The Origin and Evolution of the Solar System The Graduate Series in Astronomy Series Editors: M Elvis, Harvard–Smithsonian Center for Astrophysics A Natta, Osservatorio di Arcetri, Florence The Graduate Series in Astronomy includes books on all aspects of theoretical and experimental astronomy and astrophysics. The books are written at a level suitable for senior undergraduate and graduate students, and will also be useful to practising astronomers who wish to refresh their knowledge of a particular field of research. Other books in the series Dust in the Galactic Environment D C B Whittet Observational Astrophysics R E White (ed) Stellar Astrophysics R J Tayler (ed) Dust and Chemistry in Astronomy T J Millar and D A Williams (ed) The Physics of the Interstellar Medium J E Dyson and D A Williams Forthcoming titles The Isotropic Universe, 2nd edition D Raine Dust in the Galactic Environment, 2nd edition D C B Whittet www.com The Graduate Series in Astronomy The Origin and Evolution of the Solar System M M Woolfson Department of Physics University of York, UK Institute of Physics Publishing Bristol and Philadelphia www.com c IOP Publishing Ltd 2000 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the publisher.
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ISBN 0 7503 0457 X (hbk) 0 7503 0458 8 (pbk) Library of Congress Cataloging-in-Publication Data are available Series Editors: M Elvis, Harvard–Smithsonian Center for Astrophysics A Natta, Osservatorio di Arcetri, Florence Publisher: Nicki Dennis Commissioning Editor: John Navas Production Editor: Simon Laurenson Production Control: Sarah Plenty Cover Design: Victoria Le Billon Marketing Executive: Colin Fenton Published by Institute of Physics Publishing, wholly owned by The Institute of Physics, London Institute of Physics Publishing, Dirac House, Temple Back, Bristol BS1 6BE, UK US Office: Institute of Physics Publishing, The Public Ledger Building, Suite 1035, 150 South Independence Mall West, Philadelphia, PA 19106, USA Typeset in TEX using the IOP Bookmaker Macros Printed in the UK by Bookcraft, Midsomer Norton, Somerset www.com Contents Introduction xv PART 1 The general background 1 1 The structure of the Solar System 3 1.2 Planetary orbits and solar spin 4 1.1 Two-body motion 4 1.2 Solar system orbits 6 1.4 Angular momentum distribution 10 1.1 The terrestrial planets 10 1.2 The major planets 12 1.4 Satellite systems, rings and planetary spins 14 1.2 The Jovian system 15 1.3 The Saturnian system 18 1.4 Satellites of Uranus and Neptune 20 1.5 Spins and satellites of Mercury, Venus, Mars and Pluto 23 1.6 The Earth–Moon system 24 1.1 Characteristics of the major asteroids 30 1.2 The distribution of asteroid orbits: Kirkwood gaps 32 1.3 The compositions of asteroids 32 1.1 Falls and finds 36 1.4 Iron meteorites 38 www.com viii Contents 1.5 Isotopic anomalies in meteorites 39 1.1 Types of comet orbit 41 1.2 The physical structure of comets 43 1.3 The Kuiper belt 45 2 Observations and theories of star formation 46 2.1 Stars and stellar evolution 46 2.1 Brightness and distance 46 2.2 Luminosity, temperature and spectral class 48 2.3 The motions of stars relative to the Sun 50 2.4 The masses of stars 51 2.5 The Hertzsprung–Russell diagram and main-sequence stars 52 2.6 The spin rates of stars 54 2.7 Evolution of stars away from the main sequence 54 2.2 The formation of dense interstellar clouds 59 2.1 Dense interstellar clouds 59 2.2 Heating and cooling in the ISM 59 2.3 The pressure-density relationship for thermal equilibrium 62 2.4 Jeans’ stability criterion 63 2.5 Mechanisms for forming cool dense clouds 65 2.3 The evolution of proto-stars 72 2.1 The Hayashi model 72 2.4 Observations of star formation 75 2.2 Radio-wave observations 75 2.5 Observation of young stars 77 2.1 Identifying young stellar clusters 77 2.2 Age–mass relationships in young clusters 78 2.6 Theories of star formation 79 2.1 Stars and stellar clusters 79 2.2 A general theory of star formation in a galactic cluster 80 2.7 Planets around other stars 95 2.8 Circumstellar discs 98 3 What should a theory explain? 