifetime. Kelvin was wrong in this instance, but he contributed to advancements in many areas of science and engineering. For instance, he acquired much fame from his contributions to the successes of the early trans-Atlantic telegraph cables. (For an excellent biography of Kelvin, see David Lindley’s (2004) Degrees Kelvin: A tale of genius, invention, and tragedy.) Thomson’s estimate of about 20 million years for the lifetime of the Sun was at odds with the age of the Earth that geologist Charles Lyell (1797–1875) thought necessary from his geological research and that naturalist and biologist Charles R. Darwin (1809–1882) thought necessary from his research on biological evolution. For example, in his Origin of Species, Darwin had written that “. . . one hundred and forty million years can hardly be considered as sufficient for the development of the varied forms of life which already existed during the Cambrian period.” (Darwin, 1859:340). Based on rates of sedimentation and erosion in the Earth’s surface, geologists were arriving at ages for the Earth measured in billions of years. In 1893, for example, geologist and anthropologist William J. McGee of the United States Geological Survey published a mean estimate for the age of the Earth as 6 billion years (McGee, 1893:309). The ability to accurately measure sedimentation and erosion rates was problematic, but when the radioactivity of uranium was discovered by physicist and engineer Antoine H. Becquerel (1852–1908) in 1896, the argument for a longer time interval since the formation of the Earth became stronger. Within a decade of Becquerel’s discovery, the radioactive decay of uranium to a stable isotope of lead was being used to date minerals in rocks. In 1911, for example, geologist Arthur Holmes of the Imperial College of Science and Technology in England estimated ages in the range of 1 to 1.6 billion years for Precambrian rocks (Holmes, 1911:256). This was the state of knowledge in the fall of 1916 when Eddington published an article in the Monthly Notices of the Royal Astronomical Society (Eddington,

1.3

The Eddington-Jeans Debate

7

1916) that set off heated exchanges between himself and Jeans. Eddington believed that the evidence for the great age of the Earth, and hence the great age of the Sun, made it clear that Kelvin-Helmholtz contraction could not provide enough energy to power the Sun. There must be another unknown energy source. He decided to simply assume the presence of the unknown source and conduct an analysis based on known laws of physics. Using some speculative assumptions and approximations, Eddington came to several conclusions, among them that the total radiation of a giant star depends only on its mass (Eddington, 1917:310). Jeans strongly questioned this result. He did not see how it was possible to conduct an analysis on the internal structure of a star without knowing the source of stellar energy. In response to Eddington’s paper, Jeans wrote as follows: It is surely obvious without discussion that there can be no perfectly general laws of the type enunciated by Professor Eddington for stars supposed to be in a steady state. The rate of emission of energy, being also the rate of generation of energy in the star’s interior, must depend on the ultimate source of this energy. Clearly a star made of imaginary matter a million times more radioactive than uranium will emit energy at a greater rate than a non-radioactive star of the same mass. Hence a preliminary to any attack on the general problem must be a decision as to the source of the energy. (Jeans, 1917:37)

In 1917, neither Eddington, Jeans nor anyone else understood that nuclear fusion was the primary source of energy for all but dwarf stars. Eddington speculated about fusion in 1920, but it was not until 1939 that Cornell physicist Hans A. Bethe (1906–2005) published his paper giving the physics of nuclear fusion as being this energy source (Bethe, 1939). Bethe was awarded the 1967 Nobel Prize in physics for this work, and for his research on the theory of nuclear reactions. Despite this lack of knowledge, Eddington had begun to develop a theory on the internal structure of stars, using the concept of radiation pressure as well as ordinary thermal pressure. He began to get results that seemed to agree with observations, e.g., that the total radiation of a giant star depended only on its mass. Notwithstanding the “lifetime” dilemma, Jeans took the view that the selfgravitational contraction of solar matter was the source of the Sun’s energy. In one of his many responses to the papers of Eddington, Jeans wrote that it “appears probable that a fair approximation to actual conditions will be obtained by regarding the star as a mass of gas contracting under its own gravitation, and having no sources of energy except those of gravitational contraction.” (Jeans, 1917:37) This was in contrast to Eddington’s view that “It is well known that the hypothesis that the sun’s energy is derived from contraction leads to a value of the sun’s age which is much too small to be reconciled with estimates of the earth’s age based on studies of the radioactive minerals and other geological evidence.” (Eddington, 1917:610). As noted above, in the fall of 1920, Eddington published a paper that contained another speculation, namely that the energy source for stars resulted from the fusion of hydrogen into helium. He pointed out that the mass of the helium atom is less than the mass of the four hydrogen atoms that make up its nucleus. (The neutron had not yet been discovered, so the helium nucleus was thought to be made up of four positively charged protons and two embedded electrons.) Eddington wrote:

8

1

Introduction

Fig. 1.4 Cecilia PayneGaposchkin. Credit Schlesinger Library, Harvard Radcliffe Institute

There is a loss of mass in the synthesis [of helium from hydrogen] amounting to 1 part in 120, the atomic weight of hydrogen being 1.008 and that of helium just 4. . . . Now mass cannot be annihilated, and the deficit can only represent the mass of the electrical energy set free in the transmutation. We can therefore at once calculate the quantity of energy liberated when helium is made out of hydrogen. If 5 per cent of a star’s mass consists initially of hydrogen atoms, which are gradually being combined to form more complex atoms, the total heat liberated will more than suffice for our demands, and we need to look no further for the source of a star’s energy. (Eddington, 1920:354)

Eddington’s speculation about the fusion of helium from hydrogen led to another heated debate with physicists who maintained that the temperature at the center of a star is not high enough for the process. To which Eddington famously replied: But the helium which we handle must have been put together at some time and some place. We do not argue with the critic who urges that the stars are not hot enough for this process; we tell him to go and find a hotter place. (Eddington, 1926:301—emphasis in the original).

In the context of the history of stellar composition, Eddington’s assumption of hydrogen being 5% of a star’s mass preceded by some 5 years the definitive conclusion of British astronomer Cecilia H. Payne (Cecilia Payne-Gaposchkin; 1900–1979) (Fig. 1.4) in her Harvard PhD thesis that stars were composed primarily of hydrogen and helium (Payne, 1925). She had moved to Harvard-Radcliffe to pursue an advanced degree, which was not possible for a woman at that time at Cambridge University, where she had been an undergraduate at Newnham College of Cambridge. It took considerable time for the astronomy community to accept her results. At the time, the guiding scientific consensus was that the elemental composition of the Earth and the Sun were similar (Moore, 2020).

1.4

Eddington’s Guidelines

Arthur Eddington’s 1920 paper was derived from an address that he gave at a meeting of British scientists. Following his speculation about the source of energy within stars, including importantly the Sun, Eddington mused about scientific

1.4

Eddington’s Guidelines

9

speculation in general, and in the process, he described his approach to the development of scientific theories: I should not be surprised if it is whispered that this address has at times verged on being a little bit speculative; perhaps some outspoken friend [a likely reference to Jeans] may bluntly say that it has been highly speculative from beginning to end. I wonder what is the touchstone by which we may test the legitimate development of scientific theory and reject the idly speculative. We all know of theories which the scientific mind instinctively rejects as fruitless guesses; but it is difficult to specify their exact defect or to supply a rule which will show us when we ourselves do err. ... I think that the more idle kinds of speculation will be avoided if the investigation is conducted from the right point of view. When the properties of an ideal model have been worked out by rigorous mathematics, all the underlying assumptions being clearly understood, then it becomes possible to say that such and such properties and laws lead precisely to such and such effects. If any other disregarded factors are present, they should now betray themselves when a comparison is made with Nature. There is no need for disappointment at the failure of the model to give perfect agreement with observation; it has served its purpose, for it has distinguished what are the features of the actual phenomena which require new conditions for their explanation. ... Our model of Nature should not be like a building—a handsome structure for the populace to admire, until in the course of time someone takes away a cornerstone and the edifice comes toppling down. It should be like an engine with movable parts. We need not fix the position of any one lever—that is to be adjusted from time to time as the latest observations indicate. The aim of the theorist is to know the train of wheels which the lever sets in motion—that binding of the parts which is the soul of the engine. (Eddington, 1920:356–357)

Most space scientists would probably say that they abide by the above guidelines in the pursuit of their research. These are common-sense guidelines. But the admonition to remain objective about ones’ scientific work is difficult to achieve, as Eddington admits: . . . I suppose that an applied mathematician whose theory has just passed one still more stringent test by observation ought not to feel satisfaction, but rather disappointment-“Foiled again! This time I had hoped to find a discordance which would throw light on the points where my model could be improved.” Perhaps that is a counsel of perfection; I own that I have never felt very keenly a disappointment of this kind. (Eddington, 1920:357)

Eddington’s guidelines for science speculation, slightly modified for applicability to space science, can be summarized as follows: 1. Was the speculator rigorous in applying the appropriate science applicable to the model, 2. Did the speculator identify all the underlying assumptions used in constructing the model, and 3. Did the speculator view the model objectively, as an “adjustable engine,” as opposed to a “finished building?” For the scientific debates examined in the subsequent chapters, adherence to these guidelines by the protagonists, including the difficult third guideline of objectivity, will be assessed.

10

1

Introduction

References Baker, D. N., & Panasyuk, M. I. (2017). Discovering Earth’s radiation belts. Physics Today, 70(12), 46–51. Bethe, H. (1939). Energy production in stars. Physics Review, 55, 434. Darwin, C. (1859). The origin of species by means of natural selection of the preservation of favoured races in the struggle for life – 150th Anniversary Edition. Published in 2003 by Signet Classics. Eddington, A. S. (1916). On the radiative equilibrium of the stars. Monthly Notices of the Royal Astronomical Society, 77, 16–35. Eddington, A. S. (1917). The radiation of the stars. Nature, 99, 308–310. Eddington, A. S. (1920). The internal constitution of stars. The Observatory, 43, 341–358. Eddington, A. S. (1926). The internal constitution of the stars. Cambridge University Press. (reprinted in 1988 from the 1926 edition). Helmholtz, H. V. (1856). LXIV. On the interaction of natural forces. The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, 11(75), 489–518. Hess, V. F. (1936). Nobel lectures, physics 1922–1941 (p. 1985). Elsevier. Holmes, A. (1911). The association of lead with uranium in rock-minerals, and its application to the measurement of geological time. Proceedings of the Royal Society of London Series A, Containing Papers of a Mathematical and Physical Character, 85(578), 248–256. Jeans, J. H. (1917). The evolution and radiation of gaseous stars. Monthly Notices of the Royal Astronomical Society, 78(1), 36–47. Lemaitre, G., & Vallarta, M. S. (1933). On Compton’s latitude effect on cosmic radiation. Physics Review, 43, 87. LePrince-Ringuet, L. (1950). Cosmic rays, trans. F. Ajzenberg. Prentice-Hall . Lindley, D. (2004). Degrees Kelvin: A tale of genius, invention, and tragedy. Joseph Henry Press. McGee, W. J. (1893). Note on the “age of the Earth”. Science, 21(540), 309–310. Moore, D. (2020). What stars are made of: The life of Cecilia Payne-Gaposchkin. Harvard University Press. Payne, C. H. (1925). Stellar atmospheres: A contribution to the observational study of high temperature in the reversing layers of stars. Radcliffe College. Stanley, M. (2007). Practical mystic: Religion, science, and AS Eddington. University of Chicago Press. Thomson, W. (1862). On the age of the sun’s heat. Macmillan’s Magazine, 5(5), 288–293. Van Allen, J. A. (Ed.). (1956). Scientific uses of Earth satellites. University of Michigan Press. Van Allen, J. A. (1983). Origins of magnetospheric physics. Smithsonian Institution Press. Vernov, S. N., & Chudakov, A. E. E. (1960). Reviews of Topical Problems: Investigations of cosmic radiation and of the terrestrial corpuscular radiation by means of rockets and satellites Soviet Physics Uspekhi, 3(2), 230.

Chapter 2

Solar Wind or Solar Breeze?

2.1

Introduction

As discussed in Chap. 1, Sir Arthur Eddington (1882–1944) laid down some guidelines for speculation when researchers are in the process of advancing understandings in the sciences. These guidelines are an outgrowth of his personal research, and challenges to it, related to the internal constitution of stars. This chapter examines speculations related to physical processes external to stars. That is, in addition to the light that is emitted by the Sun and other stars, are there other emissions and, if so, what might their importance be? As this chapter relates, there were a few long-time evidences of various types for the Sun influencing Earth, but the nature of the influences was very fragmentary and non-quantitative. The ultimate understanding of the non-light emissions of the Sun—the solar wind with its plasmas and magnetic fields—that resulted from the theoretical speculations and controversies was a fundamental development in the history of space plasma physics.

2.2

Background: Evidence of Solar-Terrestrial Links

By 1939, the progress of nuclear physics had given credence to Eddington’s speculation that the process of fusing hydrogen into helium was the primary source of energy for stars (Chap. 1; Bethe, 1939, 1968). By 1957, the processes for synthesizing elements within stars was understood (Burbidge et al., 1957). Over the course of 40 years, Eddington’s project of understanding the internal structure of “so simple a thing as a star” (Stanley, 2007) had made great strides.

The original version of the chapter has been revised. A correction to this chapter can be found at https://doi.org/10.1007/978-3-031-41598-2_11 © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023, corrected publication 2024 W. D. Cummings, L. J. Lanzerotti, Scientific Debates in Space Science, Astronomy and Planetary Sciences, https://doi.org/10.1007/978-3-031-41598-2_2

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2.2.1

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Magnetic Storms

The beginning of a suspicion that the Sun affected the Earth in ways other than gravitational attraction and electromagnetic radiation can be traced to 1826. This is when German apothecary-turned-astronomer Samuel H. Schwabe (1789–1875) (Fig. 2.1) began to record the number of sunspots and groups of sunspots visible on the surface of the Sun each day that the Sun was not obscured by clouds at his observatory near Berlin. Schwabe was an amateur solar astronomer, and he was persistent. By 1843, he had discovered that the annual number of sunspots varied on roughly a 10-year cycle (Schwabe, 1844:235). He continued his observations, and in 1850 the German polymath Alexander von Humboldt (1769–1859) published a table of Schwabe’s data that convincingly demonstrated the sunspot cycle during the years 1826–1850 (Humboldt, 1850:402). In Humboldt’s table, reproduced in Fig. 2.2, the first column gives the year, the second gives the number of groups of sunspots observed, the third gives the number of spot-free days, and the fourth column gives the total number of days in the year when observations could be made. A likely Sun-Earth connection was made when researchers began to collect data on variations in the Earth’s magnetic field around the world. Influenced by Humboldt, Carl Friedrich Gauss (1777–1855) and Wilhelm Weber (1804–1891) organized a “Magnetic Union” for setting up magnetic observatories, often at locations of astronomical observatories. Irish astronomer Edward Sabine (1788–1883) (Fig. 2.3) was a principal in this effort, establishing magnetic observatories at various locations under British rule. In particular were the locations in the northern hemisphere (Toronto, Canada) and southern hemisphere (Hobarton—now Hobart, Tasmania, Australia). In addition to being in different hemispheres, the two stations were separated by 133° in longitude, and thus by about 9 h of local time. In 1851, Sabine reported that during 1843, 1844, and 1845 disturbances in the measured magnetic field had the same local-time dependence at Toronto and Hobarton. Namely, the

Fig. 2.1 Samuel Schwabe. Credit The High Altitude Observatory

2.2

Background: Evidence of Solar-Terrestrial Links

Fig. 2.2 Schwabe’s table showing the sunspot cycle. Reproduced from Von Humboldt (1874), p. 266. Credit John Oertel

Fig. 2.3 Edward Sabine. Credit Dictionary of Canadian Biography

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mean values of the amount of the disturbances was larger in the night hours than in the day hours (Sabine, 1851). The local-time effect suggested a Sun-related cause. In the following year, Sabine was able to report on the analysis of three more years of data—1846, 1847, and 1848. For the total duration of 6 years, he confirmed the local-time effect, namely that “at both stations there are fewer disturbances, and their aggregate values are less in the hours of day than in those of night” (Sabine, 1852:105). He also found that there was a seasonal dependence, namely that “the disturbances are less frequent, and have a less aggregate value in November, December, January and February at Toronto, and in May, June, July, and August at Hobarton than in the other eight months of the year” (Sabine, 1852:111–112) and that “. . . the average value of a disturbed observation is greater at both stations in the winter than in the summer months, and that it is greatest in the intermediate or equinoctial months” (Sabine, 1852: 112). The seasonal dependence added to the evidence that the Sun was implicated as the ultimate cause of the terrestrial magnetic disturbances. Finally, Sabine reported that he observed “. . . with a single exception, uninterrupted progressive increase in the amount of disturbance from a minimum in 1843 to a maximum in 1848” (Sabine, 1852:115—emphasis in the original). Relative to average values, the number and mean extent of “the disturbances in the years 1846, 1847, and 1848 were nearly twice as great as in the years 1843, 1844, and 1845” (Sabine, 1852:115—emphasis in the original). Sabine included in his article Schwabe’s most recent table showing sunspot data from 1826 through 1850, and he noted that “it is certainly a most striking coincidence, that the period, and the epochs of minima and maxima, which M. Schwabe has assigned to the variation of the solar spots, are absolutely identical with those which have been assigned to the magnetic variations” (Sabine, 1852:121). A further, and dramatic, apparent Sun-Earth connection was reported by two astronomers in Britain in 1859. Richard C. Carrington (1826–1875) and Richard Hodgson (1804–1872) independently observed a large solar flare on the morning of 1 September. This was the first observation of a white light flare, and it persisted for about 5 min. This solar event was followed some 17 h later by a great terrestrial magnetic storm (Carrington, 1859; Hodgson, 1859). Carrington regularly monitored the sun and sketched the sunspots from his personal observatory at Red Hill, south of London (Clark, 2007). He published the drawings of spots in each solar rotation from 1853 to 1861 in a major compendium (Carrington, 1863). Carrington (of whom no image is known to exist) was cautious about making a connection between the solar flare of 1 September 1859 and the subsequent magnetic storm, noting that this was a single occurrence and that “One swallow does not make a summer” (Carrington, 1859:15). A little more than three decades later Scottish physicist and engineer William Thomson (Lord Kelvin, 1824–1907) (Fig. 2.4) was more than just cautious; he argued against a connection between activity on the Sun and terrestrial magnetic storms. In his Presidential Address to the Royal Society on 30 November 1892, Lord Kelvin presented the results of a calculation of the power required of the Sun if magnetic changes on the Earth’s surface during an average magnetic storm were caused by “any possible dynamical action within the sun, or its atmosphere . . .” (Kelvin, 1892:307). He concluded that the power required would be

2.2

Background: Evidence of Solar-Terrestrial Links

15

Fig. 2.4 Lord Kelvin. Credit University of Glasgow

something like 12 × 1035 ergs/sec, whereas the total rate of energy production in the form of solar radiation was only 3.3 × 1033 ergs/sec: Thus, in this eight hours of a not very severe magnetic storm, as much work must have been done by the sun in sending magnetic waves out in all directions through space as he actually does in four months of his regular heat and light. This result, it seems to me, is absolutely conclusive against the supposition that terrestrial magnetic storms are due to magnetic actions of the sun; or to any kind of dynamical action taking place within the sun, or in connexion with hurricanes in his atmosphere, or anywhere near the sun outside. It seems as if we may also be forced to conclude that the supposed connexion between magnetic storms and sun-spots is unreal, and that the seeming agreement between the periods has been a mere coincidence (Kelvin, 1892:307–308).

In response to Lord Kelvin’s address to the Royal Society, Irish physicist George F. FitzGerald (1851–1901) (Fig. 2.5) made a couple of prescient observations. He suggested that “magnetic storms were due to something of the same kind as cathode rays” emitted from the Sun (FitzGerald, 1900:287). Cathode rays were on the mind of many physicists at this time, as British physicist and Nobel Prize laureate (1906) Joseph J. Thomson (1856–1940) had discovered the electron through experimentation with cathode ray tubes in 1897. FitzGerald also noted that: There seems to be some evidence from aurorae and magnetic storms that the Earth has a minute tail like that of a comet directed away from the Sun, the time of day of maximum magnetic disturbance being about 11 p.m., and this also being about the time of maximum auroral activity in each longitude. There are many things which seem to show that comets’ tails, aurorae, the solar corona, and cathode rays are closely allied phenomena. . . . (FitzGerald, 1900:287).

Magnetic storms at Earth were a significant topic of research in the first half of the twentieth century, both from the scientific perspective as well as from the disturbances and damages that such magnetic activity could cause to technical systems

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Fig. 2.5 George Fitzgerald. Credit Hollinger and Rockey photographers, New York

Fig. 2.6 Sydney Chapman. Credit Syun-Ichi Akasofu, University of Alaska

such as the telegraph and the growing cable and wireless telecommunications infrastructures. The eminent British mathematician Sydney Chapman (1888–1970) (Fig. 2.6) and his first graduate student, the Italian Vicenzo C. A. Ferraro (1907–1974) (Fig. 2.7), developed and published a set of papers that provided a “New Theory of Magnetic Storms,” as their work was titled (Chapman & Ferraro, 1931a, 1931b, 1933). Chapman and Ferraro were motivated to explain the large depressions in Earth’s magnetic field in equatorial regions that marked the occurrence of geomagnetic storms. They were interested in explaining how to produce a ring of electrical current circulating above the Earth that would cause a depression in the magnetic field intensity at Earth’s surface. Their approach was to examine how a stream of particles from the Sun could interact with Earth’s magnetic field (see Chap. 3). The discussion of Chapman and Ferraro of particle streams built upon prior research that postulated

2.2

Background: Evidence of Solar-Terrestrial Links

17

Fig. 2.7 V. C. A. Ferraro. Credit Maddalena Ferraro

Fig. 2.8 Solar streamers seen during the solar eclipse of 21 August 2017. From Fig. 5 of Habbal et al. (2021). Credit Shadia Habbal, University of Hawaii

that the Sun could emit a stream of particles toward Earth from a solar active (flare) region. Their first paper provided a summary of previous discussions of particle streams from the Sun by prior researchers (Chapman & Ferraro, 1931a). Such limited-in-spatial extent emissions from the Sun would be episodic, because solar flares were episodic. The Sun-originating streams that Chapman and Ferraro postulated were from discrete solar sources, not from the entire Sun (see the solar streamers shown in Fig. 2.8). Further, the streams were perfectly conducting (as the solar wind is now known to be). Their interest was not in explaining how such streams would evolve from the Sun and propagate into the interplanetary medium. The postulated solar streams near Earth orbit were their way of explaining Earthbased magnetic activity. Yet, the assumption by Chapman and Ferraro of such streams and their possible nature was another, and important, step in the growth of

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understanding the connection of Sun to Earth by physical processes other than visual emissions.

2.2.2

Comets

Prompted by the 1835 apparition of Halley’s comet (named for the British astronomer Edmond Halley, 1656–1742), German mathematician and astronomer Friedrich W. Bessel (1784–1846) began to consider the possible forces on a comet that would be consistent with the form of its head and tail. Russian astronomer Fyodor A. Bredichin (1831–1904) followed Bessel in these efforts. Among the ideas of Bessel and Bredichin was that “the physical characteristics of the comet are completely determined by two factors: the ejection of a particle (which may be a gaseous molecule) from the comet nucleus and the repulsive force of the Sun acting on such particle on its ejection” (Jaegermann, 1903; Bobrovnikoff, 1928:165). According to the Bessel-Bredichin theory, the head of a comet was formed by the emission of particles in all sunward directions to form a fountain. Arthur Eddington took up this idea in 1910 when, in anticipation of the apparition of Halley’s Comet in that year, he analyzed photographs of Comet Morehouse that had been taken with the 30-in. reflector of the Royal Observatory in Greenwich. Eddington was then 28 years old and the Chief Assistant Astronomer at the observatory. In the publication of the results of his analysis, Eddington noted that one of the most striking features of the photographs was “. . . the clear definition of the parabolic envelopes, which often appear in and near the head of the comet. . . . The great difficulty is their almost instantaneous formation, which seems to require the existence of a force of solar repulsion enormously greater than that generally accepted” (Eddington, 1910:442). In his paper, Eddington discussed the “fountain theory” of envelopes for comet heads: It is well known that when a great number of particles are projected from a point with equal velocities in all directions under gravity, the envelope of their paths is a paraboloid having the point of projection as focus. There seems to be little doubt that cometary envelopes are formed in an analogous manner. In this case the repulsion from the Sun takes the place of gravity, the particles are supposed to be projected from the nucleus with equal velocities in all directions in the hemisphere towards the Sun (those in the hemisphere away from the Sun have no part in forming the envelope), and the envelope is a paraboloid having the nucleus as focus and forming a kind of dome over it. (Eddington, 1910:443)

In his “fountain” analysis, Eddington obtained values of the ratio (μ) of the solar repulsion to its gravitational attraction in the range 180–19,000. He noted that: These values are of course much greater than the values ordinarily found. In Bredichin’s researchers μ was not greater than 36; subsequent discussions of observations by Jaegermann and others have led to values of μ up to about 80. Direct measurements of the motion of tail particles of Comet Morehouse leads to values of μ ranging under normal

2.2

Background: Evidence of Solar-Terrestrial Links

19

Fig. 2.9 Edward Barnard. Credit AIP Emilio Segrè Visual Archives

conditions to rather more than 100; and on October 1 and 2, when the motion was quite exceptionally rapid, μ may have been as great as 800. But the above results seem to show that the repulsive forces on the envelope material are generally greater still, and it will be necessary to carefully consider whether there is any possible escape from this conclusion. (Eddington, 1910:449)

Various ideas on the nature of the repulsive force required to account for the structure of comets were being discussed at the time of Eddington’s paper. In a 1910 article that was reproduced in 1935, the Irish-born astronomer Andrew C. D. Crommelin (1865–1939) listed three possible theories to explain the repulsion of the tail from the Sun: 1. Light-pressure 2. Electrical repulsion 3. Mechanical bombardment by electrons, or other tiny particles violently ejected from the sun (Crommelin, 1937). Crommelin thought that all three forces might be acting on comets. This despite FitzGerald’s observation in 1900 of “the difficulty of ascribing their repulsion by the Sun to the Maxwell pressure of radiation, in that there is no evidence that the molecules of any gas absorb more than a very minute proportion of the radiation that falls upon them” (FitzGerald, 1900:287, emphasis in the original). In the early twentieth century, other astronomers, notably among them Edward E. Barnard (1857–1923) (Fig. 2.9) of the Yerkes Observatory, began to photograph comets to analyze the behavior of comet tails. In particular, Barnard documented the instances when the tail of a comet disconnected from its head. It seems that immediately after the separation of the tail from the head a new and slender tail was shot out from the head at a different angle from that of the receding one. In the stereoscope this new tail is seen to pass behind the old one—away from us and toward the background of the stars. It was moving out much faster than the rear portion of the old tail—a peculiarity that seems to be always present in the general process of forming a new tail. . . . (Barnard, 1920:106)

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Fig. 2.10 Ludwig Biermann. Credit AIP Emilio Segrè Visual Archives, John Irwin Slide Collection

Barnard had attributed the change in direction of the comet tail after disconnection as due to a “sudden change in the direction of emission of the particles [from the comet’s head]” (Barnard, 1903:213). The solar wind, with its impact on comet tails was unknown to Barnard and other astronomers, but that would begin to be remedied in the next few decades. In 1951, German astronomer Ludwig F. B. Biermann (1907–1986), (Fig. 2.10) proposed that the repulsive force acting on comets must be particles coming from the Sun, or as he termed it, “solar corpuscular radiation” (Biermann, 1951:275). He ruled out solar radiation pressure (Biermann, 1952:252–253). Biermann pointed out that the small angle between the direction of a comet tail and the radius vector from the Sun, i.e., the aberration angle, was consistent with the assumed flow of “matter” from the Sun: Another observational fact, which may be related to the theoretical picture presented here, is this. Hoffmeister found, from the discussion of a large number of comets, that there is generally an angle between what he calls the primary tail and the radius vector sun-comet, in such a sense, that the tail appears to lag behind in the plane of the comet’s orbit by an angle of a few degrees. Since the comet is moving across the plasma in interplanetary space with a velocity of the order of 40 km/sec, the acceleration would not be exactly in the direction away from the sun, but inclined to it by an angle given by the proportion of this transverse component of its velocity to the velocity of the solar matter (of order 1,000 km/sec). . . . (Biermann, 1952:260).

Finally, Biermann pointed out that the observation of comet tails indicated that the corpuscular radiation was continuously flowing from the Sun (Biermann, 1957:109).

2.3

The Theories of Parker and Chamberlain

2.3 2.3.1

21

The Theories of Parker and Chamberlain Parker’s Solution: The Solar Wind

In 1958, physicist Eugene N. Parker (1927–2022) (Fig. 2.11) of the University of Chicago set out to explain Biermann’s observations that “gas is often streaming outward in all directions from the sun with velocities of the order of 500–1000 km/sec.” (Parker, 1958a: 664). Parker estimated that the flow of energy from the Sun via Biermann’s “solar corpuscular radiation” was 1.5 × 1029 ergs/sec, and he pointed out the problem of determining how the gas in the solar corona could escape from the Sun: Even at a coronal temperature of 3 × 106 K, the thermal velocity of a hydrogen ion is only 260 km/sec, and escape from the solar gravitational field (starting 3 × 105 km above the photosphere) requires 500 km/sec, to say nothing of leaving a residual 500–1000 km/sec at infinity (Parker, 1958a:664).

Parker speculated that the outflow of the gas from the Sun was the result of the heating of the coronal gases to ~106 K. In a similar way that Eddington proceeded to analyze the internal structure of stars without knowing the source of stellar energy, Parker proceeded to analyze the flow of gas from the Sun without knowing the details of coronal heating: We do not know quantitatively in what manner the heating of the solar corona is distributed over r [the radial distance from the center of the Sun], and in particular we do not know to what distance it extends. We shall, therefore, merely assume that T (r) [the dependence of the temperature on r] is given, rather than try to compute it by some general heat-flow equation (Parker, 1958a:666).

Parker further supposed that: the temperature is maintained (by heating mechanisms) at the uniform value T0 from r = a out to some radius r = b. Beyond r = b we suppose that the heating vanishes. Since the outward expansion of the corona consumes 1.5 × 1029 ergs/sec and the thermal conduction is

Fig. 2.11 Eugene Parker. Credit: University of Chicago Photographic Archive, [apf1-11096], Hanna Holborn Gray Special Collections Research Center, University of Chicago Library

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Fig. 2.12 Parker’s solutions for the velocity of the solar wind, parameterized by the temperature of the solar corona. Fitzpatrick (2022). Copyright (2022), from Plasma physics: an introduction (2nd edition) by Richard Fitzpatrick. Reproduced by permission of Taylor and Francis Group, LLC, a division of Informa LC. Figure redrawn by John Oertel

not capable of transporting even 1 per cent as much energy, we may reasonably take T to be negligible beyond r = b (Parker, 1958a:667).

With these simplifying assumptions, plus the assumption of spherical symmetry, Parker solved (1) the equation of motion, namely that the flux of momentum through a spherical shell around the sun must equal the outward push due to the negative gradient in thermal pressure and the inward pull due to the force of solar gravity, together with (2) the condition of continuity, i.e., that the flux of mass through a spherical shell around the Sun must not vary with the radius of the shell. Parker found that “even the 160 km/sec thermal velocities of 106 K are sufficient to push gas out of the solar gravitational field (escape velocity 500 km/sec) and give the gas an additional 500 km/sec” (Parker, 1958a:668) (see Fig. 2.12). Parker referred to the outward streaming gas from the Sun as the “solar wind” (Parker, 1958b:171). Parker’s submission of his theory results to The Astrophysical Journal was rejected by the reviewer. The journal editor, astrophysicist Subrahmanyan Chandrasekhar (1910–1995) (Nobel Prize 1983), also at the University of Chicago, sent the paper to a second reviewer, who also rejected it. Parker relates: . . .one day Chandra came to my office and said “Now see here, Parker, do you really want to publish this paper? I have sent it to two eminent referees, and they both say the paper is wrong”. (Parker, 2014:9)

Parker responded that the referees had no scientific criticism, and that Chandrasekhar “thought for a moment and then said ‘Alright, I will publish it.’” (Parker, 2014:9).

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The Theories of Parker and Chamberlain

23

While Parker’s solution was consistent with Biermann’s observations, it seemed counterintuitive that such high streaming velocities could result from relatively low thermal velocities in the solar corona.

2.3.2

Chamberlain’s Evaporation Model

To geophysicist Joseph W. Chamberlain (1928–2004) (Fig. 2.13) of the Yerkes Observatory of the University of Chicago, the Parker solution seemed counterintuitive, because it was wrong. Chamberlain speculated that the flux of plasma observed by Biermann and others was due to the evaporation of particles from the solar atmosphere. The thermal speeds cited by Parker, e.g., 160 km/sec for a coronal temperature of 1 million degrees Kelvin, are the root-mean-square speeds in a distribution of speeds called a Maxwell-Boltzmann distribution (sometimes called a Maxwell or Maxwellian distribution) (Fig. 2.14). There are many hydrogen ions in the solar corona with speeds higher than the root-mean-square speed; some of these ions will have speeds higher than required to escape from the Sun, i.e., to evaporate from the Sun’s corona. Chamberlain extended an evaporation model proposed earlier by British physicist James Jeans (1877–1946), among others, and concluded that: The hydrodynamic expansion of the corona must actually be limited by the rate of evaporation, and Parker’s large expansion velocities, presumed to correspond to a “solar wind,” result from an invalid assignment of an integration constant and an ambiguity inherent in hydrodynamic solutions. (Chamberlain, 1960: 47)

2.3.3

Parker’s Response to Chamberlain

Within a few months, Parker published a response to Chamberlain’s paper, first summarizing Chamberlain’s charge: Fig. 2.13 Joseph Chamberlain. Credit University of Chicago Photographic Archive, [apf1-11553], Hanna Holborn Gray Special Collections Research Center, University of Chicago Library

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Fig. 2.14 The MaxwellBoltzmann distribution of particle speeds in a gas with a given temperature. Credit John Oertel

In a recent paper Chamberlain (1960) has questioned our earlier application of the hydrodynamic equations (Parker 1958) to the expansion of the solar corona. Besides questioning whether the adiabatic relation can be applied beyond the region of coronal heating, he suggests that our solution of the hydrodynamic equations for the isothermal corona is not appropriate because of our singular choice of the constant of integration. Thus Chamberlain argues that the enormous expansion velocity (500 km/sec) of the corona, predicted by our special solution of the hydrodynamic equations, is incorrect. He argues that one should use the classical evaporation, or kinetic escape theory, along the lines originally proposed by Stoney, Jeans, and others if we wish to compute correctly the efflux of matter from the solar corona. (Parker, 1960:175)

As he concluded the above paragraph, Parker added that “Evaporation gives very low escape velocities and densities” (Parker, 1960:175). Parker then made it clear that he did not agree with Chamberlain’s arguments: It is the purpose of this paper to illustrate some new, singular mathematical properties of the solutions of the hydrodynamic equations and show that our original solution, yielding the large expansion velocities of the order of 500 km/sec in interplanetary space, is, in fact, the only physically correct solution. We shall go on to show that the enormous discrepancy between our hydrodynamic predictions and the results of classical evaporation theory, in which the expansion velocity goes to zero at large distances from the sun, is a consequence of the erroneous assumption in the evaporation theory that the evaporation takes place from the top of a static atmosphere. As a matter of fact, we shall find that if evaporation from the solar corona occurs at all, it is from a level at which the hydrodynamic expansion is already supersonic. (Parker, 1960:175—emphasis in the original)

Parker redeveloped his hydrodynamic treatment of the expansion of the solar corona but this time in a more general way so that he could examine the effect of varying the boundary condition for the flow at the base of the corona (Fig. 2.15). He found that: 1. The hydrodynamic treatment of the corona produced a “critical” solution in terms of the kinetic energy of the plasma bulk flow at its base 2. His former solution corresponded to this critical solution and

2.3

The Theories of Parker and Chamberlain

25

Fig. 2.15 Sketch of ψ(ξ) = Mv2/2kT as a function of ξ = r/a for various values of ψ(1). Of particular importance is the nature of the change in character of ψ(ξ) as ψ(1) increases to the critical value ψc. For ψ(1) > ψc there are no solutions going from the base of the corona, ξ = 1, beyond ξ = λ/4, where λ is the gravitational potential at r = a in units of kT0, and T0 is the temperature at the base of the corona. The maximum in ψ(ξ) at ξ = λ/4 becomes sharper and sharper as ψ(1) increases toward ψc. The solution approaches the angle ABC of the asymptotes AB and BC. On the other hand, the solution for ψ(1) = ψc is smooth across the critical point. From Fig. 1 of Parker (1960), re-drawn by John Oertel

3. This solution was the only physically reasonable one. There were no solutions for the case where the kinetic energy of the bulk flow was greater than the critical value at the base of the corona, and if the kinetic energy of the bulk flow were less than the critical value it would increase to the critical value Parker briefly examined the evaporation theory and found what he called the “error in the evaporation theory”: By fixing its attention on the fast ions, it overlooks the fact that the average thermal ion, whose bulk motion satisfies the hydrodynamic equation as the result of its high coulomb collision rate, has enough energy to give a supersonic hydrodynamic expansion at large distance from the sun. (Parker, 1960:182—emphasis in the original)

Parker thanked Chamberlain for “stimulating discussion of the problems discussed in this paper” (Parker, 1960:182). Chamberlain was perhaps stimulated by his discussion with Parker, but he was not entirely convinced.

2.3.4

Chamberlain’s Response to Parker: The Solar Breeze

In the beginning of 1961, Chamberlain published a paper in which he acknowledged that Parker had shown “that a hydrostatic model may be severely unrealistic” (Chamberlain, 1961a:675). Chamberlain therefore took a hydrodynamic approach, but one that included the first law of thermodynamics with heat conduction, which

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Parker had ignored. In additions to the equations of motion and continuity, Chamberlain’s analysis included an equation that required that the net heat flow into a volume must appear either in work done by the gas in expanding or in an increase in temperature (Chamberlain, 1961a:677). Whereas Parker frequently cited the research on comet tails by Biermann and others as evidence for a continuously flowing supersonic solar wind, Chamberlain argued that: Coronal observations offer no basis for suspecting an outward expansion of the interplanetary gas faster than several kilometers per second at the earth’s orbit— corresponding to a gentle “solar breeze.” (Chamberlain, 1961a:675)

Following his hydrodynamic/thermodynamic analysis, Chamberlain continued to maintain that the evaporation theory and the hydrodynamic approach “merely provided two approximations for describing one phenomena. . .” (Chamberlain, 1961a:676). He concluded that: a slow hydrodynamic expansion is, in fact, entirely realistic, that it gives a solution that is roughly similar to that of the kinetic or evaporative approach, and that the existence of anything stronger than a solar breeze must either be deduced observationally or postulated ad hoc—it is not an inevitable phenomenon. (Chamberlain, 1961a:680)

To a certain extent Chamberlain began to redefine his debate with Parker. He maintained that he was trying to demonstrate that a stellar wind was not inevitable. Further, while there might be a solar wind, the issue was whether it came about by a simple thermal expansion of the solar corona: High outward velocities, whether they form a more or less continuous wind or only sporadic solar streams, might be attributed to some acceleration mechanism at the sun, but probably not to thermal expansion of the corona. (Chamberlain, 1961a:676)

Despite these caveats, at the conclusion of his 1961 paper, Chamberlain predicted that the expansion velocity of the solar corona at the Earth’s orbital distance would be observed to be around 18 km/sec (Chamberlain, 1961a:687). Chamberlain’s conclusion was supported by the astronomer John C. Brandt on the basis of Brandt’s analysis of measurements of aberrations in the tails of Comet Baade 1954 h and Comet Haro-Chavira 1954 k by Donald E. Osterbrock (1924–2007) (Osterbrock, 1958). The measured aberration angle was about 45◦, which implied a plasma velocity of no more than 50 km/sec at 4 AU. (Brandt, 1961:1092). Brandt concluded that the “identification of the solar corpuscular radiation with the interplanetary gas, as proposed by Biermann (1957), should be regarded as an open question” (Brandt, 1961:1092).

2.4

The Direct Measurements of the Solar Wind

2.4

27

The Direct Measurements of the Solar Wind

The controversy between Parker and Chamberlain was short lived because of the advent of spacecraft launches and the direct observations of the interplanetary medium. These measurements began in 1959 with the launch by the Soviet Union of the robotic Luna spacecraft toward the Moon and with the launch of Venera 1 toward Venus in 1961. The initial U.S. launches of spacecraft into interplanetary space that could measure particle fluxes included those for Explorer 10 in 1961 and Mariner 2 in 1962.

2.4.1

Luna Missions

The Luna 1 spacecraft was launched by the Soviet Union on 2 January 1959. It was probably intended to impact the Moon but made a close fly by instead. Luna 2, launched on 12 September 1959, was the first spacecraft to impact the Moon. Luna 3 was launched on 4 October 1959 and was the first spacecraft to record images on the far side of the Moon. All the Luna spacecraft had so-called “three-electrode charged particle traps,” which were devices that could measure the flux of positively charged particles with energies above 15 eV. Each of the four traps on each spacecraft had a fixed orientation on the surface of the spacecraft, but the overall orientation of the spacecraft was undetermined. Therefore, the direction of particle streaming could not be determined. Nevertheless, the recording of high fluxes of positively charged particles above 15 eV well away from the Earth indicated the existence of low energy particles coming from the sun. Reports of the analyses of Luna data were published in the Russian journals Doklady Academy of Sciences and Astronomicheskii Zhurnal in the fall of 1960, at about the time Chamberlain submitted his 1961 solar breeze paper. The Doklady paper, whose lead author was Russian physicist Konstantin I. Gringauz (1918–1993) (Fig. 2.16), reported fluxes of positively charged particles in the range of 2 × 108/ cm2/sec (Gringauz et al., 1960). For Luna 2, these particles were measured right up to the moment of spacecraft impact with the Moon, so the spacecraft was assumed to be in interplanetary space. The energy of measured particles, assumed to be protons, exceeded 15 eV, which corresponds to streaming velocities of at least 50 km/sec. The Luna 2 instrumentation thus made the first direct measurement of solar corpuscular radiation that indicated that Parker’s idea of a solar wind was more likely to be correct than Chamberlain’s solar breeze. But the issue wasn’t settled.

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Fig. 2.16 Konstantin Gringauz. Credit Tamas Gombosi, University of Michigan

2.4.2

Venera 1

Venera 1 was launched by the Soviet Union on 12 February 1961. It was the first spacecraft to fly by Venus. Venera 1 carried sunward-directed ion traps with higher retarding potentials, so that lower limits of the streaming velocities of solar wind particles could be better established. About a half hour before a major magnetic storm commenced on Earth, Venera 1 recorded fluxes of positive ions coming from the solar direction of approximately 1 ×109/cm2/sec (Gringauz, 1961:550). These particles had kinetic energies greater than 50 eV, which correspond to streaming velocities of greater than about 100 km/ sec. The Venera 1 data, therefore, tended to support the solar wind model of Parker.

2.4.3

Explorer 10

The U.S. spacecraft Explorer 10 was launched on 25 March 1961 into a highly elliptical orbit about the Earth. The instruments onboard could better determine the energies of the solar wind particles. Explorer 10 carried a so-called Faraday cup for plasma measurements. The Faraday cup, as designed by physicist Herbert S. Bridge (1919–1995) (Fig. 2.17) of MIT, had the ability to measure fluxes of protons with threshold energies of 5, 20, 80, 250, 800, and 2300 eV (Bridge et al., 1962:554). Bridge and his colleagues found that beyond a distance of about 21.5 Earth radii (from the center of the Earth), the Faraday cup recorded a flux of positive particles of about 4 × 108/cm/sec at a mean energy of about 500 eV. The streaming velocity of a 500-eV proton is about 300 km/sec. Combining the flux and energy measurements of the Faraday cup, Bridge and his colleagues found that typical number densities in the solar wind ranged from 6 to 20 protons per cm3. They also noted that maximum

2.4

The Direct Measurements of the Solar Wind

29

Fig. 2.17 Herbert Bridge. Credit Fran Bagenal, University of Colorado

Fig. 2.18 Marcia Neugebauer and Conway Snyder with a version of their instrument that would be set up by the Apollo 12 astronauts to measure the solar wind impinging on the Moon. Credit Marcia Neugebauer

intensities occurred when the angle between the axis of the Faraday cup and the Sun-vehicle line was a minimum (Bridge et al., 1962:555–556). The particle measurements from Explorer 10 seemed to confirm Parker’s solar wind theory.

2.4.4

Mariner 2

Mariner 2 was launched on 27 August 1962 as a fly-by mission to the planet Venus. It was an attitude-stabilized spacecraft, so its ion detector could point directly at the Sun. The plasma analyzer was designed by physicists Marcia Neugebauer and Conway W. Snyder (1918–2011) (Fig. 2.18) of the Jet Propulsion Laboratory (which built the Mariner 2 spacecraft). Their instrument consisted of a pair of curved electrodes and an electrometer which measured the flux of incoming ions as a function of the voltage applied to the electrodes.

30

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Neugebauer and Snyder reported that “One of the principal results of the Mariner plasma experiment is the finding that there was always a measurable flow of plasma from the direction of the sun” (Neugebauer & Snyder, 1962:1095). They noted that the velocity of the streaming plasma was generally in the range of 400–700 km/sec (Neugebauer & Snyder, 1962:1095). Neugebauer and Snyder also reported that their plasma instrument recorded a fast-moving plasma front that was associated with a geomagnetic storm registered on Earth about 4½ h later. Thus, taking together all of the measurements in interplanetary space over the 3-year period, Parker’s model of the solar wind, though idealized, better described the outflow of plasma from the Sun than did the Chamberlain model of a solar breeze. Planetary astronomer Donald M. Hunten (1925–2010), in a biographical tribute to Chamberlain, made the point that both solutions (Parker’s and Chamberlain’s) could be correct, depending on the radial temperature profile assumed for the coronal atmosphere of the Sun or a star, and that “Parker’s use of a temperature profile resembling the observed one had led him to the solution that is actually valid for the sun” (Hunten, 2005:15–16). Hunten wrote that the dialog between Parker and Chamberlain improved their theoretical analyses “so that the net result was a genuine benefit to the subject” (Hunten, 2005:16). Finally, in a review article published in 1965, Parker noted that: If T(r) declines asymptotically exactly as 1/r, instead of faster or slower, the stationary equilibrium may be a subsonic expansion resembling the classical evaporation from the top of a static atmosphere. This case has been explored extensively by Chamberlain (1960, 1961a, b) . . . (Parker, 1965:672).

2.5

Eddington’s Guidelines

The speculations of Parker and Chamberlain were largely consistent with the guidelines suggested by Eddington. As discussed in Chap. 1, Eddington’s guidelines for science speculation, slightly modified for applicability to space science, are: 1. Was the speculator rigorous in applying the appropriate science applicable to the model 2. Did the speculator identify all the underlying assumptions used in constructing the model and 3. Did the speculator view the model objectively, as an “adjustable engine,” as opposed to a “finished building”? Parker and Chamberlain both developed physics-based mathematical analyses of the expanding coronal atmosphere of the Sun. Parker’s solutions produced a supersonic flow of plasma that he termed a “solar wind,” and Chamberlain’s solutions produced a sub-sonic flow, which he termed a “solar breeze”. Both worked out their idealized models with rigorous mathematics, and their underlying assumptions were clearly stated. Whether they built their models with the

2.6

Continuing Understanding

31

hope that observations would reveal flaws in them is perhaps a “counsel of perfection” that not even Eddington could keep. But both Parker and Chamberlain likely viewed their idealized models as “engines” rather than finished “buildings”. They certainly worked out the effects of “adjusting the levers” in their models. Parker and Chamberlain both contributed importantly to, and were highly recognized for, their areas of expertise in geophysics, space physics, and astrophysics. Parker’s small 1963 volume Interplanetary Dynamical Processes has been a widelyreferenced classic for studies of solar system plasmas and magnetic fields (Parker, 1963). His classic text Cosmical Magnetic Fields (2019) covers the appearance and properties of magnetic fields throughout the astronomical universe. The NASA Parker Solar Probe (PSP; launched 12 August 2018) was named in his honor, the first time a flying spacecraft was named for a living person. Chamberlain’s 1961 book Physics of the Aurora and Airglow (Chamberlain, 1961b) and his 1978 volume Theory of Planetary Atmospheres (Chamberlain, 1978) were major texts in the field.

2.6

Continuing Understanding

Following the settlement by in-situ measurements of the debate on solar wind versus solar breeze, Edmund J. Weber and Leverett Davis Jr. (1914–2003), both of Caltech, published a comprehensive theoretical analysis of solar wind flow that examined the effects of the solar wind and the embedded magnetic field on the angular momentum of the sun (Weber & Davis, 1967). While they summarize that their “model reproduces essentially Parker’s (1963) radial solution,” their full solution has been used extensively in astrophysical studies of stellar jets and winds, as well as the Sun. Studies of the Sun, a human pursuit for understanding of nature since antiquity (e.g., Vaquero & Vazquez, 2009; Vita-Finzi, 2013), can be considered in some ways a mature field of research. Yet it is one that continues to provide new understanding of stellar structure, and of the solar system and Earth’s place in it (Aschwanden, 2006, 2019). In the context of Parker’s theoretical prediction and its observational verification, extensive research on the solar wind (e.g., Cranmer et al., 2017) and its source in the solar corona has continued (e.g., Golub & Pasachoff, 2009; Antonucci et al., 2020). The solar corona is now known to not expand outward and uniformly over its entire surface area according to Parker’s model (Sect. 2.2.1, Fig. 2.8). Rather, in some areas, the solar wind speed can be considerably faster than the model would predict. In these areas, called coronal holes, the coronal magnetic field lines do not close back into the corona, but are open directly to the interplanetary medium. The largest coronal holes occur in the polar regions of the Sun and were measured over the poles by the solar polar-orbiting Ulysses spacecraft (Smith, 2008) of the European Space Agency. Coronal holes can also occur at lower solar latitudes at times. Other coronal phenomena can occur above solar active regions and solar flares. In some occurrences of flares, dynamic coronal magnetic field processes can conspire

32

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Fig. 2.19 Image of a Coronal Mass Ejection (CME) taken by the Large Angle and Spectrometric Coronagraph (LASCO) instrument on the joint NASA/ESA SOHO (Solar and Heliospheric Observatory) spacecraft. Credit SOHO/LASCO, SOHO/EIT (ESA and NASA)

to inject large clumps of coronal hot gases into the interplanetary medium (Fig. 2.19). These coronal mass ejections (CMEs) can propagate to Earth’s orbit (e.g., Manchester et al., 2017) and produce large disturbances in the magnetosphere and on Earth’s surface. The large September 1859 magnetic disturbance event is widely ascribed to a CME (but of course no spacecraft measurements were available at that time). The magnitude of such geomagnetic disturbances depends upon the orientation of the interplanetary magnetic field; when the field is oriented largely southward, disturbances can be significantly larger than they might otherwise be. In Parker’s solar wind model, the Sun’s magnetic field is stretched into the interplanetary medium by the out-flowing solar wind plasma. The field takes a spiral shape in the interplanetary medium because it remains anchored in the rotating Sun as the wind flows radially outward. The model does not ascribe any dominant north-south direction to the field. While not discussed by Parker (because it was not of relevance to his wind model), this north-south orientation that the interplanetary field can take under different solar wind conditions will be seen to be critical in the speculation and controversy regarding the nature of Earth’s magnetosphere, as described in Chap. 3. CMEs have even been measured to the edge of the solar system as evidenced by data returned from the two Voyager spacecraft. As described in Chap. 8, both Voyagers have now reached the plasma environment outside the solar cavity defined by the interaction of the solar wind with the local interstellar medium (e.g., Burlaga et al., 2008). Understanding the source of coronal heating, and thus the solar wind in its various manifestations, is one the key goals of the Parker Solar Probe. The PSP will ultimately approach within nine solar radii of the Sun (Fox et al., 2016). It is fitting that the PSP is named for the theoretician who predicted the supersonic “wind” blowing from the Sun and filling the solar system.

References

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References Antonucci, E., Harra, L., Susino, R., & Telloni, D. (2020). Observations of the solar corona from space. Space Science Reviews, 216, 53. Aschwanden, M. (2006). Physics of the solar corona: An introduction with problems and solutions. Springer. Aschwanden, M. (2019). New millennium solar physics. Springer. Barnard, E. E. (1903). Photographic observations of Borrelly’s comet and explanation of the phenomenon of the tail on July 24. The Astrophysical Journal, 18, 210. Barnard, E. E. (1920). On comet 1919b and on the rejection of a comet’s tail. The Astrophysical Journal, 51, 102. Bethe, H. A. (1939). Energy production in stars. Physical Review, 55(5), 434–456. Bethe, H. (1968). Energy production in stars. Science, 161, 541–547. Biermann, L. (1951). Kometenschweife und solare Korpuskularstrahlung. Zeitschrift für Astrophysik, 29, 274–286. Biermann, L. (1952). Physical processes in comet tails and their relation to solar activity. Liege International Astrophysical Colloquia, 4, 251–262. Biermann, L. (1957). Solar corpuscular radiation and the interplanetary gas. The Observatory, 77, 109–110. Bobrovnikoff, N. T. (1928). The present state of the theory of comets. Publications of the Astronomical Society of the Pacific, 40(235), 164–190. Brandt, J. C. (1961). On the study of comet tails and models of the interplanetary medium. The Astrophysical Journal, 133, 1091–1092. Bridge, H. S., Dilworth, C., Lazarus, A. J., Lyon, E. F., Rossi, B., & Scherb, F. (1962). Direct observations of the interplanetary plasma. Journal of the Physical Society of Japan Supplement, 17, 553. Burbidge, E. M., Burbidge, G. R., Fowler, W. A., & Hoyle, F. (1957). Synthesis of the elements in stars. Reviews of Modern Physics, 29(4), 547. Burlaga, L. F., Ness, N. F., Acuna, M. H., Lepping, R. P., Connerney, J. E. P., & Richardson, J. D. (2008). Magnetic fields at the solar wind termination shock. Nature, 454, 75–77. Carrington, R. C. (1859). Description of a singular appearance seen in the Sun on September 1, 1859. Monthly Notices of the Royal Astronomical Society, 20, 13–15. Carrington, R. C. (1863). Observations of the Spots on the Sun from November 9, 1853, to March 24, 1861, Made at Red Hill. Williams and Norgate. Chamberlain, J. W. (1960). Interplanetary gas. II. Expansion of a model solar corona. The Astrophysical Journal, 131, 47. Chamberlain, J. W. (1961a). Interplanetary gas. III. A hydrodynamic model of the corona. The Astrophysical Journal, 133, 675. Chamberlain, J. W. (1961b). Physics of the aurora and airglow. Academic. Chamberlain, J. W. (1978). Theory of planetary atmospheres. Academic. Chapman, S., & Ferraro, V. C. A. (1931a). A new theory of magnetic storms. Part I – The initial phase. Terrestrial Magnetism and Atmospheric Electricity, 36, 77–97. Chapman, S., & Ferraro, V. C. A. (1931b). A new theory of magnetic storms. Part I – The initial phase. Terrestrial Magnetism and Atmospheric Electricity, 36, 171–186. Chapman, S., & Ferraro, V. C. A. (1933). A new theory of magnetic storms. Part II – The main phase. Terrestrial Magnetism and Atmospheric Electricity, 38, 79–96. Clark, S. (2007). The sun kings. Princeton University Press. Cranmer, S. R., Gibson, S. E., & Riley, P. (2017). Origins of the ambient solar wind: Implications for space weather. Space Science Reviews, 212, 1345–1384. Crommelin, A. C. D. (1937). The origin and nature of comets. In M. Proctor & A. C. D. Crommelin (Eds.), Comets; their nature, origin, and place in the science of astronomy. The Technical Press. Eddington, A. S. (1910). The envelopes of comet Morehouse (1908c). Monthly Notices of the Royal Astronomical Society, 70(5), 442–458.

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FitzGerald, G. F. (1900). Sunspots, magnetic storms, comet tails, atmospheric electricity and aurorae. The Electrician, 46(249), 287–288. Fitzpatrick, R. (2022). Plasma Physics: An introduction. CRC Press. https://doi.org/10.1201/ 9781003268253 Fox, N. J., Vella, M. C., Bale, S. D., Decker, R., Driesman, A., Howard, R. A., Kasper, J. C., Kinnison, J., Kusterer, M., Lario, D., Lockwood, M. K., McComas, D. J., Raluafi, N. E., & Szabo, A. B. (2016). The solar probe plus mission: Humanities first visit to our star. Space Science Reviews, 204, 7–48. Gollub, L., & Pasachoff, J. M. (2009). The solar corona. Cambridge University Press. Gringauz, K. I. (1961). Some results of experiments in interplanetary space by means of charged particle traps on Soviet space probes. Space Research, II, 539. Gringauz, K. I., Bezrukikh, V. V., Ozerov, V. D., & Rybchinskii, R. E. (1960). A study of interplanetary ionized gas, energetic electrons, and solar corpuscular radiation using three electrode charged particle traps on the second Soviet cosmic rocket. Soviet Phys.: Doklady, 5, 361. See also the English translation of this paper by R. Matthews in Planetary and Space Science, 9(3), 103–107, published in 1962. Habbal, S. R., Druckmüller, M., Alzate, N., Ding, A., Johnson, J., Starha, P., et al. (2021). Identifying the coronal source regions of solar wind streams from total solar eclipse observations and in situ measurements extending over a solar cycle. The Astrophysical Journal Letters, 911(1), L4. https://doi.org/10.3847/2041-8213/abe775 Hodgson, R. (1859). On a curious appearance seen in the Sun. Monthly Notices of the Royal Astronomical Society, 20, 15–16. Humboldt, A. V. (1850). Kosmos (Vol. 3). JG Gotta’scher Verlag. Hunten, D. M. (2005). Joseph W. Chamberlain (1928-2004): A biographical memoir. The National Academies Press. Jaegermann, R. (1903). Bredichin’s Mechanische Untersuchungen uber Cometenformen, St. Petersburg Kelvin, W. T. (1892). Address to the Royal Society at their anniversary meeting, Nov. 30, 1892. Proceedings of the Royal Society of London, A, 52, 300–310. Manchester, W., IV, Kilpua, E. K., Liu, Y. D., Lugaz, N., Riley, P., Torok, T., & Vrsnak, B. (2017). The physical processes of CME/ICME evolution. Space Science Reviews, 212, 1159–1219. Neugebauer, M., & Snyder, C. W. (1962). Solar plasma experiment. Science, 138(3545), 1095–1097. Osterbrock, D. E. (1958). A study of two comet tails. The Astrophysical Journal, 128, 95. Parker, E. N. (1958a). Dynamics of the interplanetary gas and magnetic fields. The Astrophysical Journal, 128, 664. Parker, E. N. (1958b). Suprathermal particle generation in the solar corona. The Astrophysical Journal, 128, 677. Parker, E. N. (1960). The hydrodynamic treatment of the expanding solar corona. The Astrophysical Journal, 132, 175. https://doi.org/10.1086/146910 Parker, E. N. (1963). Interplanetary dynamical processes. Interscience Publishers. Parker, E. N. (1965). Dynamical theory of the solar wind. Space Science Reviews, 4(5–6), 666–708. Parker, E. N. (2014). Reminiscing my sixty year pursuit of the physics of the Sun and Galaxy. Research in Astronomy and Astrophysics, 14, 1. Parker, E. N. (2019). Cosmical magnetic fields: Their origin and their activity. Oxford University Press. Sabine, E. (1851). V. On periodical laws discoverable in the mean effects of the larger magnetic disturbances. Philosophical Transactions of the Royal Society of London, 141, 123–139. Sabine, E. (1852). VIII. On periodical laws discoverable in the mean effects of the larger magnetic disturbance—No. II. Philosophical Transactions of the Royal Society of London, 142, 103–124. Schwabe, H. (1844). Sonnenbeobachtungen im Jahre 1843. Von Herrn Hofrath Schwabe in Dessau. Astronomische Nachrichten, 21, 233.

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Smith, E. J. (2008). The global heliospheric magnetic field. In A. Balogh, L. J. Lanzerotti, & S. Seuss (Eds.), The heliosphere through the solar activity cycle. Springer. Stanley, M. (2007). So simple a thing as a star: The Eddington–Jeans debate over astrophysical phenomenology. The British Journal for the History of Science, 40(1), 53–82. Vaquero, J. M., & Vazquez, M. (2009). The sun through history. Springer. Vita-Finzi, C. (2013). Solar History: An Introduction. Springer. Von Humboldt, A., (1874). A Nabu public domain reprint of Kosmos: Entwurf Einer Physischen Weltbeschreibung. Mit Einer Biographischen Einleitung (Vol. 3). Bernhard Von Cotta. Weber, E. J., & Davis, L., Jr. (1967). The angular momentum of the sun. The Astrophysical Journal, 148, 217–227.

Chapter 3

Open Versus Closed Magnetosphere

3.1

Introduction

Prior to the discovery of the Van Allen radiation belts, both textbooks and popular books related to astronomy and physics generally depicted the space environment around Earth as “empty.” Space near Earth was filled only with Earth’s magnetic field extending outward with decreasing intensity with altitude. And the “space” was certainly considered benign as far as radiation was concerned. Cosmic rays had been discovered by Victor Hess (1883–1964) at the beginning of the twentieth century (Hess, 1912), and were an intense topic of physics research after that time (see also Chap. 1). In the same era, Norwegian scientist Kristian Birkland (1867–1917) had conducted his laboratory studies of electron beams impacting a magnetized terrella to understand aurora (Egeland & Burke, 2005). But yet, there was no reason to have a discussion as to whether any spatially finite and distinct region of space might surround Earth. Van Allen’s discovery meant that this new phenomenon and its extent around Earth required understanding. The ability to launch more satellites with increasingly sophisticated instrumentation was the enabling factor to understand what came to be called the magnetosphere. When a “boundary” to Earth’s radiation environment was found, crossed by spacecraft, its nature became a topic of intense scientific speculation and theoretical debate. That debate is discussed in this chapter.

3.2

Some Precursors

English mathematician and geophysicist Sydney Chapman (1888–1970) had been interested in Earth’s magnetism and the variations seen in magnetic records since his appointment in 1910 as a Chief Assistant at the Greenwich Observatory. This interest carried forward throughout his life in his studies of the ionosphere and of © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 W. D. Cummings, L. J. Lanzerotti, Scientific Debates in Space Science, Astronomy and Planetary Sciences, https://doi.org/10.1007/978-3-031-41598-2_3

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geomagnetism (a term that he coined). It was well recognized that large variations could occur in the magnetic records, and that these excursions appeared to be related to activity manifested by sunspots and solar flares appearing on the surface of the Sun. At the same time during the occasions of the large excursions of the magnetic field, disturbances could occur in telegraph and wireless communications systems. In an analysis of magnetic records from a wide range of observatories, Chapman concluded that all great magnetic storms commenced simultaneously “within a few seconds over the whole world” (Chapman, 1918). Chapter 2 outlines the initiation of the establishment of the world-wide network of geomagnetic observatories. Requiring an explanation were the initial increases of the geomagnetic field followed by the large decreases in the magnitude of the magnetic field in the north-south direction at the beginning of the storms, and then the slow recovery to normal. A hypothetical sheet of electrical current flowing concentrically around Earth in the space above it could produce the observed decrease in the magnetic field at Earth’s surface. Chapman and his first graduate student at Imperial College London, Vincenzo C. A. Ferraro (1907–1974), addressed the formation of such a current. They postulated that a neutral, perfectly conducting stream of particles from the Sun was involved (Chap. 2). Any magnetic fields that might occur within such a stream were not considered. Chapman and Ferraro recognized that such a stream of particles, being a perfect conductor, could not penetrate the lines of force of Earth’s magnetic field. Earth’s field would exert a retarding pressure on the stream, with the retardation being greatest on the side of the Earth facing the stream, the side toward the Sun. Prior to the now classic work of Chapman and Ferraro, British physicist Frederick A. Lindemann, 1st Viscount Cherwell (1886–1957) of Oxford, had challenged an earlier study of Chapman that postulated that a beam of alpha particles from the sun could produce magnetic storms at Earth (Lindemann, 1919). On the primary basis of such a positively charged beam being mutually repulsive, Lindemann hypothesized that an “approximately equal number of positive and negative ions are projected from the Sun in something of the form of a cloud, and that these are the cause of magnetic storms” (Lindemann, 1919:673). Indeed, this postulation of Lindemann of a “cloud”, and a cloud “emitted in the neighborhood of sunspots” (Lindemann, 1919:673), can be considered a precursor of coronal mass ejections which are largely composed of electrons and ionized hydrogen (see Chap. 2). At this time in the history of astronomy and geophysics, Cecilia H. Payne (1900–1979) had not yet presented her 1925 Harvard PhD thesis research that demonstrated that hydrogen was the dominant element by far in stars (see Chap. 1). So Lindemann could not know the chemical composition of his “cloud”. The abrupt contact between the cloud of Lindemann and the stream of particles of Chapman and Ferraro from the Sun with the Earth’s magnetic field could explain the initial positive phase of the magnetic storm (referred to as a “storm sudden commencement—SSC”). A cavity would then be formed in the solar stream, with a boundary toward the Sun at the point where there was a pressure equilibrium between the kinetic pressure of the stream and the magnetic pressure exerted on the

3.3

Dungey’s Speculation of an Open Magnetosphere

39

stream (Chapman & Ferraro, 1931, 1933). The cavity illustrated in their article resembled a comet (Chapman & Ferraro, 1931). They concluded, based on the limited data, the dimension of the cavity formed in the flowing stream to be of the order of a few earth radii. This theoretical comet-like cavity was the precursor to the complexities of Earth’s (and other planets’) magnetospheres known today.

