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LUNAR AND PLANETARY PERSPECTIVES ON THE GEOLOGICAL
HISTORY OF THE EARTH
JAMES W. HEAD III
Department of Geological Sciences, Brown University, Providence, RI 02912 USA
Abstract. During the latter part of the last century, a profound change took place in our perception of
the Earth. First, this change was holistic: Plate tectonic theory provided a unifying theme that seems
to explain disparate observations about the Earth and how it works, and lets us see the Earth as a
planet. Secondly, actually seeing the Earth from the Moon, and exploring the other planets provided
additional perspectives on our own home planet and hastened the decline of scientific terracentrism.
Thirdly, learning that the uniqueness of the Moon in terms of size and aspects of its chemistry may
be due to its derivation from the Earth as the result of a giant impact, provided a concrete filial
link. Finally, the geological record revealed by exploration of the Moon and planets has provided us
with the missing chapters in the dynamic history of the Earth. We now know that gargantuan impact
basins formed in Earth’s formative years and that impact events are likely to be the cause of many
punctuations in Earth’s biological evolution. Perspectives on ancient tectonic activity are provided
by Mercury, Venus, Mars, and the Moon, and show that the Earth has changed considerably since
its youth. Widely varying volcanic eruption styles are seen on the planets, providing insight into
how puzzling rocks from early Earth history formed. The composition of planetary atmospheres has
revealed the unusual nature of Earth’s, and its link to the evolution of life. The atmospheres of the
planets have undergone radical changes with time, providing clues to Earth’s history and destiny.
Fundamentally different hydrological cycles on Earth, Venus, Europa and Mars, and evidence for
significant changes with time, have provided insight into Earth’s history. The probable presence of
oceans on Europa and Mars has changed our thinking about the origin and evolution of life on
Earth. We no longer think of the Earth in isolation. Instead, Earth is now perceived of as a member
of a family of planets, each of which provides important missing information and perspective on
the other, and together reveal the fabric of the history of the Solar System. Future exploration and
perspectives will place our Solar System in the context of all of the others.
1. Initial Perspectives
Early observations of the heavens by humans led to awe and superstition, as unexplained and frightening appearances of comets and meteor showers profoundly
distracted people from difficult daily lives. Unusual configurations and alignments
of celestial bodies were seen in the context of animal forms and deities. Special
configurations (e.g., a bright star and crescent Moon), or unusual brightness (e.g.,
an extremely bright star over a small town in the Middle East), were seen as signs
of supreme beings, particularly when linked to unusual earthly events. A common
theme was that these signs were warnings or harbingers, and definitely related to
humans and our presence here on Earth. Although the gods who controlled these
Earth, Moon and Planets 85–86: 153–177, 2001.
© 2001 Kluwer Academic Publishers. Printed in the Netherlands.
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things were clearly superior, nonetheless, they were speaking to us. If we could not
understand these things, at least we could put them in a framework that we could
understand.
A second perspective evolved in parallel to the development of these superstitious and religious frameworks. Empirical observations of the positions of the stars,
Sun and Moon, particularly in relation to seasonal changes and cycles of growth,
led several early civilizations to attempt to understand the heavens in the context
of regular change. Later on, this perspective was sidetracked by attempts to fit the
motions of the planets into a cosmos in which Earth (read humans) occupied the
central position. Conveniently, everything revolved around the Earth, in that most
perfect of ancient Greek geometric figures, the circle. We constantly interpreted
our surroundings in terms of our most immediate frames of reference (anything in
the sky above us is a direct message to us; flat ground equals flat Earth; the known
world is the center of all activity). Acosmic terracentrism is a natural consequence
of our lack of perspective on space and time. After all, we are special.
2. The Retreat from Specialness
For Western civilization, the retreat from human specialness began with the intellectual and artistic rebirth represented by the Renaissance in the fifteenth and
sixteenth centuries. Copernicus, Tycho, Kepler, and Galileo all helped humans to
break the bonds of terracentrism and to perceive our surroundings in ever broader
frameworks of space and time. Galileo, working in Padua, applied telescopic observations to the nature and motions of the planets and satellites. These observations
took us to new dimensions of scale, thus changing our perception of the Solar
System and the place of the Earth, and laying the foundations for modern science.
Now the Sun was the center of the Solar System.
But the road was not smooth. A powerful and vengeful Catholic Church was
threatened by these new views; Giordano Bruno was burned at the stake and Galileo
was placed under house arrest and forced to recant his views. The scientific mantra
of these times might have been “Publish and perish” not “Publish or perish”. In
addition, the rich artistic and intellectual treasures produced during the Renaissance
temporarily reinforced the concept of human specialness through a triumphant selfcelebration.