100 3.1 The nature of scientific theories 100 3.1 What is a good theory? 100 3.2 The acceptance of new theories 101 3.3 Particular problems associated with the Solar System 102 3.2 Required features of theories 103 3.1 First-order features 103 3.2 Second-order features 104 3.3 Third-order features 106 www.com Contents ix PART 2 Setting the theoretical scene 109 4 Theories up to 1960 111 4.1 The historical background 111 4.1 Contributions of the ancient world 111 4.2 From Copernicus to Newton 113 4.2 Buffon’s comet theory 117 4.3 The Laplace nebula theory 118 4.1 Some preliminary ideas 118 4.2 The nebula model of Solar System formation 119 4.3 Objections and difficulties 120 4.4 The Roche model 121 4.1 Roche’s modification of Laplace’s theory 121 4.2 Objections to Roche’s theory 122 4.5 The Chamberlin and Moulton planetesimal theory 124 4.1 The planetesimal idea 124 4.2 The Chamberlin–Moulton dualistic theory 125 4.3 Objections to the Chamberlin–Moulton theory 126 4.6 The Jeans tidal theory 127 4.1 A description of the tidal theory 127 4.2 The tidal disruption of a star 129 4.3 The break-up of a filament and the formation of proto- planets 130 4.4 Objections to Jeans’ theory 131 4.7 The Schmidt–Lyttleton accretion theory 133 4.1 The Schmidt hypothesis 133 4.2 Lyttleton’s modification of the accretion theory 134 4.3 The problems of the accretion theory 135 4.8 The von Weizsäcker vortex theory 136 4.1 The basic model 136 4.2 Objections to the von Weizsäcker model 137 4.9 The major problems revealed 137 4.1 The problem of angular momentum distribution 137 4.3 Implications from the early theories 139 www.com x Contents PART 3 Current theories 141 5 A brief survey of modern theories 143 5.1 The method of surveying theories 143 5.2 The Proto-planet Theory 144 5.3 The Capture Theory 146 5.4 The Solar Nebula Theory 149 5.5 The Modern Laplacian Theory 151 5.6 Analysing the modern theories 155 6 The Sun, planets and satellites 156 6.1 Surveying extant theories 156 6.2 Formation of the Sun: dualistic theories 156 6.1 The magnetic braking of solar spin 158 6.2 The solar spin axis 162 6.3 Formation of the Sun: monistic theories 163 6.1 Removing angular momentum from a collapsing nebula 163 6.4 Formation of planets 169 6.1 Planets from the Proto-planet Theory 169 6.2 Planets from the Capture Theory 171 6.3 Planets from the Solar Nebula Theory 184 6.4 Planets from the Modern Laplacian Theory 192 6.5 Formation of satellites 195 6.1 Satellites from the Proto-planet Theory 196 6.2 Satellites from the Modern Laplacian Theory 198 6.3 Satellites from the Capture Theory 198 6.6 Successes and remaining problems of modern theories 204 6.1 The Solar Nebula Theory 204 6.2 The Accretion Theory 205 6.3 The Modern Laplacian Theory 205 6.4 The Capture Theory 206 6.5 The Proto-planet Theory 207 7 Planetary orbits and angular momentum 209 7.1 The evolution of planetary orbits 209 7.1 Round-off due to tidal effects 209 7.2 Round-off in a resisting medium 210 7.4 Commensurability of the Jovian satellite system 215 7.5 Commensurability of planetary orbits 216 7.2 Initial planetary orbits 221 7.1 The Accretion and Solar Nebula Theories 222 7.2 The Proto-planet Theory 223 7.3 The Capture Theory 223 www.com Contents xi 7.1 Angular momentum and the Proto-planet Theory 225 7.2 Angular momentum and the Modern Laplacian and Solar Nebula Theories 227 7.3 Angular momentum and the Capture Theory 228 7.4 Angular momentum and the Accretion Theory 229 7.4 The spin axes of the Sun and the planets 229 7.1 Spin axes and the Solar Nebula Theory 230 7.2 Spin axes and the Modern Laplacian Theory 232 7.3 Spin axes and the Accretion Theory 232 7.4 Spin axes and the Proto-planet Theory 233 7.5 Spin axes and the Capture Theory 234 8 A planetary