Remote Compositional Analysis: Techniques for Understanding Spectroscopy, Mineralogy, and Geochemistry of Planetary Surfaces [1 ed.] 110718620X, 9781107186200

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R E M O T E C O M P O S I T I O N A L A N A LY S I S How do planetary scientists analyze and interpret data from telescopic and spacecraft observations of planetary surfaces? What elements, minerals, and volatiles are found on the surfaces of our Solar System’s planets, moons, asteroids, and comets? This comprehensive volume answers these topical questions by providing an overview of the theory and techniques of remote compositional analysis of planetary surfaces. Bringing together eminent researchers in Solar System exploration, it describes state-of-the-art results from spectroscopic, mineralogical, and geochemical techniques used to analyze the surfaces of planets, moons, and small bodies. The book introduces the methodology and theoretical background of each technique, and presents the latest advances in space exploration, telescopic observation, and laboratory instrumentation, and major new work in theoretical studies. This engaging volume provides a comprehensive reference on planetary surface composition and mineralogy for advanced students, researchers, and professional scientists. JANICE L. BISHOP

is a senior research scientist at the SETI Institute, where she is Chair of

Astrobiology and on the Science Council. She investigates Mars surface composition, mineral spectroscopy, and volcanic alteration, and has worked with data from many martian missions, including MRO/CRISM for which she is a Co-I. She has served as editor of Icarus and special issues of American Mineralogist and Clay Minerals. She has received awards from the Clay Minerals Society, the Humboldt Foundation, and the Helmholtz Foundation, and is a Fellow of GSA and MSA. JAMES

F.

BELL III

is a professor at Arizona State University, where he specializes in

astronomy and planetary science. He studies the geology, geochemistry, and mineralogy of Solar System objects using telescopes and spacecraft. He received the Carl Sagan Medal from the American Astronomical Society for excellence in public communication in the planetary sciences, and asteroid 8146 Jimbell was named after him by the International Astronomical Union. He edited The Martian Surface: Composition, Mineralogy, and Physical Properties (Cambridge, 2008) and coedited Asteroid Rendezvous (Cambridge, 2002). JEFFREY E. MOERSCH

is a professor of Earth and Planetary Sciences and Director of the

Planetary Geosciences Institute at the University of Tennesee. His research focuses on the geology of planetary surfaces, remote sensing, terrestrial analog field work, and planetary instrument development. He has extensive spacecraft mission experience, including work as a

member of the science teams of the Mars Exploration Rover mission, the Mars Odyssey mission, and the Mars Science Laboratory mission. He has conducted astrobiology-related research in many terrestrial analog field sites, including Death Valley and the Mojave Desert, the Atacama Desert, the Andes, and the Arctic. From 2010 to 2015 he was the Mars Editor for Icarus.

C A M B R I D G E P L A N E TA R Y S C I E N C E Series Editors: Fran Bagenal, David Jewitt, Carl Murray, Jim Bell, Ralph Lorenz, Francis Nimmo, Sara Russell

Books in the Series: 1. Jupiter: The Planet, Satellites and Magnetosphere † Edited by Bagenal, Dowling and McKinnon 978-0-521-03545-3 2. Meteorites: A Petrologic, Chemical and Isotopic Synthesis † Hutchison 978-0-521-03539-2 3. The Origin of Chondrules and Chondrites † Sears 978-1-107-40285-0 4. Planetary Rings † Esposito 978-1-107-40247-8 5. The Geology of Mars: Evidence from Earth-Based Analogs † Edited by Chapman 978-0-52120659-4 6. The Surface of Mars † Carr 978-0-521-87201-0 7. Volcanism on Io: A Comparison with Earth † Davies 978-0-521-85003-2 8. Mars: An Introduction to Its Interior, Surface and Atmosphere † Barlow 978-0-521-85226-5 9. The Martian Surface: Composition, Mineralogy and Physical Properties Edited by Bell 9780-521-86698-9 10. Planetary Crusts: Their Composition, Origin and Evolution † Taylor and McLennan 978-0521-14201-4 11. Planetary Tectonics† Edited by Watters and Schultz 978-0-521-74992-3 12. Protoplanetary Dust: Astrophysical and Cosmochemical Perspectives † Edited by Apai and Lauretta 978-0-521-51772-0 13. Planetary Surface Processes Melosh 978-0-521-51418-7 14. Titan: Interior, Surface, Atmosphere and Space Environment Edited by Müller-Wodarg, Griffith, Lellouch and Cravens 978-0-521-19992-6 15. Planetary Rings: A Post-Equinox View (Second edition) Esposito 978-1-107-02882-1

16. Planetesimals: Early Differentiation and Consequences for Planets Edited by Elkins-Tanton and Weiss 978-1-107-11848-5 17. Asteroids: Astronomical and Geological Bodies Burbine 978-1-107-09684-4 18. The Atmosphere and Climate of Mars Edited by Haberle, Clancy, Forget, Smith and Zurek 978-1-107-01618-7 19. Planetary Ring Systems Edited by Tiscareno and Murray 978-1-107-11382-4 20. Saturn in the 21st Century Edited by Baines, Flasar, Krupp and Stallard 978-1-107-10677-2 21. Mercury: The View after MESSENGER Edited by Solomon, Nittler and Anderson 978-1107-15445-2 22. Chondrules: Records of Protoplanetary Disk Processes Edited by Russell, Connolly Jr. and Krot 978-1-108-41801-0 23. Spectroscopy and Photochemistry of Planetary Atmospheres and Ionospheres Krasnopolsky 978-1-107-14526-9 24. Remote Compositional Analysis: Techniques for Understanding Spectroscopy, Mineralogy, and Geochemistry of Planetary Surfaces Edited by Bishop, BellIII and Moersch 978-1-10718620-0 †

Reissued as a paperback

REMOTE COMPOSITIONAL ANALYSIS Techniques for Understanding Spectroscopy, Mineralogy, and Geochemistry of Planetary Surfaces Edited by Janice L. Bishop SETI Institute James F. Bell III Arizona State University Jeffrey E. Moersch University of Tennessee, Knoxville