3.3

Dungey’s Speculation of an Open Magnetosphere

British physicist James W. (Jim) Dungey (1923–2015) (Fig. 3.1), is the science theoretician (speculator) focused upon in this chapter. Dungey proposed that the solar magnetic field as transported to Earth orbit by the solar wind merged with the Earth’s magnetic field at the sunward boundary of the magnetosphere and again at a point behind the Earth. In Dungey’s model, the Earth’s magnetosphere was said to be “open” to the entrance of particles and magnetic fields from the sun, rather than closed to such particles and fields. In common with most speculations that have advanced the space sciences, Dungey’s was built on earlier ideas and research results of other scientists. The individuals who influenced his speculation the most were his thesis advisor, British astronomer Fred Hoyle (1915–2001) and Ronald G. Giovanelli (1915–1984), an Australian solar physics researcher who had carefully studied solar flares.

3.3.1

Giovanelli’s Research

Giovanelli began studying the relations between sunspots and solar flares in the late 1930s while working on his Master’s degree at the University of Sydney. He noted Fig. 3.1 Every effort has been made to trace the copyright holder of this photo. To make any claim to ownership, please contact the Archives and Corporate Records of Imperial College London

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Open Versus Closed Magnetosphere

Fig. 3.2 Ronald Giovanelli. Credit the Australian Academy of Science

that the probability of a flare eruption was proportional to the area covered by sunspots in a given group and to the rate of increase in this area (Giovanelli, 1939:566–567). Following World War II, Giovanelli (Fig. 3.2) continued to work on the problem of solar flares. He soon concluded that excitation of atoms in the solar chromosphere to produce the bursts of light associated with solar flares took place only when the strength of the magnetic field at the sunspot site was very small (Giovanelli, 1946:82). It occurred to him that when the general magnetic field of the Sun combined with the field of a sunspot there could be places in the chromosphere where the two fields were of the same strength but oppositely directed so as to form a null or “neutral point” in the magnetic field. He further noted that: Apart from a general magnetic field, fields from other sunspots may still be of appreciable size in the neighborhood of the spot under consideration. It is thus to be expected that there will be places where actual neutral points exist and where conditions are thus suitable for the excitations of atoms by collision. (Giovanelli, 1946:82)

Dungey remembered of Giovanelli that: Gradually he became very familiar with solar flares and developed a growing conviction that their location was usually where a magnetic null would be expected based on the polarities of the sunspots in the group. Consistently he also found that complex spot groups were more likely to develop a flare than simple spot groups . . . (Dungey, 1995:17)

3.3.2

The Contributions of Hoyle

During the late 1940s, Giovanelli continued to refine his theory of solar flares (Giovanelli, 1947, 1948). He sent drafts of his papers to Fred Hoyle (1915–2001) (Fig. 3.3) at St. John’s College, Cambridge, as Hoyle was an external examiner for Giovanelli’s PhD thesis (Dungey, 1995:17). At the same time, Hoyle was preparing

3.3

Dungey’s Speculation of an Open Magnetosphere

41

Fig. 3.3 Fred Hoyle. Credit American Institute of Physics Emilio Segrè Visual Archives

a book that would be titled Some Recent Researches in Solar Physics, in which he emphasized that his theory of solar flares “was prompted by Giovanelli’s important papers” (Hoyle, 1949:93-footnote). In a short section of his book, Hoyle discussed magnetic storms and aurorae observed on Earth. The concept of a continuously flowing solar wind had not yet fully evolved, but Hoyle considered the consequences of the interaction of the Earth’s magnetic field with a beam of ionized particles containing a magnetic field emanating from the Sun. Thinking of a simple superposition of the northward directed magnetic field in the Earth’s equator with a southward directed magnetic field in the beam coming from the Sun, Hoyle predicted that there would be two neutral points of the combined field. There would be one neutral point on the Sun-facing side of the Earth and another on the opposite side. Acceleration of particles would take place at these neutral points. Hoyle made rough estimates of the strength of the electric field that would be produced at the neutral points (the accelerating region), as well as the size of the region, and concluded that electrons could be accelerated to energies of about 40,000 eV: After leaving the accelerating region the particles move along magnetic lines of force, some of which lead into the Earth’s atmosphere. Accordingly, we expect particles with energies ~ 40,000 eV. to enter the Earth’s atmosphere. These energies are of the order necessary to give penetration by electrons to heights ~ 100 km. above the surface of the Earth. ... . . . The two lines of magnetic force that start from the Earth and pass through the neutral points determine four definite points on the Earth’s surface where an auroral display will occur. Two of these points are in the northern hemisphere and the other two are corresponding points in the southern hemisphere. . . . (Hoyle, 1949:103–104)

At about this same time, in 1947, Dungey became a PhD student of Hoyle, who suggested that Dungey further develop Hoyle’s ideas that “the current density is much greater near a magnetic null than elsewhere and that a similar setup might account for the aurora with field lines connecting magnetic nulls to the auroral zones” (Dungey, 1995:17).

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Fig. 3.4 X-type neutral point, showing the configuration of the magnetic field lines and the pattern of the plasma flow. From Fig. 15 in Stern (2002)

Dungey set out to first understand the behavior of magnetic fields in highly conducting media, following the work of Thomas G. Cowling (1906–1990) that had recently been published (Cowling, 1946a, 1946b). Cowling was an English astronomer, a student of Sydney Chapman at Oxford, and whose last position was as a professor at the University of Leeds. Dungey seems to have been struck by one of Cowling’s main points, namely the importance of Lenz’s law in conducting material; i.e., when a magnetic field changes in conducting material at rest, electric currents are induced that tend to limit the rate of change. To Dungey, Lenz’s law posed a problem for the ideas of Giovanelli and Hoyle about the relationship between the sudden eruptions of solar flares at neutral points in the solar magnetic field. Dungey recalled that: Eventually, it occurred to me that induction effects should prevent the buildup of current density. My visualizations of telling Hoyle, however, had him questioning the validity of Lenz’s law in this application so I did a thorough check with Maxwell’s equations. This showed that induction actually drives the buildup of current density at a null and to me this was a revelation. (Dungey, 1995:17)

Dungey published his ideas about electrical discharges in the vicinity of a neutral point (Fig. 3.4) in 1953 (Dungey, 1953). His mathematical treatment of the discharge instability at neutral points neglected pressure gradients in the particle population. In a subsequent paper Peter A. Sweet (1921–2005), a graduate student with Hoyle and later an astronomer at the University of Glasgow, argued that the inclusion of the effects of particle pressure was important to the process of magnetic merging.

3.3

Dungey’s Speculation of an Open Magnetosphere

3.3.3

43

The Contributions of Sweet and Parker

In his 1956 paper, Sweet acknowledged the importance of Dungey’s work, writing: Dungey indicated that the field near a neutral point is unstable and would constrict itself to produce current sheets of the narrowness required. This effect provided the germ of the ideas in the present paper although it is shown that the gas pressure and conditions far from the neutral point, neglected by Dungey, play essential parts in the development of the high currents. (Sweet, 1956:123)

The problem of magnetic merging in conducting medium is related to the conductivity of the plasma in which the magnetic field is embedded. For a perfectly conducting plasma, the magnetic field lines cannot diffuse through the plasma and merge. If the conductivity is not perfect, diffusion of the field lines through the plasma will be possible. But a finite conductivity implies particle collisions in the plasma to produce a thermal pressure and electrical resistance. The higher the electrical resistance the faster the field lines can diffuse through the plasma and merge. Sweet suggested a mechanism (Fig. 3.5) that could reduce the diffusion time. In his model the oppositely directed magnetic field lines would be pressed together “analogous to the flattening of a motor tyre when loaded” (Sweet, 1956:129). To compensate for the magnetic pressure, a thin collision layer would form between the oppositely-directed field lines, and the gas would be forced out of the ends of this layer. This would shorten the time for the magnetic merging to take place. Eugene N. Parker (1927–2022) (Chap. 2) enthusiastically endorsed Sweet’s mechanism. In his analysis published in 1957, Parker noted that: The rapid interdiffusion of two oppositely directed fields when they are pressed together by external forces arises from the fact that the field vanishes on the surface between the two oppositely directed regions, and the entire compressive stress falls on the conducting fluid. The fluid responds to the excess pressure by flowing out of the region along the lines of force, and the two oppositely directed magnetic fields approach each other more and more closely . . .(and) interdiffuse as rapidly as the efflux of fluid from between the fields allows them to approach each other. (Parker, 1957:510)

Fig. 3.5 Sweet’s Mechanism for merging of magnetic fields. From Fig. 2 in Yamada (2011)

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Sweet and Parker were focused primarily on solar flares in the chromosphere, where the conductivity of the plasma was high but finite. The primary question for them was whether Sweet’s mechanism could account for solar flares. But, without mentioning Dungey, Parker also suggested that Sweet’s mechanism might be applicable to the penetration of the “exterior” fields in the vicinity of the Earth (Parker, 1957:519). Early in the following year, Parker published his landmark paper on the supersonic expansion of the solar corona to form the solar wind (Parker, 1958a), and a few months later, he published a paper on the interaction of the solar wind with the Earth’s magnetic field (Parker, 1958b). In this second paper, Parker estimates “the depth to which the solar wind might be expected to penetrate the geomagnetic field,” (Parker, 1958b:182) and he mentions Sweet’s mechanism, but he does not reference Dungey.

3.4

Dungey’s 1961 Paper

The Pioneer V spacecraft, launched on 11 March 1960, carried a search coil magnetometer from the Space Technologies Laboratory, Inc. (STL), with geophysicist Eugene W. Greenstadt (1932-2014) as the Principal Investigator. The magnetometer was positioned to measure the magnetic field perpendicular to the spin axis of the spacecraft, which was in the plane of the ecliptic. In July of 1960 physicists Paul J. Coleman Jr. (1932–2019), Leverett Davis Jr. (1914–2003), and Charles P. Sonett (1924–2011) of STL published a report on the analysis of Pioneer V data, showing that there is a continuously present interplanetary magnetic field. Because of the magnetometer orientation on the spacecraft, the authors were able to determine that the measured field generally had a component perpendicular to the ecliptic plane (Coleman et al., 1960). This discovery, along with another realization related to electric currents in the Earth’s polar ionosphere, prompted Jim Dungey to reintroduce the idea that merging of magnetic field lines was responsible for the aurora. Dungey opened his article in the 14 January 1961 issue of Physical Review Letters as follows: The discovery [Coleman et al., 1960] of a regular interplanetary magnetic field by Pioneer V has reawakened interest in Hoyle’s [Hoyle, 1949] suggestion that the primary auroral particles are accelerated at neutral points in the combination of an interplanetary field and the geomagnetic field. Hoyle pointed out that the latitude of the aurora would depend on the distance of the neutral points from the earth and hence on the interplanetary field strength in the observed sense. The estimated particle energy was also reasonable. Dungey [1958] discusses the accelerating mechanism. Here a qualitative model of the whole field is outlined and is found to be confirmed by the observed SD current system. (Dungey, 1961:47)

Dungey realized that electric fields associated with his model (Fig. 3.6) implied a configuration of equipotential lines (and currents) in the Earth’s polar regions that matched the known system, the SD current system (per the notation used by Chapman and Bartels (1940) in their classic book on geomagnetism, the SD current

3.4

Dungey’s 1961 Paper

45

Fig. 3.6 Dungey’s model of the magnetosphere, from Fig. 1 in Dungey (1961)

Fig. 3.7 The SD polar current system. From Fig 2 in Dungey (1961), showing equipotentials in the northern hemisphere for plasma winds over the Earth’s magnetic north pole

system is the daily part of the disturbance magnetic field) that was observed on magnetically disturbed days in the Earth’s polar regions. In Fig. 3.7, the dotted circle marked A corresponds to the boundary between open and closed field lines in Dungey’s model of the Earth’s magnetosphere. Because Hall currents dominate in the Earth’s lower ionosphere, and these currents flow perpendicularly to the electric field in the ionosphere, the equipotential lines also show the current patterns in the ionosphere. In Dungey’s model, the geomagnetic field lines entering the north polar region (or exiting the south polar region) have been merged with southward directed interplanetary magnetic field lines, and the movement of solar wind plasma over the poles creates an electric field in the fixed frame of reference of the Earth. The magnetic field lines are equipotentials, so this electric field also appears in the polar ionospheres. Nine months after the publication of his paper in Physical Review Letters, Dungey presented a paper at the International Conference on Cosmic Rays and the Earth Storm, which was held in Kyoto, Japan, September 3–15, 1961. In his paper and in a response to a question from Parker following his presentation, Dungey admitted that if the interplanetary magnetic field were pointed northward, his model would predict that there would be no auroral zones (Dungey, 1962:18).

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Dungey next presented a paper at the Plasma Space Science Symposium that was held at the Catholic University of America in Washington, D.C., 11–14 June 1963. The title of his talk was “Null Points in Space Plasma”. In this paper, Dungey disagreed with the work of Sweet and Parker, noting that: The plasma in the region of the current sheet is compressed magnetically and its gas pressure, which has been neglected previously, may have an important effect on the flow. Sweet (1956) studied this aspect of the flow by considering two flat plates being pressed together and squeezing the plasma between so that it comes out in narrow jets. Parker uses this formulation to obtain numerical estimates, but I believe that the plates should be curved and this makes a big difference to the numbers . . .(Dungey, 1965a:164)

Almost all of Dungey’s talk at the symposium was about magnetic field merging as a cause of solar flares. In his last comments, Dungey referred to his model of the magnetosphere and the role of null points in auroral theory. It was these comments that provoked questions from Hannes Alfvén (1908–1995) (Nobel Prize—1970), as noted by the following exchange between Alfvén and Dungey: H. Alfvén: In the terrella experiments which seem very similar, an aurora has nothing to do with the null point. The particles seen in an aurora of course are accelerated along magnetic lines of force but they have nothing to do with the null point and whether the real aurora has anything to do with the null point is of course an open question, but I doubt it very much. J. Dungey: May I ask, when you say it has nothing to do with the null point, do you agree that the null point is connected by lines of force to the terrella? H. Alfvén: Yes. J. Dungey: And there are certain points on the terrella such that if you follow the lines of force from that point you would arrive at a null point. H. Alfvén: Yes, but you definitely have precipitation along lines of force which never reach the null point. J. Dungey: I see, but are you saying there is no precipitation along lines of force which do connect to the null point? H. Alfvén: I am not quite definite but I don’t believe it. (Dungey, 1965a:168)

As a note added in proof (of the proceedings of the conference), Dungey wrote that “Since this was written Petschek has made an important advance on the topic” (Dungey, 1965a:168).

3.5

Petschek’s Contributions

Dungey was referring to a paper that Harry E. Petschek (1930–2005), an American physicist and industrialist, had presented at a Symposium on the Physics of Solar Flares that was held at NASA’s Goddard Space Flight Center on 28–30 October 1963. In his talk, Petschek pointed out that when the Parker-Sweet model is applied to solar flares, it “leads to times for the release of energy which are too large by a factor of 10 to 100” (Petschek, 1964:425). Petschek believed that “Parker had

3.5

Petschek’s Contributions

47

Fig. 3.8 Petschek’s model of magnetic merging. From Fig. 50-3 in Petschek (1964)

overlooked a significant mechanism for the dissipation of magnetic field energy” (Petschek, 1964:425). Petschek’s mechanism is illustrated in Fig. 3.8, where the dark bowed lines represent the edge of the boundary layer between two oppositely directed magnetic fields. According to Petschek’s model, these boundary lines are slow shock waves that travel into the oncoming plasma at the same rate that the plasma is advancing toward the boundary layer. They are, therefore, standing Alfvén shock waves. In the discussion section after Petschek’s talk, Sweet expressed his wholehearted approval of Petschek’s model: I am in favor of your theory, which I thoroughly approve. Dr. Parker and I have been living with this problem for several years and have got the feel of it. Your solution struck me at once as the solution for which we have been seeking. (Hess, 1964:438)

Eugene Parker found Petschek’s innovation interesting, but he was a bit more reserved, as indicated by the exchange he had with Petschek following the talk:

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Dr. Parker: This idea is a very interesting idea that Petschek has expressed. By introducing the magnetic field ejecting the fluid, this permits him to confine the diffusion to a very narrow strip about the neutral point. And this is—as I see it, at least—the reduction of the width of that diffusion strip which then gives the enormously enhanced diffusion rates, is that right? Dr. Petschek: It is the reduction of the width and the reduction of the length which goes with it which allows the fields to go through faster there. But I think the important point is not that the magnetic field ejects the fluid but that, instead of having the change in the magnetic field diffuse into the fluid, it propagates as a wave whose propagation velocity is not limited by the conductivity of the medium. Dr. Parker: The reason I put it the way I did, still, even in this model, is that the rate at which fields come together is limited by the diffusion at the neutral point, and you have greatly enhanced that diffusion. The other way of looking at it is true, too. I am just trying to contrast what has been said before and what you are saying today. Dr. Petschek: It is limited by the diffusion. However, the wave mechanism allows you to shrink the diffusion region; and, as the conductivity increases, that region shrinks farther and farther. So that the final answer does depend on conductivity, but only logarithmically. The annihilation rate is, therefore, very insensitive to conductivity. (Hess, 1964:437–438)

In a paper presented at the AIAA Aerospace Sciences Meeting in New York during 20–22 January 1964, Petschek and his colleagues at the Avco-Everett Research Laboratory further elaborated on neutral point phenomena (Levy et al., 1964). While their paper was supportive of Dungey’s model, its focus was primarily on plasma flow within the Earth’s magnetosphere. They estimated that about 20% of the interplanetary magnetic field lines that are incident upon the Earth’s magnetopause connect with the Earth’s magnetic field. Most of the interplanetary magnetic field lines slide around the magnetosphere (Levy et al., 1964:2073).

3.6 3.6.1

The Controversies Parker’s Reservations

Eugene Parker was less supportive of Dungey’s model, as can be ascertained in part from the talk Parker gave at the Advanced Study Institute, which was held in Bergen, Norway, in 1965 from 16 August through 3 September. In his discussion of particle acceleration, Parker commented, as follows: Finally, I should remark that the neutral sheet, between two oppositely directed magnetic fields has been a favorite candidate for particle acceleration (Dungey, 1958). The neutral sheet is a region of enhanced field annihilation, perhaps unstable, with the added charm of eluding thorough analysis even to the present day. Dungey (1962) has pointed out that there is a neutral region at the subsolar point on the magnetopause whenever the interplanetary field points southward. He argues that the existence of the neutral point plays an important role in particle acceleration. I have been very pessimistic about the idea, partly because particle acceleration in a neutral sheet has not been demonstrated from first principles (Newton and Maxwell). There has been a recent development which suggests that there may be more to the neutral sheet than has been proved so far. A hydrodynamic Ohm’s law

3.6

The Controversies

49

calculation by Petschek (1963) shows a field configuration giving much more rapid annihilation at a neutral sheet than previously calculated. The calculated fields flow into each other at about 10-1 the Alfvén velocity. Dessler has pointed out that the calculated annihilation is unfortunately so rapid that it takes place in only a small fraction of the electron collision time. So Ohm’s law is inapplicable and the calculation needs to be done over again in some way. Nonetheless, the calculation indicates that there is perhaps a more rapid field annihilation than previously believed. So altogether, the neutral sheet is an interesting phenomenon and deserves both theoretical and observational attention. (Parker, 1966:316)

At the conclusion of his talk, Parker described his views on electric fields that are parallel to magnetic field lines in space plasmas: Finally, I should remark that there has always been with us the idea that there may be strong electric fields parallel to the magnetic field or to neutral sheets in the field. It is evident that such fields produce enormous particle acceleration. For precisely this reason, such fields are forbidden whenever a significant thermal plasma is present. Hence, discussion of such electrostatic acceleration requires, in most cases, the assumption that a thermal plasma is absent. Unfortunately, not every author has realized this. It is not obvious to me that a thermal plasma can be absent from the geomagnetic field with the terrestrial atmosphere nearby. The absence of thermal plasma is the key question here, and the proponents of parallel electric fields should devote their attention to it rather than to the endless obvious acceleration possibilities if parallel fields should exist. Presumably the place to expect parallel electric fields is in a region where the required current density c curl B/4π is so large as to exceed the ability of the ambient free electrons to transport it (Parker, 1966:318).

3.6.2

Dessler’s Objections

At the same conference in Bergen, Alexander J. (Alex) Dessler (1924–2023) (Fig. 3.9), an American space physicist, and F. Curtis Michel (1934–2015), an American astrophysicist, both of whom spent much of their respective careers at Rice University, presented a paper that compared various models for the Earth’s magnetosphere. The first model was that of Dungey’s shown above (Fig. 3.6). Two of the other three models are reproduced in Figs. 3.10 and 3.11. Fig. 3.9 Alex Dessler. Credit American Geophysical Union, courtesy American Institute of Physics Emilio Segrè Visual Archive

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Fig. 3.10 From Fig. 1b in Dessler and Michel (1966), Dungey’s model as elaborated upon by Levy et al. (1964)

Fig. 3.11 The model of Dessler, from Fig. 2 in Dessler and Michel (1966)

Dessler and Michel argued that the pattern of absorption of solar flare cosmic rays over the Earth’s geomagnetic poles could be one way to distinguish between the magnetospheric models: Polar cap absorption (PCA) events are caused by the bombardment of the atmosphere over the polar caps by solar flare cosmic rays. The morphology of PCA events is controlled in large part by the configuration of the magnetospheric tail. A magnetosphere whose polar cap field lines are directly connected to the interplanetary field [as in the Dungey models] should result in rather homogeneous PCA events with irregularities and inhomogeneities lasting only a few minutes. For a magnetospheric model in which there is a magnetospheric tail longer than about 1 AU and little merging between geomagnetic and interplanetary magnetic field lines the low energy (~ 5 MeV) PCA events should take several hours to become homogeneous over the polar cap (Dessler & Michel, 1966:452).

Dessler and Michel argued that:

3.6

The Controversies

51

The polar caps would be uniformly illuminated by soft cosmic rays within a few minutes following the onset of a PCA. Furthermore, a fluctuation in particle intensity at one point on the polar caps would be duplicated at other points within this same time—a few minutes. (Dessler & Michel, 1966:452)

In Dessler’s model of the magnetosphere the magnetospheric tail extends 20–50 AU beyond the earth, and there is no merging of oppositely directed magnetic field lines anywhere along the length of the tail. In such a model, solar cosmic rays would enter the Earth’s magnetospheric tail by diffusion. Because of the long time taken for the diffusion process, Dessler and Michel argued that in their model the time for solar cosmic rays to uniformly cover the Earth’s polar caps would be measured in hours following a PCA onset. They further asserted that the existing observational data supported Dessler’s model, citing a recent paper by Yukio Hakura (Hakura, 1964). After 1964, Hakura continued his research on the entry of solar cosmic rays into the polar cap atmosphere, writing in 1967 that: The initial phase of PCA consists of at least three characteristic stages that occur successively in different zones of the polar region: the first stage is observed as a slightly enhanced ionization near the geomagnetic pole; the second stage as a remarkable development of PCA in the polar cap above 65o (corrected geomagnetic latitude); and the third stage as an extension of the enhanced ionization down to latitudes of 60o or lower. A diffusion model of interplanetary space suggests that the three stages are due to differential arrival of solar electrons, protons, and α particles at the polar atmosphere. . . . Transient times of a few hours usually required for the first and second stages seem to favor the particle-diffusion model (Hakura, 1967:1461).

The data on polar cap absorption was disputed in the discussion session following the talk of Dessler and Michel, who, nevertheless, concluded that from their analysis that: No evidence is found to support the hypothesis of fast magnetic merging or annihilation of magnetic energy and neutral line formation. It is concluded that the concept of fast magnetic merging plays no significant role either in determining the configuration of the magnetosphere or in the acceleration of charged particles within the magnetosphere (Dessler & Michel, 1966:447).

Dessler and Michel included a figure (Fig. 3.12) from a paper by Norman F. Ness (Fig. 3.13), an American geophysicist then at the NASA Goddard Space Flight Center. In his figure, Ness had summarized the measurements of the magnetic field data from his magnetometer instruments on the first Interplanetary Monitoring Platform satellite (IMP 1). Dessler and Michael pointed out the similarity between Ness’s figure and the Dessler model of the magnetosphere. In a paper published in 1965, Jim Dungey took issue with the length of the tail of the magnetosphere in Dessler’s model, calling it “astronomical” (Dungey, 1965b). Dungey made the calculation of the length of the tail using features of his model: A line of force of the interplanetary field becomes attached to the earth in the region of local noon and 75° geomagnetic latitude. The terrestrial end of the line then moves over the pole to the night side and the line becomes detached again when the terrestrial end reaches a geomagnetic latitude of about 67°. The motion of the terrestrial ends of the lines shows up in

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Fig. 3.12 From Fig. 14 in Ness (1965) summarizing the findings of the first Interplanetary Monitoring Platform (IMP 1) Fig. 3.13 Norman Ness. Credit the University of Delaware, courtesy of William H. Matthaeus

numerous phenomena, such as the DS pattern, and the speed is known to be hundreds of meters per second. The formation of the tail results from the continued motion of the distant part of these lines with the solar wind, whose speed is hundreds of kilometers per second. The lines are dragged away from the sun and the length of the tail is roughly the distance traveled by the solar plasma in the time during which an individual line is attached. Since the terrestrial ends move at ~ 10-3 of the solar wind speed and have to travel nearly an earth radius, the empirical order of magnitude for the tail length is approximately 103 earth radii. The ratio of the length to width is then about 30, which does not seem excessive. Dessler’s

3.7

The Evidence and Reconsiderations

53

value of 20 to 50 AU is approximately 106 earth radii, and so the tail he proposes appears to be inordinately long (Dungey, 1965b:1753).

As the number of spaceflights increased in the late 1960s and early 1970s opportunities arose for acquiring spacecraft-based measurements of solar particle events with good time resolution simultaneously inside and outside the magnetosphere. Such direct measurement opportunities substantially improved upon the prior use of PCA events over the polar caps for arguing about the configuration of the magnetosphere. Early review articles by Louis J. Lanzerotti of AT&T Bell Laboratories, by Lanzerotti and Curtis Michel, and by George A. Paulikas of the Aerospace Corporation used time differences between the inside and outside measurements to attempt conclusions on the configuration (Lanzerotti, 1972; Lanzerotti & Michel, 1972; Paulikas, 1974). Rapid access of particles was possible evidence for reconnection of magnetic field lines at the magnetopause and/or in the magnetotail. Substantial time delays might indicate a more diffusive process—a closed magnetosphere. Spacecraft measurements provided data throughout the magnetosphere, including over the polar caps and at geosynchronous altitudes. From a table compiled of upper limit solar particle access times, Lanzerotti (1972) concluded that the “reported time delays are quite consistent with a field-line interconnection point no more than ~1000 RE behind the earth or a diffusion point substantially closer” (Lanzerotti, 1972:390). While an interesting summary, a definitive conclusion using this technique remained elusive.

3.7 3.7.1

The Evidence and Reconsiderations Dessler’s Vacuum Merging Model

By 1968, even Alex Dessler had concluded that there must be magnetic merging across the neutral sheet of the geomagnetic tail. In a paper published on 1 January 1968, Dessler noted that “lines of flux pulled into the tail during a severe magnetic storm could not return to a dipole-like configuration unless some fast-merging mechanism were active in a least some portion of the tail” (Dessler, 1968:209). Dessler’s explanation of the merging involved what he called “vacuum merging”: The plasma density in the near-earth portion of the tail is usually low because the supersonic solar wind leaves a rarefied wake region extending about 30 RE behind the dipole-like magnetosphere. The merging of oppositely directed field lines occurs when the plasma density is too low to form a current sheet of sufficient strength to prevent merging. (This mechanism is self-limiting in that the plasma energy density is built up by the merging process to be almost, but not quite, the required level.) The magnetic merging then takes place somewhat as it would in a vacuum. Merging in the neutral sheet, then, depends on the development of vacuum-like conditions immediately behind the earth. This vacuum merging mechanism preserves the essential features of a frozen-in flux that have been so successful in predicting the configuration of both the spiral interplanetary magnetic field and the extended

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magnetospheric tail. Magnetic merging does not appear to proceed when the plasma density is high enough to produce the currents required to prevent merging. (Dessler, 1968:209)

Dessler still was not persuaded of all aspects of Dungey’s model of the magnetosphere, as he did not think merging of the Earth’s magnetic field with the magnetic field carried by the solar wind took place in the sub-solar region of the magnetopause: Since this is essentially a vacuum merging mechanism, we should not expect it to apply at the magnetopause where there is a much higher plasma density (Dessler, 1968:213).

Dessler was trying to find a way of explaining magnetic field-line reconnection in the tail without losing the concept of the “frozen field” approximation; i.e., the condition for a plasma when the plasma and magnetic field move together. Years later, reflecting on the history of the concept of field-line reconnection, Jim Dungey wrote that “It must be reemphasized that reconnection is not a consequence of the frozen field approximation, but of its breakdown, which is caused by high current density” (Dungey, 1994:19,190).

3.7.2

Analyses of Fairfield, Arnoldy, Russell and McPherron

Dungey was persistent in looking for satellite data that could test his model. By means of an arrangement with the Pennsylvania State University (Penn State), he began to advise graduate students there via long distance communications (Dungey, 1994). Dungey suggested that one of his Penn State graduate students, Donald H. Fairfield, examine the relationship, if any, between the direction of the interplanetary magnetic field and magnetic disturbances in the Earth’s polar regions. (After graduation, Fairfield spent his career in space physics at the NASA Goddard Space Flight Center.) Dungey persuaded Laurence James Cahill, Jr., (1925–2013) of the University of New Hampshire to share with Fairfield his magnetic field measurements from the Explorer 12 mission. Dungey was elated with Fairfield’s results, which were published by Fairfield and Cahill in 1966. The important conclusion to be drawn from this work is that exterior fields with a southward component are associated with high-latitude disturbance, whereas northward fields tend to be associated with quiet conditions. When a northward field is present at a time of relative quiet, and is followed by change (either gradual or practically instantaneous) to a southward field, an increase in polar cap disturbance (the DS current systems) invariably occurs in the records so far examined (Fairfield & Cahill, 1966:157).

An important data analysis study by Roger L. Arnoldy (1934-2021) of the University of New Hampshire concentrated on individual geomagnetic substorms as measured by the geomagnetic Auroral Electrojet (AE) index (Davis & Sugiura, 1966) and the state of the interplanetary magnetic field at the time (Arnoldy, 1971). His results provided substantial strong evidence for the importance of a southward interplanetary magnetic field for the initiation of each substorm:

3.7

The Evidence and Reconsiderations

55

An interplanetary variable involving the southward component of the interplanetary field in the solar magnetospheric coordinate system is shown to be singularly important for the generation of substorms. (Arnoldy, 1971:5189)

Seasonal variations in geomagnetic activity had been known since at least 1852, following the work of Edward Sabine (2.2.1). His research, and later that of others, revealed semiannual variations with higher activity around the times of the equinoxes, e.g., Cortie (1912) who summarizes prior research by geomagneticians such as Edward W. Maunder (1851-1928) and Charles Chree (1860-1928). Christopher T. Russell and Robert L. McPherron of the University of California Los Angeles proposed that this phenomenon (now often termed the Russell-McPherron effect) arises from the tilt of Earth’s magnetic axis with respect to the interplanetary magnetic field in the solar equatorial plane over the course of a year: We propose, simply, that it is caused by a semiannual variation in the effective southward component of the interplanetary field. The southward field arises because the interplanetary field is ordered in the solar equatorial coordinate system, whereas the interaction with the magnetosphere is controlled by a magnetospheric system. (Russell & McPherron, 1973, 92)

3.7.3

The ISEE Evidence

As more spacecraft began to acquire data in and around Earth’s magnetosphere, as well as in the solar wind near Earth, additional results suggested that the merging between the magnetic fields of the Earth and the solar wind might be occurring. An example is the observation by the fifth Orbiting Geophysical Observatory (OGO 5) that erosion of Earth’s magnetic field began to take place on the sunward side of the magnetosphere when the interplanetary magnetic field near the Earth changed from northward pointing to southward pointing. (Aubry et al., 1970). On the theoretical side, during the time period of the development of the International Sun-Earth Explorer (ISEE) Mission, Vytenis M. Vasyliūnas of the Massachusetts Institute of Technology (and later a Director for 30 years of the Max Planck Institute for Solar System Research) published a comprehensive review paper on models of magnetic field line merging (Vasyliunas, 1975). The review article was important as it supported many theoretical discussions related to the data results that ultimately came from the mission (see below). Distance Scales in Plasmas The proton and electron distance scales in a plasma generally differ widely, primarily because the proton mass is so much larger than the electron mass. For example, if the particles have the same velocities, the radius of the circular trajectory of a proton in a uniform magnetic field (the gyroradius) is larger than the radius of that of the electron by the ratio of their masses, 1836. The electron (continued)

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distance scale for a plasma is often taken as the depth that an electromagnetic wave can penetrate into the plasma, as this is indicative of the distance that electrons would have to move to block the wave. In the outer magnetosphere this distance is of the order of a few kilometers. In the same region of space, the comparable ion distance scales are much larger, e.g., ~250 km. The magnetopause is typically moving back and forth with a speed of 10 km/s, which means that measurements related to electrons need to be made at a high time rate to capture the plasma structure in the region of the neutral line (Burch et al., 2016a, 2016b: 11). The frozen field approximation noted by Dessler (above) can be used in space plasma physics when the density of the plasma is so low that it can be considered “collisionless”; i.e., the effect of particle collisions that would cause electrical resistance can be neglected. The review paper by Vasyliūnas demonstrated that the various models for magnetic field line reconnection could be reconciled by using a more generalized form of what is known as Ohm’s law (in classical physics, an equation relating voltage to the current and electrical resistance in a circuit). For the analysis of a magnetized plasma, Ohm’s law is broadened and is usually written as a relationship between electric and magnetic fields, plasma velocities, and particle and current densities. Vasyliūnas pointed out, for example, that Dessler’s vacuum merging model is the same as the low-density limit of Petschek’s model (Vasyliunas, 1975:308). Meanwhile, Dungey continued to look for ways to test his model, preferably through direct measurements with satellites. He recognized the difficulty of a satellite measurement of a neutral line: Unfortunately, any particular line is unlikely to be hit by a spacecraft and, even considering a region defined by the magnetic field not exceeding some suitable value, the probability may be low. Moreover, the critical value may be less than the sensitivity of current magnetometers. Magnetic measurements do, however, show the neutral sheet very clearly, with a near reversal of the field over a distance which is clearly small, and should be better measured when a pair of satellites traveling in close company is launched in 1976. (Dungey, 1975:125)

Dungey also understood that the neutral sheet thinness was more related to electron distance scales than ion distance scales: . . . the sheet can be much thinner than the ion gyro-radius What thins the sheet is an electric field across the sheet which prevents the protons from going into their full gyro-radius into the sheet. (Dungey, 1978:233)

Two years after the review by Vasyliūnas, a pair of satellites that could examine the magnetopause was launched on 22 October 1977 as part of the joint NASA-European Space Agency International Sun-Earth Explorer (ISEE) Mission. One of the paired ISEE spacecraft crossed the magnetopause near the subsolar point on 8 September 1978 (Fig. 3.14). Analysis of the particle and magnetic field data gave evidence for reconnection, as discussed in a paper published by Götz

3.7

The Evidence and Reconsiderations

Fig. 3.14 From Fig. 1 in Paschmann et al. (1979). The magnetosheath with southward directed magnetic field lines is on the left (Region 1). The magnetosphere with northward-directed magnetic field lines is on the right (Region 2). The dashed line marks the magnetopause, and the shaded area the boundary layer

57 2

1

V2

V1

ISEE orbit

. I

.

.

Et

Et

Fig. 3.15 Götz Paschmann. Credit the Paschmann family, courtesy of the International Space Science Institute

Paschmann (1939–2023) (Fig. 3.15) of the Max Planck Institute for Extraterrestrial Physics in Munich, Germany, and colleagues: The concept of reconnection was introduced into magnetospheric physics by Dungey (Dungey, 1961) who proposed that reconnection should be operative at the dayside boundary of the Earth’s magnetic field, the magnetopause, as well as in the geomagnetic tail. A detailed description of the process was first provided by Petschek and coworkers (Petschek,

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1964; Levy et al., 1964) who demonstrated that reconnection may occur at substantial rates and that it leads to high-speed plasma jetting away from the reconnection sites. Jets of plasma possibly associated with reconnection have been observed in the tail (Hones et al., 1976; Frank et al., 1976) but never at the magnetopause. This negative result has added to the controversy over the nature and efficiency of the reconnection process, despite substantial indirect evidence for its occurrence. (Sonnerup, 1979) . . . (Paschmann et al., 1979, 243–244)

The Paschmann et al. paper then described data from ISEE that showed the occasional flows of high-speed plasma at the magnetopause as the satellites crossed that boundary. They further wrote that the basic reconnection configuration in the magnetopause (Levy et al., 1964; Yang & Sonnerup, 1977) is illustrated in Fig. 1 [Fig. 3–14]. In this model the magnetopause is a standing Alfvén wave emerging from the X-shaped centre of the reconnection site, referred to as the diffusion region. The magnetic fields on the two sides of the magnetopause current layer, which separates the magnetosheath and terrestrial fields, are not entirely tangential to that layer, but are connected and consequently have a small normal magnetic field component Bn. The magnetosheath plasma flows towards and across the magnetopause, or equivalently, there is a tangential electric field Et. As the plasma crosses the current sheet, its momentum changes abruptly under the action of the tangential components of the Maxwell stresses. Picturing the sharply bent field lines at the magnetopause as elastic bands acting on the plasma, the process may be likened to a magnetic slingshot, which ejects the plasma at high speed in poleward-directed jets just inside the magnetopause. (Paschmann et al., 1979:243–244)

The ISEE experimenters recognized that high time resolution was essential for investigation of the plasma conditions at a thin magnetospheric boundary that was moving in and out past the spacecraft with speeds of approximately 10 km/s. ISEE measurements of energy spectra and magnetic field were typically made every 12 s (Paschmann et al., 1979:244). The ISEE teams drew conclusions as follows: Our observations provide the first evidence for the occurrence of the plasma acceleration at the magnetopause intrinsic to the reconnection process. . . . However, in most cases with favourable magnetic field orientation no plasma acceleration is observed. (Paschmann et al., 1979, 246)

The experiment team therefore concluded that the reconnection process occurs rarely, or that the process is small scale and non-stationary. The latter would imply a low probability of being measured locally as a spacecraft crossed the magnetopause.

3.7.4

The Evidence from Polar and Cluster

The launch of NASA’s Polar satellite on 24 February 1996 gave an important opportunity for a detailed study of magnetic reconnection at the magnetopause, as this satellite was the first to carry a three-axis electric field experiment into the outer magnetosphere (Mozer et al., 2002). Forrest S. Mozer of the University of California, Berkeley, was the PI for the electric field experiment on Polar. On 1 April 2001, Polar crossed the magnetopause near the sub-solar point and at a time when the solar wind magnetic field was antiparallel to the Earth’s magnetic

3.7

The Evidence and Reconsiderations

59

Fig. 3.16 Crossing of the magnetopause by the Polar satellite. From Fig. 1 of Mozer et al. (2002)

field. Mozer and his colleagues used the electric field data in conjunction with data from the plasma and magnetic field instruments on Polar to arrive at the interpretation of the geometry of the reconnection region at the magnetopause that is summarized in Fig. 3.16. The evidence from the Polar data for the ion diffusion associated with magnetic reconnection was strong, and Mozer and his colleagues could report partial evidence for the smaller electron diffusion region. To have a better chance at differentiating between spatial features versus temporal changes in the magnetosphere and the magnetopause, the European Space Agency and NASA built four satellites (the Cluster mission) in 1996 that would orbit the Earth in a cluster. This was the first launch of the Arianespace Ariane 5 rocket, and the launch failed, losing the satellites. The agencies rebuilt the four spacecraft and launched again in the summer of 2000. This time the Cluster II mission was successful. The measurements by instruments aboard the Cluster satellites provided further details on the structure of the magnetic reconnection diffusion region of the magnetopause (Vaivads et al., 2004). Andris Vaivads of the Swedish Institute of Space Physics and his colleagues determined from Cluster data that “the diffusion region is stable on ion time and length scales” (Vaivads et al., 2004:105001). In the reported crossing, because the separation distance of the Cluster spacecraft was about 100 km, Vaivads and his colleagues had to focus on features that were on scales in between the ion and electron scales. (Vaivads et al., 2004:105001).

3.7.5

The MMS Evidence

The ISEE, Polar, and Cluster measurements confirmed some of the features of the magnetic field reconnection that were central to Dungey’s speculation. However, the

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Fig. 3.17 Jim Burch. Credit Jim Burch, the Southwest Research Institute

spacecraft instruments did not have the temporal resolution required to examine the electron-scale physics that Dungey had briefly discussed in 1978. That deficiency was remedied in 2015 with the launch of the four spacecraft of NASA’s Magnetospheric Multiscale (MMS) mission on 13 March. The Principal Investigator for the mission was James L. (Jim) Burch (Fig. 3.17) of the Southwest Research Institute, and he was the lead author of a paper published in June 2016 titled Electron-scale Measurements of Magnetic Reconnection in Space (Burch et al., 2016a, 2016b). The MMS mission incorporated the following features: 1. Four spacecraft in a closely controlled tetrahedron formation with adjustable separations down to 10 km 2. Three-axis electric and magnetic field measurements with accurate crosscalibrations allowing for the measurement of spatial gradients and time variations 3. All-sky plasma electron and ion velocity-space distributions with time resolution of 30 ms for electrons and 150 ms for ions. (Burch et al., 2016a, 2016b: aaf2939–1) Burch and his teammates reported that: By 14 December 2015, the spacecraft had crossed the magnetopause more than 2000 times. On the basis of detection of plasma jetting and heating within the magnetopause current sheets, we infer that at least 50% of the crossings encountered magnetic reconnection. Most crossings occurred in the reconnection exhaust downstream of the X-line, but a few of them passed very close to the X-line. (Burch et al., 2016a, 2016b:aaf2939-1)

The Burch team reported in detail on the crossing of the dayside magnetopause that occurred on 16 October 2015, during which the four MSS spacecraft came very near to the X-line. During this crossing, the exhaust stream north of the X-line, as well as the exhaust stream south of the X-line were observed. Plasma currents and parallel electric fields were measured in the vicinity of the X-line “as predicted for the dissipative nature of reconnection” (Burch et al., 2016a, 2016b:aaf2939–1). The team also discovered that “the X line region is important not only for the initiation of reconnection (breaking of the electron frozen-in condition), but also for electron

3.8

Eddington’s Guidelines

61

acceleration and energization, leading to much stronger electron heating and acceleration than seen in the downstream exhaust” (Burch et al., 2016a, 2016b: aaf2939–1). Finally, the MSS instrumentation provided “experimental evidence for the opening up of magnetic field lines while also demonstrating that it is the electron dynamics that drives reconnection” (Burch et al., 2016a, 2016b:aaf2939–1). Jim Dungey died on 9 May 2015, just a few months before his speculation was spectacularly confirmed by the data obtained from the instrumentation on the MSS spacecrafts. The leader of the MSS team, Jim Burch, was a graduate of Rice University’s Space Science Department, which Alex Dessler founded and chaired on three separate occasions. Though in some respects Dessler was on the wrong side of this controversy, it is a reflection of his impact on space research that a student from his department made the electron-scale measurements that were needed to finally and definitely confirm Dungey’s speculation that the Earth’s magnetosphere is open.

3.8

Eddington’s Guidelines

The guidelines of Arthur Eddington (1882–1944) (Chap. 1) for science speculation, slightly modified to make them more broadly applicable to space science, are: 1. Was the speculator rigorous in applying the appropriate science applicable to the model 2. Did the speculator identify all the underlying assumptions used in constructing the model and 3. Did the speculator view the model objectively, as an “adjustable engine,” as opposed to a “finished building?” Thus, how relevant were the Eddington guidelines to the investigators in the case of understanding Earth’s spatial boundary, the boundary of the magnetosphere? Was Dungey rigorous in applying the appropriate science to his model? David Southwood, one of Dungey’s students, has pointed out that: Plasma Physics itself was a new field when [Dungey] did his Ph.D. at Cambridge in the late 40s. The magnetosphere was a new, even unnamed regime when he began his career. At the time, ideas like frozen-in magnetic field and magnetohydrodynamic waves were opening up astrophysics as well as solar terrestrial physics. At the same time, it was becoming clear that plasmas, being perfect conductors, could lead to effects that were counter intuitive to classical electromagnetic theory. . . . (Southwood, 2015:vii)

While admitting that some of the arguments he used in developing his model were of the “hand-waving” variety, Dungey was, nevertheless, very rigorous in checking his intuitive ideas against the principles of plasma physics and electrodynamics. Did Dungey identify all the underlying assumptions used in constructing his model? Dungey was perhaps late, but nevertheless ahead of his time, in

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Fig. 3.18 From Fig. 2 in Southwood (2015). Dungey is shown “up holding” his model of the open magnetosphere, designed as a weather vane, and given to him by his former students upon his retirement from Imperial College in 1984. Credit Jeffrey Hughes, Boston University

understanding the “electron-scale” nature of the problem of reconnection and that it represented a breakdown of the frozen-in flux approximation. Did Dungey view his model objectively? Dungey was generous in his assessments of prior contributors to his speculation. He always pointed out the importance of the earlier ideas of Ronald Giovanelli on neutral points in solar flares and the extension of those ideas by Fred Hoyle as possibly applicable to the Earth’s magnetosphere. When he was criticized for not including the effects of particle pressure in the merging process, he acknowledged that shortcoming and sought to remedy it. Further, he seemed genuinely excited when Harry Petschek improved the understanding of the underlying physics of magnetic merging, noting modestly that Petschek had “specified the divergence of the outflow, whereas my flow was at the handwaving level” (Dungey, 1994:19,191). Finally, Dungey was eager to test his model as evidenced by his work to encourage a graduate student, Donald Fairfield, to perform his analysis of the possible correlation between the direction of the interplanetary magnetic field and magnetic disturbances in the Earth’s polar ionosphere. In conclusion, Dungey (Fig. 3.18) viewed his model objectively, as, in the words of Eddington, an “adjustable engine” as opposed to a “finished building”.

References

3.9

63

Continuing Understanding

Since the confirmation of magnetic merging between the solar wind magnetic field and Earth’s magnetic field, considerable research effort has been expended to better understand when and where magnetic merging takes place on the surface of the magnetopause. (See the review article by Trattner et al., 2021.) Extensive and broad research about and in the space environment around Earth, the magnetosphere, continues because of the importance of understanding Earth’s place in the solar system. Research about the magnetosphere is conducted for several important reasons: (a) to better understand the plasma physics of the space around earth; (b) for the interrelations of basic laboratory plasma physics and measured space plasma processes; (c) for the interrelations of Earth’s plasma environment that are studied in situ and astrophysical plasma environments that can only be remotely observed; and (d) for the implications and the impacts of what is now called “space weather” in the magnetosphere on the multitude of technical systems that fly in it or whose operations depend upon knowledge of its environment. A central aspect of investigation, by measurements and by theory, remains the radiation environment—the radiation belts. A recent example is the dual spacecraft Van Allen Probes mission (Fox & Burch, 2014) whose objectives were both pure science and the applications of the knowledge for improving models of the quite variable trapped radiation—and of practical space weather understandings in general. The Ionosphere Connection Explorer (ICON) mission (2020) studies the ionosphere—the dynamic region that separates Earth’s atmosphere from the space environment (the magnetosphere). It is often recognized that, like studies of Earth’s atmosphere, a multitude of measuring points in the magnetosphere environment are necessary to separate spatial and temporal effects, especially for forecasting changes in the radiation environment. The magnetosphere is a huge environmental volume, much larger than Earth’s atmosphere. Small spacecraft, called CubeSats that are highly targeted to specific research topics and spatial regions, are being designed and planned to accomplish aspects of these objectives. This is an area of space research around Earth that will grow substantially in the future.

References Aubry, M. P., Russell, C. T., & Kivelson, M. G. (1970). Inward motion of the magnetopause before a substorm. Journal of Geophysical Research, 75(34), 7018–7031. Arnoldy, R. L. (1971). Signature in the interplanetary medium for substorms. Journal of Geophysical Research, 76, 5189–5201. Burch, J. L., Moore, T. E., Torbert, R. B., & Giles, B. (2016a). Magnetospheric multiscale overview and science objectives. Space Science Reviews, 199(1), 5–21. Burch, J. L., Torbert, R. B., Phan, T. D., Chen, L. J., Moore, T. E., Ergun, R. E., et al. (2016b). Electron-scale measurements of magnetic reconnection in space. Science, 352(6290).

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Chapman, S. (1918). An outline of a theory of magnetic storms. Proceedings of Royal Society A, 95, 61–83. Chapman, S., & Bartels, J. (1940). Geomagnetism, Vol. I: Geomagnetic and Related Phenomena. Oxford University Press. Chapman, S., & Ferraro, V. C. A. (1931). A new theory of magnetic storms. Part I – The initial phase. Terrestrial Magnetism and Atmospheric Electricity, 36, 171–186. Chapman, S., & Ferraro, V. C. A. (1933). A new theory of magnetic storms. Part II – The main phase. Terrestrial Magnetism and Atmospheric Electricity, 38, 79–96. Coleman, P. J., Jr., Davis, L., & Sonett, C. P. (1960). Steady component of the interplanetary magnetic field: Pioneer V. Physical Review Letters, 5(2), 43. Cortie, A. L. (1912). Sun-spots and terrestrial magnetic phenomena, 1898-1911. Monthly Notices of the Royal Astronomical Society, 73, 52–60. Cowling, T. G. (1946a). Alfvén’s theory of sunspots. Monthly Notices of the Royal Astronomical Society, 106(5), 446–456. Cowling, T. G. (1946b). The growth and decay of the sunspot magnetic field. Monthly Notices of the Royal Astronomical Society, 106(3), 218–224. Davis, T. N., & Sugiura, M. (1966). Auroral electrojet activity index AE and its universal time variations. Journal of Geophysical Research, 71(3), 785–801. Dessler, A. J. (1968). Magnetic merging in the magnetospheric tail. Journal of Geophysical Research, 73(1), 209–214. Dessler, A. J., & Michel, F. C. (1966). Magnetospheric models. In B. M. Mccormac (Ed.), Radiation trapped in the earth’s magnetic field (pp. 447–456). Springer. https://doi.org/10. 1126/science.150.3697.785 Dungey, J. W. (1953). Conditions for the occurrence of electrical discharges in astrophysical system. Philosophical Magazine, 44, 725–738. Dungey, J. W. (1958). Cosmic electrodynamics. Cambridge University Press. Dungey, J. W. (1961). Interplanetary magnetic field and the auroral zones. Physical Review Letters, 6(2), 47. https://link.aps.org/doi/10.1103/PhysRevLett.6.47 Dungey, J. W. (1962). The interplanetary field and auroral theory. Journal of the Physical Society of Japan Supplement, 17, 15. Dungey, J. W. (1965a). Null points in space plasma. In Proceedings of the plasma space science symposium (pp. 160–169). Springer. Dungey, J. W. (1965b). The length of the magnetospheric tail. Journal of Geophysical Research, 70(7), 1753–1753. Dungey, J. W. (1975). Some remaining mysteries in the aurora. Quarterly Journal of the Royal Astronomical Society, 16, 117–127. Dungey, J. W. (1978). The history of the magnetopause regions. Journal of Atmospheric and Terrestrial Physics, 40, 231–234. Dungey, J. W. (1994). Memories, maxims, and motives. Journal of Geophysical Research: Space Physics, 99(A10), 19189–19197. Dungey, J. W. (1995). Origins of the concept of reconnection and its application to the magnetopause: A historical view. Geophysical Monograph Series, 90, 17. Egeland, A., & Burke, W. J. (2005). Kristian Birkland: The first space scientist. Springer. Fairfield, D. H., & Cahill, L. J., Jr. (1966). Transition region magnetic field and polar magnetic disturbances. Journal of Geophysical Research, 71(1), 155–169. Frank, L. A., Ackerson, K. L., & Lepping, R. P. (1976). On hot tenuous plasmas, fireballs, and boundary layers in the earth’s magnetotail. Journal of Geophysical Research, 81(34), 5859–5881. Fox, N., & Burch, J. L. (2014). The Van Allen probes mission. Springer. Giovanelli, R. G. (1939). The relations between eruptions and sunspots. The Astrophysical Journal, 89, 555. Giovanelli, R. G. (1946). A theory of chromospheric flares. Nature, 158(4003), 81–82.

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Giovanelli, R. G. (1947). Magnetic and electric phenomena in the Sun’s atmosphere associated with sunspots. Monthly Notices of the Royal Astronomical Society, 107(4), 338–355. Giovanelli, R. G. (1948). Chromospheric flares. Monthly Notices of the Royal Astronomical Society, 108(2), 163–176. Hakura, Y. (1964). Patterns of polar cap blackouts drawn in geomagnetic coordinates corrected by the higher terms of spherical harmonic development. Report on Ionosphere Space Research Japan, 18. Hakura, Y. (1967). Entry of solar cosmic rays into the polar cap atmosphere. Journal of Geophysical Research, 72(5), 1461–1472. Hess, V. F. (1912). Uber beobachtungen der durchdringenden strahlung bie sieben freiballonfahrten. Physikalische Zeitschrift, 13, 1084–1091. Hess, W. N. (1964). AAS NASA symposium on the physics of solar flares: Proceedings of a symposium held at the Goddard space flight center, Greenbelt, Maryland, October 28–30, 1963 (Vol. 50). National Aeronautics and Space Administration. Hones, E. W., Jr., Bame, S. J., & Asbridge, J. R. (1976). Proton flow measurements in the magnetotail plasma sheet made with IMP 6. Journal of Geophysical Research, 81(1), 227–234. Hoyle, F. (1949). Some recent researches in solar physics. Cambridge University Press. ICON Mission. (2020). The ionosphere connection explorer (ICON) mission. Space Science Reviews, 216, 1572–9672. Lanzerotti, L. J. (1972). Solar energetic particles and the configuration of the magnetosphere. Reviews of Geophysics and Space Physics, 10, 379–393. Lanzerotti, L. J., & Michel, F. C. (1972). Solar particle access to the magnetosphere-how? Comments on Astrophysics and Space Physics, 4, 161–165. Levy, R. H., Petschek, H. E., & Siscoe, G. L. (1964). Aerodynamic aspects of the magnetospheric flow. AIAA Journal, 2(12), 2065–2076. Lindemann, F. A. (1919). Note on the theory of magnetic storms. Philosophical Magazine, 38, 669–684. Mozer, F. S., Bale, S. D., & Phan, T. D. (2002). Evidence of diffusion regions at a subsolar magnetopause crossing. Physical Review Letters, 89(1), 015002. https://doi.org/10.1103/ PhysRevLett.89.015002 Ness, N. F. (1965). The earth’s magnetic tail. Journal of Geophysical Research, 70(13), 2989–3005. https://doi.org/10.1029/jZ07013p02989 Paulikas, G. A. (1974). Tracing of high-latitude magnetic field lines by solar particles. Reviews of Geophysics and Space Physics, 12, 117–128. Parker, E. N. (1957). Sweet’s mechanism for merging magnetic fields in conducting fluids. Journal of Geophysical Research, 62(4), 509–520. Parker, E. N. (1958a). Dynamics of the interplanetary gas and magnetic fields. The Astrophysical Journal, 128, 664. Parker, E. N. (1958b). Interaction of the solar wind with the geomagnetic field. The Physics of Fluids, 1(3), 171–187. Parker, E. N. (1966). Particle effects in the geomagnetic field. In B. M. McCormac (Ed.), Radiation trapped in the earth’s magnetic field (pp. 302–320). Springer. Paschmann, G. B. U. O., Sonnerup, B. Ö., Papamastorakis, I., Sckopke, N., Haerendel, G., Bame, S. J., et al. (1979). Plasma acceleration at the Earth’s magnetopause: Evidence for reconnection. Nature, 282(5736), 243–246. https://doi.org/10.1038/282243a0 Petschek, H. E. (1964). Magnetic field annihilation. In AAS NASA Symposium on the Physics of Solar Flares: Proceedings of a Symposium Held at the Goddard Space Flight Center, Greenbelt, Maryland, October 28–30, 1963 (Vol. 50, p. 425). NASA Special Publication. Russell, C. T., & McPherron, R. L. (1973). Semiannual variation of geomagnetic activity. Journal of Geophysical Research, 78, 92–108. Sonnerup, B. U. O. (1979). Magnetic field reconnection. In L. J. Lanzerotti, C. F. Kennel, & E. N. Parker (Eds.), Solar System Plasma Physics (Vol. Ill, pp. 45–108). North Holland Publishing.

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Southwood, D. (2015). Introduction: Jim Dungey and magnetospheric plasma physics. In D. Southwood, S. W. Cowley, & S. Mitton (Eds.), Magnetospheric plasma physics: The impact of Jim Dungey’s research (Vol. 41, pp. vii–xiii). Springer. https://doi.org/10.1007/978-3-31918359-6 Stern, D. P. (2002). A millennium of geomagnetism. Reviews of Geophysics, 40(3), 1–1. https://doi. org/10.1029/2000RG000097 Sweet, P. A. (1956). The neutral point theory of solar flares. In B. Lehnert (Ed.), Electromagnetic phenomenon in cosmical physics (pp. 123–140). Cambridge University Press. Trattner, K. J., Petrinec, S. M., & Fuselier, S. A. (2021). The location of magnetic reconnection at Earth’s magnetopause. Space Science Reviews, 217(3), 1–47. Vaivads, A., Khotyaintsev, Y., André, M., Retino, A., Buchert, S. C., Rogers, B. N., Décréau, P., Paschmann, G., & Phan, T. D. (2004). Structure of the magnetic reconnection diffusion region from four-spacecraft observations. Physical Review Letters, 93(10), 105001. Vasyliunas, V. M. (1975). Theoretical models of magnetic field line merging. Reviews of Geophysics and Space Physics, 13, 303–336. Yamada, M. (2011). Understanding the dynamics of magnetic reconnection layer. Space Science Reviews, 160(1), 25–43. https://doi.org/10.1007/s11214-011-9789-5 Yang, C. K., & Sonnerup, B. U. Ö. (1977). Compressible magnetopause reconnection. Journal of Geophysical Research, 82(4), 699–703.

Chapter 4

Influx of Small Comets into Earth’s Atmosphere

4.1

Introduction

Images in the ultraviolet of Earth’s atmosphere from his instrument on the mediumaltitude Dynamics Explorer-1 satellite in 1986 led Professor Louis A. Frank (1938–2014) (Fig. 4.1) of the University of Iowa to speculate that house-size small comets are constantly entering the upper atmosphere. Such an influx of small objects would provide water for the planet’s lakes, rivers and oceans. The finding would revolutionize understandings of sources of Earth’s water, and of the comet-like population of the solar system. The scientific debate spurred by Frank’s speculation was unusually animated within the university space research community, and it lasted for 17 years.

4.2 4.2.1

The Beginning of the Controversy The Decision of a Journal Editor

In January 1986, Alexander J. (Alex) Dessler (1924–2023) became the Editor-inChief of the Geophysical Research Letters (GRL) published by the American Geophysical Union. Dessler decided to make the GRL more lively and interesting by publishing controversial “forefront” science. In a GRL editorial at the beginning of his editorship, Dessler wrote: The most interesting research papers are the ones most likely to be controversial. In contrast, routine research is rarely the subject of animated conversations among researchers. Papers describing forefront research commonly challenge conventional wisdom. While they are the ones most essential in pushing research forward and enlarging the sphere of knowledge at the expense of ignorance, they are also the ones most likely to be shown to be incorrect. (Dessler, 1986a:1)

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 W. D. Cummings, L. J. Lanzerotti, Scientific Debates in Space Science, Astronomy and Planetary Sciences, https://doi.org/10.1007/978-3-031-41598-2_4

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Fig. 4.1 Louis Frank. Credit University Relations records, University of Iowa Libraries, Iowa City, Iowa

Fig. 4.2 The cover of the April 1986 issue of Geophysical Research Letters showing the Earth as seen by the UV imager on Dynamics Explorer 1 (Frank et al., 1986a, 1986b)

4.2.2

The Initial Papers by Frank and His Colleagues

Alex Dessler’s strategy for making the GRL more lively was soon tested, and the comments he made in his initial editorial proved to be prescient. In April of 1986, against the advice of reviewers (e.g., Kerr, 1997a), Dessler published two papers in GRL by Louis Frank and his graduate student, John B. Sigwarth (1960–2010), and an Iowa research physicist, John D. Craven (1941–2021). The cover of the journal issue that contained their papers reproduced a figure from their research on what they came to interpret as “atmospheric holes.” (Fig. 4.2). The first article (Frank et al., 1986a) was a description of some of the observations made by Frank’s instrumentation on the Dynamics Explorer 1 (DE-1) spacecraft (a satellite with a highly

4.2

The Beginning of the Controversy

69

Fig. 4.3 Sequence of images of what Frank et al. characterized as an atmospheric hole. The central vertical scan lines of consecutive frames are separated in time by 72 s and scan lines are telemetered from right-to-left and left-to-right sequentially for the frames. The positions of the atmospheric hole are circled. From Fig. 2 of Frank et al. (1986a)

elongated polar orbit around Earth with an initial apogee altitude of 3.65 Earth radii (23,290 km) and a perigee altitude of 570 km). Frank et al. summarized their observations as follows: Images of the earth’s dayglow emissions at ultraviolet wavelengths, primarily those of atomic oxygen at 130.4 nm, reveal regions of transient intensity decreases to ~ 5% to 20% of surrounding values over areas estimated to be ~ 2000 km2. The duration of these transient decreases in intensities is ~ several minutes. Approximately 10 of these events occur each minute in the day side upper atmosphere. (Frank et al., 1986a:303)

The paper contained some examples of the transient spots, one of which is shown in Fig. 4.3. Much of their article on the observations was devoted to analyses and arguments to counter anticipated criticisms that the transient spots were due to telemetry noise or counting statistics. The second paper (Frank et al., 1986b) was an interpretation of these observations, which the authors summarized as follows: Large, transient decreases of atmospheric dayglow intensities at ultraviolet wavelengths, primarily the atomic oxygen emissions at 130.4 nm, are interpreted in terms of an influx of heretofore undetected comet-like objects. The primary composition of these comet-like objects is water snow or clathrate in the form of a fluffy aggregate. These small comets are covered with a dust mantle and the tensile stress at fracture is estimated to be ~ 0.1 dyne/ cm2. The water molecules that form the absorbing blanket for ultraviolet emissions arrive at the top of the Earth’s atmosphere as a piston of gas with bulk speed ≤ 20 km/sec. The mass of each of these comet-like objects is ~ 108 gm, or ~ 100 tons. The global influx rate is ~ 20 comets per minute. The global mass accretion rate by the earth’s atmosphere is ~ 1012 kilograms per year, and sufficient to replace the atmospheric mass in ~ 5 × 106 years. The earth and the other bodies in the solar system would be thus more strongly coupled to cometary matter than presently thought. (Frank et al., 1986b:307)

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Influx of Small Comets into Earth’s Atmosphere

A Deluge of Comments Following the Opening Papers

During the remainder of 1986 and through 1987, the GRL published more than ten articles commenting on the papers and the conclusions by Frank and his colleagues. Frank and his colleagues replied to almost all of these comments. These comments and responses are given in abbreviated form in Table 4.1.