Later in the millennium, a physical, geological and biological renaissance began
to reveal the true age of the Earth, the concept of ‘deep’ time, and the role of longterm biological evolution. Newton introduced quantitative approaches to testing
scientific ideas. Geologists began to understand the extent and temporal immensity
of the history of the Earth and how events had changed with time. Darwin, a
geologist by training, outlined the nature of biological evolution, and explicitly
and implicitly, the place and role of humans. By the latter two centuries of the last
millennium, the Sun was accepted as the center of the Solar System, the motions
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of the planets and satellites were well known, the age of the Earth was known to
be over 4.5 billion years, and humans were generally, and begrudgingly, accepted
as the product of biological evolution measured over geological time scales. The
retreat from human specialness was well underway. But all was not lost; we were
still the crowning achievement of this biological evolution, we were still at the top
of the tree of life.
3. The Influence of the Exploration of Inner and Outer Space on Perception
During the latter part of the final century of the last millennium, profound changes
took place in our perception of the Earth as we explored inner and outer space.
First, this change was holistic. Prior to this time, the geology of the Earth was
seen as regional in nature. Mountain belts and volcanoes were classified, compared
and contrasted, to look for common themes in their formation and evolution. But
there were no unifying themes in geological sciences for how the planet worked.
Concepts like continental drift, put forth to explain the close fit of many continental
margins with each other, were seen as eccentric or untestable, primarily because
the outer parts of the interior of the Earth were thought to be solid and immobile.
Exploration of inner space (the floors of the oceans and the structure of the interior
of the Earth) in the years following World War II forever changed our concepts
of our own planet. Seafloor exploration revealed that the ocean floors were very
young geologically and completely unlike the continents. Probing of the Earth’s interior revealed chemical and mechanical layers in the interior. The outermost of the
mechanical layers was a lithosphere, overlying a more mobile substrate called the
asthenosphere. The lithosphere was comprised of many adjacent plates, was created at mid-ocean ridges, moved laterally, and was destroyed at subduction zones,
where the lithosphere was bent downward and reentered the interior of the planet.
This paradigm of “global plate tectonics” showed that the seafloor was spreading
apart at amazing geological rates, and that continents were forming and breaking
apart as a result of this motion. Earthquakes, mountain belts and volcanoes could
all be placed in the context of geological activity at the boundaries of these plates.
Plate tectonic theory provided a unifying theme that seemed to explain disparate
observations about the Earth and how it works, and for the first time, it let us see the
Earth as a planet. A few years of reflection led to the awareness that the dynamism
implied by plate tectonics explained the lack of abundant rocks from early Earth
history. Two-thirds of the present surface of the Earth formed in the last 5% of the
history of our planet! Most of the chapters in the book of Earth history had been
destroyed.
The second revolution in our perspective came from the exploration of outer
space. The launches of Sputnik and Yuri Gagarin made us look upward again, but
this time we were prepared to see the cosmos in a broader context of space and
time. Early Soviet images of the lunar farside showed a face of a nearby planetary
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Figure 1. Earth from space. Image taken by Apollo astronauts on their way to the Moon. NASA
photograph.
body previously unseen by life on Earth. As astronauts took their first tentative
steps toward the Moon, they looked back in awe at an Earth suspended in the
black vastness of the cosmos (Figure 1). Their wistful descriptions of the Earth
from the Moon reminded us all of the vastness of space and the specialness of, not
us, but our planet. We saw the Earth as a beautiful blue sphere, with no political
boundaries and a tenuous and fragile environment. The Apollo photographs of the
Earth became an icon for this new awareness. It was rapidly dawning on humans
that we were part of a larger planetary environment and that our very activities were
destroying it. Words like ecology and environmentalism were in vogue. But human
specialness still prevailed; ecology was commonly defined as “the relationship
between humans and their environment”. We were still “top dog”.
Scientific terracentrism was also still rampant. Several decades of successful
application of plate tectonic theory to scientific problems on Earth rapidly brought
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our knowledge of recent geologic history to encyclopedic proportions. But again,
we were very highly collimated in our perceptions of time. What about the other
80% of Earth history? What happened in the formative years? How did the Earth
we observe today get to be the way it is? Where might it be going in the future?
How does the Earth compare to the other planets? Could there be information there
that might provide a broader perspective on our own home planet? These questions
were on the minds of only a very few scientists.
