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Stuart Ross Taylor
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Solar System Evolution
A New Perspective
This completely rewritten new edition begins with an historical perspective
of the place of the solar system in the universe. Evidence from meteorites is
used to describe how the planets were formed and the giant planets are considered in the light of the asteroids and why Pluto is not a planet.
Explanations on why Earth and Venus turned out so differently, and how
Mars and Mercury are the survivors of many similar bodies, are also discussed. The formation of the Moon in a giant impact leads to an assessment
of the importance of collisions and impacts in the solar system. It is concluded that our solar system is the end-product of many accidental and
chance events. This leads to the philosophical discussion of whether planets
like our Earth are likely to be found elsewhere in the universe.
ROSS TAYLOR took degrees in chemistry and geology. He is Emeritus Professor
and Visiting Fellow at the Australian National University, Canberra,
Australia. He has received the premier awards of the Geochemical Society
(Goldschmidt Medal) and of the Meteoritical Society (Leonard Medal), as well
as the Bowen Award of the American Geophysical Union and the Gilbert
Award of the Geological Society of America. Asteroid 5670 has been named
Rosstaylor in his honor. He is a foreign Associate of the National Academy of
Sciences of the United States of America and a Fellow of the Australian
Academy of Science.
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Solar System Evolution
A New Perspective
SECOND EDITION
An inquiry into the chemical composition, origin
and evolution of the solar system
Stuart Ross Taylor
The Australian National University,
Canberra, Australia
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Stuart Ross Taylor
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CAMBRIDGE UNIVERSITY PRESS
Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo
Cambridge University Press
The Edinburgh Building, Cambridge CB2 2RU, UK
Published in the United States of America by Cambridge University Press, New York
www.cambridge.org
Information on this title: www.cambridge.org/9780521641302
© Cambridge University Press 1992
© Stuart Ross Taylor 2001
This book 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 1992
Reprinted 1994
Second edition 2001
This digitally printed first paperback version 2005
A catalogue record for this publication is available from the British Library
Library of Congress Cataloguing in Publication data
Taylor, Stuart Ross
Solar system evolution : a new perspective : an inquiry into the chemical
composition, origin, and evolution of the solar system / Stuart Ross Taylor. –
2nd ed.
p. cm.
Includes bibliographical references and indexes.
ISBN 0 521 64130 6
1. Solar system. 2. Solar system–Origin. 3. Cosmochemistry. I. Title.
QB501.T25 2001
523.2–dc21
00-068903
ISBN-13 978-0-521-64130-2 hardback
ISBN-10 0-521-64130-6 hardback
ISBN-13 978-0-521-67566-6 paperback
ISBN-10 0-521-67566-9 paperback
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Contents
Preface to the First Edition
Preface to the Second Edition
Acknowledgments
Prologue
Chapter 1 A brief history
1.1 The pre-Copernican view
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1.3 Laplace and his followers
1.4 Tidal theories
1.5 Solar accretion theories
1.6 Nebular theories
1.7 Notes and references
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Chapter 2 The universe
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2.1 The scale and structure of the universe
2.3 Notes and references
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Chapter 3 Stars
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3.1 Star formation
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1.1.1 The Greeks
1.1.2 Ptolemy
1.2 The Copernican Revolution
1.2.1 Tycho Brahe and Kepler
1.2.2 Clockwork systems
1.2.3 Galileo
1.2.4 Descartes and Newton
1.2.5 Kant
2.1.1 Titius–Bode Rule
2.1.2 Galaxies
2.1.3 Is the universe uniform?
2.1.4 Density
2.1.5 The age of the universe
2.2 Element synthesis
2.2.1 Deuterium
2.2.2 Helium
2.2.3 Lithium, beryllium and boron
2.2.4 Synthesis of the heavy elements
3.1.1 Pre-main sequence evolution
3.1.2 Binary and multiple stars
3.1.3 Red dwarfs
3.1.4 Brown dwarfs
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3.2 Early violent stellar activity
3.4 Notes and references
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Chapter 4 The solar nebula
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4.1 The initial concept
4.2 Molecular clouds and interstellar dust
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3.2.1 T Tauri stars
3.2.2. FU Orionis stars
3.2.3 Bipolar outflows
3.3 The formation of the Sun
3.3.1 Solar composition
3.3.2 The solar corona
3.3.3 The fate of the Sun
4.2.1 Cores
4.2.2 Interstellar dust
4.3 Presolar material
4.3.1 Diamond
4.3.2 Silicon carbide
4.4 Separation of nebulae
4.5 Dust disks around stars
4.5.1 Disks around pre-main sequence stars
4.5.2 HL Tauri disk
4.5.3 Disks around mature stars
4.6 Nebular collapse, nebular lifetime, and angular momentum
transfer
4.6.1 Nebular lifetimes
4.6.2 The angular momentum problem
4.6.3 Nebular mass
4.7 Nebular structure and temperature
4.8 Notes and references
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Chapter 5 Composition and chemical evolution of the solar
