A B I T A B L E Z O N E S , AND T I M E S ,
I N T H E UN I V E R S E
Because our limitations of the Universe deal with time, we must pose our question
in a temporal sense: Are there times that are habitable in the Universe? As
we will see in the next chapters, life (at least life as we know it) requires many
elements that had to be created after the Big Bang (the advent of the Universe,
some 15 billion years ago). Twenty-six elements (including carbon, oxygen,
nitrogen, phosphorus, potassium, sodium, iron, and copper) play a major role
in the building blocks of advanced life, and many others (including the heavy
R A R E E A R T H
30
radioactive elements such as uranium) play an important secondary role by
creating, deep within Earth, heat indirectly necessary for life. All of these elements
were created within the centers of stars—often in exploding stars, or
supernovae—rather than in the Big Bang itself, so they were not present in
sufficient abundance for perhaps the first 2 billion years or more of the Universe.
Then, the “habitable zone” of the Universe, in the sense of time, began
only after its first 2 billion years. The early history of the Universe was also
dominated by objects known as quasars, which would have been very dangerous.
The early Universe must have been lifeless or at least empty of advanced
life, and quite remarkably, there are also limits on the time during
which the Universe can exhibit Earth-like planets that provide adequate life
support for advanced life. The geological activity on Earth that is so important
in controlling the atmospheric temperature via the CO2–rock cycle is
driven by the heat liberated by the radioactive decay of uranium, thorium,
and potassium atoms. These elements are produced by supernovae explosions,
and their rate of formation is declining with time. In our galaxy, stars
that form at present have less of these radioisotopes than the sun did when it
formed 4.6 billion years ago. It is entirely possible that any true Earth clones
now forming around other stars would not have enough radioactive heat to
drive plate tectonics, a key process that helps stabilize Earth’s surface temperature.
Our definition of a universe habitable zone is based on time, and
though intriguing, it is still a bit unsatisfying. Is there some geographic,
rather than temporal, component of the Universe that favors or is poisonous
to life? If we could map the Universe, would we find favorable and unfavorable
regions, just as we do in stellar systems and in our galaxy? In other words,
is life uniformly distributed throughout the Universe, or are there regions
where it will exist and others where it will not? We cannot yet answer questions
such as this, but some remarkable new discoveries have enabled us at
least to address them.
For 10 days in December 1995, the Hubble Space Telescope in orbit
around Earth focused its large mirror on a small region in space. A total of 342
Habitable Zones of the Universe
31
exposures were made in the vicinity of Ursa Major, the Big Dipper. The area
of space examined is tiny: from our perspective, only 1⁄30 the size of the full
Moon. The target area in this small region—now known as the Hubble Deep
Field—was a sprinkling of galaxies. The Hubble Deep Field appears to be one
of the richest windows into distant galaxies known in the sky.
The results of these 10 days of photography have been nothing short of
spectacular—and in a sense revolutionary. The photographs revealed galaxies
3 to 15 times fainter—and thus proportionally more distant—than any
previously observed. More than 1500 individual galaxies can be identified in
the photos. The light from these faint objects has come to us from the deep
past—from periods long before our own galaxy formed, and our own sun.
The most distant galaxies visible in these photos probably date to some time
during the first few billion years after the start of the Universe, and hence
they may antedate life anywhere. It is unlikely that any of the stars in these
galaxies could have Earth-like planets because the heavy elements to build
them were not yet abundantly available. We thus may be seeing images of the
prebiotic Universe.
Another insight gleaned from the Hubble Deep Field is that older
galaxies seem to have more irregular shapes than newer galaxies. From 30%
to 40% of the most distant galaxies (and hence of the the oldest galaxies) are
unusual or deformed compared to those nearest our own galaxy. The galaxies
of the early Universe are quite different from newer galaxies. Does galaxy
morphology affect habitability? And has habitability changed through time?
An even more surprising result was the finding that the various distances
from Earth of the many galaxies seen in these photos cluster around a few values.
Galaxies appear to be concentrated in great bubble-like or sheet-like structures
with vast voids between. We might ask whether regions along these great
sheets of galaxies have higher or lower hospitality to life. A key to habitability
in various galaxies may be their abundance of heavy elements. Planets that form
around metal-poor stars may be too small to retain oceans, atmosphere and
plate tectonics. Metal-poor planets may not be able to support or maintain animal
life, for reasons that we will detail in later chapters. It is known that entire
galaxies are metal-poor and hence likely devoid of animal life.