4. The New and Present Perspective on Earth History
Seeing the Earth from the space, walking on the Moon, holding samples from
other planetary bodies in our hands, and exploring a host of other planets has
indeed provided additional perspectives on our own home planet and hastened
the decline of scientific terracentrism. Apollo astronauts completed extensive geological traverses on the Moon (Figure 2). Samples returned from these carefully
planned scientific expeditions provided the first documentation of the nature and
processes operating in the first one-half of Solar System history (Figure 3). We
began to understand that this early history is unlike that seen in later stages of
planetary evolution. Geology as a science, in its early development, had to define
itself against the “catastrophism” of the great biblical flood. Thus, “uniformitarianism”, the concept that geological processes have operated at about the same rate
throughout geological history, was developed. No special circumstances, no “Deus
ex machina”, no catastrophic events, were required. A second concept developed at
this time added to the underpinnings of geological thought. Geological processes
observed to operate today (e.g., volcanism, stream activity, glaciers, etc.) have been
operating throughout geological time, and thus “the present is the key to the past”.
But the expanded geological record provided by the Moon began to yield important perspectives on these underpinnings. Processes such as impact cratering,
which occur so infrequently in recent geological history as to not be part of the
geologist’s awareness, were found to dominate earlier planetary history (Figure 4).
Individual impact craters were certainly catastrophic locally, and perhaps globally.
And clearly the relative proportions of processes operating during different times
in planetary history could vary widely.
Laboratory analysis of the returned lunar samples provided a further perspective. The elements were the same, and the minerals were familiar, but the proportions were generally different. Rocks that dominated the lunar highlands (anorthosites) were rare and poorly understood on Earth. The maria were made of basalts, a
common rock type on Earth, but the proportions of titanium within them were virtually unheard of on Earth. And most importantly, the lunar rocks were extremely
dry and had unusual isotopic ratios. Two stunning conclusions were reached from
these and other data. First, it appears likely that the Moon formed from the impact
of a Mars-sized body into the very early Earth. The melting and ejection of this
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Figure 2a. Astronauts exploring the Moon: (a) Apollo 15 Commander Dave Scott examines the
geology of the base of the Apennine Mountains. (b) Apollo 16 Commander John Young jumps a few
feet off the lunar surface to get a better view of the Cayley Formation in the Descartes highlands. (c)
Apollo 17 Lunar Module Pilot Harrison H. “Jack” Schmitt samples a large boulder at the base of the
Taurus Littrow Mountains. NASA Apollo photographs.
material into Earth orbit ultimately resulted in the re-collection of the debris to
form the Moon. The uniqueness of the Moon in terms of its size and chemistry may
thus be due to its derivation from the Earth as the result of a giant impact. In what
may have been the ultimate catastrophic event in our local frame of reference, the
Moon may indeed have been born from stripping of the outer layers of the Earth.
The Earth–Moon system may represent a concrete filial link (Figure 5). And this
must have forever changed the course of the evolution of the Earth.
Secondly, the anorthositic crust of the Moon formed early in lunar history and
appears to be the result of heat associated with intense impact bombardment. The
energy associated with the accretion of the Moon may have melted the outer several
hundreds of kilometers of the Moon and produced a molten rock (magma) ocean.
Low density crystals floated to the top to produce the anorthositic crust. Could the
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Figure 2b. Continued.
Earth have undergone a similar type of global melting and early crustal evolution?
No Earth rocks have been found dating from this period of planetary history. Could
the other planets provide clues?
The geological record revealed by exploration of the Moon and other planetary
bodies has indeed provided us with many of the missing chapters in the dynamic
history of the Earth (Figure 3). We now know that even hundreds of millions of
years after the accretion of the planets, gargantuan impact basins were forming
on planetary surfaces, including Earth’s. The Orientale Basin on the lunar western
limb is almost 1000 km in diameter and is among the larger (but not the largest)
of the impact structures there. Its rings form a prominent bull’s-eye pattern and
its ejecta influences almost an entire lunar hemisphere. Although the depth of
excavation is not yet well constrained, it is obvious that some of these impacts
must have penetrated to great depths to excavate material from deep within the
interior. The influence of the millions of cubic kilometers of ejecta on the early
atmosphere and surface is as yet not fully conceivable. Such planetary-scale events
were not uncommon in the first third of Solar System history. It is interesting to
speculate as to how human culture and religion might have evolved differently if
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Figure 2c. Continued.
this gigantic unblinking eye had been directly facing Earth, rather than hidden on
the limb (Figure 3).