4.3 4.3.1

Editorial Comments Dessler’s Article

Toward the end of 1986, given the deluge of Comments and Replies that Geophysical Research Letters had received on the small-comet hypothesis, Editor-in-Chief, Alex Dessler, apparently feeling the need to defend his editorial experiment with the journal, wrote a brief article in which he attempted to answer the question “Why should GRL publish controversial papers that seek to overturn conventional wisdom?” (Dessler, 1986b:1363). Dessler’s answer was that, while controversial speculations are usually wrong, “The importance for scientific progress of the occasional new idea that proves correct is out of all proportion to their number,” and “it is best to get new ideas into the open literature where they can be discussed, attacked, tested, or supported as the will of the community and the soundness of the idea dictate” (Dessler, 1986b:1363). Finally, Dessler wrote that: Exposure of conflict is a proper function for a letters journal that aims to publish new, timely, and interesting research results. The Commentaries section is one of the most important parts of GRL. It provides a forum for discussion and testing of new and controversial ideas. Rather than restricting such debate to private exchanges between authors and referees, the publication of Comments and Replies in Commentaries allows the broad scientific community to hear both sides of an issue and form an independent judgment. (Dessler, 1986b:1363)

4.3.2

Paul Feldman’s Article

Through 1986 and most of 1987 the debate over the small-comet hypothesis had been conducted primarily at meetings of the American Geophysical Union (AGU) and within the pages of Geophysical Research Letters. At the end of 1987, Professor Paul D. Feldman (1939–2022), a comet specialist at Johns Hopkins University, published an article in the News and Views section of Nature (Feldman, 1987). Feldman referred to Frank’s small comets as “Comet-like objects” (CLOs). Feldman both recapitulated the research controversy and discussed the editorial decision and policy of Dessler. Feldman concluded his article as follows:

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Editorial Comments

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Table 4.1 Discussions of Frank et al. papers Commenter Thomas M. Donahue (1941–2004). U. Michigan

David P. Rubincam NASA Goddard Space Flight Center

Christopher P. Mc Kay. NASA Ames Research Center

William B. Hanson (1923–1994). U. Texas Dallas.

Talbot A. Chubb (1923–2011). U.S. Naval Research Laboratory.

George C. Reid (1929–2011); Susan Solomon. Aeronomy Laboratory, NOAA.

Paul M. Davis, U. California Los Angeles

Comment Disposal problem for water molecules deposited near 100 km. Absence of water reservoir on Mars. Donahue (1986) Presented calculations that mantles covering small comets are too thin to suppress evaporation into solar wind. Rubincam (1986) Implies skewed distribution of comets: vastly too many small comets. Also, thermal instability, blowing off of mantle (Fanale & Salvali, 1984). McKay (1986). Found no such events in examining 85 h data from Atmospheric Explorers C, E, and Dynamics Explorer (DE) 2. Should produce visual display every half hour on Earth on nightside. Hanson (1986). Argued four reasons for an instrumental artifact: two based upon instrument characteristics; two based upon appearance in data, including no altitude or look direction effects. Chubb (1986). Entry of objects at meteoric speeds likely leads to dissociation of H2O, resulting in downward flux of OH. Still required influx of small comets 30 times smaller than Frank et al. Reid and Solomon (1986). An influx on moon consistent with proposed influx on Earth would result in “five orders of magnitude” more seismic signals than recorded in 8 years of seismic signals from Apolloinstalled seismograph

Response Refined altitude for water deposition down to 55 km. Periodic warming of Mars atmosphere for enhanced outflow. Frank et al. (1986c). Mantles might not be dust typically found in interplanetary space. Solar wind ion bombardment or UV could alter surface (e.g., Cheng & Lanzerotti, 1978). Frank et al. (1986d) Skewed distribution ignored. Assumed mantle primarily of carbon and interior tensile strength of snow at Earth. Frank et al. (1986e).

Nocturnal display ignored. Reported to find one event when DE2 directly below DE1-observed event; reported to find four similar events in 755 min of data. Frank et al. (1986f).

Referred to Frank et al. (1986a) for inflight performance. Found one event in successive scans, arguing for prograde orbits. Frank et al. (1986g).

No response

Response included in response to Nakamura et al., below

(continued)

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Table 4.1 (continued) Commenter

Comment

Yosio Nakamura and Jurgen P. Oberst, U. Texas Austin, Stephen M. Clifford and Bruce G. Bills, USRA Lunar Science Institute (LPI).

(Nakamura et al., 1982). Davis (1986). Cited seismic impact of spent Saturn IV booster on Moon and concluded that small comet impacts could not “escape detection by the lunar seismic network”. Nakamura et al. (1986).

Donald E. Morris, U. California Lawrence Berkey Laboratory

Four criticisms, two related to evaporation rate under assumed mantle; based on deep sea iridium abundance, dust fraction would have to be far less than known comets; prograde orbit model inconsistent with evolution of long period comet orbits. Morris (1986)

Steven Soter, Cornell U.

Noted that small comet model implied 25 million comets within spherical volume bounded by Moon’s orbit. MIT space surveillance telescope should have detected 250 per hour during certain surveillance periods. Soter (1987). Should be large and easily detected amounts of hydrogen (via Lyman α) in interplanetary space unless very thick dust mantles. Donahue (1987). Analyzed 182 dark pixel images provided by Iowa group. No statistical darkening of adjacent pixels; no statistical spacecraft altitude effect. Cragin et al. (1987).

Thomas M. Donahue, U. Michigan.

Bruce L. Cragin, William B. Hanson, R. Richard Hodges, Jr., Donald R. Zuccaro, U. Texas Dallas.

John T. Wasson (1934–2020) and Frank T. Kyle, U. California Los Angeles.

Citing four prior comments in 1986, criticized Iowa group “tailoring” the properties of the small comets, conflicting “evidence that

Response

Response to both Davis and Nakamura et al. Amplitude of wave generated from impact and its frequency (too high) to be detected by seismic array. Further stated that surface of Moon suffers less damage by small comet impacts than from meteoroids. Frank et al. (1986h). Reiterated position for thin, carbon-based mantles (i.e., less dust, and hence less iridium, than in ordinary comets); prograde orbit comets (from original comet shower with origins about 20,000 AU*) would have larger orbit changes introduced by Jupiter and giant outer planets than retrograde orbit comets. Frank et al. (1986i). MIT telescope observations conducted when flux of small comets low and physical characteristics of comets make them hard to detect with optical telescopes. Frank et al. (1987a).

No response

Asserted the use of an over simplified model of instrument response to “small, opaque objects against. . .fluctuating. . .dayglow”, and “insufficiently thorough treatment of the observations”. Frank et al. (1987b). Asserted that similar criticisms addressed in prior replies. Cloud of comets at distances ranging from 100 AU to 1000 AU with small comets consisting mainly of (continued)

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Table 4.1 (continued) Commenter

Comment

Response

chondritic dust is a major constituent of comets”. Wasson and Kyte (1987).

volatiles at the more distant reaches of the cloud. Frank et al. (1987c).

*

Astronomical Unit, sometimes abbreviated as A.U., a.u., or au

It is fair to ask whether the editor of Geophysical Research Letters, Alex Dessler, should have published the papers of Frank et al., apparently against the advice of referees. The answer is far from clear; the issue raised is not without interest. But by allowing individual replies to each criticism of the original idea, Dessler placed the burden of proof on critics to show that their objections were valid. The debate has thus been turned inside out. The burden of proof should have been squarely on Frank et al. to examine other data sets or otherwise to find positive evidence for the existence of these objects. (Feldman, 1987:518–519)

4.4

Initial Attempts by Others to Find Small Comets

Other scientist began to try to find positive evidence for the existence of small comets. One search examined data from the Swedish Viking satellite. Another used data from a ground-based telescope.

4.4.1

The Viking Sweden Satellite Images

The first Swedish satellite, Viking Sweden, was launched into a polar orbit on 22 February 1986. The Principal Investigator for the ultraviolet imager instrument on the satellite was physicist Clifford D. Anger of the University of Calgary. Anger brought extensive experience in imaging studies of Earth’s aurora from the ground. Anger and his colleagues reported on the scientific results of the ultraviolet imager (Anger et al., 1987). Their report was primarily concerned with images of the aurora. They noted, however, that they had not observed any obvious atmospheric “holes” of the kind reported by Louis Frank and his colleagues. They further commented that a more extensive analysis of their data would need to be done “to determine whether the Viking UV data can support or deny the existence of this phenomenon” (Anger et al., 1987:386). Later in 1988, Frank and John Craven published in the Reviews of Geophysics a lengthy review of the imaging results from the Dynamics Explorer 1 satellite (Frank & Craven, 1988). Part of their review dealt with the small-comet hypothesis. They noted the objections that had been made by a number of scientists, and they repeated their answers to these objections. In the case of the Viking data, Frank and Craven argued that the Viking images that had been published displayed limb darkening,

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rather than limb brightening, and that this indicated that the long-wavelength response of the Viking ultraviolet imager had been inadequately suppressed. Because of this failure the detection of atmospheric holes is not possible with Viking unless there is some small portion of the field of view that is not overwhelmed with long wavelength radiation or instrumental scattering of light (Frank & Craven, 1988:268).

Richard A. Kerr, a writer for Science magazine, reported on a visit and the follow up by Frank, Sigwarth, and Craven to the University of Calgary in January of 1988 (Kerr, 1988). In that visit, the Iowa group obtained some Viking data from John Sandy Murphree and Leroy L. Cogger (1937–2022), who were colleagues in the physics department with Anger on the Viking UV imager team. At the spring meeting of the American Geophysical Union in 1988, Frank, Sigwarth, Craven, Murphree and Cogger were listed as co-authors of a presented paper that asserted that the Viking UV instrument had detected the atmospheric holes. Murphree had not attended the meeting, however, and when Kerr contacted him in Calgary, Kerr reported that: Murphree cannot support Frank’s conclusions. “I’m fairly convinced that these are just instrumental effects,” he says. He expects to ask shortly that his and Cogger’s names be dropped from the paper. Murphree sees one major problem with the Iowa interpretation of the data. The two groups cannot agree on how to calculate the probability of such dark spots occurring by chance, as part of the background of instrumentation noise. At the moment their different approaches yield probabilities that differ by a factor of 10 billion, says Murphree. (Kerr, 1988:1403–1404)

Frank and his colleagues published their analysis of the Viking UV data without Murphree and Cogger in GRL. They reported that: Images taken with the two ultraviolet cameras on board the Viking spacecraft were examined for evidence of transient decreases of earth’s ultraviolet dayglow. Comparison of near-limb observations of dayglow intensities with those at smaller angles to the nadir with the camera sensitive to OI 130.4 nm emissions supports the existence of transient decreases in the nearnadir dayglow. However, the amount of near-nadir imaging is severely limited and only several significant events are found. More decisive confirmation of the existence of such transient decreases must await a larger survey from another spacecraft. The diameters of these regions as detected with Viking are ~ 50–100 km. Occurrence frequencies, intensity decreases, and dimensions for these clusters of darkened pixels are similar to those previously reported for such events, or ‘atmospheric holes’, as seen in images of the ultraviolet dayglow with Dynamics Explorer 1. (Frank et al., 1989:1457)

4.4.2

Cragin’s Comments on the Claims That Atmospheric Holes Are Seen in the Viking Data

Frank and his colleagues had asserted that the ratio of atmospheric holes observed by the Viking satellite near the limb of the Earth’s atmosphere to the holes observed at near nadir angles was similar to the ratio observed by the Dynamics Explorer

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1 satellite. They stated that this was additional proof that the holes were real as opposed to instrumental artifacts (Frank et al., 1989:1460). In addition, Frank and his colleagues gave physical reasons why there should be fewer holes observed near the limb than when the satellite instrumentation was looking directly down into the Earth’s atmosphere (Frank et al., 1989:1458). The issue for Bruce L. Cragin of the University of Texas at Dallas was the selection by Frank and his colleagues of the darkness threshold for the determination of an atmospheric hole. Cragin argued that Frank and his colleagues had “selected the detection threshold most favorable to their hypothesis” (Cragin, 1990:1174). With the threshold selected by the Iowa colleagues for a limited set of Viking data, there were seven holes observed at near-nadir directions and only one at near-limb directions. Had a different threshold been chosen, and one that to Cragin was equally plausible, Frank et al. “would have found 33 near-nadir and 25 near-limb events” (Cragin, 1990:1174). In the reply to Cragin’s comment, Frank and his colleagues admitted that if they had used Cragin’s darkness threshold, “the confidence level is a very low 67% that there is a statistically significant difference in the rates of limb-viewing and nadirviewing events” (Frank et al., 1990a:1175). Frank and his colleagues argued, however, that: The purpose of the analysis of the Viking images is to determine whether or not there is a value of [the threshold darkness parameter] for which there is a statistically significant difference between limb-viewing and nadir-viewing measurements. (Frank et al., 1990a:1175)

With this approach, as Cragin had remarked, “It is therefore not nearly as surprising as [Frank et al.] makes it seem that they were able to obtain results in agreement with their theory” (Cragin, 1990:1174).

4.4.3

The University of Arizona Spacewatch Camera at Kitt Peak

Early in 1988, Clayne M. Yeates (1936–1991) of the Jet Propulsion Laboratory conducted a search for the small comets by sluing one of the telescopes at the Kitt Peak National Observatory. He had determined a slue rate that he had calculated would record the images of the small comets as short streaks of light. The telescope was equipped with a charge-coupled device (CCD) and was known as the Spacewatch camera. It was used to survey for asteroids and comets. Yeates wanted to record images of the small comets on two successive images, and he described the method and criteria he used as follows: The camera was set in motion . . . but two images were taken with a readout in between while the camera remained in motion. . . . From an observed streak in the first image the angular speeds in right ascension and declination are determined with an ambiguity in direction. Thus the required location of the same object in the second image must be precisely in one of

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two possible locations. Not only must its location be precisely correct, but the second streak must have the same length as and be collinear with the first streak, and both streaks must have the same brightness. (Yeates, 1989:1193)

In the spring of 1988, Yeates announced that he had found pairs of images that fit his criteria for small comets of the kind that had been proposed by Frank and his colleagues. At the December 1988 meeting of the AGU, Frank, Sigwarth, Craven, and Yeates made a poster presentation of their analysis of the successive exposures of the images of the supposed small comets. The four submitted a corresponding paper to EOS, the newsletter of the AGU (Frank et al., 1988). Yeates submitted a more detailed paper on his Spacewatch experiment to Geophysical Research Letters, but it was rejected by the GRL editors (Kerr, 1989:171). (Alex Dessler was still Editor-in-Chief of GRL.) Yeates then submitted his paper to the journal Planetary and Space Science, which accepted it (Yeates, 1989:1193). Some prominent astronomers familiar with the use of CCDs were unconvinced that Yeates had recorded images of Frank’s small comets. Anton M. J. (Tom) Gehrels (1925–2011), who took the Spacewatch images for Yeates, thought the images were “not convincing” (Kerr, 1989:171). Similarly, Eugene M. Shoemaker (1928–1997) of the U.S. Geological Survey told Kerr that: He [Yeates] is pushing right against the noise limit. When you look for rare things, you can find all kinds of flukes. They don’t look convincing to me. (Kerr, 1989:171)

4.5

Publication of The Big Splash

Perhaps convinced by the evidence from the Viking satellite data and that of the Spacewatch camera at Kitt Peak, Louis Frank and Patrick A. Huyghe, a free-lance science writer, published a book in 1990 with the main title of The Big Splash, and a long subtitle—A Scientific Discovery That Revolutionizes the Way We View the Origin of Life, the Water We Drink, the Death of the Dinosaurs, the Creation of the Oceans, the Nature of the Cosmos, and the Very Future of the Earth Itself. In the first pages of his book, Frank and Huyghe laid out the implications of his smallcomet hypothesis: It would mean that our lakes, rivers, and oceans were not formed as we thought early in the earth’s history. It would mean that the substances necessary for the origin of life on this planet may well have arrived from space. It would mean that periodic increases in cometary showers could have caused the ice ages and been responsible for the death of many species, including the dinosaurs. It would mean that the planet Venus never had a primitive ocean, as many scientists suspect, and that the Martian landscape was formed by water, which may one day again flow on its surface. And it would mean that the volume of water on the Earth is slowly increasing and may one day submerge the planet entirely. It would mean, in sum, that our cherished notion regarding the Earth’s isolation from the rest of the solar system and the universe will have to be discarded. (Frank & Huyghe, 1990:3–4)

In The Big Splash, Frank and Huyghe reviewed the history of the small-comet controversy, giving some background details that did not come out in the journal

4.6

The Review Papers of Dessler and of Frank

77

publications. (It is clear from the text of the book that Frank was the primary author, as it is written as if by Frank in the first person.) Frank also made clear that he felt that he had been mistreated by his scientific colleagues. At the beginning of Chap. 22 in The Big Splash, Frank wrote about his musings while working in the early hours of the mornings. I thought of several brontosauri that made the effort to enter a quicksand swamp to overwhelm a comrade who was foolish enough to wander there in the first place. Their comrade’s name might have been Frank, and those entering the swamp could have been Donahue, Hanson, and the others. One named Dessler could have pointed the way to the conflict. Along the edge of the swamp would stand a multitude of other creatures with lesser capabilities and courage but all secure in the final outcome since the struggle was so one-sided. They would throw a few rocks at the hapless comrade without knowing why. The brontosauri would thrash and struggle and finally sink slowly into the swamp. So it was with the small comet debate that took place in Geophysical Research Letters. (Frank & Huyghe, 1990:176)

4.6

The Review Papers of Dessler and of Frank

But by the time of the publication of The Big Splash in 1990, the controversy over the small-comet hypothesis was far from over. In the August 1991 issue of Reviews of Geophysics (3 years after the review by Louis Frank and John Craven in the same journal), Alex Dessler published a long review article on the small-comet hypothesis (Dessler, 1991). Dessler reviewed the various aspects of the hypothesis of Frank, John Sigwarth, and Craven (for which he used the initials FSC). Included in Dessler’s review were most of the topics addressed in the extensive comments and responses, such as the disruption, vaporization, and event rate; ionospheric and atmospheric effects; and solar-system effects, including lunar impacts, interplanetary water vapor and the insulation of small comets. Dessler regarded “the nondetection of small-comet lunar impacts to be one of the most persuasive facts leading to the conclusion that small comets, as described by FSC, do not exist” (Dessler, 1991:367). Dessler reviewed FSC’s explanation of why they concluded that small-comet impacts on the Moon are relatively benign and then expressed his skepticism through an analysis of a large object striking the moon: A 100-ton object traveling at 12.6 km/s has a kinetic energy of 8 × 1012 J, which is the energy equivalent of a 2-kiloton TNT explosion. . . . There is nothing soft or fluffy about any substance impacting at 12–17 km/s. . .at lunar impact speeds of 12–17 km/s a water molecule in an ice crystal has a kinetic energy of 21 ± 7 eV, and a silicon dioxide molecule in a rock has a kinetic energy of 72 ± 24 eV, energies that exceed molecular binding energies by a factor of approximately ~102. (Dessler, 1991:368–369)

Dessler argued that an object the size of a small house impacting the lunar surface would release the equivalent energy of a 2 kiloton nuclear bomb. He noted that the resulting crater would be the size of a football stadium.

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Dessler then reviewed other evidence, including the Viking satellite results, quoting a personal communication from the Viking UV imager Principal Investigator Sandy Murphree: “the fact that depressed pixels could be found in the few such [calibration] examples we have is I think strong evidence against a geophysical source for the pixel depression [i.e., dark spots].” (Dessler, 1991:374)

In his discussion of the observations of Clayne Yeates using the Spacewatch camera of Tom Gehrels at Kitt Peak, Dessler reproduced the statement of disagreement that Gehrels had posted alongside the poster by Frank and his colleagues at the Fall meeting of the AGU in 1988. Gehrels’s statement reads as follows: Evaluation of Cometesimals, November 1988, Tom Gehrels. If there indeed were as many cometesimal events in the noise of our CCD frames as L. A. Frank and C. M. Yeates believe, I would expect a noticeable distribution in their size and distance such that occasionally we should see a convincingly bright event. The images that have been shown to us are unconvincing, and this includes the observation of 19 April 1988 for repeated events. Anyone can look at the pictures released by Yeates and Frank and decide for oneself. Before believing the discovery of cometesimals, a new type of population in the solar system, one would want to see a few good images. Independent observations are needed. (signed) Tom Gehrels, Spacewatch PI*. (Dessler, 1991:376) *Principle Investigator

Finally, Dessler concluded that the best explanation of Frank’s dark spots is that they are instrumental artifacts. In February of 1993, Frank and Sigwarth responded to Dessler’s review paper by publishing their own review in the Reviews of Geophysics (Frank & Sigwarth, 1993). John Craven, Frank’s former research collaborator at the University of Iowa, had moved to the University of Alaska in Fairbanks in 1991. In general Frank and Sigwarth repeated the arguments they, with Craven, had made in the many replies to critical comments from others. Here and there, however, they modified their model in response to Dessler’s analyses. For example, they lowered the altitude where the small comets were disrupted by electrostatic forces from 3000 km to 1000 km. On the other hand, Frank and Sigwarth added arguments based on experiments that had not been completed when Dessler wrote his review paper. One of these arguments was based on an experiment that used the microwave radiometer at the Pennsylvania State University (Penn State) to detect bursts of water vapor in the mesosphere and thermosphere. A project to analyze these transient bursts was carried out by Michael F. Bonadonna, a meteorologist in the United States Air Force, for his M.S. thesis at Penn State. In the abstract of his thesis, Bonadonna wrote: This study examines the short-term variability of upper atmospheric water vapor with the intent of examining a proposed extraterrestrial water vapor source. This source would be provided by an influx of the small (12 m in. (sic) diameter) comets described by Frank et al. (1986). A ground-based microwave (22.235 GHz) radiometer located at Penn State has been measuring the thermal emission of upper atmospheric water vapor since 1984. Over 22,000 20-minute brightness temperature spectra from the period of Nov. 1984 through Dec. 1988

4.7

Polar Satellite Results

79

were analyzed for statistically significant, transient increases of the amounts of water vapor. This signature could indicate the presence of the cometary water vapor source. Individual 20-minute spectra were compared to the local 12-hour mean and variance spectra in a search for this excess signal signature or event. The analysis yielded 111 significant events which could have been caused by the cometary water vapor. The rate of detection (2.9 days between events) compares favorably with what could be expected from the small comet theory (1.8 days/events). This result is also comparable to the 4.1 days/events obtained by Adams (1988) using a small subset of this data base. After exploring alternate explanations for the observed phenomena, it is concluded that these results support the existence of the small comet hypothesis. (Bonadonna, 1990:Abstract)

This research was published in Bonadonna et al. (1990). There appear to have been no follow-ups to this type of investigation and its positive implication for the small comets. On the topic of orbits for the small comets, Frank and Sigwarth accused Dessler of misciting an earlier paper by Frank, Sigwarth, and Craven: Dessler [1991, p. 372] miscites Frank et al. [1986i] in stating that “one sixth of the small comets are in retrograde orbits” in order to base his claim that at least two bright, high speed impacts should be seen each night by a ground observer. No such statement is made in Frank et al. [1986i]. (Frank & Sigwarth, 1993:14)

While it is true that the wording “one sixth of the small comets are in retrograde orbits” does not appear in Frank et al. (1986i), what does appear seems to amount to the same thing. FSC wrote: The velocity change, Δv, for a comet due to the gravitational perturbation from an object with mass M, relative velocity V and closest approach distance R is given by the proportionality Δv / M/RV (cf. Hills, 1981). Of interest here is the dependence on 1/V that greatly favors prograde orbits in the vicinity of the outer planets. For example, for a comet orbit with perihelion at Jupiter’s orbit and aphelion at 500 A.U., V for retrograde orbits is a factor of ~ 6 greater than that for prograde orbits. If the aphelian position decreases to 10 A.U. during the shower then this factor increases to ~ 15. Thus during the declining phase of a cometary shower, the significantly more rapid diffusion of prograde orbits in the planetary system should be reflected in a corresponding dominance of prograde orbits at the earth. (Frank et al., 1986i:1485)

In the summary section of their review article, Frank and Sigwarth looked forward to 1994, when “the Earth’s ultraviolet dayglow is to be revisited by cameras of significantly improved temporal and spatial resolution when they are launched on the Polar spacecraft of the International Solar Terrestrial Physics Mission” (Frank & Sigwarth, 1993:25–26).

4.7

Polar Satellite Results

The Polar satellite was not launched until February 1996, about 15 years after the launch of both Dynamics Explorer satellites in August 1981. As the name implies, the satellite was launched into a polar orbit, with an initial apogee altitude of 50,510 km and perigee altitude of 5170 km (Frank & Sigwarth, 1997a:2423).

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Included among the 11 scientific instruments on Polar were 2 imaging systems that could cover the far-ultraviolet wavelength range necessary for dayglow observations. Louis Frank was the PI for the Visible Imaging System (VIS), which included an ancillary Earth sensor for the far-ultraviolet wavelengths within a passband of 124–149 nm (Frank et al., 1995). George K. Parks of the University of Washington was the PI for the Ultraviolet Imager (UVI), which incorporated a narrow-band filter that detected emissions at 130.4 ± 4.0 nm, i.e., designed to detect atomic oxygen emissions at 130.4 nm (Parks et al., 1997:3109). Both the VIS with its UV Earth sensor and the UVI were mounted on a despun platform on the Polar satellite (Parks et al., 1998:3063). As explained by Parks, however: After Polar was launched, it was discovered that the despun platform has an unresolved 0.4° wobble in one direction which degrades the UVI pixel resolution to approximately 1 × 10 pixels. However, this wobble turns out to be serendipitous for the purpose of studying the dark pixels because an external source reaching the UVI will now be modulated by the wobble. We can look for this signature to determine if the source of the dark pixels is internal or external to the instrument. (Parks et al., 1997:3109)

4.7.1

Frank and Sigwarth Analyses of Polar Data

When the data from the Visible Imaging System began to be analyzed, Frank and John Sigwarth announced their results at the 1997 Spring meeting of the American Geophysical Union, and they felt fully vindicated. Richard Kerr of Science reported from the AGU meeting as follows: Frank has used a satellite camera with sharper resolution to produce more detailed images that confirm the existence of these dark spots to the satisfaction of other scientists. The new data even seem to show an influx of water. “Now, you’re faced with overwhelming evidence,” says Frank. “We’ve verified [the spots] from five different viewpoints” (Kerr, 1997a).

Frank and Sigwarth published a series of four papers in the 1 October 1997 issue of Geophysical Research Letters. The abstract of their first paper reads as follows: Ten years ago transient decreases in Earth’s far-ultraviolet dayglow were reported for global images acquired with the high-altitude, polar-orbiting spacecraft Dynamics Explorer 1. These decreases were observed primarily in the atomic oxygen emissions at 130.4 nm. The diameters of these dark spots, or atmospheric holes, were in the range of 50 to 100 km. Recently a sophisticated camera for imaging Earth’s far-ultraviolet dayglow, with far greater spatial and temporal resolution than its predecessor, was launched with the Polar spacecraft. The images from this spacecraft provide irrefutable evidence that these atmospheric holes are a geophysical phenomenon. (Frank & Sigwarth, 1997a:2423)

The VIS system was recording clusters of dark spots, instead of the single spots that were observed by the lower resolution cameras on Dynamics Explorer 1. Frank and Sigwarth pointed out that the only correction to their data was:

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Polar Satellite Results

81

Fig. 4.4 From Fig. 1 of Frank and Sigwarth (1997b), showing a putative atmospheric hole (arrow) as recorded by the UVI camera on the Polar satellite

The removal of easily identified ionization events in the CCD due to the energetic charged particles. When the spacecraft is outside of the radiation zones then the numbers of such ionization events are in the range of 30 to 200 per image. These pixel responses are replaced by the respective mediums of the adjacent pixels. (Frank & Sigwarth, 1997a:2423)

As can be seen in Fig. 4.4, one has to have a practiced eye to pick out the clusters of dark spots from the mottled images produced by the UVI camera system. Frank and Sigwarth also noted that their instruments occasionally recorded “the same atmospheric hole in two consecutive frames” of data (Frank & Sigwarth, 1997a:2426) and “the ‘double vision’ that is expected for viewing real objects from misalignment of the center-of-gravity and spin axis of the spacecraft.” (Frank & Sigwarth, 1997a:2425). In the second paper of the series, Frank and Sigwarth claimed to show examples of atmospheric holes as simultaneously recorded at the same location by the Iowa Earth camera of the VIS system and the University of Washington UVI camera. The abstract for the second paper of Frank and Sigwarth reads as follows: The Polar spacecraft carries two cameras which are capable of viewing earth’s far-ultraviolet dayglow. One of these two cameras was programmed into a special operating mode during 12 April and 30 July 1996 in order to obtain simultaneous images of transient decreases of dayglow emissions from atomic oxygen at 130.4 nm. During the 76 minutes of usable imaging the two cameras acquired five sets of frames for which a transient decrease was detected by each camera, and the transient decrease occurred at the same geographical position in the dayglow. These series of observations provide strong evidence for the identification of atmospheric holes as a geophysical phenomenon. (Frank & Sigwarth, 1997b:2427)

In the third paper of the series, Frank and Sigwarth give examples of transient trails of far-ultraviolet emissions taken by the Earth Camera on the Polar satellite (see Fig. 4.5). The abstract of this paper reads as follows: Transient trails of emissions at far-ultraviolet wavelengths have been detected by the Earth Camera on board this polar spacecraft. These emissions are interpreted in terms of resonantly scattered solar radiation from atomic oxygen at 130.4 nm. The temporal durations of the emission trails are typically tens of seconds. The maximum brightnesses of the shorter trails

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Fig. 4.5 From Fig. 1 of Frank and Sigwarth (1997c), showing the trail in the upper right-hand side of the image that Frank and Sigwarth interpreted as “the record of the final moments of the disintegration of a small comet.” (Frank & Sigwarth, 1997c, Fig. 1 caption)

are usually lesser than those of the longer trails which indicate that the shorter trails are farther from the spacecraft. The rate of occurrence of these trails is approximately 5 to 10 each day. These events are interpreted as the signatures of the disruption and rapid dissipation of small comets in the vicinity of the Earth. (Frank & Sigwarth, 1997c:2431)

In the fourth paper of their series, Frank and Sigwarth noted that two of the cameras of the Visible Imaging System instrument had a filter wheel with 12 individual filters. One of these filters was centered at 308.5 nm, and they gave examples of trails of emissions of the OH radical at 308.5 nm (Frank & Sigwarth, 1997d:2435). As Frank and Sigwarth explain: Because there is no strong fluorescence emissions from water at ultraviolet and visible wavelengths the OH emissions act as proxy for the presence of the cometary water molecules. (Frank & Sigwarth, 1997d:2435)

The abstract of their fourth paper reads as follows: The results are reported for a successful search for the OH emissions associated with an influx of small comets into Earth’s upper atmosphere with a camera on board the Polar spacecraft. The spatial distribution of the OH emissions are characterized by a bright core of intensities at or less than the spatial resolution of the camera which is surrounded by a larger dim region of luminosity. The Earth’s shadow is employed in order to obtain a coarse determination of the altitudes of these OH trails, i.e., an altitude range < 3000 km. (Frank & Sigwarth, 1997d:2435)

According to Richard Kerr, reporting for Science magazine: Although Frank’s observations are being vindicated, he has a long way to go toward persuading the community that these black dots are actually the remains of midget comets. “There are two quite separate questions,” says atmospheric physicist Donald Hunten of the University of Arizona, another early critic. “One is, are the spots real? Okay, they’re real. The next question is whether Lou’s explanation is valid. No, it certainly isn’t valid. It is very easy to put forward five objections to the small-comet explanation, any one of which rules it out.” (Kerr, 1997a:1333)

Kerr notes, however, that some of Frank’s former critics were taking the view that Frank had evidence of a real phenomenon. Kerr quoted Thomas Donahue as follows: “It’s very impressive observational work,” acknowledges atmospheric physicist Thomas Donohue of the University of Michigan, “that I don’t think leaves us much room for doubt. There are little somethings releasing a lot of oxygen, and they show the signature of

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hydroxyl in emission. It’s hard to imagine what other than water” they would be. (Kerr, 1997a:1333)

Kerr also noted Donahue as saying “I still have all the problems I ever had with the amount of water involved” (Kerr, 1997a:1333).

4.7.2

The Washington Post Article

Following the Spring meeting of the AGU in 1997 at which the Iowa Polar spacecraft results were presented, Frank and Patrick Huyghe published an article in the 13 July 1997 (Sunday) edition of The Washington Post (Frank & Huyghe, 1997). Frank considered the Polar satellite results to be a confirmation of the speculation he and his colleagues had made more than 10 years earlier in their interpretation of dark spots in the Dynamics Explorer 1 satellite data. In addition to recounting his perceived victory, however, Frank took the opportunity to express his opinion that he had been ill treated by the space research community, as is indicated from the following excerpts of the article: The universe is what it is. I don’t bury observations that stand in the way of conventional wisdom. I don’t gloss over things I don’t understand. I will not compromise my integrity. Unfortunately, this stance has made me the target of scientific vandalism. ... . . . . After I presented my findings on the small comets in 1986, the scientific community did its best to extinguish my career. In the past decade, I’ve been unable to get any other projects off the ground. Before the small-comet findings became public, my success in this regard was envious; I was able to get instruments on board several major spacecraft—Polar, Galileo and Geotail. But after my small-comet announcement, I got nothing. I had my ongoing projects, such as the one on Polar that eventually produced the confirmatory data. But the new projects I proposed went nowhere even those that had nothing to do with small comets ... . . . . Many of my colleagues labeled me as a crank for my unwavering defense of the small comets, and I was blackballed from the community. Awards and honors with my name on them were cancelled. It is public knowledge, for instance, that I was not elected to the prestigious National Academy of Sciences for this very reason. (Frank & Huyghe, 1997)

4.7.3

Analyses of Polar Data by Parks and Colleagues

While some former critics shared Donahue’s view that Frank’s instruments on the Polar satellite were seeing something “real,” other critics shared, or eventually developed, the view that had been expressed by Alex Dessler in his 1991 review paper. That is, that Frank’s data on “atmospheric holes” were best explained as an instrument artifact.

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Fig. 4.6 George Parks. Photo courtesy of George Parks, University of Washington

Fig. 4.7 The distribution of the number of dark clusters per UVI image as a function of the size of the clusters, parameterized by the degree of darkness as measured by the standard deviation from the mean. From Fig. 2 in Parks et al. (1997)

This was notably the view of George Parks (Fig. 4.6), the PI for the UVI camera (Torr et al., 1995) on Polar. In December of 1997, Parks and his colleagues published a paper in which they wrote that: Examination of the UVI images has revealed that dayglow images are indeed spotted with single and multiple dark pixels. But is a snowball the only explanation for these dark pixels? To learn more about the dark pixels, we have examined the calibration images obtained from the same camera just before the instrument was launched. We find that dark pixels similar to those in dayglow images also exist in calibration images. This strongly indicates that the source of the dark pixels is instrumental. For further verification, a statistical analysis found the dark pixels from dayglow and calibration images have nearly identical shaped occurrence patterns. (Parks et al., 1997:3109)

By “occurrence patterns,” Parks and his colleagues meant the plots of the number of clusters of dark spots per image versus the size of the spots (see Fig. 4.7). Parks and his colleagues also noted that “Calibration data indicate that the dark pixel occurrence distribution changes with the image intensifier gain . . .” (Parks

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Fig. 4.8 An image of an ultraviolet star recorded with the UVI camera on the Polar satellite, showing the effect of the wobble motion of the spacecraft. From Fig. 3 in Parks et al. (1997)

Fig. 4.9 The angular distribution of cluster pairs as recorded by the UVI camera on the Polar satellite. The wobble is along 0°. From Fig. 5 in Parks et al. (1997)

et al., 1997:3110), giving an additional indication that the dark spots were an instrumental artifact. Parks and his colleagues used the despun platform 0.4° wobble to help determine if the dark spots were originating from an external source: One clear determination whether a dark pixel is of geophysical origin is the evidence of spacecraft wobble. The image of a small source originating external to the UVI appears as a dual-peaked function (dumbbell shape) with about a ten-pixel spread in the direction of the wobble motion. . . . Any small source with a duration longer than 6 seconds evidences the characteristic dual-peaked function. (Parks et al., 1997:3111) (See Fig. 4.8)

Parks et al. showed that dark cluster pairs recorded by the UVI did not preferentially align with the wobble direction (see Fig. 4.9), as they should have if the source of the dark pixels was external to the spacecraft. Finally, with regard to the bright streaks reported by Frank and Sigwarth (1997c), Parks and his colleagues noted that: One known source of structured bright streaks is cosmic ray noise. Cosmic rays lose energy as they propagate through the camera system. The ionization tracks they produce frequently appear in the UVI images as bright spots, long and short streaks and other complex shapes. We can distinguish between real comets and penetrating radiation because the former is wobble modulated and the latter is not, owing to a very short interaction time. After examining numerous bright streaks in UVI images, we conclude that virtually all of them can be explained as ionization tracks due to cosmic rays and other types of penetrating radiation. (Parks et al., 1997:3112)

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Fig. 4.10 Occurrence distributions of dark-pixel events as a function of the size of the event for VIS and UVI images. Distributions are shown for dark pixel events that satisfy the 2σ criterion identified by temporal and spatial averaging. From Fig. 2 in Parks et al. (1998)

Continuing his reporting on the small-comet controversy, Richard Kerr wrote, following the publication of the Parks et al. paper, that the UVI imaging team: conclude that “there is no scientific evidence from UVI that snowballs pelt Earth.” The spots are not clouds of water left from high-altitude impacts of small comets, they say, but simply artifacts produced inside the camera—so much snow on a UV television. (Kerr, 1997b:1217)

Kerr also wrote that Alex Dessler, former editor of GRL, stated that: “Parks’ paper is absolutely devastating,” says longtime small-comet critic Alexander Dessler, a space physicist at the University of Arizona, Tucson. (Kerr, 1997b:1217)

After Parks and his colleagues submitted their 1997 paper, they received 1 h of Iowa VIS data from the Frank team that was concurrent with the UVI data they had analyzed for their paper. They subsequently published a paper on a comparison of the dark pixels from the VIS images that overlapped those from the UVI camera (Parks et al., 1998). The last sentence of the abstract of the paper containing their results is blunt. The conclusion of this study is that neither VIS nor UVI provide any scientific evidence that the dark pixels are geophysical. (Parks et al., 1998:3063)

This conclusion is supported by their analyses in the paper, in which they noted the similarity in the occurrence distributions for the VIS and UVI camera systems as shown in Fig. 4.10, Parks and his colleagues asked: How can two independent cameras behave so similarly when the pixel resolution of VIS (0.08°) is half that of UVI (0.04°)? How can the occurrence distributions of the dark pixels not show any dependence on the cameras’ pixel resolutions if the source is external to the camera? The only plausible explanation for this similarity is that the source of the dark pixels is internal in both cameras. . . . (Parks et al., 1998:3065)

Parks and colleagues then explain their “plausible explanation” by examining the differences in pixel resolution that exist between the two instruments. They note that the projected area of an Iowa VIS instrument pixel is four times that of the Berkeley

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Fig. 4.11 Occurrence distributions of dark-pixel events for UVI (top) and VIS (bottom) as a function of the altitude for the same events shown in Fig. 4.10. From Fig. 3 in Parks et al. (1998)

UVI pixel area. Therefore, an image of an atmospheric source by the VIS will have four times fewer pixels than will an UVS image. The result would be, as they write, that the . . . the two distributions in Figure 2 [Fig. 4.10] should be offset by a factor of 4 if the source were external (the UVI curve should be shifted by a factor of 4 to the right). There is no reasonable explanation for why the offset was not observed if the source were external. This result is a clear demonstration that dark pixels are internal to the cameras. (Parks et al., 1998:3065)

In Fig. 4.11, Parks and his colleagues showed a lack of altitude dependence in the images of dark pixels: The various distributions for spatial averaging taken at different altitudes are virtually identical and no height dependence is observed. If the source of the dark pixels is external to the camera, one would expect the distributions to change since the source will look smaller when the spacecraft is further away. Again, the only plausible explanation for the

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nondependence of the altitude is that the dark pixels are produced in the camera. (Parks et al., 1998:3065)

Finally, Parks and his colleagues found no evidence that the dark-cluster pairs recorded by VIS or UVI were preferentially aligned with the direction of the wobble of the spacecraft. They noted as well that the high occurrence rate of the dark pixels for VIS and UVI would yield a high probability for accidental coincident events reported by Frank and Sigwarth (1997b) (Parks et al., 1998:3066).

4.7.4

Computer Simulations of Polar Pixel Responses by the Berkeley Group

In October of 1998 a group led by physicist Forrest S. Mozer (Fig. 4.12) of the Space Sciences Laboratory at the University of California, Berkeley, published two papers. Mozer, a pioneer in developing instruments for electric field measurements in space plasmas, was the principal investigator of the Electric Fields Instrument (EFI) on Polar. In the first paper Mozer and his colleagues used computer simulations to analyze the possibility of an instrumental source for the dark-pixel clusters in the Polar VIS and UVI experiments (McFadden et al., 1998). The abstract from the first paper reads in part as follows: To check for instrumental effects, we simulated the polar VIS and UVI response to uniform illumination and calculated the rate of dark pixel clusters. The simulation includes the finite size and exponential amplitude of light pulses from the experiments’ image intensifiers and the removal of contamination by penetrating radiation. The dark pixel cluster size distributions from the computer simulations reproduce the distributions observed in both of the Polar imagers. . . . These instrumental effects fully explain the dark pixel clusters without the need for invoking a geophysical phenomena. (McFadden et al., 1998: 3705)

In the second paper (Mozer et al., 1998), the authors used a catalog of about 700,000 atmospheric-hole data produced by Frank and Sigwarth to test whether the number

Fig. 4.12 Forrest Mozer. Courtesy of Forrest Mozer, University of California at Berkeley

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Fig. 4.13 VIS image collected over the North Pole, illustrating the dayglow, the Aurora Oval and 10 dark pixel clusters, of which two are viewed against the dark sky and five are viewed over the dark earth. From Fig. 4 in Mozer et al. (1998)

of recorded pixel clusters varied in a systematic way with the altitude of the Polar satellite. The abstract of the second paper summarizes their study in part: A geometrical requirement of the small-comet hypothesis is that the number of pixels in a typical cluster must vary by a factor >100 with spacecraft altitude because of the inversesquare law of the apparent cluster area versus distance. We find no systematic variation of cluster size with spacecraft altitude. The Iowa catalog data are consistent with instrument noise because neither the size distribution nor the event rate of dark pixel clusters depend on altitude. (Mozer et al., 1998:3713)

Mozer and his colleagues used the methods of Frank and Sigwarth to process a day’s worth of raw data provided to them by the Iowa group. One of their resulting images is shown in Fig. 4.13 to make the point that: The appearance of “atmospheric holes” where there should be none, demonstrates again that the so-called atmospheric holes are, instead, noise. (Mozer et al., 1998:3715—figure caption)

Mozer and his colleagues demonstrated how the automated process used by Frank and Sigwarth for removal of bright pixels caused by penetrating radiation: . . . changed the result from no dark pixels within a [given area] before removal of penetrating radiation to five dark pixels afterwards, and thus produced the “atmospheric hole.” (Mozer et al., 1998:3716)

The Berkeley authors concluded their paper with the following charge: The Iowa methods also produce atmospheric holes against backgrounds of the dark sky or the dark Earth as illustrated in Fig. 4 [Fig. 4.13]. In this image, the Earth is observed from above the north polar region. Dayglow and the auroral oval are seen, and 10 dark pixel clusters are found by the Iowa methods. Three or four of these dark pixel clusters are above the dayglow, two are viewed against the dark sky, and four or five are viewed against the dark Earth. Two of the clusters over the dayglow and three of the clusters over the dark Earth were produced by removing the penetrating radiation. Events over the dark Earth or against the dark sky have been excluded from the Iowa catalog by definition because they are both meaningless and devastating in the context of the small-comet hypothesis. (Mozer et al., 1998:3716)

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Influx of Small Comets into Earth’s Atmosphere

The Response of Frank and Sigwarth

In the 1 January 1999 issue of the Journal of Geophysical Research, Frank and Sigwarth published a long rebuttal to the analyses by the research groups at the University of Washington and at the University of California, Berkeley. They pointed out the steps they had taken to eliminate the possibility that the atmospheric holes were the result of instrumental effects, and they emphasized the data that suggested the holes were real geophysical phenomena. Their abstract listed studies of four instrumental effects that they examined: (1) evaluation of random rates for a period in mid-January for which there were no atmospheric holes, thus providing an excellent inflight calibration of the instrument’s noise, (2) the effects in the image is due to energetic electrons in the outer radiation zone, (3) the nonuniform sensitivities for the pixels of the sensor, or “hot spots,” and (4) the contributions of longer wavelength radiation from the atmosphere to the camera’s responses. (Frank & Sigwarth, 1999a:115)

They summarized in the abstract that instrumental effects were not major contributors to their results. The abstract also summarized what they termed geophysical effects: (1) a strong altitude dependence on the frequency of atmospheric holes, (2) a substantial local time variation of rates which favored locations in the local morning sectors of the atmosphere relative to those in the evening, (3) increasing sizes of the atmospheric holes as the spacecraft altitude decreases, and (4) large seasonal variations in the hole rates during the period November 1997 through late January 1998 which were remarkably similar to those observed during the same months but 16 years earlier with Dynamics Explorer 1. (Frank & Sigwarth, 1999a:115)

In introductory remarks, Frank and Sigwarth dismissed the validity of observations of darkened pixels made by the UVI camera (they claimed the spatial resolution was insufficient for detection) as well as the charge by Parks that simultaneous sightings by their UVI system and the Iowa VIS were coincidental: simultaneous sightings were not coincidental because during a 64-minute interval on April 12 only ten holes were observed in 116 VIS images and eleven holes in 53 UVI images. Only two sets of simultaneous sightings were available due to the nonsynchronous timings of the frames from the two cameras. For these two sets of frames, the holes were sighted at the same position in the dayglow with the two cameras. (Frank & Sigwarth, 1999a:116)

With regard to the wobble of the Polar satellite, Frank and Sigwarth pointed out that they had eliminated the effect by periodically shuttering the VIS Earth Camera in synchronization with the spacecraft spin, so that only one of the two “dumbbell” images was recorded (Frank & Sigwarth, 1999a:117). With regard to the dependence of the rate of occurrence of the atmospheric holes on spacecraft altitude, Frank and Sigwarth argued that the strong local time dependence of the rate of occurrence of hole detection made it important that a specific atmospheric area be identified. If this were not done, they claimed that local time variations and varying viewing geometries would confuse any conclusions. They

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Fig. 4.14 The average number of holes per sector for each Earth Camera image as a function of spacecraft altitude. From Fig. 11 in Frank and Sigwarth (1999a), see also Frank and Sigwarth (1999b)

then stated that their “. . . strong observed variations of rates as a function of altitude reject the instrument artifact hypothesis.” (Frank & Sigwarth, 1999a:129). Frank and Sigwarth showed an example of the dependence of atmospheric holes on satellite altitude for a particular sector of the sunlit Earth (Fig. 4.14). To further demonstrate the altitude and local-time dependence of the observation of atmospheric holes, Frank and Sigwarth included a plate comparing the occurrence rate in terms of holes per pixel when the Polar satellite was between 3.0 and 5.0 RE and when it was between 5.0 and 8.0 RE in altitude (Fig. 4.15). Noting the altitude dependence and the local-time dependence, with most of the events occurring in the late-morning sector of the sunlit Earth, Frank and Sigwarth deemed that the case was closed:

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Fig. 4.15 Comparison of occurrence rates of atmospheric holes in two different altitude ranges. From Plate 9 in Frank and Sigwarth (1999a)

The atmospheric holes must be due to geophysical effects, and not due to instrument artifacts. (Frank & Sigwarth, 1999a; Figure caption of Plate 8)

Frank and Sigwarth concluded their paper with a summary of arguments against the atmospheric holes being instrument artifacts. They pointed out that the period in mid-January 1997 during which no atmospheric holes were detected provided the equivalent of a postlaunch laboratory calibration that established the camera’s noise performance. This allowed them to understand how varying the criterion for pixel darkening created false hole detections due to such instrumental effects as penetrating radiation and sensor “hot spots.” While no obvious correlation was found between electron intensities and the occurrence of atmospheric holes, they used the data from the onboard charged particle detector to reject images taken when the spacecraft was subject to high electron intensities. They understood that for clouds with sufficiently high altitudes, the Polar Earth Camera could record clouds in its images and that a sufficiently high near-ultraviolet response from the clouds would preclude detection of atmospheric holes. They saw no correlation between the positions of atmospheric holes and cloud features, however, and they argued that since high-altitude clouds typically cover only about 5–10% or less of a dayglow image, the loss of area was acceptable. (Frank & Sigwarth, 1999a:138–139).

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Their summary then turned to their arguments that the clusters of darkened pixels corresponded to a real geophysical phenomenon. They argued that the demonstrated decrease in the occurrence rate for the holes was owing to the fact that fewer holes can be detected with increased altitude because the apparent size of the holes decreases below the spatial resolution of the camera. They noted that the typical global rates in the range of a few tens per minute for the atmospheric holes was the same for those recorded by the Polar Earth Camera as the ones recorded by the camera on the Dynamics Explorer 1 satellite, even though the camera designs were entirely different. They pointed to the strong local-time dependence of the occurrence rates of atmospheric holes as evidence for a real geophysical phenomenon, and they noted that the same local-time effect was observed with the Dynamics Explorer 1 camera. They argued that the low occurrence rate for atmospheric holes when the Polar satellite was at very low altitudes was because a relatively small fraction of the dayglow area was viewed at these altitudes and, hence, smaller numbers of real object were recorded. They further noted that the atmospheric-hole size was shown to increase with decreasing altitude, as expected. Finally, they pointed to the seasonal variation of the occurrence rates of atmospheric holes as evidence of a real geophysical phenomenon, and, further, the same seasonal variation was observed by the Dynamics Explorer 1 satellite camera. (Frank & Sigwarth, 1999a:140). For Frank and Sigwarth, the argument was over: The sources of instrumental effects for detection of atmospheric holes with the Earth Camera on board the Polar spacecraft have been comprehensively investigated. The automated determination of the strong geophysical effects reported in this paper validates the reality of the atmospheric holes. (Frank & Sigwarth, 1999a:140)

4.8

Search for Small Comets Using Radar

A few months after the publication of the rebuttal by Louis Frank and John Sigwarth (1999a), Stephen H. Knowles and his colleagues of the Naval Research Laboratory and the University of Florida published the results of their search for small comets in the Journal of Geophysical Research. They employed the Naval Space Surveillance System (NAVSPASUR), also known as the Fence. The Fence Surveillance System was an operational system until 2013 that used radar to detect space objects in low-Earth orbit (Knowles et al., 1999a). A multi-static system, NAVSPASUR consisted of three transmitter sites and six receiver sites stretching across the southern continental United States. The radar design was fan-shaped, narrow (0.1 degree) in the north-south direction and horizon to horizon in the east-west direction. The Knowles group used the characteristics of small comets that had been published by Frank and his colleagues. They also noted that they used parameters that were more conservative from the point of view of radar detectability. Radar data were obtained during three periods in 1997. The analyses involved primarily comparing the speed versus altitude of about 12,000 objects that had not been correlated

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with known satellites or other targets. Orbiting objects are gravitationally bound to the Earth and must have speeds below the Earth’s escape velocity, i.e., below about 11 km/s. The putative small comets are expected to have speeds in the range of 16.6–20 km/s (Dessler, 1991:362). The search by the Knowles group found that all the observed velocities of the 12,000 objects fell well below the anticipated cometary velocities. They concluded that “. . . the radar observations are inconsistent with the small-comet hypothesis” (Knowles et al., 1999a:12641). A few months later in 1999, Frank and Sigwarth published a Comment on the paper by the Knowles group. (Frank & Sigwarth, 1999c). In their Comment, they claimed that: In order to increase the radar cross section for the small comets, Knowles et al. [1999a] assume that their dust content is similar to that of the well-known large comets. (Frank & Sigwarth, 1999c:22605)

Knowles and his colleagues published a Reply in the same issue of the Journal of Geophysical Research in which the Comment by Frank and Sigwarth appeared (Knowles et al., 1999b). Knowles and his colleagues contended that the above statement by Frank and Sigwarth was untrue. In their Reply, Knowles et al. used the initials KMGG to refer to their prior paper (Knowles et al., 1999a): Frank and Sigwarth [this issue] state that in order to increase their estimates of the radar cross section, KMGG assumed that the small comet dust content is similar to normal comets. KMGG did present one example of a cross section for a dust-containing small comet for illustration purposes only, since ices are believed to have condensed on dust in the standard scenario for solar system formation. However, the actual conclusions of KMGG were based on calculations of radar cross sections for small comets without dust, in conformance with the model Frank and Sigwarth [1993]. (Knowles et al., 1999b:22609)

Another example of incorrect statements by Frank and Sigwarth is their assertion that: The model of the comets used by Knowles at al. employs the smallest density for the water snow, 0.02 g/cm3, in order to yield the largest radar cross section. (Frank & Sigwarth, 1999c:22606)

In their Reply, Knowles and his colleagues pointed out that: Frank and Sigwarth [this issue] claim that KMGG chose a cometary mass density of 0.02 g cm-3 in order to produce a larger radar cross section. . . . we derived that density based on the 12-m diameter used by Frank and Sigwarth and their recommended mass of 2 × 107 g. KMGG also pointed out that increasing the mass density to the Frank and Sigwarth [1993] recommended (but inconsistent) value of 0.1 g cm-3 approximately doubles the radar cross section. Thus a mass density of 0.02 g cm-3 produces a smaller cross section than 0.1 gm cm-3, not larger. (Knowles et al., 1999b:22609–22610)

Based on their false, or at best, misleading, statements, Frank and Sigwarth concluded their reply to the radar search by the Knowles group as follows: We have used values for the physical parameters for small comets in the published literature and the corresponding radar cross sections as given by Knowles et al. [1999b] and conclude that detection of the small comets with the naval radar is very unlikely. (Frank & Sigwarth, 1999c:22607)

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Orbital Analysis Critique of Harris

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Nevertheless, their final sentence suggested that even for Frank and Sigwarth, the case for small comets was not yet closed: The uncertainties associated with this radar search for the small comets provide additional impetus for resolving the issue of the existence of small comets by ground-based observations with optical telescopes. (Frank & Sigwarth, 1999c:22607)

4.9

Orbital Analysis Critique of Harris

In August of 2000, Alan W. Harris of the Jet Propulsion Laboratory published a paper in the Journal of Geophysical Research in which he criticized the small-comet hypothesis based on an orbital analysis of planetary objects impacting the Earth. The abstract of his article reads as follows: Frank and Sigwarth [1993, 1999a] have promoted the hypothesis of a population of small “house-sized” comets in near-parabolic orbits that are nearly tangent with the Earth’s and rain down on the Earth at a rate of ~20/min. Their hypothesis is based on “holes” in the dayglow of the Earth’s atmosphere, which they first claimed to see in images from the Dynamics Explorer 1 satellite, and more recently in images from the Polar satellite. One of the key aspects of the claimed observations is that the atmospheric holes appear to be more numerous on the morning hemisphere of the Earth, to which Frank and Sigwarth draw analogy to radar meteors, which show a similar asymmetry. In this paper, I show that the distribution of orbits posited by Frank and Sigwarth yield a maximum flux in the afternoon, the opposite asymmetry from the claimed observations. The only orbits which yield the correct diurnal asymmetry, and preserve acceptably low entry speeds, are orbits interior to the Earth’s having perihelia near the orbit of Mercury. Thus any relationship of “atmospheric holes” to “small comets,” or icy bodies from the outer solar system, can be ruled out. (Harris, 2000:18575)

In their reply to Harris, Louis Frank and John Sigwarth made an argument based on the observability by orbiting spacecraft of the water clouds resulting from comet impact (Sigwarth & Frank, 2000). They illustrated their point with the two accompanying figures (Figs. 4.16 and 4.17) (Sigwarth & Frank, 2000:8582). The authors argued that a water cloud that would form from an impact at local evening would quickly disappear below the optically thick oxygen atmosphere before it could expand and be observable by satellite cameras as a 50 km cloud. Hence, relatively low rates of detection are expected for local evening. In contrast for an impact at local morning, following disruption at 1000 km, the water cloud has time to form and be visible in late morning hours with a spacecraft camera. Thus, detection of atmospheric holes is favored at these local times.

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Fig. 4.16 Example of a small-comet impact at local evening. From Fig. 1 in Sigwarth and Frank (2000)

4.10

The Iowa Optical Search for Small Comets

For some time, Louis Frank had been encouraging additional optical searches for small comets. Some 10 years after the search conducted by Clayne Yeates, Frank persuaded two colleagues at the University of Iowa to use the Iowa Robotic Observatory (IRO) near Sonoita, Arizona, to search for small comets. The Iowa colleagues were Astronomy Professors Robert L. Mutel (Fig. 4.18) and John D. Fix (Fig. 4.19). Between 24 September 1998 and 11 June 1999, Mutel and Fix used the Iowa imaging system, which included a sensitive CCD camera, to search for the small comets. The abstract of their paper in the Journal of Geophysical Research details their negative results: We have conducted an extensive optical search for small comets with the characteristics proposed by Frank et al. [1986a] and Frank and Sigwarth [1993, 1997a]. The observations were made using the 0.5-m reflector of the Iowa Robotic Observatory between September 1998 and June 1999. The search technique consisted of tracking a fixed point in the ecliptic plane at ±9° geocentric solar phase angle. The telescope scan rate was chosen to track objects moving prograde at 10 km s-1 relative to the Earth at a distance of 55,000 km. The camera was multiply shuttered to discriminate against trails caused by cosmic rays and

4.10

The Iowa Optical Search for Small Comets

Fig. 4.17 Example of a small-comet impact in the local morning. From Fig. 2 in Sigwarth and Frank (2000)

Fig. 4.18 Robert Mutel. Credit University of Iowa

sensor imperfections. Of 6143 total images, we selected 2713 which were suitable for detection of objects with a magnitude 16.5 or brighter with 120 pixel trails. The sensitivity and reliability of the visual detection scheme were determined by extensive double-blind tests using synthetic trails added to over 500 search images. After careful visual inspection of

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Fig. 4.19 John Fix. Credit University Relations records, University of Iowa Libraries, Iowa City, Iowa

all images, we found no trails consistent with small comets. This result strongly disagrees with previous optical searches of Yeates [1989] and Frank Sigwarth and Yeates [1990b], whose detection rates and magnitudes, when converted to the present search, predict 65 ± 22 detections. We conclude that at 99% confidence, the number density of any prograde objects in the ecliptic plane brighter than magnitude 16.5 with speeds near 10 km s-1 have a number density less than 5% of the small-comet density derived by Frank Sigwarth and Yeates [1990b]. Any object fainter than this magnitude limit with a mass corresponding to the small-comet hypothesis (M > 20,000 kg) must have either an implausibly low geometric albedo (p < 0.01) or a density larger than that of water. (Mutel & Fix, 2000:24907)

Frank and John Sigwarth analyzed the images obtained by Mutel and Fix, and they claimed to have found evidence for small comets. They published their positive results in the Journal of Geophysical Research in March of 2001, stating that, “There were sightings of nine small comets in the set of 1500 usable images which were gained from the IRO” (Frank & Sigwarth, 2001a:3665). Though Frank and Sigwarth, in this paper, used the same images that had been used by Mutel and Fix (see Mutel et al., 2001:Sia 5–1), Frank and Sigwarth did not mention or cite the 2000 paper by Mutel and Fix. Frank was unhappy with the negative search results from his Iowa colleagues (Mutel, personal communication—2021), and he and Sigwarth published a rebuttal paper (Frank & Sigwarth, 2001b). Mutel found that rebuttal paper “quite flawed” (Mutel, personal communication—2021), and he and his colleagues published a Reply in the same issue of the Journal of Geophysical Research (Mutel et al., 2001). Finally, in 2003, Mutel and Fix published a Comment (Mutel & Fix, 2003) on the 2001 paper by Frank and Sigwarth in which the latter authors had reported a positive result from their use of the IRO to search for small comets. In their Comment, Mutel and Fix referred to their published negative search result (Mutel & Fix, 2000) as “MF” and the published positive search result by Frank and Sigwarth (2001a) as “FS.” In their 2003 Comment, Mutel and Fix provided some historical background, including the fact that the detections reported by FS used the same images that had been published by MF. They summarized first how the optical search with the Iowa

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99

Robotic Observatory began, and that it was mutually agreed to be conducted for 8 months independently of the Frank/Sigwarth group. They also agreed that all MF images would be shared with the FS group so that the FS group could carry out any independent analyses: In August 1998, FS suggested to one of us (R. L. M.) That the 0.5-m telescope at the Iowa Robotic Observatory would be a suitable instrument for a new optical search for small comets. They suggested using a form of the “skeet shoot” method originally used by Yeates [1989] and by Frank, Sigwarth and Yeates [1990b] in previous optical searches. . . . (Mutel & Fix, 2003:SIA 5-1)

Mutel and Fix then summarized their investigation with the IRO. That is, after their accumulation and study of 2713 images, in contrast with the Frank and Sigwarth findings, MF reported that no trails could be detected to a limiting magnitude of 16.5 (120 pixel trails). We concluded that if a population of small comets existed the number density must be less than 5% of the number density of small comets derived by Frank Sigwarth and Yeates [1990b]. Frank and Sigwarth [2001b] subsequently challenged this conclusion based on a disagreement concerning the calculation of the effective search volume.. . . . (Mutel & Fix, 2003:SIA 5-1)

In reply to this challenge of Frank and Sigwarth (2001b), Mutel and Fix (Mutel et al., 2001) generalized their search volume to include a range of ecliptic inclinations and perihelions for the orbits of the small comets. They wrote that relaxing the assumptions that small comets orbits lie close to the ecliptic reduces the expected number of detections. However, the expected number was sufficiently large for any plausible range of orbital parameters that the lack of any detections meant that the conclusions of MF were not substantially altered. (Mutel & Fix, 2003:SIA 5-1)

They further importantly noted that the . . .nine detections reported by FS are all fainter than the detections limit of MF and are therefore not in direct conflict with our null result. . . . (Mutel & Fix, 2003:SIA 5-3)

Mutel and Fix wrote that they did not get responses to their requests for data on the nine claimed small comet detections by Frank and Sigwarth (2001b) and thus they could not conduct an independent study of the FS events. They wrote that, nevertheless FS illustrates trails for two of the detections; another trail position was obtained from an illustration in a previous talk by Frank on small comets given in February 1999 . . . . We carefully examined all six remaining original images with candidate trials listed by FS but were unable to find detectable trail signatures on any of the images. . . . (Mutel & Fix, 2003: SIA 5-6)

Having analyzed three of the nine claimed trail detections reported by FS, Mutel and Fix concluded that “all three claimed detections fail one or more independent criteria required for a valid detection.” (Mutel & Fix, 2003:SIA 5-6). Frank and Sigwarth replied in an article in the Journal of Geophysical Research (Frank & Sigwarth, 2003) in which they concluded that “. . . our present analyses of the claims by Mutel and Fix (2003) show that there is no significant merit to their arguments. . . .” (Frank & Sigwarth, 2003:SIA 6-6).

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Despite this comment from Frank and Sigwarth, Robert Mutel viewed the negative result of the optical search he and John Fix conducted to be the definitive conclusion of the small-comet controversy that had raged for 17 years (Mutel, personal communication—2021).

4.11

The Tragic Aftermath

At the celebration held in Iowa City on 9 October 2004 to honor James A. Van Allen on the occasion of his 90th birthday, more than 200 of his colleagues, former students, and friends attended (Baker et al., 2013). One of Van Allen’s former students, Louis Frank, could not attend, however, owing to a severe decline in his mental condition. Frank continued to live for another decade, dying on 16 May 2014. Soon after his last exchange (in 2003) with Mutel and Fix, he was never again able to work as a scientist. Following are excerpts from Frank’s colleagues at the University of Iowa on the occasion of his death (https://clas.uiowa.edu/faculty/louis-frank): Donald Gurnett, a UI Professor of Physics, said Frank was a “contemporary,” and the two began working for Van Allen shortly after the launch of Explorer 1. “He turned into a real leader in space research,” Gurnett said. “An extraordinary leader, I would say, in developing instruments for space research.” Gurnett said he was best known for his early measurements of the particles that cause the northern lights, and developing a “very successful” low energy particle detector. Gurnett said Frank was a great scientist, and it was sad to see “a loss of his productive life.” William Kurth, a research scientist in the UI Department of Physics and Astronomy, revered Frank for his early work on the plasma instrumentation for the Galileo Mission to Jupiter and the Japanese Geotail spacecraft. Kurth said another “outstanding” contribution of Frank’s was the global imaging of Earth’s auroral zones and atmosphere, which led to our understanding of the auroras. “The last several years of his life, it was a shame,” he said. “He had such a brilliant mind, and to have him be in the state that he was in for so long, I think, was quite sad. It was a loss for everyone.”

Soon after the last exchange about small comets, John Sigwarth moved to NASA’s Goddard Space Flight Center, where he worked until his sudden death at the age of 49 of an aortic aneurysm on 13 December 2010 (https://www.dyersvillecommercial. com/obituaries/dr-john-benedict-sigwarth/article_0b546988-5d15-572d-bb57-3 fdc1ff89341.html).