R A R E E A R T H
32
T H E END O F P L A N E T A R Y HA B I T A B I L I T Y
For nearly all of Earth history, life was limited to creatures so small that they
are invisible to the naked eye. Casual inspection during all that time would
have suggested that it was a failed planet. In other planetary systems, primitive
life might flourish but never advance to the point where forests and flying
animals even get a serious chance to evolve. Stars with short lifetimes, unstable
planetary atmospheres, changes in orbital or spin axis, massive
extinctions, impacts, crustal catastrophes, the cessation of plate tectonics, or
any of a whole raft of other problems could prevent the evolution of advanced
life or its prolonged survival. And on Earth itself, complex life has
thrived only for the last 10% of the planet’s existence.
Perhaps the most predictable aspect of advanced life (if it exists) on
other planets around other stars is that its tenure is limited and that eventually
any such life—and even some of the planets—will perish. Like individual organisms,
planets and their grand environments have life spans. All planets with
life eventually become extinct. This final outcome may be brought about by
external sources such as impacts or a nearby supernova, by internal effects
such as atmospheric or biological catastrophe, or (if all else fails) by increase
in the brightness of the central star. This will be the ultimate fate of Earth: Life
on our planet will eventually be roasted out of existence. The sun is slowly getting
brighter. It is now 30% brighter than it was in the early history of the
planet. Over the next 4 billion years it will double in brightness. Even if life
survives this travail, it will soon be stilled. About 4 billion years from now, the
sun will begin to expand rapidly in size, and its brightness will dramatically increase.
The sun will become a red giant, as did the stars Antares in the constellation
Scorpio and Betelgeuse in the constellation Orion. In a billion-year
time span, its brightness will increase over 5000 times.
At the very beginning of this process, Earth’s oceans will vaporize, driving
our precious water supply into space. In the final stages of its transformation
into a red giant, the sun will expand to the point where it will nearly
reach the orbit of Earth. The Universe will be one living planet poorer.
Habitable Zones of the Universe
33
SUMMARY
A review of habitable zones—for animals as well as microbes, and in the
galaxy and Universe as well as around our sun—leads to an inescapable conclusion:
Earth is a rare place indeed. Perhaps the most intriguing finding of this
line of research is that Earth is rare as much for its abundant metal content as
for its location relative to the sun. As we will see in the next chapter, the
metal-rich core of our Earth is responsible for much of its hospitality to life.
Building
a Habitable
Earth
The Earth is the only world known, so far, to harbor life.
There is nowhere else, at least in the near future, [to which]
our species could migrate.
—Carl Sagan, Pale Blue Dot
Most of the Universe is too cold, too hot, too dense, too vacuous,
too dark, too bright, or not composed of the right elements to
support life. Only planets and moons with solid surface materials
provide plausible oases for life as we know it. And even among planets with
surfaces, most are highly undesirable. As we noted in the Introduction to
this book, of all yet known celestial bodies, Earth is unique in both its physical
properties and its proven ability to sustain life. The success of Earth in
supporting life for billions of years is the result of a remarkable sequence of
physical and biological processes; knowledge of these processes is our main
source of insight into the possibilities of life elsewhere. In this chapter we
will describe the formation and evolution of the planet Earth. Understanding
how Earth attained its life-giving properties will provide a framework for
35
3
R A R E E A R T H
36
understanding what is required for life and how likely it is to exist on other
bodies.
Using Earth to generalize about what life requires is, of course, fraught
with uncertainty. Lacking knowledge of any extraterrestrial life forms, we
cannot be confident that we understand the optimal or even the minimal conditions
necessary to support life beyond this planet. But our planet is an uncontested
success in terms of the abundance and variety of life it sports, even
though it was certainly sterile soon after its formation. How did that change
come about, and what were the physical attributes of Earth that allowed it to
become so rich with life?
Earth is the only location in the universe that is known to have life, but
it is only one of perhaps millions of habitats in our galaxy, and trillions in the
Universe, that might also harbor life. From the biased viewpoint of Earthlings,
however, it does appear that Earth is quite a charmed planet. It has the
right properties for the only type of life we know, it formed in the right place
in the solar system, and it underwent a most remarkable and unusual set of
evolutionary processes. Several of its neighbors in the solar system even
played highly fortuitous, supporting roles in making Earth a congenial habitat
for life. The near-ideal nature of Earth as a cradle of life can be seen in its
prehistory, its origin, its chemical composition, and its early evolution. What
are the most important factors that allowed Earth to support advanced life?