collision 237 8.1 Interactions between proto-planets 237 8.1 Probabilities of interactions leading to escape 237 8.2 Probabilities of interactions leading to a collision 242 8.3 Numerical calculation of characteristic times 243 8.2 The Earth and Venus 244 8.1 A planetary collision; general considerations 245 8.2 A collision between planets A and B 246 9 The Moon 251 9.1 The origin of the Earth–Moon system 251 9.1 The fission hypothesis 251 9.2 Co-accretion of the Earth and the Moon 254 9.3 Capture of the Moon from a heliocentric orbit 255 9.4 The single impact theory 256 9.5 The Earth–Moon system from a planetary collision 261 9.2 The chemistry of the Earth and the Moon and formation of the Moon 263 9.1 Possible models of Moon formation 265 9.3 The physical structure of the Moon 267 9.1 Hemispherical asymmetry by bombardment 269 9.2 A collision history of the Moon 271 9.4 Mascons and basalts in mare basins 274 9.5 Volcanism and the evolution of the Moon 276 9.6 Calculations of thermal evolution 278 9.2 The induction model of lunar magnetism 285 9.com xii Contents 10 Smaller planets and irregular satellites 294 10.1 Mars according to accretion theories 296 10.2 Mars according to the planet-collision hypothesis 296 10.3 The Martian crust 298 10.4 The COM–COF offset 300 10.5 Polar wander on Mars 302 10.3 A general description of Mercury 303 10.1 Mercury and accretion theories 305 10.2 Mercury and the Capture Theory 306 10.4 Neptune, Pluto and Triton 307 10.1 Encounter scenarios for the Neptune–Triton–Pluto system 308 10.2 Comments on the Neptune–Triton–Pluto system 311 10.6 Summary 314 11 Asteroids, meteorites and comets 316 11.5 Information from meteorites 325 11.6 Isotopic anomalies in meteorites 326 11.1 Oxygen isotopic anomalies 327 11.2 Magnesium in meteorites 328 11.3 Neon in meteorites 330 11.4 Anomalies in silicon carbide grains 331 11.5 The deuterium anomaly 332 11.7 Explanations of isotopic anomalies in meteorites 332 11.1 A planetary collision origin for isotopic anomalies 334 11.8 Comets—a general survey 354 11.1 New comets and the Oort cloud 357 11.9 The inner-cloud scenario 364 11.10 Kuiper-belt objects 366 11.11 Comets from the planetary collision 367 11.12 Ideas about the origin and features of small bodies 368 www.com Contents xiii PART 4 The current state of theories 371 12 Comparisons of the main theories 373 12.1 The basis of making comparisons 373 12.2 The Proto-planet Theory reviewed 374 12.3 The Modern Laplacian Theory reviewed 376 12.4 The Solar Nebula Theory reviewed 377 12.5 The Capture Theory reviewed 379 12.6 General conclusion 383 APPENDICES I The Chandrasekhar limit, neutron stars and black holes 386 II The Virial Theorem 391 III Smoothed particle hydrodynamics 393 IV The Bondi and Hoyle accretion mechanism 398 V The Poynting–Robertson effect 401 References 402 Index 408 www.com Introduction Since the time of Newton the basic structure of the solar system and the laws that govern the motions of the bodies within it have been well understood. One central body, the Sun, containing most of the mass of the system has a family of attendant planets in more-or-less circular orbits about it.
In their turn some of the planets have accompanying satellites, including the Earth with its single satel- lite, the Moon. With improvements in telescope technology, and more recently through space research, knowledge of the solar system has grown apace. Since the time of Newton three planets have been discovered and also many additional satellites. A myriad of smaller bodies, asteroids and comets, has been discovered and a vast reservoir of comets, the Oort cloud, stretching out half way towards the nearest star has been inferred.
Spacecraft reaching out into the solar system have revealed in great detail the structures of all the types of bodies it contains— the gas giants, terrestrial planets, comets, asteroids and satellites, both with and without atmospheres.