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nebula
5.1 CI chondrites and the primitive nebula
5.1.1 Comets as pristine samples of the nebula
5.2 Volatile elements
5.2.1 The rare gases
5.2.2 Oxygen isotopes
5.2.3 Chemical reactions in the nebula
5.3 Homogeneity or heterogeneity?
5.3.1 Heliocentric zoning
5.4 Gas loss from the inner nebula
5.5 Volatile-element depletion in the early inner nebula
5.5.1 Condensation or evaporation?
5.5.2 Water
5.5.3 Deuterium/hydrogen ratios
5.6 Notes and references
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Chapter 6 The evidence from meteorites
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6.1 The most-ancient samples
6.2 Refractory or calcium–aluminum inclusions (CAIs)
6.9 Notes and references
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Chapter 7 Building planets
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7.1 The collapse of clockwork solar systems
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6.2.1 Composition
6.2.2 Fremdlinge
6.2.3 Rims
6.2.4 FUN inclusions
6.3 The matrix of chondrites
6.3.1 Composition
6.3.2 Source of matrix
6.4 Chondrules
6.4.1 Textures and cooling history
6.4.2 Composition
6.4.3 Complementary compositions
6.4.4 Depletion mechanisms
6.4.5 Oxygen isotopes
6.4.6 Ages of chondrules
6.4.7 Chondrule precursor material
6.5 Models for the origin of chondrules and CAIs
6.5.1 Nebular origin of chondrules
6.6 Chondrites and other primitive meteorites
6.6.1 Composition
6.6.2 Hydration
6.6.3 Enstatite chondrites
6.6.4 Accretion temperatures for chondrites
6.7 Chronology
6.7.1 Extinct radionuclides
6.7.2 Sources of short-lived nuclides
6.7.3 The beginning of the solar system, or T0
6.7.4 Chondrite “ages”
6.7.5 Rb–Sr and Mn–Cr isotopic systems
6.8 Fractionated meteorites and parent bodies
6.8.1 Howardites, eucrites, and diogenites (HEDs)
6.8.2 Ureilites
6.8.3 Angrites and Angra dos Reis (ADOR)
6.8.4 Aubrites
6.8.5 Pallasites and mesosiderites
6.8.6 Iron meteorites
7.1.1 Planetary eccentricity
7.2 Differences between the inner and outer planets
7.2.1 Forming giant planets
7.2.2 Giant gaseous protoplanets
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CONTENTS
7.3 Formation of the giant planets by the planetesimal hypothesis 171
7.3.1 Significance of the asteroid and Edgeworth–Kuiper belts
171
7.3.2 Early formation of Jupiter
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7.3.3 “Snow lines”
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7.3.4 Saturn, Uranus and Neptune
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7.3.5 Limits to growth
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7.3.6 Extrasolar planets
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7.4 The inner solar system
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7.4.1 Accretion from dust or from a hierarchy of objects
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7.4.2 Evidence from cratering
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7.4.3 Planetary obliquities and spins
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7.5 Planetesimals
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7.5.1 Fractionation in precursor planetesimals
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7.6 Accretion of planetesimals
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7.6.1 Accretion of the inner planets
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7.6.2 Meteorites as planetary building blocks
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7.7 The long-term stability of the solar system
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7.8 Notes and references
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Chapter 8 The giant planets
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8.1 Four giant cores
8.6 Notes and references
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Chapter 9 Satellites and rings
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9.1 Miniature solar systems?