Earth has offered (1) at least trace amounts of carbon and other important
life-forming elements, (2) water on or near the surface, (3) an appropriate atmosphere,
(4) a very long period of stability during which the mean surface
temperature has allowed liquid water to exist on its surface, and (5) a rich
abundance of heavy elements in its core and sprinkled throughout its crust
and mantle regions.
Earth is actually the final product of an elaborate sequence of events
that occurred over a time span of some 15 billion years, three times the age
of Earth itself. Some of these events have predictable outcomes, whereas others
are more chaotic, with the final outcome controlled by chance. The evolutionary
path that led to life included element formation in the Big Bang and
Building a Habitable Earth
37
in stars, explosions of stars, formation of interstellar clouds, formation of the
solar system, assembly of Earth, and the complex evolution of the planet’s interior,
surface, oceans, and atmosphere. If some god-like being could be
given the opportunity to plan a sequence of events with the express goal of
duplicating our “Garden of Eden,” that power would face a formidable task.
With the best intentions, but limited by natural laws and materials, it is unlikely
that Earth could ever be truly replicated. Too many processes in its formation
involved sheer luck. Earth-like planets could certainly be made, but
each outcome would differ in critical ways. This is well illustrated by the fantastic
variety of planets and satellites that formed in the solar system. They all
started with similar building materials, but the final products are vastly different
from each other. Just as the more familiar evolution of animal life involved
many evolutionary pathways with complex and seemingly random
branch points, the physical events that led to the formation and evolution of
the physical Earth also required an intricate set of nearly irreproducible circumstances.
Any construction project requires that building materials be on site before
the actual construction begins. The formation of Earth was no different.
Hence the first step is assembling the raw materials.
CR E A T I O N O F T H E E L E M E N T S
Although we understandably date Earth’s history from the origin of the
planet, a considerable “prehistory” preceded Earth’s formation. One of the
most important aspects of this period was the origin of the chemical elements.
The elements are the building blocks of both planets and life. Consider
that in a sort of cosmic reincarnation, every atom in our bodies resided
inside several different stars before the formation of our sun and has been part
of perhaps millions of different organisms since Earth formed. Planets, stars,
and organisms come and go, but the chemical elements, recycled from body
to body, are essentially eternal.
R A R E E A R T H
38
All but a minute fraction of the atoms in the planet Earth and in its inhabitants
were produced, long before Earth formed, by an intricate set of astrophysical
processes. A most remarkable aspect of our prehistory is that the
processes of element formation were universal and that they provided fairly
similar starting materials for most planets’ building processes, wherever these
occurred. Planets and the life they harbor may develop great variations, but
their initial stores of building blocks were similar, largely because of the relative
abundance of the various chemical elements. By looking at this prehistory,
we can gain insight into the range of possible planets and life habitats that
might form in different places and at different times in the Universe.
The cosmic choreography that led to the formation of Earth, all other
bodies in the Universe, and (ultimately) life began with the Big Bang, the
very “beginning of time.” The Big Bang is what nearly all physicists and astronomers
believe is the actual origin of universe. Born in an instant, the entire
universe started out as an environment of incredible heat and density, but
subsequent expansion led to rapid cooling and more rarefied conditions. During
the first half-hour, conditions existed that produced most of the atoms
that are still the major building blocks of the stars—mainly hydrogen and helium,
atoms that make up over 99% of the normal (visible) matter in the universe.
In itself however, the Big Bang generated little chemical diversity. It
gave us little or nothing beyond hydrogen, helium, and lithium to fill the periodic
table. It did not produce oxygen, magnesium, silicon, iron, and sulfur,
the elements that constitute more than 96% of the mass of our planet. It did
not produce carbon, a chemically unique element whose versatile ability to
form complex molecules is the basis for all known life. But the Big Bang did
produce the raw material (hydrogen) from which all heavier and more interesting
elements would later form.