University Printing House, Cambridge CB2 8BS, United Kingdom One Liberty Plaza, 20th Floor, New York, NY 10006, USA 477 Williamstown Road, Port Melbourne, VIC 3207, Australia 314–321, 3rd Floor, Plot 3, Splendor Forum, Jasola District Centre, New Delhi – 110025, India 79 Anson Road, #06–04/06, Singapore 079906 Cambridge University Press is part of the University of Cambridge. It furthers the University’s mission by disseminating knowledge in the pursuit of education, learning, and research at the highest international levels of excellence. www.cambridge.org Information on this title: www.cambridge.org/9781107186200 DOI: 10.1017/9781316888872 © Cambridge University Press 2020 This publication is in copyright. Subject to statutory exception and to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published 2020 Printed in the United Kingdom by TJ International Ltd. Padstow Cornwall A catalogue record for this publication is available from the British Library. Library of Congress Cataloging-in-Publication Data Names: Bishop, Janice L., editor. | Bell, Jim, 1965– editor. | Moersch, Jeffrey E., 1966– editor. Title: Remote compositional analysis : techniques for understanding spectroscopy, mineralogy, and geochemistry of planetary surfaces / edited by Janice L. Bishop (SETI Institute), James F. Bell, III (Arizona State University), Jeffrey E. Moersch (University of Tennessee, Knoxville). Description: Cambridge ; New York, NY : Cambridge University Press, [2020] | Includes bibliographical references and index. Identifiers: LCCN 2019002564 | ISBN 9781107186200 (alk. paper) Subjects: LCSH: Planets – Spectra. | Astronomical spectroscopy. | Planetary science. Classification: LCC QB603.S6 R46 2019 | DDC 523.4028/7–dc23 LC record available at https://lccn.loc.gov/2019002564 ISBN 978-1-107-18620-0 Hardback

Cambridge University Press has no responsibility for the persistence or accuracy of URLs for external or third-party internet websites referred to in this publication and does not guarantee that any content on such websites is, or will remain, accurate or appropriate.

Contents List of Contributors Foreword CARLÉ M. PIETERS AND PETER A. J. ENGLERT

Preface Acknowledgments

Part I Theory of Remote Compositional Analysis Techniques and Laboratory Measurements 1 Electronic Spectra of Minerals in the Visible and Near-Infrared Regions GEORGE R. ROSSMAN AND BETHANY L. EHLMANN

2 Theory of Reflectance and Emittance Spectroscopy of Geologic Materials in the Visible and Infrared Regions J O H N F. M U S TA R D A N D T I M O T H Y D . G L O T C H

3 Mid-infrared (Thermal) Emission and Reflectance Spectroscopy: Laboratory Spectra of Geologic Materials MELISSA D. LANE AND JANICE L. BISHOP

4 Visible and Near-Infrared Reflectance Spectroscopy: Laboratory Spectra of Geologic Materials JANICE L. BISHOP

5 Spectroscopy of Ices, Volatiles, and Organics in the Visible and Infrared Regions D A L E P. C R U I K S H A N K , LY U B A V. M O R O Z , A N D R O G E R N . C L A R K

6 Raman Spectroscopy: Theory and Laboratory Spectra of Geologic Materials SHIV K. SHARMA AND MILES J. EGAN

7 Mössbauer Spectroscopy: Theory and Laboratory Spectra of Geologic Materials M . D A R B Y D YA R A N D E L I Z A B E T H C . S K L U T E

8 Laser-Induced Breakdown Spectroscopy: Theory and Laboratory Spectra of

Geologic Materials SAMUEL

M.

CLEGG,

R YA N

B.

ANDERSON,

AND

NOUREDDINE

MELIKECHI

9 Neutron, Gamma-Ray, and X-Ray Spectroscopy: Theory and Applications THOMAS

H.

PRETTYMAN,

PETER

A.

J.

E N G L E RT,

AND

NAOYUKI

YA M A S H I T A

10 Radar Remote Sensing: Theory and Applications J A K O B VA N Z Y L , C H A R L E S E L A C H I , A N D Y U N J I N K I M

Part II Terrestrial Field and Airborne Applications 11 Visible and Near-Infrared Reflectance Spectroscopy: Field and Airborne Measurements ROGER N. CLARK

12 Raman Spectroscopy: Field Measurements PA B L O S O B R O N , A N U PA M M I S R A , F E R N A N D O R U L L , A N D A N T O N I O SANSANO

Part III Analysis Methods 13 Effects of Environmental Conditions on Spectral Measurements E D WA R D C L O U T I S , P I E R R E B E C K , J E F F R E Y J . G I L L I S - D AV I S , J Ö R N H E L B E RT, A N D M A R K J . L O E F F L E R

14 Hyper- and Multispectral Visible and Near-Infrared Imaging Analysis WILLIAM

H.

FA R R A N D ,

ERZSÉBET

MERÉNYI,

AND

MARIO

C.

PA R E N T E

15 Thermal Infrared Spectral Modeling JOSHUA L. BANDFIELD AND A. DEANNE ROGERS

16 Geochemical Interpretations Using Multiple Remote Datasets SUNITI

K A R U N AT I L L A K E ,

LY N N

M.

C A RT E R ,

LY D I A J . H A L L I S , A N D J O E L A . H U R O W I T Z

H E AT H E R

B.

FRANZ,

Part IV Applications to Planetary Surfaces 17 Spectral Analyses of Mercury SCOTT L. MURCHIE, NOAM R. IZENBERG, AND RACHEL L. KLIMA

18 Compositional Analysis of the Moon in the Visible and Near-Infrared Regions C A R L É M . P I E T E R S , R A C H E L L . K L I M A , A N D R O B E RT O . G R E E N

19 Spectral Analyses of Asteroids JOSHUA

P.

E M E R Y,

CRISTINA

A.

THOMAS,

VISHNU

R E D D Y,

AND

NICHOLAS A. MOSKOVITZ

20 Visible and Near-Infrared Spectral Analyses of Asteroids and Comets from Dawn and Rosetta M.

CRISTINA

DE

SANCTIS,

FA B R I Z I O

C A PA C C I O N I ,

ELEONORA

AMMANNITO, AND GIANRICO FILACCHIONE

21 Spectral Analyses of Saturn’s Moons Using the Cassini Visual Infrared Mapping Spectrometer B O N N I E J . B U R AT T I , R O B E R T H . B R O W N , R O G E R N . C L A R K , D A L E P. CRUIKSHANK, AND GIANRICO FILACCHIONE

22 Spectroscopy of Pluto and Its Satellites D A L E P. C R U I K S H A N K , W I L L I A M M . G R U N D Y, D O N A L D E . J E N N I N G S , C AT H E R I N E

B.

OLKIN,

S I LV I A

P R O T O PA PA ,

DENNIS

C.

REUTER,

B E R N A R D S C H M I T T, A N D S . A L A N S T E R N

23 Visible to Short-Wave Infrared Spectral Analyses of Mars from Orbit Using CRISM and OMEGA SCOTT

L.

A RV I D S O N ,

MURCHIE,

JEAN-PIERRE

JANICE

B I S H O P,

L.

BIBRING,

JOHN

C A RT E R ,

R AY M O N D BETHANY

E. L.

E H L M A N N , Y V E S L A N G E V I N , J O H N F. M U S TA R D , F R A N C O I S P O U L E T, L U C I E R I U , K I M B E R LY D . S E E L O S , A N D C H R I S T I N A E . V I V I A N O

24 Thermal Infrared Spectral Analyses of Mars from Orbit Using the Thermal Emission Spectrometer and Thermal Emission Imaging System VICTORIA

E.

H A M I LT O N ,

PHILIP

R.