Such spectacular examples of impact events obscure the fact that smaller projectiles were much more abundant and that they dominated the geological record
of early planetary history (Figure 4). The lunar geologic record of impact flux,
known from the samples returned by Soviet Luna and US Apollo missions, shows
a monotonic decrease in the rate of cratering as a function of time (Figure 7).
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Figure 3. Comparative geological records of different planets. All planetary bodies formed at essentially the same time, about four and a half billion years ago. Plotted is the percentage of the presently
exposed surface that dates from different times in the history of the Solar System. The Earth’s surface
is dominated by the young seafloor and continental deposits ringing ancient cratons. The record of
the Moon, Mars and Mercury formed in the first half of Solar System history and is still preserved
today. Impact cratering and volcanism dominate these one-plate planets. On Venus, the surface has a
young Earth-like age, but does not display plate tectonic features.
Implicit in the knowledge of this flux is the fact that impact cratering is an ongoing
and recurring geological process throughout the history of the planets, including
Earth. If we view Earth history backwards from the perspective of recent geological events, most Earth scientists would relegate impact craters to the category
of minor curiosity. When viewed from the perspective of the past history of the
planets, planetary scientists see impact cratering as an ongoing process operating
at many scales, and having substantial geological, environmental, and biological
consequences. These two disparate views did not begin to be reconciled until distinctive geochemical anomalies similar to those seen in meteorites were detected
in sediments at the Cretaceous–Tertiary (K–T) boundary. The demise of the dinosaurs and the formation of this distinctive world-wide geological boundary is now
thought to be due to the impact of a bolide that formed a crater in the Yucatan.
Subsequent investigations have shown that impact events are likely to be the cause
of many other punctuations in Earth’s biological evolution. The road to the top of
the tree of life may not have been direct.
What about other geological processes? On Earth, the destruction of the early
chapters of history have obscured the origin of plate tectonics. We know it has
been operating for at least hundreds of millions of years, but when and how did
it start? Perspectives on ancient tectonic activity are provided by Mercury, Venus,
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Figure 4. Lunar craters on the heavily cratered lunar farside. The 75 km diameter King crater, with
its lobster-claw-like central peaks, is seen near the center of the picture. NASA Apollo 16 image.
Mars, and the Moon, and these records show that the Earth has changed considerably since its youth. The Moon, Mars and Mercury all have heavily cratered
surfaces that formed and were modified predominantly in the first half of Solar
System history (Figure 4). The stability of these surfaces, and the lack of features
associated with plate tectonics on Earth, indicate that these bodies are “one-plate
planets”. Their outer mechanical layers, or lithospheres, stabilized early on into
one continuous global plate. This stability preserved the important record of early
planetary history that we see today. Tectonic movement on these one-plate planets
was then largely vertical, with loading by volcanic deposits, subsidence and flexure
on the Moon, broad uplift by mantle plume activity on Mars, and minor global
shrinkage to produce spectacular scarps on Mercury. Why do these bodies differ so
from the Earth? The surface area to volume ratio means that they are good radiators,
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Figure 5. A view of the Earth–Moon system from the Galileo spacecraft as it returned from Venus
and the asteroid belt. The Moon is closer to the viewer than the Earth and a significant portion of the
lunar farside is seen. NASA Galileo photo.
losing heat very efficiently. This, together with their small diameters, results in their
lithospheres becoming a relatively large percentage of their radii early in history.
It is then extremely hard to start the subduction that apparently resulted in plate
tectonics on Earth. Breaking a thick rigid layer and pushing it into the interior on a
small planet is not easy.
But what about Venus, the most Earth-like of the planets in terms of its size,
density, and position in the Solar System? Does Venus have plate tectonics? Exploration of Venus was motivated by just such questions and following numerous
missions by the Soviet Union and the US, the Magellan mission obtained global
high-resolution radar images in the 1990s. These spectacular images (Figure 8)
revealed mountain ranges, rift zones, and an extremely young surface geologically
(Figure 3), general properties that were very similar to the Earth and its plate
tectonic system. But most surprisingly, there was no supporting evidence for plate
Figure 6. The Orientale Basin on the Moon (left), which formed by impact about four billion years ago. The basin is almost 1000 km in diameter. NASA
Lunar Orbiter photo. A montage of how the Orientale Basin might have appeared if it had formed at the sub-Earth point and was constantly, and unblinkingly,
looking down on us (right).
