Physics and Chemistry of the Solar System

559 Pages·1995·29.148 MB·English

by  John S. Lewis (Auth.)

Physics and Chemistry
of the Solar System
John S. Lewis
Department of Planetary Sciences
University of Arizona
Tucson, Arizona
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Library of Congress Cataloging-in-Publication Data
Lewis, John S.
Physics and chemistry of the solar system / John S. Lewis.
p. cm.
Includes index.
ISBN 0-12-446740-7 (case) ISBN 0-12-446741 -5 (paper)
1. Solar system. 2. Planetology. 3. Astrophysics.
4. Cosmochemistry. I. Title.
QB501.L497 1995
523.2-dc20 94-31854
CIP
PRINTED IN THE UNITED STATES OF AMERICA
95 96 97 98 99 00 QW 9 8 7 6 5 4 3 2 1
Preface
This book is based on the structure, scope, and philoso­ This book does not closely approximate the level and
phy of a sophomore-junior level course taught at the Mas­ scope of any previous book. The most familiar texts on the
sachusetts Institute of Technology (MIT) by the author and planetary sciences are Exploration of the Solar System, by
Professor Irwin I. Shapiro from 1969 to 1982. Although the William J. Kaufmann, III (MacMillan, New York, 1978), a
content of that course varied greatly over the years in re­ non-mathematical survey of the history of planetary explo­
sponse to the vast new knowledge of the Solar System pro­ ration, Moons and Planets, by William K. Hartmann (Wads-
vided by modern Earth-based and spacecraft-based experi­ worth, Belmont, CA, 1972, 1983, and 1993), a scientific
mental techniques, the philosophy and level of presentation tour of the Solar System with high-school-level mathemati­
remained very much the same. cal content, and Meteorites and the Origin of Planets, by
In this book, as in that planetary physics and chemistry John A. Wood (McGraw-Hill, New York, 1968), a fine
course, I shall assume that the reader has completed one year qualitative introduction that is similarly sparing of mathe­
of university-level mathematics, chemistry, and physics. The matics and physics. Several other nonmathematical texts are
book is aimed at several distinct audiences: first, the upper- available, including Introduction to the Solar System, by
division science major who wants an up-to-date appreciation Jeffrey K. Wagner (Saunders, Philadelphia, 1991), Explor­
of the present state of the planetary sciences for "cultural" ing the Planets, by W. Kenneth Hamblin and Eric H. Chris­
purposes; second, the first-year graduate student from any of tiansen (Macmillan, New York, 1990), The Space-Age Solar
several undergraduate disciplines who intends to take gradu­ System, by Joseph F. Baugher (Wiley, New York, 1988), and
ate courses in specialized areas of planetary sciences; and The Planetary System, by planetary scientists David Mor­
third, the practicing Ph.D. scientist with training in physics, rison and Tobias Owen (Addison-Wesley, Reading, MA,
chemistry, geology, astronomy, meteorology, biology, etc., 1988). The scope of the present text is broader and the level
who has a highly specialized knowledge of some portion of higher than any of these books.
this material, but has not had the opportunity to study the History has arranged recent lulls in planetary probe
broad context within which that specialty might be applied launch schedules that strongly influence both the timing
to current problems in this field. and the scope of this book. From 1982 to 1986 there was a
ix
X Preface
gap in the acquisition of planetary data by American space­ Meteors), and Chapter VIII (Meteorites and Asteroids), and
craft. This drought was interrupted in 1986 by the Voyager- might fairly be entitled 'The Solar System beyond Mars."