CHRISTENSEN,

JOSHUA

L.

B A N D F I E L D , A . D E A N N E R O G E R S , C H R I S T O P H E R S . E D WA R D S , A N D S T E V E N W. R U F F

25 Thermal Infrared Remote Sensing of Mars from Rovers Using the Miniature Thermal Emission Spectrometer

S T E V E N W. R U F F, J O S H U A L . B A N D F I E L D , P H I L I P R . C H R I S T E N S E N , TIMOTHY

D.

GLOTCH,

VICTORIA

E.

H A M I LT O N ,

AND

A.

DEANNE

ROGERS

26 Compositional and Mineralogic Analyses of Mars Using Multispectral Imaging on the Mars Exploration Rover, Phoenix, and Mars Science Laboratory Missions J A M E S F. B E L L I I I , W I L L I A M H . FA R R A N D , J E F F R E Y R . J O H N S O N , K J A R TA N M . K I N C H , M A R K L E M M O N , M A R I O C . PA R E N T E , M E L I S S A S. RICE, AND DANIKA WELLINGTON

27 Mössbauer Spectroscopy at Gusev Crater and Meridiani Planum: Iron Mineralogy, Oxidation State, and Alteration on Mars RICHARD

V.

MORRIS,

CHRISTIAN

SCHRÖDER,

G Ö S TA R

K L I N G E L H Ö F E R , A N D D AV I D G . A G R E S T I

28 Elemental Analyses of Mars from Rovers Using the Alpha-Particle X-Ray Spectrometer R A L F G E L L E RT A N D A L B E RT S . Y E N

29 Elemental Analyses of Mars from Rovers with Laser-Induced Breakdown Spectroscopy by ChemCam and SuperCam NINA

L.

LANZA,

ROGER

C.

WIENS,

S Y LV E S T R E

MAURICE,

AND

JEFFREY R. JOHNSON

30 Neutron, Gamma-Ray, and X-Ray Spectroscopy of Planetary Bodies THOMAS

H.

PRETTYMAN,

PETER

A.

YA M A S H I T A , A N D M A R G A R E T E . L A N D I S

31 Radar Remote Sensing of Planetary Bodies JEFFREY J. PLAUT

Index

J.

E N G L E RT,

NAOYUKI

Contributors

Editors Janice L. Bishop Carl Sagan Center, SETI Institute, Mountain View, CA, USA James F. Bell III School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA Jeffrey E. Moersch Earth and Planetary Science Department, University of Tennessee, Knoxville, TN, USA

Contributing Authors David G. Agresti Department of Physics, University of Alabama at Birmingham, Birmingham, AL, USA Eleonora Ammannito Agenzia Spaziale Italiana, Via del Politecnico snc, Rome, Italy Ryan B. Anderson Astrogeology Science Center, United States Geological Survey, Flagstaff, AZ, USA Raymond E. Arvidson Department of Earth and Planetary Sciences, Washington University in St. Louis, St. Louis, MO, USA Joshua L. Bandfield formerly, Space Science Institute, Boulder, CO, USA Pierre Beck Institut de Planétologie et d’Astrophysique de Grenoble, Université Grenoble Alpes, SaintMartin-d’Hères, Grenoble Cedex, France James F. Bell III School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA Jean-Pierre Bibring Institut d’Astrophysique Spatiale, Orsay Cedex, France Janice L. Bishop Carl Sagan Center, SETI Institute, Mountain View, CA, USA Robert H. Brown University of Arizona, Tucson, AZ, USA Bonnie J. Buratti Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA Fabrizio Capaccioni

Istituto di Astrofisica e Planetologia Spaziali, INAF, Rome, Italy John Carter Institut d’Astrophysique Spatiale, Orsay Cedex, France Lynn M. Carter Department of Planetary Sciences, University of Arizona, Tucson, AZ, USA Philip R. Christensen School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA Roger N. Clark Planetary Science Institute, Tucson, AZ, USA Samuel M. Clegg Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM, USA Edward Cloutis Department of Geography, University of Winnipeg, Winnipeg, MB, Canada Dale P. Cruikshank NASA Ames Research Center, Moffett Field, CA, USA M. Cristina De Sanctis Istituto di Astrofisica e Planetologia Spaziali, INAF, Rome, Italy M. Darby Dyar Department of Astronomy, Mount Holyoke College, South Hadley, MA, USA, and Planetary Science Institute, Tucson, AZ, USA Christopher S. Edwards Department of Physics and Astronomy, Northern Arizona University, Flagstaff, AZ, USA Miles J. Egan Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, HI, USA Bethany L. Ehlmann Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA Charles Elachi

Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA Joshua P. Emery Earth and Planetary Science Department, University of Tennessee, Knoxville, TN, and Astronomy and Planetary Science Department, Northern Arizona University, Flagstaff, AZ, USA Peter A. J. Englert Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, HI, USA William H. Farrand Space Science Institute, Boulder, CO, USA Gianrico Filacchione Istituto di Astrofisica e Planetologia Spaziali, INAF, Rome, Italy Heather B. Franz NASA Goddard Space Flight Center, Greenbelt, MD, USA Ralf Gellert Department of Physics, University of Guelph, Guelph, ON, Canada Jeffrey J. Gillis-Davis Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, HI, USA Timothy D. Glotch Department of Geosciences, Stony Brook University, Stony Brook, NY, USA Robert O. Green Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA William M. Grundy Lowell Observatory, Flagstaff, AZ, USA Lydia J. Hallis School of Geographical and Earth Sciences, University of Glasgow, Glasgow, Scotland Victoria E. Hamilton Department of Space Studies, Southwest Research Institute, Boulder, CO, USA

Jörn Helbert Deutsches Zentrum für Luft und Raumfahrt e.V. (DLR), Berlin, Germany Joel A. Hurowitz Department of Geosciences, Stony Brook University, Stony Brook, NY, USA Noam R. Izenberg Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA Donald E. Jennings NASA Goddard Space Flight Center, Greenbelt, MD, USA Jeffrey R. Johnson Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA Suniti Karunatillake Planetary Science Lab, Geology and Geophysics, Louisiana State University, Baton Rouge, LA, USA Yunjin Kim Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA Kjartan M. Kinch Astrophysics and Planetary Science, Niels Bohr Institute, University of Copenhagen, Denmark Rachel L. Klima Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA Göstar Klingelhöfer formerly, Institut für Anorganische und Analytische Chemie, Johannes GutenbergUniversität, Mainz, Germany Margaret E. Landis Planetary Science Institute, Tucson, AZ, USA Melissa D. Lane Fibernetics LLC, Lititz, PA, USA Yves Langevin Institut d’Astrophysique Spatiale, Orsay Cedex, France