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Figure 7. The record of the impact flux on the Moon. Comparison of the number of craters on different geologic units can provide a relative time scale of events. Return of samples from well-known
places on regional units is required to provide the basis for the absolute time scale. The absolute time
scale derived from Apollo and Luna samples shows that in the first few hundred million years of
lunar history, the flux was extremely high, decreasing exponentially between 4 and 3 billion years
ago. Although considerably diminished from its early values, the impact flux, and individual events,
are still a very important part of the geological processes operating on planetary surfaces. Return of
samples from well known units on other planets will provide the exact time scale for those bodies in
the future. The number of craters on the vertical axis refers to craters larger than 1 km in diameter
per million square kilometer area.
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Figure 8. Tectonic deformation in the mountains of Venus. In this view of a huge dome in the Freya
Montes region of Ishtar Terra, numerous tectonic features are testimony to the intense deformation
accompanying the creation of this and adjacent tessera terrain. Ringing the dome to the east and west
are broad folds caused by shortening and contraction. On top of the dome are seen a set of intersecting
extensional structures (graben) indicating that the dome underwent stretching and collapse. Width of
the image is about 75 km. NASA Magellan radar image.
tectonics! No globe-encircling system of plate boundaries, no evidence for some
very young surfaces (where plates were forming), and no evidence for older surfaces (where mature plates were being subducted and destroyed). The distribution
of impact craters could not be distinguished from a completely random one, and
the global density of craters was so low as to suggest that the surface was only
several hundred million years, not billions of years, old.
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How could this be? How could this contrast so much with the Earth, where the
young average age (Figure 3) is a combination of the older continental surfaces
and the very young ocean basins? Geophysicists set about to try to understand this
and one of the models they came up with was radically different than our previous
thinking. In the absence of plate tectonics, could the vertical buildup of crust on
a one-plate planet, if it continues long enough on a large body like Venus, lead to
periodic density inversion, vertical foundering of the outer layer, and catastrophic
resurfacing of the planet? Could this be how plate tectonics started on the Earth?
Among the competing ideas to this is the concept of episodic plate tectonics: in
this view, periods in Venus history are alternatively plate-tectonic dominated, and
one-plate-planet dominated, and at present we are in a one-plate phase. A third
alternative is that plate tectonics previously characterized the surface of Venus,
constantly destroying old terrain and producing new, but that due to continuing
heat loss over geologic time, the lithosphere thickened, froze up in recent history,
and plates stopped moving relative to one another. In this view, Venus is now
and forevermore a one-plate planet. Could this be the fate of Earth in its future?
These radical ideas are still hotly debated in the scientific community, and no firm
consensus has emerged. But the richness of the alternatives has opened our eyes to
several new ways of thinking about the history of the Earth. Could there be major
changes in the style of global tectonic activity with time? Could global changes and
long-term loss of heat from the interior be episodic rather than monotonic? Could
the abundance of water on Earth be a critical factor in plate tectonics?
Widely varying volcanic eruption styles are seen on the planets, providing insight into how unusual rocks, such as very iron-rich fluid lavas called komatiites,
formed early in Earth history. Gigantic lava flows on the Moon are equivalent to
40,000 times the annual output of Kilauea volcano on Hawaii. Venus displays huge
flows (Figure 9) that have resurfaced thousands of square kilometers in very short
periods of time. These types of eruptions, uncommon on Earth today, may explain
the nature, origin and associations of rock types seen in past history. Indeed, the
massive outpourings of lava on Venus are now thought to have put so much gas into
the atmosphere that surface temperatures increased substantially. As this thermal
wave passed into the crust, the style of global tectonic activity is thought to have
been influenced. Imagine, atmosphere changes causing changes in the style of
deformation of planetary crusts!
Massive edifices on Mars rise to over 20 km height (Figure 10) and dwarf the
puny Hawaii. The stable one-plate planets can build these large edifices over longlived sources or hot spots, and lay out the complete sequence of deposits with
time. This is in contrast to the Earth, where such volcanoes are smaller, formed in
production-line-like manner, moved laterally away from the source, and then are
subducted and destroyed.
Determination of the composition of planetary atmospheres has revealed the
unusual nature of Earth’s, and its link to the evolution of life. The atmosphere of
Venus and Mars is predominantly carbon dioxide in contrast to the nitrogen and
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Figure 9. Huge bright and dark lava flows in the Lada Terra region of Venus converge on a low
point in a north–south ridge, and pour through this saddle and out into the surrounding plain. These
flows have traveled almost 700 km eastward from their source region. The total area of the flow
field exceeds 500,000 km2 , similar to some ancient flood basalt provinces on Earth. NASA Magellan
radar image.