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4.12

Eddington’s Guidelines

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Eddington’s Guidelines

As discussed in all of the chapters of this book, some of the speculations that moved forward the disciplines of the space sciences and the controversies that accompanied these speculations are examined. The point of departure is the speculation of Arthur Eddington (1882–1944) (Chap. 1) in the early part of the twentieth century about the internal constitution of stars. In the midst of the controversy that erupted following his speculation, Eddington laid down some guidelines for science speculation that have been slightly modified to make them more broadly applicable to space science: 1. Was the speculator rigorous in applying the appropriate science applicable to the model 2. Did the speculator identify all the underlying assumptions used in constructing the model and 3. Did the speculator view the model objectively, as an “adjustable engine,” as opposed to a “finished building?” The nearly two decades of debates on the validity of the small comet hypotheses of Frank and his colleagues concluded with a consensus that Frank’s adversaries were correct—the “atmospheric holes” in the Dynamics Explorer 1 and Polar satellite data were instrumental artifacts. Such instrumental effects are not unusual in CCD instrumentation flown on spacecraft. Why was the controversy so prolonged? Do the guidelines for science speculation laid down by Arthur Stanley Eddington have anything to say about this controversy? It would appear that guideline number 3 is the one that is most applicable to the speculation of Frank and his colleagues. There is perhaps a fine line for a scientist between, on the one hand, mounting a rigorous defense of what one believes and, on the other hand, striving to meet Eddington’s guideline of objectivity. In this case, however, by his own admission the prestige of membership in the National Academy of Sciences and possible other honors were at stake, and that likely was a major cause for Frank to dig in deeper and deeper to defend his original thesis against the many subsequent criticisms. Had Frank and his colleagues treated their “discovery” of atmospheric holes more objectively, allowing more sincerely that their interpretation of the data might be in error, perhaps the controversy might have subsided much more quickly. As might be expected given the outcome of the controversy, there has been no continuing research on this topic of small comets introducing water to Earth during contemporary times. The origins of Earth’s water in the history of the formation of Earth and of the solar system remains an interesting and important area of research, especially in the context of the origins of life on the planet (Alexander et al., 2018).

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Chapter 5

Origin of the Moon

5.1

Introduction

Until Galileo Galilei turned his newly developed telescope to the planet Jupiter, Earth’s companion Moon was the only celestial object known to circle one of the six planets that had been watched by humans for eons. Afterwards, it was not until 1877 that the two small moons of Mars were discovered by Asaph Hall III (1829–1907) with the telescope at the United States Naval Observatory (although Jonathan Swift had announced in 1726 in Gulliver’s Travels that Mars had two satellites). Earth’s relatively large Moon has long featured in many of the cultures and folklores of humanity, as well as in some religions. The scientific disciplines of geology and astronomy benefit from centuries of research through which the evolutions of ideas and technology have been developed and widely shared. Before the exploration of the Moon by the six Apollo missions that landed on it, the discipline of lunar and planetary science was less well developed than several other areas of astronomy, and it lacked a vigorous research community. This chapter discusses the origin of the Moon. In particular, the speculation that the Earth-Moon system was created as a result of a collision between a Mars-size body and the proto-Earth is outlined and discussed. For a more detailed account of this speculation and the attendant debates, see Evolving Theories on the Origin of the Moon (Cummings, 2019), from which much of the content of this chapter was adapted. Chapter 6 discusses the controversies related to the speculation of deep dust layers on the surface of the Moon that could affect the Apollo missions.

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 W. D. Cummings, L. J. Lanzerotti, Scientific Debates in Space Science, Astronomy and Planetary Sciences, https://doi.org/10.1007/978-3-031-41598-2_5

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Origin of the Moon

Pre Space-Age Speculations on the Origin of the Moon

From time to time, geologists or astronomers put forward ideas about the Moon, its features and origins, but there was little or no follow up from others. An example is the work of Grove K. Gilbert (1843–1918) (Fig. 5.1), a distinguished American geologist who in the summer of 1892 made observations of the Moon with the 26½ in. refracting telescope of the U.S. Naval Observatory in Washington, D.C. Gilbert concluded that the craters of the Moon were not volcanoes, as was commonly thought, but were caused by impacts of other solar system bodies (Gilbert, 1893). Gilbert’s work was ignored for decades, as can be demonstrated by the fact that the American astronomer Ralph B. Baldwin (1912–2010) (Fig. 5.2) was not aware of Gilbert’s work when he also made careful observations of the Moon’s surface and came to the same conclusions in 1941. Baldwin’s assertion that the great lunar basins, such as Imbrium, Serenitatis, and Sinus Iridum (Baldwin, 1943:119) (see Fig. 5.3) were caused by giant impacts was not accepted by many distinguished astronomers. He had a difficult time finding a journal willing to publish his initial work (Baldwin, 1942; Marvin, 2003). The lack of a significant community of lunar Fig. 5.1 G. K. Gilbert. Credit the U.S. Geological Survey

Fig. 5.2 Ralph Baldwin. Courtesy of John Wood

5.2

Pre Space-Age Speculations on the Origin of the Moon

109

Fig. 5.3 Lunar Basins. Credit NASA/GSFC/Arizona State University

and planetary scientists prior to the Apollo era, and the dialog that would have taken place within it, left standing a few old hypotheses for the origin of the Moon.

5.2.1

Co-accretion

Perhaps the leading one of these older hypotheses was called “co-accretion” or “binary accretion.” In the U.S., Harold C. Urey (1893–1981), a physical chemist and Nobel laureate for the discovery of deuterium (1934), discussed the formation of the solar system in terms of a slowly rotating cloud of dust and gas that first formed the Sun and then after flattening into a disk, called the accretion disk, formed the planets (Urey, 1952:105–107). Separately, Otto Y. Schmidt (1891–1956) (Fig. 5.4) and his colleagues at the Institute of Theoretical Geophysics in Moscow developed a

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Fig. 5.4 Otto Schmidt. Courtesy of Mir Titles

version of this idea that influenced planetary scientists following the Apollo explorations. Schmidt studied the formation of the Moon and other planetary satellites in the context of the overall process for the formation of the solar system. Following Schmidt’s death in 1956, his colleagues published a third edition of his book, A Theory of Earth’s Origin: Four Lectures. The book lays out Schmidt’s belief that: The satellites are formed in one single process together with the planets. During the process of planet formation, when particles encountered the bigger planet embryos, some of them lost their velocity to such an extent in collisions that they were captured from the swarm and began to revolve around the planets. In this way a condensation, a swarm of particles, was formed near the planet embryo and revolved about it on elliptical orbits. These particles also collided amongst themselves, thus changing their orbits. In these swarms, processes similar to the formation of planets took place on a smaller scale. The majority of the particles fell on to the planet and were absorbed by it, but some of them formed a swarm around the planet and accumulated to form independent embryos, the future satellites. The exception is the ring of Saturn which consists of small particles that have not been able to agglomerate on account of the tidal action of Saturn in whose immediate vicinity they are (an unformed satellite). As the orbits of the particles forming a satellite were averaged, the satellite acquired a symmetrical, almost circular orbit in the equatorial plane of the planet and could not fall on it. In this way satellites appeared around the planets. Thus we see that the formation of the satellites was a by-product of the formation of the planets . . . . (Schmidt, 1958:58–59)

Schmidt’s model was refined by his students, among them Evgenia L. Ruskol (1927–2017) (Fig. 5.5). Ruskol studied the origin of the Moon, and she envisaged the formation of the Moon from the smaller bodies in the proto-Earth’s extended envelope. From the discussion of the scheme of formation of the satellite swarm encircling the earth, we find that the growth of the moon must have begun at a short distance from the earth, since the density of matter in the swarm was at a peak near the earth. . . .The swarm would have to have grown most rapidly at an earth mass of 0.3 to 0.5 of its present mass . . . .

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Fig. 5.5 Evgenia Ruskol. Credit Alexander Safronov, courtesy of the Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences. (The Russian Academy of Sciences presented this photo of Evgenia Ruskol in the book: Russian Astronomers. 1917–2021. RAS, 2022, 659рp)

Roche’s Limit The Earth exerts tidal forces on the Moon in the same way that the Moon exerts tidal forces on the Earth. When the Moon was in a formative state, so that its various parts were held together loosely by mutual gravitational attraction, there was a minimum distance that the Moon could approach the Earth without being torn apart by the tidal forces from the Earth. That distance is called the Roche limit, named for the French astronomer, Édouard Roche, who first derived it in 1848. The concept of a Roche limit applies to all bodies in the solar system. For the Earth, the Roche limit is a little less than a distance of 3 Earth radii from the Earth’s center. (Cummings, 2019:59) As a study of the earth’s rate of accumulation shows, the growth of the earth proceeded at a far more rapid rate in the beginning than later. At most, 100 to 200 million years were required to form 99% of the earth’s mass. During this same time interval, the formation of the satellite swarm must have been completed for the most part. The accumulation of the major bulk of the moon’s mass would have necessarily taken no longer a time than the accumulation of the earth. The difference in the ages of the earth and moon could not be greater than 200 million years in the light of this argument.

The earth-moon distance must have always exceeded the Roche limit. . . . Since the bulk of the mass of the swarm was included within a range of 10 earth-radii, we may arrive at the conclusion that the moon was formed basically at a distance of 5–10 earth-radii. (Ruskol, 1963:226)

5.2.2

Fission

A second commonly held view about the origin of the Moon was called “fission,” or more properly “rotational fission.” The person primarily responsible for developing

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Fig. 5.6 George Darwin. Credit J. Russell and Sons

this theory was the English astronomer George H. Darwin (1845–1912) (Fig. 5.6), who was the second son of the English naturalist and biologist Charles R. Darwin. George Darwin and others understood that the ocean tides observed on Earth were caused by the Moon, and to a lesser extent the Sun. He reasoned that if the early Earth were a molten, viscous mass with an orbiting Moon, then the main body of the Earth would rotate beneath stationary tidal bulges, just as today’s Earth rotates beneath tidal bulges in the ocean’s surface. Assuming that the early Earth was a viscous liquid, there would be frictional losses of energy within the Earth as it rotated beneath the tidal bulges. This loss of energy would come from the rotational kinetic energy of the Earth, so that the Earth’s rate of spin about its axis would be reduced. Because of the conservation of angular momentum in the Earth-Moon system, the loss of spin angular momentum of the Earth would be compensated by an increase in orbital angular momentum of the Moon about the Earth-Moon center of mass. This increased orbital angular momentum of the Moon would be accomplished by an increase in the Earth-Moon separation distance. Darwin then argued that if in the early life of the Earth-Moon system the Moon was moving away from the Earth, there must have been an earlier time when the Earth and Moon were closer together. Therefore, “if the moon and the earth were ever molten viscous masses, then they once formed parts of a common mass.” (Darwin, 1879:535). Conservation of angular momentum implied that the common mass of the protoEarth and Moon must have been rotating with a period of between 4 and 5 h. Darwin argued that this time interval might have been close to twice the period of oscillation of a liquid sphere with the same overall mass density, but with mass concentrated at its center. The Sun raises tides on the Earth owing to the greater gravitational force it exerts on the nearer Sun-facing side of the Earth as opposed to the far side, and Darwin speculated that a resonance between these tides and the natural oscillation of

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Fig. 5.7 Possible evolution of shapes for a rapidly rotating Earth and separated Moon. Reproduced with permission from Fig. 1 of Wise, D. U. (1963). Copyright 1963 by the American Geophysical Union

a liquid proto-Earth could cause a rupture that would result in the Moon revolving around the Earth (Fig. 5.7). (Darwin, 1879:537).

5.2.3

Capture

A third idea for the formation of the Moon was the “capture” hypothesis. When one planetary body passes near another one, it cannot go into orbit around the second body unless the kinetic energy of the former body is reduced. This concept has become familiar during the age of planetary exploration, when the kinetic energy of a spacecraft is reduced by the firing of retro-rockets to allow the spacecraft to go into orbit around a planet. In 1955, a German teacher in a girls school in Hanover, Horst Gerstenkorn (1923–1981), developed a detailed theory published in Zeitschrift für Astrophysik for how an approaching planetary body could have been captured by the Earth to become the Moon (Gerstenkorn, 1955). Gerstenkorn speculated that the pre-captured Moon passed close to the Earth so that enough energy would be lost through tidal effects to allow its capture. To account for the current angular momentum of the Earth-Moon system, Gerstenkorn proposed that the planetary body that would become the Moon approached the Earth so that it would initially orbit in the opposite direction to the Earth’s rotation. The inclination of the orbit would gradually increase, which would tend to increase the angular momentum of the system. To keep the angular momentum of the Earth-Moon system constant, the Moon would orbit closer to the Earth until its orbital plane crossed over Earth’s poles. At this point

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the orbiting Moon would begin to make a positive contribution to the angular momentum of the Earth-Moon system. The Earth’s spin rate would be decreasing owing to the loss of energy caused by tidal effects, however, and this would keep the angular momentum of the Earth-Moon system constant. The Moon’s orbital plane would gradually move toward its current near-equatorial plane and the orbital radius of the Moon would increase to compensate for the continued loss of spin angular momentum of the Earth. (See Field, 1963:349–354 for more details on Gerstenkorn’s theory.)

5.3

The Initial Speculation of a Giant Impact

A fourth idea for the origin of the Moon might be called “collisional fission” or “collisional capture” or, as described below, the “giant impact” hypothesis. Until the 1970s, few scientists considered this possibility, though the idea had its modern origin in the mid 1940s. At that time the eminent Princeton University astronomer, Henry N. Russell (1877–1957), wrote to the Canadian-American geologist Reginald A. Daly (1871–1957) (Fig. 5.8), who was a professor emeritus at Harvard, suggesting that: it might be worthwhile to study the question whether the main part of the moon’s substance represents a planetoid which, after striking the earth with a glancing, damaging blow, was captured (Daly, 1946:108).

Daly conducted the study suggested by Russell, and he published his results in the Proceedings of the American Philosophical Society in 1946. Daly speculated that: . . . a ‘planetoid,’ captured because of tangential, slicing, collision with the liquid earth, brought with it so much angular momentum as to ensure its perpetuation as a separate, revolving body—the moon we know. . . . initially liquid fragments were exploded out of the

Fig. 5.8 Reginald Daly. Photograph courtesy of the Cabot Science Library, Harvard University

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planet, well beyond Roche’s limit. Many of these were gravitationally aggregated by the pull of master fragment or captured ‘planetoid’ to make the substance of our moon, and the somewhat diminished earth felt a prolonged rain of other earth-fragments, large and small (Daly, 1946:118).

Daly’s paper was ignored for 46 years (long after the Apollo missions), another example of the absence of a vibrant lunar science research community during that time. In 1992, Ralph Baldwin and American geologist Donald E. Wilhelms published a review of Daly’s work in the Journal of Geophysical Research, pointing out that: . . . Daly’s paper explicitly discussed the idea that a collision between the brand-new Earth and a planet-sized body led to the formation of the Moon. Yet his paper is not discussed in any of the modern works on the origin of the Moon, . . .(Baldwin & Wilhelms, 1992:3837).

By the mid 1960s, strong objections had been raised to the co-accretion, rotational fission, and capture hypotheses for the formation of the Moon. The high angular momentum of the Earth-Moon system was not easily explained by the co-accretion theory. Theoretical analysis indicated that internal friction in a liquid proto-Earth would not allow the tidal resonances required for rotational fission (Jeffreys, 1917:117). The capture hypothesis of Gerstenkorn seemed unlikely. It required an enormous initial (positive) angular momentum for the Earth prior to the capture of a body that was bringing angular momentum with an opposite sign to what would become an Earth-Moon system with a very high (positive) angular momentum. (See Cummings (2019) for further details on these objections.) There were counter arguments to these objections, however, and all three primary models were at least marginally viable as the Apollo exploration of the Moon drew near. But, as Harold Urey noted in 1963, “. . . there is no model for the origin of the moon that is not complicated and does not appear to be very highly improbable” (Urey, 1963:164). And in 1965, Ralph Baldwin echoed Urey: We are thus left on the multipointed horns of a dilemma. There is no existing theory of the origin of the moon which gives a satisfactory explanation of the earth-moon system as we know it. The moon is not an optical illusion or a mirage. It exists and is associated with the earth. Before 4.5 billion years ago, the earth did not exist. Somehow in this period of time, the two bodies were formed and became partners. But how? (Baldwin, 1965:42–43)

Many scientists anticipated that the upcoming Apollo missions would provide the answer to the question of the Moon’s origin.

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The Apollo Explorations of the Moon The Formation of the Lunar Science Institute

James E. Webb (1906–1992) (Fig. 5.9) was the second Administrator of the National Aeronautics and Space Administration (NASA), and he had a great influence on NASA-university relations. Webb realized that NASA would mount increasingly complex missions that would benefit from the involvement of university researchers and at the same time require more off-campus participation of these researchers. Webb sought a new mechanism for the involvement of university professors that would mitigate the disruption of their teaching and training responsibilities in the new discipline of space science. Webb turned to the United States National Academy of Sciences (NAS) to help him find a solution to the problems that he anticipated. Specifically, Webb asked the president of the NAS, Frederick Seitz (1911–2008), for help. Seitz worked with the academic community, and the result of their deliberations was the formation of the Lunar Science Institute (LSI) near the Manned Spacecraft Center (later re-named the Johnson Space Center) in the city of Clear Lake south of Houston. To manage the LSI and other institutes and programs as needed, NASA organized an independent non-profit organization that was incorporated on 12 March 1969 as the Universities Space Research Association (USRA) (Cummings, 2009). The first director of the LSI, geologist William W. Rubey (1898–1974), began to bring to the institute scientists who would carry out lunar research and assist other researchers around the world in the analysis of the soon-to-be acquired lunar samples. They would also, more generally, conduct research in a discipline that would come to be well defined as lunar and planetary science. There was not much time for the development of the LSI as an entity that would assist both NASA and the university research community in these endeavors. The first Apollo landing would take place on 20 July 1969, just months after NASA’s establishment of USRA and the LSI. Fig. 5.9 James Webb. Credit NASA, courtesy of AIP Emilio Segrè Visual Archives

5.4

The Apollo Explorations of the Moon

5.4.2

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Key Results of the Apollo Explorations

Between July 1969 and December 1972 six Apollo explorations brought to Earth laboratories 382 kg (842 pounds) of lunar samples (rocks and soil) for analysis by scientists from all over the world. In addition, data from surface instrument packages installed and left on the Moon by the astronauts, including seismometers, heat-flow sensors, and retro-reflectors, were analyzed. Major results of the analyses of lunar data include (see Cummings, 2009:104–107): 1. The age of the Moon was determined to be 4.6 billion years, similar to the age of the Earth and other objects in the solar system (Chamberlain, 1972:11). 2. The ages of the basaltic lavas from the lunar maria that were visited varied from site to site and were younger than the ages determined for the rocks in the lunar highlands by at least 0.5 billion years. The younger basalts must have been produced by melting by radioactive heating deep below the surface of the Moon subsequent to its formation. Liquid lava is about 10% less dense than solid basalt, so the basaltic lava would have risen to the surface of the Moon and filled preexisting lunar basins that had been created by giant impacts (Wood, 1970:6497 ff). 3. Most of the samples taken from the lunar highlands were anorthosites, an unusual rock type that is not likely to be produced as a condensate from the primitive solar nebula of gas and dust. It can only be formed by fractional crystallization (Wood et al., 1970:980). 4. There had been melting of the outer 100 km or more of the Moon, with the result that lunar material had “differentiated,” with lighter layers on top of heavier ones (Chamberlain, 1972:25). 5. The rate of impacts by bodies causing craters on the Moon was much higher during the first 0.5 billion years following the Moon’s formation than the average rate of impacts over the life of the Moon (Chamberlain, 1972:25). 6. The lunar samples were significantly deficient in volatile elements (Lunar Sample Analysis Planning Team, 1970:450), including water in the mineral structure of the rocks (Chamberlain, 1972:7), and enriched in most of the refractory elements, compared to their presumed abundance in the primitive solar system (O’Keefe, 1970:634). The lunar samples were also deficient in iron and iron-loving elements, called siderophiles (O’Keefe, 1970:634). 7. The outward heat flow measured at the Apollo 15 and Apollo 17 sites suggested a global heat flux of about 18 ergs/cm2s (Langseth et al., 1976:3170). The outward heat flow through the surface of the Earth has been estimated at 47 ± 2 × 1012 W (Davies & Davies, 2010:5), which corresponds to an average outward heat flux over the Earth’s surface of about 92 ergs/cm2s. 8. Lunar magnetic data suggested that the Moon had a fluid core 3–4 billion years ago (Chamberlain, 1972:25).

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The Various Theories of the Origin of the Moon

The analyses of the lunar samples and other Apollo data certainly gave a clearer picture of the structure of the Moon, but the fundamental question of the origin of the Moon was not immediately settled. Generally, the protagonists of the pre-Apollo theories for the formation of the Moon tried to adjust their theories to accommodate the lunar data.

5.5.1

Urey

Harold Urey (Fig. 5.10) held a number of different views on the origin of the Moon during the course of the debate, but as far back as 1955 he had argued that: . . . very probably the moon was accumulated at low temperatures from a primitive dust cloud of solar composition with the iron in oxidized states and that the concentrations of radioactive substances within the moon are sufficiently low that melting has never occurred. In fact, I believe that present temperatures are so low that the interior of the moon has a high strength and that such low temperatures require the moon to have been formed at low temperatures and never to have been melted at any time (Urey, 1955:427).

At least in the beginning of the pre-Apollo debate over the origin of the Moon, Urey’s views were similar to those of Otto Schmidt. By 1971, Urey was still arguing that the deep interior of the Moon was cold but agreed that: The surface of the Moon has been melted to a considerable depth, possibly to some 200 km. This liquid had crystallized slowly producing a layer of anorthosite which crystallized and floated on the molten silicates like ice on water (Urey, 1972:626).

Fig. 5.10 Harold Urey. Credit: University of Chicago Photographic Archive, [apf1–08421], Hanna Holborn Gray Special Collections Research Center, University of Chicago Library

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The Various Theories of the Origin of the Moon

5.5.2

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Ringwood

Prior to the Apollo explorations, Alfred Edward (Ted) Ringwood (1930–1993) (Fig. 5.11), geologist and geochemist of the Australian National University, had proposed a “precipitation hypothesis” as a variation of the co-accretion theory. Following the return of the Apollo 11 samples, Ringwood argued that his precipitation hypothesis best fit the new data affecting theories of the origin of the Moon. He suggested that: . . . during the later stages of accretion of the earth, a massive primitive atmosphere developed which was hot enough to selectively evaporate a substantial portion of the silicates which were accreting upon the earth. Subsequently, the atmosphere was driven away by particle radiation from the sun as it passed through a T-Tauri phase. The relatively non-volatile silicate components were precipitated close to the earth to form a swarm of planetesimals or moonlets, as the atmosphere was dissipated, and the moon accreted from these chemically fractionated planetesimals. The more volatile components of the terrestrial atmosphere were precipitated at lower temperatures, further from the earth, as fine smoke particles and were lost from the earth-moon system with the escaping gases. The hypothesis appears capable of explaining the low density of the moon, the inferred fractionation of relatively volatile elements between the earth and moon, and the different oxidation states of the terrestrial and lunar mantles. The above “precipitation” hypothesis thus implies a close genetic relationship between the earth and moon. The precipitation and fission hypotheses are also closely connected since the material now in the moon is regarded as having been derived ultimately from the earth— not the solid mantle, but from the massive primitive terrestrial atmosphere. (Ringwood, 1970:131)

5.5.3

O’Keefe and Wise

Planetary scientist John A. O’Keefe (1916–2000) (Fig. 5.12) of NASA’s Goddard Space Flight Center and geologist Donald U. Wise of Franklin and Marshall College Fig. 5.11 Alfred Ringwood. Credit Australian Academy of Science

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Fig. 5.12 John O’Keefe (left) with a colleague, Louis Walter, at NASA’s Goddard Space Flight Center. Credit NASA

drew different conclusions from the Apollo 11 data. O’Keefe and Wise favored the rotational fission hypothesis. When the initial analyses of Apollo 11 samples were made public, Wise wrote an article for the Journal of Geophysical Research in which he argued that: Roasting of a newly fissioned moon adjacent to a tidally heated incandescent earth, may account for a lunar magmatic source depleted in volatile alkali elements and enriched in refractory elements as suggested by first analyses of Apollo 11 specimens. (Wise, 1969:6044)

O’Keefe soon wrote a paper in which he stated: . . . the Apollo 11 data support the idea that the moon was formed by the breakup of the earth, and that they suggest that after the breakup, the moon went through a heating episode that boiled away most of its mass. . . .(O’Keefe, 1970:633).

O’Keefe discussed the relative abundances of elements found in the Apollo 11 samples. In particular, he pointed out the low levels of nickel in the lunar samples, asking “Where did the moon’s nickel go?”. He argued that: . . . It is essentially certain that the moon’s nickel is not concentrated, like the earth’s, in an iron-nickel core. In fact there can be at most a very small amount of free iron (whose density is about 7.8) in the moon. It has been found that the mean density of the moon is approximately 3.34 g cm-3; this is almost the density of the earth’s mantle. This density is actually slightly less that the density of the crustal rocks at the Apollo 11 site. Any significant mixture of metallic iron would give the moon a higher over-all density than is observed. The logical conclusion is then that the moon’s nickel is in the earth’s core and that the moon formed by fission of the earth after the core-mantle separation had taken place (O’Keefe, 1970: 634–635).

O’Keefe recognized that the conservation of angular momentum implies that the current total angular momentum of the Earth-Moon system must be the same as the angular momentum of the Earth prior to fission. However, the current amount of

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angular momentum in the Earth-Moon system is not enough to cause the fission. Therefore, O’Keefe speculated that there was more angular momentum in the Earth prior to its fission than exists in the current Earth-Moon system, but that: . . . after the separation of the earth and moon, the strong tidal interaction between the two bodies heated both, but especially the moon, and caused the loss of large amounts of mass and angular momentum from the moon. . . . If we suppose that over half of the moon’s mass was boiled away at this point, then we explain simultaneously the deficiency of volatile elements in the moon, the enhancement of the refractory elements, the loss of angular momentum, and a great loss of mass which Lyttleton (1953) has shown is required if we are to understand the formation of the moon by fission. (O’Keefe, 1970: 634–635).

5.6 5.6.1

The Giant Impact Speculation Hartmann and Davis

While the developing lunar science community continued to be unaware of Reginald Daly’s hypothesis, a collisional fission hypothesis began to be discussed. Initially this was in the context of variations of other theories of lunar origin. In the mid-1970s, however, the collisional fission or “giant impact” idea began to be treated as an independent theory to explain the lunar geochemical data. In 1975, planetary scientist and space artist William K. Hartmann (Fig. 5.13) and astronomer Donald R. Davis published a paper in the journal Icarus in which they pointed out that: Collision of a large body with the Earth could eject iron-deficient crust and upper mantle material, forming a cloud of refractory, volatile-poor dust that could form the Moon. (Hartmann & Davis, 1975:504).

Hartmann and Davis further argued that:

Fig. 5.13 William Hartmann. Credit Gayle Hartmann

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This model has an important philosophically satisfying aspect. There has always been difficulty in accounting for all properties of all satellite systems by a single evolutionary theory. Jupiter and Saturn have “miniature” solar systems with retrograde outriders. Uranus has its spin and satellites’ angular momentum vectors radically altered. Earth is a “dual” planet with a relatively huge satellite. Mars has only two tiny moons. Venus and Mercury have none. This heterogeneity becomes more satisfyingly accountable if it is viewed as the product of events involving statistics of small numbers. Does the second-largest planetesimal in each system hit the planet after 107 years or 108 years? Is it large or small? Does it hit the planet dead center? Retrograde? A glancing blow prograde? Or is it captured? Or is it destroyed by a planetesimal-planetesimal collision so that it has no appreciable effect on the planet other than to produce many small craters? Or does it hit a preexisting satellite of the planet, perhaps converting it to several small satellites? Only one of these kinds of fates can befall the second-largest planetesimal. And this fate, the product of small-number statistical chance encounters, may determine whether the planet acquires a tilted axis, a massive circumplanetary swarm of dust, a captured satellite, or perhaps loses a larger satellite, gaining small fragmentary satellites. This model can thus account for the iron depletion, refractory enrichment, and volatile depletion of the Moon, and at the same time account for the Moon’s uniqueness; the Moon may have originated by a process that was likely to happen to one out of nine planets. (Hartmann & Davis, 1975:512–513)

5.6.2

Cameron and Ward

Canadian-American astrophysicist Alastair G. W. (Al) Cameron (1925–2005) (Fig. 5.14) and planetary scientist William R. Ward (1944–2018) had independently developed ideas for a giant impact theory, which they discussed at the Seventh Lunar Science Conference in 1976. A key constraint on the origin of the Earth-Moon system is the abnormally large value of the specific angular momentum of the system, compared to that of the other planets in the solar system. At an early stage, when the Moon was close to the Earth, most of the angular momentum resided in the spin of the Earth. This spin was presumably imparted by a collision with a major secondary body in the late stages of accumulation of the Earth, with Fig. 5.14 Al Cameron. Credit the Yeshiva University Archives and the AIP Emilio Segrè Visual Archives, Physics Today Collection

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the secondary body adding its mass to the remainder of the proto-earth. The collisional velocity must have been close to 11 km/sec, and if the impact parameter was one earth radius, then the mass of the impacting body was comparable to that of Mars. It is probable that the largest accumulative collision should have involved a mass of this order, but the size and location of the impact parameter would have been a matter of chance. It is likely that both bodies would have been differentiated and possibly molten at the time of impact. (Cameron & Ward, 1976:120)

Cameron and Ward also pointed out that after the collision . . . the mantle material of both bodies in the region of the collision would shock-unload predominantly in the forward direction relative to the collision velocity and much of the material would vaporize. The subsequent motion of this material is not just a set of ballistic trajectories; the early motion of the material is entirely governed by gas pressure gradients in the vapor which is expanding into a vacuum. (Cameron & Ward, 1976:120)

The common understanding was that material ejected from the Earth below the escape velocity would fall back to Earth. Cameron and Ward argued, however, that much of the ejected material would escape from the Earth owing to the expanding hot gas envelope that would be created by the impact.

5.6.3

Clayton

At the Sixth Lunar and Planetary Science Conference, the Canadian-American cosmochemist Robert N. Clayton (1930–2017) (Fig. 5.15) and his colleagues from the University of Chicago introduced a new constraint on theories for the formation of the Moon. This constraint was made possible because of new technologies for the separation of isotopes of chemical elements. With 8 protons and 8 neutrons, 16O is the most abundant isotope of oxygen. But 17O and 18O are stable isotopes of oxygen present in minute amounts together with 16O. Clayton and his colleagues had

Fig. 5.15 Robert Clayton. Credit University of Chicago Photographic Archive, apf11973, Hanna Holborn Gray Special Collections Research Center, University of Chicago Library

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developed methods to determine with high accuracy the ratios of 18 16 O/ O in a given sample of material. In his talk Clayton argued that:

17

O/16O and

Due to the inhomogeneous distribution of the stable isotopes of oxygen at the time of condensation and accretion in the solar nebula, it is possible to identify bodies which formed from a common region of the nebula and distinguish them from bodies formed in other regions. On this basis, the moon is in the same group as the earth and the differentiated meteorites (achondrites, mesosiderites, pallasites, irons), and is unrelated to the ordinary chondrites or the carbonaceous chondrites. . . . The fact that the moon and the earth lie on the same mass-fractionation line would not be surprising except for the observation that most of the other analyzed samples of the solar system do not. (Clayton & Mayeda, 1975:1761)

5.6.4

Safronov and Wetherill

In the 1950s and 60s, Victor S. Safronov (1917–1999) (Fig. 5.16), worked on a model for the formation of the planets in the solar system. This model, developed at the Institute of Theoretical Geophysics in Moscow, was referred to by planetary scientists as the “Russian model” or the “Safronov model” for the formation of the solar system. An important feature of this refinement of the model of Otto Schmidt was that the accretion of the planets and their satellites occurred in two stages. First, asteroid-sized intermediate bodies were formed from the dust component of the solar nebula. In a second stage these bodies aggregated to form the planets and their satellites (Safronov, 1958, 1964, 1968; Levin, 1972). In the 1970s and 1980s, George W. Wetherill (1925–2006) (Fig. 5.17), a geochemist at the Carnegie Institution of Washington, furthered the work on the formation of the terrestrial planets in the solar system. Following the work of Safronov and his colleagues, Wetherill assumed the existence of planetesimals in the early solar system. Given the proposals of Hartmann and Davis (Sect. 5.6.1) and

Fig. 5.16 Victor Safronov. Credit Alexander Safronov, courtesy of the Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences

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Fig. 5.17 George Wetherill. Credit Alan Boss and the American Astronomical Society

Cameron and Ward (Sect. 5.6.2), the question for Wetherill was the probability of a collision with the Earth of a Mars-size body. Wetherill argued that what was needed was an accretional model that could give “fairly definite predictions concerning the size distribution, orbits, and evolution of large bodies in the early solar system” (Wetherill, 1976:3255). In 1975, Wetherill published in the Annual Review of Nuclear Science what he described as a “tentative synthesis” for the formation of the solar system and planets. This synthesis prominently featured planetesimals in his version of the Safronov model: 1. The nebular period had a duration of 100–150 million years, following which time condensation of solid matter took place. 2. Planetesimals up to ~100 km in radius accreted rapidly (~104 years) . . . as a consequence of gravitational instability of solid matter in the central plane of the nebula. 3. The interiors of at least some of these planetesimals were at temperatures of 500–1600 °C within ≤107 years, resulting in metamorphism and/or igneous differentiation. 4. Some of the planetesimals cooled quickly (~107–108 years) either because of their original small size or because of disruption during an early heavy bombardment associated with the formation of Jupiter and Saturn. 5. Most of these planetesimals and their debris accreted further to form planets and large satellites on a very uncertain time scale of 104–108 years. The importance of gravitational energy of accretion as a heat source is critically dependent on the length of this time scale. 6. The larger surviving planetesimals and planetary objects combined to evolve internally in the subsequent planetary era. (Wetherill, 1975:324) Using digital computers with ever increasing power, Wetherill and his colleagues began to develop such an accretion model. These models could follow the orbits and

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collisions of the planetesimals and the development of the terrestrial planets over millions of simulated years.

5.6.5

The Kona Conference

In October of 1984, the Lunar and Planetary Institute (formerly the Lunar Science Institute) held a conference in Kona, Hawaii, that marked a turning point in the effort to understand how the Moon was formed. At that conference, Wetherill gave a review paper (Wetherill, 1986), in which he reported on his computer simulations of terrestrial planet formation. In his computer simulations, Wetherill’s initial state consisted of 500 bodies with a range of masses determined based on the theoretical planetesimal work of Safronov and others. The initial eccentricity of the elliptical orbit of each body about the Sun was assigned a random value between 0 and 0.05, the initial inclination a random value between 0 and 0.025, and the initial semi-major axis a random value between 0.7 and 1.1 astronomical units (Wetherill, 1986:525). Wetherill described how his computer simulations worked: . . . The evolution of the system is assumed to result from close two-body encounters between planetesimals in crossing orbits . . . When the encounter distance between two planetesimals becomes less than the sum of their physical radii, the bodies are assumed to merge to form a larger body with mass equal to the sum of the masses. Otherwise, an encounter between two bodies results in gravitational perturbation to new orbits. As the calculation progresses the number of bodies becomes smaller . . . . Eventually, only bodies in noncrossing orbits will remain, the calculation is then terminated, and the surviving bodies are considered to be the final planets resulting from that particular accumulation calculation (Wetherill, 1986:527–528).

Wetherill found that his simulations generally resulted in the formation of four or less small planetary bodies with masses greater than 1026 g (1.4 lunar masses). (See an example of his simulations in Fig. 5.18.) In half of his simulations the number of “large” planets (>2 × 1027 g) is three instead of the observed two (Earth and Venus) (Wetherill, 1986:536 and 538). Typically, one or two impacts of bodies more massive than Mars occur for each accumulation, and about three more massive than Mercury. . . . These giant impacts occur most frequently after the accumulation has proceeded for 1-15 m.y. In this time interval from 15% to 70% of the mass of the Earth had already formed. . . . Rather than considering giant impacts as a somewhat radical suggestion, if one is skeptical about the reality of the phenomenon, a good starting point would be to consider it a normal phenomenon that one should, at least naively, expect during planetary formation. (Wetherill, 1986:540–543)

Wetherill further concluded that: . . . for a wide range of initial conditions, terrestrial planet accumulation was characterized by giant impacts, ranging in mass up to 3 times the mass of Mars, at typical impact velocities of ~9 km/sec. These large planetesimals and the impacts they produce are sufficient to explain the unexpectedly large angular momentum of the Earth-Moon system. (Wetherill, 1986:519)

5.6

The Giant Impact Speculation

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Fig. 5.18 An example of Wetherill’s simulations. Accumulations are shown for a simulation after 64 million years. From Fig. 3 (F) in Wetherill (1986)

Wetherill also noted that: It is particularly interesting that these large planetesimals provide in a natural way the giant impacts proposed by Hartmann and Davis (1975) and Cameron and Ward (1976) as a way of forming the Moon. . . . Although it would be presumptuous to conclude that these large planetesimals and impacts were inevitable consequences of planet formation, their probable occurrence imposes obligations of explicitly considering their consequences in any discussion of the early history of the Earth and the Moon. (Wetherill, 1986:548)

For many at the Kona Conference, the presentation by Wetherill made the giantimpact hypothesis seem much more plausible. As noted above, increasing computer power allowed Wetherill to carry out his simulation runs in reasonably short times. Other researchers were also carrying out computer simulations with the goal of examining collisions of the type that had been suggested by Hartmann and Davis and by Cameron and Ward. These simulations started with one spherical collection of “particles” on a collision path with another

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spherical collection of “particles.” The computer programs would then track the trajectories of individual “particles” before, during, and after the collision. See an example simulation in Fig. 5.19. Immediately after the Kona Conference, these simulations were rather limited. With increasing computer technology, however, the simulations became ever more sophisticated and were able to include more realistic physical models and processes.

5.7

Conclusions

In the follow-on book for the 1984 Kona conference on the origin of the Moon, William Hartmann reviewed the fit between the “giant impact” theory and the properties and constraints on lunar origin. • Iron deficiency and gross similarity to Earth’s upper mantle: After Apollo, lunar rock geochemistry led to the consensus that the lunar material crudely resembles Earth’s mantle . . . • Volatile depletion: The volatile depletion pattern of the Moon has always been difficult to explain in detail. However, at a first-order level, it appears consistent with a strong heating of most lunar material, probably in pulverized form to allow volatile escape, perhaps to temperatures of 1400–1800 K, and possibly additional chemical processing. . . . The hypothesis of an impact ejecting hot, finely disseminated material thus appears to be a step forward in understanding lunar volatiles . . ., but the chemistry of impact processing clearly requires further study. • Angular momentum considerations: As Cameron and Ward (1976) emphasize, a giant impact provides a plausible mechanism to explain the unusually high value of angular momentum in the Earth/Moon system, relative to other planets. . . . Indeed, a large impact is the ideal mechanism to produce Earth’s final spinup to the effective period of 4.1 h, matching the angular momentum of the present system . . . • Oxygen isotope ratios: . . . lunar samples . . . fall on the chemical mass fractionation line characteristic of Earth materials and are indistinguishable from Earth . . . In summary, the O-isotope data require that the Moon formed from material that originated in the same terrestrial “feeding zone” that contributed material to the Earth, and not as far away as the “feeding zone” of Mars. • Bulk iron content: . . . The estimated bulk elemental iron content of the Earth’s mantle and the Moon are: Earth mantle: 7% iron by weight . . . Moon:7–9% iron by weight . . . . . . The similarity is predictable if the Moon formed from ejected upper mantle material (especially if some projectile iron were added), but is an odd coincidence in other theories. • Density: The mean densities of the Moon (3.344 ± 0.002 g/cm3) and [Earth’s] upper mantle (3.3–3.4 g/cm3) are virtually identical . . . . This is directly explained

5.7

Conclusions

129

Fig. 5.19 A numerical simulation showing snapshots of events taking place between the impact and formation of a clump in orbit. From Fig. 2 in Benz et al. (1986)

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Fig. 5.20 Jeffrey Taylor. Courtesy of Jeffrey Taylor

if the Moon formed from ejected upper mantle material, but is an odd coincidence in other theories. (Hartmann, 1986:582–586, emphasis in the original) All the questions surrounding the origin of the Moon were not answered at the 1984 Kona Conference. An example being whether the ejected material that formed the Moon largely originated in the Earth’s mantle or, on the other hand, was mainly composed of material from the impactor. Despite lingering uncertainties, continued research on the origin of the Moon has been heavily influenced by the Kona Conference, as one of the organizers of the conference, University of Hawaii planetary scientist G. Jeffrey Taylor (Fig. 5.20), noted. The conference was revolutionary. The traditional ideas for lunar origin were tossed aside by almost all attendees in favor of the giant impact hypothesis. Beyond the giant impact hypothesis being a good idea, several factors came into play to raise it to its pedestal. The three old ideas (fission from the Earth, capture, and binary accretion) had their adherents, but most of us were dissatisfied with all of the old hypotheses. Each had serious flaws. Computer methods had improved significantly, so simulations of the giant impact could be done. Our understanding of impact processes was stronger than ever because of experiments and studies of large terrestrial craters. Finally, and perhaps most important, our ideas of how planets accumulated had achieved a new paradigm that depicted planets accumulating from objects that were themselves still accumulating, leading to several large bodies near each other. In this view, a giant impact was almost certain to happen. At the end of the three-day conference, the traditional hypotheses were discarded by most of us—a revolution in our thinking! . . . the giant impact idea . . . provides the context in which we think about planet formation, much the way plate tectonics provides the context in which we try to understand the geology of the Earth. (Taylor, 1998)

5.8

5.8

Eddington’s Guidelines

131

Eddington’s Guidelines

The initial speculation of Henry Norris Russell and Reginald Daly about a giant impact that could have resulted in the Earth-Moon system had something of the character of the speculations of Arthur Eddington (1882–1944) (Chap. 1) on stellar interiors and Eugene N. Parker (1927–2022) (Chap. 2) on the solar wind in the sense that all were largely the work of single individuals. The working out of the later speculations about a giant impact of the Earth by a Mars-size body, reintroduced by Hartmann and Davis and by Cameron and Ward, were different in character in that a world-wide group of researchers was involved. This was also the evolution toward the increasing use of ever-larger computer resources for sophisticated calculations and modeling of solar system phenomena. The competitive collaboration was due in part to the distribution by NASA of lunar samples to laboratories around the world, and in part to the role played by the Lunar Science Institute (now the Lunar and Planetary Institute) in helping to move forward and structure the discipline of lunar science. With these considerations in mind, the work of the space-age speculators on the origin of the Moon in the context of Eddington’s guidelines as slightly modified for the space sciences can be discussed: 1. Was the speculator rigorous in applying the appropriate science applicable to the model 2. Did the speculator identify all the underlying assumptions used in constructing the model and 3. Did the speculator view the model objectively, as an “adjustable engine,” as opposed to a “finished building?” Were the speculators rigorous in applying the appropriate science to their models? Eddington and Parker worked in relatively few subdisciplines of science as they carried forward the results of their speculations. On the other hand, as James Webb was early to recognize, the challenges of much of space research would be very interdisciplinary in nature. The problems inherent to the giant-impact speculation involve research in planetary orbital dynamics; chemistry; solar-system isotopic analyses; solar wind effects on the lunar surface; radioactive dating; and lunar and terrestrial geology including lunar stratigraphy, magnetism, seismology, and thermal history. With the help of a world-wide collection of experts in, and results from, diverse subject areas, Hartmann, Davis, Cameron, and Ward were able to apply the appropriate science to their models. Did the speculators identify all the underlying assumptions used in constructing their models? As in the prior criterion, the very interdisciplinary nature of the problem makes this assessment difficult. The question of the origin of the Earth-Moon system did not lend itself to an analytical model, even one where assumptions and approximations had to be made, as was the case with the models of Eddington and Parker.

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Fig. 5.21 Hartmann’s “trilogy diagram,” from Fig. 2 in Hartmann (1986)

Did the speculators view their models objectively? The evidence in this category is mixed, as can be illustrated with two examples. On the first day of the 1984 Kona Conference, Jeffrey Taylor gave an invited talk that was titled What Were the Earliest Lunar Differentiation Events? (See Cummings, 2019:169 ff) Taylor’s talk reviewed the evidence for and against a lunar magma ocean. Aware of the complications in the evidence and that there were some skeptics of the concept of a global magma ocean in the audience, Taylor presented a balanced review. For so doing, he was criticized after the talk by Cameron, who thought that Taylor should have been more forceful in support of the global magma ocean concept, given that it was a necessary consequence of the giant impact model that was blossoming at the conference (personal communication from Jeffrey Taylor, June, 2014). It would appear, therefore, that Cameron was not completely objective about the global impact model. On the other hand, Hartmann presented a “trilogy diagram” as a part of one of his talks at the Kona Conference. This diagram (Fig. 5.21) illustrates that the giant impact model gives different outcomes depending on the details of the impact of the Mars-size body with the proto-Earth. It suggests that Hartmann and his colleagues viewed the model as an “adjustable engine,” in the words of Eddington, as opposed to a finished “building.”

5.9

5.9

Continuing Understanding

133

Continuing Understanding

The two-body collision hypothesis of Moon’s origin remains a key topic of lunar scientific research and speculation. Analyses of the isotope abundances of elements in Earth’s crust and in the Apollo samples have raised considerable controversy about details of the body (the Moon) that might result from such a collision. Analyses of lunar samples have shown that Earth and Moon have very similar isotopic compositions. As noted in the excerpt from Robert Clayton above, this would not be surprising except that most of the other samples from the solar system do not have the same isotopic composition as Earth’s mantle. As the sophistication of computer simulations for various models of the two-body collision continues to increase, researchers are trying to discover how the isotopic equilibration might have occurred. For example, a modeling study of a re-impact of the proto-Earth and the original collisional body up to a million years after an initial impact attempts to address this isotopic abundance question (Asphaug et al., 2021). As noted by Jeffrey Taylor, however, the giant impact idea has given planetary science a new paradigm, and follow-on research on the origin of the Earth-Moon system has generally been consistent with this new basic understanding. In space science as in other disciplines, however, research theories are tentative. There is always the possibility that further research can completely alter a field (the oft-quoted examples of special and general relativity in physics and the double helix in bioscience). In addition, there is always the potential that a research community might prematurely adopt a flawed paradigm. William Hartmann has given a good example of this latter possibility in his review of the ongoing debate about what has been called the Late Heavy Bombardment of the Moon (Hartmann, 2019). Graham Ryder (1949–2002) (about whom more will appear in Chap. 7) and others argued that the dating of rocks produced by melts in the aftermath of impacts on the Moon indicated that there was a period of about 150 million years in duration when the Moon was heavily bombarded (Ryder, 1990). This period was thought to be perhaps 450 million years after the formation of the Moon, so it was labeled the Late Heavy Bombardment (LHB). Prior to this period there were few impacts and following the LHB the rate of impacts was much less than during the “spike” of the LHB. On the other side of the debate, Hartmann and others argued that absence of impact melts does not necessarily mean the absence of impacts. There could have been a huge rate of impacts in the early history of the Moon that destroyed evidence of most of the rocks formed from impact melts until the rate of impacts decreased to allow some such rocks to survive. Hartmann has also pointed out that one class of meteorites, namely enstatites, have very similar isotopic composition to Earth’s mantle (Hartmann, 1986). The source of these meteorites, called E-type asteroids, are thought to reside in a belt inward (sunward) of the main asteroid belt. Hartmann suggested that if, during migration of the outer planets, including Jupiter, the asteroid belt was disturbed (as some have suggested) then the proto-Earth might have been impacted by a large

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E-type asteroid to form the Earth-Moon system. This would perhaps explain the similarity in isotopic composition of the Earth and Moon. An intense but declining rate of bombardment of the newly formed Moon by other large bodies would follow for hundreds of millions of years (see Hartmann, 2019, Sect. 16). The LHB debate continues. An important point of Hartmann is that the early acceptance of the LHB paradigm has influenced the thinking not only of planetary scientists but of scientists in other disciplines as well, e.g., bio-scientists studying the origin of life on Earth, when perhaps the paradigm was in error. The admonition is that, despite apparent paradigm shifts, scientists must continue research in areas that remain under valid debate. A vibrant lunar and planetary research community exists to advance discussions on the lunar origin and other topics of planets and planetary satellites. The plans by several nations to send orbiters and robotic landers, as well as humans, to the moon in the coming years has kept lunar research an important discipline. Identifying water ice reservoirs in lunar polar regions is an important research topic, together with locating appropriate landing spots near them. At the same time, orbiting and landing instruments can continue to more precisely characterize the space environment around the Moon, its solar wind-sputtered “atmosphere”, and probe by radio means the lunar interior. The search for, and finding of, meteorites on Earth (especially on Antarctic icefields) that have been blasted from the Moon by cosmic impacts continue importantly to advance lunar research.

References Asphaug, E., Emsenhuber, A., Cambioni, S., Gabriel, T. S., & Schwartz, S. R. (2021). Collision chains among the terrestrial planets. III. Formation of the Moon. The Planetary Science Journal, 2(5), 200. Baldwin, R. B. (1942). The meteoritic origin of lunar craters. Popular Astronomy, 50, 365–369. Baldwin, R. B. (1943). The meteoritic origin of lunar structures. Popular Astronomy, 51, 117. Baldwin, R. B. (1965). A fundamental survey of the Moon (pp. 42–43). McGraw-Hill. Baldwin, R. B., & Wilhelms, D. E. (1992). Historical review of a long-overlooked paper by R A. Daly concerning the origin and early history of the Moon. Journal of Geophysical ResearchPlanets, 97(E3), 3837–3843. Benz, W., Slattery, W. L., & Cameron, A. G. W. (1986). Short note: Snapshots from a threedimensional modeling of a giant impact. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 59–101). Lunar and Planetary Institute. Cameron, A. G. W., & Ward, W. R. (1976). The origin of the Moon [Abstract]. In Abstracts of papers submitted to the seventh lunar science conference (pp. 120–122). Lunar and Planetary Institute, p. 120. Chamberlain, J. W. (1972, July). Post-Apollo Lunar Science: Report of a study by the lunar science Institute (p. 11). Lunar and Planetary Institute. Clayton, R. N., & Mayeda, T. K. (1975). Genetic relations between the Moon and meteorites. In R. B. Merrill (Ed.), Proceedings of the Sixth Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 6, v. 2, pp. 1761–1769) (p. 1761). Pergamon Press.

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Cummings, W. D. (2009). A documentary history of the formation of USRA. Universities Space Research Association. Cummings, W. D. (2019). Evolving theories on the origin of the Moon. Springer. Daly, R. A. (1946). Origin of the Moon and its topography. Proceedings of the American Philosophical Society, 90(2), 104–119. Darwin, G. H. (1879). On the precession of a viscous spheroid, and on the remote history of the Earth. Philosophical Transactions of the Royal Society of London, 170, 447–539. Davies, J. H., & Davies, D. R. (2010). Earth’s surface heat flux. Solid Earth, 1(1), 5–24. p. 5. Field, G. B. (1963). The origin of the Moon. American Scientist, 51(3), 349–354. Gerstenkorn, H. (1955). Über Gezeitenreibung beim Zweikörperproblem. Zeitschrift für Astrophysik, 36, 245–274. Gilbert, G. K. (1893). The Moon’s face: A study of the origin of its features. Bulletin of the Philosophical Society of Washington, 12, 241–292. Hartmann, W. K. (1986). Moon origin: The impact-trigger hypothesis. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 579–608). Lunar and Planetary Institute. pp. 582–586. Hartmann, W. K. (2019). History of the terminal cataclysm paradigm: Epistemology of a planetary bombardment that never (?) happened. Geosciences, 9(7), 285. Hartmann, W. K., & Davis, D. R. (1975). Satellite-sized planetesimals and lunar origin. Icarus, 24(4), 504–515. p. 504. Jeffreys, H. (1917). The resonance theory of the origin of the Moon. Monthly Notices of the Royal Astronomical Society, 78, 116–131. Langseth, M. G., Keihm, S. J., and Peters, K. (1976). Revised lunar heat-flow values. In R. B. Merrill (Ed.), Proceedings of the Seventh Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 7, v. 3) (pp. 3143–3171). : Pergamon Press., p. 3170. Levin, B. J. (1972). Origin of the Earth. Tectonophysics, 13(1-4), 7–29. Lunar Sample Analysis Planning Team: Arnold, J, Arrhenius, G., Eglinton, G., Frondel, C., Gast, P., MacGregor, I., Pepin, R., Strangway, D., Walker, R., Wasserburg, G., & Zill, P. (1970). Summary of Apollo 11 Lunar Science Conference. Science, 167(3918), 449–451. p. 450. Lyttleton, R. A. (1953). The stability of rotating liquid masses. Chapter X. Cambridge University Press. Marvin, U. B. (2003). Oral histories in meteoritics and planetary science: X. Ralph B. Baldwin. Meteoritics and Planetary Science, 38(S7), A163–A175. O’Keefe, J. A. (1970). Apollo 11: Implications for the early history of the solar system. Eos, Transactions of American Geophysical Union, 51(9), 633–636. p. 634. Ringwood, A. E. (1970). Origin of the Moon: The precipitation hypothesis. Earth and Planetary Science Letters, 8(2), 131–140. Ruskol, E. L. (1963). The origin of the Moon. II. The growth of the Moon in the circumterrrestrial swarm of satellites. Soviet Astronomy, 7(2), 221–227. Ryder, G. (1990). Lunar samples, lunar accretion and the early bombardment of the Moon. Eos, Transactions of American Geophysical Union, 71(10), 313–323. Safronov, V. S. (1958). On the growth of the terrestrial planets. Voprosy Kosmogonii, 6, 80–94. (In Russian) Safronov, V. S. (1964). The growth of terrestrial planets. Problems of Cosmogeny, 6, 71–88. (This is an English version of a paper with a similar title as Safronov’s 1958 paper.) Safronov, V. S. (1968). Evolution of the Protoplanetary Cloud and Formation of the Earth and the Planets. (The English translation of this book was published in 1972.) Schmidt, O. (1958). A theory of the Earth’s origin: Four lectures (pp. 58–59). Foreign Languages Publishing. Taylor, G. J. (1998, December 31). Origin of the Earth and Moon [Online comment]. Planetary Science Research Discoveries. http://www.psrd.hawaii.edu/Dec98/reminisces.html. Urey, H. C. (1952). The planets: Their origin and development. Yale University Press.

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Urey, H. C. (1955). Some criticisms of “On the origin of the lunar surface features” by G.P. Kuiper. Proceedings of the National Academy of Sciences USA, 41(7), 423–428. Urey, H. C. (1963). The origin and evolution of the solar system. In D. P. LeGalley (Ed.), Space Science (pp. 123–168). Wiley. Urey, H. C. (1972). Evidence for objects of lunar mass in the early solar system. In H. Alfvén, Z Kopal, and H. C. Urey (Eds.), Proceedings of a Conference on Lunar Geophysics at the Lunar Science Institute, Houston, Texas, October 18–21, 1971 (The Moon. 4(3–4)) (pp. 383–389). Reidel. Wetherill, G. W. (1975). Radiometric chronology of the early solar system. Annual Review of Nuclear Science, 25(283–328), 324. Wetherill, G. W. (1976). The role of large bodies in the formation of the Earth and Moon. In R. B. Merrill (Ed.), Proceedings of the Seventh Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 7, v. 3) (pp. 3245–3257). Pergamon Press, p. 3255. Wetherill, G. W. (1986). Accumulation of the terrestrial planets and implications concerning lunar origin. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 519–550). Lunar and Planetary Institute. Wise, D. U. (1963). An origin of the Moon by rotational fission during formation of the Earth’s core. Journal of Geophysical Research, 68(5), 1547–1554. https://doi.org/10.1029/ jZ068i005p01547 Wise, D. U. (1969). Origin of the Moon from the Earth: Some new mechanisms and comparisons. Journal of Geophysical Research, 74(25), 6034–6045. Wood, J. A. (1970). Petrology of the lunar soil and geophysical implications. Journal of Geophysical Research, 75(32), 6497–6513. Wood, J. A., Dickey, Jr., J. S., Marvin, U. B., & Powell, B. N. (1970). Lunar anorthosites and a geophysical model of the Moon. In A. A. Levinson (Ed.), Proceedings of the Apollo 11 Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 1, v. 1, pp. 965–988) (p. 980). Pergamon Press.

Chapter 6

Lunar Dust

6.1

Introduction

The origin of the Moon and the nature of its visible physical features have puzzled and motivated speculations by humans for millennia (Chap. 5; see, e.g., Cummings, 2019). The beginnings of space flight, and the political objective for humans to travel to the moon as articulated by United States President John F. Kennedy, motivated even more scientific discussion and research related to Earth’s companion. A major issue in the discussions that initially arose was the nature of the surface material on the Moon that exploratory robotic landing missions and the following human footsteps would encounter. While over intellectual history the origin of the Moon has been the central topic, new speculations of its origin did not significantly come to the fore in the Apollo era until lunar samples were returned by the astronauts and were analyzed. The discussions during the design stage of the Apollo program regarding the surface material, especially in the maria, were not purely intellectual exercises but were central to the planning and design of landing vehicles, and of human mobility on the surface. This chapter discusses the controversy that arose when astrophysicist Thomas Gold (1920–2004) (Fig. 6.1) of Cornell University proposed that the large basin areas on the Moon’s surface (Figs. 5.3 and 6.2), so visible even with the naked eye, were filled with deep dust layers caused by the meteoroids that had bombarded the lunar surface over the life of the Moon. That is, Gold hypothesized that these areas were not filled with lava from lunar volcanoes or by the impacts of bolides of various sizes. And, therefore, human exploration on the surface could be seriously impacted by deep layers of dust.

The original version of the chapter has been revised. A correction to this chapter can be found at https://doi.org/10.1007/978-3-031-41598-2_11 © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023, corrected publication 2024 W. D. Cummings, L. J. Lanzerotti, Scientific Debates in Space Science, Astronomy and Planetary Sciences, https://doi.org/10.1007/978-3-031-41598-2_6

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Fig. 6.1 Thomas Gold. Credit the Division of Rare and Manuscript Collections, Cornell University, courtesy of AIP Emilio Segrè Visual Archives

Fig. 6.2 A photo of the Moon taken by the Lunar Reconnaissance Orbiter Camera, showing the darkcolored maria and the lighter highlands. Credit NASA/ GSFC/Arizona State University

6.2 6.2.1

Background on the Nature of the Lunar Surface The Majority View: Lunar Volcanoes

From the late eighteenth century into the first half of the twentieth century, the dominant view was that lunar craters were volcanic and the maria were lava flows from these volcanoes (see examples in Hershel & Banks, 1787; Dana, 1846; and Nasmyth & Carpenter, 1903; for a more comprehensive review of the evolution of ideas about lunar surface features, see Cummings, 2019). Objections had been raised, however, that the lunar craters didn’t resemble the classic picture of a volcano as a symmetrical mountain with a small orifice at its summit, exemplified by the

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Gold’s Speculation and the Ensuing Controversy

139

Vesuvius volcano (Fauth, 1909). In 1846, however, James D. Dana (1813–1895) a geologist and volcanologist of Yale University had published a paper in the American Journal of Science and Arts in which he argued that the lunar volcanoes were of the Kilauea type, rather than the Vesuvian. Dana had recently returned from an expedition in the Pacific, where he observed the Kilauea volcano as a large open shallow pit of lava. He concluded that: A map of the moon, if there is any truth in these views, should be in every geological lecture room; for no where can we have a more complete or magnificent illustration of volcanic operations (Dana, 1846:347).

6.2.2

The Minority View: Impact Craters

In the nineteenth century and before, a minority of scientists argued that the lunar craters were caused by impacts of bolides of various sizes on the Moon (see an early example of this idea in Gruithuisen, 1825). A few scientists had done model experiments to support this view (Althans, 1839; Meydenbauer, 1877; Thiersch & Thiersch, 1879). In 1892 a distinguished American geologist made a persuasive case for the impact origin of the large Imbrium basin (Gilbert, 1893). Unfortunately, the paper by this geologist, G. K. Gilbert (1843–1918), who spent most of his career with the U.S. Geological Survey, was unknown to scientists who studied the Moon, or was ignored by them, for about 50 years. In 1942, Ralph B. Baldwin (1912–2010), an astronomer at the Dearborn Observatory of Northwestern University, published a paper in Popular Astronomy (Baldwin, 1942), in which he repeated the observations and conclusion made by Gilbert, being unaware of Gilbert’s earlier paper (Marvin, 2003:A168). Baldwin’s paper and his subsequent analyses set the course for the eventual acceptance of the impact origin of lunar craters. In a 1946 paper on the impact theory, however, a noted geologist Robert S. Dietz (1914–1995), pointed out that “Nearly all astronomy texts . . . favor volcanism, inasmuch as it appears to have a large majority of proponents.” (Dietz, 1946:359). Dietz contributed importantly to many areas of geology, including—and especially—to meteoroid and asteroid impacts on Earth and the Moon and to “seafloor spreading,” for which he coined the phrase (Koppes, 1995).

6.3 6.3.1

Gold’s Speculation and the Ensuing Controversy Gold’s 1955 Paper

Apparently, Thomas Gold believed that an alternative to lava flows was not being considered. His alternative hypothesis was that the large basins on the Moon could have been filled not by lava but by fine material (dust) that had been created by the

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bombardment of meteoroids over the life of the Moon. In his 1955 paper in the Monthly Notices of the Royal Astronomical Society titled “The Lunar Surface” (Gold, 1955), Gold began to elaborate on his speculation. He first pointed out that overlapping craters on the Moon give a relative time scale for the ages of the craters, i.e., it is easy to determine the time order of the impacts that caused the craters. Gold noted that: Crater rims and other detail vary greatly in the degree of sharpness, and it is found in all cases of overlapping groups of craters that when there is a clear difference between the members in this respect then it is the youngest one on the criterion of overlap which shows the sharpest features. (Gold, 1955:586–587)

To explain the fact that younger craters have sharper features than older ones, Gold argued that: A process of erosion has taken place which has resulted in gradual changes . . . that must have been progressing through the major part of the interval of time during which the craters that are now visible were formed. (Gold, 1955:587)

Gold argued that this same bombardment was responsible for the erosion of the presumably initially sharp rims and other features of lunar craters. Gold’s speculation faced two problems. (1) How did the dust that was created by the impacts that caused the erosion of craters get transported to the low-lying maria? (2) What accounted for the difference in color of the dark maria and the light craters of the highlands areas of the Moon? In his 1955 Monthly Notices paper, Gold offered some suggestions for how a very thin layer of the lunar surface might be “fluidized,” so that it could flow from higher to lower elevations. If there was a high rate of bombardment by small meteoric dust particles in the past, the hot gas generated in each strike would have agitated the surface layer of the Moon and served to move the dust. Gold also suggested that electrical forces might play a role in transporting the dust particles: Electrical forces are also a possibility. These can arise either as a consequence of photoemission of electrons from the surface due to the ultraviolet light of the Sun, or they could arise in some larger electrical process connected with solar events of the type that cause the aurorae and magnetic storms on the Earth. . . . While the average positive charge will be inadequate to lift particles off, it is not clear that the chance distributions in small localities could not do it. (Gold, 1955:600–601)

To explain the color difference between the dark maria and the lighter highlands regions of the Moon (see Fig. 6.2), Gold suggested the possibility that: The darker colour is produced by the action of the solar X-rays on material that has been on the surface for a comparatively long time, whilst the denuded highlands keep presenting material that has until recently been protected. (Gold, 1955:602)

Gold noted that small craters in the maria show a lighter color, and he argued that “radiation-induced coloration is generally lost by heating, and this would explain why fine powder that has been involved in an explosion is again turned to a lighter shade” (Gold, 1955:602).

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Urey’s Response

Harold C. Urey (1893–1981), a 1934 Nobel-laureate physical chemist, responded to Gold’s paper with an article published in December 1956 (Urey, 1956). It was Urey’s view that the Moon was cold and had always been cold, thus having no volcanism (Urey, 1955). Urey approved of Gold’s model to the extent that it did not require a hot interior of the Moon. As discussed above, central to Gold’s speculation was the idea that the maria were not lava flows on the surface of the Moon but, rather, were dust that had migrated from highland regions to the interior of massive craters such as Imbrium, Serenitatis, and Tranquillitatis. The dust would have darkened as it lay in the basins owing perhaps to long exposure to solar X-rays (Gold, 1955). In Gold’s model, then, there was no need to assume that the maria were beds of lava that had seeped upward from the interior of a Moon that was hot and fluid, having been heated by radioactive decay of various elements. Urey disagreed, however, with Gold’s “proposed methods for the formation of the dust and for its distribution over the surface of the Moon . . .” (Urey, 1956:232). Urey thought that the maria were lavas that resulted from local melting at the time of the impacts that created the large basins on the Moon. Citing experiments on color changes caused by the heating of meteorites in the laboratory, Urey suggested that the dark color of the maria was the result of heating during the collisional process that formed the basins (Urey, 1956:233).