9.2 Satellite classification
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8.1.1 Composition
8.1.2 Heat flows
8.1.3 Magnetic fields
8.2 Atmospheres
8.2.1 Deuterium/hydrogen ratios
8.3 Origin of our giant planets
8.3.1 On the usefulness of Jupiter
8.4 Planet X
8.5 Extrasolar giant planets
8.5.1 Detection of extraterrestrial planets
8.5.2 Orbital distribution
8.5.3 Mass distribution
8.5.4 Composition
8.5.5 The pulsar planets
8.5.6 Formation of extrasolar planets
9.2.1 Regular satellites
9.2.2 Captured satellites
9.3 Galilean satellites
9.3.1 Io
9.3.2 Europa
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9.9 Notes and references
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Chapter 10 The refugees
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10.1 Pluto and Charon
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9.3.3 Ganymede
9.3.4 Callisto
9.4 Saturnian satellites
9.4.1 Titan
9.5 Uranian satellites
9.6 Neptunian satellites
9.6.1 Triton
9.7 Planetary subnebulae
9.7.1 The cosmic junkyard
9.8 Planetary rings
9.8.1 The jovian ring system
9.8.2 Saturn’s rings
9.8.3 The uranian rings
9.8.4 The neptunian ring system
9.8.5 Ring origin
10.1.1 Size, density and internal structure
10.1.2 Atmospheres and surfaces of Pluto and Charon
10.1.3 Origin of Pluto
10.1.4 Origin of the Pluto–Charon pair
10.1.5 Pluto and the solar-nebular composition
10.2 The Centaurs
10.3 Comets
10.3.1 The Edgeworth–Kuiper Belt
10.3.2 The Oort Clouds
10.3.3 Comet Halley
10.3.4 Formation sites for comets
10.3.5 Deuterium/hydrogen ratios
10.3.6 Cometary ices
10.3.7 Comets from outside the solar system?
10.4 The asteroids
10.4.1 The asteroid belt
10.4.2 Kirkwood Gaps
10.4.3 Zonation in the asteroid belt?
10.4.4 Asteroid families
10.4.5 The Apollos, Atens, and Amors (near-Earth asteroids)
10.4.6 The spectrophotometric paradox
10.4.7 The “missing olivine” problem
10.4.8 Individual asteroids
10.4.9 Other asteroid belts?
10.4.10 Origin of the asteroid belt
10.5 Asteroidal and cometary dust
10.5.1 CP and CS particles
10.5.2 The zodiacal cloud
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10.6 Notes and references
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Chapter 11 The survivors: Mercury and Mars
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11.1 Mercury
11.3 Notes and references
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Chapter 12
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10.5.3 Zodiacal dust and the search for extrasolar planets
10.5.4 The terrestrial record
10.5.5 Dust as a solar-system sample
11.1.1 Internal structure
11.1.2 Surface composition
11.1.3 Evidence for planetary contraction
11.1.4 Bulk composition
11.1.5 Origin of Mercury
11.2 Mars
11.2.1 Shape and surface topography
11.2.2 Tharsis
11.2.3 Obliquity
11.2.4 Martian interior
11.2.5 Martian (SNC) meteorites
11.2.6 Bulk composition
11.2.7 Accretion and evolution
11.2.8 Plate tectonics?
11.2.9 Crust and dust
11.2.10 Atmosphere and water
11.2.11 Life on Mars?
11.2.12 Phobos and Deimos
The twins: Venus and the Earth
12.1 Venus
12.1.1 Crustal composition
12.1.2 Impact craters and the age of the surface
12.1.3 Heat loss and rates of volcanism
12.1.4 A one-plate planet
12.1.5 The geological history of Venus
12.1.6 Bulk composition
12.1.7 Primitive oceans on Venus?
12.1.8 A close relative of the Earth?
12.2 The Earth
12.2.1 Formation and early history
12.2.2 An early crust?
12.2.3 The oceanic crust
12.2.4 The continental crust
12.2.5 Comparison with other planetary crusts
12.2.6 The terrestrial mantle
12.2.7 Mantle composition
12.2.8 The siderophile-element paradox
12.2.9 The terrestrial core
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12.3 Notes and references
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Chapter 13 The Moon
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13.1 Some physical properties
13.2 The lunar crust
13.7 Notes and references
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Chapter 14 The role of impacts
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14.1 A reluctant conversion
14.2 Mechanics of cratering
14.7 Notes and references
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Chapter 15 Epilogue: on the difficulty of making Earth-like
planets
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12.2.10 The source of terrestrial water
12.2.11 The atmosphere
13.2.1 The highlands
13.2.2 The maria
13.2.3 Lunar cratering
13.3 Lunar mantle and core
13.4 Lunar bulk composition
13.4.1 Volatile elements
13.4.2 Vanadium, chromium and manganese
13.4.3 Depletion of siderophile elements
13.4.4 Refractory lithophile elements
13.5 Evolution of the Moon
13.5.1 Ancient lunar magnetic field
13.5.2 Atmosphere
13.6 Hypotheses of lunar origin
13.6.1 Pre-Apollo ideas
13.6.2 Large-impact model for the origin of the Moon
13.6.3 Eucrites and the Moon
14.2.1 Types of craters
14.2.2 Large basins
14.3 Surface histories of the planets and satellites
14.3.1 The early intense bombardment
14.3.2 Basin ages
14.4 Lunar cataclysms?
14.5 The impactor population in the early solar system
14.6 Cratering flux since the heavy bombardment
14.6.1 Periodic extinctions
14.6.2 The Torino scale
14.6.3 Venus
14.6.4 Mars
15.1 The plurality of worlds?
15.1.1 Nebular constraints
15.1.2 Consequences of the early formation of Jupiter
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15.2 The random nature of inner planet formation
15.3 Is the continental crust of the Earth unique?
15.4 Synopsis
15.5 Notes and references
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Name index
Subject index
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15.2.1 Effects of impacts on biological evolution
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Preface to the First Edition
The true dimensions of the terrestrial globe were revealed mostly in the
fifteenth and sixteenth centuries, principally through the technical
development of truly ocean-going vessels and the magnetic compass,
which enabled the exploration of the oceans. Even with these advantages, the real extent of the Great Southern Continent or terra australis
incognita had to await the voyages of Cook in the eighteenth century.
This flood of new geographic knowledge replaced the medieval view of
the world (although Flat-Earth societies still persist). Such understanding was gained only very slowly. A detailed knowledge of the topography
of the ocean floors and our understanding of their composition and
origin has been obtained only a little ahead of our radar pictures of the
surface of Venus.
Our exploration of the solar system is at a similarly heroic stage. The
distant points of light, barely resolvable in telescopes, have been
revealed through the use of space vehicles, the latter-day equivalent of
the Portuguese caravels, as separate worlds, with an astonishing
amount of diversity.
The information has been rapidly and widely disseminated, electronic media having superseded the printing press of the Renaissance.
Everyone is informed of the striking new discoveries. Although no
Eldorados have emerged, the pictures reveal a plurality of worlds
unimagined by the Elizabethans. Every satellite has turned out to differ
in some significant feature from its neighbor: “. . . the sense of novelty
would probably not have been greater if we had explored a different
solar system” [1].