Uranus flyby and by five spacecraft encounters with Halley's The third and final part comprises Chapter IX (The Airless
comet (two Soviet, two Japanese, and one from the European Rocky Bodies), Chapter X (Mars, Venus, and Earth), Chap­
Space Agency), but the drought again resumed until it was ter XI (Planets and Life about Other Stars), and Chapter XII
broken by the Voyager-Neptune encounter, the Soviet- (Future Prospects). This part could be called, "The Inner
Phobos missions in 1989, and the Magellan mission to Venus Solar System."
in 1990. The launch of the Galileo orbiter and probe to Ju­ As presently structured, the book is a broad survey of
piter, long scheduled for 1986, was severely delayed by the the Solar System suitable for reference use or as background
explosion of the space shuttle orbiter Challenger, the result­ reading for any course in Solar System science. Using this as
ing two-year grounding of the entire shuttle fleet, and the a textbook, a Planetary Sciences course taught in a trimester
subsequent cancellation of the high-energy Centaur G upper setting could use one part in each term. In a two-semester
stage intended for launching heavy planetary missions from program, either an inner Solar System course (parts 1 and 3)
the shuttle. The European-American Ulysses solar mission, or an outer Solar System course (parts 1 and 2) could be
not instrumented for intensive planetary studies, flew by Ju­ taught. The most ambitious and intensive program, and the
piter in February 1992, returning only data on its magnetic most similar to the way the course was structured at MIT,
and charged-particle environment. The combined effects of would be to teach parts 2 and 3 in two semesters, reserving
the unaccustomed slowdown in the rate of collection of new most of the material in part 1 for use as reference reading,
data about the planets from 1982 to 1989 and the absence of not lecture material.
large-scale data return activity since 1990 have been not only This book is written in fond memory of the approxi­
to inhibit progress in research, but also to encourage would- mately 350 students who took the course at MIT, and who
be writers of textbooks who wish to avoid the instant obso­ have unanimously and vocally deplored the lack of a text­
lescence of their work. book for it. I can only plead that the rate of advance of
This text may be regarded as a collection of three parts. knowledge in the planetary sciences has until recent years
The first of these parts contains Chapter I (Introduction), been so large that, once half the text for a book this size has
Chapter II (Astronomical Perspective), Chapter III (General been written, it becomes obsolete and requires replacement
Description of the Solar System), and Chapter IV (The Sun at a rate equal to the best writing speed of a harried professor.
and the Solar Nebula). This first part might be called "Gen­ I extend my particular thanks to Irwin Shapiro for his many
eral Properties and Environment of Our Planetary System." years of cheerful, devoted, always stimulating, and some­
It is roughly equivalent to a brief introductory astronomy times hilarious collaboration on our course, and for his gen­
book emphasizing the concerns of planetary scientists, not erous offer to allow me to write "his" half of the text as well
stellar or galactic astronomers. The second part contains as "mine."
Chapter V (The Major Planets), Chapter VI (Pluto and the
Icy Satellites of the Outer Planets), Chapter VII (Comets and John S. Lewis
I. Introduction
Nature and Scope of Planetary Science stars, the formation of planets, the thermal and outgassing
history of planetary bodies, and the involvement of geo-
When asked in an interview to give his viewpoint on the chemical processes in the origin of organic matter. The con­
frontiers of science, the famous physicist Victor Weisskopf nection between life and planetary environments is so fun­
commented that the most exciting prospects fell into two damental that it has been given institutional recognition: it is
categories, the frontier of size and the frontier of complexity. not widely known outside the field, but research on the origin
A host of examples come to mind: cosmology, particle phys­ of life in the United States is a mandate of the National Aero­
ics, and quantum field theory are clearly examples of the ex­ nautics and Space Administration.
tremes of scale and are all clearly among the most exciting Wherever we begin our scientific pilgrimage throughout
frontiers of science. Biology, ecology, and planetary sciences the vast range of modern science, we find ourselves forced
are equally good examples of the frontier of complexity. to adopt ever broader definitions of our field of interest. We
When we peruse the essential literature of planetary sci­ must incorporate not only problems on the frontier of com­
ence, we find that we must continually face three main is­ plexity, but also problems from both extreme frontiers of
sues. First, we are concerned with the origin and nuclear and scale. In this way, we are compelled to trespass across many
chemical evolution of matter, from its earliest manifestations hallowed disciplinary boundaries.
