Table Of ContentA look back at Earth by Voyager I in February 1990 from above the lunar highlands. Photograph ASP·
14·2383, courtesy of Lyndon B. Johnson Space Center, NASA, Houston. Photograph provided by R. E.
Stevenson, Del Mar
E. Uchupi K.O. Emery
Morphology of the
Rocky Members
of the Solar System
With a Foreword by Robert S. Dietz
With 150 Figures
Springer-Verlag
Berlin Heidelberg New York
London Paris Tokyo
Hong Kong Barcelona
Budapest
Professor Dr. ELAZAR UCHUPI
Professor Dr. KENNETH O. EMERY
Woods Hole Oceanographic Institution (W.H.O.I.)
Department of Geology and Geophysics
Woods Hole, MA 02543
USA
ISBN 978-3-642-87552-6 ISBN 978-3-642-87550-2 (eBook)
DOl 10.1007/978-3-642-87550-2
Library of Congress Cataloging-in-Publication Data. Uchupi, Elazar. Morphology of the rocky
members of the solar system 1 E. Uchupi, K.O. Emery. p. cm. Includes bibliographical references
and index. ISBN-13: 978-3-642-87552-6
l. Planets- Geology. 2. Satellites. 3. Satellites- Outer planets. 4. Planetology. I. Emery, K.O.
(Kenneth Orris), 1914- . II. Title. QB603.G46U23 1993 559.9'2-dc20 93-30159
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© Springer-Verlag Berlin Heidelberg 1993
Softcover reprint of the hardcover 1st edition 1993
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Begin at the Beginning,
and Go On
Till You Come to the End: Then Stop.
The King in ALlCE IN WONDERLAND
Foreword
These words are written on the SOOth anniversary of Columbus'
discovery of the New World. Surely the deep-space exploration of other
worlds in our Solar System over the past few decades is an event of
similar magnitude. Man has traveled far enough to see Spaceship Earth
suspended alone in black space. And he has voyaged even farther to
marvel at the crescent Earth rising over the Moon's cratered terrain.
Instrumented spacecraft have toured the entire Solar System even
beyond the ninth planet Pluto. This work of science Morphology of the
Rocky Members of the Solar System is an inquiry about our extended
home. As with the Darwinian and Copernican paradigms, the nature of
our planetary system, as the extended world around us, has great
significance for those who ponder the human condition. The deep-space
views of our Planet Ocean with its sweeping clouds, and moving oceans
and creeping continents must rank as the greatest photograph ever
taken. Viewing Spaceship Earth hanging in the vast void is an almost
frightening experience. We are so alone! It is easy to understand why so
many are attracted to a simpler account of origins, like the allegorical
tale of creation written in heroic style (but eschewing math, maps,
figures, tables, references, and evidence) in the first eleven chapters of
Genesis.
This treatise examines the morphology of the six rocky planets and
their 27 satellites from a broad perspective. Aspects examined are their
internal constitution and volcanic/tectonic activity (endogenic pro
cesses); their surface modification by wind, water, ice, mass-wasting, etc.
(exogenic processes); and their modification by impact crate ring (an
exotic process). Remarkably, asteroid/cometary cratering was regarded
as a nonprocess on Earth until recently but now these star scars
(astroblemes) appear to dominate surface morphology on terrestrial
(rocky) planets.
The landing of man on the Moon during the 1960s is best known.
But the grand tour of the outer planets (Voyagers 1 and 2), the Mars
missions, and the 23 encounters with Venus topped off with its mapping
by Magellan are events of far greater scientific value. There are those
who decry these expensive feats as a waste of money, but what a
comment it would be on the human race if, although we had the
technology to do so, we did not try! Mark Twain once chided scientists
VIII Foreword
for spinning so many speculations from a paucity of data. But in
reading this synthesis, one is impressed by the ingenuity of scientists in
understanding the planets although we have certain ground truth only
from lunar rocks. The usual behest is "Let's go and see" but better
advice would seem to be "Let's think about it." Some personal com
ments on the Moon by astronauts ("Wow, fantastic!") are not really
meaningful. In the bibliography of about 1000 references, interpreta
tions from instrumental results overwhelm personal observations.
The Earthlike planets and their satellites have orbited the Sun since
its nuclear furnace burst into flame 4.55 Ga (billion years ago). This
book should appeal to those having a nontrivial interest in Earth
history, astronomy, or the grand scheme of things. Crammed with facts
and figures, it is not easy reading but nevertheless its perusal is richly
rewarding. The interpretations are constrained by the current data set
which is expanding rapidly. Some conclusions may not stand the test of
time as it is the nature of science to be tentative, falsifiable, testable, and
corrigible, so we can expect a better and more exact understanding as
we approach the twenty-first century.