This comment on the jovian system reveals a fundamental truth.
Lurking behind the photographs and radar images is a new observation,
uncomfortable for Homo sapiens. What combination of circumstances
could reproduce in some other planetary system the detail observed in
our own solar system or lead to the assembly and evolution of a clone of
the Earth? Whether one contemplates the battered face of Mercury, the
crumpled crust of Venus, our own water-sheathed planet, the vast
deserts and gigantic landscapes of Mars, or the profusion of distinct icy
satellites of the giant planets, the uniqueness of the individual planets
and satellites informs us of a sober fact: The system is unlikely to be
duplicated in detail elsewhere.
Other planetary systems doubtless exist. That they would resemble
ours in any but the broadest detail is only a remote possibility. Too many
chance events have occured to bring our system to its present condition.
Jacques Monod [2] remarked on the uniqueness of the evolutionary path
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PREFACE TO THE FIRST EDITION
for life and of the apparently unsurmountable difficulties of duplicating our version of intelligent life elsewhere. The planets and satellites
are rich in diversity and the difficulty of producing clones of our present
solar system makes duplication as unlikely as the possibility of finding
an elephant on Mars.
Despite the cornucopia of new information about the solar system,
it is curious how little effect this has had on the development of theories for its origin. The Apollo and Luna data from the Moon had almost
no impact on theories of the origin of the solar system, or indeed of the
Moon, until comparatively recently. The reason for this is partly the
unusual circumstances of lunar origin and evolution and the very large
amount of new data from separate fields that had to be assimilated.
The complexity of the solar system is not in accord with theories that
start from some simple initial condition. Such hypotheses do not
predict the diversity of the present system. Thus, while it is tempting to
look for grand unified theories to explain our solar system, the basic
approach is wrong and the trail is false. Too many chance events occur.
Among the many singular events that determined the outcome, a few
examples may be cited: the size of the fragment of the molecular cloud
that became the solar nebula; the formation of the Sun as a single
rather than a double or multiple star; the huge size of Jupiter and its
controlling influence on the asteroid belt and Mars; the random accretion events that made the Earth larger than Venus; the unique collisions
that produced the bone-dry Moon and the high density of Mercury (a red
herring for grand theories); the differences among the satellite systems;
and the hundred other curious features of the solar system such as
Hyperion tumbling chaotically until the end of time. All are the result
of events that might readily have taken a different turn.
Scientists and philosophers have been attempting to find an explanation for the origin of the solar system since the question was first
raised by the Greeks, but a universally acceptable scenario has yet to
appear. A major reason for this failure to solve one of the oldest scientific
questions has lain in the approach adopted. Another planetary system
will differ in numbers and sizes of planets and satellites. Thus, the quest
for a universally applicable solution to the origin of the system is not
fruitful. Attention is better directed to explaining how the various
aspects of the system came about.
In this book I have attempted to provide this and to describe our
current understanding of the origin and evolution of the solar system.
In this respect, I have followed the path of most previous investigators
in trying to account for the existence of the planets, satellites, asteroids,
and comets. The formation and evolution of the Sun as a quite normal
G-type star is well enough understood, so in this account it is mainly an
offstage companion to the discussion. The book is biased toward a geochemical point of view because of my field of interest. Cosmochemistry
and geochemistry are not, however, without their difficulties, and
those readers who wish to find a simple statement of the composition of
the Earth will be disappointed to learn that many problems yet remain.
I have, nevertheless, been unable to resist treading among other areas,
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xv
conscious that such a path is fraught with as many perils as those
recorded by John Bunyan (1628–1688) in The Pilgrim’s Progress.
The subject is rendered very difficult by the wide variety of contrary
opinions held among investigators and the many alternative explanations offered for the same set of data. Sometimes the data are suspect. It
is thus surprising to find that the students of cratering frequently disagree on the numbers of craters involved. One might have hoped that at
least the database was agreed on. However, it is a common error to
underrate the difficulties of subjects outside one’s experience. The
diversity of opinion is only exceeded by the strength with which the
opposing views are held and defended, supporting the thesis that subjectivity is more common in science than hoped for [3]. This has not
made my task easier in trying to reach some conclusions about the
status of our understanding of the solar system.
Readers may complain that I have chosen to cover too broad a canvas
and that the scope of the book should be restricted to areas of immediate competence. Thus, they may question why topics as disparate as
Chiron, comets, crustal development, calcium-aluminum inclusions,
chondrules, and Copernican Revolution, and lunar cataclysms are all
discussed.
The problem is that detail in one area may provide a crucial constraint in another. If one paints with too broad a brush, error may creep
in. Grand theories are useless if they cannot explain the minute details.
Thus, the concept of giant gaseous protoplanets condensing from fragments of the nebula was viable until it was demonstrated that Jupiter
and Saturn were not of the solar composition required by the theory.
The rare-gas isotopic abundances in the terrestrial planets rule out the
popular concept of late-accreting veneers of carbonaceous chondrites.
Consequently, an apparently minor detail from one topic may negate
extensive work from another, just as the presence of the europium
depletion in lunar mare basalts removed the model, developed from
experimental petrology, that these lavas were derived from a primitive
unfractioned lunar interior.
