This page intentionally left blank PLANETARY CRUSTS: THEIR COMPOSITION, ORIGIN AND EVOLUTION Planetary Crusts is the first book to explain how and why solid planets and satellites develop crusts. This extensively referenced and annotated volume presents a geo- chemical and geological survey of the crusts of the Moon, Mercury, Venus, the Earth and Mars, as well as the distinct crusts of the asteroid Vesta and the satellites Io, Europa, Ganymede, Callisto, Titan and Triton. Spanning a much wider compass than mere descriptions of the diverse crusts encountered throughout the Solar System, the book begins with a discussion of the nature of Solar System bodies and their formation. The authors then adopt a comparative approach to investigate the many current controversies surrounding the development and evolution of planetary crusts. These include the origin of the Moon and Mercury, the nature of the Mercurian plains, the exotic chemistry of Mars, differences in the geological histories of Venus and Earth, the significance of the rare earth element europium, the primitive crusts on the Earth, the onset of plate tectonics, the composition of the mantle, the origin of granites, why Ganymede differs from Callisto, and many other debated topics. The authors conclude that stochastic processes dominate crustal development, and the book ends with a discussion of the likelihood of Earth-like planets and plate tectonics existing else- where in the cosmos. Written by two of the world’s leading authorities on the subject, this book presents an up-to-date survey of the numerous scientific problems surrounding crustal development. It is a key reference for researchers and students in geology, geochemistry, planetary science, astrobiology, and astronomy. Stuart Ross Taylor was born in New Zealand and is now an Emeritus Professor at the Australian National University. He is a trace element geochemist and carried out the initial analysis of the first lunar sample returned to Earth at NASA, Houston in 1969. He has a D. Sc. from the University of Oxford, is a Foreign Member of the US National Academy of Sciences, and has received the Goldschmidt Medal of the Geochemical Society, the Leonard Medal of the Meteoritical Society, and the Bucher Medal of the American Geophysical Union. He is the author of 6 other books including Solar System Evolution, Second edition (Cambridge University Press, 2001). Asteroid 5670 is named Rosstaylor in his honour. Scott M. McLennan is Professor of Geochemistry at the State University of New York at Stony Brook. He conducts research into the geochemistry of sedimentary rocks, and has published 140 papers in the fields of geochemistry, planetary science and sedimentology. Since 1998, he has applied laboratory experiments and data returned from missions to Mars to understand the sedimentary processes of that planet, and is on the science teams of the 2003 Mars Exploration Rover and 2001 Mars Odyssey missions. He received a Presidential Young Investigator Award from the National Science Foundation in 1989 and a NASA Group Achievement Award as part of the Mars Exploration Rover Science Operations Team in 2004. Professors Taylor and McLennan are also the authors of The Continental Crust: Its Composition and Evolution (1985). PLANETARY CRUSTS: THEIR COMPOSITION, ORIGIN AND EVOLUTION STUART ROSS TAYLOR Department of Earth and Marine Sciences Australian National University Canberra, Australia AND SCOTT M. MCLENNAN Department of Geosciences State University of New York at Stony Brook, Stony Brook, NY, USA CAMBRIDGE UNIVERSITY PRESS Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo Cambridge University Press The Edinburgh Building, Cambridge CB2 8RU, UK Published in the United States of America by Cambridge University Press, New York www.cambridge.org Information on this title: www.cambridge.org/9780521841863 © S. R. Taylor, S. M. McLennan 2009 This publication is in copyright. Subject to statutory exception and to the provision of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published in print format 2008 ISBN-13 978-0-511-46382-2 eBook (EBL) ISBN-13 978-0-521-84186-3 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. Ross dedicates this book to Angelo in the hope that he will find the planets just as interesting as has his grandfather. Scott dedicates the book to his wife Fiona and daughter Kate. Contents Preface page xv Acknowledgments xx List of abbreviations xxii Prologue 1 Notes and references 4 1 The planets: their formation and differentiation 5 1.1 Planetary formation 5 1.2 The solar nebula and the giant planets 8 1.2.1 The depletion of the volatile elements in the inner nebula 11 1.3 Planetesimals and the accretion of the terrestrial planets 14 1.4 The random nature of terrestrial planet formation 18 1.4.1 Meteorites and planetary composition 20 1.4.2 Uncompressed density and bulk planetary compositions 21 1.5 Types of crusts 22 1.6 Geochemical processes during crust formation 23 1.6.1 Europium as a universal tracer 25 Synopsis 26 Notes and references 27 2 A primary crust: the highland crust of the Moon 32 2.1 The composition of the Moon 32 2.2 The lunar surface 36 2.3 Structure of the crust 37 2.3.1 Tectonics 38 2.3.2 Lunar stratigraphy 38 2.4 Craters and multiring basins 40 2.4.1 A lunar cataclysm? 