through the appearance of atoms, molecules, minerals, and Further, as we seek an evolutionary account of the emer­
organic matter. Second, on the cosmic scale, the origin and gence of complexity from simplicity, we become able to see
evolutionary fate of the universe emerges as a theme. Third, more clearly the threads that lead from one science to an­
we are confronted with the problem of understanding the ori­ other. It is as if the phenomena of extreme scale in physics
gin and development of life. In each case, we are brought existed for the express purpose of providing a rationale for
face to face with the spontaneous rise of extreme complexity the existence of astronomy. The other disciplines evolve
out of extreme simplicity and with the intimate interrelation­ logically from cosmic events:
ship of the infinitesimally small and the ultimately large. The astronomical universe, through the agency of nu­
Further, our past attempts at addressing these three great clear reactions inside stars and supernova explosions, popu­
problems have shown us that they are remarkably inter­ lates space with atoms of heavy elements, which are the basis
twined. The very issue of the origin of life is inextricably tied of chemistry.
up with the chemistry of interstellar clouds, the life cycles of The course of spontaneous chemical evolution of
I
2 I. Introduction
interstellar matter produces both mineral grains and or­ and systems far removed from thermodynamic equilibrium.
ganic molecules, giving rise to geochemistry and organic Think of it: systems slightly perturbed from equilibrium
chemistry. spontaneously relax to the dullest conceivable state, whereas
Solid particles accrete to form large planetary bodies systems far from equilibrium spontaneously organize them­
and give us geology. selves into structures optimized for the minimization of dis­
Radioactive elements formed in stellar explosions are order and the maximization of information content!
incorporated into these planets, giving life to geophysics. It is no wonder that the whole idea of evolution is so
Melting, density-dependent differentiation, and outgas- magical and counterintuitive to so many people and that the
sing take place, and atmospheres and oceans appear; petrol­ critics of science so frequently are able to defend their posi­
ogy, meteorology, and oceanography become possible. tions by quoting the science of an earlier century. We often
Organic matter is formed, accumulated, concentrated, hear expressed the idea that the spontaneous rise of life is as
and processed on planetary surfaces, and biology is born. improbable as that a printshop explosion (or an incalculable
Planetary science may then be seen as the bridge be­ army of monkeys laboring at typewriters) might accidentally
tween the very simple early universe and the full complexity produce an encyclopedia. But have we ever heard that this
of the present Earth. Although it partakes of the excitement argument is obsolete nonsense, discredited by the scientific
of all of these many fields, it belongs to none of them. It is progress of the 20th century? Sadly, there is a gap of a cen­
the best example of what an interdisciplinary science should tury between the scientific world view taught in our schools
be: it serves as a unifying influence by helping to dissolve and the hard-won insights of researchers on the current fore­
artificial disciplinary boundaries and gives depth and vi­ front of knowledge. The great majority of all people never
brancy to the treatment of evolutionary issues in nature that learn more than the rudiments of Newtonian theory, and
transcend the concerns and the competence of any one of the hence are left unequipped by their education to deal with
parent sciences. But there is more: planetary sciences is cen­ popular accounts of modern science, which at every interest­
trally concerned with the evolutionary process and hence ing turn is strikingly non-Newtonian. News from the world
with people's intuitive notion of "how things work." There of science is, quite simply, alien to them. The message of
is as much here to unlearn as there is to learn. modern science, that the universe works more like a human
We, in the late 20th century, still live under the shadow being than like a mechanical wind-up toy, is wholly lost to
of the clockwork, mechanistic world view formulated in the them. Yet it is precisely the fundamental issues of how things
17th century. Even the education of scientists is dedicated work and how we came to be, what we are, and what may
first and foremost to the inculcation of attitudes and values become of us that are of greatest human interest. The modern
that are archaic, dating as they do from Newton's era in the artist or writer of the 20th century often asserts modernity by
17th century, viewpoints that must be unlearned after sopho­ preaching the sterility of the universe and the alienation of
more year. We are first led to expect that the full and precise the individual from the world. But this supposed alienation
truth about nature may be extracted by scientific measure­ of the individual from the Universe is, to a modern scientist,
ments; that the laws of nature are fully knowable from the an obsolete and discredited notion.