The reverse of this particular coin is that the mere accumulation of
the staggering amount of detailed observations in the solar system is of
little use unless there is some unifying concept; the discovery of the
Periodic Table was needed to make sense out of the bewildering array of
properties of the chemical elements.
Although this book treads into far more nebulous territory than, for
example, Lunar Science: A Post-Apollo View [4], it might be recalled that the
state of our understanding of the Moon at that time was also beset with
many misconceptions and false trails, a particularly good example
being the many ingenious but erroneous explanations advanced to
explain the europium anomaly [5].
It therefore behooves workers in this interesting field to consider all
the details if they wish to avoid error, unless they want to make all the
planets out of CI carbonaceous chondrites, or the Moon from the
Earth’s mantle. In this context, I had hoped, somewhat naïvely when
beginning this task, to conclude with definitive compositions at least
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PREFACE TO THE FIRST EDITION
for the terrestrial planets. What has emerged in reality is that even the
major-element compositions of the inner planets are poorly constrained. A solution will have to wait until the problems of the moments
of inertia of Mars and Mercury, the nature of the lower mantle of the
Earth, and that of the light element in the core, among many others, are
resolved.
However, it is easy to become fascinated with the curious landscapes
of Miranda and Triton, the large crater on Mimas, the enormous layered
deposits in Valles Marineris, the crumpled terrain of Venus, the rare gas
abundances in the atmospheres of the terrestrial planets, the Kirkwood
Gaps in the asteroid belt, the relationship of Pluto and Charon, the composition of Comet Halley, or that of interplanetary dust. While contemplating all these marvels, it is not difficult to become lost among the
trees, forgetting that the principal purpose of this book it to understand
the origin of the system, and to integrate all these wonders into a coherent explanation.
Thus, it is not my intention to provide a detailed travelogue or
Cook’s Tour of the solar system, tempting though that option may be,
since there is already a multitude of good technical books and papers
covering both the detail and overviews of the solar system [6]. I assume
that most readers are familiar with the spectacular images and I have
not attempted to reproduce many of these or provide much mathematical treatment. This is available in the material referenced and it
seems pointless to fill up pages with equations of use only to a few specialists.
Rather, I have attempted a commentary on the problems of its origin
and evolution. I had orginally planned to start the book with a description of the present solar system, and proceed from there into more nebulous regions, moving backwards in time into the unknown. I was
wisely persuaded to begin instead with the solar nebula, and to bring
the story forward in chronological order, a decision which has proven
robust. However, as one of the editors of Meteorites and the Early Solar
System remarked, “. . . good reasons could be found for placing every
chapter before every other chapter” [7].
In general, I have attempted to reference the most recent reviews and
texts; the subject is moving so rapidly that older material quickly
becomes of historical interest. In many areas, the subject has come
clearly into focus only in the past two or three years as the avalanche of
data from the past two decades has been assimilated. Accordingly,
much of the older literature is now of historical, rather than scientific,
interest [8]. Literature coverage extends to April 1990, with a few later
additions.
The deluge of new information and the emergence of some agreement on the general scenario of solar system origin and evolution has
turned the subject in a few years from one having the status of a hobby,
“. . . pursued by eccentric, elderly gentlemen who fight with one
another’s theories of the origin of the solar system” [9] to one in which
quite detailed scientific questions can be posed. In this context, I have
been selective in giving more attention to those studies that fall within
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the general theme of the planetesimal hypothesis, rather than to give
an overview of all competing ideas. Here I have attempted to produce a
broad survey pointing to the many questions of interest that can be
addressed and the regions in which our understanding is inadequate.
Even in those areas such as the surface geology of Mars, where we have
much basic knowledge and understanding, many significant questions
(for example, the relative importance of sedimentary vs. igneous
processes, or the early climate and role of ice) remain obscure. Only a
sample return, followed by a manned mission, will supply the answers
to these particular questions.
The wealth of new detail leads to the complaint, common to many
scientific disciplines, that there is now too much information for any
individual to comprehend. In this view, it was simpler in past ages when
the corporate body of knowledge was so much smaller.
However, it is doubtful that this is true; scientists in the Renaissance
had to deal with a staggering burden of topics such as astrology,
alchemy or numerology, on which Newton wasted so much of his time.
In any subject where there are no unifying principles, as in chemistry
before the Periodic Table revealed an essential simplicity or biology
before evolution cast its revealing light, it is impossible for an individual to be an expert in every nook and cranny. Nevertheless, given some
general principles of physics and chemistry as a guide, it is possible to
discern the forest, even through the thickest underbrush, just as the
architectural unity and splendour of a great building may be glimpsed
from a distant prospect.
Among the other intellectual baggage that has been discarded, the
differences between the relative ages of the universe and the solar
system have clearly distinguished their origins; we no longer have to
account for them together. The age of the solar system, obtained from
meteorites as 4560 m.y., is only about one-quarter of the age of the
visible universe. This fact – a relatively recent discovery – separates the
origin of the Earth and solar system from that of the universe. This
knowledge has become so ingrained in thinking that it is surprising to
recall that as recently as 1950, the age of the Earth, established by isotopic dating of rocks, approached and sometimes exceeded the astronomical estimates for the age of the universe.