6.3.3

“More Treacherous than Quicksand”

On 29 July 1955, the Eisenhower White House announced that the United States intended to orbit some small satellites during the International Geophysical Year, which was planned for 1 July 1957 through 31 December 1958 (see Chap. 1: Introduction). Many individuals and organizations took the mental leap forward to human exploration of space. For example, the United States Air Force Office of Scientific Research (AFOSR) convened its first symposium on astronautics on 18–20 February 1957 in San Diego. The major event, of course, was in the fall of the year, on 4 October 1957, when the Soviet Union launched Sputnik 1, the Earth’s first artificial satellite. Gold was among the speakers at the Second Annual AFOSR Astronautics Symposium that was held in Denver, Colorado, in April of 1958. He spoke in the session on The Earth’s Moon, and his subject was lunar dust. Gold noted that for a given size, older craters have “a much lower height of rim and a much more rounded profile of the crater edge. They also have more frequently a remarkably flat interior” (Gold, 1959: 261). Gold’s point was that the lower rim heights of the older craters suggested loss of material by erosion:

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The quantities of material that would have to be removed by such an erosion process are by no means small. A large proportion of the surface area of the high ground is occupied by crater walls, which will then at one time have been 2 or 3 km higher than they are now. The quantity of material that is missing, that must have been moved, might well be of the order of 1 km average depth. Unless this material has left the Moon altogether, it has to be identified with some parts of the present surface. (Gold, 1959:261)

Apparently as a counter argument to Urey’s idea that the maria were lava flows that were created when bolides struck the Moon to create the great basins, Gold outlined his own view of a possible process for the production of lava beds on the Moon: The supposed lava flows would seem to have occurred in most cases much later than the formation of the crater which they flooded, for we can observe the number of subsequent small impacts that have occurred on the rim and in the interior, and in general that number is much greater on the rim. The process of a major impact can thus not have been directly responsible for the flooding, but one would have to have it that the depression of each crater bowl became later a weak spot through which lava could get to the surface. This would have to apply to many craters of very different sizes without the hydrostatic unbalance necessary to cause the flooding having shown itself in any other way. Also, the flooding must all have occurred at a temperature well above the melting point of lava and rather quickly if the generation of steep lava slopes, which are most common on Earth, is to be so completely avoided. . . . (Gold, 1959:262)

Gold thought this process was unlikely. However, the subsequent Apollo explorations and analyses proved his hypothesis to be largely accurate, particularly that lava flooding of the great basins occurred long after the basins were created by giant impacts (see Heiken et al., 1991:19–21). Skeptical of his own model for lava as the material corresponding to the lunar maria, Gold continued to pursue his alternative explanation of the erosion and migration of dust to smoothly fill the low-lying areas of the Moon. He discussed various possible processes for erosion and migration of dust particles, and he referred to a British radar experiment that showed that the Moon’s surface was smooth at a scale of 10 cm. Gold concluded his talk with remarks that were to inflame the controversy about his speculation that the Moon was covered with a deep layer of dust: At a meeting on astronautics, it is appropriate to discuss in what condition this dust surface might be. As I have said, at a depth of a few hundred feet it would almost certainly be quite solid. But that conclusion will hardly satisfy the astronaut who wishes to make such an area his landing ground. The top few feet may well be extremely loose and more treacherous than quicksand. (Gold, 1959:265)

He concluded by proposing that if large accumulations of dust existed on the lunar surface they could be identified by exploding a “substantial bomb”. The dust thrown up above the moon by the bomb explosion could be seen from Earth, illuminated by the Sun. Fortunately, Gold’s idea about setting off a bomb on the Moon was never carried out. Instead, the National Aeronautics and Space Administration (NASA) sent several probes to impact the Moon (the Ranger series) and another set of landers (the Surveyor series) to make sure that the surface of the Moon had sufficient bearing

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strength for the Apollo landers. The results of these experiments are discussed below.

6.3.4

Whipple

The next speaker in the Earth’s Moon session of the second AFOSR symposium was Fred L. Whipple (1906–2004) (Fig. 6.3), a Harvard University astronomer and expert on asteroids and comets. His topic was the lunar dust layer. Whipple argued that the influx of small meteoritic dust particles and the: slow rain of dust, molecules and atoms from meteoritic impacts . . . plus a comparable proton bombardment at energies of tens of thousands of volts . . . will produce a thin layer of extremely heterogeneous and badly organized structure of the surface of the small grains. This heterogeneous matter will act as a cement between the edges of the grains, penetrating the dust interstices to a few grains’ thickness below the upper surface. It appears inevitable to the writer that such dust and gas will form a low-density semiporous matrix, weak compared to normal sedimentary rocks on the Earth, but strong compared to a layer of dust, perhaps comparable to sand. (Whipple, 1959:270–271)

Whipple dismissed Gold’s hypothesis of mobile lunar dust, writing: Flow of ‘liquid dust’ as postulated by Gold seems to be an impossible process in view of the cementing power of corpuscular radiation and the slow rain of heavier molecules, atoms and ions. The Moon cannot carry a strong electric charge to produce electrostatic transport because interplanetary space is a surprisingly good conductor, 103 electrons/cm3 near the surface and perhaps 102 electrons/cm3 at great distances from the Moon. (Whipple, 1959:271)

Whipple concluded that: Loose dust on the lunar surface is practically non-existent. The surface away from the immediate areas of large and new craters (~ 100m or larger dia.) should have the structural strength of desert sand or possibly greater. It should be sufficiently cemented or consolidated that it will not blow out dangerously in a rocket jet. To the human encased foot or under

Fig. 6.3 Fred Whipple. Credit Astronomical Society of the Pacific, courtesy of AIP Emilio Segrè Visual Archives, Physics Today Collection

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vehicles the surface should be ‘crunchy’ and allow minimal imprint. . . . (Whipple, 1959:271)

Whipple was right about the lunar surface in some respects, but he was wrong about the non-existence of dust and the effect of a rocket exhaust on it. As discussed below, the dust blown up by the rockets for the Apollo 12 lunar lander was so dense that as the lander descended to within 60 ft of the surface the pilot was “flying blind” and had to perform an instrument landing (Wagner, 2006:1).

6.3.5

Kuiper

The final speaker in the Earth’s Moon session of the second AFOSR symposium was astronomer Gerard P. Kuiper (1905–1973) of the Yerkes Observatory of the University of Chicago. Kuiper had recently used the 82-in. telescope of the McDonald Observatory in Texas to make a careful study of the features of the Moon. In his talk at the symposium, he used photographs from the McDonald Observatory, as well as some other observatories, to describe surface features of the Moon, particularly the mare regions. He discussed mare features such as ridges, rills, and domes that had traditional geological explanations in the context of lava fields. He concluded his analysis with the following statements: In summary, there is overwhelming evidence that the traditional view that the mare floors are lava fields is correct. The alternative hypothesis recently advanced that the maria instead are low-lying basins filled with perhaps one mile of dust resulting from surface erosion elsewhere ignores known facts. If surface erosion had led to the production of vast quantities of dark-colored dust, now found in the maria, then all lunar impact craters would be partially filled with such dark dust also. On the contrary, the oldest craters on the Moon are at full Moon, when albedo differences are best seen, indistinguishable from the bright surrounding uplands. The only craters having dark bottoms appear to have been formed contemporaneously with Mare Imbrium. Perhaps the strongest evidence that the maria have been molten is given by the frequent occurrence and the structural detail of the ridges. These ridges occur in the maria only and are clearly a result of compressional forces. The fact that in many locations they have cracked open and often have given rise to dike-like extrusions established the igneous nature of these ridges. Furthermore, . . . , the maria are not flatbottomed as has been supposed but show, besides the ridges, intricate structural features everywhere and are totally unlike the flat, featureless basins which one would expect from a semi-fluid mass of charged dust particles. Also, the numerous small meteor craters found in maria, with numbers compatible with the great age attributed to these basins (about 4.5 billion years) as well as the delicate detail of scars, gauches, crater rays etc. found on the mare floors, rules out that these are covered with a thick coat of dust. . . . (Kuiper, 1959:301– 302, emphasis in the original)

The evidence cited by Kuiper that the great basins on the Earth-facing side of the Moon were filled by lava would seem to have settled the issue. But Thomas Gold did not easily change his mind, and he would later provide counter arguments to some of those provided by Kuiper.

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The Space Race

In response to continued space achievements by the Soviet Union, President Eisenhower and the U.S. Congress created the National Aeronautics and Space Administration on 29 July 1958. By the next year NASA, through the Jet Propulsion Laboratory, began the development of the Ranger series of spacecraft, which were intended to take and transmit to Earth high-resolution photographs of the lunar surface before the spacecraft impacted the Moon. On 25 May 1961, a few months after his inauguration, President John F. Kennedy gave a speech to a joint session of Congress in which he announced the goal of putting a man on the Moon by the end of the decade and returning him safely. Later that year, on 12 September, President Kennedy reiterated this goal during a major speech at Rice University. The space race had begun, and there was now a heightened interest on the Moon and its surface features.

6.3.7

Gold’s 1962 Paper

Meanwhile, with the growing urgency of understanding the surface strength of the Moon because of the anticipated lunar landings, Gold continued to pursue his dust hypothesis despite the negative reception it had received from Urey, Whipple, and Kuiper. In his next paper on the subject of the lunar surface (Gold, 1962), Gold again took up the issue of the Moon’s coloration, noting that “It is generally true that the high ground is light and the low flat ground is dark” (Gold, 1962:433). Apparently alluding to Whipple’s hypothesis, Gold argued that infalling material should have given a uniformly-colored coating to the lunar surface over the lifetime of the Moon, adding that: Even the present rate of infall, which presumably is lower than the rate has been in the remote past, would already suffice to do this in a few million years. Such a conclusion is clearly ruled out unless there are other processes which all the time make for a differentiation between low and high ground. (Gold, 1962:433)

Gold once again connected his argument for color differences on the Moon with his speculation about the creation and migration of dust across the lunar surface: It seems that a satisfactory explanation can be found only if one allows the possibility of transportation of material over the lunar surface. In that case the following sequence of events may take place: material may constantly be removed from the high ground, presumably in finely powdered form, and migrate to the low ground. All the ionizing radiation from the Sun and the particle bombardment which the Moon receives is absorbed in a very thin layer of material on the surface, and this material therefore will show very substantial radiation damage. It is known that most silicates change to a darker color as a consequence of such crystallographic damage. If the dark color is now attributed entirely to radiation damage, then those areas whose material has longest been exposed on the surface will appear the darkest. On the slopes of the highlands fresh material would constantly be exposed as the erosion process proceeds. On the low ground the top layer will be composed of material

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which had been on the surface for the period of at least an entire migration. Areas of sediment would therefore be dark, eroding areas would be light. The rate of infalling meteoritic powder would have to be low enough by comparison with the surface migration so as not to dominate the coloration. (Gold, 1962:433–434)

6.3.8

Interpretation of the Ranger Photos

The Ranger missions that were first launched in 1961 continued into 1965. The first engineering test missions (Rangers I and II) suffered launch failures. The next set of missions (Rangers III, IV, and V) were launched in 1962. They either missed the Moon (Rangers III and V) or failed before impact (Ranger IV). Launch and spacecraft failures on the first six missions, typifying the learning process of the very early space program, prevented good data from being obtained. The cameras on Ranger VI failed before impact, but the remaining three Ranger missions, VII, VIII, and IX, were fully successful. Ranger VII impacted the Moon on 31 July 1964 after transmitting some 4000 images of the lunar surface. Ranger VIII impacted the lunar surface on 20 February 1965, and Ranger IX impacted the Moon on 24 March 1965. All the Ranger spacecraft carried six television cameras that were constructed the same but used different exposure times, fields of view, lenses, and scan rates. Perhaps the most striking result of the Ranger missions was that they revealed that craters caused by impacts were the dominant feature of the lunar surface. Crater sizes returned by the Ranger camera images ranged from the great mare-filled basins down to the limit of resolution of the cameras just prior to impact. About 2 months after Ranger VII returned pictures from the Moon, Thomas Gold published a paper in Science magazine (see Fig. 6.4 from Fig. 1 in his article) in which he argued that his speculation about dust on the Moon had been verified. He suggested that erosion had resulted in the rounded shallow crater profiles, which had originally been sharp edged. Fig. 6.4 From Fig. 1 of Gold’s Science article – Ranger Moon Pictures: Implications. The scale of the image is approximately 2.6 km on a side. Photo courtesy of the Lunar and Planetary Institute

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He further pointed out that The fact that crater formation proceeds more readily than in solid rock is also displayed by the appearance of the multitude of small secondary craters seen in the Ranger pictures. There it is certain that impact speeds are low, well below the lunar orbital speed. Nevertheless most of these are also circular or only slightly deformed. This must mean that the energy liberated even at speeds of less than 1 km per second is still enough to excavate a crater that is very large compared to the projectile. (Gold, 1964:1047)

Gold argued that low-speed impacts on a hard surface would not result in the shallow circular craters that are observed in the Ranger photos. The Ranger pictures thus appear to show mostly a uniform, fine-grained material of low structural strength near the surface and in the first few meters below the surface. They show no hint of any transition to a different material below, such as a change in the appearance of deeper craters or an occasional outcrop of something like a rock. It is therefore most likely that one is seeing the same type of material at all the depths excavated by the craters, but very probably in progressively greater compaction and cementation at the greater depths. The Ranger pictures have clearly strengthened the case for dust being the main constituent of the lunar lowlands by not showing any rock formations. There is no case for discussion of a two-layer model. . . . without any clear signs of firm rock the pictures must lead to more concern about sinkage on impact or dust blowing in rocket exhausts in future operations on the lunar surface. (Gold, 1964:1047–1048)

Gold wrote his Science article after the Ranger VII photos became available but before the Ranger VIII and IX missions had occurred. On 15–16 April 1965, less than a month after the Ranger IX mission, a meeting was held at NASA’s Goddard Space Flight Center that brought together scientists and others who were keenly interested in, and actively involved in studying, the nature of the lunar surface. The five scientists who were asked to discuss their interpretations of the Ranger photographs were Harold Urey, geologist Eugene M. Shoemaker (1928–1997), lunar astronomer and cartographer Ewen A. Whitaker (1922–2016), Gerard Kuiper, and Thomas Gold. All but Gold were members of the Ranger Experimenter Team, headed by Kuiper. Each of the five scientists stressed the importance of different features revealed by the Ranger photos. Urey’s analysis: Urey was interested in how the Moon was formed and its subsequent history, and many of his remarks were related to this primary interest. Regarding the possibility of dust on the Moon, Urey pointed to so-called dimple craters, which are relatively small funnel shaped lunar craters, as evidence for a deep layer of dust on the Moon: Of course, materials on the surface of the moon would hardly flow easily like sand, and so one must expect that bouncing from one point to another occurs as a result of micrometeorites and small macrometeorites. The dimple is then the absence of material coming back from the hole in the bottom and reflects the pattern of scattered material from the point on the moon’s surface. If this explanation proves to be correct (and I think Ranger IX photographs show many of these dimples) it would mean that there is a considerable layer of finely divided material on the surface of the moon, some 10 or 20 m, as indicated by this phenomenon, without any indication that this necessarily is the maximum thickness. (Urey, 1965:8)

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Fig. 6.5 Eugene Shoemaker. Credit U.S. Geological Survey, courtesy American Institute of Physics Emilio Segrè Visual Archives, Physics Today Collection, AAS Collection

Urey referred to Gold’s hypothesis that the maria were filled with dust and noted that he had suggested that possibly the large smooth areas of the moon were produced by fragmented material from the great collisions that produced the maria. (Urey, 1965:20)

He further wrote that This, of course, is consistent with a cold origin for the moon and a relatively cold history since. It was necessary, however, to account for some way by which all of the dust left the mountain tops; I suggested that water was briefly present on the moon and that rains washed the dust from the mountains. I have said not much about it in the years since; we can make many suggestions about the moon, but we have rather great difficulty in proving that what we say is more than just possibilities. (Urey, 1965:20)

Urey expressed his unhappiness about the view of many that the maria are lava flows because of the thermal calculations and my expectation that a small object such as the moon would be much colder than the earth; hence the probability of extensive volcanism would be considerably less. . . . I have been very much intrigued by Gold’s dust ideas. (Urey, 1965:20)

None of the other speakers mentioned Gold by name, much less gave him any credit for his ideas. Shoemaker’s analysis: A principal interest of Eugene Shoemaker (Fig. 6.5) was the development of a model of the lunar surface that could be used to predict “coherence and bearing strength, properties important in the problem of landing spacecraft on the moon . . .” (Shoemaker, 1965:23). Shoemaker’s model featured a debris layer, but not a deep layer of dust. Based on his observations of the Ranger VII photographs, along with information obtained from Earth-based telescopes, Shoemaker concluded that: A layer of shattered and pulverized rock covers more than 95 percent of the mare, . . . The fragments in this layer or blanket of shattered rock have been derived by ejection from craters, most of them nearby, but some lying great distances away. . . .

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,. . . Progressing upward from the base, the layer has been stirred or reconstituted an increasing number of times by smaller and smaller and more and more numerous cratering events. The uppermost millimeter of the debris layer is probably reorganized once every 10 to 100 years by the formation and filling of minute craters. The average grain size of the debris layer tends to decrease from base to top because fragments in the upper part have been shocked and broken a greater number of times and have been ejected, on the average, from smaller craters. . . . The debris layer typically varies in thickness from a few tens of meters to less than a millimeter. . . . Between craters less than 100 m in diameter, the average thickness of the debris layer is probably between ½ and 1 m. The porosity of the debris layer is expected to be of the order of 90 per cent at the surface. It decreases rapidly with depth in most places, probably to less than 50 percent at depths of a few tens of centimeters. Beneath these depths, the debris will have been compacted by shocks propagated from numerous small impact events. The uppermost few millimeters of the debris layer is conceived as a fragile open network of loosely stacked, very fine grains. It is probably compressible under loads of the order of a few tens of grams per square centimeter. The bearing strength and shear strength of the material increase rapidly with depth, and at depths of a few tens of centimeters are probably similar to the bearing and shear strength of moderately consolidated dry alluvium on Earth. (Shoemaker, 1965:77)

Whitaker’s analysis: Whitaker, a British-born astronomer, and a member of the University of Arizona’s Lunar and Planetary Laboratory, which was led by Kuiper, concluded that: The maria and other dark areas appear to be best explained by fluid flows rather than ash flows, debris deposits, or dust aggregations. There is no reason to think the flows are not lava. ... The soft-edge craters seen in the higher resolution Ranger photographs appear to be better explained as subsidence features, somewhat analogous to terrestrial karsts, rather than secondary or tertiary impact features. ... The average depth of finely divided debris on the lunar surface may be of the order of 1 meter . . . Dr. Kuiper has computed a bearing strength of the order of 1 ton per square foot for the floor of Alphonsus, based on certain assumptions in the interpretation of the final Ranger IX frames. (Whitaker, 1965:79)

Gold’s analysis: Gold began his talk with a comment that must have further unsettled the NASA scientists and managers who were planning the human exploration of the Moon: The degree of toughness of the moon’s surface is a matter of great concern to those in technology, and optical investigations just are not capable of giving us the answer. They can give us various hints and clues . . . , but they cannot tell us whether it is material we can walk on and sink only a few inches in, or something we would wallow in and have great difficulty in even taking a step in. . . . (Gold, 1965:107)

At the time of this meeting, Gold headed the Center for Radio Physics and Space Research at Cornell University, which managed the 305-meter Arecibo radio telescope in Puerto Rico. To provide background for his discussion of the Ranger photographs, Gold used some data from the Arecibo radar probe operating at a wavelength of 70 centimeters, as well as thermal data from a 1930 paper (Pettit and

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Nicholson, 1930) by Edison Pettit (1889–1962) and Seth B. Nicholson (1891–1963). He noted that the data from Arecibo’s radar signal bounced off the moon and reflected back to Earth receivers indicated that much of the lunar surface is very porous to a several wavelength depth, except for the young craters that are solid rock. He also noted that “the same kind of information on porosity of the ground is contained in the thermal data, namely that the moon cools much too fast to be solid rock (Gold, 1965:112). Gold added that The exception to the above are the young craters which behave more like solid rock in every respect. After all, if a crater is made by a large impact, the pressures that are exerted on the ground during the excavation are in the range of megabars. At these pressures the rock is not only completely compacted, it actually suffers pressure-induced phase changes as well. Whatever the original composition of the area was when the crater Tycho was blasted, its walls were made into some absolutely solid compacted material. The same must be true for all the other craters if they were made by the same process. The anomaly of the moon is not that the young craters behave more like solid rock. The anomaly is in the rest of the moon, which has suffered some degradation down to a depth at which radar waves get absorbed. (Gold, 1965:112)

He stated that the older craters, once new and hard, now absorb radar signals and that an understanding is required as to the degradation process over time to a radio signal depth of several meters. Gold acknowledged the occurrence of many surface features in the maria such as rilles, wrinkles, ridges, and irregular depressions. He speculated that the cause of these surface features was underlying ice. That is, that the surface of the Moon was underlain by the equivalent of permafrost as is the case in the polar regions of Earth. Gold noted that these features were observed on the mare regions of the Moon, but not on the highlands, and that a mechanism is needed to account for the distortions. I personally believe that the most likely reason for this is that on the moon ice underlies low regions of the ground where water has slowly come up from the interior. Owing to the very low temperatures, more than 30°C below freezing, that the moon’s surface has, the water cannot penetrate freely to the surface, but will always accumulate underneath and make subterranean ice, possibly in great quantity. For the temperature conditions and overlying debris conditions that would obtain there, this ice will not readily evaporate. It will be able to maintain itself . . . for long periods of time comparable with geologic time. The distortions that are characteristic of mare ground may be due to the underlying crevasses and the underlying movement of plastic ice, which is in contrast to the solid of the surrounds. (Gold, 1965:120)

6.3.9

The First Soft Landings on the Moon

Luna 9: The Luna 9 spacecraft of the Soviet Union was the first to make a soft landing on to the surface of the Moon. The landing occurred on 3 February 1966 (nearly a year after the last NASA Ranger mission), and television cameras on the spacecraft soon began to make a panoramic survey of its surroundings.

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The fact that Luna 9 did not sink beneath a deep layer of dust on the lunar surface must have relieved many at NASA who were planning for soft landings on the Moon by a series of Surveyor spacecraft followed by the Apollo landers. Thomas Gold was not sanguine, however. In an article that he published in Science magazine with Bruce W. Hapke, a colleague in Cornell’s Center for Radiophysics and Space Research, he continued to warn of a possibly dangerous lunar dust surface. During its first day of operation, Luna 9 shifted its position on the lunar surface by several inches. Gold and Hapke pointed out that Luna 9 had moved during an interval of a few hours and that: Plastic deformation time constants in any kind of rock material are many orders of magnitude longer. No significant deformation can be envisioned that would take a matter of hours to occur and then trigger a movement of larger amplitude. The only exception that we can now suggest would be time constants in the nature of those of an hourglass. If the lunar ground underneath Luna 9, and perhaps underneath most areas, had many hollow spaces, such as those that would be produced by all the many secondary impacts at moderate speeds, then it is conceivable that the additional loading may cause many such cavities to cave in. The time constants of caving in may then be given, just as in the case of an hourglass, by the many small particles that have to fall in sequence before a major structural change is made. If the addition of a new weight on the surface opens up a few new cracks in a very porous structure of not very firmly cemented small particles, then little collapses and landslides may appear until eventually the structural properties are so changed that a major collapse takes place. (Gold & Hapke, 1966:291)

The Surveyor Missions: NASA’s follow-on to the Ranger probes was a series of larger spacecraft, the Surveyors, also under the responsibility of the Jet Propulsion Laboratory. The Surveyors were to make soft landings on the Moon, take photographs, and carry out other experiments to better understand the characteristics of the lunar surface. Five of the seven Surveyor spacecrafts successfully landed on the Moon. Four landed near the equator of the Earth-facing side of the Moon in mare areas that were being considered for the Apollo landing sites, and one landed in the lunar highlands near the crater Tycho. Surveyor I landed in the southwest part of Oceanus Procellarum on 2 June 1966. A camera that was focused on one of the landing pads of the spacecraft showed that the pad rested on the lunar surface with sinkage of the order of a few centimeters. Similarly, Surveyors III, IV, VI, and VII made only small depressions in the lunar surface when they landed. The final report of the Surveyor program noted that the soil at all landing sites was found to be “predominantly fine grained, granular, and slightly cohesive” (Surveyor Program Results, 1969:167). The report also noted that the lunar soil had a tendency to adhere to parts of the Surveyor spacecrafts. This tendency problem of lunar dust adhering to landed spacecraft and to the suits of the astronauts would become more apparent during the Apollo explorations of the Moon. In an appendix to the final report of the Surveyor program, a “lunar sunset phenomenon” was described as follows: Television pictures, taken by several Surveyors a few minutes after local lunar sunset, showed the lunar western horizon highlighted by the Sun as a thin, bright, jagged, discontinuous line. (Surveyor Program Results, 1969:413)

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As discussed below, this line of light has been interpreted as scattering of sunlight from a thin layer of dust electrostatically levitated at the sunlit/shadow boundaries in the terminator zone (Rennilson & Criswell, 1974).

6.3.10

The Apollo Explorations

The Apollo 11 lander touched down on the southwestern edge of Mare Tranquillitatis on 20 July 1969. Blowing dust caused by the rockets of the lander partially obscured the surface of the Moon from a height of about 100 feet down to the surface (Wagner, 2006:1). Astronauts Neil A. Armstrong (1930–2012) and Edwin E. Aldrin, Jr. were able to walk on the dusty surface of the Moon with little difficulty, however (Fig. 6.6). Gold said that he never claimed that the astronauts would be swallowed up in the dust of the Moon’s surface, but he warned of that possibility. On the other hand, he did suggest that if he were walking on the Moon, he would want to use a tie line, as he related in a 1983 interview with historian David H. Devorkin of the Smithsonian’s National Air and Space Museum: I did say to NASA and many people in public also, that if I were going to the moon, I would prefer to be on a line, like you would be walking on a glacier, because I said that just like a glacier with fresh snow on it the moon has never had weather to test the strength of any particular piece of surface. There could be loose fluffy bridges of dust or dust, analogous to snow, which could have accumulated in curious ways somewhere, and you might fall into something. It might be disastrous. And that’s probably still true now, because we haven’t walked all that much on the moon. I would still prefer to be on a line if I were to walk on the moon for fear that untested ground may be treacherous. That’s all I said. The actual ground would be fairly firm, crunchy was the word, like a cake, I said, but it would be composed to great depth out of material with small grains, and I believe that that is true. (DeVorkin, 1983)

The rock samples brought back from Mare Tranquillitatis looked like charcoal briquettes. They were found to be igneous rocks that crystallized 3.7 billion years ago, long after the Moon was formed some 4.5 billion years ago. Faced with this Fig. 6.6 Footprint of an Apollo 11 Astronaut on the Moon. Courtesy of NASA

6.3

Gold’s Speculation and the Ensuing Controversy

153

data, Gold still maintained that the mare basins were filled with dust and that the rock samples were excavated from below this layer of dust when craters were formed: The mare basins represent deep deposits three to six kilometers in depth (judging from the seismic evidence and from the appearance of submerged craters) and those deposits may consist of a similar material as is found on the surface, though somewhat compacted and cemented at the greater depths. There is crystalline rock below this, at least in some regions. Within this deep deposit[,] rocks have been distributed by major impacts in which they were either generated, or previously existing crystalline rock at some depth was excavated. The origin of the crystalline rock may be early volcanism, or perhaps more likely it was produced by the heating of very major impacts such as the ones that created the mare basins. During the entire accumulation process of the mare basin the surface would have looked much as it does now, with a sprinkling of rocks among the deposit of fine powder. (Gold, 1971:3–4)

Gold was right to point out the hazards of dust on the Moon. Later in 1969, the Apollo 12 Lunar Lander touched down in another mare region, Oceanus Procellarum. Blowing dust during the landing was worse than for the Apollo 11 landing. The Apollo 12 Mission report indicates that below an altitude of about 60 ft: Degradation of visibility continues until the surface is completely obscured and conditions are blind. . . . At 25 seconds before touchdown, the dust cloud is quite dense, although observations of the film show some visibility at the surface. From the pilot’s point of view, however, visibility is seen to be essentially zero at this time, which corresponds to an altitude of about 40 feet . . . obscuration occurred at an altitude of about 50 feet is confirmed. The Commander considered visibility to be so completely obscured at this point that he depended entirely on his instruments for landing cues. (Wagner, 2006:2)

Dust was present as a surface layer for all of the Apollo missions that landed on the Moon. The dust proved to be a problem from the standpoint of dust contamination inside and outside the Lunar Modules as well as in the Command Modules. The last human mission was Apollo 17. Commander Eugene A. Cernan (1934–2017) commented on the problem of lunar dust in his technical debrief: I think dust is probably one of our greatest inhibitors to a nominal operation on the Moon. I think we can overcome other physiological or physical or mechanical problems except dust. (Wagner, 2006:iii)

Harrison H. (Jack) Schmitt (Fig. 6.7) was a crew member on the Apollo 17 mission, and he was the only trained geologist to land on and explore the lunar surface. Schmitt agreed with Cernan about the hazard presented by lunar dust for future explorations of the Moon, but he was a bit more optimistic that the dust problem could be overcome (Schmitt, 2006). . . . With over half the mass of the regolith made up of abrasive particles less than 100 μm in diameter, this dust will penetrate into any space, fabric, bearing or moving parts not specifically sealed against it. Habitats and their internal systems will need to be designed either to tolerate dust or to prevent its intrusion, and to do either indefinitely. . . . Our Apollo experience indicates that dust control can be carried out successfully on the moon. The pressure suits and the interior of the lunar modules were exposed to dust and tolerated it without significant degradation in performance for over three days. Suit glove and helmet bearings had circumferential scratches from dust, but showed no significant change in leak rates during pre-excursion tests. . . .

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Fig. 6.7 Jack Schmitt Credit NASA

Fig. 6.8 Lunar Horizon Glow as photographed by Surveyor 7. Credit NASA/ JPL-Caltech/University of Arizona Lunar and Planetary Laboratory/Gary Rennilson

On the problem side of the Apollo experience, however, dust accumulated on mirrored and other thermal control surfaces and had to be removed frequently—but only partially successfully—to avoid excess heating. Some equipment did overheat. Exposed connectors on various tools eventually jammed on the third day of use after repeated matings and dematings. Suit visors required to be cleaned between excursions to remove electrostatically adhering dust. Dust also prevented the knife-edge indium seals of any of the sample return containers (rock boxes) from maintaining a vacuum during storage in the spacecraft and during transit to the Lunar Receiving Laboratory. (Schmitt, 2006:1224)

Schmitt also noted that “Continuous exposure of lungs and other internal organs to micron- and submicron-sized mineral and glass particles, sizes too small to be cleared through normal processes, may cause long term health problems” (Schmitt, 2006:317). Gold seems to have been right, as well, about his contention that electrostatic forces could facilitate the movement of dust on the lunar surface. In 1974, Justin J. Rennilson of Caltech and David R. Criswell (1941–2019) of the Lunar Science Institute published a paper in which they reported on a careful analysis of the Surveyor photos taken during lunar sunset. They summarized their results as follows: Each of the Surveyor 7, 6, and 5 spacecraft observed a line of light along its western lunar horizon following local sunset. It has been suggested that this horizon-glow (HG) [See Fig. 6.8] is sunlight, which is forward-scattered by dust grains (~ 10 μ in diam, ~ 50 grains cm-3) present in a tenuous cloud formed temporarily (≤ 3 h duration) just above sharp sunlight/shadow boundaries in the terminator zone. Electrically charged grains could be levitated into the cloud by intense electrostatic fields (> 500 V cm-1) extending across the

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Assessments of Gold

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sunlight/shadow boundaries. Detailed analysis of the HG absolute luminance, temporal decay, and morphology confirm the cloud model. . . . Electrostatic transport is probably the dominant local transport mechanism of lunar surface fines. (Rennilson & Criswell, 1974:122)

While there might be migration of dust by the process of electrostatic levitation, it appears from chemical and other analyses that most dust is generated locally. From the Lunar Sourcebook: All lunar soils have a minor exotic component derived from some distance away, but most of these soils appear to have been derived largely from bedrock in their immediate vicinity. (Heiken et al., 1991:288)

and: The sharp contrasts in color and chemical compositions at highland-mare contacts, inferred from optical observations and orbital x-ray fluorescence data on the regolith, imply that lateral transport and mixing could not have been extensive, even over more than 3 b.y. (Heiken et al., 1991:287)

It seems that Harold Urey’s assessment of Gold in 1966 was prophetic. Urey said of Gold’s speculation about dust on the lunar surface that: Like all proposals of this kind that any of us make, they are likely to be only partly right, and we ought to be immensely pleased if they are only partly right. I think Gold has made a great contribution in calling our attention to the possibility of dust on the surface of the moon. (Urey, 1965:20)

6.4

Assessments of Gold

Gold received the Gold Medal of the Royal Astronomical Society, and he was elected to the Royal Society in 1964. He was elected to the U. S. National Academy of Sciences in 1968. In their biographical memoir of Gold for the National Academy of Sciences, the renowned English-born astronomers/astrophysicists Margaret Burbidge (1919–2020) and Geoffrey Burbidge (1925–2010) wrote that Gold was one of the most outstanding physicists of his time, with unmatched versatility (Burbidge & Burbidge, 2006:11). Other views on Gold’s contributions have been published at various times by contemporaries, and they gave conflicting perspectives. As examples, some perspectives are provided from Donald E. Wilhelms, Lawrence A. Taylor, and Bruce Hapke, individuals who worked in the area of lunar studies and had ample contacts with Thomas Gold.

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Wilhelms

In his book To a Rocky Moon, the lunar geologist of the U.S. Geological Survey Donald Wilhelms wrote of Gold: He never obtained his Ph.D. “ticket” that buys professional status. His status was enhanced, however, in 1948 when he enjoyed success as coformulator (with Fred Hoyle and Hermann Bondi) of the (now-unpopular) steady-state theory of the universe. In a paper published in 1955 the scientific world learned of another interesting idea of Gold’s that it would not soon forget. He favored the impact hypothesis for crater origin and realized that the differences in sharpness of upland craters were the result of erosion. From this impeccable starting point he concluded that the eroded material was just about right to constitute the maria. Small impacts and “electrostatic forces” rising from such otherworldly phenomena as solar radiation would loosen the dust and keep it moving until it settled down in the maria basins. The dust is darkened by radiation damage. His mathematics fit his ideas perfectly, of course, as mathematics can always be made to do. Gold clung tenaciously to his idea of oceans of lunar dust even after the Apollo missions had returned many kilograms of solid rock from the lunar maria. . . . His creative imagination was sometimes vindicated, as in 1968 when the astronomical establishment scornfully rejected his interpretation that the just-discovered pulsars are fast-spinning neutron stars, only to have the idea proved correct a few months later and gain a Nobel Prize for its discoverers. But the Gold-dust straw man cost the community of lunar scientists and engineers considerable time and money. (Wilhelms, 1993: 27)

6.4.2

Taylor

The idea that Gold’s speculation was costly to NASA and the lunar science community was voiced by another lunar geologist, Lawrence Taylor (1938–2017) of the University of Tennessee: Those of us who were around during the early Apollo days know well about the “Gold Dust Theory,” that cost NASA beaucoup dollars. And the electrostatic fluffiness of the lunar soil was not a problem to landing on the Moon, but may [have] contributed to the dust that was observed to cling to the astronauts’ suits, as well as to the “rock boxes” such that they all leaked. (Taylor, 2000:71)

6.4.3

Hapke

Bruce Hapke, a geologist who was a postdoctoral research associate colleague of Gold at Cornell and later a long-time faculty member at the University of Pittsburgh, gives a more balanced assessment. Hapke points out that before the Apollo landings, most lunar geologists did not think that dust covered the surface of the Moon: Gene Shoemaker was quoted in an article in National Geographic (circa 1963) as saying that the surface was covered with cobbles (fist-sized chunks of rock). I still have a copy of the cover of the Houston telephone directory (circa 1964), which was a NASA publicity photo

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showing a spacesuited astronaut walking on the NASA geologists’ best guess of the Lunar surface: volcanic ash consisting of centimeter-sized rocks. Following the Soviet Luna 9 landings, Gerard Kuiper, the preeminent planetary astronomer of his time, held a news conference in which he proclaimed that the surface was obviously volcanic ‘aa’ (rough scoriaceous) lava, adding that this would “tear an astronaut’s boots to shreds.” Even after the unmanned Surveyor landings on the moon, the NASA geologists continued to insist that the regolith was course-grained. ... The notion that the maria deposits were so unconsolidated that a spacecraft would sink out of sight in them was not a part of [Gold’s] original model, nor was it essential to it. Unfortunately, Gold had a penchant for the dramatic, and over-emphasized this possibility prior to the Apollo missions. This is the part that was not taken seriously by most persons. Even Gold considered it unlikely, as he admitted to me in private conversations. However, since he was a member of President Kennedy’s President’s Science Advisory Council, NASA was forced to take his suggestions seriously, much to the chagrin of some of the Astrogeologists. However, the deep dust model remained viable up until the Apollo landings because there was little direct evidence for volcanism in the pre-Apollo data. . . .It was only after petrologic examination of the Apollo 11 rocks that it became clear that the maria were volcanic flood basalts. Unfortunately, Gold was never able to accept this result. ,,, Why was there this personal animosity on the part of some NASA Astrogeologists towards Gold’s model . . . ? There appears to be two main reasons. First, one must realize that there were two groups of scientists investigating the moon in the decades before Apollo. One group consisted of persons with a background in astrophysics, who studied the moon by quantitative remote sensing methods. Their optical, thermal and radar data were all consistent with Gold’s model in its less extreme form. . . . By contrast, the other group was the NASA Astrogeology Branch put together by Shoemaker. These persons were trained in classical geology . . . . The two groups did not speak the same language and did not understand each other’s reasoning. Consequently the Astrogeologists tended to ignore the astrophysicists’ arguments. Second, Gold came from the British School of debating, in which opponents take great delight in sarcastically skewering each other with rapier wit. While this makes for great entertainment at scientific meetings, it does not enhance communication among persons holding opposing points of view who are not used to this debating style. The NASA Astrogeologists were often the butt of Gold’s sarcasm, and I think it got to the point that they either ignored or deprecated anything he said. . . . Finally. It should be emphasized that many aspects of Gold’s model are correct after all. The craters are of impact origin. While the maria did turn out to be lava flows, the flow surfaces are buried under several meters of dust. There are, indeed, erosional and depositional processes operating on the lunar surface, including electrostatic levitation, although these processes are not as effective as Gold hypothesized. And there is ample evidence for his predicted darkening process, which today is called “space weathering.” I think it is time for the devil to be given his due. (Hapke, 2006)

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Eddington’s Guidelines

As discussed in Chap. 1, Arthur Eddington (1882–1944), in his 1920 paper on the internal constitution of stars, identified criteria for productive scientific speculation. The three main guidelines that Eddington suggested, modified for consideration of topics in the space sciences, were: 1. Was the speculator rigorous in applying the appropriate science applicable to the model 2. Did the speculator identify all the underlying assumptions used in constructing the model and 3. Did the speculator view the model objectively, as an “adjustable engine,” as opposed to a “finished building?” One could argue that the dominant “appropriate science” for models of the lunar surface is geology. Regarding guideline 1, then, was Thomas Gold rigorous in applying the science of geology to his model? Gold certainly did apply some geological concepts as he developed his model. However, in reading the analyses of the Surveyor data by Gerard Kuiper, Gold and the others, there appears to be a significant difference in the breadth and depth of geological understanding between Kuiper and Gold. A significant part of the difference in understanding could have arisen by tensions between scientific groups with different language and different training (astrophysicists versus classical geologists), as Bruce Hapke wrote (Sect. 6.4.3). Regarding guideline 2, Hapke (Sect. 6.4.3) notes that Gold’s original model was logical and incorporated what was then known about the Moon (developed beginning 15 years before the Apollo 11 landing). Gold incorporated other assumptions as he further developed his model, mainly as he reacted to objections and criticisms of others. In the context of guideline 3, one might argue that by generating explanations “on the fly” in response to criticisms (for example that subsurface ice could be the cause of various surface features in the floors of basins and craters) Gold was “adjusting the levers” of his model (and not “finishing a house”). That is, Gold could be viewed as considering his model as a work in progress, “an engine” in Eddington’s terminology, the workings of which was his primary interest, rather than the construction of a “finished building.” However, as some have argued, Gold would never admit that he was wrong, or could be wrong. As noted above, overlaying this debate on the nature of the lunar surface was the tension between scientific groups with different training (astrophysicists versus geologists). In fact, this debate between the scientific groups has similarity to the tensions that existed (and still exist) between physicists and classical paleontologists in the debates over the cause of extinctions at the Cretaceous-Paleogene boundary (Chap. 7). In both cases, lunar dust and life extinctions, the protagonists often did not speak the same scientific “language” and brought quite different investigative techniques to their research endeavors.

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Eddington described two types of scientific speculators toward the end of the 1920 paper: Some will prefer to take the interpretation which seems to be most immediately indicated and at once adopt that as an hypothesis; others will rather seek to explore and classify the widest possibilities which are not definitely inconsistent with the facts. (Eddington, 1920:356)

Thomas Gold was definitely in the second of Eddington’s guidelines of scientific speculators. Indeed, in a memorial to Gold, his colleague British mathematician and cosmologist Hermann Bondi (1919–2005) wrote that “Tommy Gold will long be remembered as a singular scientist who stepped into any field where he thought an option was being overlooked” (Bondi, 2004).

6.6

Continuing Understanding

Interest in the moon, its origins, its interior and surface, and its role in the eventual human exploration of the solar system, has continued since the days of the Apollo program. In the more than five decades since humans have last stood on the moon and put their footprints in its dust, several nations have sent missions to circle it and to land robotic science packages on it. Of particular interest have been investigations from orbiting vehicles to determine whether water, especially in the form of ice, exists on or near the surface. Water in the form of ice has now been identified in permanently shaded areas in lunar polar regions. A number of missions (e.g., Clementine, Lunar Prospector, and Lunar Reconnaissance Orbiter (LRO) from the United States and Chandrayaan-1 from India) have returned data with suggestions of lunar water ice. Chandrayaan-1 was the first most definitive of these in 2009. These missions established the polar regions as main target areas of future instrument landings to understand the spatial and concentration extents of the ices. Infrared detection instrumentation used on the 2.7 m telescope on the NASA SOFIA aircraft reported in 2020 evidence of water on the sunlit lunar surface. This result is still in the process of additional confirmation. Given the objective of several nations to explore the moon, and land their citizens on its surface, it can be expected that gaining deeper scientific knowledge of the moon will continue to be a central area of research in the space science investigations of the solar system.

References Althans, C. L. (1839). Grundzüge zur Gänzlichen Umgestaltung der Bisherigen Geologie, oder Kurze Darstellung der Weltkörper– und Erdrindenbildung. Karl Bädeker. Baldwin, R. B. (1942). The meteoritic origin of lunar craters, with plate III. Popular Astronomy, 50, 356.

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Bondi, H. (2004). Thomas Gold (1920–2004). Nature, 430(6998), 415–415. Burbidge, G., & Burbidge, M. (2006). Thomas Gold 1920 - 2004, National Academy of Sciences, Washington, D.C. Cummings, W. D. (2019). Evolving theories on the origin of the Moon. Springer. Dana, J. D. (1846). ART. XXX. --On the Volcanoes of the Moon. American Journal of Science and Arts (1820-1879), 2(6), 335. DeVorkin, D. (1983). Transcript: AIP-sponsored interview of Thomas Gold. https://www.aip.org/ history-programs/niels-bohr-library/oral-histories/28197 Dietz, R. S. (1946). The meteoritic impact origin of the Moon’s surface features. Journal of Geology, 54(6), 359–375. Eddington, A. S. (1920). The internal constitution of the stars. The Observatory, 43(557), 341–358. Fauth, P. (1909). The Moon in modern astronomy: A summary of twenty years selenographic work, and a study of recent problems (Transl. Joseph McCabe). A. Owen. Gilbert, G. K. (1893). The Moon’s face: A study of the origin of its features. Philosophical Society of Washington. Gold, T. (1955). The lunar surface. Monthly Notices of the Royal Astronomical Society, 115(6), 585–604. Gold, T. (1959). Dust on the moon. Vistas in Astronautics, 2, 261–266. Gold, T. (1962). Processes on the lunar surface. The Moon, 14, 4340. Gold, T. (1964). Ranger moon pictures: implications. Science, 145(3636), 1046–1048. https://doi. org/10.1126/science.145.3636.1045 Gold, T. (1965). The moon’s surface. In The Nature of the Lunar Surface (p. 107). Gold, T. (1971). Evolution of Mare surface. NASA Technical Report. Gold, T., & Hapke, B. W. (1966). Luna 9 pictures: implications. Science, 153(3733), 290–293. Gruithuisen, F. V. P. (1825). Gedanken und Ansichten über die Ursachen der Erdbeben nach der Aggregations Theorie der Erde. J. L. Schrag. Hapke, B. (2006). Commentary on the history of the fairy castles. https://www.hq.nasa.gov/alsj/ Fcastles.htm Heiken, G. H., Vaniman, D. T., & French, B. M. (1991). Lunar Sourcebook: A user’s guide to the Moon (753 p). Research supported by NASA. Cambridge University Press. No individual items are abstracted in this volume. Hershel, W., & Banks, J. (1787). An account of three volcanoes in the Moon. Philosophical Transactions of the Royal Society of London, 77, 229–232. Koppes, S. (1995). Memorial for Robert Sinclair Dietz. Meteoritics, 30, 474. Kuiper, G. P. (1959). The exploration of the moon. Vistas in Astronautics, 2, 273–313. Marvin, U. B. (2003). Oral histories in meteorites and planetary science: X. Ralph B. Baldwin. Meteoritics and Planetary Science, 38(S7), A163–A175. Meydenbauer, A. (1877). Über die bildung der Mondoberfläche. Sirius, 10(N.F. 5), 180. Nasmyth, J., & Carpenter, J. (1903). The Moon, considered as a planet, a world, and a satellite (4th ed.). John Murray. Pettit, E., & Nicholson, S. B. (1930). Lunar radiation and temperatures. The Astrophysical Journal, 71, 102–135. Rennilson, J. J., & Criswell, D. R. (1974). Surveyor observations of lunar horizon-glow. The Moon, 10(2), 121–142. Surveyor Program Results. (1969). NASA SP 184. National Aeronautics and Space Administration. Schmitt, H. H. (2006). Return to the Moon: Exploration, enterprise, and energy in the human settlement of space. Copernicus books. Shoemaker, E. M. (1965). Preliminary analysis of the fine structure of the lunar surface in Mare Cognitum. In The Nature of the Lunar Surface (p. 23). Taylor, L. A. (2000). The lunar dust problem: A possible remedy. Space Resources Roundtable II, 1070, 71. Thiersch, H. W. J., & Thiersch, A. (1879). Die Physiognomie des Mondes. C. H. Bech’sche Buchhandlung.

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Urey, H. C. (1955). Some criticisms of “On the origin of the lunar surface features” by G. P. Kuiper. Proceedings of the National Academy of Sciences of the United States of America, 41(7), 423. Urey, H. C. (1956). The Moon’s surface features. The Observatory, 76, 232–235. Urey, H. C. (1965). Observations on the Ranger photographs. In The Nature of the Lunar Surface (p. 3). Wagner, S. (2006). The Apollo experience lessons learned for constellation lunar dust management. NASA/TP-2006-213726. NASA Johnson Space Center. Whipple, F. L. (1959). On the lunar dust layer. Vistas in Astronautics, 2, 267–272. Whitaker, E. A. (1965). The surface of the Moon. In The Nature of the Lunar Surface (p. 79). Wilhelms, D. E. (1993). To a Rocky Moon: A geologist’s history of lunar exploration (497 p). University of Arizona Press.

Chapter 7

Did the Chicxulub Impact Cause the Cretaceous Extinctions?

7.1

Introduction

The Cretaceous-Tertiary (K-T or sometimes K/T or KT), now called CretaceousPaleogene, K-Pg) extinction event of some 66 million years ago in Earth’s history is popularly known as the event that marked the end of dinosaurs. In fact, it is commonly understood that the event also marked the mass extinction of the order of three quarters of the plant and animal species on Earth at the time. The Cretaceous era, from about 145 to 66 million years ago was a markedly warm period in Earth’s history, with the polar regions devoid of ice. Geological studies show that the end of the Cretaceous was world-wide. Dinosaurs thrived in the Cretaceous but went extinct at its end. In 1979, the paleobiologist Dale A. Russell (1937–2019) reviewed, as he called it, “the enigma of the extinction of the dinosaurs” (Russell, 1979). He discussed nine theoretical models for the sudden disappearance of the dinosaurs at the end of the Cretaceous. Several of those models involved astrophysical processes as known at the time that could affect Earth and its atmosphere. One model involved the “collisions of comets and large meteorites” with Earth. As discussed below, there is support in the geologic record for this collision hypothesis, namely a thin layer of sediment dating to about 66 million years that is found in both terrestrial and ocean rocks. A high level of iridium is found in the layer everywhere. Since iridium is more common in asteroids than in Earth’s crust, many scientists have attributed the extinction event to the impact of a large asteroid or comet on Earth some 66 million years ago. The discovery in the 1990s of a large crater (now called the Chicxulub crater), centered near the boundary of the Yucatán peninsula and the Gulf of Mexico and dated to approximately the right time provided support to the asteroid/comet hypothesis for the extinction. Nevertheless, there was considerable, and extensive, debate about the role of an extraterrestrial body in the Cretaceous extinction event. Other hypotheses for the extinction have been proposed and examined, including volcanism from the large © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 W. D. Cummings, L. J. Lanzerotti, Scientific Debates in Space Science, Astronomy and Planetary Sciences, https://doi.org/10.1007/978-3-031-41598-2_7

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Deccan Traps in India, and possibly climate change. This chapter discusses the scientific speculations and debates about the extinction event. The very intense and high-level intellectual debates and discussions importantly involved leading paleontologists, physicists, and geophysicists, all experts in their fields.

7.2

The Speculation of Luis and Walter Alvarez and Their Colleagues

In 1980, Luis Walter Alvarez (1911–1988) (Fig. 7.1—left), a Nobel-laureate physicist at the University of California, Berkeley, and the Lawrence Berkeley Laboratory, his geologist son Walter Alvarez (Fig. 7.1—right), nuclear chemist Frank Asaro (1927–2014), and paleontologist Helen Vaughn Michel published an article in Science magazine titled “Extraterrestrial Cause for the Cretaceous-Tertiary Extinction” (Alvarez et al., 1980). The group reported on their finding large increases of the element iridium in deep-sea sediments that were exposed in Italy, Denmark, and New Zealand (see Fig. 7.1). The iridium was deposited in a thin clay layer that separated two distinct geological periods, the Cretaceous and the Paleogene. The Alvarez group proposed that the iridium came from an asteroid that impacted the Earth 65 million years ago, which is the time that marked the end of the Cretaceous in geological history. They estimated the diameter of the asteroid at Fig. 7.1 Luis and Walter Alvarez showing the K-T boundary layer near Gubbio, Italy. Credit U.S. Department of Energy

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The Controversy over the Speculation

165

about 10 km and that the impact would have injected about 60 times the asteroid’s mass into the Earth’s stratosphere, where it would be distributed worldwide and remain for several years. They proposed that the resulting darkness would suppress photosynthesis and cause the extinctions recorded in the geological record. As noted by geologist David A. Kring (more about him below): This [Cretaceous–Paleogene] boundary corresponds to one of the greatest mass extinctions in Earth’s history. At least 75 percent of the species on our planet, both in the seas and on the continents, were extinguished forever. The most famous of the vanquished are the dinosaurs. However, these giants were only a small fraction of the plants and animals that disappeared. In the oceans, more than 90 percent of the plankton was extinguished, which inevitably led to the collapse of the oceanic food chain. (Kring, 2020)

Interestingly, some 7 years prior to the Alvarez et al. publication, Harold C. Urey (1893–1981) (Chaps. 5 and 6), in the context of the production of tektites found on Earth, had discussed the environmental and other impacts resulting from a comet colliding with Earth (Urey, 1973). From his quantitative analysis, he wrote: . . .a very great variation in climatic conditions covering the entire Earth should occur and very violent physical effects should occur . . . .I suggest that the termination of a geological period would result and a new one would begin. (Urey, 1973:32)

He further commented that: . . .it does seem possible and even probable that a comet collision with the Earth destroyed the dinosaurs and initiated the Tertiary division of time. (Urey, 1973:32)

Urey’s speculations appear to have had little, if any, follow up in the years prior to the iridium discoveries and the asteroid (not comet) hypothesis for the end of the Cretaceous.

7.3 7.3.1

The Controversy over the Speculation The First Snowbird Conference

The speculation by the Alvarez team caused enormous interest and immediate controversy. To channel the discussion, the Lunar and Planetary Institute (LPI) organized and sponsored a topical conference titled The Conference on Large Body Impacts and Terrestrial Evolution: Geological, Climatological, and Biological Implications. The conference was co-sponsored by the U. S. National Academy of Sciences and held in Snowbird, Utah, from 19 to 22 October 1981. This was the first “Snowbird Conference,” as subsequent conferences on this topic area came to be called. Among the more than 60 papers presented at the 4-day meeting was a paper by the Alvarez group. The majority of papers given at the conference aimed to further explore the idea that had been put forward by Luis and Walter Alvarez and their colleagues.

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Fig. 7.2 Thomas Schopf. Credit University of Chicago Photographic Archive, apf10982, Hanna Holborn Gray Special Collections Research Center, University of Chicago Library

One was not. The abstract of the paper by paleontologist Thomas J. M. Schopf (1939–1984) (Fig. 7.2) of the University of Chicago read in part: The first step in any scientific program is to determine the problem to be solved. The often popular view that thousands of species of dinosaurs went extinct in the space of a year or two, worldwide, is not true. The firm evidence is that during the last 2 to 3 m.y. of the latest Cretaceous . . . a total of about 16 species . . . which had been living along the margins of a large seaway (which once extended from the Gulf of Mexico to the Arctic Circle) died off as the seaway dried up. Elsewhere in the world, local populations of dinosaurs had evidently died out before the latest Cretaceous both in Mongolia and in southern Europe. Possibly a species persisted in northern Europe into the latest Cretaceous. Seen in this light, the extinction of the dinosaurs is a perfectly understandable phenomenon—indeed no different than the fate of millions and millions of previous species. The reason why the extinction of the dinosaurs has attracted so much non-scientific attention by scientists and others is that (1) it doesn’t cost anyone anything, (2) it sounds impressive, (3) it’s basically a rather unimportant scientific problem though a rather important popular problem, and (4) hard paleontological data are difficult to obtain. (Schopf, 1981:49)

The seaway referenced by Schopf is called the Western Interior Seaway (shown in Fig. 7.3 and absent in Fig. 7.4), and Schopf’s views were well established in the field of paleobiology. In 1975, for example, the American paleontologist and worldrenowned dinosaur expert Robert T. Bakker (Fig. 7.5), then with the University of Colorado Museum, had written in a Scientific American article that the likely reason for the extinction of the dinosaurs: . . . is the draining of shallow seas on the continents and a lull in mountain-building activity in most parts of the world, which would have produced vast stretches of monotonous typography. Such geological events decrease the variety of habitats that are available to land animals, and thus increase competition. They can also cause the collapse of intricate, highly evolved ecosystems; the larger animals seem to be the more affected. At the end of the Permian similar changes had been accompanied by catastrophic extinctions among therapsids and other land groups. Now, at the end of the Cretaceous, it was the dinosaurs that suffered catastrophe; the mammals and birds, perhaps because they were so much smaller, found places for themselves in the changing landscape and survived. (Bakker, 1975:77)

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Fig. 7.3 The Earth’s surface about 95 million years ago. Credit Professor Chris Scotese (2021)

Fig. 7.4 The Earth’s surface about 65 million years ago. Credit Professor Chris Scotese (2021)

Schopf’s skepticism about the impact hypothesis as the cause of the extinctions at the end of the Cretaceous was shared by other paleobiologists. In a 1981 paper titled, Out with a Whimper, Not a Bang, William A. Clemens Jr. (1932–2020), J. David Archibald, and Leo J. Hickey (1940–2013) pointed out that: . . . the global pattern of relatively few extinctions in the tropics with increasing frequency of exterminations to the north is just the reverse of what would be expected [under the Alvarez hypothesis]. Dormancy and carry-over mechanisms evolved in response to climatic stress and are assumed to have been, then as now, less well developed in the tropics. In addition, plants eliminated from northern floras are those of more tropical affinity, like palms. . . . This multiplicity of patterns of extinction strongly argues against any hypothesis invoking some kind of catastrophic, short, sharp shock as the causal factor of the terminal Cretaceous

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Fig. 7.5 Robert Bakker, shown working with a student at a paleo site near Seymour, Texas, in 2019. Credit Michael Rathke, Houston Museum of Natural Science

extinctions. These paleobiological data suggest the Cretaceous-Tertiary transition was a period of several tens of thousands if not hundreds of thousands of years in duration, characterized by interaction of a complex of physical and biological factors producing a high net rate of decrease in biotic diversity within both the terrestrial and marine biotas. (Clemens et al., 1981:296–297)

This was just the beginning of widespread resistance to the ideas of the Alvarez group by paleontologists. In 2002, David B. Weinreb of Yale University wrote a review article in the Journal of Young Investigators (a peer reviewed journal targeted to undergraduate research and researchers) about historical changes in the theories of extinction and evolution. At the 1985 meeting of the Society of Vertebrate Paleontologists, a survey conducted by The New York Times revealed that a meager 4% of paleontologists were convinced that a meteorite impact had brought about the mass extinction, although 90% were receptive to the theory that an impact had indeed occurred at the K-T boundary. In short, paleontologists were convinced by the Alvarez report that an impact had occurred, but they doubted the link between the meteorite crash and the global extinction. (Weinreb, 2002:6–7)

Weinreb quoted the harsh judgement of Luis Alvarez of the quality of science produced by paleontologists. I don’t like to say bad things about paleontologists, but they’re really not very good scientists. They’re more like stamp collectors. . . I can say these things about some of our opponents because this is my last hurrah, and I have to tell the truth. I don’t want to hold these guys up to too much scorn. But, they deserve some scorn because they’re publishing scientific nonsense. (Weinreb, 2002:6–7)

Weinreb also quoted the contrary view of Robert Bakker: “The arrogance of these people is simply unbelievable. They know next to nothing about how real animals evolve, live, and become extinct. But, despite their ignorance, the geochemists feel that all you have to do is crank up some fancy machine and you’ve revolutionized science. The real reasons for the dinosaur extinctions have to do with temperature and sea level changes, the spread of diseases by migration and other complex events. In effect, they’re saying this: we high-tech

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people have all the answers, and you paleontologists are just primitive rockhounds.”. (Bakker as quoted by Weinreb, 2002:6–7) Bakker’s charge of arrogance may have been overblown, but it was true that many geochemists were applying new technologies that had been developed for the analyses of lunar samples.

7.3.2

The Search for Evidence of the Crater

Meanwhile at the Lunar and Planetary Institute, scientists were keeping their heads down as the controversy raged, continuing their research on crater formation, conducting workshops and conferences, and training new planetary scientists. The 1983 class of LPI Summer Interns (Fig. 7.6) contained an undergraduate from Indiana University, David Kring, who would become involved in investigations that led to the identification of a crater that was caused by the kind of impact the Alvarez group had envisaged. Kring obtained his PhD at Harvard University and then joined the staff at the University of Arizona, where he worked on K-Pg impact research, among other topics. In 2006, Kring returned to the LPI as a scientist.