Nina L. Lanza Space and Remote Sensing, Los Alamos National Laboratory, Los Alamos, NM, USA Mark Lemmon Texas A&M University, College Station, TX, USA Mark J. Loeffler Department of Physics and Astronomy, Northern Arizona University, Flagstaff, AZ, USA Sylvestre Maurice Institut de Recherche en Astrophysique et Planétologie, Toulouse, France Noureddine Melikechi Kennedy College of Sciences, University of Massachusetts Lowell, Lowell, MA, USA Erzsébet Merényi Department of Statistics, and Department of Electrical and Computer Engineering, Rice University, Houston, TX, USA Anupam Misra Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, HI, USA Lyuba V. Moroz University of Potsdam, Potsdam, Germany Richard V. Morris Exploration and Integration Science Division, NASA Johnson Space Center, Houston, TX, USA Nicholas A. Moskovitz Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA, and Lowell Observatory, Flagstaff, AZ, USA Scott L. Murchie Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA John F. Mustard Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, USA Catherine B. Olkin

Southwest Research Institute, Boulder, CO, USA Mario C. Parente Department of Electrical and Computer Engineering, University of Massachusetts at Amherst, Amherst, MA, USA Carlé M. Pieters Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, USA Jeffrey J. Plaut Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA Francois Poulet Institut d’Astrophysique Spatiale, Orsay Cedex, France Thomas H. Prettyman Planetary Science Institute, Tucson, AZ, USA Silvia Protopapa Southwest Research Institute, Boulder, CO, USA Vishnu Reddy Lunar and Planetary Laboratory,University of Arizona, Tucson, AZ, USA Dennis C. Reuter NASA Goddard Space Flight Center, Greenbelt, MD, USA Melissa S. Rice Western Washington University, Bellingham, WA, USA Lucie Riu Institut d’Astrophysique Spatiale, Orsay Cedex, France A. Deanne Rogers Department of Geosciences, Stony Brook University, Stony Brook, NY, USA George R. Rossman Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA Steven W. Ruff

School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA Fernando Rull University of Valladolid/Unidad Asociada UVa-CSIC Centro de Astrobiología, Valladolid, Spain Antonio Sansano University of Valladolid/Unidad Asociada UVa-CSIC Centro de Astrobiología, Valladolid, Spain Bernard Schmitt Université Grenoble Alpes, Saint-Martin-d’Hères, France Christian Schröder Biological and Environmental Sciences, University of Stirling, Stirling, Scotland, UK Kimberly D. Seelos Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA Shiv K. Sharma Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, HI, USA Elizabeth C. Sklute Department of Astronomy, Mount Holyoke College, South Hadley, MA, USA, and Planetary Science Institute, Tucson, AZ, USA Pablo Sobron Impossible Sensing, St. Louis, MO, USA, and Carl Sagan Center, the SETI Institute, Mountain View, CA, USA S. Alan Stern Department of Space Studies, Southwest Research Institute, Boulder, CO, USA Cristina A. Thomas Planetary Science Institute, Tucson, AZ, USA Christina E. Viviano Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA Danika Wellington School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA

Roger C. Wiens Space and Remote Sensing, Los Alamos National Laboratory, Los Alamos, NM, USA Naoyuki Yamashita Planetary Science Institute, Tucson, AZ, USA Albert S. Yen Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA Jakob van Zyl Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

Chapter Reviewers S. MICHAEL ANGEL

(University of South Carolina)

GABRIELA ARNOLD

(German Aerospace Center, DLR-Berlin)

J A M E S W. A S H L E Y

(Jet Propulsion Laboratory)

JOSHUA L. BANDFIELD ADRIAN BROWN

(Plancius Research)

B O N N I E J . B U R AT T I JAMES BYRNE

formerly, (Space Science Institute)

(Jet Propulsion Laboratory)

(Universität Tübingen) (Smithsonian National Air and Space Museum)

BRUCE CAMPBELL

DONALD B. CAMPBELL LY N N M . C A R T E R

(University of Arizona)

SAMUEL M. CLEGG E D WA R D C L O U T I S E D D Y D E G R AV E

(Cornell University)

(Los Alamos National Laboratory)

(University of Winnipeg)

(University of Ghent)

M. CRISTINA DE SANCTIS M . D A R B Y D YA R

(Istituto di Astrofisica e Planetologia Spaziali)

(Mount Holyoke College and Planetary Science Institute)

C H R I S T O P H E R S . E D WA R D S BETHANY L. EHLMANN

(Northern Arizona University)

(California Institute of Technology)

W I L L I A M H . FA R R A N D

(Space Science Institute)

ABIGAIL A. FRAEMAN

(Jet Propulsion Laboratory)

P AT R I C K J . G A S D A

(Los Alamos National Laboratory)

M A RT H A S . G I L M O R E

(Wesleyan University)

TIMOTHY D. GLOTCH

(Stony Brook University)

WILLIAM M. GRUNDY

(Lowell Observatory)

V I C T O R I A E . H A M I LT O N CRAIG HARDGROVE AMANDA HENDRIX

(Southwest Research Institute)

(Arizona State University)

(Planetary Science Institute)

TA K A H I R O H I R O I

(Brown University)

BRIONY HORGAN

(Purdue University)

MELISSA D. LANE L U C Y F. L I M

(Fibernetics LLC)

(NASA Goddard Space Flight Center)

PA U L G . L U C E Y

(University of Hawai‘i at Mānoa) (University of Arizona)

R O B E RT L . M A R C I A L I S THOMAS MCCORD

(Bear Fight Institute)

FRANCIS M. MCCUBBIN

(NASA Johnson Space Center)

L U C Y A N N A . M C FA D D E N H A R R Y Y. M C S W E E N

(University of Tennessee)

NOUREDDINE MELIKECHI A L B E RT E . M E T Z G E R

(University of Massachusetts Lowell)

(Jet Propulsion Laboratory)

JOSEPH R. MICHALSKI D O U G L A S W. M I N G

(NASA Goddard Space Flight Center)

(University of Hong Kong)

(NASA Johnson Space Center)

A N D R Z E J W. M I Z I O L E K

(US Army Research Laboratory, retired)

GARETH A. MORGAN

(Planetary Science Institute)

R I C H A R D V. M O R R I S

(NASA Johnson Space Center)

(Brown University)

J O H N F. M U S TA R D ION PRISECARU

(Bruker, WMOSS)

VISHNU REDDY

(Lunar and Planetary Laboratory) (Planetary Science Institute)

R O B E RT C . R E E D Y

A. DEANNE ROGERS TED L. ROUSH

(Stony Brook University)

(NASA Ames Research Center) (Northern Arizona University)

M A R K S A L VAT O R E

CHRISTIAN SCHRÖDER SHIV K. SHARMA

(University of Hawai‘i at Mānoa)

G R E G G A . S WA Y Z E

(US Geological Survey Boulder)

S T E FA N I E T O M P K I N S T O O N VA N A L B O O M ANNE VERBISCER A L I A N WA N G

(University of Stirling)

(Colorado School of Mines)

(University of Ghent)

(University of Virginia)