analysis of experimental results; and that it is possible to pre­ The problems of evolutionary change and ultimate
dict the entire course of future events if, at one moment, we origins are not new concerns. Far from being the private do­
should have sufficiently detailed information about the dis­ main of modern science, they have long been among the
tribution and motion of matter. Quantum mechanics and chief philosophical concerns of mankind. Astronomy and as­
relativity are later taught to us as a superstructure on New­ trology were the parents of modern science. The earliest hu­
tonian physics, not vice versa. We must internally turn our man records attest to mankind's perpetual fascination with
educations upside down to accommodate a universe that is origins:
fundamentally quantum mechanical and relativistic, within
Who knows for certain and can clearly state
which our "normal" world is only a special case.
Where this creation was born, and whence it came?
All of these issues come to bear on the central question The devas were born after this creation,
of the evolution of the cosmos and its constituent parts. Most So who knows from whence it arose?
of us have had a sufficient introduction to equilibrium ther­
No one knows where creation comes from
modynamics to know that systems spontaneously relax to Or whether it was or was not made:
highly random, uninteresting states with minimum potential Only He who views it from highest heaven knows;
energy and maximum entropy. These are the classical con­ Surely He knows, for who can know if He does not?
clusions of Gibbs in the 19th century. But very few of us are RigvedaX 129.6-7
Circa 3000 BC
ever privileged to hear about the development of nonequilib-
rium thermodynamics in the 20th century, with its treatment We need no longer regard our origins as complete mys­
of stable dissipative structures, least production of entropy, teries. We can now use the observational and theoretical
Guide to the Literature 3
tools of modern science to test rival theories for their faith­ Guide to the Literature
fulness to the way the universe really is. Some theories,
when tested by the scientific method, are found to give in­ It is difficult, as we have seen above, to draw a tidy line
accurate or even blatantly wrong descriptions of reality and around a particular portion of the scientific literature and
must be abandoned. Other theories seem to be very reliable proclaim all that lies outside that line to be irrelevant. Still,
guides to how nature works and are retained because of their there are certain journals that are more frequently used and
usefulness. When new data arise, theories may need to be cited by practitioners of planetary science. Every student
modified or abandoned. Scientific theories are not absolute should be aware both of these journals and of the powerful
truth and are not dogma: they are our best approximation of abstracting and citation services now available.
truth at the moment. Unlike dogma, scientific theories cannot Astronomical observations, especially positional mea­
survive very long without confronting and accommodating surements, orbit determinations, and the like, which are
the observed facts. The scientific theories of today are sec­ carried out using Earth-based optical, radio, and radar tech­
ondary to observations in that they are invented—and modi­ niques, are often published in the Astronomical Journal.
fied—by human beings in order to explain observed facts. Infrared spectroscopic and radiometrie observations and a
They are the result of an evolutionary process, in which the broad range of theoretical topics often appear in the Astro-
"most fit" theories (those that best explain our observations) physical Journal. The most important journals devoted to
survive. Our plan of study of the Solar System mirrors this planetary science in the broad sense are Icarus and the Jour­
reality. nal of Geophysical Research. Two journals are devoted to
This book will begin with what little we currently know relatively quick publication of short related papers: Geo­
about the earliest history of the universe and trace the evo­ physical Research Letters and Earth and Planetary Science
lution of matter and its constructs up to the time of the take­ Letters. Two general-purpose wide-circulation journals also
over of regulatory processes on Earth by the biosphere. We frequently publish planetary science papers, including spe­
introduce the essential contributions of the various sciences cial issues on selected topics: Science and Nature. The most
in the order in which they were invoked by nature and build important western European journal for our purposes is
complexity upon complexity stepwise. Otherwise, we might Astronomy and Astrophysics.