Fig. 7.6 Summer Interns at the LPI for 1983. David Kring is standing in the background, third from left. Credit the LPI

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Fig. 7.7 Glen Penfield, analyzing data during the PEMEX project. Credit Glen Penfield

Completely independent of the discussions revolving around the implications of the Alvarez group, oil exploration was in progress by the Mexican national oil company, Petróleos Mexicanos (PEMEX) in southern Mexico. Geophysicist Glen T. Penfield (Fig. 7.7) worked for a company called Aero Services that had been hired by PEMEX to conduct surveys of the magnetic and gravitational fields related to potential drilling sites. Antonio Camargo-Zanoguera was a project manager for PEMEX, and the two of them gave a paper at the 51st annual meeting of the Society of Exploration Geophysicists (SEG), held in Los Angeles on 11–15 October 1981. They described finding a large, buried, circular structure on the northwestern margin of the Yucatán peninsula of Mexico. The report of the discovery of this crater site at the SEG meeting was made the weekend before the first Snowbird Conference was held and was unknown at Snowbird: A recent survey collected approximately 50,000 km of high sensitivity aeromagnetic data at 500 m altitude over the Campeche bank and Yucatan platform. In conjunction with gravimetric studies and data from three Pemex wells, this survey indicated the presence of two concentric zones of igneous material beneath the central Yucatan platform. The central zone, characterized by numerous high amplitude (approaching 1000 γ), short wavelength magnetic anomalies, and a gravity high, has a diameter of approximately 60 km and is centered near the town of Progreso on the northern Yucatan coast. Well data indicates the presence of massive andesites [a type of volcanic rock] of Jurassic or Cretaceous age. Modeling the magnetic and gravity data indicates in excess of 3 km of high-density highly magnetic material in this central zone. The depth to the top of the zone is on the order of 1100 m below the ground surface. Concentric with this area is an outer zone characterized by low amplitude (5 to 20 γ), short wavelength magnetic anomalies, and a gravity low surrounded by a weak gravity high. This outer zone is approximately 200 km in diameter, and the well data suggest the presence of intercalated volcanics and limestones. (Penfield & Camargo, 1982:448–449)

In their abstract, among the possible explanations for the structure, Penfield and Camargo listed “a mid-plate igneous plume, or astrobleme,” the latter being the scar left on the surface of the Earth by the impact of an asteroid or comet. As noted above, Penfield and Camargo were exploring for oil deposits when they carried out their survey and subsequent analysis of the PEMEX data. Such hydrocarbon exploration was the primary interest of the members of the SEG, and their

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Fig. 7.8 Carlos Byars. Credit Deborah Leibham

meeting was not attended by geologists and geophysicists who were involved in the exploration of the Moon and planets of the solar system. Thus, the initial discovery of what came to be called the Chicxulub crater was unknown to lunar and planetary scientists for the next 10 years, though the news from the SEG presentation was published in the Houston Chronicle, a major newspaper read in the area around the Lunar and Planetary Institute. A few months after Penfield and Camargo presented their paper at the meeting of the Society of Exploration Geophysicists, Carlos Byars (1936–2016) (Fig. 7.8), a journalist for the Houston Chronicle, reported on the Penfield-Camargo discovery in an article published in the Sunday edition of the paper (13 December 1981) with the headline “Mexican site may be link to dinosaur’s disappearance.” (Verschuur, 1997:23). The radio astronomer and author Gerrit L. Verschuur explained how Byars came to know about the buried crater in the Yucatán peninsula. Byars had been working as a writer in the oil industry and knew Penfield. Once, when Byars was visiting his office, Penfield showed him the gravity and magnetic maps that revealed the buried crater. Byars soon after got a job with the Houston Chronicle, where he published his article (Verschuur, 1997:23). Though the article was apparently not known or acted upon by researchers who later searched for the crater, as discussed below Byars would play an important role in connecting these researchers with Penfield and Camargo. In the fall of 1984, the LPI held a conference in Kona, Hawaii, titled the Origin of the Moon, and the Institute subsequently published a book with the same title. As discussed in Chap. 5, an outcome of this conference was a near-consensus view by the lunar and planetary research community that the Moon was created by a collision of a Mars-size planetary body with the Earth very early in the history of the Earth. The presentations and discussions at the conference helped to open the minds of researchers to the inevitability of cosmic impacts on Earth. In a paper for the

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Fig. 7.9 Joanne Bourgeois. Credit Joanne Bourgeois, University of Washington

conference, planetary scientist and space artist William K. Hartmann of the University of Arizona wrote: An example of the problem of class-predictable events in planetary science is the probable Cretaceous-ending asteroid impact. Since the 1960s, asteroid statistics have implied such events every few 107–108 years, but we could not convincingly tie specific geologic effects to specific impacts. In the absence of such evidence, impacts of this size tended to be ignored; as scientists, we should have pursued the geologic and climatic consequences of these class-predictable events instead of waiting for iridium-rich layers to take us by surprise. (Hartmann, 1986:587–588)

Ever since the announcement of the impact hypothesis by the Alvarez group, many research groups had been searching for the impact site. In the same era, other research groups were looking for or studying large volcanic sources (such as, especially, the Deccan Traps in India) that might explain the extinctions at the end of the Cretaceous. In the spring of 1987, the LPI held its 18th annual Lunar and Planetary Science Conference in Houston. Reports of some of the searches for evidence of the impact crater, and studies, were given at the conference. Alan R. Hildebrand and William V. Boynton of the Lunar and Planetary Laboratory of the University of Arizona reported on their analysis of rare-earth-element abundances in the CretaceousPaleogene impact fall-out layers at various sites around the globe. They suggested that the impact site was in the eastern Pacific Ocean basin (Hildebrand & Boynton, 1987:427–428). A year later, however, a group of researchers led by geophysicist Joanne Bourgeois (Fig. 7.9) of the University of Washington discovered an enormous tsunami deposit at sites near the Brazos River in Texas. The deposits were dated at the end of the Cretaceous period (see Fig. 7.10). The authors wrote: Conditions for depositing such a sandstone layer at these depths are most consistent with the occurrence of a tsunami about 50 to 100 meters high. The most likely source for such a tsunami at the Cretaceous-Tertiary boundary is a bolide-water impact. (Bourgeois et al., 1988:567)

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Fig. 7.10 From Fig. 1 in Bourgeois et al. (1988). Closed circles represent sites with coarse-grained deposits at the K-Pg boundary. Open circles represent sites where there is a discontinuity at the K-Pg boundary, presumably a gap in the sediment record caused by tsunami erosion

7.3.3

The Second Snowbird Conference

In the fall of 1988, the LPI co-sponsored with the U. S. National Academy of Sciences the second Snowbird Conference, which had the title Global Catastrophes in Earth History: An Interdisciplinary Conference on Impacts, Volcanism, and Mass Mortality. Various possible mechanisms for the end of the Cretaceous period were discussed at the conference. These end-of-Cretaceous-period mechanisms included the gradual transition favored by most paleontologists, as well as more catastrophic causes such as Earth impacts by asteroids or comets or large-scale volcanism episodes. Among the 60 talks and 67 posters given at the conference was a paper by Peter W. Francis (1944–1999), a visiting scientist at the LPI and Kevin C. A. Burke (1929–2018), who was the Director of the Institute. These LPI researchers discounted volcanism as a cause unless large episodes might have triggered ocean current circulation patterns that would cause global climatic changes. Paleontologists J. David Archibald and Laurie J. Bryant, both of San Diego State University and the Museum of Paleontology at the University of California, Berkeley, reported on their examination of the vertebrate record, concluding: The extinction patterns among the vertebrates do not appear to be attributable to any single cause, catastrophic or otherwise. The earliest Paleocene fauna can be understood as a Late Cretaceous fauna simply altered by withdrawal of the Western Interior Sea and by the formation of extensive swamps that replaced well-drained terrestrial environments. (Archibald & Bryant, 1988:5)

Luiz Alvarez died on 1 September 1988, more than a month before the second Snowbird Conference. Walter Alvarez gave the team’s presentation in which they wavered a bit about the certainty of an asteroid impact as the sole cause of the end of

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the Cretaceous period. This is perhaps because they did not yet know of a suitable impact crater to support their hypothesis. In a paper at the conference, Alan Hildebrand and William Boynton continued to favor an oceanic impact for an asteroid or comet, but now argued that the putative impact occurred in the ocean near North America: All available evidence is consistent with an impact into oceanic crust terminating the Cretaceous Period. Although much of this evidence is incompatible with endogenic origin, some investigators still feel that a volcanic origin is possible for the K/T boundary clay layers. Following the dictum that remarkable hypotheses require extraordinary proof this latter view may still be reasonable, especially since the commonly cited evidence for a large impact stems from delicate clay layers and their components (i.e., no catastrophic deposits), and the impact site has not yet been found. Impact sites have been suggested all over the globe, but are generally incompatible with known characteristics of the boundary clay layers. We feel the impact is constrained to have occurred near North America by: the occurrence of a 2 cm thick ejecta layer only at North America locales, the global variation of shocked quartz grain sizes peaking in North America . . . and possibly uniquely severe plant extinctions in the North American region. Also the ejecta layer may thicken from north to south. . . . A new constraint on the impact location comes in the form of impact wave deposits; giant waves are a widely predicted consequence of an oceanic impact. Impact wave deposits have not been found elsewhere on the globe, suggesting the impact occurred between North and South America. (Hildebrand & Boynton, 1988:76)

In a paper for the follow-on book for the second Snowbird Conference, a group of researchers from the LPI, led by Virgil L. (Buck) Sharpton, argued against a single oceanic impact: Understanding the crustal signature of impact ejecta contained in the Cretaceous Tertiary (K/T) boundary layer is crucial to constraining the possible site(s) of the postulated K/T impact event. The relatively unaltered clastic constituents of the boundary layer at widely separated outcrops within the Western Interior of North America are not compatible with a single oceanic impact but require instead an impact site on a continent or continental margin. On the other hand, chemical compositions of highly altered K/T boundary layer components in some marine sections have suggested to others an impact into oceanic crust. We suspect that post-depositional alteration within the marine setting accounts for this apparent oceanic affinity. If, however, this is not the case, multiple simultaneous impacts, striking continent as well as ocean floor, would seem to be required. (Sharpton et al., 1990:394)

7.3.4

The “Discovery” of the Chicxulub Crater

Gerrit Verschuur reports the following sequence of events that ultimately led to the discovery of the site of the impact of the asteroid that occurred at the end of the Cretaceous. While the search for the impact crater was ongoing during the 1980s: Carlos Byars, the Houston Chronicle journalist, began attending the lunar and planetary science conferences held at the Johnson Spacecraft Center on an annual basis. He had been doing stories about the K/T impact event and extinction of species and had heard many presentations about potential candidate craters that were supposed to mark where the asteroid or comet had slammed into the earth 65 million years ago. At every conference

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Fig. 7.11 Alan Hildebrand at a Brazos River site, pointing to the K-Pg boundary. Credit Alan Hildebrand

he buttonholed some of the scientists and pointed out that there was a crater not on their list that merited a closer look. Every year he was apparently ignored. “They regarded me as a good, competent science writer, but no expert on craters,” admits Byars. (Verschuur, 1997:25–26)

In early 1990, Alan Hildebrand (Fig. 7.11), then a graduate student at the University of Arizona, met Carlos Byars and listened to his story (Verschuur, 1997:26). Hildebrand soon connected with Glen Penfield, then at the Aerogravity Division of Carson Services. Thus, by the spring of 1990, at least some members of the planetary science community had become aware of the 1981 paper by Penfield and Camargo at the meeting of the Society of Exploration Geophysicists, with its evidence for a buried impact structure on the edge of the Yucatán peninsula. Alan Hildebrand and William Boynton mention the Penfield and Camargo paper in their Science article of 18 May 1990, which reported an ~50 cm-thick ejection layer on the southern peninsula of Haiti (Hildebrand & Boynton, 1990:843). In their Science paper, Hildebrand and Boynton argued that their “trace element, isotopic, and mineralogic studies indicate that the proposed impact at the Cretaceous-Tertiary (K/T) boundary occurred in an ocean basin, although a minor component of continental material is required” (Hildebrand & Boynton, 1990:843). They favored an ~300-km-diamaeter candidate structure in the Colombian Basin of what is now the Caribbean Sea, though they cited the Penfield-Camargo buried impact structure on the Yucatán peninsula as a possible alternative. The following year, David Kring and William Boynton, both of the University of Arizona, reported finding glass spherules produced by impact-induced shock

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melting in the K/Pg boundary sediments of Haiti (Kring & Boynton, 1991). From their analysis of the samples, they concluded that, The composition of the glass can best be reconciled with a continental margin terrane, consistent with studies of shocked mineral phases reported elsewhere. The thickness of the deposit in which the impact spherules occur indicates the source of the ejecta was in the proto-Caribbean region. (Kring & Boynton, 1991:1737)

Kring and Boynton favored the Yucatán site as the location of the giant impact crater at the end of the Cretaceous era (Kring & Boynton, 1991:1741). After Alan Hildebrand connected with Glen Penfield, the two of them, together with David Kring, Mark Pilkington (Geological Survey of Canada), Antonio Camargo-Zanoguera (PEMEX), Stein B. Jacobsen (Harvard), and William Boynton collaborated. They collected the magnetic and gravity-field data, some of the core samples from previous PEMEX drillings, and data from analysis of ejecta from sites around the Caribbean. At the 1991 spring meeting of the Lunar and Planetary Science Conference, Kring reported on their analysis of two core samples retrieved from a PEMEX well that was offset from the center of the Chicxulub crater by about 50 km. The presence of shocked quartz crystals in one of the samples confirmed the impact nature of the Chicxulub crater (Kring et al., 1991). In the fall of 1991, the entire team published their results in a paper in the journal Geology titled Chicxulub Crater: A possible Cretaceous/Tertiary boundary impact crater on the Yucatán Peninsula, Mexico. In the paper, they concluded: The Chicxulub crater is the largest probable impact crater on Earth. Its position and targetrock composition satisfy many of the characteristics required for the K/T crater, and it may have a K/T boundary age. This impact may have caused the K/T extinctions. (Hildebrand et al., 1991:871)

While Hildebrand, Kring and their colleagues found shocked quartz grains in Chicxulub rocks, indicative of an impact, they were unable to precisely date the Chicxulub crater. The following year, however, a team led by Buck Sharpton that included Graham Ryder (1949–2002) and Benjamin C. Schuraytz of the LPI, Brent Dalrymple of the U. S. Geological Survey, and Luis Marín and Jaime UrrutiaFucugauchi of the Universidad Nacional Autónoma de Mexico (UNAM) were able to determine the age of some of the PEMEX melt-rock core samples as 65.2 ± 0.4 million years at the 95% confidence level (Sharpton et al., 1992:820–821). The Sharpton team measured enhanced concentrations of iridium in some of the sections of the core samples. For many researchers, this age determination, coupled with an independent and identical result the same year by Paleontologist Carl C. Swisher III of Rutgers University (Swisher et al., 1992) ended any remaining uncertainty about whether or not the buried Chicxulub structure (see Fig. 7.12) was the long-sought impact crater that had been postulated by the Alvarez group in 1980 from their global iridium studies. Sharpton and Paul D. Spudis (1952–2018) at the LPI, and their colleagues at the University of Houston, UNAM, and PEMEX continued to study the Chicxulub structure through a cooperative research agreement between the LPI and the Instituto de Geofisica of UNAM. Building on their accumulated knowledge of lunar and

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Fig. 7.12 Chicxulub Crater from Fig. 2 B of Sharpton et al. (1993)

Fig. 7.13 Schrödinger Crater on the Moon. Credit NASA and the LPI

planetary crater structures (see, for example, Fig. 7.13) over many years, Sharpton and his colleagues published an article in Science magazine in the fall of 1993 with the following abstract: The buried Chicxulub impact structure in Mexico, which is linked to the Cretaceous-Tertiary (K-T) boundary layer, may be significantly larger than previously suspected. Reprocessed gravity data over Northern Yucatán reveal three major rings and parts of a fourth ring, spaced similarly to those observed at multi-ring basins on other planets. The outer ring, probably corresponding to the basin’s topographic rim, is almost 300 kilometers in diameter,

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indicating that Chicxulub may be one of the largest impact structures produced in the inner solar system since the period of early bombardment ended nearly 4 billion years ago. (Sharpton et al., 1993:1564)

7.4

Continuation of the Controversy

The impact site had been located and its structure continued to be examined by additional drillings. Some of these were conducted through the collaborations between the LPI and UNAM. However, the debate persisted as to whether the impact caused the extinctions at the end of the Cretaceous. In 1994, the LPI sponsored the third Snowbird Conference, which was actually held in Houston, Texas, and titled New Developments Regarding the K/T Event and Other Catastrophes in Earth History. The follow-on publication was titled The Cretaceous-Tertiary Event and Other Catastrophes in Earth History. At the conference, the impact data were well presented by various research groups. The dissenters, while admitting the reality of the impact, were reaching other conclusions about the K-Pg extinctions. For example, paleontologist J. David Archibald wrote in his abstract, “Single-cause theories of extinction, such as a bolide impact and its corollaries, fail to explain the pattern of vertebrate extinctions at the KT boundary” (Archibald, 1994:7). In their abstract, paleontologists Jose Guadalupe Lopez-Oliva and Gerta Keller, both of Princeton University, stated, “Our study indicates that the biotic effects of the KT boundary event on planktic foraminifera in the northeastern Mexico sections were not as catastrophic as predicted from a large bolide impact on Yucatan” (Lopez-Oliva & Keller, 1994:73). In a separate paper, Keller (Fig. 7.14) argued: Fig. 7.14 Gerta Keller. Credit Gerta Keller, Princeton University

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Conclusions and Reflections

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One of the most important recent developments in KT boundary studies is the growing awareness that (1) the mass extinction associated with this event is not the result of a single catastrophe, (2) that extinctions occurred over an extended time period and were selective rather than random within organismal groups as well as between different groups, and (3) that the biotic effects were most severe and sometimes limited to tropical-subtropical regions while high-latitude faunas and floras escaped virtually unscathed. (Keller, 1994:57)

The LPI’s Graham Ryder wrote an article for the follow-on book that responded to the conclusions made by many of the paleontologists: Many counter-revolutionary papers (i.e., those that deny an impact cause) over the last decade . . . give the impression that it is those who invoke an impact who have required a particular paleontological significance, for instance that impact proponents claim abrupt extinction. Yet it was never the case that an impact was inferred and that then there was a search for associated extinctions. It is an ironic reversal that some paleontologists chose to reduce the significance of the boundary after the impact was inferred. Rather than evaluate the record in the light of an impact, they chose to construct straw men. (Ryder, 1996:35)

The LPI co-sponsored the fourth Snowbird Conference, which was held in Vienna, Austria, in 2000. It was titled Catastrophic Events and Mass Extinctions: Impacts and Beyond. There was continued discussion of the Chicxulub impact at the conference, but the main focus was on whether or how short-term, high-energy impacts influence biological evolution on Earth.

7.5

Conclusions and Reflections

In 2010, 40 senior researchers from 12 countries joined in the publication of a review article for Science magazine titled The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary (Schulte et al., 2010). Following their world-wide, exhaustive research, the authors concluded: The Cretaceous-Paleogene boundary ~65.5 million years ago marks one of the three largest mass extinctions in the past 500 million years. The extinction event coincided with a large asteroid impact at Chicxulub, Mexico, and occurred within the time of Deccan flood basalt volcanism in India. Here, we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction. Notably, a single ejectarich deposit compositionally linked to the Chicxulub impact is globally distributed at the Cretaceous-Paleogene boundary. The temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations (for example, darkness and cooling) lead us to conclude that the Chicxulub impact triggered the mass extinction. (Schulte et al., 2010:1214)

Two decades earlier, Walter Alvarez had reflected on the significance of the research topic that he and his father and others at Berkeley had started: . . . this extraordinary event has led to new kinds of thinking in every branch of science it has touched. In biology, it required thinking about non-Darwinian mechanisms of evolution. In geology, it forced a reevaluation of the central geological doctrine of “uniformitarianism” or “gradualism,” which for 150 years had discouraged any thinking about catastrophic events. In chemistry, it focused on iridium, an almost comically obscure element, and created a

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demand for very fast analytical capabilities at the parts-per-trillion level. And new problems have been opened up in ecology, geophysics, astrophysics and atmospheric science, as well. (Alvarez, 1990:94)

7.6

Eddington’s Guidelines

The guidelines of Arthur Eddington (1882–1944) for science speculation, slightly modified to make them more broadly applicable to the space sciences, are: 1. Was the speculator rigorous in applying the appropriate science applicable to the model 2. Did the speculator identify all the underlying assumptions used in constructing the model and 3. Did the speculator view the model objectively, as an “adjustable engine,” as opposed to a “finished building?” Were Walter and Luis Alvarez and their colleagues rigorous in applying the appropriate science to their model? As noted above, some prominent paleontologists were vocal in their opposition to the speculation made by the Alvarez group. They argued that the group did not rigorously incorporate in their model the then current paleontological evidence for the extinction of the dinosaurs and other species. Did the Alvarez group identify all the underlying assumptions used in constructing their model? The basic assumptions of the impact speculation were that the source of excess iridium found at the K-Pg boundary layer was a single asteroid that collided with the Earth some 65 million years ago and that this collision brought about the extinctions of species at the end of the Cretaceous period. Ancillary assumptions were that the collision: would inject about 60 times the object’s mass into the atmosphere as pulverized rock; a fraction of this dust would stay in the stratosphere for several years and be distributed worldwide. The resulting darkness would suppress photosynthesis, and [result in the extinctions]. (Alvarez et al., 1980:1095)

The discussion of their model in their 1980 Science article seems to have covered all of their underlying assumptions. Did the Alvarez group view their model objectively? Eight years after their initial speculation, the Alvarez group gave a talk at the Second Snowbird Conference in which they discussed possible modifications to their model, including the possibility of two major impacts rather than a single impact (Alvarez et al., 1988). Such discussions suggest that they viewed their model as a work in progress, rather than a finished product. As in the debate on the nature of the lunar surface (Chap. 6), tensions between scientific groups with different training affected the controversy of the origin of the extinctions at the Cretaceous-Paleogene boundary. In the case of the lunar surface, it was geologists versus astrophysicists while paleontologists versus physicists and

7.7

Continuing Understanding

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geophysicists were the primary groups in the extinction hypothesis. In both cases the protagonists often did not speak the same scientific “language” and brought different investigative techniques to their research endeavors.

7.7

Continuing Understanding

In his 1990 reflection, Walter Alvarez did not mention paleontology as one of the scientific disciplines affected by the discovery that he and his father made (Alvarez, 1990, quoted in Sect. 7.5). But, at least recently, paleontology has been affected. In 2019, paleontologist Robert A. DePalma, then at the University of Kansas and the Palm Beach Museum of Natural History (Fig. 7.15) and 11 co-authors, including Walter Alvarez (Fig. 7.16), published a paper in the Proceedings of the National Academy of Sciences in which they reported on evidence discovered at the Tanis site in the Hell Creek Formation in southwestern North Dakota of mass deaths within minutes to hours of the Chicxulub impact. (DePalma et al., 2019) The authors explained the significance of their discovery, as follows: The Chicxulub impact played a crucial role in the Cretaceous–Paleogene extinction. However the earliest postimpact effects, critical to fully decode the profound influence on Earth’s biota, are poorly understood due to a lack of high-temporal-resolution contemporaneous deposits. The Tanis site, which preserves a rapidly deposited, ejecta-bearing bed in the Hell Creek Formation, helps to resolve that long-standing deficit. Emplaced immediately (minutes to hours) after impact, Tanis provides a postimpact “snapshot,” including ejecta accretion and faunal mass death, advancing our understanding of the immediate effects of the Chicxulub impact. Moreover, we demonstrate that the depositional event, calculated to have coincided with the arrival of seismic waves from Chicxulub, likely resulted from a Fig. 7.15 Robert DePalma. Credit Robert DePalma, Tanis Paleo Heritage Conservancy

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Fig. 7.16 Walter Alvarez and Robert DePalma showing the K-Pg boundary at the Tanis site. Credit Robert DePalma, Tanis Paleo Heritage Conservancy

seismically coupled local seiche [a standing wave oscillating in a body of water]. (DePalma et al., 2019:8190)

It is now certain that the Chicxulub crater is the one that the Alvarez group identified from iridium deposits and that impacted Earth at the time of the CretaceousPaleogene boundary. Considerable research continues related to both the impact crater and its ejections, and in paleontology and extinctions. On the impact crater, additional drillings and studies are being pursued by David Kring and others (see for example Kring et al., 2020). Studies of other impact-produced phenomena such as the Gorgonilla Island (Colombia) spherule layer (e.g., Bermudez et al., 2016; Mateo et al., 2020) and the identification of tsunami “megaripples” in the subsurface of Louisiana are in active investigation (Kinsland et al., 2021). In paleontology, the Deccan Trap volcanism possibility for climate effects preceding the Cretaceous-Paleogene boundary and end-Cretaceous mass extinction remains a quite active research area (e.g., Keller et al., 2020; Nava et al., 2021). Still under much discussion from the on-going research in both areas is the relative importance of the impact versus the volcanism (and accompanying climate change) hypotheses in the extinction process. This includes time scales, in particular the fact that a gradual decline is evident in the fossil record prior to the Chicxulub impact event. A research paper in the Proceedings of the National Academies of Sciences by Theodore Green, Paul R. Renne, and C. Brenhin Keller (2022) examined the five largest mass extinction events on Earth. They report a “correlation between large igneous provinces and periods of Phanerozoic faunal turnover.” After their extensive analysis, they conclude that their data are consistent with the hypothesis that the Chicxulub impactor increased the severity and brevity of the K-Pg extinction, but the role of the Deccan Traps as a preimpact stressor and extinction mechanism should not be minimized. (Green et al., 2022:7)

Such wide-ranging research on mass extinctions on Earth is expected to continue in the future. The controversy over the relative importance of volcanism versus asteroid impact as the cause of the K-Pg extinctions is typical of some of the other debates

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chronicled in this book and is consistent with the manner in which frontier science proceeds.

References Alvarez, W. (1990). Interdisciplinary aspects of research on impacts and mass extinctions; A personal view. In V. L. Sharpton & P. D. Ward (Eds.), Global catastrophes in earth history; An interdisciplinary conference on impacts, volcanism, and mass mortality (pp. 93–97). Geological Society of America Special Paper 247; p. 94. Alvarez, L. W., Alvarez, W., Asaro, F., & Michel, H. V. (1980). Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science, 208(4448), 1095–1108. Alvarez, W., Asaro, F., Alvarez, L. W., & Michel, H. V. (1988). The debate over the CretaceousTertiary boundary (p. 1). LPI Contribution 673. Lunar and Planetary Institute. Archibald, J. D. (1994). Testing KT extinction hypotheses using the vertebrate fossil record. [Abstract]. In Papers presented to the conference new developments regarding the KT event and other catastrophes in Earth history (p. 7). LPI Contribution 825, 6–7. Lunar and Planetary Institute. Archibald, J. D., & Bryant, L. J. (1988). Limitations on K-T mass extinction theories based upon the vertebrate record [Abstract]. In Abstracts presented to the topical conference global catastrophes in earth history: An interdisciplinary conference on impacts, volcanism, and mass mortality (pp. 4–5). Lunar and Planetary Institute; p. 5. Bakker, R. T. (1975). Dinosaur Renaissance. Scientific American, 232(4), 58–78. p. 77. Bourgeois, J., Hansen, T. A., Wiberg, P. L., & Kauffman, E. G. (1988). A tsunami deposit at the Cretaceous-Tertiary boundary in Texas. Science, 241(4865), 567–570. p. 567. https://doi.org/ 10.1126/science.241.4865.567 Bermúdez, H. D., García, J., Stinnesbeck, W., Keller, G., Rodríguez, J. V., Hanel, M., Hopp, J., Schwarz, W. H., Trieloff, M., Bolivar, L., & Vega, F. J. (2016). The Cretaceous–Palaeogene boundary at Gorgonilla Island, Colombia, South America. Terra Nova, 28(1), 83–90. Clemens, W. A., Archibald, J. D., & Hickey, L. J. (1981). Out with a whimper not a bang. Paleobiology, 7(3), 293–298. pp. 296–297. DePalma, R. A., Smit, J., Burnham, D. A., Kuiper, K., Manning, P. L., Oleinik, A., et al. (2019). A seismically induced onshore surge deposit at the KPg boundary, North Dakota. Proceedings of the National Academy of Sciences, 116(17), 8190–8199. Green, T., Renne, P. R., & Keller, C. B. (2022). Continental flood basalts drive Phanerozoic extinctions. Proceedings of the National Academy of Sciences, 119(38), e2120441119. Hartmann, W. K. (1986). Moon origin: The impact-trigger hypothesis. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 579–608). Lunar and Planetary Institute. pp. 587–588. Hildebrand, A. R. and Boynton, W. V. (1987). The K/T impact excavated oceanic mantle: Evidence from REE abundances [Abstract]. In Abstracts of papers submitted to the eighteenth lunar and planetary science conference (pp. 427–428). Lunar and Planetary Institute. Hildebrand, A. R., & Boynton, W. V. (1988). Impact wave deposits provide new constraints on the location of the K/T boundary impact [Abstract]. In Abstracts presented to the topical conference global catastrophes in earth history: An interdisciplinary conference on impacts, volcanism, and mass mortality (pp. 76–77). Lunar and Planetary Institute; p. 76. Hildebrand, A. R., & Boynton, W. V. (1990). Proximal Cretaceous-Tertiary boundary impact deposits in the Caribbean. Science, 248(4957), 843–847. Hildebrand, A. R., Penfield, G. T., Kring, D. A., Pilkington, M., Camargo, Z., & A., Jacobsen, S. B., and Boynton, W. V. (1991). Chicxulub crater: A possible Cretaceous/Tertiary boundary impact crater on the Yucatán Peninsula, Mexico. Geology, 19, 867–871.

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Keller, G. (1994). Global biotic effects of the KT boundary event: Mass extinction restricted to low latitudes? [Abstract]. In Papers presented to new developments regarding the KT event and other catastrophes in Earth history (LPI Contribution 825) (pp. 57–58). Lunar and Planetary Institute; p. 57. Keller, G., Mateo, P., Monkenbusch, J., Thibault, N., Punekar, J., Spangeberg, J., Abramovich, S., Ashckenazi-Polivoda, S., Schoene, B., Eddy, M. P., Samperton, K. M., Khadri, S. F. R., & Adatte, T. (2020). Mercury linked Deccan Traps volcanism, climate change and the end-Cretaceous mass extinction. Global and Planetary Change, 194. https://doi.org/10.1016/ j.gloplacha.2020.103312 Kinsland, G. L., Egedahl, K., Strong, M. A., & Ivy, R. (2021). Chicxulub impact tsunami megaripples in the subsurface of Louisiana: Imaged in petroleum industry seismic data. Earth and Planetary Science Letters, 570, 117063. Kring, D. (2020). Chicxulub impact event. https://www.lpi.usra.edu/science/kring/Chicxulub/ Kring, D. A., & Boynton, W. V. (1991). Altered spherules of impact melt and associated relic glass from the K/T boundary sediments in Haiti. Geochimica et Cosmochimica Acta, 55(6), 1737–1742. Kring, D. A., Hildebrand, A. R., & Boynton, W. V. (1991). The petrology of an Andestic Melt Rock and a Polymict Breccia from the interior of the Chicxulub Structure, Yucatan, Mexico. Abstracts of the Lunar and Planetary Science Conference, 22, 755. Kring, D. A., Tikoo, S. M., Schmieder, M., Riller, U., Rebolledo-Vieyra, M., Simpson, S. L., et al. (2020). Probing the hydrothermal system of the Chicxulub impact crater. Science Advances, 6(22). Lopez-Oliva, J. G., & Keller, G. (1994). Biotic effects of the KT boundary event in northeastern Mexico [Abstract]. In Papers presented to new developments regarding the KT event and other catastrophes in Earth history (LPI Contribution 825) (pp. 72–73). Lunar and Planetary Institute; p. 73. Mateo, P., Keller, G., Adatte, T., Bitchong, A. M., Spangenberg, J. E., Vennemann, T., & Hollis, C. J. (2020). Deposition and age of Chicxulub impact spherules on Gorgonilla Island, Colombia. GSA Bulletin, 132, 215–232. Nava, A. H., Black, B. A., Gibson, S. A., Bodnar, R. J., Renne, P. R., & Vanderkluysen, L. (2021). Reconciling early deccan traps CO2 outgassing and pre-KPB global climate. Proceedings of the National Academy of Sciences, 118(14). Penfield, G. T., & Camargo, A. (1982). Definition of a major igneous zone in the central Yucatan platform with aeromagnetics and gravity [Abstract]. Geophysics, 47(4), 448–449. Russell, D. A. (1979). The enigma of the extinction of the dinosaurs. Annual Review of Earth and Planetary Sciences, 7, 163. Ryder, G. (1996). The unique significance and origin of the Cretaceous-Tertiary boundary: Historical context and burdens of proof. In G. Ryder, D. Fastovsky, & S. Gartner (Eds.), The Cretaceous-Tertiary event and other catastrophes in Earth history (pp. 31–38). The Geological Society of America, Special Paper 307, p. 35. Schopf, T. J. M. (1981). Extinction of the dinosaurs [Abstract]. In Papers presented to the conference on large body impacts and terrestrial evolution: Geological, climatological, and biological implications (p. 49). Lunar and Planetary Institute; p. 49. Schulte, P., Alegret, L., Arenillas, I., Arz, J. A., Barton, P. J., Bown, P. R., et al. (2010). The Chicxulub asteroid impact and mass extinction at the Cretaceous-Paleogene boundary. Science, 327(5970), 1214–1218. Scotese, C. R. (2021). An atlas of Phanerozoic paleogeographic maps: The seas come in and the seas go out. Annual Review of Earth and Planetary Sciences, 49, 679–728. Sharpton, V. L., Schuraytz, B. C., Burke, K., Murali, A. V., & Ryder, G. (1990). Detritus in K/T boundary clays of western North America; Evidence against a single oceanic impact. In V. L. Sharpton & P. D. Ward (Eds.), Global catastrophes in earth history; An interdisciplinary conference on impacts, volcanism, and mass mortality (pp. 349–357). Geological Society of America Special Paper 247; p. 394.

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Sharpton, V. L., Dalrymple, G. B., Marín, L. E., Ryder, G., Schuraytz, B. C., & Urrutia, J. (1992). New links between the Chicxulub impact structure and the Cretaceous/Tertiary boundary. Nature, 359(819–821), 820–821. Sharpton, V. L., Burke, K., Camargo-Zanoguera, A., Hall, S. A., Lee, D. S., Marin, L. E., et al. (1993). Chicxulub multiring impact basin: Size and other characteristics derived from gravity analysis. Science, 261(5128), 1564–1567. https://doi.org/10.1126/science.261.5128.1564 Swisher, C. C., III, Grajales-Nishimura, J. M., Montanari, A., Margolis, S. V., Claeys, P., Alvarez, W., Renne, P., Cedillo-Pardo, E., Maurrasse, F. J.-M. R., Curtis, G. H., Smit, J., & McWilliams, M. O. (1992). Coeval 40Ar/29Ar ages of 65.0 million years ago from Chicxulub crater melt rock and Cretaceous-tertiary boundary tektites. Science, 257(5072), 954–958. Urey, H. C. (1973). Cometary collisions and geological periods. Nature, 242, 32–33. Verschuur, G. L. (1997). Impact!: The threat of comets and asteroids. Oxford University Press. Weinreb, D. B. (2002). Catastrophic events in the history of life: Towards a new understanding of mass extinctions in the fossil record: Part I. Journal of Young Investigators (HTML), 6, 7.

Chapter 8

Size of the Solar System

8.1

Introduction

Before the launches of the first artificial satellites, theoretical speculations existed as to the nature of what is now called the heliosphere—the region of space in the local interstellar medium that is controlled by plasma and magnetic field emissions from the Sun. If the Sun emitted more than light in the form of photons, how might these emissions, including the photons over many different wavelengths, interact with any interstellar material that might exist in the vicinity of the Sun and its circling planets? In this chapter, the speculations and scientific controversies related to this region surrounding the Sun are examined. Specifically, the many different estimates of the heliocentric distances to the boundary of the heliosphere—where interstellar space begins—are examined, i.e., the distances to what are called the termination shock and the heliopause.

8.2 8.2.1

The Early Speculations Leverett Davis Jr.

The Fourth International Cosmic Ray Conference was held in Guanajuato, Mexico, in September of 1955. Some 20 years prior, Sydney Chapman (1888–1970) and Vincenzo Ferrarro (1907–1974) had postulated a stream of particles from the sun to produce a ring current around Earth as an explanation for measurements of magnetic disturbances during a magnetic storm (Chaps. 2 and 3). Also, by this date, Ludwig Biermann (1907–1986) had argued on the basis of his observations of comet tails that there must be a continuous flow of what he called “corpuscular radiation” from the Sun (Chap. 2).

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 W. D. Cummings, L. J. Lanzerotti, Scientific Debates in Space Science, Astronomy and Planetary Sciences, https://doi.org/10.1007/978-3-031-41598-2_8

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Fig. 8.1 Leverett Davis. Credit the American Astronomical Society

At the meeting in Guanajuato a discussion took place about the cavity or bubble that corpuscular radiation from the Sun might carve out of the surrounding interstellar medium. Leverett Davis Jr. (1914–2003) (Fig. 8.1) of the California Institute of Technology (Caltech) participated in the Guanajuato meeting. In the same year he published in the Physical Review his estimate on the size of the cavity. Davis used a pressure balance argument, i.e., that the outward momentum flux of the solar corpuscular radiation must be balanced at some heliocentric distance by the restraining pressure of the interstellar medium in the Milky Way galaxy. Davis assumed that the galactic magnetic field in the vicinity of the solar system was the dominant restraining force. He argued that the galactic field was not present near Earth because it was known that solar cosmic rays could reach Earth. He further noted that a postulated cavity in the galactic field was also supported by the observed deflection of comet tails and that solar activity was known to produce geomagnetic activity on Earth. Davis argued that: The size of the cavity can be estimated by balancing the momentum flux of the diverging corpuscular emission against the lateral pressure, B2/8π, of the magnetic field. The momentum flux is due mainly to protons and at the distance r from the sun may be written as npvp2mp(re/r)2, where np is the number of protons per unit volume at r = re = 1.5X1013 cm = 1 A.U. (astronomical unit), the radius of the earth’s orbit, vp is their mean radial velocity, and mp is the proton mass. . . . . The order of magnitude of vp is 108 cm sec-1 (Kiepenheuer, 1953:437) and that of B may be taken to be 10-5 gauss. That of np is taken by Unsöld and Chapman (Unsöld & Chapman, 1949) and by Biermann (Biermann, 1951) to be 103 cm-3 at ordinary times and 105 cm-3 during large magnetic storms. (Kiepenheuer’s value (Kiepenheuer, 1953:437), np = 1 cm-3, seems low.) Thus momentum balance is attained at r = 3 × 1016 10 - 3 np

½

105 B 10 - 8 vp cm,

which is 2000 A.U. if each item in parentheses is unity as expected. (Davis, 1955:1440)

There is a typo in Davis’s equation for r. It should have been written as:

8.2

The Early Speculations

r = 3 × 1016 10 - 3 np

189 ½

105 B

-1

10 - 8 vp cm:

The typo does not affect his initial estimate of the radius of the cavity, i.e., 2000 AU. (An astronomical unit (AU) is the average distance from the center of the Sun to the center of the Earth. It is also abbreviated by various authors as “A.U.”, “a.u.” or “au”.)

8.2.2

The Termination Shock: Francis Clauser

The Fourth Symposium on Cosmical Gas Dynamics: Aerodynamic Phenomena in Stellar Atmospheres, was held in Varenna, Italy, from August 18 through August 30, 1960. Leverett Davis, Ludwig Biermann, and Eugene Parker (1927–2022) were among the participants, as was Francis H. Clauser (1913–2013) (Fig. 8.2), a distinguished American aeronautical engineer. In a discussion session at the conference, Clauser pointed out a feature of the plasma flow from the Sun that later would be called the termination shock wave: You can see this in your own wash tub [See Fig. 8.3], if you allow the water from the faucet to strike a flat plate there. You will find it goes down a column, and spreads out, and reaches a supersonic water velocity. You find that the flow spreads into a very thin, high speed layer; and then out at a certain distance, it goes through a shock-wave, in which the height of the layer increases manyfold, and the velocities become very low. This is my picture of what happens in the stars, that you get this supersonic outflow, and that out at a great distance there is a shock-wave, that converts the flow back to a higher density, higher pressure, lower velocity flow. (Thomas, 1960:306–307)

After Clauser’s comments in the discussion session, Armin J. Deutsch (1918–1969), who was an astronomer based at the Mt. Wilson and Palomar observatories in California, asked:

Fig. 8.2 Francis Clauser. Credit Special Collections, Sheridan Libraries, Johns Hopkins University

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Fig. 8.3 Termination shock in a sink. Photo taken by one of the authors (Cummings)

first, whether in the astronomical context it’s possible to give now some estimate of the order of magnitude of the radius of the standing shock-wave; and second, would you expect that this shock would lead to any observational consequences? (Thomas, 1960:307)

Eugene Parker of the University of Chicago had recently published his theory of the solar wind (Chap. 2), and he responded to Deutsch’s questions: You can estimate the shock position by the following argument. Coming out from the sun is a flow that has essentially constant velocity after about (20 to 30) solar radii. Thus, density falls off as 1/r2 and can be computed from its estimated value at the earth. The condition giving the shock mentioned by Clauser is simply that the pressure of the solar wind after passing through the shock must balance the interstellar pressure. The pressure across the shock is essentially ρv2. Take a velocity of a few hundred km/s and a density of some 102 particles/cm2 [This is a typo; cm2 should be cm3] at the earth. If the interstellar pressure is 1014 dyne/cm2, the radius of the shock is 5000 a.u. If we introduce a magnetic pressure—which Biermann suggested might be a factor of 103 higher than the gas pressure—the radius is reduced roughly by 103, or to about 160 a.u. (Thomas, 1960:307–308).

So, Parker’s initial rough estimate of the heliocentric distance to the termination shock was about 160 AU. He further wrote that the most important consequences of a shock would be on cosmic rays. The outward convection of in-coming galactic cosmic rays by the disordered magnetic fields in the outflowing solar plasma would likely occur on the sunward (up-wind) as well as down-wind side of a shock boundary. His comments on the modulation of galactic cosmic rays presaged later discussion of this effect in estimating the distance to the shock.

8.2.3

Revised Estimates

By the early 1960s instrumentation on spacecraft orbiting the Earth had begun to make direct measurements of the confinement of Earth’s magnetic field by the solar wind to form the magnetosphere (Chap. 3). Leverett Davis drew upon these measurements to make an argument about the confinement of the solar wind by the surrounding galactic magnetic field. He argued as follows (his notation of magnetic

8.2

The Early Speculations

191

field intensity in units of a gamma (γ) corresponds to more currently used terminology of the field intensity in units of nanoTesla, nT, where 1 nT = 1γ): At Guanajuato it was suggested that the plasma from the sun might make a large cavity or bubble in the galactic magnetic field. I would like to suggest that this bubble is smaller and its boundary more irregular than previously suggested. It is now possible to arrive at a zeroth order estimate of the radius of the cavity, making relatively few assumptions. Space probe observations have shown that at solar maximum in the direction of the sun the geomagnetic field is not drastically affected by the solar plasma until the field’s strength drops to about 10 – 20 γ, but that at somewhat smaller fields it is completely pushed aside. Thus we would expect a galactic field of 10 – 20 γ to be pushed back to about 1 A.U. from the sun. It seems plausible that the velocity of the solar wind would be nearly independent of distance from the sun and that its density would be inversely proportional to the square of the radius. Since the pressure of the galactic magnetic field is B2/8π, the radius of the cavity would be inversely proportional to the strength of the galactic field. If the latter is 1 – 2 γ [1 γ = 10-5 gauss = 10-9 Tesla = 1 nT], the radius of the bubble should be 5 – 20 A.U. (Davis Jr., 1962:544)

He then noted that the interface between the solar wind plasma and the galactic field might be subject to interchange plasma instabilities, possibly resulting in an even smaller cavity: Hence it would not be surprising if a space probe sent to the neighborhood of Jupiter, particularly at solar minimum, should discover evidence of the galactic magnetic field and plasma; on the other hand, it may be necessary to go beyond Uranus, particularly if the galactic field should be weaker than 1 γ in the neighborhood of the sun. (Davis Jr., 1962:544)

So, by 1962, Leverett Davis had revised his initial estimate of the radius of the solarproduced cavity in the surrounding interstellar medium from 2000 AU down to 5–20 AU. Based on the observations of comets, John C. Brandt and Richard W. Michie (1931–1969) of the University of California, Berkeley, gave an estimate of the heliocentric distance to the termination shock of 2 AU. In the Physical Review Letters in 1962, Brandt and Michie argued as follows: Near 2 a.u., the velocities [of solar wind particles] undergo an order of magnitude decrease and remain low out to some 5 a.u. (the limit of the observations). The observational reason for this decrease is that, interior to 2 a.u., comet tails point nearly in the radial direction, while outside of 2 a.u., they make an angle of ≈ π/4 with the radial direction. The velocities in the inner region are approximately those expected on the basis of Parker’s solar wind model (Parker, 1960), and exterior to 2 a.u. they are comparable to or less than the velocities found on Chamberlain’s solar breeze model (Chamberlain, 1961). ... The transition region near 2 a.u. seems to be the shock transition from fast to slow expansion velocities (Parker, 1961); at 2 a.u., an order-of-magnitude balance can be obtained between the dynamic pressure in the wind and the pressure in the interstellar medium due to a 3γ magnetic field (1 γ = 10-5 gauss). (Brandt & Michie, 1962:195)

The estimate of 2 AU by Brandt and Michie for the distance to the termination shock (just beyond Mars) appears to be the smallest among the many early speculations offered by various space researchers. In his book, Interplanetary Dynamical Processes, published in 1963, Eugene Parker introduced the additional factor of the pressure of galactic cosmic rays in his

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pressure-balance argument to revise downward his earlier estimate of the distance to the termination shock from 160 AU to 45–90 AU. Parker argued as follows: The solar wind will continue its supersonic flow outward from the sun until its pressure or energy density decreases to the level of the ambient interstellar pressure, denoted by pi. At such a distance from the sun the solar wind is expected to be diverted by the interstellar gases and magnetic fields. The diversion takes place at a distance of the order of rD, where rD = rE ð1=2Þ NE Mv2 =pi

½

The interstellar pressure pi is composed of the hydrostatic pressure of the interstellar gas, pG, the hydrostatic pressure of the cosmic ray gas, pc, and the interstellar magnetic field pressure pB. The conventional numbers for the interstellar gas are 1 hydrogen atom/cm3 at about 102 ° K (for HI regions of space), so that pG is of the order of 1.4 × 10-14 dyne/cm2. The cosmic ray pressure is believed to be equal to, or slightly less, than the magnetic field pressure, with both pressures of the order of 2-4 × 10-12 dyne/cm2. Only a fraction of the cosmic ray pressure is exerted on the solar wind (Parker, 1958), of course, because a portion of the pressure penetrates all the way through the inner solar system. Thus the interstellar magnetic field pressure is probably the dominant pressure (assuming the conventional figure of 10-5 gauss). In any case the estimate of pi lies in the range 1-4 × 10-12 dyne/cm2. with the interstellar gas pressure contributing very little. With NE = 10/cm3 and v = 300 km/sec, gives rD = 45-90 a. u., i.e., the solar wind is diverted somewhere in the vicinity of the orbit of Pluto, or beyond. (Parker, 1963:114–115)

8.2.4

Neutral Hydrogen and Charge Exchange

In Parker’s analysis described above, the interstellar gas was insignificant in determining the pressure balance at the interface boundary with the solar wind. However, the flow of neutral hydrogen from the galaxy into interplanetary space began to be considered in addition to the pressures of the galactic magnetic field and cosmic rays. In 1962, a Canadian astronomer, Donald C. Morton and an American physicist, James D. Purcell (1912–1986), both working at the U. S. Naval Research Laboratory, published a paper in Planetary and Space Science giving the results of the detection of Lyman-α radiation from rocket flights above the atmosphere during 1955 and 1957 (the era of the International Geophysical Year, IGY). The measured Lyman-α radiation was indicative of the presence of neutral hydrogen atoms. Morton and Purcell interpreted their data as showing (1) a fraction of the source had a high temperature (7000 K), (2) was above the altitude of the rockets, and (3) was corotating with the Earth (Morton & Purcell, 1962:457). As discussed below, this latter interpretation—that the high-temperature component of neutral hydrogen was part of the Earth’s hydrogen corona (also called the telluric corona)— would be disputed. An argument would be made that the residual neutral hydrogen found by Morton and Purcell was of interplanetary origin (Patterson et al., 1963).

8.2

The Early Speculations

193

Fig. 8.4 Thomas Patterson. Credit History of Aviation Collection, Special Collections and Archives Division, Eugene McDermott Library, The University of Texas at Dallas (UT Dallas)

Meanwhile, in a 1963 paper, William Ian Axford (1933–2010), Alexander Dessler (1924–2023), and Benjamin Gottlieb discussed the problem of the termination of the solar wind and magnetic field. They addressed the importance of the process of charge exchange between solar wind protons and interstellar neutral hydrogen atoms inside the heliosphere moving toward the Sun (due to the motion of the solar system relative to the local interstellar medium). After the charge exchange the now neutral, but high energy, solar wind particles would exit into the galaxy. The now charged lower-energy interstellar hydrogen atoms would be picked up by the solar wind magnetic field and carried along with the other solar wind particles away from the Sun, toward a termination shock. The result would be a lowering of the total kinetic energy in the solar wind and a consequent decrease in the ram pressure of the solar wind. This would decrease the distance to a shock. The interaction between the solar wind and the galactic magnetic field and interstellar medium results in the formation of a shock wave at the heliocentric distance of the order of 50 a.u. . . . (Axford et al., 1963:1268)

In their Astrophysical Journal paper, Axford and co-authors also stated that: the effects of charge-exchange and perhaps recombination make distances larger than about 100 a.u. unlikely. (Axford et al., 1963:1275)

and: The interplanetary shock transition is almost surely beyond the orbit of Jupiter (i.e., >5 a.u., at least during periods of moderate and high solar-wind activity). The interstellar hydrogen atom density would have to be rather large if S [the distance to the termination shock] is smaller than 5 a.u. (or the galactic magnetic field would have to be much larger than commonly believed) (Axford et al., 1963:1277).

Later in 1963, a young Irish physicist, Thomas N. L. Patterson (Fig. 8.4), and Francis S. Johnson (1918–2009) (Fig. 8.5) and William B. Hanson (1923–1994) (Fig. 8.6), who were his mentors at the Southwest Center for Advanced Studies (later to become the University of Texas at Dallas) published a paper in Planetary and Space Science in which they reinterpreted the Lyman-α results of Morton and Purcell. After summarizing the characteristics of the Morton and Purcell detector

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Fig. 8.5 Francis Johnson. Credit Roderick Heelis, UT Dallas

Fig. 8.6 William Hanson. Credit Roderick Heelis, UT Dallas

(which could have its atomic hydrogen filter turned on and off in flight), they remarked that with the filter turned off and the detector looking upward: the 15 per cent of the response due to radiation with wavelengths greater than 0.04 Å from the line center cannot have been due to scattering in the telluric corona; it must have come from space. The only tenable source appears to be the scattering of solar Lyman-α radiation by interplanetary hydrogen (Shklovsky, 1959), for which Doppler shifts would be expected to shift the radiation largely outside the passband of the filter. (Patterson et al., 1963:768)

They then discussed their determination in the context of the Axford/Dessler/ Gottlieb work: Owing to the effectiveness of solar ionizing radiation in ionizing interplanetary hydrogen, or of solar-wind charge-exchange and driving it out of the interplanetary space, one can maintain significant concentrations of interplanetary hydrogen only by continually renewing it. . . . interplanetary hydrogen atoms must have high velocities (~ 300 km/sec) if they are to approach the sun closely enough to contribute to the observed distribution of scattered Lyman-α radiation. Thus, the picture presented by Axford et al. describes a source of the very type needed to explain the observations of Morton and Purcell. Conversely, Morton and

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Purcell’s observations provide a striking confirmation of the essential accuracy of the picture presented by Axford et al. (Patterson et al., 1963:768)

Patterson, Johnson, and Hanson, seemed particularly pleased that their interpretation of the observations of Morton and Purcell confirmed the accuracy of the recentlypublished theory of Axford, Dessler, and Gottlieb that featured charge exchange between solar wind protons and interstellar neutral hydrogen. These early space researchers, Dessler, Johnson, and Hanson, had been colleagues at the Southwest Center for Advanced Studies and, before that, at the Lockheed Research Laboratory in Palo Alto, California. Patterson et al. assumed that most of the charge exchange between solar-wind ions and galactic neutral hydrogen occurred just beyond the termination shock. To supply enough neutral hydrogen near the Earth to account for the observed Lyman-α radiation that they attributed to the Morton and Purcell measurements, Patterson et al. required that the termination shock be located at about 20 AU from the Sun (about the orbit of Uranus): It is found that when the solar wind velocity and concentration at 1 A.U. are respectively 400 km sec-1 and five cm-3, the concentration of interplanetary neutral hydrogen in the vicinity of the earth is about 0.02 cm-3 with the shock front located at about 20 A.U. from the Sun (Patterson et al., 1963:767)

As discussed below, the assumption by Patterson et al. that most of the charge exchange occurred in a thin shell just beyond the termination shock was challenged, with the result that the estimated distance to the shock was substantially reduced from their 20 AU estimate to about 5 AU. (Hundhausen, 1968).

8.2.5

Additional Speculations on the Effect of Galactic Cosmic Rays

The intensity of galactic cosmic rays observed at Earth was known to vary with the 11-year cycle of solar activity indexed by the number of sunspots seen on the Sun’s surface. Fewer galactic cosmic rays were observed during the peak of the sunspot cycle when the surface of the Sun was most active and the flux of the solar wind was high than during the sunspot minimum part of the cycle, when the flux of the solar wind was low. Also observed was a frequent modulation in the intensity of galactic cosmic rays that has an approximate 27-day period. This is consistent with the idea that active regions on the Sun’s surface would cause enhanced streams of solar plasma to flow into interplanetary space and be observed at the Earth with the cadence of the 27-day rotation period of the Sun. In a paper published in Planetary and Space Science in 1965, Ian Axford (Fig. 8.7) described one effect of galactic cosmic rays on the solar wind: We suggest that the 11-year and 27-day modulations of the galactic cosmic ray intensity are the result of a radial gradient in the density of the cosmic rays produced by a balance between inwards diffusion and outwards convection of the particles by the solar wind. As a

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Fig. 8.7 Ian Axford. Credit Joy Axford; Gary Zank, University of Alabama Huntsville

consequence of the existence of this gradient the density of cosmic rays in the galaxy might be somewhat larger than the density observed near the earth, even at sunspot minimum. Furthermore, the work done in pushing the cosmic rays away from the sun might result in a substantial decrease of the solar wind velocity at great distances from the sun and thereby affect the size of the cavity produced by the interaction of the solar wind with the interstellar medium. (Axford, 1965:116)

Axford and a Cornell University graduate student, R. C. Newman, soon analyzed the effect of “cosmic ray friction” on the solar wind in more detail: The characteristics of the cavity within which the solar wind flows supersonically are of considerable interest since it is believed that this is the region in which most of the modulation of the galactic cosmic ray intensity takes place. . . . We find that if our model calculations can be taken seriously, the process can limit the radius of the cavity to 30–40 A.U. without leading to any serious conflict with observations. However, if the cavity extends only to 15–20 A.U., as suggested by Patterson et al. (1963), some more effective mechanism is needed and one such possibility will be discussed briefly. (Axford & Newman, 1965:73)

Axford and Newman then discussed that the distance to the termination shock could be reduced from 30–40 AU to the Patterson et al. 15–20 AU value by the cooling of the compressed solar wind plasma due to the charge exchange with the interstellar hydrogen atoms as previously proposed by Axford, Dessler, and Gottlieb.

8.3

To the Outer Planets—and Beyond: Pre-voyager Speculations

In 1964, Gary A. Flandro (Fig. 8.8), an American aeronautical engineer, began to investigate ways to use gravitational assists from the large gas planets to make possible “tours” of the outer part of the solar system. Flandro was a Caltech graduate student and Jet Propulsion Laboratory (JPL) employee at the time. In 1966, Flandro published in the journal Astronautica Acta the results of his analysis:

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Fig. 8.8 Gary Flandro. Credit Gary Flandro

. . . . Contrary to popular belief, indirect ballistic trajectories involving close approach to one or more intermediate planets may not require longer flight duration than is characteristic of direct transfer orbits. In fact, significant reduction of both required flight time and launch energy results if efficient use is made of the energy which can be gained during a midcourse planetary encounter. From the point of view of a passing space vehicle, the intermediate planet appears as a field of force moving relative to the inertial heliocentric coordinate system. Thus, work is done on the spacecraft, and its heliocentric energy may be increased or decreased depending upon the geometric details of the encounter. This paper describes the application of energy derived in this fashion, utilizing gravitational perturbations from Jupiter, for reduction of required launch energy and flight duration for exploration missions to all of the outer planets of the solar system. (Flandro, 1966:329)

He then wrote that the latter half of the 1970s abounds in interesting multiple planet opportunities due to the similar heliocentric longitudes of the major planets during this time period. Trajectories to Saturn, Uranus, Neptune, and Pluto using the midcourse energy boost from Jupiter are best initiated in the years 1978, 1979, 1979, and 1977 respectively. Flight time reductions range from one half the required direct trajectory duration for Earth-Jupiter-Saturn missions to as much as 85% of the direct transfer time for Pluto flights via Jupiter. Many multiple-target trajectories are also possible. Of particular interest is the 1978 Earth-Jupiter-Saturn-Uranus-Neptune “grand tour” opportunity which would make possible close-up observation of all planets of the outer solar system with the exception of Pluto in a single flight. (Flandro, 1966:329)

Following up on Flandro’s analyses, NASA convened an Outer Planets Working Group in 1969. This group proposed a two spacecraft “Grand Tour” mission with one spacecraft visiting Jupiter, Saturn and Pluto while the second would encounter Jupiter, Uranus and Neptune. Total mission time for both would be about 8 years. The projected cost of the mission (near $1B including launch costs in that year dollars) and the beginning of funding for the space shuttle program caused the concept to die. There was considerable basic scientific research interest in flying to the outer planets. In particular, the discovery of kilometric radio emissions from Jupiter in 1974 by Bernard F. Burke (1928–2018) and Kenneth L. Franklin (1923–2007), both at that time at the Carnegie Institution of Washington D.C., opened a new frontier in studies of planetary astronomy (Burke & Franklin, 1955). At the time of their

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discovery, the radio signals were attributed to giant thunderstorms in Jupiter’s atmosphere. Following the discovery of Earth’s radiation belts by the U.S. Explorer 1 and 3 spacecraft (Chaps. 1 and 10), it was evident to space physicists that the radio signals from Jupiter must be from a radiation environment (albeit a strong one) around the planet. Such an environment required space missions to explore and understand. Approximately in parallel as the Jet Propulsion Laboratory was re-studying the possibility of some type of grand tour program, the NASA Pioneer spacecraft program, led by the NASA Ames Research Center, planned and launched two spacecraft. These, Pioneer 10 (March 1972) and Pioneer 11 (April 1973), traveled beyond the orbit of Mars, through the asteroid belt (Sect. 8.4.1). A program titled Mariner Jupiter-Saturn was approved in 1972 for JPL. This mission would have two spacecraft: one targeted specifically for studies of Jupiter, Saturn, and Saturn’s moon Titan (the only moon known at the time to have an atmosphere), and the second for the same objectives if the first was not successful. If the first was successful, the option of a “grand tour” going on to Uranus and Neptune using gravity assist from Saturn, as outlined almost a decade earlier by Gary Flandro, would be pursued. In March 1977, prior to the Mariner spacecraft launches in August and September of 1977, the spacecraft were renamed Voyager 1 and Voyager 2.

8.3.1

Dessler’s Review

In 1967, Alex Dessler published a lengthy review in Reviews of Geophysics of what was then known about the solar wind. In this article, Dessler coined the term “heliosphere,” which he defined as “the region of interplanetary space where the solar wind is flowing supersonically” (Dessler, 1967:33). For Dessler, then, the heliopause, i.e., the boundary of the heliosphere, was the termination shock. Dessler termed the region beyond the termination shock the “boundary shell.” That region is now called the heliosheath, and the “heliosphere” is now used to describe the region of interplanetary space that includes the part bounded by the termination shock as well as the heliosheath. In his review, Dessler noted that the Sun is moving relative to the local interstellar medium and that interstellar neutral hydrogen would then be flowing into the heliosphere from a particular direction, the apex. If the boundary shell/heliosheath is thick: the momentum of the directed flow of hydrogen into the apex side of the boundary shell is transferred to the boundary shell magnetic field by charge-exchange collisions. The maximum pressure of the atomic hydrogen would be ρHvH2 where ρH is the mass density of the atomic hydrogen and vH is the velocity of this hydrogen relative to the boundary shell. (Dessler, 1967:34)

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Using his assumed interstellar magnetic field (1 nT) as the confining pressure, Dessler arrived at a distance to the termination shock of ~60 AU, considerably beyond planet Neptune. Using both his assumed magnetic field pressure and the assumed pressure resulting from the inflowing interstellar hydrogen (at 20 km/s with a number density of 1 hydrogen atom/cm3), he arrived at a distance of ~30 AU, about the orbit of Neptune (Dessler, 1967:35). Dessler pointed out that because the Sun is moving relative to the local interstellar medium, there is no single distance to the termination shock, but, rather, the distance would be a function of the angle of any measurement location from the apex.

8.3.2

Jovian Radio Emissions

The 22.2 MHz radio emissions from Jupiter that were discovered in 1955 by Burke and Franklin were a large surprise in terms of planetary studies at the time. The radio emissions remained a major objective of study by radio and planetary scientists afterwards. One of the intriguing findings reported during solar cycle 19 (1954–1964) was the apparent inverse relationship between the probability of 18 MHz Jovian radio emissions that were being regularly measured and the Zurich sunspot number (Smith & Carr, 1964:90, Figs. 4–7). Further, there was also found to be a positive relationship between individual radio bursts and large solar flare events (Smith & Carr, 1964:88–89). At Bell Laboratories, Louis J. Lanzerotti and Mike Schulz were studying Earth’s magnetosphere. They were intrigued by the reports of Jupiter’s radio emissions. They discussed the possible relationships to a radiation environment around the planet, and the nature of the radiation. They speculated in the journal Nature that: if the decametric radio emission is taken as an index of Jovian magnetospheric activity, the short term positive correlation and the long term negative correlation with solar activity can be accounted for qualitatively by assuming that the average position of the heliosphere boundary moves inward to about 5 AU during solar minimum and outward to perhaps 8 or 10 AU during solar maximum (Lanzerotti & Schulz, 1969:1055).

8.3.3

Estimates Based on Cosmic Ray Modulation

On 23 February 1956, a large solar flare near the beginning of solar cycle 19 produced a very large solar cosmic-ray event. The flare products (the solar wind was unknown at that time) that impacted Earth produced a huge geomagnetic storm with aurora seen to as low as 40 degrees latitude over North American, with accompanying radio signal blackouts. Cosmic-ray intensities from the flare event were continuously measured in an array of five latitudinally-spaced neutron monitoring stations that had been established earlier by the University of Chicago. High energy neutrons are a by-product of the collision of cosmic rays with atmospheric

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Fig. 8.9 John Simpson. Credit American Geophysical Union (AGU), courtesy of AIP Emilio Segrè Visual Archives

molecules. The neutron monitors were invented by John A. Simpson (1916–2000) (Fig. 8.9) of the Enrico Fermi Institute for Nuclear Studies at the university. Simpson and his colleagues Peter Meyer (1920–2002) and Eugene Parker published an influential paper in the Physical Review (Meyer et al., 1956) that focused on an analysis of the February 1956 cosmic ray event. Measurements of the time of arrival of particles captured by the neutron monitors showed initial strong streaming from the Sun (as would be expected). Then, after some time, the data from the several monitors recorded particles coming from all directions, including flowing back toward the Sun. The neutron-monitor measurements of the solar particles from the solar flare event lasted several hours, long after the flare-related activity on the Sun had subsided. To explain the cause of the backscatter of solar-produced particles, and the long life of the measured intensities, Meyer, Parker, and Simpson proposed a model that featured a magnetic field-free region from the Sun to the inner edge of the boundary of a heliocentric shell of disordered galactic magnetic field. The kinks in the disordered field would act as scattering centers for the cosmic rays. The authors concluded that the arrival times and the rates of decline in cosmic ray intensities after the peak flux had occurred were caused by the existence of a backscattering shell with an inner radius of 1.4 AU and an outer radius of 5 AU (Meyer et al., 1956:777). The 1956 cosmic ray event and the subsequent Chicago paper analyzing its properties predated the theoretical development by Eugene Parker of the concept of the solar wind. Following the confirmation of Parker’s theoretical research by interplanetary spacecraft instrumentation (Chap. 2), it was realized that the region from the Sun to the Earth’s orbit was not free of magnetic fields, and that the galactic magnetic field, whatever its magnitude, would be excluded from what came to be called the heliosphere. Nevertheless, the 1956 paper by the Chicago group proved to be very influential long into the future for studies of solar and galactic cosmic ray propagation. The work planted the seed for the idea of cosmic rays being scattering by irregularities in the solar wind magnetic field. The work also supported the idea

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Fig. 8.10 George Gloeckler. Credit George Gloeckler, University of Michigan

for a scattering region that reached 5 AU in heliocentric distance, an idea that influenced cosmic-ray research for more than a decade. In early 1967, a graduate student of John Simpson’s, Joseph J. O’Gallagher, published a single author (as was the practice at the University of Chicago) paper in the Astrophysical Journal that was based upon his PhD thesis (O’Gallagher, 1967). This paper used Mariner IV particle data to determine the radial gradient of cosmic rays in the heliosphere as the spacecraft traveled to Mars. Over the heliocentric distance between Earth and Mars (about 0.55 AU), the measured positive radial gradient was concluded to be so high that if it extended to 5–10 AU the energy density of galactic cosmic rays would be about 2 eV/cm3. This value was about double the generally accepted value for the energy density of cosmic rays in the interstellar medium. From the gradient determined from the density of galactic cosmic rays over the distance interval of 0.4 AU and using a model for the inward diffusion and outward convection of cosmic rays in the heliosphere, O’Gallagher estimated that “at solar minimum the galactic [cosmic ray] spectrum is not significantly modulated beyond ~5 a.u.” (O’Gallagher, 1967:675). The high value (2 eV/cm3) of the implied energy density of the interstellar cosmic rays led O’Gallagher and Simpson to suggest that the modulation region for cosmic rays did not extend beyond 5–10 AU. If the outer edge of the modulation region for cosmic rays was assumed to be the heliopause, then the heliocentric distance to the heliopause would be 5–10 AU (O’Gallagher and Simpson, 1967:826). Later in 1967, George M. Gloeckler (Fig. 8.10), who was a Postdoctoral Fellow at the Enrico Fermi Institute of the University of Chicago following his prior graduate studies with John Simpson, published a paper in the Astrophysical Journal with Jack Randolph (Randy) Jokipii (1939–2022) (Fig. 8.11). Jokipii was at Chicago at the time as a Postdoctoral Fellow with Eugene Parker. Gloeckler and Jokipii pointed out that if the local interstellar energy density were 2 eV/cm3, then: the interstellar magnetic field intensity would have to exceed 1 γ (10-5 G) in order to confine the cosmic rays in a quasi-equilibrium configuration. Parker (1966) has shown, however,

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Fig. 8.11 Randy Jokipii. Credit Joe Giacolone, Lunar and Planetary Laboratory, University of Arizona

Fig. 8.12 Arthur Hundhausen. Credit the American Physical Society and the American Institute of Physics Emilio Segrè Visual Archives

that increasing the magnetic field intensity leads to difficulties in holding together the combined magnetic-field cosmic-ray system by gravity. (Gloeckler & Jokipii, 1967:L44)

8.3.4

Solar Wind and Neutral Hydrogen Densities

Arthur J. Hundhausen (Fig. 8.12) of the Los Alamos National Laboratory reviewed the situation at the time, especially as related to interstellar neutral hydrogen densities. He offered an analysis based on his thesis research at the University of Wisconsin that yielded 5 AU as the estimated heliocentric distance to the termination shock. In his paper in Planetary and Space Science Hundhausen stated that: on the basis of this model that most of the neutral hydrogen in the vicinity of the Earth does not originate (in charge exchanges between solar wind protons and interstellar neutral

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hydrogen) near the shock boundary of the heliosphere, as assumed by Patterson, Johnson, and Hanson, but in a shell far beyond the shock. The neutral hydrogen density near the earth can then be maintained only if the shock is near 5 A.U. This reduced value for the radius of the heliosphere implies an interstellar magnetic field larger by an order of magnitude than that conventionally accepted. It is, however, in good agreement with recent estimates of the size of this region based on the solar modulation of galactic cosmic rays. (Hundhausen, 1968:783)

He then concluded with the implications for the interstellar magnetic field magnitude at the location of the solar system and/or the neutral hydrogen density: This result is in sharp conflict with the estimate of 50 ~ A.U. based on the balance of the solar wind dynamic pressure and the background pressure of the interstellar magnetic field. For the solar wind density and flow velocity considered here, a shock at 5 A.U. implies a background pressure equivalent to an interstellar field of 10 γ, an order of magnitude greater than the conventional figure of 10-5 gauss or 1 γ (Parker, 1963, p. 115, Axford et al., 1963). Thus either the interstellar magnetic field in the neighborhood of the sun is much larger than expected, or the picture of the neutral hydrogen distribution, based on the ideas presented in Axford et al., (1963) and developed by Patterson et al., (1963) is grossly incorrect. (Hundhausen, 1968:790, 792)

Arthur Hundhausen thus recognized that something might be wrong with the model that led him to speculate that the termination shock was near 5 AU. Eugene Parker (Fig. 2.10) had a more definite opinion that the model was wrong. Eugene Parker, in a 1969 review paper in Space Science Reviews on the solar wind, addressed the idea of a close-by, even 5 AU, termination of the heliosphere. He wrote that suggestions of a 5 AU termination shock have been based on indications that (a) cosmic-ray modulation ceases at some distance of the order of 3-5 AU, (b) comet tail behavior, and (c) the neutral hydrogen indicated by the night time Lα line observed in the anti-solar direction, etc. (Parker, 1969:349)

Parker noted that a 5 AU termination would require an interstellar pressure (order 4 × 10-10 dynes/cm2) that would have “explosive” effects on the disk of the Milky Way galaxy. He further discussed the density of the neutral atoms and their velocity at the boundary that would be involved with charge exchange with the solar wind at 5 AU location. He concluded that “the inference (from cosmic-ray modulation, etc.) that the wind terminates at 5 AU is unjustified.” (Parker, 1969:349). Parker then addressed the studies of Lα by Hundhausen (1968) and Patterson et al. (1963), stating that We think the problem, with the possibility, of observing the terminus of the wind in the Lα line is of sufficient interest and importance as to merit further inquiry. In this connection it is interesting to note that Williams (1965) has calculated the size of the region that the sun would produce in a neutral interstellar medium at rest with respect to the sun. He finds that the ionization at the outer boundary of the region drops off only gradually, because of the . . . radiation from the sun at wavelengths in the neighborhood of 200 Å. The distance at which the ionization level falls to one half is calculated to be 1500 AU if the surrounding interstellar medium has a density of one atom/cm3, 400 AU if 10 atoms/cm3, 120 AU if 102, and 50 AU if 500. The mean interstellar neutral gas density in the disk of the galaxy is inferred from 21 cm radio observations to be about 1/cm3 (Schmidt, 1963) and probably rather less in the neighborhood of the sun (Münch & Unsöld, 1962), indicating that neutral hydrogen may not get within 103 AU of the sun. (Parker, 1969:349–350)

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Parker’s conjecture, and that of others, that high-energy solar radiation might ionize the interstellar neutral hydrogen well beyond the solar system was based on calculations that ignored the flowing of the neutral hydrogen into the heliosphere. A year after the publication of Parker’s review, two German scientists made calculations of the penetration into the heliosphere of interstellar neutral hydrogen that took into account the velocity of the interstellar gas relative to the solar system (Blum & Fahr, 1970). P. W. Blum and Hans-Jörg Fahr of the Institute for Astrophysics and Extraterrestrial Research in Bonn calculated that, far from being totally ionized to great distances from the Sun, interstellar neutral hydrogen penetrates deep into the heliosphere. The results of Blum and Fahr implied that the termination shock should be located near 80 AU (Blum & Fahr, 1970:280). As Axford later pointed out, if there is relative motion between the neutral interstellar hydrogen and the heliosphere, “the situation is drastically altered, and even with quite modest speeds of the order of 20 km/s it can be shown that the neutral interstellar gas can penetrate almost unattenuated into the inner solar system within the orbit of Jupiter” (Axford, 1972:623). Basically, the sunward-moving neutral hydrogen doesn’t allow the time required for full ionization by solar radiation of the near interstellar region beyond the heliosphere. In 1972, at the NOAA Aeronomy Laboratory, Thomas E. Holzer, a former graduate student of Axford’s, published his quantitative analysis of the interaction between the solar wind and the neutral hydrogen of the interstellar medium (Holzer, 1972). Assumptions included in Holzer’s calculations were that a constant-density atomic hydrogen gas (cold, with temperature ~ 100 K) exists throughout the heliosphere beyond 5 AU, and the flow speed of the interstellar gas was between 0 and 40 km/s. The interplanetary magnetic field beyond 5 AU was assumed to be normal to the solar wind flow (as is now known to be the case from deep space investigations by Pioneers 10,11 and Voyagers 1,2). Holzer determined that the coupling between the solar wind and the neutral hydrogen gas was provided primarily by charge exchange been the solar wind protons and the neutral gas. Using both analytical and numerical methods, Holzer concluded that The slowing of the supersonic solar wind, coupled with the compressibility introduced into the subsonic flow, serves to reduce the estimate of the minimum shock distance from some 100 AU to less than 50 AU. (Holzer, 1972:5428)

Holzer estimated that a lower limit to the minimum shock distance might be about 25 AU.