The temperature of the Universe, during its first half-hour, was above
50 million degrees Celsius. At this temperature, positively charged protons
(the nuclei of hydrogen) could occasionally collide with enough energy to
overwhelm the electrostatically repulsive effects of their like positive charges
and fuse together to form helium. This simple fusion process is the secret of
Building a Habitable Earth
39
the stars. It is the reason why the night sky is not dark, the reason why Earth’s
surface is not frozen, the reason why planets can exist; it is the energy source
that powers life on Earth. This process commonly occurs inside stars, but it
was also the major nuclear reaction in the Big Bang. In stars the fusion of hydrogen
to yield helium provides a critical long-term energy source, but in the
Big Bang, helium production was a mere footnote to the grand events that
had just preceded it. In addition to being the first nuclear reaction to produce
new elements, the formation of helium from hydrogen (thermonuclear fusion)
has handed advanced life a double-edged sword. On the positive side,
fusion is the only known process that could be used in future reactors to provide
truly long-term energy sources for advanced civilizations. (Fossil fuel
and solar power could not possibly supply Earth’s human population, at its
present rate of energy consumption, for more than a few thousand years. Fusion
reactors using hydrogen from the ocean could, in principle, produce
nearly unending supplies of energy.) On the other hand, bombs based on the
fusion of hydrogen are one of the surest means of destroying advanced life
forms on a planet-wide scale.
The fusion of hydrogen to form helium was the end of the road for element
production during the Big Bang. The key process that would lead from
helium to the production of heavier elements could not occur under the conditions
that prevailed in the early Universe. When the temperature was high
enough to produce them, the spatial density of atoms was too low and the reaction
rates too small. Thus it was not possible for Earth-like planets to form
in the early Universe, because their formation depends on elements heavier
than helium. During the first 15% of the age of the Universe, a period of over
2 billion years, stars could form, but there was not enough dust and rocks for
them to have terrestrial planets. When modern telescopes are used to observe
more and more distant objects, we are actually seeing further and further back
into the early history of the Universe. If it were possible to detect life with a
telescope, we would observe a “dead zone” beyond a certain distance—
beyond a certain time, that is, when the Universe was without life or planets
or even the elements to produce them.
R A R E E A R T H
40
The trick for getting from helium to the generation of planets, and ultimately
to life, was the formation of carbon, the key element for the success of
life and for the production of heavy elements in stars. Carbon could not form
in the early moments following the Big Bang, because the density of the expanding
mass was too low for the necessary collisions to occur. Carbon formation
had to await the creation of giant red stars, whose dense interiors are
massive enough to allow such collisions. Because stars become red giants only
in the last 10% of their lifetimes (when they have used up much of the hydrogen
in their cores), there was no carbon in the Universe for hundreds of
millions to several billion years after the Big Bang—and hence no life as we
know it for that interval of time.
Carbon formation requires three helium atoms (nuclei) to collide at essentially
the same time: a three-way collision. What actually happens is that
two helium atoms collide to form the beryllium-8 isotope, and then, within a
tenth of a femtosecond (1/10,000,000,000,000,000 second) before this highly
radioactive isotope decays, it must collide with and react with a third helium
nucleus to produce carbon. Carbon has a nucleus composed of six protons
and six neutrons, the cumulative contents of three helium atoms. Once carbon
had been made, however, heavier and heavier elements could be formed.
The production of heavier and more interesting elements occurred in the
fiery cores of stars where temperatures ranged from 10 million to over 100
million degrees Celsius. The sun is currently producing only helium, but in
the future, in the last 10% of its lifetime, it will produce all of the elements
from helium to bismuth, the heaviest nonradioactive element in nature. Elements
heavier than bismuth are all radioactive, and most are produced by the
decay of uranium and thorium. The elements heavier than bismuth were produced
in the cores of stars ten times more massive than the sun that underwent
supernova explosions, dramatic events in which a star brightens by a
factor of 100 billion over a period of a few days.
The sequence of element production in the Big Bang and in stars provided
not only the elements necessary for the formation of Earth and the
other terrestrial planets but also all of the elements critical for life—those actually
needed to form living organisms and their habitats. Among the most
Building a Habitable Earth
41
Argon
Sodium
Aluminum
Helium
Hydrogen
Neon
Carbon
Nitrogen
Oxygen
Sulfur
Silicon
Iron
Magnesium
Figure 3.1 The relative proportions (by number) of the most abundant elements in the Sun. Hydrogen
and helium and the elements resting directly on top of the hydrogen cube dominate the composition
of stars and Jovian planets. The terrestrial planets could not efficiently incorporate these elements and
are composed largely of oxygen and the elements resting on the helium cube.
important of these elements were: iron, magnesium, silicon, and oxygen to
form the structure of Earth; uranium, thorium, and potassium to provide radioactive
heat in its interior; and carbon, nitrogen, oxygen, hydrogen, and
phosphorus, the major “biogenic” elements that provide the structure and
complex molecular chemistry of life. The production of elements inside stars,
along with continued recycling between stars and the interstellar medium,
produced a relative proportion of the different elements known as the “cosmic
abundance,” the approximate elemental composition of the sun and most
common stars. This composition is approximately 90% hydrogen and 10%
helium, leavened with carbon, nitrogen, and oxygen at around 0.1% each and
magnesium, iron, and silicon at roughly 0.01% each (see Figure 3.1). Earth itself
exhibits similar relative abundance of iron, magnesium, and silicon and
has some oxygen but can claim only trace amounts of the other cosmically
abundant elements. The elements carbon, oxygen, hydrogen, and nitrogen
dominate its biotic inhabitants, or life.