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Figure 10. Perspective view of Olympus Mons, a gigantic volcano on Mars about 600 km in diameter
and over 20 km high. Viking Orbiter photomosaic overlain on Mars Orbiter Laser Altimeter altimetric
data. Vertical exaggeration is about twenty times.
oxygen-rich Earth atmosphere. Analysis of ancient Earth rocks and inventory of
carbon dioxide stored in carbonate rocks (such as limestones) on the Earth shows
that the Earth originally had as much carbon dioxide as other planets, but the
evolution of life has changed the atmosphere considerably.
In addition, analysis of the planetary atmospheres and the geological record of
the planets shows that they have undergone radical changes with time, providing
clues to Earth’s history and destiny. The polar caps of Mars have been recognized
for over a hundred years, but recent observations of the Moon and Mercury reveal
evidence for volatile-rich polar ice deposits there. Planetary degassing products
and cometary impact debris apparently migrate to the polar cold traps and produce
deposits even in the extreme thermal environment of Mercury. The prospect of
sampling the geologic record of volatiles contained in these caps is extremely
exciting. The larger and more accessible polar caps of Mars contain a stratigraphic
record of many hundreds of layers (Figure 11) which could provide the keys to
understanding recent climatic change there.
Earth has always been thought of as the water planet. Water dominates the surface, is abundant (occurring in glaciers, rivers, lakes and oceans covering almost
two-thirds of the planet), is extremely significant in weathering, and is thought
to be essential in the formation and nurturing of life. But recent exploration has
shown that water may have played a very important role on other planets too, and
that oceans may not be the exclusive purview of Earth. Although liquid water is not
now stable under present conditions on Mars, we see evidence for ancient glacial
deposits, streams, rivers, and lakes. Indeed recent evidence is consistent with the
presence of a huge ocean filling the northern lowlands of Mars earlier in its history
(Figure 12). On Europa, the second of the Galilean satellites (Figure 13), we now
have evidence that a global ocean covers the surface and that it is frozen over, but
likely still liquid today below the surface. These new perspectives on environments
have changed our frame of reference in thinking about the formation and evolu-
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Figure 11. Layers in the north polar cap of Mars seen in a Mars Orbiter Camera image. These layers,
as small as a few meters thick, are thought to be related to changing conditions on the surface and
greater and lesser amounts of deposition of ice (bright) and dust (dark). These layers may be related
to the same kind of obliquity variation in the orbital axis that are responsible for the ice ages on
Earth.
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Figure 12. Hypothesized position of an ocean in the northern lowlands of Mars in its earlier history. In
this topographic map of the northern hemisphere of Mars derived from Mars Orbiter Laser Altimeter
(MOLA) data, the black area is the low topography proposed to have been occupied by a huge
standing body of water. The Tharsis region is seen as a high on the right. Much of the water may
have entered the basin from outflow channels entering at the lower left. NASA MOLA data.
tion of life on Earth. Radically different hydrological cycles on Earth, Mars, and
Europa, and evidence for significant changes with time, have also provided insight
into Earth’s history.
5. The Lessons for the History of Earth, Our Home Planet
How have these new views changed our perception about the history of the Earth?
We now know that the following themes must be considered in the reconstruction
of those missing chapters of Earth history.
− Formation of the Earth from accretion of planetesimals.
− Derivation of the Moon from the Earth as the result of a gigantic (Mars-sized)
impact event.
• Late addition of a large amount of material from elsewhere in the Solar
System.
• Stripping of the early atmosphere of the Earth.
• Loss from the early Earth of a considerable amount of the solid upper
layers.
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Figure 13. Galileo image of the surface of Europa in the anti- Jovian region. Note the general lack of
impact craters and the cracked nature of its frozen water-ice surface layer. The dark wedge-shaped
area in the middle of the image is about 15 km wide, and represents the cracking and opening of
the European crust, much in the way sea-floor spreading operates on Earth. The icy layer seen in
this image likely overlies a global ocean at depth. The width of the image is about 170 km. NASA
Galileo image.
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• Massive changes in the internal constitution and thermal structure of early
Earth.
• Does this event explain the differences between Earth and Venus?
• What are other implications of this concrete filial link?
− Subsequent continuing high impact flux.
− Formation of large impact basins excavating deep into the planet and spreading ejecta widely, influencing the atmosphere and any biota.
− Delivery of rocks (and any available microbes) from the surfaces of other
planetary bodies to Earth as meteorites.
− Launch of rocks (and any available microbes) from the Earth’s surface to other
planetary bodies as meteorites.
− Effusion of large volcanic outpourings over short periods of time, influencing
the atmosphere, hydrosphere, and biosphere.