(Washington University in St. Louis)

SHOSHANA Z. WEIDER J A M E S J . W R AY

(Imperial College London)

(Georgia Institute of Technology)

Foreword carle´ m. pieters and peter a. j. englert

So much has happened in the 25 years since the last collection of expert papers documenting principles and products of remote compositional analyses were brought together as a book! Of course, it is not that the physics and chemistry governing properties of planetary materials has changed much in a generation. However, great advances have been made during the intervening decades as new instruments were built and new spacecraft were flown across the Solar System. This is coupled with advances in information extraction techniques and instrument technology that enable the measurement of these properties with increasing detail both in the laboratory and remotely. Consequently, understanding nuances of the diagnostic properties forming the basis for compositional analyses has grown in leaps and bounds along with remarkable and expanding new data from the inner to outer Solar System, including both rocky and icy bodies with and without an atmosphere. The 1993 book Remote Geochemical Analyses: Elemental and Mineralogical Composition was compiled near the end of the last millennium following a symposium bringing together planetary scientists across many disciplines. At that time, remote planetary exploration techniques were just beginning to grow in importance and impact. The impetus for bringing together information and technical background in one book was to make the scientific basis for this relatively new field readily available across a growing community. The initial discussions in Remote Geochemical Analysis laid the foundation for years of basic exploration of Solar System bodies in all their diversity and mystery. Subsequent expansion and maturation of remote sensing data obtained using telescopes and increasingly sophisticated exploratory spacecraft opened a wide range of data types and approaches with which to obtain information and understand the diverse and complex bodies of our Solar System. Today, several decades later, the initial reconnaissance of the Solar System is complete. We have now looked at everything from Mercury to the Kuiper belt at least once. That has taught us there is a LOT more to learn. The path that exploration has taken has provided profound insight, awesome discoveries, and continuous inspiration. Nevertheless, it necessarily has not been a linear or complete process. We are now embarking on an era of detailed and serious exploration, that cries for in-depth knowledge of, and comparisons between, the diverse rocky, hydrated, icy, and gaseous bodies of our Solar System (including Earth) – and even planets of other star systems. Although the exploration focus and xix

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Foreword

resulting data acquired have been uneven, a plethora of fundamental questions are posed and remain unanswered regarding the composition of each planetary body we have come to know. In parallel with the quest for deeper scientific understanding, modern technology provides increasingly sophisticated instruments to measure compositional properties remotely, and such exploration tools promise many exciting decades ahead. The chapters in this completely new Remote Compositional Analysis: Techniques for Understanding Spectroscopy, Mineralogy, and Geochemistry of Planetary Surfaces provide a diverse and enormously updated taste of what we know and what we have been able to learn about the composition of planetary bodies using remote sensing techniques over the last few decades. Remote compositional analysis has become a mature interdisciplinary field of science and has evolved into an indispensable component of Earth and planetary exploration.

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Preface

The field of remote sensing is integral to exploration of our Solar System and encompasses results acquired from telescopic, spacecraft, and landed missions. The remote sensing techniques described in this book span a range of processes, compositions, and planetary bodies. It is a dramatically updated version of the original first edition from 1993. Since then, significant advances have occurred in space exploration, including dozens of new missions from NASA, ESA, and other space agencies around the world; substantial advances in telescopic and laboratory instrumentation, and major new work in theoretical studies have also occurred. As a result, every topic has been updated from the original book edited by C. M. Pieters and P. A. J. Englert, and new content has been added to reflect major advancements since 1993. This work sprung out of a 1988 symposium on mineral spectroscopy organized by L. M. Coyne with assistance from J. L. Bishop that was sponsored by the Division of Geochemistry of the American Chemical Society. The purpose of that symposium was to bring together an interdisciplinary, international community to foster spectral identification of minerals. We have attempted to provide an introduction to the field of planetary surface composition and mineralogy for upper-level undergraduates, graduate students, or professional researchers just moving into this topic. This book is organized into four sections including (I) theory and laboratory measurements, (II) terrestrial field and airborne applications, (III) analysis methods, and (IV) applications to planetary surfaces. Among the types of remote sensing techniques covered are visible to infrared reflectance spectroscopy, infrared emission spectroscopy (also called thermal infrared spectroscopy), Raman spectroscopy, Mössbauer spectroscopy, Laser-Induced Breakdown Spectroscopy (LIBS), neutron spectroscopy, X-ray spectroscopy, gamma-ray spectroscopy, and radar. The basic premise of each technique, information on how to perform measurements, and example spectra of rocks, regolith, minerals, and volatiles are provided. This book covers the minerals, elements, and molecules found on airless rocky bodies including Mercury, the Moon, and asteroids. It describes the kinds of volatiles (ices, organics, hydrated minerals) found on the surfaces of our Solar System’s planets, moons, asteroids, and comets, and how they are related to volatiles on Earth. Finally, several chapters specifically focus on the composition and processes taking place on Mars, the planet most studied besides Earth. xxi

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1 Electronic Spectra of Minerals in the Visible and Near-Infrared Regions george r. rossman and bethany l. ehlmann

1.1 Origin of Electronic Spectra of Minerals Many of the spectral features of minerals in the visible to near-infrared region (VNIR; defined here as ~0.4–2.5 µm) arise from electronic transitions within and between transition elements and the anions chemically bound to them. Thousands of minerals have color or wavelengthvariable properties in this portion of the spectrum. Metal ions including vanadium, chromium, manganese, iron, cobalt, nickel, and copper, usually in either the 2+ or 3+ oxidation state, are responsible for the color of many minerals. However, only a few of these elements, typically iron, titanium, and oxygen, are important in most remote sensing applications of rocky bodies. Many features arise from electronic transitions of electrons between the d orbitals of a metal ion, while some spectroscopic features arise from interactions between atoms. 1.2 Units Wavelengths are commonly expressed in nanometers (nm) or micrometers (μm) and, in older literature, Ångstrom units. Literature on mineral spectroscopy and mineral chemistry often uses nm, while the remote sensing literature typically uses µm. The conversion among them is: 1000 nm = 1 μm = 10,000 Å.