One is comforted on this journey by the steady convergence of
scientific ideas toward some kind of consensus, as new facts are
acquired. Science is in this way distinct from most other human activities, which display the opposite tendency of divergence with time, a
process most clearly revealed by the multitude of religious and philosophical systems.
Stuart Ross Taylor
Houston, Texas
December 1990
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PREFACE TO THE FIRST EDITION
Notes and references
1. Smith B. A. et al. (1979) Science, 204, 951.
2. Monod J. (1974) Chance and Necessity, Collins, 187 pp.
3. Mitroff I. (1974) The Subjective Side of Science: A Philosophical Enquiry into the
Psychology of the Apollo Moon Scientists, Elsevier, New York, 329 pp.
4. Taylor S. R. (1975) Lunar Science: A Post-Apollo View, Pergamon, New York,
372 pp.
5. Ibid., pp. 154–159.
6. There are so many good texts that it would be invidious to choose among
them. A good beginning is Morrison D. and Owen T. (1987) The Planetary
System, Addison-Wesley, Reading, Massachusetts, 600 pp.
7. Kerridge J. F. (1988) in Meteorites and the Early Solar System (eds. J. F. Kerridge
and M. S. Matthews), Univ. of Arizona, Tucson, p. xv.
8. The fate of most scientific work is to be forgotten. The typical scientific
paper has a half-life of five years, forming a curious contrast to the dominant egos of most workers in the field. The correct is incorporated into the
general body of knowledge; the erroneous is mostly ignored.
9. Wetherill G. W. (1988) in LPI Tech. Rpt. 88–04, p. 81.
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Preface to the Second Edition
My purpose in writing this book is to enquire into the solar system and
how it came to be. So much progress has been made in the past decade
that this book has been completely rewritten from the first edition. The
seven large chapters in that edition have been restructured into fifteen
smaller ones that deal more readily with the increased flood of information.
As in the first edition, I have tried to place the solar system in the
broader context of the universe. My excuse for venturing into fields such
as cosmology is to reinforce the point that the solar system is a relative
newcomer in the universe and came about through a fortunate
sequence of chance events. A secondary purpose is to try to overcome
the narrow and potentially hazardous specialization that is endemic in
science and that I talk about in the Prologue. I have attempted to educate
myself in fields remote from my own through discussions with many
colleagues, listed in the Acknowledgments.
The book does not follow the usual descriptive arrangement of starting with Mercury and marching stolidly out through the giant planets
to Pluto. Instead, the unconventional arrangement I have adopted here
has arisen naturally as I have tried to explain how the system arose from
the solar nebula and why the various bodies happen to be where they
are. The result is that there are many associations that may at first sight
seem surprising.
I begin with a brief historical outline that traces the development of
thinking about the solar system. This is followed by comments about
the place of the solar system in the universe, the formation of stars and
of the solar nebula. The important and unique information provided by
meteorites on the early evolution of the nebula next sets the stage for
considering the formation of the planets themselves. Although long
considered as an inevitable process that would produce the solar
system as a result of some general law, this view of the formation of
“clockwork” planetary systems has been superseded by a recognition
that random events have predominated. This accumulation of planets
from short-lived disks around stars separates the planetary scientist
from the astronomer, who deals with the simpler processes that
produce the stars from the collapse of gas clouds. Early violent solar
activity swept away the gas and volatile elements from the inner
nebula, out to the vicinity of Jupiter. This local increase in density so far
from the Sun enabled that planet to form a large core quickly enough
to capture the fleeing gas by gravitational attraction. Disks around
young stars are short-lived, so that giant Jupiter was the earliest planet
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PREFACE TO THE SECOND EDITION
to form, followed by Saturn and a little later by the ice giants, Uranus
and Neptune, who formed just too late to collect gas. The giant planets
are here lumped together rather than separating the gas giants from
the ice giants as is customary. These massive bodies probably formed
much closer together than where they now find themselves and their
present masses are the result of chance rather than the consequence of
some pre-ordained plan. At this point, the problems of forming the
giant extrasolar planets are considered. A multitude of satellites of the
giants formed close upon their parents.
Left over after these major events that shaped the system were the
icy bodies in the outer parts of the nebula, where the amount of material was too small to produce more planets. Pluto and the comets are the
famous examples, while Uranus and Neptune, migrating outwards,
swallowed up much of the material so that the solar system seems to terminate abruptly at the orbit of Neptune. Meanwhile Jupiter, on the
sunward side, swept up so much material that only a depleted zone of
rubble, now the asteroid belt, was too scattered by the giant to form a
planet. For the next 50 or 100 million years, the rocky rubble left over in
the inner solar system collected itself into larger bodies, of which Mars
and Mercury are survivors. Earth and Venus were more fortunate, the
Earth in particular acquiring the Moon from a chance collision with
something bigger than Mars. Collisions dominated the growth of all the
planets as their variable tilts and rotation rates testify. So the formation
of the Earth, that turned out wildly different from its twin, Venus,
demonstrates the importance of chance events in making planets and
emphasises the difficulties of forming a duplicate of our planet elsewhere, a message reinforced by the startling diversity among the extrasolar planets.