− Possible episodic (not just monotonic) heat loss from the interior, regional
and perhaps global in scale; this would influence the atmosphere and surface
temperatures, and perhaps the tectonic style.
− Internal density instabilities causing global changes in volcanism and tectonism, and possible resurfacing of the entire planet. Could this initiate plate
tectonics?
− Planetary contraction due to thermal evolution (cooling or phase changes).
Could this initiate plate tectonics?
− Continuing role of impact events throughout the history of the Earth; potential large-scale modification of the atmosphere and the biota at numerous but
random times.
In the coming decades, these concepts derived from the last forty years of Solar
System exploration will be folded into our ongoing reconstruction of the missing
chapters in Earth history to produce a radically different picture of the formative
years of our own home planet. In the words of T. S. Elliot, “We will not cease from
exploration, and the end of all of our exploring will be to arrive where we started
and know the place for the first time”.
6. New Perspectives in Time and Space
Terracentrism and human specialness are on the run, being replaced by new scales
of insight in time and space. We no longer think of the Earth in isolation. Instead,
Earth is now perceived of as a member of a family of planets, each member of
which provides important missing information and perspective on the other, and
together reveal the fabric of the history of the Solar System. Our perception of
life has changed radically from the photosynthesis-based, mammalian-dominated
tree, leading inexorably to humans, the ultimate evolutionary achievement. Life is
now seen everywhere on Earth, even in the most extreme environments, from hightemperature deep-sea volcanic vents, to the deepest, hottest mines, deriving energy
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form a host of different reactions and eating toxic waste and radioactive material.
Indeed life is now thought by many to have originated in such high-temperature
environments. Assessment of the terrestrial biota underlines again that bacteria are
phenomenally important in terms of sheer numbers, biomass, consistency and survivability. Mars rocks may contain microfossils. Rocks can be readily transported
from planet to planet throughout geological history, and the interiors of rocks are
survivable environments for microorganisms.
Rather than the crowning achievement of organic evolution, humans are now
seen as just another marginal species. Bacteria rule. Evolution is not directed, and
certainly not directed toward us. Evolution is a random process involving multiple chance and chaotic events. Life may have arrived on Earth from Mars inside
meteorites almost 4 billion years ago. Humans may owe their day in the Sun to a
chance impact that terminated a successful global population of reptiles and paved
the way for opportunistic mammals. Humans could easily suffer the same fate as
the dinosaurs. Bacteria would still rule.
We have had just a few years to enjoy this recently-acquired Solar Systemcentered perspective. Ongoing and future exploration and perspectives will place
our Solar System in the context of all of the others that are now known to exist
around other stars, and the many more soon to be discovered. The continuing retreat
from terracentrism and human specialness is underlined by the incredible diversity
of solar system arrangements that have been encountered in the last few years. Of
course, our quest is driven by the desire to find a planet “like Earth”.
7. Into the Future: Beyond Human Specialness
We have analyzed the historical self-perception of the role of humans and examined
our past perceptions in understanding Earth and the cosmos. These assessments
show some progress, but abundant mistakes, wrong turns, and prejudices. The
record demonstrates that human thinking is almost by definition, limited in space
and time (Figure 14). Indeed, limited thinking in space and time is a fundamental
characteristic of almost all animal species, and is almost certainly an inheritance
from our genetic forebearers. Our daily lives are so dominated by short-term local
events that we must struggle mightily to break these bounds. In the short-term,
local environment, we are the dominant species. But as we have expanded our
ability to probe over greater distances and longer time scales, and understand what
we see in these dimensions, human specialness disappears. In this millennium,
humans will continue to try to turn the tide of the retreat from human specialness,
and will do their utmost to avoid surrender. But the odds are not with us. Not too
many years ago we were the center of the cosmos and the star atop the tree of
life. Now we are not. But this should not be a threat. The past tells us that we
have a very exciting ride ahead as we probe to ever greater dimensions of space
and time, explore outer and inner space, and perhaps other dimensions as well.
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Figure 14. The density of humans occupying different parts of the space and time scales. Most
humans are concerned with their immediate surroundings (space) and with their short term activities
(time). Few individuals are considering broader dimensions of the space and time scales, and even
those occupy it only for a short period of time. The decisions that are made and the actions that are
taken in the lower left part of the diagram may have immense consequences for the upper right hand
portion. Similarly, inaction in this area, and inattention to the broader scales of space and time, can
have equally important effects. Modified from D. H. Meadows et al., The Limits to Growth, Universe
Books, New York, 1974.