(1.1)

The spectrum can also be presented in energy units, usually wavenumbers, which are the reciprocal of the wavelength, and are usually expressed in reciprocal cm. The advantage of wavenumbers is that absorptions are symmetrical in energy coordinates but not in wavelength coordinates. Spectroscopic energies can also be expressed in electron volts, but this is more commonly encountered in the physics literature.
Wavenumbers cm–1 ¼ 10; 000; 000=nm ¼ 10; 000=μmÞ ð1:2Þ 1000 nm ¼ 1 μm ¼ 10; 000 cm–1 ; 400 nm ¼ 0:4 μm ¼ 25;000 cm–1 1 cm–1 ¼ 1:23984  10–4 eV; 8065:54 cm–1 ¼ 1 eV ¼ 1239:8 nm: Spectra are usually displayed in either reflectance units or absorbance units. Reflectance spectra must be taken in comparison to a standard. In a laboratory setting, the standard can

3

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be a colorless polytetrafluoroethylene-based plastic such as Spectralon®, aluminum, or, in the near-IR (NIR) region, gold. Spectra are presented as the percent (0–100%) or fraction (0–1) of sample reflectance relative to the standard versus wavelength. In spacecraft applications, the comparison standard is typically the solar flux. Reflected light data collected by spacecraft are typically expressed as I/F [radiance/(solar irradiance/π)]. For a given viewing geometry, these data can be further corrected for angular dependencies in scattering properties (see Hapke, 1981, for definitions of different types of reflectance). Even though remotely obtained spectra are the composite response of many components in a field of view, many fundamental studies of mineral spectra are conducted with single crystals. In chemistry, such studies are usually presented in absorbance units, where Absorbance = ‒log10(Transmission).

(1.3)

An absorbance of 1 means that 10% of the incident light is passing through the crystal; an absorbance of 2 means that 1% of the light passes through. Beer’s law formulations (I = I0e–αd) are also sometimes used to derive an absorption coefficient (α) where the intensity of initial light (I0) is compared to the intensity after transmission (I) through a given thickness of material (d). Because most mineral crystals are anisotropic, fundamental studies of single crystals usually measure the spectrum with polarized light vibrating along the fundamental optical directions of the crystal. The refractive indices of a crystal for light traveling in different directions relative to the crystal axes form an optical indicatrix, mathematically, an ellipsoidal surface. Crystals that belong to the orthorhombic, monoclinic, and triclinic crystal systems will have three independent spectra that can display very different absorption properties (biaxial indicatrix). Crystals in the tetragonal and hexagonal systems will have two different spectra (uniaxial indicatrix), while isotropic, cubic crystals will have only one spectrum (spherical indicatrix). In general, spectra can be named either according to the crystal axes in which the vibration occurs (e.g., E\\c or the c-spectrum) or by the symbol for the index of refraction that would be measured in the vibration direction. For biaxial crystals with three independent optical directions we have the α, β, and γ spectra (also called the X, Y, and Z spectra). For uniaxial crystals with two different spectra there are two independent optical orientations: the E⊥c direction, also called the ω-spectrum, and the E\\c direction, which is also called the ε-spectrum.

1.3 Crystal Field Transitions The spectra of metal ions, particularly those of first-row transition elements, Ti through Cu, are often interpreted with the use of Crystal Field Theory. The d-orbital electrons are the valence (outermost) electrons in the case of these metals. For an isolated transition metal ion, electrons occupy any d orbital with equal probability. However, in a mineral, electrostatic fields produced by the anions (usually oxygen) surrounding the central metal ion separate the metal ion’s d orbitals into different energy levels. This allows the d-orbital

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electrons to undergo transitions between orbital energy states. Their transitions to different energy levels under the influence of VNIR light give rise to much of the color we see and the spectra we measure of minerals. This can be understood for the case of a metal ion surrounded by six oxide ions (ligands) arranged in perfect octahedral symmetry. If the metal ion were floating in free space with no oxide or other anions near it, all five orbitals in the 3d level would have the same energy (Figure 1.1a). But when the metal ion is in an octahedral arrangement of oxide ions, the d orbitals split into groups of two different energies (Figure 1.1b) reflecting the different interactions the d-orbital electron clouds have with the surrounding ligands. Iron in the 2+ oxidation state has six electrons, the valence electrons, in the 3d orbital. In an octahedral coordination environment, these go into the 3d orbitals as pictured in Figure 1.2a because the electrons are energetically more stable when pairing of electrons is minimized. An electronic transition will occur when light of an appropriate energy interacts with the Fe2+ ion and promotes an electron from a lower energy orbital to a higher energy

Figure 1.1 Energy diagram for 3d orbitals and their electron probability clouds. (a) Orbitals in free space. (b) Orbitals in an octahedron of oxide ions. (c) The electron clouds of the d orbitals in relationship to the oxide ions in an octahedral arrangement.

Figure 1.2 Electron configurations for Fe2+. (a) The ground state in an octahedral coordination environment. (b) The spin-allowed excited state that gives rise to the primary NIR absorption bands. Here, the total number of unpaired electrons has not changed in the excited state. (c) A spinforbidden state in which the total number of unpaired electrons has changed in the electronic excitation. A comparison of the relative splitting of ground state d orbitals for Fe2+ ion in (d) regular octahedral, (e) regular tetrahedral, and (f) a representative distorted coordination.

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orbital (Figure 1.2b). Each configuration of the electrons is an electronic state of the system. If the total number of unpaired electrons is not changed during the transition from the electronic ground state to a higher energy state, this is called a spin-allowed transition. If the total number of unpaired electrons changes, this transition is called a spin-forbidden transition because such a transition is about 100 times less likely to occur than a spinallowed one. The intensities of electronic bands relate to their spin-allowed or spin-forbidden properties. Spin-forbidden transitions produce absorption bands that are commonly much weaker than the spin-allowed bands. However, interactions between cations, as explained in a following section, can dramatically increase the intensity of formally spin-forbidden bands and produce other features of high intensity when cations in different oxidation states interact. In addition, electronic transitions that involve transfer of charge from anions to cations (Sections 1.4 and 1.5) can also be of much higher intensity, but are usually centered in the ultraviolet portion of the spectrum. Qualitative predictions of the spectrum of metal ion complexes can be obtained from Tanabe–Sugano diagrams. These diagrams usually present energy states for complexes in ideal, octahedral coordination that can be used to interpret the number of spin-allowed and spin-forbidden absorption bands and their widths, and, with suitable experimental parameters, can provide predictions of where bands will occur. Most ions in minerals are not in ideal octahedral coordination, so these diagrams often do not accurately interpret mineral spectra, but they do indicate which absorption bands will split into multiple components for metal ions in crystal sites of low symmetry. These diagrams, along with other concepts previously discussed, are reviewed in more detail in several books and articles about mineral spectroscopy (Karr, 1975; Burns, 1993; Rossman, 2014). Another important factor in determining the number and wavelengths of absorption bands from a metal ion is the symmetry and distances of the ions surrounding the central metal ion. The number and energies of absorption bands strongly depend on the symmetry (Figure 1.2d–f). In a perfectly regular octahedron, Fe2+ will have one possible transition from the lower to the higher set of orbitals (Figure 1.2b). In a perfectly regular tetrahedral coordination environment, the energy difference between the orbitals will be smaller; consequently, the absorption will occur at longer wavelengths, but still with only one absorption band. However, coordination environments of ideal symmetry are almost never encountered. In nearly all minerals, the metal ion is in a coordination environment distorted from ideal symmetry. In such cases, the energies of the orbitals will split and multiple absorption bands will be possible. This fact is crucial for understanding the relatively broad nature of absorption bands in spectra of common rock-forming minerals. For example, in olivine the broad Fe-related electronic absorption observed is, in reality, a set of overlapping absorptions, caused by the existence of numerous 6-coordinated sites of different dimensions and symmetries that occur as the atoms around the iron vibrate due to thermal energy. In pyroxene, the wavelength of the Fe-related electronic absorption in the distorted 6-coordinated M(2) site shifts systematically with Ca, Fe, and Mg substitution that changes the dimensions of the octahedral site.