8.3.5

Evidence of the Penetration of Interstellar Neutral Hydrogen Deep into the Heliosphere: OGO 5 Measurements

The presence of interstellar neutral hydrogen in the heliosphere was soon confirmed by direct measurements with the fifth Orbiting Geophysical Observatory (OGO 5)

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satellite. OGO 5 was launched into a highly eccentric Earth orbit on 4 March 1968. The spacecraft carried two photometer instruments that could measure Lyman α radiation, one from the University of Colorado, Boulder, and the other from the French National Center for Scientific Research (C.N.R.S.). OGO 5 provided a stable (non-spinning) platform for its instruments, but NASA enabled slow rolls of the spacecraft on three occasions so that the photometers could perform sky surveys. The sky surveys showed broad maxima and minima in Lyman α radiation that appeared to move along the plane of the ecliptic as the Earth and its orbiting OGO 5 spacecraft progressed around the Sun through the year. The University of Colorado team concluded that: The most likely origin of the observed emission is scattered of solar Lyman alpha from interstellar hydrogen that is swept into the vicinity of the earth from the direction of the apparent apex of motion of the solar system. The observed displacements of the maxima and minima along the ecliptic over a six month period are explained by a parallax effect due to the Earth’s motion in its orbit. This explanation requires that the effective scattering region be located well within the solar system. (Thomas & Krassa, 1971:218)

The C.N.R.S. investigators largely agreed with these conclusions: We interpret the 50° apparent displacement of the maximum region between the September 1969 and April 1970 observations as the parallax effect of the Earth’s motion on its orbit, as evidence that the emission in this direction originates from a distance of ≈ 3 A.U. from the Sun. . . . Our results give strong support to the theory that in its motion towards the apex, the sun crosses neutral atomic hydrogen of interstellar origin, giving rise to an apparent interstellar wind; . . . . (Bertaux & Blamont, 1971:200)

Thus, there was little doubt that neutral hydrogen was flowing into the heliosphere. The question remained as to the magnitude of the flow. Parker had noted in his review that a heliopause distance of 5 AU would imply that the density of neutral hydrogen in near interstellar space would have to be around 10/cm3. In the same era that the OGO 5 results were reported, Thomas R. McDonough and Neil M. Brice (1934–1974) from Cornell University wrote that there was evidence that the interstellar hydrogen density was likely to be of the order of 0.05/cm3: It has previously been suggested that the solar wind might terminate at distances of 5 AU to 20 AU from the Sun, and that the solar wind might be drastically slowed down by charge exchange and photoionization of interstellar hydrogen atoms which approached the Sun. However, recent satellite measurements of resonantly scattered Lyman alpha radiation, together with pulsar dispersion and Faraday rotation measures, imply very small values for the interstellar hydrogen density (0.05 cm-3) and magnetic field strength (3 μG). As a result, the solar wind is not expected to be slowed down by more than about 30% inside the termination distance, which is expected to be about 100 AU. (McDonough & Brice, 1971:505)

McDonough and Brice arrived at their conclusion of a termination shock of about 100 AU for quiet solar conditions by using solar wind parameters of 300 km/s for the velocity and 5 particles/cm3 for the density of protons at Earth’s orbit, and a galactic magnetic field of 0.3 nT (McDonough & Brice, 1971:508).

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The Experts are Polled on the Distance to the Termination Shock

In February 1990 Voyager 1, about 40 AU from Earth, turned its camera on for the last time and captured a picture of planet Earth in the vastness of the universe—a “pale blue dot” as Carl Sagan called this stunning photo of where humanity exists and Voyager originated. Some 8 months earlier, a major conference had occurred at the University of New Hampshire. This meeting gathered more than a hundred researchers active in studies of the heliosphere, especially the outer heliosphere. Data from the Pioneers and Voyagers were discussed, and theoreticians presented new ideas and speculations on what the Voyagers might measure as they moved ever farther from Earth. The stimulation of the data presentations and the theoretical discussions led to the suggestion to take a poll of the attendees as to at what distance Voyager 1 might encounter the termination shock, and thus enter into the heliosheath. The results of the polling are shown in Fig. 8.13. The majority of the poll results lie in the 55–70 AU range, with a mean of 61 AU.

8.3.7

Heliosphere Radio Emissions

Early in July 1992, with Voyager 1 at 50.8 AU and Voyager 2 at 39 AU and separated by about 45 degrees in heliolatitude, strong radio emissions in the 2–3 kHz range began to be recorded in the Plasma Wave System (PWS) instruments

Fig. 8.13 Poll of the experts on the distance to the termination shock. Courtesy of Tom Krimigis

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Fig. 8.14 Donald Gurnett. Credit the American Geophysical Union and the American Institute of Physics Emilio Segrè Visual Archives

Fig. 8.15 William Kurth. Credit William Kurth, the University of Iowa

on both spacecraft. The emissions could be measured above instrument noise level for almost a year. Prior to this very strong and persistent event, only one other strong event had been reported, measured in 1983–1984 (Kurth et al., 1984). The quite unusual occurrence of such emissions, global in extent as measured by both Voyagers, implied that an unusual event must have occurred in the solar wind environment of the heliosphere. Donald A. Gurnett (1940–2022) (Fig. 8.14), William S. Kurth (Fig. 8.15) and their colleagues relate that an about 3-week interval in May–June 1991—a year prior to their first measurement of the 1992 radio emissions—saw six major flare events on the Sun (Gurnett et al., 1993). The combined plasma emissions from these flare events in the form of coronal mass ejections with the accompanying interplanetary shock waves produced, as the shocks passed Earth, one of the largest decreases of cosmic rays measured by neutron monitors on the planet. Large decreases in the intensities of cosmic rays were also measured by the cosmic ray detectors on both Voyagers as the merged shock waves passed them in late 1991 (with Voyager 1 at 46 AU and Voyager 2 at 35 AU at that time).

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From considerations of the possible plasma conditions in the distant heliosphere and beyond a terminal shock, Gurnett et al. proposed that the emissions could be produced by the interaction of the traveling interplanetary shock wave with the heliopause. Some reasonable knowledge of the speed of the traveling shock wave could be obtained from the time interval between the solar events and when the global shock passed the two Voyagers. However, because the traveling shock would likely be slowed after it encountered the termination shock and before it met the heliopause, and the unknown knowledge of the location of the termination shock, their derivation of the distance of the heliopause had some uncertainties. Nevertheless, given reasonable ranges in value for the distance to the termination shock, and possible values of the reduced speed of the traveling shock after encountering it, Gurnett et al. estimated that the heliopause was located between 116 and 177 AU.

8.4 8.4.1

Direct Measurements of the Termination Shock and the Heliopause Pioneers 10 and 11

The Pioneer 10 spacecraft was launched on 2 March 1972. Through Gary Flandro’s approach of making close fly-bys of Jupiter and then Saturn, Pioneer 10 was the first spacecraft to achieve enough velocity to leave the solar system. Pioneer 10 carried magnetic field and charged particle measuring instruments that could help settle the controversies about the distance to the termination shock and the heliopause. The instrumentation on Pioneer 10 was able to detect shock waves in the solar wind and the standing bow shock sunward of Jupiter’s magnetic field (analogous to the bow shock of Earth’s magnetosphere). But until the end of its mission on 31 March 1997, when the spacecraft was 67 AU from the Sun, the spacecraft instruments did not detect the termination shock. The numerous speculations that the termination shock was in the range of 2–20 AU from the Sun turned out to be considerably in error. Pioneer 11 was launched on 6 April 1973. It was identical to Pioneer 10, except that it carried two magnetic field detection instruments, one using the same design as the Pioneer 10 magnetometer and the other with a different design. Pioneer 11 also used close approaches (gravity assists) to Jupiter and Saturn to boost its velocity. Thus, like Pioneer 10, it was also designed with the capability to leave the solar system. The received signal strengths of both Pioneer spacecraft declined as they receded farther and farther from Earth. At some point data could no longer be received from their instruments. The last contact with Pioneer 11 was on 30 September 1995, when the spacecraft was about 44 AU from Earth and Pioneer 10 was still returning data. It would be about a decade before Voyager 1 finally achieved the goal of encountering the termination shock.

8.4

Direct Measurements of the Termination Shock and the Heliopause

8.4.2

209

Crossing of the Termination Shock by Voyagers 1 and 2

The Pioneer spacecraft paved the way for the exploration of the heliosphere by the Voyager spacecraft (Fig. 8.16). Voyager 1 was launched on 5 September 1977, a few weeks after the launch of Voyager 2 on 20 August 1977. Both spacecraft carried the necessary magnetic field and charged particle instruments to determine any heliosphere shock that might be encountered. The spacecraft also carried imaging instruments that transmitted back to Earth detailed pictures of the four giant icy outer solar system planets and some of their moons (Voyager 1 at Jupiter and Saturn, and Voyager 2 at Jupiter, Saturn, Uranus, and Neptune). The plasma spectrometer experiment on Voyager 1 failed in 1980. However, the magnetometer instrument and the two particle instruments on Voyager 1 were able to definitively determine the crossing of the termination shock at a heliocentric distance of approximately 94 AU. The data from the magnetometer instrument team led by Leonard F. Burlaga (Fig. 8.17) of NASA’s Goddard Space Flight Center clearly showed the shock crossing on 16 December 2004 (Fig. 8.18). A steady Fig. 8.16 Vision of the heliosphere. Credit NASA

Fig. 8.17 Leonard Burlaga. Courtesy of Leonard Burlaga

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Fig. 8.18 Daily averages of magnetic field strength B (a), azimuthal angle λ (b), and elevation angle δ (c) as a function of time measured in days from the beginning of 2004. The angles are in heliographic coordinates. DOY day of year. From Fig. 1 in Burlaga et al. (2005) Fig. 8.19 Edward Stone with model of Voyager spacecraft. Credit Edward Stone, courtesy of Deborah Miles

magnetic field was measured in the heliosheath following the crossing. The plasma wave instrument recorded evidence of the termination shock prior to and as Voyager 1 crossed the shock (Gurnett and Kurth, 2005). Charged particle data from the Voyager 1 particle instruments provided evidence for the shock crossing (Decker et al., 2005; Stone et al., 2005). The cosmic ray instrument team led by Edward C. Stone (Fig. 8.19) and Alan C. Cummings (Fig. 8.20), both of Caltech, showed a steady and increasing particle flux in the heliosheath following the shock crossing (Fig. 8.21), attributed to galactic cosmic rays as the spacecraft moved through the heliosheath toward the heliopause.

8.4

Direct Measurements of the Termination Shock and the Heliopause

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Fig. 8.20 Alan Cummings. Courtesy of Suzette Cummings

Although Voyager 2 was launched before Voyager 1, its trajectory and planetary energy boosts caused it to have a slightly lower speed through the distant heliosphere (3.2 AU per year for Voyager 2 versus 3.6 AU per year for Voyager 1) (Gurnett & Kurth, 2008:78). Voyager 2 crossed the solar wind termination shock on 30 August 2007, 2 years and 8½ months after the crossing by Voyager 1. There were a few crossings and re-crossings as the termination shock moved back and forth across Voyager 2 between 30 August and 1 September 2007 at a heliocentric distance of 83.7 AU. Voyager 1 crossed the termination shock at the northern heliographic latitude of 34.3°, whereas Voyager 2 crossed the shock at the southern latitude of -27.5°. The fact that Voyager 2 crossed the termination shock about 10 AU closer to the Sun than did Voyager 1 was likely due, in part, to differences in the solar wind pressure in different parts of the solar cycle. Another difference could as well be the quite different heliolatitudes of the two crossings.

8.4.3

Crossing of the Heliopause by Voyagers 1 and 2

It took an additional 7 years and 8 months for Voyage 1 to reach and cross the heliopause on 25 August 2012 at a distance from the Sun of 121.6 AU. At the time of the crossing, the principal scientists for Voyager were unwilling to identify the boundary as the heliopause, calling it instead the “heliosheath depletion region” (Burlaga et al., 2013). At this boundary, the counting rates for “hot” heliosheath particles, as measured by the low energy charged particle instrument, dropped to instrumental background levels (Fig. 8.22b). The instrument data from this team, led by Stamatios (Tom) Krimigis (Fig. 8.23) of the Johns Hopkins Applied Physics Laboratory showed a clear increase in the galactic cosmic-ray intensity as the spacecraft exited the heliosheath (Fig. 8.22a) (Krimigis et al., 2013:144).

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Fig. 8.21 (a) Intenstites of energetic termination shock particles (TSPs). (b) Azimuthal streaming index. (c) Intensities (5-day moving averages) of ~10 Mev electrons and ~35 Mev H. (d) Intensities (5-day moving averages) of He ions and galactic cosmic rays (GCR) with energy >70 Mev/ nucleon. From Fig. 1 in Stone et al. (2005)

A primary reason for the hesitancy in calling this boundary the heliopause was that, while the magnetic field strength measured by Voyager 1 doubled across the boundary, from 0.2 nT to 0.4 nT, its direction did not change (Burlaga et al., 2013:147). The approximate doubling of the magnetic field strength but no change in direction at the heliopause was also observed by Voyager 2 (Fig. 8.24), which crossed the heliopause in its southern hemisphere on 5 November 2018 at a heliocentric of approximately 119.0 AU (Burlaga et al., 2019). The doubling of the magnetic field strength was understood to be a consequence of pressure balance, i.e., the loss of heliosheath particle pressure was compensated by the increase in magnetic field energy density (Burlaga et al., 2013:146).

8.4

Direct Measurements of the Termination Shock and the Heliopause

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Fig. 8.22 (a) Hourly averages of galactic cosmic ray (GCR) activity and the pronounced boundary crossing of 25 August 2012 (day 238) by Voyager 1. GCR error bars are ±1σ. (b) Intensities of lowto medium-energy ions and low-energy electrons. From Fig. 1 in Krimigis et al. (2013) Fig. 8.23 Tom Krimigis. Courtesy of Tom Krimigis

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Fig. 8.24 (a) One-hour averages of the magnetic field strength, (b) the azimuthal angle λ of the direction of the magnetic field, (c) the elevation angle δ of the direction of the magnetic field, and (d) the 6-h averages of the counting rate for >0.5 MeV/nucleon particles from day 250 to day 365, 2018, as measured by Voyager 2. From Fig. 1 of Burlaga et al. (2019)

8.5

Eddington’s Guidelines

It was not until some observations of terrestrial phenomena (such as geomagnetic storms as recorded by geomagnetic measuring instruments) and extraterrestrial phenomena (orientations of comet tails as recorded visually and by telescopes) that researchers were definitively prompted to consider in detail particulate emissions from the Sun. In terms of the solar system itself, if there were such emissions, how did they interact with the local interstellar medium (about which little was even qualitatively known)? And, if such interactions occurred, what was the implied spatial limit to the Sun’s influence in the solar system? There were a multitude of researchers, beginning with Leverett Davis, who speculated on the distance to the edge of the solar system. As context, and keeping in mind the guidelines of Arthur Eddington (1882–1944) for “science speculation,” modified for applicability to space science (Chap. 1): 1. Was the speculator rigorous in applying the appropriate science applicable to the model

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2. Did the speculator identify all the underlying assumptions used in constructing the model and 3. Did the speculator view the model objectively, as an “adjustable engine,” as opposed to a “finished building?” The largely theoretical speculations that were initially made were based upon only poorly (if that) known physical quantities in the solar system and in the interstellar environment. As described, the speculations of the distance of a termination shock at the edge of the solar system varied widely over many years. And the conclusions arrived at depended strongly upon the data used and the parameters assumed for data that did not as yet exist. The scientists did in general identify the underlying assumptions and data used in arriving at their conclusions. The speculations came closer to the distances later measured by the two Voyager spacecraft as more data were gained from satellite measurements on the nature of the interplanetary medium and on the cosmic rays that populated it. Large differences did occur among speculators who used different sets of observations and assumptions of the solar system and the interstellar medium. Nevertheless, using the sets of data and assumptions they thought most applicable to the problem, the researchers tended to follow the Eddington guidelines.

8.5.1

Estimates Based on Pressure Balance Models

The researchers who used straightforward pressure balance models fared reasonably well. Their speculations would have been more reasonable if they had known, and then been able to use, more accurate values for the solar wind parameters, especially the density. For example, in 1955 Leverett Davis thought that his assumed value for the momentum flux past the Earth was overestimated. After measurements became available, his estimate of 1000/cm3 for the typical solar wind proton number density np near the Earth was seen to be high by a factor of about 200 (Ness, 1968). Had Davis used a value of np = 5/cm3, he would have arrived at an estimated value of the heliosphere boundary r ≈ 140 A.U., rather than 2000 A.U. Davis’s revised estimate in 1962 of the radius of the cavity carved out of the galactic medium by the solar wind assumed that the geomagnetic field at Earth’s magnetopause was 10–20 nT. In 1963, however, Laurence J. Cahill Jr. (1924–2013) and P. G. Amazeen, both of the University of New Hampshire, published their analysis of magnetometer data from the Explorer 12 satellite, which crossed the magnetopause on the Earth’s sunlit side several times during both magnetically active and quiet times. They found that immediately inside Earth’s magnetopause, “the field magnitude is just doubled by the solar wind pressure” (Cahill & Amazeen, 1963). In other words, the pressure of the solar wind compressed Earth’s magnetic field so that it was double the dipole value it would have had in the absence of the solar wind. That means that at a geocentric distance of about 10 Earth radii at the sub-solar

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point in the geomagnetic equator, Earth’s magnetic field is typically 60 nT, rather than the 10–20 nT assumed by Davis. Furthermore, the Voyager spacecraft found that the magnetic field just outside the heliopause was 0.4–0.6 nT, rather than the 1–2 nT assumed by Davis. Putting these measured values into Davis’s simple model would have yielded estimates of the heliocentric distance near the termination shock of about 100–150 AU (assuming that the pressure at the heliopause is transmitted unabated through the heliosheath). As the pressure balance models were refined and used with more accurate parameters as was the case with the 1971 estimate of Thomas McDonough and Neil Brice, the estimate of the distance to the termination shock was found to be about 100 AU (McDonough & Brice, 1971:505). The inclusion of inflowing neutral hydrogen into the pressure balance model in the work of P. W. Blum and Hans-Jörg Fahr resulted in the predicted distance to the termination shock of about 80 AU (Blum & Fahr, 1970:280). Despite the detail in the calculations of Thomas Holzer, his models could only predict a minimum distance to the termination shock, which he set at about 50 A.U. (Holzer, 1972:5428).

8.5.2

Estimates Based on the Observations of the Orientation of Comet Tails

By 1965, John Brandt had moved to the Kitt Peak National Observatory, where he worked with Michael J. S. Belton (1934–2019) on a catalogue of comet-tail orientations (Belton & Brandt, 1966). Using the data in this more complete catalogue, both Belton and Brandt concluded that the prior conclusion regarding a transition in solar wind velocities at 2 AU was no longer supported by the data on comet-tail orientation (Belton, 1965:461; Brandt, 1967:217). By 1995, measurements from several spacecraft, including Pioneer 10, Pioneer 11, Voyager 2, and the eighth Interplanetary Monitoring Platform (IMP 8) demonstrated that the solar wind speed did not decrease by an order of magnitude somewhere between 1 and 43 AU (the location of Voyager 2 at that time) (Richardson et al., 1995:325). Fifty-day averages of the solar wind speed presented by John D. Richardson and colleagues at MIT were found to vary between 350 and 600 km/s (Richardson et al., 1995:326); no large change was measured near 2 AU.

8.5.3

Estimates Based on Lyman-α Measurements

As discussed above, the speculations based on the measurements of Lyman α from neutral hydrogen were in error because the source of the neutral hydrogen was assumed to be derived from charge exchange between solar wind protons and neutral

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hydrogen, rather than directly from interstellar neutral hydrogen penetrating the heliosphere. As the European astronomer, Rosine Lallement, explained: We now know that this interpretation of the residual Lyman-alpha emission was . . . not correct, since the emission is due to hydrogen atoms coming directly from interstellar space and approaching the Sun, and not to neutralized solar wind protons. The authors were misled by the fact that the residual 15% seemed to be constant over the whole sky, while in the case of a directed flow, the hydrogen cell effect would have to depend on the angle with the Earth’s orbital motion, modulating the Doppler shift and the absorption. In fact, this is due to the date of the rocket launch: it happened during the time when the earth is moving upwind, adding to the interstellar wind velocity. In such a case, the region where there is some absorption of the interstellar emission is concentrated in a very narrow band (a few degrees) along a great circle of zero Doppler shift, and if by chance the field of view (FOV) crossed this band, it affected only a few data points, going unnoticed by Morton and Purcell. If the launch had been repeated at another date, they would have noticed the variation with direction of the residual absorption, yielding other theories, in particular a preferential direction of the flow of emitting atoms. (Lallement, 2001:197)

8.5.4

Estimate Based on Jupiter Radio Emissions

Following the discovery at 22 MHz by Bernard Burke and Kenneth Franklin, Jupiter radio emissions in the decametric band from about 8 MHz to about 40 MHz began to be monitored extensively from Earth by amateur radio enthusiasts and professional radio astronomers. The lower decametric “limit” at Earth is determined by Earth’s ionosphere cutting off the lower frequencies; the upper “limit” is due to the Jupiter radio signals falling off in amplitude at higher frequencies. Radio-detecting instruments on the two Voyager spacecraft that encountered Jupiter measured in detail— about 25 years after Burke and Franklin—the relationships of the radio emissions with the radiation belts and Jovian satellite influences, especially the satellite Io with its extensive volcanism (Chap. 10). Over the years NASA has sponsored a project, Radio Jove, to encourage amateurs to monitor radio emissions from Jupiter, the Sun, and other strongly emitting celestial objects. After the report during cycle 19 of the possible solar cycle dependence of the radio emissions from Jupiter, little further evidence of such a dependence was reported. Subsequent studies, especially with the two Voyager encounters, demonstrated that the time dependencies of the decametric (and other) radio emission were a complex mix of many factors, including the planet’s magnetic tilt with respect to an observer, the location of some of the major moons of the planet, and the state of the radiation belts. At the time, the speculation from the radio emissions of the heliosphere boundary to be at 5–10 AU was not far out of line with other boundary discussions. But the speculation was in error largely due to the need for further data, especially the in-situ data finally acquired from Voyager.

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Estimates Based on Cosmic-Ray Gradients

Five years after the publications of the papers by Joseph O’Gallagher and John Simpson in which they estimated that the cosmic-ray modulation region could not extend beyond 5–10 AU, O’Gallagher wrote a review paper in Reviews of Geophysics (O’Gallagher, 1972). In this comprehensive review, he described the results of more than 20 measurements of the heliocentric radial gradient of cosmic rays obtained between 1960 and 1971. He noted that “The striking feature of the measurements is the apparent wide degree of disagreement.” His own 1967 determination of the gradient in particles with energies between 20 and 30 MeV/nucleon was the highest in the data set at +500% per AU. (O’Gallagher, 1972:822). To his credit, O’Gallagher studied the possible reasons that his 1967 measurements were well outside the range of subsequent measurements by others. In his review, O’Gallagher noted several requirements for determining reliable measurements of the radial gradient of cosmic-ray intensities. These included: 1. A reliable detector system, demonstrably free from background and instrumental drifts 2. The need for a reference point measurement to separate temporal from spatial changes owing to the movement through the 11-year cosmic-ray modulation cycle 3. The need to remove the effects of fluctuating contributions to the interplanetary spectrum from particles of solar origin and 4. The need to recognize and account for fluctuations in the modulated density caused by advancing or corotating magnetic shocks. (O’Gallagher, 1972:831–832) In a paper published in 1987, Alan Cummings and colleagues used simultaneous cosmic-ray measurements from Voyagers 1 and 2 and Pioneer 10. They derived a radial gradient of 0.95 ± 0.12%/AU for galactic cosmic rays (Cummings et al., 1987:175). Two years later, at a COSPAR meeting in 1989, R. Walker Fillius of the University of California, San Diego, gave his estimate of the spatial distribution of the cosmic ray intensity, based on multiple spacecraft measurements (Fillius, 1989). Fillius noted that: The spatial dependence of cosmic ray intensities in the heliosphere is slowly being revealed by intercomparisons among several spacecraft widely spread throughout the interplanetary space. Up to the beginning of this year, important spacecraft included Helios 1 and 2 in to 0.3 AU. IMP 8 and ICE near 1 AU, Pioneer 11 out to 25 AU, Voyager 2 to 25 AU, Voyager 1 to 32 AU, and Pioneer 10 to 43 AU. The radial gradient has been studied most extensively, and recent determinations during the approach to solar minimum set its value from 1 to 3%/AU for integral gradients (above some threshold energy), with remarkable independence of the threshold. (Fillius, 1989:(4)209)

Thus, the speculations of O’Gallagher and Simpson on the distance to outer edge of the cosmic ray modulation region (~10 AU) and the supposed location of the

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heliopause, was based on an enormous overestimate of the average radial gradient in the intensities of galactic cosmic rays as they entered the solar system.

8.5.6

Estimates Based on Measurements in the Far Heliosphere

The range of estimates of the heliopause by Gurnett et al. (1993) was based upon plasma wave measurements in the deep heliosphere, with reasonable assumptions made as to the plasma conditions at and beyond a termination shock. The Voyager 1 spacecraft at the time was about half the distance to the later-encountered heliopause. Even so, the range deduced was quite large and spanned many of the theoretical estimates of distances at and beyond about 100 AU that had been made in the past.

8.5.7

Summary

The mean value of the distance to the termination shock determined by the poll of experts at the New Hampshire conference in 1989 (Sect. 8.3.6) proved to be considerably low. The reasons for these and earlier speculations about a short distance to the termination shock proved to be quite varied. A significant number of different physical processes were invoked to arrive at the several determinations over the years. One tie to Eddington’s guidelines, however, might relate to his admonition that speculators should be “rigorous in applying the appropriate science applicable to their models.” In 1969, Eugene Parker pointed out that the speculators who proposed very short distances to the heliopause did not seem to realize the implications of their model. For example, that the required large galactic magnetic field strength might not be consistent with stability considerations for the galaxy. Parker’s critique of this case suggests that the “appropriate science applicable to a given model” should include a careful examination of the wider implications of the model.

8.6

Continuing Understanding

As time progressed from the initial derivations of Leverett Davis, on-going research produced new data and potential new understandings of the physical environments of the solar system and, especially, the neutral hydrogen of the local interstellar medium. Additional spacecraft data and its analysis sharpened prior estimates and new theoretical and detailed modeling techniques arose (see, e.g., Zank et al., 2022,

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for detailed discussions of the theoretical and modeling advances during this time). These advances arose over the decades following Davis in 1955 to the mid-1970s as the Pioneers and Voyagers were sent on their way and to the 2000s as Voyager data continued to be received. This is also how science progresses: speculations resulting in new research to confirm or modify or refute the speculations, leading to yet further understandings—and often new questions. Following the crossing of the termination shock, the two Voyagers passed through and provided plasma measurements of the extensive heliosheath region. Plasma physical processes operative in this region beyond the shock continued to be studied (e.g., Decker et al., 2012; Cummings et al., 2021). The size of the solar system and thus the location of the boundary (the heliopause) with the interstellar medium depends upon the configuration of the heliosphere. Is the heliosphere shaped more like Earth’s magnetosphere, with a possibly extended “tail” (as schematically illustrated in Fig. 8.16)? In this case, the exit the Voyagers made was on the “front,” in the direction of motion of the solar system with respect to the local interstellar cloud. A spacecraft launched in the opposite direction of the Voyagers might not have encountered a boundary as yet (the Pioneer 10 mission trajectory was oriented in the direction of a possible heliosphere tail, but the spacecraft lost power long before it could make measurements of any distant region). The crossing, finally, of the heliopause has encouraged substantial further theoretical research on the nature of the interstellar medium. Actual data were now available on the interactions of the very local interstellar medium with the solar system, and of the medium itself, at least at the two locations crossed by Voyager (e.g., Krimigis et al., 2019; Dialynas et al., 2021, 2022; Richardson et al., 2022; Mostafavi et al., 2022). Neutral particle detection with Energetic Neutral Atom (ENA), instruments on the Interstellar Boundary Explorer (IBEX) and Cassini spacecraft have provided important data that can give indication of the shape of the heliosphere. Data from ENA investigations show considerable variation of the ENA intensities over the spatial boundary of the heliosphere with time and with solar wind intensity over a solar cycle. The IBEX data revealed a “ribbon” structure over the heliosphere (McComas et al., 2009), and have tended to favor the magnetosphere analogy for the heliosphere’s shape (McComas et al., 2020). Data returned from the ENA instrument the Ion and Neutral Camera (INCA) on the Cassini spacecraft, obtained prior to Saturn encounter and while it encircled the planet for nearly 20 years, have been interpreted to suggest that the heliosphere may be more of a diamagnetic bubble in the local galaxy (e.g., Westlake et al., 2020). ENA data continue to be actively examined in order to “picture” the shape of the heliosphere (e.g., Galli et al., 2017, 2022; Dialynas et al., 2022; Kleimann et al., 2022). The planned launch in 2024 of the Interstellar Mapping and Acceleration Probe (IMAP) will provide additional fundamental information and insights from ENA data about the boundary region of the heliosphere (McComas et al., 2018). The New Horizons mission (that encountered Pluto in 2015 at about 39 AU) could send a signal of its crossing of a heliosphere boundary if its instruments and plutonium power source are still active in the 2030s. An eventual interstellar probe mission,

References

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especially if it were to use the Sun for a gravity assist, could exit the heliosphere much more rapidly than any of the previous missions (McNutt Jr. et al., 2022). Gary P. Zank, in an examination of the Voyager 1 results after its crossing of the heliopause in 2012, expressed how the new results from the heliosphere and into the local interstellar medium “will be useful to the new field of astrospherical astrophysics” Zank (2015). That is, stellar objects can produce “astrospheres,” analogous to the heliosphere. As data continued to be returned by the Voyagers, these two missions launched decades prior established the frontier in Earth’s place in the Milky Way galaxy.

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Cummings, A. C., Stone, E. C., & Webber, W. R. (1987). Latitudinal and radial gradients of anomalous and galactic cosmic rays in the outer heliosphere. Geophysical Research Letters, 14(3), 174–177. Cummings, A. C., Stone, E. C., Richardson, J. D., Heikkila, B. C., Lal, N., & Kota, J. (2021). No stagnation region before the heliopause at Voyager 1? Inferences from new Voyager 2 results. The Astrophysical Journal, 906, 126. Davis, L., Jr. (1955). Interplanetary magnetic fields and cosmic rays. Physical Review, 100(5), 1440. Davis, L., Jr. (1962). The effect of solar disturbances and the galactic magnetic field on the interplanetary gas. Journal of the Physical Society of Japan Supplement, 17, 543. Decker, R. B., Krimigis, S. M., Roelof, E. C., Hill, M. E., Armstrong, T. P., Gloeckler, G., Hamilton, D. C., & Lanzerotti, L. J. (2005). Voyager 1 in the foreshock, termination shock, and heliosheath. Science, 309(5743), 2020–2024. Decker, R. B., Krimigis, S. M., Roelof, E. C., & Hill, M. E. (2012). No meridional plasma flow in the heliosheath transition region. Nature, 489, 124–127. Dessler, A. J. (1967). Solar wind and interplanetary magnetic field. Reviews of Geophysics, 5(1), 1–41. Dialynas, K., Krimigis, S. M., Decker, R. B., & Hill, M. E. (2021). Ions measured by Voyager 1 outside the heliopause to ~28 au and implications thereof. The Astrophysical Journal, 917. Dialynas, K., Krimigis, S. M., Decker, R. B., Hill, M., Mitchell, D. G., Hsieh, K. C., Hilchenbach, M., & Czechowski. (2022). The structure of the global heliosphere as seen by in-situ ions from the Voyagers and remotely sensed ENAs from Cassini. Space Science Reviews, 218. Fillius, W. (1989). Cosmic ray gradients in the heliosphere. Advances in Space Research, 9(4), 209–219. Flandro, G. A. (1966). Fast reconnaissance missions to the outer solar system utilizing energy derived from the gravitational field of Jupiter. Astronautica Acta, 12(4), 329–337. Galli, A., Wurz, P., Schwadron, N. A., Kucharek, H., Mobius, E., Bzowski, M., Sokol, J. M., Kubiak, M. A., Fuselier, S. A., Funsten, H. O., & McComas, D. J. (2017). The downwind hemisphere of the heliosphere: Eight years of IBEX-Lo observations. The Astrophysical Journal, 815, 2. Galli, A., Baliukin, I. I., Bzowski, M., Izmodenov, V. V., Kornbleuth, M., Kurcharek, H., Mobius, E., Opher, M., Reisenfeld, D., Schwadron, N. A., & Swaczyna, P. (2022). The heliosphere and local interstellar medium from neutral atom observations at energies below 10 kev. Space Science Reviews, 218, 31. Gloeckler, G., & Jokipii, J. R. (1967). Solar modulation and the energy density of galactic cosmic rays. The Astrophysical Journal, 148, L41. Gurnett, D. A., & Kurth, W. S. (2005). Electron plasma oscillations upstream of the solar wind termination shock. Science, 309(5743), 2025–2027. Gurnett, D. A., & Kurth, W. S. (2008). Intense plasma waves at and near the solar wind termination shock. Nature, 454(7200), 78–80. Gurnett, D. A., Kurth, W. S., Allendorf, S. C., & Poynter, R. L. (1993). Radio emissions from the heliopause triggered by an interplanetary shock. Science, 262, 199–203. Holzer, T. E. (1972). Interaction of the solar wind with the neutral component of the interstellar gas. Journal of Geophysical Research, 77(28), 5407–5431. Hundhausen, A. J. (1968). Interplanetary neutral hydrogen and the radius of the heliosphere. Planetary and Space Science, 16(6), 783–793. Kiepenheuer, K. O. (1953). Solar activity. In G. P. Kuiper (Ed.), The Sun. University of Chicago Press. Kleimann, J., Dialynas, K., Fraternale, F., Galli, A., Heerrikhuisen, J., Izmodenov, V., Kornbleuth, M., Opher, M., & Pogorelov, N. (2022). The structure of the large-scale heliosphere as seen by current models. Space Science Reviews, 281.

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Chapter 9

Sources of Gamma-Ray Bursts

9.1

Introduction

The advent of the scientific use of sounding rockets following WWII, and the opening of space research with the launching of satellites, enabled an explosion of research in measuring electromagnetic radiation from astronomical objects. Of special interest enabled by space flight technologies were those electromagnetic frequencies on both sides of the visual band that are blocked by the ionosphere and by chemical constituents of Earth’s atmosphere. Gamma rays are the most energetic of photons, classified as those photons with energies greater than 100 keV. These photons originate from sources such as the decay of highly excited nuclear species and from electron and positron annihilations. The first measurements of gamma rays from the Milky Way galaxy (the galaxy in which the solar system resides) were achieved in the late 1960s by the American Orbiting Solar Observatory-3 (OSO-3) satellite (Clark et al., 1968). A serendipitous discovery of highly intense and short-lived bursts of gamma rays was made by instruments on satellites launched in the late 1960s for the detection of clandestine nuclear explosions in space. This chapter discusses the theoretical interpretations of the bursts that accompanied their continued measurements into the 1970s. The theoretical speculations and debates involved the origins of the bursts; that is, their sources. Were the sources of the bursts in the neighborhood of the Milky Way—the “galactic” hypothesis, or were the sources at great distances from it—the “cosmological” hypothesis. The location of the sources is obviously of great importance in achieving an eventual understanding of very energetic processes operative in the universe. It is difficult to determine the individuals who initially made these two speculations. However, the chapter examines the arguments that were made during a debate between representatives of both sides, and subsequent developments in the science.

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 W. D. Cummings, L. J. Lanzerotti, Scientific Debates in Space Science, Astronomy and Planetary Sciences, https://doi.org/10.1007/978-3-031-41598-2_9

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9.2

The Discovery of Gamma–Ray Bursts

Sources of Gamma-Ray Bursts

Very short-lived bursts of gamma rays had been detected in the late 1960s and early 1970s by instruments on the U.S. Vela satellites. The Vela program operated from 1963 to 1984, with spacecraft placed in high (118,000 km) circular orbit around the Earth to watch for possible violations of the 1963 treaty that banned nuclear weapon tests in outer space (the treaty also banned tests in the atmosphere and under water). While the satellites were emplaced to monitor the Soviet Union’s compliance with the treaty, researchers at the Los Alamos National Laboratory quickly realized that the gamma-ray bursts their instruments were detecting did not come from Soviet nuclear explosions. In 1973, Ray W. Klebesadel, Ian B. Strong, and Roy A. Olson of the Los Alamos National Laboratory reported that the analysis of Vela satellite data over the 3-year period July 1969–July 1972 demonstrated that measured gamma rays with bursts as short as 0.1 s and as long as 30 s did not come from the Earth or the Sun (Klebesadel et al., 1973). So where did they come from? The energy carried by a photon is proportional to the frequency of the photon, or inversely proportional to its wavelength. Gamma rays have the highest frequency and shortest wavelength of all the photons. Their wavelength is comparable to the size of an atomic nucleus. The energy of a gamma ray is hundreds of thousands times the energy of a photon in the “visible” range, i.e., a photon to which the human eye responds. Earth’s atmosphere protects human and other life from the otherwise lethal effects of gamma rays and other high frequency radiation that occur from processes on the Sun and from other extra-terrestrial sources. Because of their high energy, gamma rays cannot be easily focused, which means that localizing gamma-ray sources is difficult. While accurately pinpointing individual gamma-ray bursts was not possible with instrumentation available in the 1970s, determining the distribution of the energies of gamma-ray bursts could be achieved. If the sources of gamma-ray bursts were distributed uniformly throughout an unchanging space, the number, N, of sources emitting fluences (ergs/cm2) greater than a given amount, S, of energy would increase with the increase in volume, i.e., as the cube of distance, while the fluence would decrease by the inverse square law for radiating sources, i.e., as the square of distance. Because of these volume (3) and inverse-square (2) factors, the slope of a plot of the log(N) vs log(S) curve for such a distribution of sources should be -3/2. It was soon shown from the Vela and other data that the -3/2 slope described the strong gamma-ray bursts (Fishman et al., 1978). In 1975 and 1977, Gerald J. Fishman (Fig. 9.1) and his colleagues at NASA’s Marshall Space Flight Center (MSFC) in Huntsville, Alabama, conducted two high altitude balloon flights from the National Scientific Balloon Facility in Palestine, Texas. They flew gamma-ray detectors that could measure relatively low fluences, i.e., weaker bursts with energies in the range from 30 to 200 keV (Fishman et al., 1978). The idea was to test whether the distribution of sources for gamma-ray bursts gave a value of -3/2 for the slope of the log(N) vs log (S) curve for weak bursts as was found for the strong bursts.

9.3

The Burst and Transient Source Experiment (BATSE)

227

Fig. 9.1 Gerald Fishman with the BATSE instrument. Credit NASA

Fishman and his colleagues reported that no burst candidates were found on either balloon flight (Fishman et al., 1978:L14). This finding led Fishman and his colleagues to conclude that the sources of gamma-ray bursts were unlikely to be at extra-galactic distances, as they wrote in summarizing their results: Two balloon flights of large-area scintillation crystal detector arrays indicate that the rate of weak γ-ray bursts is significantly below that expected from a uniform distribution of burst sources. This result, combined with the data from stronger bursts, gives strong evidence for a galactic confinement of burst sources. . . . (Fishman et al., 1978:L13)

Thus, Fishman and his colleagues at MSFC were among the first to argue that the sources of gamma-ray bursts were in the neighborhood of the Milky Way galaxy.

9.3

The Burst and Transient Source Experiment (BATSE)

To continue to probe the source of gamma-ray bursts Fishman and his team at MSFC proposed the Burst And Transient Source Experiment (BATSE) to be flown as an instrument on the Gamma Ray Observatory (GRO) satellite. The GRO was later re-named the Compton Gamma Ray Observatory (CGRO) in honor of the American physicist and 1927 Nobel Prize winner Arthur H. Compton (1892–1962). In the 1930s Compton led a major effort for the studies of the intensities of cosmic rays with geographical latitude (Chap. 1). The CGRO was one of NASA’s “Great Observatories,” a series of four large space observatories designed to examine the universe from the infrared to the gamma-ray regime. (The set of Great Observatories also includes the Hubble Space Telescope, the Spitzer Space Telescope, and the Chandra X-ray Observatory.) Fishman turned to the Universities Space Research Association (USRA) to help him enlarge his team, which would work on various aspects of BATSE. In January 1991, USRA hired Chryssa Kouveliotou (Fig. 9.2) to work with Fishman and others

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Fig. 9.2 Chryssa Kouveliotou. Credit NASA

on the BATSE team at MSFC. Kouveliotou is an expert on gamma-ray bursts; she was the first student with a PhD thesis on the subject. She took a leave of absence from the University of Athens, Greece, to help develop BATSE data analysis software and to analyze and interpret these data. There wasn’t much time for development work. The CGRO with BATSE on board was launched on the space shuttle Atlantis on 5 April 1991. The BATSE instrument was activated on 21 April 1991 and began to record gamma-ray bursts at a rate of about one per day. The CGRO ceased operations in 2000. To further explore the question about the distance to the sources of gamma-ray bursts, as well as other questions about them, NASA and USRA co-sponsored the first Huntsville Gamma-Ray Burst Workshop in the fall of 1991. Over 130 scientists from around the world attended and discussed their research in the context of the recently acquired BATSE data. Prior to BATSE, scientific consensus was that the sources for gamma-ray bursts were, as Fishman and his colleagues put it, confined by the Milky Way galaxy. If the sources were actually in the Milky Way, a plot of the positions of the gamma-ray bursts would show most of them near the disk of the galaxy. Perhaps the most important result to come out of this first Huntsville Gamma-Ray Burst Workshop was that gamma-ray bursts seemed to be coming from sources that are randomly distributed across the entire sky, i.e., the distribution seemed to be isotropic, and not confined to the galactic plane. The first scientific paper by members of the BATSE team was published in Nature on 9 January 1992 (Meegan et al., 1992). The team ran tests on the data from 153 gamma-ray bursts that occurred between 21 April 1991 and 31 October 1991. The results demonstrated analytically that there is no statistically significant deviation from isotropy across the sky. In Fig. 9.3, which was presented in the BATSE team’s paper, the format of the plot allows for a two-dimensional presentation of the whole sky, from the north to the south galactic pole in latitude and a full 360 degrees in galactic longitude. The figure shows the positions of the 153 gammaray bursts.

9.3

The Burst and Transient Source Experiment (BATSE)

229

Fig. 9.3 Measured distribution of gamma-ray bursts as of 1992. From Fig. 2 of Meegan et al. (1992)

Fig. 9.4 Gamma-ray bursts observed by BATSE over its entire mission. Credit NASA

As more gamma-ray bursts were observed over the life of the CGRO, the isotropic distribution was confirmed. During the entire mission, BATSE observed 2704 gamma-ray bursts (Fig. 9.4). The BATSE results now suggested to many astronomers that the sources of the gamma-ray bursts must be “at cosmological distances,” i.e., at distances beyond the Milky Way galaxy, and the event distribution should follow the isotropic distribution of distant galaxies. If the sources really were associated with distant galaxies, then the BATSE results implied that gamma-ray bursts are some of the brightest explosions, if not the brightest, in the observable universe. Some researchers argued at the time, however, that the sources might be neutron stars in a halo or corona around the Milky Way. This theory is plausible, in part,

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because of the extraordinary characteristics of neutron stars. A neutron star is the remnant of a massive star that has used up its inner nuclear fuel, the burning of which kept it supported against the force of its self-gravity. Once the nuclear fuel has been spent, the stellar core collapses into an incredibly dense ball in which electrons and protons are squeezed together into neutrons and other sub-atomic species. As the core collapses, the down-rushing, outer layers of the star collide with the neutron core, and then rebound, producing an explosive outpouring of energy called a “supernova.” The Crab Nebula in the Milky Way is an example of this process: a massive star that exploded as a supernova and was observed by Chinese astronomers in 1054 CE. The neutron star left behind by the Crab supernova is about 30 km across, has a spin rate of about 30 revolutions per second, and is located near the center of the nebula in the constellation Taurus. The halo (or corona) theory posits that, depending on the energy of the explosion and whether the explosion was asymmetric, a supernova could impart a highvelocity kick sufficient to propel a neutron star out of the Milky Way galaxy. These ejected neutron stars could eventually form a nearly isotropic halo/corona around the galaxy. In principle, as a result, these ejected neutron stars could produce the distribution of gamma-ray bursts measured by BATSE.

9.4

The Debate

On 22 April 1995, a debate was held in the Baird Auditorium of the Smithsonian Institution’s Natural History Museum. The title of the debate was The Distance Scale to Gamma Ray Bursts. Seventy-five years earlier (1920) in the same auditorium Heber D. Curtis (1872–1942) of the University of Michigan and Harlow Shapley (1885–1972) of the Mount Wilson Observatory (and, beginning in 1921, Director of the Harvard College Observatory) participated in an historic debate titled The Scale of the Universe (Nemiroff, 1995:1131). With analogies to the gamma-ray burst controversy, Shapley argued that distant nebulae were small and were within the region of the Milky Way; Curtis argued for them being independent galaxies at large distances. Following some introductory talks before the 1995 debate, Gerald Fishman provided an overview of what was known about gamma-ray bursts. He showed a sky distribution chart, similar to the one shown in Fig. 9.4, though not as completely filled in, and he illustrated the deviation from the -3/2 power law with the graphic shown in Fig. 9.5. Fishman concluded his talk with the following summary: The consensus opinion of the locale of the sources of gamma-ray bursts has changed from a fraction of a galactic scale height to either an extended galactic halo or to cosmological distances. . . . A wealth of new data on time profiles, spectral characteristics, and burst distributions has thus far failed to provide conclusive evidence on the distance scale, central objects(s) or emission mechanism(s) for the classical gamma-ray bursts. The isotropy and inhomogeneity of the bursts shows only that we are at or near the center of the apparent burst distribution. For the galactic halo models, the apparent isotropy of observed bursts requires

The Debate

Fig. 9.5 The intensity distribution of gamma-ray bursts, showing the clear deviation from the -3/2 power law that would be expected if the gamma-ray bursts were homogeneously distributed in Euclidean space. From Fig. 4 in Fishman (1995)

231 Peak flux distribution: 1024 ms. timescale 1000

100 Number of Bursts

9.4

10

1 0.1

1.0

10.0

100.0

Photons/cm2/sec: 50–300 keV

that the distribution radius be significantly greater that the distance between Earth and the Galactic Center . . . The required source luminosity is of order 1042 ergs s-1 (for isotropic emission).

z In astronomy, z denotes the fractional change in the wavelength λ of photons caused by the relative motion of the source (s) of the emission of the photons and the observer (o). z=

ð λo - λ s Þ λs

For photons emitted from nearby stars, z can be either positive or negative because of the local motion of the star relative to the Sun. For example, the Andromeda galaxy, which is the closest big galaxy to the Milky Way and which is moving toward the Milky Way, has a z value of -0.001. Photons emitted far from the Milky Way, however, have z values that are positive because of the expansion of the universe. For example, the Sombrero galaxy has a z value of +0.0034. The more distant the galaxy, the larger the z value. Currently, the most distant galaxy known is GN-z11, with a z value of +11.09. (The z values quoted above were obtained from the NASA/IPAC Extragalactic Database.)

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Fig. 9.6 Bohdan Paczyński. Credit R. P. Matthews, Princeton University, courtesy of AIP Emilio Segrè Visual Archives, Physics Today Collection

The observed isotropy is a necessary requirement of cosmological models. The apparent inhomogeneity would result from redshift effects, and possibly source evolution. Satisfactory fits can be found using standard candle luminosities, standard cosmologies, and no source evolution. The weakest bursts originate from sources that are at redshifts of about z ~ 1 to 2 and the fundamental luminosity is of order 1050 to 1052 erg s-1, assuming isotropic emission. (Fishman, 1995:1150)

The debate about the distance to the source of gamma-ray bursts began with a presentation by Bohdan Paczyński (1940–2007) (Fig. 9.6), a distinguished Polish astronomer who had immigrated to the United States in 1981 and joined Princeton University. Paczyński was an early advocate for a cosmological distance scale for the sources of gamma-ray bursts (see Paczyński, 1991), and he took that side of the debate. His starting point was that “we do not know what gamma-ray bursters are and what makes them burst” (Paczyński, 1995:1167). Paczyński, therefore, did not base his arguments on any of the various models for gamma-ray bursts. He took “a standard astronomical approach to the distance determination” (Paczyński, 1995:1167), summarizing his argument as follows: The positions of over 1000 gamma-ray bursts detected with the BATSE experiment on board the Compton Gamma Ray Observatory are uniformly and randomly distributed in the sky, with no significant concentration to the galactic plane or to the galactic center. The strong gamma-ray bursts have an intensity distribution consistent with a number density independent of distance in Euclidean space. Weak gamma-ray bursts are relatively rare, indicating that either their number density is reduced at large distances or that the space in which they are distributed is non-Euclidean. In other words, we appear to be at the center of a spherical and bounded distribution of bursters. This is consistent with the distribution of all objects that are known to be at cosmological distances (like galaxies and quasars), but inconsistent with the distribution of any objects which are known to be in our galaxy (like stars and globular clusters). If the bursters are at cosmological distances then the weakest bursts should be redshifted, i.e., on average their durations should be longer and their spectra should be softer than the corresponding quantities for the strong bursts. There is some evidence for both effects in the BATSE data. At this time the cosmological distance scale is strongly favored over the galactic one, but is not proven. A definite proof (or disproof) could be provided with the results of a search for very weak bursts in the Andromeda galaxy (M31) with an instrument ~ 10 times more sensitive than BATSE. If the bursters are indeed at

9.4

The Debate

233

Fig. 9.7 Donald Lamb. Credit the University of Chicago

cosmological distances then they are the most luminous sources of electromagnetic radiation known in the Universe. At this time we have no clue as to their nature, even though well over a hundred suggestions have been published in the scientific journals. An experiment providing ~ 1 arcsecond positions would greatly improve the likelihood that counterparts of gamma-ray bursters are finally found. A new interplanetary network would offer the best opportunity. (Paczyński, 1995:1167)

Paczyński elaborated on the reason for searching for weak gamma-ray bursts around the nearby Andromeda galaxy (M31): Note that all types of objects known to exist in our galaxy are always found in other galaxies as soon as the detectors are sensitive enough to uncover those objects at extragalactic distances. Therefore, we may have full confidence that if the known gamma-ray bursts are associated with our galaxy there should also be gamma-ray bursts associated with M31, which is the nearest giant spiral, pretty much like our own (Atteia & Hurley, 1986). This issue cannot be resolved by BATSE, at least not yet (Hakkila et al., 1994), but a new experiment with ~ 10 times BATSE’s sensitivity should resolve the issue (See Harrison et al., 1995). If a concentration of very weak bursts towards M31 is detected we shall have to accept the fact that the gamma-ray bursters are in the corona of our Galaxy, establishing a new type of a distribution of astronomical objects. If no signature of M31 is found we shall have to accept the cosmological origin of the bursts. (Paczyński, 1995:1174)

Donald Q. Lamb (Fig. 9.7) of the Enrico Fermi Institute of the University of Chicago then presented the case for the galactic origin of gamma-ray bursts. Lamb based his argument on: . . . the recent discovery that many neutron stars have high enough velocities to escape from the Milky Way. These high velocity neutron stars form a distant, previously unknown Galactic “corona.” This distant corona is isotropic when viewed from Earth, and consequently, the population of neutron stars in it can easily explain the angular and brightness distribution of the BATSE bursts. If this were all of the evidence that we considered, we could not distinguish the cosmological and Galactic hypotheses. I contend that we can go further, by considering other important evidence. I draw attention to the many similarities between soft gamma-ray repeaters, which are known to be high-velocity neutron stars, and gamma-ray bursts. I point out that the source of the famous 1979 March 5 event, which is a high-velocity neutron star 50 kpc [a kiloparsec (kpc)—corresponds to 3,260 light years] away from us, demonstrates that high-velocity neutron stars are capable of producing bursts

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which have the energy, the duration, and the spectrum of gamma-ray bursts. Finally, I comment that high-velocity neutron stars in a distant Galactic corona can account for cyclotron lines and repeating, and naturally explain the absence of bright optical counterparts in gamma-ray burst error boxes, whereas all of these present major difficulties for cosmological models. I conclude that when we consider all of the evidence, it adds up to a strong case for the Galactic hypothesis. (Lamb, 1995:1152)

Toward the end of his talk, Lamb discussed avenues of research that might help to distinguish between the Galactic and cosmological hypotheses. He noted, as did Paczyński, that lack of observations of bursts in the vicinity of the Andromeda Galaxy would be compelling evidence for the cosmological model. Nevertheless, he thought a definitive result would require instruments 50 times more sensitive than those of BATSE.

9.4.1

The Source of Soft Gamma-Ray Repeaters

Lamb based much of his argument on the similarities between the characteristics of gamma-ray bursts and soft gamma-ray repeaters (SGRs). SGRs had been discovered in March 1979 from measurements made on several U.S. and Soviet satellites, including the U.S. Pioneer Venus Orbiter around Venus. SGRs were found not to be “normal” gamma-ray bursts; their photons were less energetic in low-energy gamma-ray and hard X-ray ranges, and repeated bursts came from the same astronomical region. In his argument, Lamb noted that SGRs had been identified to be high-velocity neutron stars. SGRs represented a small number of gamma-ray bursts that were found to repeat at irregular intervals. SGRs typically had a very short (a fraction of a second) high-energy burst followed by a longer, fainter glow of lower-energy X-rays (Schilling, 2002:199). The BATSE team took the position that there are varieties of gamma-ray bursts, and they had been trying to identify some of the categories. Chryssa Kouveliotou was the lead author for the team on an important paper published in The Astrophysical Journal in 1993 titled Identification of Two Classes of Gamma-ray Bursts (Kouveliotou et al., 1993). In the paper, she and her BATSE colleagues demonstrated that the short bursts of gamma-rays, defined as those with durations less than 2 seconds, had a larger proportion of higher energy photons than the bursts with longer durations. The short bursts were said to have “hard energy spectra,” while the bursts of longer duration were said to have “soft energy spectra.” Kouveliotou suggested that SGRs represent a completely different phenomenon from either of the other two categories of gamma-ray bursts, and she set out to understand them. Kouveliotou’s success in this endeavor is an interesting story of space research in itself. Suffice to say here that she gave experimental confirmation to a theory of Robert C. Duncan of the University of Texas at Austin and Christopher Thompson of the University of Toronto that SGRs are the result of large fractures in the crust of highly magnetized neutron stars (they called them “magnetars”) within the Milky Way (Duncan & Thompson, 1992; Thompson & Duncan, 1996; Kouveliotou et al.,

9.5

Eddington’s Guidelines

235

2003). The source of SGRs, then, would be “starquakes”, similar to the sudden energy released by earthquakes, but trillions of times more powerful. For her work in proving the existence of magnetars, Chryssa Kouveliotou shared with Christopher Thompson and Robert Duncan the 2003 Rossi Prize of the High Energy Astrophysics Division of the American Astronomical Society. Kouveliotou was elected to the U.S. National Academy of Sciences in 2013, and she received the prestigious Shaw Prize of the Shaw Prize Foundation in Hong Kong in 2021. For his contributions to gamma-ray astronomy through BATSE, Gerald Fishman was awarded the Rossi prize in 1994 and the Shaw Prize in 2011.

9.5

Eddington’s Guidelines

As discussed in Chap. 1, Arthur Eddington laid down some guidelines for science speculation that have been slightly modified to make them more broadly applicable to space science: 1. Was the speculator rigorous in applying the appropriate science applicable to the model 2. Did the speculator identify all the underlying assumptions used in constructing the model and 3. Did the speculator view the model objectively, as an “adjustable engine,” as opposed to a “finished building?” The speculations in this chapter are those highlighted by the debate between Bohdan Paczyński and Donald Lamb; i.e., simply whether the sources of gamma-ray bursts are (1) confined in some way by the Milky Way, or (2) are at cosmological distances. Paczyński argued only from astronomical, rather than astrophysical, evidence, since he claimed that the origin of gamma-ray bursts was completely unknown. Lamb, on the other hand, used evidence other than the isotropic distribution of the sources across the sky and the known distribution of peak energies of the bursts. He drew analogies with soft gamma-ray repeaters, for example, which turned out to be misleading, as shown by the later research of Robert Duncan, Christopher Thompson, and Chryssa Kouveliotou. The debate of Paczyński and Lamb did not resolve the question of the distance to the sources of “classical” gamma-ray bursts; i.e., those other than the soft gamma-ray repeaters. Accumulating evidence soon began to support the cosmological hypothesis (Sect. 9.6). However, detailed models of gamma-ray bursts were not at issue, so Eddington’s questions of whether the speculators used the appropriate science and identified all their assumptions for their models is moot. It seems that both speculators were willing to concede to the other’s position should future evidence require it, and both identified possible ways to check their speculations. In this sense their debate would seem to satisfy Eddington’s third guideline, namely that the protagonists should be as objective as possible.