Before 1950 historical geology concerned mainly the Phanerozoic
Era beginning in the Cambrian with a clearly legible fossil record.
Recently, the veil has been lifted from the Precambrian with radio
metric dating providing chronological control. Even so, our knowledge
hits a blank wall about 3.0 Ga ago. Fortunately, an earlier readable
history appears to be preserved on the Moon, Mercury, and especially
Mars although the Hadean Era (4.55 to 4.0 Ga ago) is once again wiped
out by accretionary bombardment. Yet, the large variety of planetary
bodies gives hope that initial events such as core formation eventually
can be worked out. Planets are heat engines, so the generation and the
ridding of internal heat are basic to their evolution. It is understandable
that small bodies, whose surface areas are large relative to their volume,
radiate heat readily and so remain as solid coreless rocks in space. But
why does not Venus, with a planetary radius like that of Earth, exhibit
plate tectonics? Many mysteries remain to be explained.
Why is Earth so special? The presence of an ocean and an
atmosphere is a partial answer. Earth uniquely has three volumes or
"spheres" ~ hydrosphere, atmosphere, and lithosphere (liquid, gaseous,
and solid states of matter). Geometrically, volumes intersect~forming
plains; thus there is the First Surface (land/air), the Second Surface
(air/water), and the Third Surface (water/ocean floor). It is in the nature
of things that energy exchanges take place primarily across surfaces;
hence their importance in the scheme of things. Only Earth has a
hydrosphere which is a substance needed for the existence of life. Our
planet is a remarkable place. There may be other similar planets in our
universe, but even in a galaxy of ten billion stars the number of Earth
like planets must be a small number. Can we ever get there? Probably
Foreword IX
not, for if so, they would already be here. The question remains: where is
everybody?
Nearly three decades ago we learned how our world works - by
plate tectonics and uniquely so. Among Solar System members, there is
a carapace of eight major lithospheric plates and many lesser ones
which drift and rotate a few centimeters per year interacting along their
boundaries by subducting, shearing, or spreading apart. Mountains,
volcanoes, earthquakes, and even the continents and ocean basins are
the result. Hypsographic bimodality is unique to Earth and is the
hallmark of a highly evolved planet where basalt is extruded as a partial
melt of mantle peridotite and then is remelted to create the sialic rocks
of the granitoid continental plateaus. A convecting body like the Sun is
understandable and so is a solid rock in space which has gravitationally
collapsed into a sphere like the Moon. But a planet with surficial plates
drifting only as fast as one's fingernails grow is nonintuitive. This must
be why we were so slow in understanding our own planet.
The Moon was the first celestial body to be a source of wonder. To
ancient Greeks, it was a mirror reflecting a map of the Mediterranean
region, but in 1609 Galileo's telescope discerned the presence of moun
tains and maria ('seas'). Before 1965 there was a consensus that the
lunar craters were volcanic and the Apollo project was biased toward
the search for such plutonian features. But it quickly became evident
that lunar highlands or terrae are saturated with impact craters. Even
the maria are giant waterless impact basins dating back to the terminal
bombardment nearly 4 billion years ago. The Moon is so dry that
basalts erupted then are less weathered than Hawaiian flows extruded
yesterday. With the drift of the continents over the eons Mother Earth
has changed her expression slowly, but the Man-in-the-Moon virtually
forever has retained a fixed stare. Mercury, like the Moon, is saturated
with craters and became quiescent 3 billion years ago. At the University
of Illinois in the late 1930s, this writer proposed writing a doctoral
thesis about the lunar surface, but it was rejected as not a suitable
subject for geological study. Not in their wildest dreams did anyone
then anticipate that 30 years hence man would walk on the lunar
surface. Now space travel is taken for granted.
Mars, the red planet, has long been the subject of unconstrained
speculation and science fiction. Sadly, Percival Lowen's ideas about
canals and intelligent life were proved totally wrong when viewed in the
stark reality of the Mariner and Viking spacecraft missions. Mars is
now dry, cold, and inhospitable to life although there may once have
been shallow lakes whose deposits may one day reveal fossil evidence of
extinct primitive life. Mars is unique in having polar ice caps that
strongly wax and wane yearly. Its two small satellites, Phobos and
Deimos, appear to be captured asteroids. Although it lacks surficial
water we do know that Mars has ice and a thin atmosphere sufficient to
x
Foreword
generate dust storms which obscure the surface. It also has a dense core
and has evolved a basaltic crust but apparently lacks granitic highlands.