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Group theory provides symbolic names for each of the electronic states of the system. These names convey the spin state, degeneracy, and symmetry of the electron cloud (for further reading see Harris & Bertolucci, 1989; Cotton, 1990). For example, in a perfectly octahedral coordination environment, the Fe2+ ground state has a 5T2g symmetry designation, where the T indicates that the state is triply degenerate, the 5 is the number of unpaired electrons +1, and the 2g relates to the symmetry of the electron cloud. The excited state has a designation 5Eg, where the E symbolizes a doubly degenerate state. A single absorption band occurs when the electron is promoted from the 5T2g state to the 5Eg state. In coordination environments of lower symmetry, the T state can split into three different electronic states and the E state can split into two, each with a different energy. In orthopyroxene, the electronic ground state of Fe2+ in the low symmetry M(2) site splits into three different states labeled 5A1, 5A2, and 5B2, and the excited state splits into a 5B1 and a 5A1 state, each of which is no longer degenerate (Goldman & Rossman, 1977). Electronic absorption bands can be temperature sensitive. They typically broaden at higher temperatures and sharpen at lower temperatures. Fundamental studies of minerals and chemicals are often conducted at liquid nitrogen or even liquid helium temperatures to sharpen absorptions and allow determination of band centers at high spectroscopic resolution. Particularly for targets below ~150 K, consideration of shifts may be relevant in interpretation of remotely collected spectra. Absorptions can also shift position or change intensity as mineral sites are distorted and metal–oxygen bond distances change at elevated temperatures (e.g., Aronson et al., 1970; Sung et al., 1977). High-temperature spectra are important in planetary science for interpreting the composition of bodies that are several hundreds of degrees warmer than Earth such as Mercury, Venus, and lavas on Jupiter’s moon, Io.

1.4 Oxygen-to-Metal Charge Transfer Another common feature in the spectra of many minerals is the oxygen-to-metal charge transfer transition. This feature arises from absorption of photons with enough energy to transfer charge density from an oxygen ligand to the central metal ion. Oxygen-to-iron charge transfer is most commonly encountered in common rock-forming minerals where the band is usually centered in the ultraviolet region. The higher the charge state of the central metal ion, the lower the energy of the absorption band will be. Oxygen-to-Fe3+ charge transfer bands sometimes tail into the visible portion of the spectrum, where they absorb in violet and blue and often produce a rusty orange-red color. Oxygen-to-metal charge transfer absorptions are normally much more intense than those arising from transitions within the d orbitals of metal ions.

1.5 Intervalence Charge Transfer Intervalence Charge Transfer (IVCT) refers to a process in which two metal ions in close proximity to each other in a structure transfer an electron between them, thereby

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temporarily changing the oxidation state of both cations. Absorption bands in the optical spectrum from IVCT can be comparatively intense, and only a little IVCT produces spectroscopic features and color in the visible spectral region. In the geological world, only two such interactions are commonly encountered: Fe2+–Fe3+ and Fe2+–Ti4+. A third, Ti3+–Ti4+, is occasionally found in meteorites. For these interactions to occur, cations need to be adjacent to each other in the mineral structure, often sharing a common edge or face of the coordination polyhedron. Both Fe2+–Fe3+ and Fe2+–Ti4+ IVCT are particularly common in terrestrial minerals such as micas, pyroxenes, amphiboles, and tourmalines and are the origin of the dark color of many minerals including magnetite and ilmenite. Fe2+–Fe3+ IVCT in sites of nearoctahedral coordination is found in the 630–820 nm region. Fe2+–Ti4+ IVCT (Figure 1.3a) is typically found in the 425–460 nm region for 6-coordinated nearoctahedral cations such as in pyroxenes (Mao et al., 1977; Mattson & Rossman, 1988). The Ti3+–Ti4+ IVCT, observed in pyroxenes and hibonite from meteorites (Dowty & Clark, 1973; Burns & Vaughn, 1975), occurs near 690 nm in meteoritic hibonite from Murchison (Rossman, 2019). In a number of terrestrial minerals adjacent sites may have different coordination polyhedra including edge-shared octahedra and tetrahedra in cordierite or edge-shared octahedra and distorted cubes in garnets. In these cases, the wavelengths of the IVCT bands will differ from those of the edge-shared octahedra. A number of different mineral examples are reviewed in Burns (1981).

Figure 1.3 Transmission spectra. (a) Clinopyroxene from the Angra dos Reis meteorite showing Fe2+–Ti4+ IVCT near 480 nm and the Fe2+ features near 1000 and 1200 nm discussed in Section 1.6. (Modified from Mao et al., 1977.) (b) A 200 μm thick augite crystal showing the absorption bands from Fe2+ in the geometrically distorted M(2) site near 1000 and 2400 nm, and the weaker bands from Fe2+ in the nearly octahedral M(1) site near 970 and 1200 nm. Weak absorption from Cr3+ appears near 450 and 650 nm. (c) A 200 μm thick diopside crystal showing comparatively weak absorption bands from Fe2+ in the geometrically distorted M(2) site near 1000 and 2400 nm, and the stronger bands from Fe2+ in the nearly octahedral M(1) site near 1000 and 1200 nm. Absorption near 800 nm arises from Fe2+–Fe3+ IVCT.