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9.6 9.6.1

9

Sources of Gamma-Ray Bursts

Continuing Understanding The Resolution of the Question of the Distance Scale for Gamma-Ray Bursts

The science of cosmic gamma-ray bursts (GRBs) was ultimately found to be much larger and more complex than might have been expected at the time of their surprising discovery by the Vela satellite program. As described, the subsequent investigations required sophisticated instrumentation and creative data analyses and theory. Some investigations are still very much frontier research in progress. The sequence of events that led to a confirmation of the cosmological hypothesis is described in Levan (2018). First, it was discovered that there is an afterglow of lower energy photons following a gamma-ray burst. The afterglow is thought to arise from the plasma heated by the gamma-ray burst in the host galaxy of the burster. Second, satellites were launched with instrumentation that could deliver accurate positions of the afterglows to arcminute error boxes and consequently identify the host galaxy. The Italian and Dutch satellite BeppoSAX (1996–2002) was the first of these satellites (Levan, 2018:1-10, 1-11). In a memorial to the high-energy astrophysicist Johannes A. (Jan) van Paradijs (1946–1999) (Fig. 9.8), the Dutch astronomer Edward P. J. van den Heuvel related the inside story of how van Paradijs and his students made the first optical identification of a gamma-ray burst (van den Heuvel, 2003). In 1997, van Paradijs spent 7 months of the year at his home institution, the University of Amsterdam, and 5 months as a professor at the University of Alabama in Huntsville, where he worked closely with the BATSE team that included his wife, Chryssa Kouvelioutou. On 28 February 1997, BeppoSAX recorded a gamma-ray burst (GRB 970229), and the

Fig. 9.8 Jan van Paradijs. Credit Chryssa Kouvelioutou

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Fig. 9.9 From Fig. 6 of van den Heuvel (2003). The left-hand picture was taken by the William Herschel Telescope 20.8 h after the detection of the GRB on 28 February 1997. The right-hand picture was taken by the Isaac Newton Telescope on 8 March 1997. OT stands for Optical Transient. Credit Edward van den Heuvel

accurate arc-minute position of its hard X-ray afterglow, recorded by BeppoSAX’s Wide-Field hard X-ray cameras, was given by the BeppoSAX team to Jan van Paradijs for radio follow-up. In his tribute to van Paradijs, van den Heuvel writes about what happened next: Jan at the time was in Huntsville and had passed the position of the error circle of this GRB on to his graduate student Titus Galama in Amsterdam for immediate radio follow-up with the Westerbork Synthesis Radio Telescope. Paul Groot, one of Jan’s other graduate students, carrying out PhD research on Cataclysmic Variables, happened to be in the same room as Galama that same evening when the GRB position arrived. Groot suddenly realized that for that very night of 28 February Jan had been granted time on the William Herschel Telescope at La Palma [one of the Canary Islands of Spain] for studying the WFC [Wide Field Camera on the BeppoSAX satellite] error box of an earlier BeppoSAX GRB, that had occurred on 11 January 1997. These observations were being carried out by Jan’s former graduate student John Telting, who was now staff astronomer at La Palma. Paul Groot desperately tried to call Jan in Huntsville to ask him whether it would be OK to ask Telting to point the telescope at the error box of GRB 970228. He could, however, not reach Jan, as Jan happened to be teaching a class in Huntsville (where it is 7 hours earlier than in Amsterdam). Paul then decided himself to alert La Palma and ask them to take a CCD frame of the error box with the William Herschel Telescope. He asked Telting to take another exposure of the same field 8 days later, which was done with the Isaac Newton Telescope. The comparison of the February 28 and March 8 fields (Figure 6) [reproduced in Fig. 9.9] immediately showed the fading optical afterglow of the GRB. Thus Jan’s team had realized the first ever optical identification of a Gamma Ray Burst (van Paradijs et al., 1997; van den Heuvel, 2003:15–16)

Van Paradijs and his colleagues reported their discovery in a letter to Nature magazine (van Paradijs et al., 1997). They pointed out that the fading remnant of the burst seemed to be associated with a faint galaxy: Here we report the detection of a transient and fading optical source in the error box associated with the burst GRB970228, less than 21 hours after the burst. The optical transient appears to be associated with a faint galaxy, suggesting that the burst occurred in that galaxy and thus that γ-ray bursts in general lie at cosmological distance. (van Paradijs et al., 1997:686)

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An attempt was made to take a spectrum of the optical afterglow of GRB970228 with the twin Keck telescopes atop Mauna Kea on the island of Hawaii so that the fractional change in the wavelength of spectral lines, z, could be determined. By the time the spectrum measurement for GRB970228 was attempted, however, the afterglow had grown so faint that the determination of the redshift wasn’t successful. (van den Heuvel, 2003:16). A few months later an optical counterpart for a gamma-ray burst (GRB970508) was found for which the spectra of the host galaxy could be obtained. In a paper written shortly before the death of van Paradijs and published in 2000 in the Annual Reviews of Astronomy and Astrophysics, van Paradijs and his colleagues wrote: Optical spectroscopy obtained with the Keck telescope revealed the presence of absorption lines of Mg II, FeII, and MgI [i.e., multiple-ionized atoms of iron and magnesium] . . . redshifted by z = 0.835 (Metzger et al., 1997). The subsequent discovery of [OII] and [Ne III] [multiple ionized atoms of oxygen and neon] emission lines in the spectrum at the same z = 0.835 [see Metzger et al., 1997a] established the presence of an underlying host galaxy. . . . This result unambiguously established that GRBs originate at cosmological distances, and terminated the discussion on the GRB distance scale . . . (van Paradijs et al., 2000)

An object with a z value of 0.835 is indeed a long way from the Milky Way galaxy. For comparison, the cluster of galaxies denoted by Abell 2667, found in 2007 by astronomers using the Hubble Space Telescope, is about 3.3 billion light years from Earth (NASA/IPAC Extragalactic Database), and its z value is only about 0.23 (Cortese et al., 2007). (As the name implies, a light year is the distance light travels in a year. The diameter of the disk of the Milky Way galaxy is about 100,000 light years.) The implication of observing gamma-ray bursts coming from such great distances is that their central engines must generate, in a matter of seconds, more energy than the Sun could emit over billions of years, assuming that the gamma-rays are emitted at the same rate in all directions.

9.6.2

The Physical Mechanism(s) for GRBs

The details of gamma-ray bursts remain a subject of intense scientific enquiry. Further research has supported the observation by Kouveliotou and others that there are two classes of gamma-ray bursts, one of long duration and the other of short duration. The models for these two types of gamma-ray bursts, first proposed in 1993 by astronomer Stanford E. (Stan) Woosley (Fig. 9.10) of the University of California, Santa Cruz, seem to be on the right track (see Levan, 2018:1–5). Woosley concluded that: Gamma-ray bursts may be produced at cosmic distances by the collapse of “failed” supernova, especially the collapse to a black hole of a Wolf-Rayet star. Shorter and/or less energetic bursts are possible from merging neutron stars, merging white dwarfs, and black holes merging with neutron stars. The burst is generated as neutrino emission from a massive accretion disk produces a relativistically expanding bubble of radiation and pairs along the

References

239

Fig. 9.10 Stan Woosley. Credit S. E. Woosley

rotational axis. Burst luminosities would be roughly 1050 ergs s-1 beamed into about 10% of the sky . . .(Woosley, 1993:276)

Regarding the longer duration bursts that might be produced by massive stars collapsing to a black hole rather than to a neutron star (a “failed” supernova), Woosley points out that (the symbol Mʘ below is for the mass of the Sun) The model has the appeal of something that is likely to happen with regularity in nature. If the signature of a 5 Mʘ black hole accreting stellar masses of material in a minute is not a gamma-ray burst, what is it? Bodenheimer and Woosley (1983) asked the question which still has not been answered: “What is the observational signature of a ‘failed’ supernova?” Indeed, the term ‘failed’ supernova has been repeatedly used in quotes throughout this paper because one can hardly call an event which yields more power than any other phenomenon in the modern universe a failure. In neutrinos, one expects ~ 1054 ergs, as much as three supernovae. In kinetic energy 1051 ergs may be developed if the jet is rich in baryons. If it is not, then these are the brightest gamma-ray sources in the universe by a very wide margin (Woosley, 1993:276–277).

Research continues not only on the various possible mechanisms that could produce the energy associated with gamma-ray bursts, but also on the use of gamma-ray bursts to further probe the universe (Levan, 2018).

References Atteia, J. L., & Hurley, K. (1986). Extragalactic gamma-ray bursts. Advances in Space Research, 6(4), 39–43. Bodenheimer, P., & Woosley, S. E. (1983). A two-dimensional supernova model with rotation and nuclear burning. The Astrophysical Journal, 269, 281–291. Clark, G. W., Garmire, G. P., & Kraushaar, W. L. (1968). Observation of high-energy cosmic gamma rays. The Astrophysical Journal, 153, L203–L207. Cortese, L., Marcillac, D., Richard, J., Bravo-Alfaro, H., Kneib, J. P., Rieke, G., et al. (2007). The strong transformation of spiral galaxies infalling into massive clusters at z≈ 0.2. Monthly Notices of the Royal Astronomical Society, 376(1), 157–172. Duncan, R. C., & Thompson, C. (1992). Formation of very strong magnetized neutron stars: Implications for gamma-ray bursts. The Astrophysical Journal, 392, L9–L13.

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Fishman, G. J. (1995). Gamma-ray bursts: An overview. Publications of the Astronomical Society of the Pacific, 107(718), 1145. https://doi.org/10.1086/133672 Fishman, G. J., Meegan, C. A., Watts, J. W., Jr., & Derrickson, J. H. (1978). New limits on gammaray bursts. The Astrophysical Journal, 223, L13–L15. Hakkila, J., Meegan, C. A., Pendleton, G. N., Fishman, G. J., Wilson, R. B., Paciesas, W. S., et al. (1994). Constraints on galactic distributions of gamma-ray burst sources from BATSE observations. The Astrophysical Journal, 422, 659–670. Harrison, F. A., Cook, W. R., III, Prince, T. A., Schindler, S. M., Hailey, C. J., & Thorsett, S. E. (1995). Andromeda: A mission to determine the gamma-ray burst distance scale. In EUV, XRay, and Gamma-Ray instrumentation for astronomy VI (Vol. 2518, pp. 223–234). SPIE (Note Paczynski cited an earlier version of the Andromeda proposal.). Klebesadel, R. W., Strong, I. B., & Olson, R. A. (1973). Observations of gamma-ray bursts of cosmic origin. The Astrophysical Journal, 182, L85–L88. Kouveliotou, C., Meegan, C. A., Fishman, G. J., Bhat, N. P., Briggs, M. S., Koshut, T. M., Paciesas, W. S., & Pendleton, G. N. (1993). Identification of two classes of gamma-ray bursts. The Astrophysical Journal, 413, L101–L104. Kouveliotou, C., Duncan, R. C., & Thompson, C. (2003). Magnetars. Scientific American, 288(2), 35–41. p. 40. Lamb, D. Q. (1995). The distance scale to gamma-ray bursts. Publications of the Astronomical Society of the Pacific, 107(718), 1152. Levan, A. (2018). Gamma-ray bursts. IOP Publishing. Meegan, C. A., Fishman, G. J., Wilson, R. B., Paciesas, W. S., Pendleton, G. N., Horack, J. M., Brock, M. N., & Kouveliotou, C. (1992). Spatial distribution of γ-ray bursts observed by BATSE. Nature, 355(6356), 143–145. https://doi.org/10.1038/355143a0 Metzger, M. R., Cohen, J. G., Chaffee, F. H., & Blandford, R. D. (1997). GRB 970508. International Astronomical Union Circular, 6676, 3. Metzger, M. R., Djorgovski, S. G., Kulkarni, S. R., Steidel, C. C., Adelberger, K. L., Frail, D. A., Costa, E., & Frontera, F. (1997a). Spectral constraints on the redshift of the optical counterpart to the γ-ray burst of 8 May 1997. Nature, 387(6636), 878–880. Nemiroff, R. J. (1995). The 75th anniversary astronomical debate on the distance scale to gammaray bursts: an Introduction. Publications of the Astronomical Society of the Pacific, 107(718), 1131. Paczynski, B. (1991). Cosmological gamma-ray bursts. Acta Astronomica, 41, 257–267. Paczynski, B. (1995). How far away are gamma-ray bursters? Publications of the Astronomical Society of the Pacific, 107(718), 1167. Paradijs, J. V., Kouveliotou, C., & Wijers, R. A. (2000). Gamma-ray burst afterglows. Annual Review of Astronomy and Astrophysics, 38(1), 379–425. Schilling, G. (2002). Flash!: The Hunt for the Biggest Explosions in the Universe. (Translated by Naomi Greenberg-Slovin.) (p. 199). Cambridge University Press. Thompson, C., & Duncan, R. C. (1996). The soft gamma repeaters as very strongly magnetized neutron stars. II Quiescent neutrino, X-ray, and Alfven wave emissions. The Astrophysical Journal, 473, 322–342. van den Heuvel, E. P. (2003). The scientific life and work of Jan van Paradijs. In From X-ray binaries to gamma-ray bursts: Jan van Paradijs Memorial Symposium (Vol. 308, p. 3). Van Paradijs, J., Groot, P. J., Galama, T., Kouveliotou, C., Strom, R. G., Telting, J., et al. (1997). Transient optical emission from the error box of the γ-ray burst of 28 February 1997. Nature, 386(6626), 686–689. Woosley, S. E. (1993). Gamma-ray bursts from stellar mass accretion disks around black holes. The Astrophysical Journal, 405, 273–277.

Chapter 10

Reflections on Space Science Research

10.1

Introduction

The foregoing chapters have identified and discussed eight interesting and influential controversies in space science research beginning early in the space age. All the controversies had important impacts on the topic areas underlying the controversies. The resolutions of most of the conflicts resulted in altering or defining future research directions, and substantially furthered the understanding of nature. Some of the controversies had impacts on other disciplines. Eugene N. Parker’s elucidation of the nature of the solar wind, for example, influenced not only how space researchers viewed the heliosphere, but also how astronomers viewed other stellar systems. As also mentioned in terms of continuing research on the solar wind, a stellar “breeze”—not a wind—might exist in the case of other stars. The open-closed Earth’s magnetosphere controversy put a spotlight on the importance of magnetic field merging in fundamental plasma physics. Such a plasma process likely occurs in astrophysical systems throughout the universe. Unfortunately, the application of plasma physics processes studied in situ by instrumentation in the solar system can only be done theoretically for other astrophysical entities. This realization emphasizes the importance of detailed studies of the contents of the solar system by robotic (and ultimately human in some cases) means in order to arrive at understanding of many astronomical observations in the Milky Way galaxy and beyond. The Chicxulub controversy added considerably to the idea of the importance of catastrophic collisions by asteroids and comets on solar system bodies, including Earth and its moon. It is worthwhile to recall that as recently as about three centuries ago, the idea of rocks falling from the heavens was viewed as preposterous. The Chicxulub controversy also forced two very disparate communities of researchers— paleontologists and astronomers—to face one another both as adversaries and as seekers of ultimate understanding of nature.

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 W. D. Cummings, L. J. Lanzerotti, Scientific Debates in Space Science, Astronomy and Planetary Sciences, https://doi.org/10.1007/978-3-031-41598-2_10

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The controversy of a continued contribution to Earth’s water budget by the infall of small comets likely persisted longer than it would have if the guidelines of Arthur Eddington had been followed. The controversy was ultimately resolved by unassailable data from both ground-based radar and optical telescopes. With the exception of the group that proposed the idea, the resolution was satisfactory to the research community at large; the controversy faded from attention and interest. Of course, in terms of continuing research, the origin(s) of Earth’s vast reservoir of water, which enables life as humans know it, remains unresolved and is an important topic related to extraterrestrial research.

10.2

Other Important Space-Related Discoveries

Not included as separate chapters in this book are two other important discoveries during the early years of space exploration that merit mention, though there was little or no debate or controversy associated with them. One has the distinction of being the first important space-related discovery in what is now called the “Space Age.” The discovery of the radiation belts by James A. Van Allen, which led to the study of magnetospheric physics (Van Allen, 1983), was largely unrelated to prior speculations. And from this discovery resulted the concept of planetary (and even broader astrophysical) magnetospheres, confirmed by subsequent planetary-directed space missions. Another set of spectacular discoveries were focused on the moons of the outer planets of the solar system. As discussed below, the theory of the tidal heating of Jupiter’s moon Io (one of the planet’s four moons discovered by Galileo in 1610) was published a mere 3 days before the observation of Io’s volcanism by the Voyager 1 imaging system. There was no time for any controversies to arise.

10.2.1

Discovery of the Earth’s Radiation Belts

The launch of Sputnik I on 4 October 1957 generated considerable concern in the United States, which was in a “cold war” with the Soviet Union. Sputnik I weighed only about 175 pounds (80 kg), but the launch of Sputnik II on 3 November 1957 demonstrated that the Soviet Union had much larger launch capacity. As noted by the political historian, Walter A. McDougall, the payload for Sputnik II. weighed 1,121 pounds [510 kilograms], but since the satellite remained attached to the spent upper stage, the weight placed into orbit was of the order of six tons. The satellite contained geophysical equipment, a life support system, and a live dog named Laika. (McDougall, 1985:150)

Concern in the U.S. began to evolve into panic that the country was in danger of a potential nuclear attack via an intercontinental missile. President Dwight

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243

Fig. 10.1 James Van Allen. Credit Drake Hokanson, courtesy of University Relations records, University of Iowa Libraries, Iowa City, Iowa

D. Eisenhower went on television to try to calm Americans, but many in the press and elsewhere were demanding a technological response from the U.S., hence an acceleration of the U.S. satellite programs. The U.S. response came on 6 December 1957 with the attempted launch of the Vanguard rocket of the Naval Research Laboratory. The event was to be televised. Much publicity was made of how the Soviet space program was secret and conducted by the military, whereas the U.S. program was open and civilian in character. Enormous dismay was felt across the country as the Vanguard rose a few feet off the launch pad and then settled back in flames. For several years, Professor Van Allen (Fig. 10.1) and his colleagues at the State University of Iowa, as well as other American researchers, had been using the German V-2 rockets, which the U.S. had confiscated after World War II, to launch scientific instruments into the upper atmosphere (Chap. 1). (The V-2 rockets had been built by Germany and used as missiles against England, carrying 1000 pound bombs.) Van Allen was particularly interested in determining what was the nature of cosmic rays, i.e., what kinds of particles they were, and what was their flux. Getting at this question from measurements on the ground was difficult, because when cosmic rays hit the atmosphere of the earth, they generate secondary energetic particles, which collide with other particles in the atmosphere and generate more particles, etc., etc. By getting above the atmosphere, and above altitudes where balloons could reach (Chap. 1), Van Allen could measure the primary cosmic rays directly, without the confusion of the secondaries. When the opportunity arose to put a Geiger counter on a satellite, Van Allen was ready, and one of his graduate students, George H. Ludwig (1927–2013) (Fig. 10.2), had such an instrument for the Vanguard rocket. When Vanguard failed, Eisenhower let the group of German engineers in Huntsville, Alabama, (in what is now NASA’s Marshall Space Flight Center) attempt the launching of a satellite on the Jupiter C rocket that had been developed by the Army. This Army rocket group was led by Wernher von Braun (1912–1977), who had invented the V-2 rocket and led the German Rocket Team for Hitler (e.g., Neufeld, 1995). Van Allen was able to get his

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Fig. 10.2 George Ludwig. Credit AIP Emilio Segrè Visual Archives

student’s experiment transferred to the new Explorer satellite (built by the Jet Propulsion Laboratory, JPL) that was to be launched by the Army’s rocket. Explorer I with Van Allen and Ludwig’s instrument was launched successfully on 31 January 1958. The first U.S. satellite weighed 10.5 pounds (4.8 kg). (The little dog, Laika, a passenger in Sputnik II, probably weighed that much.) The U.S. couldn’t brag about being the first nation to orbit a satellite or being able to put the most weight in orbit, so some national politicians began to tout U.S. prowess in the miniaturization of electronics. Van Allen’s Geiger-Müller tube was small, less than an inch in diameter and about 4 in. long. It was a gas-filled tube with a wire running down the center and charged by a battery relative to the exterior of the tube. Such a device records the ionization of the gas caused by a charged particle (cosmic ray) that passes through the tube. The pulse of current that results is a record of the passage of the charged particle. After an energetic charged particle passes through the tube, the tube returns to its former condition. In Van Allen’s Geiger counter, the time for recvery to record the next event (“dead time”) was about 100 ms. (Van Allen et al., 1958:589). There was such a rush to get Explorer I in orbit that Van Allen’s graduate student, George Ludwig, didn’t have a chance to get all his equipment in place. In particular, he didn’t have the opportunity to put a tape recorder in the payload. Hence, data could only be taken when the satellite passed over 1 of 16 receiving stations that had been established around the world. Van Allen and his team could only receive about 2 min of good data when Explorer I was over a receiving station. George Ludwig relates the following events immediately after the launch of Explorer 1: A few days after Explorer 1 was launched, . . . scientists at the Jet Propulsion Laboratory (JPL), primarily Conway Snyder and Phyllis Buwalda, were carefully checking the quality of the initial data. As quickly as possible, they gathered the verbal comments from the station operators and took a look at the data tapes as they arrived to determine the condition of the operating instruments and to measure the satellite internal temperatures. In the process they observed on 5 February that the Geiger-Müller (GM) counter rates appeared at a few times to

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Fig. 10.3 Results from Explorer 1, showing expected counts at low satellite altitudes and no counts at high altitudes. From Fig. 4 in Van Allen et al. (1958)

be zero. Conway immediately notified Bill Pickering [the Director of JPL], who in turn called Van Allen, starting the conversation along the lines, “I have bad news for you. Conway Snyder has looked at the data, and there are no counts. Your instrument appears to have failed.” Van Allen . . . was noncommittal during that conversation. He had considerable confidence in our instrument and was greatly concerned that premature interpretation of the data might be problematic. (Ludwig, 2013:324).

Van Allen’s confidence was to be rewarded. He and his University of Iowa colleagues soon noticed something peculiar in the limited data from Explorer 1. When the satellite was at a low altitude in its orbit, about 500 km above the earth, the Geiger counter would record about 30 counts per second, which is what Van Allen expected for cosmic rays, based on his earlier research with sounding rockets. On the other hand, when Explorer I was at a high altitude, about 2500 km above the earth, the Geiger counter seemed to be reporting no counts at all (Fig. 10.3). Van Allen’s group had Geiger counters prepared to go on the Explorer II and Explorer III satellites. The launch of Explorer II failed, but Explorer III was launched successfully on 26 March 1958. Explorer III included a tape recorder. With a tape recorder as part of the instrument, data could be taken continuously and rapidly played back (while the tape was being rewound) when the satellite was over a receiving station. In this way, the data from the Geiger counter could be examined almost continuously as the satellite moved through its orbit. The data for a complete orbit of Explorer III indicated, again, that when the satellite was at a relatively low point in its orbit, the counting rate was what might reasonably be expected for cosmic rays. As the satellite moved along in its orbit and

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Fig. 10.4 Sample results of tape recorder readout from Explorer III. From Fig. 6 in Van Allen et al. (1958)

got farther from the earth, the counting rate began to rise and then suddenly dropped to zero (Fig. 10.4). Van Allen’s group considered three possible effects that could account for the data being received from the Geiger counters on Explorers I and III. They thought of, and discounted, the possibility of malfunctions of the instruments. They considered another possibility, namely that the satellite passed through regions where very few cosmic rays could reach. They concluded that this was extremely unlikely, as “a magnetic field the order of one gauss extending over thousands of kilometers and remaining unbelievably free of local irregularities would be required to exclude a significant fraction of the cosmic radiation” (Van Allen et al., 1958:591). They then came to the following conclusion: The possibility that we firmly believe is correct is that the geiger tube encountered such intense radiation that dead time effects reduced the counting rate essentially to zero. (Van Allen et al., 1958:591)

In other words, the gas in the Geiger-Müller tube was not being given a chance to recover between intrusions of energetic charged particles. The Van Allen team put a spare flight unit in front of an X-ray beam in their Iowa laboratory and showed that at a rate beyond 30,000 per second of incident radiation (X-rays are as good at triggering a Geiger counter as are energetic charged particles) the count rate of the Geiger counter would drop to zero (Ludwig, 2013:334). So, on 1 May 1958, Van Allen announced the discovery of a belt of intense radiation surrounding the Earth, even though his Geiger counters had actually indicated zero counts per second in the region of space where the Van Allen

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247

Radiation Belts existed. Later space flights by the Iowa team and by other investigators validated the intense-radiation conclusion over and over again. Explorer I had encountered protons that are trapped in the Earth’s magnetic field and that have energies in the millions of electron volts. And this first major discovery at the launch of the “Space Age” occurred largely unrelated to prior speculations, debates or controversies. When the story of the discovery of the radiation belts is related to graduate students studying space science, the accompanying message should be, “Become thoroughly familiar with how your scientific instrumentation behaves under a variety of conditions,” with a corollary message, “If you are a good, careful scientist, don’t be afraid to stand your ground and state your conviction.”

10.2.2

Tidal Heating of Planetary Moons

At times a novel theory or concept in space science can be proposed and validated in short order, before any significant debates arise and before Eddington’s criteria are relevant. A prime example of such an occurrence was the theoretical speculation by Stanton J. (Stan) Peale (1937–2015) (Fig. 10.5) with two colleagues that the surface of Jupiter’s moon Io would have evidence of volcanism. This arose from their analysis that the magnitudes of tidal dissipation caused by Jupiter could completely dominate the thermal history of the moon (Peale et al., 1979). Peale and his colleagues pointed out that Io, Europa, and Ganymede are locked in resonant orbits around Jupiter. For every orbit of Ganymede around Jupiter, Europa makes two, and for every orbit of Europa around Jupiter, Io makes two. Like our Moon with respect to the Earth, Io always presents the same face to Jupiter. Tidal bulges are thus produced on opposite sides of Io. If Io were the only moon of Jupiter, the tidal bulges would be fixed, and there would be no internal motion of material

Fig. 10.5 Stan Peale. Credit American Astronomical Society

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Fig. 10.6 Linda Morabito. Credit NASA/JPL

within the moon, just as there is relatively little internal motion of material within Earth’s Moon. Peale and his colleagues showed, however, that the two other moons of Jupiter cause small perturbations in Io’s orbit around Jupiter, and thus the bulges and other material interior to Io can move slightly. As Peale and his colleagues pointed out, such tidal-produced movement of material will cause heating within Io, so that a major fraction of the moon might have melted. Thus, they made the following speculation: The implications of the orbital resonances of the inner three Galilean satellites are profound for the thermal state of Io. [Our] calculations suggest that Io might currently be the most intensely heated terrestrial-type body in the solar system. The surface of the type of body postulated here has not yet been directly observed, and although the morphology of such a surface cannot be predicted in any detail, one might speculate that widespread and recurrent surface volcanism would occur, leading to extensive differentiation and outgassing. (Peale et al., 1979: 894)

The paper of Peale and colleagues was submitted on 26 January 1979 and published on 2 March 1979 in Science magazine. The closest approach of the NASA Voyager 1 mission to Jupiter and to Io occurred on 5 March of that year, passing on that day about 0.3 of a Jupiter radius from the moon. Four days after closest encounter, after almost sleepless nights (as she relates) reducing and processing images returned from Io, Linda A. Morabito (Fig. 10.6), Jet Propulsion Laboratory astronomer, made a discovery that changed existing views of solar system bodies (Morabito, 2012). Morabito, Cognizant Engineer for the Optical Navigation Image Processing System, identified that in one of the images taken for purposes of spacecraft navigation, “something anomalous emerged off the limb of Io”. It was an active

10.3

Eddington’s Guidelines: Conclusions

249

Fig. 10.7 Volcano on Io. Credit NASA

volcano (Fig. 10.7). And there was another in the same image, between the dark and sunlit hemispheres. Additional volcanoes were quickly found in other images taken of the moon (Morabito et al., 1979). The Morabito discovery dramatically validated the concluding sentence from Peale et al., written some 2 months prior: “Voyager images of Io may reveal evidence for a planetary structure and history dramatically different from any previously observed” (Peale et al., 1979:894). The idea of tidal heating of planetary moons has resulted in continuing research related to other moons in the solar system. Jupiter’s moon Europa has been of particular interest. Peale and his colleagues noted that it is possible that tidal dissipation in the icy crust of that moon preserves liquid water (Cassen et al., 1979:731). This paper appeared shortly after the success of their Io prediction. Data from the magnetometer instrument on the Galileo orbiter spacecraft provided strong evidence that liquid water existed in the interior of Europa (Khurana et al., 1998; Kivelson et al., 2000). A radiation hardened NASA mission, dubbed “Europa Clipper,” is under planning to conduct a detailed investigation of the moon and its possible liquid water interior.

10.3

Eddington’s Guidelines: Conclusions

Arthur Eddington described three guidelines that researchers should follow as they develop scientific models. These guidelines are basically common sense guidelines that most researchers tend to follow without thinking of them. Slightly modified to make them more broadly applicable to space science as discussed in prior chapters, they are: 1. Was the speculator rigorous in applying the appropriate science applicable to the model

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2. Did the speculator identify all the underlying assumptions used in constructing the model and 3. Did the speculator view the model objectively, as an “adjustable engine,” as opposed to a “finished building?” The examination of the debates covered in the chapters of this book suggests that Eddington’s third guideline for scientific speculation is the most difficult to follow. This is likely true for all disciplines of science, not just space research. However, the record for the objectivity of space researchers in the debates covered in this book seems to be reasonably good. On the positive side: • Both Eugene Parker and Joseph Chamberlain (Chap. 2) eventually recognized that “adjusting the levers” of their models could result in either supersonic or sub-sonic solutions to the flow of stellar solar winds. • James Dungey (Chap. 3) was quick to recognize and applaud improvements in his model of the Earth’s magnetosphere, and he encouraged his students and others to make further observations to test his ideas about merging of magnetic field lines. • The post-Apollo researchers on the origin of the Moon (Chap. 5) had reasonable debates, and adjustments to their consensus “giant-impact” model for the origin of the Earth-Moon system continue. • The Chicxulub debates (Chap. 7) perhaps rose to the level of controversy, but both sides appear to have agreed that the impact of the asteroid occurred and contributed to the end-of-Cretaceous extinctions, even if it might not have been the sole cause. The debates were loud and acrimonious at times, but they seem to be based on objective science, albeit from different disciplines with very different cultures. • As with most of the other cases studied in this book, space-based measurements were necessary to settle the debate over the distance to the termination shock and the heliopause (Chap. 8). The debates were conducted objectively, and they were based on knowledge at the time of the space environment and underlying science. • In their debate on gamma-ray bursts (Chap. 9), both Bohdan Paczyński and Donald Lamb well illustrated the scientific objectivity that Eddington was calling for in his third criterion. They agreed, for example, that the absence of gamma-ray bursts coming from a halo around a nearby galaxy (Andromeda) would settle the debate in favor of the cosmological origin of the bursts. On the negative side: • Louis Frank’s zeal for prestige, e.g., membership in the U.S. National Academy of Sciences, was likely a factor unrelated to objective consideration of his smallcomet hypothesis (Chap. 4). Though a previously well-regarded experimentalist, he also appeared to be less than objective in his assessment of the strengths and the potential weaknesses of his optical instrumentation. The lunar dust controversy (Chap. 6) perhaps falls in the middle:

References

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• The treatment of Thomas Gold by some of his adversaries during the lunar dust controversy suggests that something other than objective science was in play. This is evident, for example, in the denigration of Gold and his speculations by Donald Wilhelms (Wilhelms, 1993:26). On the other hand, Gold was quite reluctant to admit that some of his speculations about the possibility of astronauts sinking into lunar dust were exaggerated. Gold had at least two prior experiences on different science topics where his initial position was rejected but ultimately proved to be correct, and this history might have influenced his behavior. There is a fine line between maintaining a position, as James Van Allen did while others were premature in thinking that his instrument had failed, and objectively maintaining a scientific position after contrary data become evident. Where Eddington’s guidelines were followed, the resolution of controversies in space research led to further research; in fact, to further important research in most cases. The further research has generally arisen because the process of resolution has uncovered aspects of nature that were not apparent before. Examples of this include the continuing investigations of solar emissions from the sun and the mechanisms for their expulsion, the role of catastrophic impact processes in the evolution of the bodies of the solar system, and basic plasma physics processes in astrophysics. Where Eddington’s third guideline was not followed, the resolution of the controversy did not lead to additional or deeper understandings, e.g., in the origins of Earth’s water. The debate went on far longer, and occupied more effort, than should have been the case. The negative consequences associated with this exception likely proves the value of following the Eddington guidelines in conducting and discussing future space research.

References Cassen, P., Reynolds, R. T., & Peale, S. J. (1979). Is there liquid water on Europa? Geophysical Research Letters, 6(9), 731–734. Khurana, K. K., Kivelson, M. G., Stevenson, D. J., Schubert, G., Russell, C. T., Walker, R. J., & Polanskey, C. (1998). Induced magnetic fields as evidence for subsurface oceans in Europa and Callisto. Nature, 395(6704), 777–780. Kivelson, M. G., Khurana, K. K., Russell, C. T., Volwerk, M., Walker, R. J., & Zimmer, C. (2000). Galileo magnetometer measurements: A stronger case for a subsurface ocean at Europa. Science, 289(5483), 1340–1343. Ludwig, G. H. (2013). Opening space research: Dreams, technology, and scientific discovery (Vol. 62). Wiley. McDougall, W. A. (1985). The heavens and the earth: A political history of the space age. Basic Books. Morabito, L. A. (2012). Discovery of volcanic activity on Io. A historical review. arXiv preprint arXiv:1211.2554. Morabito, L. A., Synnott, S. P., Kupferman, P. N., & Collins, S. A. (1979). Discovery of currently active extraterrestrial volcanism. Science, 204(4396), 972–972. Neufeld, M. J. (1995). The rocket and the reich. The Free Press.

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Peale, S. J., Cassen, P., & Reynolds, R. T. (1979). Melting of Io by tidal dissipation. Science, 203 (4383), 892–894. Van Allen, J. A. (1983). Origins of magnetospheric physics. Smithsonian Institution Press. Van Allen, J. A., Ludwig, G. H., Ray, E. C., & McIlwain, C. E. (1958). Observation of high intensity radiation by satellites 1958 Alpha and Gamma. Journal of Jet Propulsion, 28(9), 588–592. Wilhelms, D. E. (1993). To a Rocky Moon: A geologist’s history of lunar exploration. University of Arizona Press.

Correction to: Scientific Debates in Space Science

Correction to: Chapters 2 and 6 in: W. D. Cummings, L. J. Lanzerotti, Scientific Debates in Space Science, Astronomy and Planetary Sciences, https://doi.org/10.1007/978-3-031-41598-2_2 https://doi.org/10.1007/978-3-031-41598-2_6 This book was inadvertently published with incorrect sentences in the book. It has now been corrected as follows: p. 28–Last line, “protons per cm2.” has been updated as “protons per cm3.” p. 154–Third line from the bottom, “ 50 grains cm-2” has been updated as “ 50 grains cm-3”.

The updated versions of the chapters can be found at https://doi.org/10.1007/978-3-031-41598-2_2 https://doi.org/10.1007/978-3-031-41598-2_6 © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 W. D. Cummings, L. J. Lanzerotti, Scientific Debates in Space Science, Astronomy and Planetary Sciences, https://doi.org/10.1007/978-3-031-41598-2_11

C1

Author Index

A Aldrin, E.E. Jr., 152 Alfvén, H., 46 Alvarez, L., 168, 173, 180 Alvarez, L.W., 164 Alvarez, W., 173, 179–181 Amazeen, P.G., 215 Anger, C.D., 73 Archibald, J.D., 167, 173, 178 Armstrong, N.A., 152 Arnoldy, R.L., 54 Asaph Hall III, 107 Asaro, F., 164 Axford, I., 195 Axford, W.I.

B Bakker, R.T., 166, 168 Baldwin, R.B., 108, 115, 139 Barnard, E.E., 19 Becquerel, A.H., 6 Belton, M.J.S., 216 Bessel, F.W., 18 Bethe, H.A., 7 Biermann, L., 187 Biermann, L.F.B., 20 Bills, B.G., 72 Birkland, K., 37 Blum, P.W., 204, 216 Bonadonna, M.F., 78 Bondi, H., 156, 159 Bourgeois, J., 172 Boynton, W.V., 172, 174–176

Brandt, J.C., 26, 191, 216 Bredichin, F.A., 18 Brice, N.M., 205, 216 Bridge, H.S., 28 Bryant, L.J., 173 Burbidge, G., 155 Burbidge, M., 155 Burch, J.L., 60 Burke, B.F., 197, 217 Burke, K.C.A., 173 Burlaga, L.F., 209 Buwalda, P., 244 Byars, C., 171, 174, 175

C Cahill, L.J. Jr., 54, 215 Camargo-Zanoguera, A., 170, 176 Cameron, A.G.W. (Al), 122 Carrington, R.C., 14 Cernan, E.A., 153 Chamberlain, J.W., 23, 250 Chandrasekhar, S., 22 Chapman, S., 16, 37, 187 Chree, C., 55 Chubb, T.A., 71 Clauser, F.H., 189 Clayton, R.N., 123, 133 Clemens, W.A. Jr., 167 Clifford, S.M., 72 Cogger, L.L., 74 Coleman, P.J. Jr., 44 Compton, A.H., 1, 227 Cortie, A.L., 55

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 W. D. Cummings, L. J. Lanzerotti, Scientific Debates in Space Science, Astronomy and Planetary Sciences, https://doi.org/10.1007/978-3-031-41598-2

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254 Cowling, T.G., 42 Cragin, B.L., 72, 75 Craven, J.D., 68, 73, 77 Criswell, D.R., 154 Crommelin, A.C.D., 19 Cummings, A.C., 210, 218 Curtis, H.D., 230

D Dalrymple, B., 176 Daly, R.A., 114, 121, 131 Dana, J.D., 139 Darwin, C.R., 6, 112 Darwin, G.H., 112 Davis, D.R., 121 Davis, L. Jr., 31, 44, 188, 190, 214, 215, 219 Davis, P.M., 71 DePalma, R.A., 181 Dessler, A.J. (Alex), 49, 53, 67, 70, 77, 83, 193, 198 Deutsch, A.J., 189 Devorkin, D.H., 152 Dietz, R.S., 139 Donahue, T.M., 71, 82 Duncan, R.C., 234, 235 Dungey, J., 44, 51, 54, 250

E Eddington, A.S., 4, 8, 11, 18, 60, 100, 131, 158, 214, 235, 242, 249 Einstein, A., 5 Eisenhower, D.D., 242

F Fahr, H.-J., 204, 216 Fairfield, D.H., 54, 62 Feldman, P.D., 70 Ferraro, V.C.A., 16, 38, 187 Fillius, R.W., 218 Fishman, G.J., 226, 230, 235 FitzGerald, G.F., 15 Fix, J.D., 95 Flandro, G., 198, 208 Flandro, G.A., 196 Francis, P.W., 173 Frank, L.A., 67, 68, 73, 76, 77, 80, 93, 95, 99, 250 Franklin, K.L., 197, 217

G Galama, T., 237

Author Index Galilei, G., 107 Gauss, C.F., 12 Gehrels, T., 78 Gerstenkorn, H., 113 Gilbert, G.K., 108, 139 Giovanelli, R.G., 39, 62 Gloeckler, G.M., 201 Gold, T., 137, 139, 144, 147, 151, 155, 158, 250 Gottlieb, B., 193 Green, T., 182 Greenstadt, E.W., 44 Gringauz, K.I., 27 Groot, P., 237 Gurnett, D.A., 100, 207

H Hakura, Y., 51 Halley, E., 18 Hanson, W.B., 71, 193 Hapke, B., 155, 158 Hapke, B.W., 151 Harris, A.W., 95 Hartmann, W.K., 121, 128, 133, 172 Hess, V.F, 1, 37 Hickey, L.J., 167 Hildebrand, A., 174–176 Hildebrand, A.R., 172 Hodges, R.R. Jr., 72 Hodgson, R., 14 Holmes, A., 6 Holzer, T.E., 204, 216 Hoyle, F., 39, 62, 156 Hundhausen, A.J., 202, 203 Hunten, D.M., 30, 82 Huyghe, P.A., 76, 83

J Jacobsen, S.B., 176 Jeans, J.H., 6, 24 Johnson, F.S., 193 Jokipii, J.R. (Randy), 201

K Keller, C.B., 182 Keller, G., 178 Kennedy, J.F., 137, 145 Kerr, R.A., 74, 80, 82, 85 Klebesadel, R.W., 226 Knowles, S.H., 93 Kouveliotou, C., 227, 234–236 Krimigis, S. (Tom), 211

Author Index Krimigis, T., 206 Kring, D., 169, 175, 176, 182 Kring, D.A., 165 Kuiper, G.P., 144, 147, 158 Kurth, W.S., 100, 207 Kyle, F.T., 72

L Lallement, R., 217 Lamb, D.Q., 233, 235, 250 Lanzerotti, L.J., 53, 199 Lindemann, F.A., 38 Lindley, D., 6 Lopez-Oliva, J.G., 178 Lord Kelvin, 6, 14 Ludwig, G.H., 243, 244 Lyell, C., 6

M Marín, L., 176 Maunder, E.W., 55 Mc Kay, C.P., 71 McDonough, T.R., 205, 216 McDougall, W.A., 242 McGee, W.J., 6 McPherron R.T., 55 Meyer, P., 200 Michel, F.C., 49 Michel, H.V., 164 Michie, R.W., 191 Millikan, R.A., 1 Morabito, L.A., 248 Morris, D.E., 72 Morton, D.C., 192 Mozer, F.S., 58, 88 Murphree, J.S., 74, 78 Mutel, R.L., 95

N Nakamura, Y., 72 Ness, N.F., 51 Neugebauer, M., 29 Newman, R.C., 196 Nicholson, S.B., 150

O Oberst, J.P., 72 O’Gallagher, J.J., 201, 218 O’Keefe, J.A., 119 Olson, R.A., 226

255 Osterbrock, D.E., 26

P Paczyñski, B., 232, 235, 250 Parker, E.N., 21, 43, 47, 48, 131, 189, 190, 200, 201, 203, 219, 241, 250 Parks, G., 83 Parks, G.K., 80 Paschmann, G., 57 Patterson, T.N.L., 193 Paulikas, G.A., 53 Payne, C.H., 38 Payne-Gaposchkin, C., 8 Peale, S.J. (Stan), 247 Penfield, G.T., 170, 175, 176 Petschek, H.E., 46, 62 Pettit, E., 148–149 Pickering, W., 3, 245 Pilkington, M., 176 Planetary science, xii Purcell, J.D., 192

R Reid, G.C., 71 Renne, P.R., 182 Rennilson, J.J., 154 Richardson, J.D., 216 Ringwood, A.E., 119 Rubey, W.W., 116 Rubincam, D.P., 71 Russell, C.T., 55 Russell, D.A., 163 Russell, H.N., 114, 131 Ryder, G., 133, 176, 179

S Sabine, E., 12 Safronov, V.S., 124 Sagan, C., 206 Schmidt, O.Y., 109, 118 Schmitt, H.H., 153 Schopf, T.J.M., 166 Schulz, M., 199 Schuraytz, B.C., 176 Schwabe, S.H., 12 Seitz, F., 116 Shapley, H., 230 Sharpton, B., 176 Shoemaker, E.M., 76, 147, 148 Sigwarth, J.B., 68, 77, 80, 93, 95, 97, 100 Simpson, J.A., 200, 201, 218

256 Snyder, C.W., 29, 244 Solomon, S., 71 Sonett, C.P., 44 Soter, S., 72 Spudis, P.D., 176 Stanley, M., 5 Stone, E.C., 210 Strong, I.B., 226 Sweet, P.A., 42 Swift, J., 107 Swisher, C.C., III, 176

T Taylor, G.J., 130 Taylor, J., 132, 133 Taylor, L.A., 155, 156 Telting, J., 237 Thompson, C., 234, 235 Thomson, J.J., 15 Thomson, W., 6, 14

U Urey, H.C., 109, 115, 118, 141, 147, 155, 165 Urrutia-Fucugauchi, J., 176

V Vaivads, A., 59 Van Allen, J.A., 2, 37, 99, 242, 243, 251 van den Heuvel, E.P.J., 236

Author Index van Paradijs, J.A. (Jan), 236 Vasyliûnas, V.M., 55 Vernov, S.N, 2 Verschuur, G.L., 171, 174 Virgil L. (Buck) Sharpton, 174 von Braun, W., 3, 243 von Helmholtz, H.L.F., 6 von Humboldt, A., 12

W Walter, L., 120 Ward, W.R., 122 Wasson, J.T., 72 Webb, J.E., 116, 131 Weber, E.J., 31 Weber, W., 12 Weinreb, D.B., 168 Wetherill, G.W., 124 Whipple, F.L., 143 Whitaker, E.A., 147 Wilhelms, D.E., 115, 155, 156, 251 Wise, D.U., 119 Woosley, S.E. (Stan), 238

Y Yeates, C.M., 75, 78, 95

Z Zank, G.P., 221 Zuccaro, D.R., 72

Subject Index

A Accretional model, 125 Advanced Study Institute, 48 Aerospace Corporation, 53 AFOSR Astronautics Symposium, 141, 142, 144 Afterglow, 236, 237 Age of the Moon, 117 AIAA Aerospace Sciences Meeting, 48 American Astronomical Society, 235 American Geophysical Union (AGU), 67, 70, 74, 78, 80 American Journal of Science and Arts, 139 Andromeda galaxy (M31), 231–234, 250 Angular momentum, 115, 128 conservation of, 112, 120 orbital, 112 spin, 112 Annual Review of Nuclear Science, 125 Annual Reviews of Astronomy and Astrophysics, 238 Apex (of the heliosphere), 198, 199, 205 Apollo astronauts, 151 data, 118 era, 137 explorations, 115–117, 142, 151–155 landers, 142, 151 Apollo 11, 119, 120, 152, 153, 157 Apollo 12, 144, 153 Apollo 15, 117 Apollo 17, 117, 153 landing sites, 116, 151, 156 missions, 107, 115, 153, 157

program, 137 Asteroid(s), 75, 139, 163, 164, 170, 172–174, 180, 241 belt, 133, 198 impact, 173, 182 Astronautica Acta, 196 Astronomical unit (AU), 189 Astrophysical Journal, 193, 201 Astrospheres, 221 Atmospheric dayglow, 69 Atmospheric holes, 68, 73, 74, 81, 83, 89, 90, 92, 93 Atomic oxygen (O), 69, 80, 81 Aurora, aurorae, 41, 44, 46, 73, 100, 199 Auroral Electrojet (AE) index, 54 Auroral oval, 89 Auroral zone(s), 41, 100 Avco-Everett Research Laboratory, 48

B Basaltic lavas, 117 Basins, 137, 139, 141, 142, 144, 158 Bell Laboratories, 53, 199 Bolide(s), 137, 139, 172, 178 Boundary shell, 198 Burst And Transient Source Experiment (BATSE), 227–230, 232, 233, 235, 236

C Carnegie Institution of Washington, 124, 197 Cathode rays, 15

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 W. D. Cummings, L. J. Lanzerotti, Scientific Debates in Space Science, Astronomy and Planetary Sciences, https://doi.org/10.1007/978-3-031-41598-2

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258 Center for Radio Physics and Space Research, 149, 151 Charge-coupled device (CCD), 75, 78, 80, 95, 101, 237 Charge exchange (s), 193, 195, 196, 198, 202–205, 216 Chicxulub, 171, 176, 179, 241, 250 Comet(s), 18–20, 69, 73–76, 95, 101, 163, 165, 173, 174, 241 Baade, 26 fountain theory of, 18 Halley’s, 18 Haro-Chavira, 26 Morehouse, 18 Cometesimal(s), 78 Comet tails, 19, 187, 188, 191, 203, 214, 216 Coronal holes, 31 Coronal mass ejections (CMEs), 32, 38, 207 Cosmic ray(s), 1, 2, 37, 50, 51, 85, 188, 190, 192, 196, 199–201, 215, 243–245 Cosmic ray pressure, 192 Cosmological distance(s), 229, 232, 235, 238 Cosmological hypothesis, 225, 235 Cretaceous, 163, 166, 167, 170, 172–174, 176, 178, 179, 181, 182 Cretaceous-Paleogene (K-Pg), 158, 163, 165, 169, 172, 178–182 Cretaceous-Tertiary (KT), (K/T), or (KT), 163, 164, 168, 174–179 CubeSats, 63 Current sheet, 53

D Dayglow, 80, 81, 84, 89, 92 Dearborn Observatory, 139 Deccan flood basalt volcanism, 179 Deccan Traps, 164, 172, 182 Dynamics Explorer, 79

E Earth-Moon system, 112, 113, 115, 120, 126, 131, 133, 250 Earth’s dayglow, 69 Electric Fields Instrument (EFI), 88 Energetic Neutral Atom (ENA) Instrument, 220 Enrico Fermi Institute, 200, 201, 233 Enstatites, 133 EOS, 76 European Space Agency, 31, 59 Explorer satellite, 244

Subject Index F Fence Surveillance System, 93 Flares, 17 Fourth International Cosmic Ray Conference, 187 Fourth Symposium on Cosmical Gas Dynamics: Aerodynamic Phenomena in Stellar Atmospheres, 189 French National Center for Scientific Research (C.N.R.S.), 205 Frozen field, 54, 56

G Galactic, 205 cosmic ray(s), 190, 191, 195, 196, 200, 201, 203, 210, 211, 219 field, 188, 191 hypothesis, 225, 234 magnetic field, 188, 190–193, 200, 219 Galaxy, 193 Galileo, 83, 100 Gamma (γ) (unit of magnetic field strength), 191, 201, 203 Gamma ray(s), 225, 226 Gamma-ray burst(s) (GRB), 226–228, 230, 232–235, 250 Gauss (unit of magnetic field strength), 191 Geiger counter, 243–246 Geiger-Müller tube, 244, 246 General Relativity, 5 Geological periods Cambrian, 8 Cretaceous, 164 Jurassic, 170 Paleocene, 173 Paleogene, 164 Permian, 166 Precambrian, 6 Geological Survey of Canada, 176 Geology (journal), 176 Geomagnetic activity, 188 field, 49, 191 storm(s), 30, 199, 214 substorm(s), 54 Geophysical Research Letters (GRL), 67, 68, 70, 74, 77, 80, 86 Giant impacts, 108, 114, 117, 121, 126–128, 133, 142 Giant impact theory, 122

Subject Index Gravitational assists, 196 Greenwich Observatory, 37

H Hall currents, 45 Harvard College Observatory, 230 Heliocentric distance, 187, 188, 190, 201, 202, 209 Heliopause, 187, 198, 208, 210, 211, 216, 219, 220, 250 Heliophysics, xii, 4 Helios 1 and 2, 218 Heliosheath, 198, 206, 210, 211, 216, 220 Heliosphere, 187, 193, 198, 200, 201, 203, 204, 217, 220 Helium, 7 Hell Creek Formation, 181 Houston Chronicle, 171, 174 Houston Museum of Natural Science, 168 Huntsville Gamma-Ray Burst Workshop, 228 Hydrogen, 7, 38, 194

I IMP 1, 51 IMP 8 and ICE, 218 Institute for Astrophysics and Extraterrestrial Research, 204 Institute of Physics of the Earth, 124 Institute of Theoretical Geophysics, 109, 124 Instituto de Geofisica, 176 Instrumental artifacts, 78, 83, 84, 91 Instrumental effects, 88, 90 International Conference on Cosmic Rays and the Earth Storm, 45 International Geophysical Year (IGY), 2, 141, 192 Interplanetary hydrogen, 194 magnetic field, 44, 45, 48, 53–55, 62, 204 medium, 17, 215 space, 72, 192, 194, 198 Interplanetary field, 51 Interplanetary neutral hydrogen, 195 Interstellar cosmic rays, 201 hydrogen, 193, 196, 199, 205 magnetic field, 199, 201, 203 magnetic field pressure, 192 medium, 188, 196, 204, 220 neutral hydrogen, 193, 195, 198, 202, 204, 217 pressure, 190, 192, 203

259 Ion and Neutral Camera (INCA), 220 Iowa Robotic Observatory (IRO), 95, 98 Iridium, 72, 163–165, 176, 179, 180, 182 Isaac Newton Telescope, 237 Isotopic compositions, 133

J Jet Propulsion Laboratory, 29, 75, 95, 151, 196, 198, 244, 248 Johns Hopkins Applied Physics Laboratory, 211 Johnson Space Center, 116, 174 Journal of Geophysical Research, 89, 93–95, 97, 99 Journal of Young Investigators, 168 Jovian radio emissions, 199 Jupiter’s radio emissions, 199, 217

K Keck telescopes, 238 Kelvin-Helmholtz contraction theory, 6, 7 Kitt Peak National Observatory, 75, 76, 78, 216 Kona Conference, 125–128, 130, 132

L Laika, 2 Late Heavy Bombardment (LHB), 133 Lawrence Berkeley Laboratory, 164 Lenz’s law, 42 Local interstellar medium, 32, 187, 193, 198, 214, 219–221 Lockheed Research Laboratory in Palo Alto, 195 Los Alamos National Laboratory, 202, 226 Lunar and Planetary Institute (LPI), 126, 131, 165, 169, 171–174, 176, 178, 179 Lunar and Planetary Laboratory, 149, 172 Lunar and Planetary Science Conference, 123, 172, 176 Lunar horizon glow, 154 Lunar landings, 145 Lunar origin models, 121 binary accretion, 109 capture, 113, 115 co-accretion, 109, 115, 119 collisional capture, 114 collisional fission, 114, 121 fission, 111, 120 giant impacts, 108, 114, 117, 121, 126–128, 133, 142 precipitation, 119

260 Lunar origin models (cont.) rotational fission, 111, 115, 120 two-body collision, 133 Lunar Receiving Laboratory, 154 Lunar samples, 137 Lunar Science Conference, 122 Lunar Science Institute (LSI), 116, 126, 131, 154 Lunar surface features, 137, 140, 145, 150, 151, 153, 157, 158 basalts, 117 basins, mare, 108, 117, 150, 153 Imbrium, 108, 139, 141, 144 Mare Tranquillitatis, 152 Oceanus Procellarum, 151, 153 Serenitatis, 108, 141 Sinus Iridum, 108 Tranquillitatis, 141 crater(s) Tycho, 150, 151 craters, 138, 140, 146, 157, 158 dimple craters, 147 dust, 137, 139, 141–145, 147, 148, 151–153, 156–158, 251 erosion, 140, 141, 145, 146 flood basalts, 157 highlands, 117, 140, 145, 150, 151 ice, 150, 159 impacts, 77 lava, 137, 139, 141, 142, 144, 149 lava beds, 142 lava flows, 138, 139, 142, 148 lowlands, 147 maria, 117, 137, 138, 140–142, 144, 148, 157 regolith, 157 volcanism, 139, 141, 148, 163, 173, 182, 217, 247 volcano(es), 137–139 water ice, 159 Lyman alpha, 205 Lyman-α, 72, 192–195, 205, 216 Lα, 203 M Magnetic disturbances, 14, 32, 54, 187 field(s), 12, 38, 42, 48, 187, 200, 211 field-line reconnection, 54, 56, 59 field lines, 43, 44, 49 field merging, 46, 241 merging, 43, 51

Subject Index pressure, 43, 190, 199 storm(s), 15, 28, 38, 41, 53, 187 storm sudden commencement, 38 Magnetopause, 48, 56, 58–60, 63, 215 Magnetosheath, 57 Magnetosphere(s), 39, 45, 46, 48, 49, 51, 53–55, 57, 59, 62, 63, 190, 199, 220, 242, 250 Magnetospheric physics, 242 Magnetospheric tail, 51 Manned Spacecraft Center, 116 Mars-size body, 107, 125, 131, 171 Max Planck Institute for Extraterrestrial Physics, 57 Max Planck Institute for Solar System Research, 55 Maxwell-Boltzmann distribution, 23 McDonald Observatory, 144 Meteoroid(s), 137, 139, 140 Milky Way, 233, 235 Milky Way galaxy, 188, 203, 221, 225, 227–229, 238 Momentum flux, 188 Monthly Notices of the Royal Astronomical Society, 140 Moon, 4, 27, 72, 77 Mount Wilson Observatory, 230 MSS, 60, 61 Museum of Paleontology, 173

N NanoTesla (nT), 191 National Academy of Sciences (NAS), 3, 83, 101, 116, 155, 165, 173, 235, 250 National Aeronautics and Space Administration (NASA), 59, 116, 142, 145, 205 Ames Research Center, 198 Astrogeology Branch, 157 Goddard Spaceflight Center (GFSC), 46, 51, 54, 100, 119, 147, 209 Marshall Spaceflight Center (MSFC), 226–228, 243 National Scientific Balloon Facility, 226 Nature, 70, 199, 228, 237 Naval Space Surveillance System (NAVSPASUR), 93 Neutral hydrogen, 192, 202–205, 216, 219 Neutral line, 51, 56 Neutral point(s), 38, 41, 42, 44, 48 Neutral sheet, 48, 53, 56 Neutron monitors, 200 Neutron stars, 229

Subject Index NOAA Aeronomy Laboratory, 204 Nobel Prize, 1, 7, 15, 22, 46, 109, 140, 164, 227 Nuclear fusion, 7 Null points, 46

O OH, 71 OH emissions, 82 Ohm’s law, 48, 56 Origin of Species, 6 Origin of the Moon, 108–114, 118, 119, 130, 131, 137 Outer Planets Working Group, 197

P Palm Beach Museum of Natural History, 181 Palomar observatory, 189 PCA events, 50, 53 PEMEX, 170, 176 Petschek’s mechanism, 47 Physical Review, 188, 200 Physical Review Letters, 44, 45, 191 Planetary and Space Science, 192, 193, 195, 202 Planetary astronomy, 197 Planetary bodies (other than Earth and Moon) Europa, 247, 249 Ganymede, 247 Io, 217, 242, 247–249 Jupiter, 72, 79, 100, 107, 122, 133, 191, 193, 197–199, 204, 208, 209, 217, 242, 247, 248 Mars, 71, 122, 123, 126, 191, 198, 201 Mercury, 122, 126 Neptune, 197–199, 209 Pluto, 192, 197, 220 Saturn, 122, 197, 198, 208, 209, 220 Uranus, 122, 191, 195, 197, 198, 209 Venus, 76, 122 Planetary science, xii Planetesimals, 124–126 Plasma Space Science Symposium, 46 Plasma Wave System (PWS), 206 Polar cap(s), 51 Polar cap absorption (PCA), 50, 51 Polar ionosphere, 44, 45 Popular Astronomy, 139 Proceedings of the American Philosophical Society, 114 Proceedings of the National Academy of Sciences, 181, 182

261 Q Quicksand, 142

R Radial gradient of cosmic rays, 201, 218 Radiation belts, 37, 63, 198, 217, 242, 247 Radioactive decay, 141 Radio emissions, 206, 217 Radio signals from Jupiter, 198 Reconnection of magnetic field lines, 53 Reviews of Geophysics, 73, 77, 78, 198, 218 Roche limit, 111 Rockets, 243 Arianespace Ariane 5, 59 Jupiter C, 243 Titan, 198 V-2, 2, 243 Vanguard, 243 Royal Astronomical Society, 155 Royal Observatory, 18 Royal Society, 14, 15 Russian Academy of Sciences, 111

S Schrödinger Crater, 177 Science, 74, 82, 146, 147, 151, 164, 175, 177, 179, 180, 248 Scientific American, 166 SD current system, 44 Small comet(s), 67, 70, 75, 78, 79, 82, 83, 100 Small-comet hypothesis, 70, 73, 77, 79, 95, 250 Small-comet impacts, 77 Smithsonian Institution’s Natural History Museum, 230 Smithsonian’s National Air and Space Museum, 152 Snowbird Conference, 165, 173, 174, 178–180 Society of Exploration Geophysicists (SEG), 170, 171, 175 Society of Vertebrate Paleontologists, 168 Soft gamma-ray repeaters (SGRs), 233–235 Solar, 188 activity, 188, 195, 199 breeze, 25–26, 30, 191 chromosphere, 40, 44 corona, 24, 26, 31, 44 corpuscular radiation, 20, 21, 188 cosmic rays, 199 eclipse, 5 flare(s), 38, 40, 44, 46, 50, 199, 200 ionizing radiation, 194

262 Solar (cont.) plasma, 191 system, 32, 76, 117, 124, 159, 171, 177, 188, 193, 197, 204, 209, 214, 215, 219, 220, 225, 241, 242, 248 wind, 11, 22, 24, 26–32, 41, 44, 52, 54, 58, 190–193, 195, 198–200, 203–205, 215 wind plasma, 45, 191 wind protons, 195, 202, 216 wind ram pressure, 193 wind velocity, 196, 216 Solar wind magnetic field, 193, 200 Solar wind pressure, 211 Sombrero galaxy, 231 Southwest Center for Advanced Studies, 193, 195 Southwest Research Institute, 60 Soviet Union, 27, 28, 141, 145, 150, 226, 242 Space astronomy, xii Spacecraft, space missions Atmospheric Explorer(s) C, E, 71 BeppoSAX, 236 Cassini, 220 Chandrayaan-1, 159 Chandra X-ray Observatory, 227 Clementine, 159 Cluster II mission, 59 Cluster mission, 59 Compton Gamma Ray Observatory (CGRO), 227–229 Dynamics Explorer 1 (DE-1), 67, 68, 71, 73, 74, 80, 83, 90, 92, 95, 101 Dynamics Explorer 2 (DE-2), 71 Explorer 1, 3, 100, 198, 244–247 Explorer 2, 245 Explorer 3, 198, 245, 246 Explorer 10, 28–29 Explorer 12, 54, 215 Galileo, 8 Gamma Ray Observatory (GRO), 227 Geotail, 100 Hubble Space Telescope, 227, 238 International Solar Terrestrial Physics Mission, 79 International Sun-Earth Explorer (ISEE), 55, 56, 58, 59 Interplanetary Monitoring Platform (IMP), 51, 216 Interstellar Boundary Explorer (IBEX), 220 Interstellar Mapping and Acceleration Probe (IMAP), 220 Ionosphere Connection Explorer (ICON), 63

Subject Index Luna 1, 27 Luna 2, 27 Luna 3, 27 Luna 9, 150, 151, 157 Lunar Prospector, 159 Lunar Reconnaissance Orbiter (LRO), 159 Magnetosphere Multiscale (MMS) mission, 60 Mariner 2, 29–30 Mariner 4, 201 New Horizons, 220 Orbiting Geophysical Observatory (OGO 5), 55, 204, 205 Orbiting Solar Observatory (OSO-3), 225 Parker Solar Probe (PSP), 31, 32 Pioneer 10, 198, 208, 218, 220 Pioneer 11, 198, 208, 218 Pioneer spacecraft program, 198 Pioneer V, 44 Pioneer Venus Orbiter, 234 Polar, 58, 59, 79–93, 95 Ranger, 142, 145, 147, 151 Ranger mission, 150 Ranger VI, 146 Ranger VII, 146–148 Ranger VIII, 146, 147 Ranger IX, 146, 147, 149 Rangers I and II, 146 Rangers III, , and V, iv, 146 Spitzer Space Telescope, 227 Sputnik 1, 141, 242 Sputnik 2, 2, 3, 242, 244 Sputnik 3, 3 Surveyor I, 151 Surveyors III, , , and VII, iv, vi, 151 Ulysses, 31 Van Allen Probes, 63 Vela, 226, 236 Venera 1, 28 Viking, 73–76, 78 Voyager 1, 198, 206–209, 211, 218, 242, 248 Voyager 2, 198, 206, 207, 209, 211, 216, 218 Spacecraft wobble, 85 Space Science Reviews, 203 Space Sciences Laboratory, 88 Space Technologies Laboratory (STL), 44 Spacewatch, 76 Spacewatch camera, 75, 76, 78 Space weather(ing), 63, 157 Sun, 6–8, 11, 12, 14, 17, 19–21, 31, 32, 38, 193, 200, 204 Sunspot(s), 12, 14, 38, 40, 195, 199

263

Subject Index Supernova, 230 Surveyor, 142, 151, 157 Surveyor data, 158 Surveyor Missions, 151 Surveyor program, 151 Surveyor spacecrafts, 151 Swedish Institute of Space Research, 59 Sweet’s Mechanism, 43 Symposium on the Physics of Solar Flares, 46

T Tanis, 181 Tanis Paleo Heritage Conservancy, 182 Tanis site, 181 Termination shock, 187, 190–193, 195, 196, 198, 199, 202–206, 208, 209, 211, 215, 216, 219, 220, 250 Termination shock wave, 189 Terrella experiments, 46 Tesla (unit of magnetic field strength), 191 The Big Splash, 76–77 The New York Times, 168 The Washington Post, 83 Tidal bulges, 112, 247 dissipation, 249 heating, 242, 249

U Ultraviolet (UV), 67, 69, 73, 74, 79 Ultraviolet Imager (UVI), 80, 81, 83, 85, 86, 88, 90 United States (U.S.) Air Force, 78 Air Force Office of Scientific Research (AFOSR), 141 Geological Survey (USGS), 6, 76, 139, 156, 176 Naval Research Laboratory, 93, 243 United States Naval Observatory, 107 Universities and Colleges Alabama in Huntsville, 236 Alaska, 78 Amsterdam, 236 Arizona, 75–76, 82, 86, 149, 169, 172, 175 Athens, 228 Australian National University, 119 Calgary, 73, 74 California, Berkeley, 58, 88, 90, 164, 173, 191 California Institute of Technology (Caltech), 31, 154, 188, 196, 210 California, Los Angeles, 55

California, Santa Cruz, 238 Cambridge University, 8 Catholic University of America, 46 Chicago, 21–23, 123, 144, 165, 199, 201, 233 Colorado, 205 Cornell, 137, 149, 196, 205 Florida, 93 Fordham, 1 Franklin and Marshall, 119 Glasgow, 42 Harvard, 114, 143, 169 Harvard-Radcliffe, 8 Hawaii, 130 Houston, 176 Indiana, 169 Imperial College London, 38 Imperial College of Science and Technology, 6 Iowa, 67, 78, 95, 243, 245 Kansas, 181 Leeds, 42 Massachusetts Institute of Technology (MIT), 28, 55, 72, 216 Michigan, 82, 230 New Hampshire, 54, 206, 215 Newnham College, 8 Northwestern University, 139 Oxford, 42 Pennsylvania State, 54, 78 Pittsburgh, 156 Princeton, 114, 232 Rice, 49, 145 Rutgers, 176 San Diego State, 173 St. John’s College, Cambridge, 40 Sydney, 39 Tennessee, 156 Texas at Austin, 235 Texas at Dallas, 75, 193 Toronto, 235 Universidad Nacional Autónoma de México (UNAM), 176, 178 Washington, 80, 81, 90, 172 Wisconsin, 202 Yale, 139, 168 Universities Space Research Association (USRA), 116, 227, 228 University of Colorado Museum, 166 Upper Atmosphere Research Panel, 2

V Vacuum merging, 53, 56 Van Allen Radiation Belts, 246

264 Visible Imaging System (VIS), 80–82, 86, 88, 90 Volcanism (other than lunar volcanism), 148, 163 Voyager spacecraft, 32, 249 V-2 Panel, 2

W Westerbork Synthesis Radio Telescope, 237 Western Interior Seaway, 166, 173 William Herschel Telescope, 237

Subject Index X X-line, 60

Y Yerkes Observatory, 19, 23, 144 Yucatán, 163, 170, 171, 175–177

Z Zeitschrift für Astrophysik, 113