I have attempted to summarise the literature through to the end of
1999 with some outliers into the 21st century. Readers will observe that
there is a bimodal distribution in the dates of the papers, a common
feature in second editions, that first peak toward 1990 and then toward
1999. As in the first edition, I have tried to refer to the more recent works
for two reasons. Firstly, reference to them will in turn enable the reader
to refer back to the previous literature on the topic. A more serious
reason is that the subject is moving so rapidly that published papers on
topics such as the extrasolar planets, the source of water on the Earth or
the origin of Uranus and Neptune in our own system rapidly become
dated as new information appears. Thus in many areas, papers more
than a couple of years old have already entered the realm of past history,
interesting only in reflecting the thinking of the period.
However, the purpose of references in a book such as this is also to
refer to primary sources, so that I have included a number of the citations from the first edition for this reason. I have excluded all references
to conference abstracts and other grey literature except in a very few
unavoidable cases where the information is not available elsewhere. I
have followed the convention adopted in the first edition of listing references by number in order of appearance in each chapter, rather than
breakup the continuity of the text with names and dates. I list first
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xxi
authors only, followed by et al., except for two-author papers, as it is
pointless in a work such as this to provide lists that on occasion extend
to 30 names. With over 1300 references, often to fields remote from my
speciality, it is inevitable that I have missed some relevant sources. My
apologies for any such oversights.
I have continued to include tables of relevant data, but two developments have altered the more pressing need that I perceived for presenting such material in the first edition. One is the appearance of The
Planetary Scientist’s Companion (Oxford University Press, 1998) that contains much relevant information. The second is the vast amount of upto-date data available on the Internet that include, for example, lists of
extrasolar planets, centaurs and transneptunian objects. Both these
developments have made data tabulations less crucial, particularly as a
gap of a year or so exists between compiling data and its appearance in
book form. In many cases, as with the basic petrological data on meteorites, little has changed in the decade since the previous edition. In
others, ranging from updated information on the Galilean satellites to
the appearance of the extrasolar planets, the data are new.
This great increase in the availability of comprehensive data-sets
thus frees the author to concentrate on understanding rather than
merely cataloguing the material. The task of the author, however,
differs from that of the compiler. He has a responsibility to the reader to
present his own, perhaps biased, interpretation rather than merely
to tabulate previous efforts. As one of my colleagues memorably
remarked, I prefer to make my own mistakes than to repeat the errors of
others. Thus there is still a need for critical evaluation of the mass of
available data. This applies particularly to geochemistry where the
problem of sampling planetary bodies remains formidable. With this
excuse, I have given here my own assessment of such debatable issues as
the composition of the terrestrial mantle and crust, the Moon and that
of the primitive solar nebula.
Stuart Ross Taylor
Canberra
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Acknowledgments
xxii
© Cambridge University Press
My wife Noël has provided continuous help and support over many
years, without which I could not have undertaken this work.
The Department of Geology, Australian National University, has provided facilities through a Visiting Fellowship that have enabled this
book to be written. I also thank the Department of Nuclear Physics,
Research School of Physical Sciences, at this University for assistance. I
thank Clementine Krayshek for drafting the figures and tables and the
Research School of Earth Sciences at this University for providing this
service.
I am grateful to the following scientists for discussions, comments,
supplying preprints or reprints and generally assisting me in this
endeavour:
Craig Agnor, Richard Arculus, Pawel Artymowicz, Neta Bahcall, Gibor
Basri, Willy Benz, Rick Binzel, Alan Boss, Adrian Brearley, Roland Buser,
Paul Butler, Natalie Cabrol, Al Cameron, Robin Canup, Pat Cassen, Chris
Chyba, Bob Clayton, Harold Connolly, Dale Cruikshank, Kimberly Cyr, Don
Davis, Thérèse Encrenaz, Tezer Esat, Paolo Farinella, Maria Firneis, Mike
Gaffey, Daniel Gautier, Brett Gladman, Guillermo Gonzalez, Jitendra
Goswami, Ron Greeley, Richard Grieve, John Guest, Alex Halliday, Greg
Henry, Roger Hewins, Munir Humayun, Bill Irvine, Rhian Jones, Amir
Kahn, Bill Kaula, Gero Kurat, Harold Levison, Mike Lipschutz, Jack
Lissauer, Gunter Lugmair, Jonathon Lunine, Renu Malhotra, Geoff Marcy,
Ursula Marvin, Francesco Marzani, Michel Mayor, Lucy-Ann McFadden,
Bill McKinnon, Scott McLennan, Andy McWilliam, Patrick Michel, Thierry
Montmerle, Andrew Nelson, Horton Newsom, Marc Norman, Ulrich Ott,
Herbert Palme, Jim Papike, Jon Patchett, Arch Reid, Kevin Righter, Sara
Russell, Alex Ruzika, Ed Scott, Frank Shu, Chip Shearer, Alex Shukolyukov,
Dave Smith, Ann Sprague, Jeff Taylor, Larry Taylor, Mark Thiemens, Chris
Tinney, Allan Treiman, David Trilling, Virginia Trimble, Stéphane Udry,
Don VandenBerg, Harri Vanhala, Meena Wadhwa, Malcolm Walter, David
Wark, Jerry Wasserburg, George Wetherill, Don Wilhelms, John Wood,
Tetsuo Yamamoto, Ed Young and Maria Zuber.