42 2.5 Composition of the lunar highland crust 44 vii viii Contents 2.5.1 Anorthosites 45 2.5.2 KREEP 48 2.5.3 The Mg-suite 50 2.5.4 Lunar highland terranes 51 2.5.5 The Cayley Plains: a cautionary tale 53 2.5.6 Tektites and the Moon 54 Synopsis 54 Notes and references 55 3 A secondary crust: the lunar maria 61 3.1 The maria 61 3.1.1 Mare basalt ages 63 3.2 Composition of the mare basalts 64 3.2.1 The interior of the Moon 68 3.3 Origin of the mare basalts 71 3.3.1 An impact origin? 72 3.4 The magma ocean 73 3.4.1 The depletion in europium 75 3.4.2 Depth of melting 76 3.5 Large-impact model for lunar origin 77 Synopsis 80 Notes and references 81 4 Mercury 86 4.1 The planet 86 4.1.1 The composition and internal structure of Mercury 87 4.2 Origin of Mercury 88 4.3 Surface structure 89 4.3.1 The heavily cratered terrain 90 4.3.2 The intercrater plains 90 4.3.3 The Caloris Basin: a mercurian cataclysm? 91 4.3.4 The smooth plains 92 4.4 The origin of the plains: a Cayley Plains analog? 93 4.4.1 Lobate scarps 94 4.5 The crust of Mercury 95 4.5.1 Primary and secondary crusts on Mercury? 97 4.5.2 Atmosphere 98 Synopsis 98 Notes and references 99 5 Mars: early differentiation and planetary composition 103 5.1 The origin of Mars 103 5.1.1 A volatile-rich and oxidized planet 104 Contents ix 5.2 The interior of Mars 104 5.2.1 Core 105 5.2.2 Mantle 106 5.2.3 Crust 108 5.3 Martian stratigraphy 108 5.4 Cratering record and the age of the martian surface 110 5.4.1 Crustal dichotomy 111 5.4.2 Quasi-circular depressions 111 5.4.3 Tharsis and Valles Marineris 112 5.5 Early plate tectonics? 114 5.5.1 Crustal magnetization and plate tectonics 114 5.6 Samples from Mars 115 5.6.1 Martian meteorites 115 5.6.2 Shergottite crystallization ages 121 5.7 Early differentiation on Mars and magma oceans 122 5.8 Multiple reservoirs and the age of the earliest crust 125 5.9 The composition of Mars 126 5.9.1 A cautionary note 130 Synopsis 131 Notes and references 132 6 Mars: crustal composition and evolution 141 6.1 Sampling martian crust 141 6.2 Crustal dimensions 142 6.2.1 Hypsometry 143 6.3 Igneous diversity in a basaltic crust 144 6.3.1 SNC meteorites and crustal contamination 146 6.3.2 Hemispheric dichotomy, Surface Types 1 and 2 and martian andesites 148 6.3.3 Gusev plains and Meridiani Planum 150 6.3.4 Alkaline volcanism and the Columbia Hills 150 6.4 The sedimentary rock cycle on Mars 152 6.4.1 Water, wind and ice 152 6.4.2 Surficial processes 155 6.4.3 Soils and dust 157 6.4.4 Sedimentary rocks on Mars 159 6.4.5 Meteoritic components 162 6.5 Bulk composition of the crust 163 6.5.1 Compositional evolution of the martian surface 166 6.6 Heat flow and crustal heat production 167 6.6.1 Compositional variation with depth 167 x Contents 6.7 Crustal evolution on Mars 168 6.7.1 Tertiary crusts on Mars? 170 Synopsis 171 Notes and references 172 7 Venus: a twin planet to Earth? 181 7.1 The enigma of Venus 181 7.2 Surface features of Venus 182 7.2.1 Plains 182 7.2.2 Channels 183 7.2.3 Volcanoes 184 7.2.4 Coronae 185 7.2.5 Tesserae 186 7.2.6 Ishtar Terra and Aphrodite Terra 186 7.3 Impact craters and the age of the surface 186 7.4 Heat production and rates of volcanism 190 7.4.1 A one-plate planet 191 7.5 Crustal composition 192 7.5.1 Pancake domes: rhyolites on Venus? 194 7.5.2 The differentiation of Venus 196 7.6 The geological history of Venus 197 7.6.1 Water on Venus 199 Synopsis 201 Notes and references 202 8 The oceanic crust of the Earth 207 8.1 The sea floor and plate tectonics 207 8.2 Structure of the oceanic crust 208 8.3 Mid-ocean ridges 209 8.3.1 Formation processes at mid-ocean ridges 211 8.4 Mid-ocean ridge basalts (MORB) 211 8.4.1 Interaction with seawater 215 8.5 Oceanic island basalts (OIB) 216 8.6 Composition of the oceanic crust 217 8.7 Mantle structure 218 8.7.1 Mantle plumes 220 8.8 Composition of the Earth 222 8.8.1 Core 223 8.8.2 Mantle 223 Synopsis 226 Notes and references 227 Contents