be so overawed by the complexity of Earth, our first view of Russian research papers frequently appear first (or in
nature, that we might despair of ever gaining any under­ prompt translation) in English. The most important Soviet
standing. journals are Astronomiche skii Zhurnal (Soviet Astronomy to
This approach should also dispel the notion that we are the cognoscenti), Kosmicheskii Issledovaniya (Cosmic Re­
about to understand everything. It is quite enough to see that search), and Astron. Vestnik (Solar System Research), all of
there are untold vistas for exploration and more than enough which appear in English translation with a delay of several
of the Real to challenge our most brilliant intellects and most months.
penetrating intuitions. Other journals containing relevant research articles in­
Let us approach the subject matter covered herein with clude Physics of the Earth and Planetary Interiors, the
the attitude that there are a number of fundamental principles Proceedings of the Lunar and Planetary Science Confer­
of nature, of universal scope, that allow and force the evolu­ ences, the Journal of the Atmospheric Sciences, Planetary
tionary process. With our senses at the most alert, willing to and Space Science, Geochimica et Cosmochimica Acta, the
entertain the possibility of a host of hypotheses and deter­ Russian-language Geokhimiya, Meteoritics, Origins of Life,
mined to subject all theories and observations alike to close and perhaps 50 other journals that are usually a bit far from
scrutiny, we are challenged to grasp the significance of what the center of the field. Many space scientists keep abreast of
we see. Let us cultivate the attitude that the ultimate purpose the politics and technology of space exploration by reading
of the planetary sciences is to uncover enough of the blue­ Aviation Week and Space Technology, which often prints fu­
prints of the processes of evolution so that we will be able to ture news and juicy rumors.
design, build, and operate our own planetary system. Very valuable service is also rendered by several review
Like it or not, we are assuming responsibility for the publications, such as the Annual Review of Earth and Plane­
continued stability and habitability of at least one planet. The tary Science, Space Science Reviews, Reviews of Geophysics
scale of human endeavor has now become so large that our and Space Physics, and the Annual Review of Astronomy
wastes are, quite inadvertently, becoming major factors in and Astrophysics.
global balances and cycles. Soon our scope may be the Books on the planetary sciences have an unfortunate
whole solar system. The responsible exercise of our newly tendency to become obsolete during the publication process.
acquired powers demands an understanding and conscious­ Nonetheless, many books have useful coverage of parts of
ness superior to that which we have heretofore exhibited. the material in the field, and a number of these will be cited
Now is the time for us to learn how planets work. at the relevant places in the text.
4 I. Introduction
It is often valuable to track down the history of an idea back to an old issue of the Irish Astronomical Journal. Be
or to see what publications are following a lead established eclectic: do not fear journals with Serbian or Armenian
in a landmark paper of several years ago. For these purposes, names. The contents are most likely in English; if not, then
every scientist should become familiar with the uses of the almost certainly in French, German, or Russian, often con­
Science Citation Index. Depending upon one's own particu­ veniently equipped with an English abstract.
lar interests, any of a number of other abstracting services Planetary science is a genuinely international endeavor,
and computerized databases may be relevant. The reader is and to make the most of the available resources one must be
encouraged to become familiar with the resources of the willing to dig deep and write lots of letters to colleagues
most accessible libraries. Every research library has Chemi­ abroad. One must be prepared to face the hardship of back-
cal Abstracts, Biological Abstracts, etc. to-back conferences in Hawaii and Nice, of speaking en­
For the diligent searcher, there will be an occasional gagements three days apart in Istanbul and Edmonton, and
gem captured from Tetrahedron Letters or the publications of January trips to Moscow balanced against summer work­
of the Vatican Observatory, and surely one cannot claim to shops in Aspen. I suppose that this is part of our training as
be a planetary scientist until one has followed a long trail thinkers on the planetary scale.