The Tharsis region is a great bulge with volcanic edifices and capped by
Olympus Mons, the highest mountain in the Solar System. Extending
radially away from Tharsis is Valles Marineris, a giant gash of un
certain origin. Like nearly all extraterrestrial bodies, Mars is a museum
of archaic landforms; major geological activity ceased more than three
eons ago. Although no surface samples have been returned from Mars,
certain meteorites called SNCs (eucrites or shocked basaltic rocks)
probably are martian samples blasted off its surface by asteroidal
impact and then orbited into space for millions of years before colliding
with Earth. The Mars Orbiter launched in 1992 will mark a new return
to Mars and promises a wealth of new information provided by new
sophisticated instrumentation. Will man ever visit this most Earthlike
of planets? Very likely it is in the offing although decades away. It is
even remotely conceivable that we can modify the martian environment
or perhaps ourselves make Mars a second habitable oasis in space.
Presently, Mars, with its complex landforms and varied history, re
mains a wonderful planet for scientific contemplation.
Radar imaging has pierced the thick cloud veil of Venus and
poured out an avalanche of new data from the Magellan
mission - more than from all previous space flights combined. Revealed
is an arid searing (500°C) terrain where lead would melt and which
apparently was resurfaced catastrophically some 500 million years ago.
Volcanic constructs dominate the scene, but there are also nearly 1000
impact craters. The term astrobleme used on Earth to describe the
eroded or buried remains of an impact crater is quite unnecessary on
Venus as nearly all such features are pristine with little degradation. In
contrast to Mars with an atmospheric pressure only 1% that of Earth,
venusian air is 100 times as thick as Earth's and is dominantly carbon
dioxide. Thus, the surface has a shield against impacting bodies equival
ent in effect to 1 km of water. The ice-ball comets probably break up
high in the atmosphere and only the larger asteroids get through. Mead,
the largest impact crater, is 275 km across. Volcanoes abound but it is
doubtful that any are active, for in spite of their fresh appearance they
are mostly tens and even a few hundred million years old. The overall
generation oflava is probably only about 10% of the rate on Earth. As
a wholly different kind of world, Venus holds fascination but is more
like Hell than Heaven.
The Copernican revolution was a blow to man's ego. Jerusalem
was not the center of the universe nor was the Earth for it revolved
around the Sun. The Sun, in turn, was an ordinary main sequence star
in the vast Milky Way, an ordinary galaxy among billions in the
universe. But comparative planetology reveals that the Earth is never
theless remarkably unique. Life-friendly habitats capable of housing a
Foreword XI
biosphere are a one in a billion chance. Not only were conditions right
on Earth for biopoiesis (the origin of life) but the environment was
stable for eons permitting the evolution from dawn species, to photo
synthesis, to the eucaryotic cell, to multicellular organisms, to verteb
rates, and to sentient man. Surely, the greatest disappointment of our
generation is not finding even primitive life on Mars or elsewhere. There
seems little chance that we ever can communicate with intelligent life
elsewhere because, if it were possible, they would have contacted us
already.
The surface of a sphere, the perfect geometric form, is finite but
boundless. All points on a sphere are geometrically equal but hardly so
for Earth's ecology. Most of our globe is so inhospitable that higher life
is confined to the green oases. We regard our environment or biosphere
as benign, but oxygen, a product oflife, is a highly corrosive poison, and
water is the greatest of all solvents. Through evolution life has adapted
to this milieu although magmatic rocks rapidly weather to sand and
clay. We live under stress which causes time's arrow of evolution to fly
faster sacrificing individuals for the good of the species along the way.
Land is more stressful than the sea, requiring locomotion and coping
with desiccation, gravity, freezing, etc. Evolution proceeds faster on
land so that the mammals which now dominate the ocean returned to
the sea after evolving on land. As the Earth winds down, life winds up
but a quartz crystal from the Precambrian is the same yesterday, today,
and forever.
It is said that we must leave home to understand the place from
whence we came. The bizarre collection of neighboring planetary
bodies emphasizes the unbelievable uniqueness of Earth with its vast
blue planet-encircling ocean of liquid water from which rise the tan
island continents. A hypothetical Man-in-the-Moon (I suppose he
should be called a lunatic) would see no sign of life on Earth except for
coral reefs. He would wonder at the shallow continental shelves and
whether rising sea level would drown the continents. But land life has
existed since the Silurian and so it never will be completely drowned
because plate tectonics provide a feedback mechanism which relates
the freeboard of continents to the depth of the oceans. Earth's surface
temperatures (permitting water to occur in all three states of matter)
and atmospheric composition always have been conducive to the origin
and evolution of life. Although some other stars may have stable
planetary systems, other habitable abodes such as our must be rare in
the extreme.
The new chapters in today's standard textbooks of geology are
plate tectonics, man's role as a geological agent (environmental geo
logy), and planetology. Clearly, we must accept the new paradigm of
astrocatastrophism - the role of impacting asteroids and comets in
shaping the external morphology of the planetary bodies. This is most