In fact, early in this millennium it will likely be conclusively shown that the very
exploration ethic that we think of as uniquely human, is instead a manifestation of
the survival of species. Of course, this is not so obvious to us as yet in species other
than ourselves.
The importance of maintaining an open mind to new and seemingly heretical
ideas cannot be understated. The very things that bind us perceptually to a small
area of space and time (Figure 14) also make us threatened by and resistant to new
and changing ideas. The socialization and acculturation processes work heavily
against individual and creative thinking. We must recognize this and strive to overcome these effects. But progress is clearly being made and the times are very, very
exciting. And the future is as promising as it is incomprehensible. Several years
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ago, a student entered my office in utter frustration, having been unable to find a
thesis topic on which no previous work had been done. In answer to her query, my
colleague simply said, “Oh, don’t worry, almost everything is not yet known”.
Acknowledgements
I would like to acknowledge the influence of the following individuals and their
writings: E. O. Wilson, Carl Sagan, Ray Bradbury, Rodney Brooks, Isaac Asimov,
Thomas S. Kuhn, T. S. Elliot, Arthur C. Clarke, Stephen Jay Gould, David R.
Scott, John Young, Harrison Schmitt, John McPhee, Lionel Wilson, and Richard S.
Williams, Jr. Discussions with many students at Brown University have challenged,
developed and sharpened these thoughts.
Selected Readings
Allegre, C.: 1992, From Stone to Star, A View of Modern Geology, Harvard University Press, CT.
Alvarez, W.: 1997, T. Rex and the Crater of Doom, Princeton University Press, Princeton.
Beatty, J. K., Petersen, C. C., and Chaikin, A. (eds): 1999, The New Solar System, Sky Publishing
Corporation, Cambridge, MA.
Boorstein, D. J.: 1983, The Discoverers, Random House, New York.
Chaikin, A.: 1994, A Man on the Moon, Viking Press, New York.
Ciba Foundation: 1996: Evolution of Hydrothermal Ecosystems on Earth (and Mars?), John Wiley
& Sons Ltd, West Sussex.
Goldsmith, D.: 1997, Worlds Unnumbered: The Search for Extrasolar Planets, University Science
Books, Sausalito, CA.
Goldsmith, D.: 1997, The Hunt for Life on Mars, Penguin, New York.
Greeley, R.: 1993, Planetary Landscapes, Chapman & Hall, New York.
Grinspoon, D. H.: 1997, Venus Revealed: A New Look below the Clouds of Our Mysterious Twin
Planet, Addison-Wesley, Reading, MA.
Harland, D. M.: 1999, Exploring the Moon: The Apollo Expeditions, Springer Praxis, London.
Hartmann, W. K.: 1993, Moons and Planets, Wadsworth, Belmont, CA.
Krupp, E. C.: 1983, Echoes of the Ancient Skies, Harper and Row, New York.
Light, M.: 1999, Full Moon, Alfred A. Knopf, New York.
MacKenzie, F. T.: 1998, Our Changing Planet: An Introduction to Earth System Science and Global
Environmental Change, Prentice Hall, NJ.
McSween, H. Y.: 1995, Stardust to Planets: A Geological Tour of the Universe, St. Martin’s Griffin,
New York.
Morrison, D. and Owen, T.: 1988, The Planetary System, Addison-Wesley Publishing Company,
Reading MA.
Norton, O. R.: 1998, Rocks from Space, Mountain Press Publishing, Missoula, MT.
Oreskes, N.: 1999, The Rejection of Continental Drift: Theory and Method in American Earth
Science, Oxford University Press, New York.
Sagan, C.: 1982, Cosmos, Random House, New York.
Shirley, J. H. and Fairbridge, R. W.: 1997, Encyclopedia of Planetary Sciences, Chapman & Hall,
London.
Spudis, P. D.: 1996, The Once and Future Moon, Smithsonian Institution Press, Washington, DC.
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Strom, R. G.: 1987, Mercury: The Elusive Planet, Smithsonian Institution Press, Washington, DC.
Ward, P. D. and Brownlee, D.: 2000, Rare Earth: Why Complex Life Is Uncommon in the Universe,
Copernicus, New York.
Weissman, P. R., McFadden, L., and Johnson, T. V. (eds.): 1999, Encyclopedia of the Solar System,
Academic Press, San Diego.
Wilhelms, D. E.: 1993, To a Rocky Moon: A Geologist’s History of Lunar Exploration, University of
Arizona Press, Arizona.
Zebrowski, E.: 1997, Perils of a Restless Planet: Scientific Perspectives on Natural Disasters,
Cambridge University Press, Cambridge.