II. Astronomical
Perspective
Introduction Solar System, the mean distance of the Earth from the Sun,
is called an astronomical unit (AU) and has a length of
We cannot study the Solar System without some knowl­ 149,597,870 km.
edge of the universe in which it resides, and of events that In order to measure the enormously larger distances be­
long predate the Solar System's existence, including the very tween the Sun and nearby stars, we must make use of the
origin of matter and of the universe itself. We shall therefore apparent motion of nearby stars relative to more distant stars
begin by tracing the broad outlines of present understanding produced by Earth's orbital motion about the Sun. Figure II. 1
of the origin and evolution of the universe as a whole, in­ shows how the relative motions of the star and the Sun
cluding the synthesis of the lighter elements in the primor­ through space are separated from the effects due to Earth's
dial fireball, galaxy and star formation, the evolution of stars, annual orbital motion. The angular amplitude of the oscilla­
explosive synthesis of the heavier elements in supernova ex­ tory apparent motion produced by the Earth's orbital motion
plosions, and astronomical evidence bearing directly on the is called the parallax (/?), which is inversely proportional to
origins of stellar systems and their possible planetary com­ the distance of the star. The parallax is so small that it is
panions. No attempt is made to describe every current theory always measured in seconds of arc ("), and hence the most
bearing on these matters. Instead, the discussion cleaves convenient distance scale is
closely to the most widely accepted theories and selects sub­
d(pc) = I/O, (ILI)
ject matter for its relevance to the understanding of our own
planetary system. where the unit of distance (inverse arc seconds) is called a
parsec (pc). The distance to the nearest stars is about one
parsec. From Fig. II. 1 it can be seen that 1 pc is 1 AU/sin
Distance Scales in the Universe (1"), or 206,264.8 AU (3.08568 X IO13 km). Because only a
handful of nearby stars have parallaxes large enough to be
Distances within the Solar System, such as the distance measurable to a precision of ±1%, this precision in speci­
from Earth to the Moon or to the other terrestrial planets, can fying the size of a parsec is gratuitous: 2X105 AU or
now be measured by radar with a precision of about one part 3 X 1013 km is entirely adequate for most purposes.
in 108. The basic yardstick for measuring distances in the We shall see later how such distance determinations per-
5
6
II. Astronomical Perspective
Earth.
a.
Earth 6 months later.
1 parsec
tan 6=sin0=0=l AU/lpc= 1/206,264.8
à^W0^':-i'Â-
o
M:
"proper" motion due
to relative motion of
apparent motion due to
the star and the Sun
Earth's motion about the Sun
d
Αζ=ρ=1/α(ρο)
Αη=Αζ cosy
Figure II. I Planetary and stellar distance scales. The mean distance of the Earth from the Sun,
1.5 X 108 km, is defined as 1 astronomical unit (AU). The stellar distance unit, the parsec (pc), is the
distance from which the radius of Earth's orbit subtends 1 s of arc, as shown in a. The apparent motion
of a nearby star against the background of much more distant stars is shown schematically in b. This
motion is composed of a "proper" motion due to the relative translational velocity of the Sun and the
star, combined with a projected elliptical motion due to the annual orbital excursions of Earth about the
Sun (c). A nearby star lying near the plane of Earth's orbit will oscillate back and forth along a straight
line in the sky; one close to the pole of Earth's orbit will describe an almost circula rpath. At intermediate
ecliptic latitudes, elliptical paths are seen. When the effect of proper motion is removed, the ratio of the
semimajor axis to the semiminor axis of the projected ellipse is easily calculated from the ecliptic lati­
tude of the star, as in d.
mit the calculation of the absolute luminosities (erg s~l) of For the present it suffices to state that there exists a class of
stars and how correlations of spectral properties with lumi­ variable stars, called Cepheid (SEFF-ee-id) variables, whose
nosity provide a very useful scheme for describing stars in luminosities have been found to be directly related to their
terms of the relationships between their intrinsic properties. period of light variation (see Fig. II.2). This means that, once