R A R E E A R T H
42
The processes that occurred during the billions of years of Earth’s “prehistory”
when its elements were produced are generally well understood. Elements
are produced within stars; some are released back into space and are
recycled into and out of generations of new stars. When the sun and its planets
formed, they were just a random sampling of this generated and reprocessed
material. Nevertheless, it is believed that the “cosmic abundance”
mix of the chemical elements—the elemental composition of the sun—is representative
of the building material of most stars and planets, with the major
variation being the ratio of hydrogen to heavy elements.
The dominant atoms that formed Earth were silicon, magnesium, and
iron, with enough oxygen to oxidize fully (from compounds such as MgO,
magnesium oxide) most of the silicon and magnesium and part of the iron.
Earth’s oxygen content is 45% by weight but 85% by volume. Other elements
are rare, but some play very critical roles. Carbon is a trace element in Earth,
but as we have noted, it is the key element for terrestrial life, and its rich chemical
properties are probably the basis of any alien life as well. Hydrogen is also
a trace element in planet Earth; still its gifts include the oceans and all water, the
essential fluid of terrestrial life. Other important trace elements are uranium,
potassium, and thorium. The decay of these radioactive elements heats Earth’s
interior and fuels the internal furnace that drives volcanism, the vertical movement
of matter within its interior, and the drift of continents on its surface.
The “cosmic abundance” pattern is well known in the scientific literature,
but it actually isn’t as “cosmic” as its name implies. It is actually the “solar
abundance” pattern, because it is based on measurements of the composition
of the sun and solar system. Many stars are similar in composition, but there
is variation, mainly in the abundance of the heavier Earth-forming elements
relative to hydrogen and helium. The sun is in fact somewhat peculiar in that
it contains about 25% more heavy elements than typical nearby stars of similar
mass. In extremely old stars, the abundance of heavy elements, may be as
low as a thousandth of that in the sun. Abundance of heavy elements is
roughly correlated with age. As time passed, the heavy-element content of
the Universe as a whole increased, so newly formed stars are on the average
more “enriched” in heavy elements than older ones. There are also systematic
Building a Habitable Earth
43
variations within the Milky Way galaxy. Stars in the center of the galaxy are
richer in metals (astronomical slang for elements heavier than helium) than
stars at the outer regions.
The abundance of heavy elements enters into Rare Earth considerations
because it influences the mass and size of planets. If Earth had formed around
a star with lower heavy-element abundance, it would have been smaller because
there would have been less solid matter in the annular ring of debris
from which it accumulated. Smaller size can adversely influence a planet’s
ability to retain an atmosphere, and it can also have long-term effects on volcanic
activity, plate tectonics, and the magnetic field. If the sun were older, if
it were further from the center of the galaxy, or even if it were a typical onesolar-
mass star (equal to the mass of the sun), then Earth would probably be
smaller. If Earth were just a little smaller, would it have been able to support
life for long periods of time?
Of all these properties of the solar system, perhaps the most curious—
and at the same time the least appreciated—is that it is so rich in metals. Recent
studies by Guillermo Gonzalez and others have shown that the sun is
quite rare in this respect. Metals are necessary attributes of planets: Without
them there would be neither magnetic fields nor internal heat sources. And
metals may also be a key to the development of animal life: They are necessary
to important organic constituents of animals (such as copper and iron
blood pigments). How did we get our surplus treasure trove of metals?
CO N S T R U C T I O N O F P L A N E T E A R T H
The matter produced in the Big Bang was enriched in heavier elements by cycling
in and out of stars. Like biological entities, stars form, evolve, and die.
In the process of their death, stars ultimately become compact objects such as
white dwarfs, neutron stars, or even black holes. On their evolutionary paths
to these ends, they eject matter back into space, where it is recycled and further
enriched in heavy elements. New stars rise from the ashes of the old.