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1.6 Spectra of Key Minerals There are currently more than 5400 known mineral phases, but only a small number of them contribute electronic absorptions routinely associated with remotely sensed spectra in the VNIR region. These phases include pyroxenes, olivines, feldspars, iron-bearing layered silicate minerals, and iron oxides. A number of other phases such as iron carbonates, iron sulfates, and other sulfur species are occasionally encountered. While they are only components contributing to the whole spectroscopic signature of an object, these minerals and their electronic absorptions carry important information for revealing the geological history of an object. In this section, we review the spectra of select, important phases. Many examples of the spectra of mineral single crystals with other cations are presented in Rossman (2014). Iron is the element most commonly causing absorptions in the VNIR spectral region and is responsible for the color of common rock-forming minerals. In the primary igneous minerals, iron is usually found in the 2+ oxidation state, often either in sites that are somewhat distorted from ideal octahedral 6-coordination or in irregular sites of higher coordination number. Frequently, iron occurs in more than one distinct site in the crystal structure of the host mineral. Sulfur species of mixed oxidation state are important on some outer Solar System bodies (e.g., Io: Nash et al., 1980; Carlson et al., 1997). Other metal cations such as V, Cr, Mn, Ni, and Cu are important contributors to the spectra of terrestrial minerals and are responsible for the spectacular colors of many museum-quality minerals. To date, they have not played a significant role in remotely sensed spectra of other planetary bodies. 1.6.1 Pyroxenes Pyroxenes, (Ca, Mg, Fe)2(Si, Al)2O6, are important minerals in many planetary bodies and are an excellent example of how structural distortion affects spectral properties. The two components of the pyroxene absorption bands of Fe2+ become increasingly separated as the sites become more distorted from octahedral geometry due to cation substitutions. In the case of the pyroxene M(2) site, the two components can be separated by about 1000 nm (Figure 1.3b). The spectrum of augite in Figure 1.3b, a terrestrial clinopyroxene, shows prominent absorptions at about 1000 nm in the beta polarization and near 2300 nm in the alpha direction. These two bands arise from Fe2+ in the M(2) site of pyroxene, which is highly distorted from an octahedral geometry. Two weaker bands near 970 and 1200 nm are due to Fe2+ in the less distorted M(1) site. Small, sharp spin-forbidden transitions are observed at wavelengths less than 1.0 µm. In contrast, the spectrum of diopside in Figure 1.3c has comparatively little contribution from the M(2) site and is primarily dominated by Fe2+ absorption from the M(1) site. Pyroxenes are among the most widespread rock-forming minerals in the Solar System. The absorptions caused by Fe2+ in distorted M(1) and M(2) sites can be detected in remote sensing reflectance spectra and related to pyroxene crystal chemistry (Figure 1.4a–c),

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Figure 1.4 Reflectance spectra of pyroxenes. (a) Spectra of a variety of pyroxenes from Klima et al. (2011). (b) Pyroxene absorption band positions systematically shift with the crystal chemistry, here represented on a pyroxene quadrilateral. Apices are diopside (CaMgSi2O6), hedenbergite (CaFeSi2 O6), enstatite (Mg2Si2O6), and ferrosilite (Fe2Si2O6). (c) These changes have been essential for identifying distinct geologic units with low-Ca and high-Ca pyroxene (LCP and HCP) on Mars (spectra from Mustard et al., 2005), pyroxenes on Vesta (spectra from DeSanctis et al., 2012), and high-Ca pyroxenes in lunar lavas (spectra from Pieters, 1986).

which in turn can be related to magmatic processes occurring on Solar System bodies. For example, 1 µm and 2 µm absorptions in dark lunar mare terrains were used to establish their volcanic origin and map distinct lava flows (e.g., Pieters, 1978; Staid et al., 2011; Whitten & Head, 2015). Strong pyroxene absorption bands observed for the asteroid Vesta and its family were used to identify it as the parent body for the HED meteorite suite and later mapped with spacecraft data (e.g., McCord et al., 1970; DeSanctis et al., 2012). On Mars, an observed transition from older lavas with low-Ca to younger lavas with high-Ca pyroxenes is inferred to result from thermal evolution of the martian mantle (Mustard et al., 2005; Baratoux et al., 2013). For more reading on pyroxene spectroscopy, see Klima et al. (2011).

1.6.2 The Olivine Series The spectrum of forsterite provides another example of the role of Fe2+ in two distinct sites in the crystal of (Mg, Fe)2SiO4. Each of the 6-coordinated sites for the metal cations, known as the M(1) and M(2) sites, is significantly distorted from purely octahedral symmetry. Consequently, each site produces a pair of Fe2+ NIR absorption bands, which correspond to the crystal field splitting between the lower energy orbitals and the excited states (Figure 1.5). Because olivine is orthorhombic, the 3 spectra in Figure 1.5a represent polarizations along the a-, b-, and c-axes of the crystal which correspond to the γ, α, and

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Figure 1.5 Absorption spectra. (a) Olivine (forsterite) from San Carlos, Arizona. (b) Fe-bearing plagioclase feldspar from Lake County, Oregon.

β spectra, respectively. The absorption in the 700–1600-nm region of the forsterite spectrum represents spin-allowed bands of Fe2+. There are absorption bands centered near 830 nm, 1060 nm, 1100 nm, and 1310 nm. The two most intense bands displayed in the γ-spectrum are from iron in the M(2) site. Weak features at less than 800 nm are either spinforbidden bands of Fe2+ or features from other minor components. The diffuse reflectance spectrum convolves the three spectra, as shown in Figure 1.6. The band positions shift with increasing Fe/(Mg + Fe) ratios. Figure 1.6a compares the spectra of an Mg-poor and an Mgrich olivine. Olivine spectroscopy is further discussed in Sunshine et al. (1998), Isaacson et al. (2014), and Chapters 4 and 18.

1.6.3 Feldspars Plagioclase feldspars (e.g., CaAl2Si2O8) can have iron substitution and thus an Fe2+ absorption in the NIR region (Figure 1.5b). The dominant absorption centered near 1300 nm arises from Fe2+ in the Ca site, which is significantly distorted from any standard coordination geometry. Features in the 300–500 nm region are from Fe3+ in the Al sites, and features near 3000 nm are from the OH content of the feldspar. The Fe3+ bands are absent in the spectrum of lunar plagioclase returned by the Luna 20 mission (Bell & Mao, 1973; Chapter 18). Plagioclase spectroscopy is further discussed in Cheek (2014).

1.6.4 Spinels The spinel group minerals of the general formula (XY2O4) are phases that commonly contain Fe2+ in a tetrahedral environment, substituting for Mg. Because there is less

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Figure 1.6 Reflectance spectra of iron-containing phases. (a) Spectra of Fe2+ in olivine of two compositions (from Sunshine & Pieters, 1998), plagioclase feldspar (from the NASA/Keck RELAB database at Brown University, spectra by C.M. Pieters), and a spinel (from Cloutis et al., 2004). OMCT = oxygen to metal charge transfer; IVCT = intervalence charge transfer. See text for absorption attributions. (b) Fe(II) and Fe(III) create prominent absorptions in Fe-bearing glasses (from Cannon et al., 2017).

electrostatic repulsion from the four oxygen atoms surrounding the iron compared to six oxygen atoms in octahedrally coordinated iron, the lowest energy (longest wavelength) Fe2+ absorption bands occur at lower energies (longer wavelengths) than those from 6-coordinated Fe2+ (Figure 1.7a).

1.6.5 Ferric Oxides Ferric oxides contain VNIR absorption features due to both crystal field splitting and charge transfer. Hematite (Fe2O3) has a prominent absorption near 860 nm and a shoulder at 630 nm due to Fe3+ in a site of near-octahedral symmetry (Figure 1.7b). Starting at 530 nm, the visible light wing of a strong UV-visible charge transfer of oxygen–Fe3+ dominates the spectrum, making the reflectance very low and obscuring remaining crystal field absorptions. These properties change for nanocrystalline hematite, which has particle sizes