Robin Brett, US Geological Survey, Tezer Esat, Australian National
University, Chris Koeberl, University of Vienna and Scott McLennan, SUNY,
Stony Brook have continued to be actively helpful. Brian Mason and the late
Louis Ahrens, Gene Shoemaker and Harold Urey all contributed to setting
me on this path. It has been a pleasure to work with Cambridge University
Press during the production of the book. I am grateful to Dr Brian Watts for
meticulous copy-editing that has materially improved this book. I thank Dr
Simon Mitton for his continuing encouragement and support.
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Prologue
A principal task in writing in the first edition of this book was to
examine the series of events that led to the formation of the solar
system. The conclusion, so greatly illuminated by the previous three
decades of planetary exploration by spacecraft, was that random events
had predominated in the construction of the great variety of planets
and satellites. Thus it was unlikely that duplicates might be found elsewhere. This was in contrast to earlier views that the solar system was as
orderly as a clock and that, given sufficient computer power, one might
simulate the construction of such a clockwork system from first principles according to the laws of physics and chemistry.
In the 1992 edition, I commented that “the . . . common occurrence
of disks around young . . . stars strengthens the case for the existence of
other planetary systems. If so, would they resemble our own? Would we
see something like the Galilean satellite system of a few equal-sized
planets, systems with one giant planet, or a single brown dwarf companion?” After contemplating the satellite systems around our giant
planets, I concluded that “no simple sequence of reproducible events
has occurred in our solar system. Other planetary systems . . . will be different in detail to our own. What their satellites might look like is only
for bold spirits to predict” (p. 251).
This concept was reinforced in dramatic fashion in 1995 by the
unequivocal discovery of extrasolar planets. Now that the question of
the existence of other worlds, that has been around for 24 centuries
since Democritus, has been answered, I wish to take a different tack in
this edition. Nature is able to make a variety of planets in surprising
orbits so that the problem posed by Lucretius and most dangerously by
Giordano Bruno has moved on to the matter of their habitability. Is
there life out there and what are the chances both of finding it and communicating with the “little green men” that are so desperately hoped
for by many of the human inhabitants of this planet?
Here I have tried to enquire into this question by studying the detail
in our planetary system and making some comparisons with the new
ones that are being discovered daily. What emerges once again is the
random and stochastic nature of events leading to the formation of
planetary systems and the immense difficulties in constructing a duplicate of the Earth. Added to this problem of forming habitable planets
are the biological problems, both of the origin of life and its evolution.
It thus becomes exceedingly difficult to be optimistic about the possibilities of finding duplicates of the Earth or any extraterrestrial life that
remotely resembles Homo sapiens.
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PROLOGUE
In a work of this scope, it is impossible to be expert in the many
diverse fields that I cannot resist straying into. My excuse for wandering
into regions as remote from my speciality as cosmology, where a mere
student of rocks has little business, is that science has become dangerously fragmented. This is understandable. The difficulties of becoming
expert even in some tiny corner of science are well enough known. We
all tend to get into our own particular foxholes, secure in our understanding of alkenes, atomic particles, ants, andesites or asteroids.
This leaves the way open for salesmen of simplistic solutions that
purport to explain everything in terms that a young child could follow.
Of these, creationism in its various incarnations represents the most
serious threat. The whole endeavor of science and ultimately the very
survival of our civilized society that has arisen from the study of the
natural world in the past four centuries is at risk. One writer has commented that “though science is stronger today than when Galileo knelt before
the Inquisition, it remains a minority habit of mind and its future is very much
in doubt. Blind belief rules the millennial universe, dark . . . as space itself” [1].
One is reminded of what has been called “the failure of nerve” of the
ancient world. Following the scientific advances of ancient Greece and
the major progress at the Museum and Library (Mouseion) at
Alexandria, there followed a retreat into the comfort of myths. Much of
the great library at Alexandria, that had found shelter in the Temple of
Serapis from civil wars in the 3rd century AD, was destroyed by a
Christian mob in 391 AD. The remaining books were burnt in 646 AD by
the Arab conquerors of the city [2]. Book burning seems to be a popular
and widespread habit, judging from more recent examples that have
included both Nazi Germany and the Red Guards of China.
There followed a retreat from objective truth into medieval modes
of thought as the Dark Ages began. The stage was set by Saint Augustine
(354–430 AD) who ordained that as God had created everything, “it is
not necessary to probe into the nature of things” as the Greeks had
done. This attitude still survives.
Will faith once again smash science, as happened in the ancient
world? This seductive pathway is not without its difficulties. As Harvey
Brooks (1971) has remarked “if the modern era has created social and cultural
conditions in which the enterprise of science is no longer viable, it has sown the
seeds of its own disintegration and decay, to be followed by the disappearance of
a large fraction of the world’s present population and a decline in the material
conditions of human life. It is a mere detail whether this will come about first
through some ecological disaster, through the decay and demoralisation of the
technological structure, or through a military holocaust” [3].
References
1. Ferris, T. (1997) The New Yorker, April 14, p. 31.
2. Forster, E. M. (1925) Alexandria, A History and a Guide, Peter Smith, London
(considered the best guide book ever written).
3. Brooks, H. (1971) Science, 174, 21.
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