This is why we say that each of the individual atoms in Earth and in all of its
R A R E E A R T H
44
creatures—including us—has occupied the interior of at least a few different
stars. Just before the sun formed, the atoms that would form Earth and the
other planets existed in the form of interstellar dust and gas. Concentration
of this interstellar matter formed a nebular cloud, which itself then condensed
into the sun, its planets, and their moons.
Let’s take a closer look at what happened. The formation process began
when a mass of interstellar material became dense and cool enough to grow
unstable and gravitationally collapse into itself to form a flattened, rotating
cloud—the solar nebula. As the nebula evolved, it quickly assumed the form
of a disk-shaped distribution of gas, dust, and rocks orbiting the proto-sun, a
short-lived juvenile state of the sun when it was larger, cooler, and less massive
and was still gathering mass. The planets formed from this nebula, even
though the nebula itself existed for only about 10 million years before the
majority of its dust and gas either formed large bodies or was ejected from the
solar system.
It would be highly informative to examine similar nebulae around other
young stars, but their distance from us is so great, and their size so small, that
their details cannot yet be directly imaged with telescopes. Ground-based
and space-borne telescopes have, however, revealed several lines of evidence
suggesting that disks surround newly forming stars. Among this evidence is a
peculiar and spectacular phenomenon that has only recently begun to be understood.
Young stars show jets of material radiating away from them. These
“bipolar nebulae” are gaseous objects resembling two giant turnips, each with
its apex pointing toward the star. The jets appear to be gas ejected perpendicular
to disks that apparently exist around the central star. Thus as stars form,
they paradoxically also eject matter back into space. The presence of a disk
in the equatorial plane of the star forces the ejected material into jets along
the polar axes of the spinning system of star and disk.
In the solar nebula, 99% of the mass was gas (mostly hydrogen and helium),
and the heavier elements that could exist as solids made up the remaining
1%. Some of the solids were surviving interstellar dust grains; others
were formed in the nebula by condensation. This gas played a major role in
Building a Habitable Earth
45
forming the sun, Jupiter, and Saturn. All of the other planets, the asteroids,
and the comets formed primarily from the solids. Solids were only a trace
component of the nebula as a whole, but they could undergo a concentration
process that gas could not. As the nebula evolved, dust, rocks, and larger solid
bodies separated from the gas and became highly concentrated, forming a
disk-like sheet in the mid-plane of the solar nebula, in some ways resembling
the rings of Saturn.
One of the fundamental processes that led to the production of planets
was accretion, the collision of solids and their sticking to one another to form
larger and larger bodies. This complex process involved the formation, evolution,
destruction, and growth of vast numbers of bodies ranging in size
from sand grains to planets. Most of the mass of a planet was accreted from
materials in its “feeding zone”—a ring section of the solar nebula disk that extended
roughly halfway to the nearest neighboring planets. If viewed from
above, the concentric feeding zones could be imagined as a target, with one
planet forming in each radial band. The composition of solids varied with distance
from the sun, so the nature of each planet was critically influenced by
its feeding zone.
The accretion process was responsible for unique and very important
aspects of Earth. An enigma of Earth’s formation is its composition and particular
location in the solar system. As we saw in Chapter 2, Earth formed
within the habitable zone of the sun. A grand paradox of terrestrial planets is
that if they form close enough to the star to be in its habitable zone, they typically
end up with very little water and a dearth of primary life-forming elements
such as nitrogen and carbon, compared to bodies that formed in the
outer solar system. In other words, the planets that are in the right place, and
thus have warm surfaces, contain only minor amounts of the ingredients necessary
for life. The accretion process accumulated solids from the nebula, but
the composition of solid dust, rocks, and planetesimals in the nebula varied
with distance from the sun. At Earth’s distance from the center of the solar
nebula (see Figure 3.2), the temperature was too high for abundant carbon,
nitrogen, or water to be bound in solid materials that could accrete to form
R A R E E A R T H
46
Jupiter Saturn
Mars
Earth
Venus
Mercury
asteroid belt
Uranus
Neptune
Pluto
Kuiper belt comets
Figure 3.2 The plan of the solar system. The rhythmic geometry of planetary spacings results from planet
formation from annular “feeding zones.” The asteroid belt and the Kuiper belt of comets are regions where planet
growth processes failed and original planetesimals are still preserved. The planet orbits are shown to scale, illustrating
how Earth and the other terrestrial planets occupy only the tiny central portion of the solar system.
(The sizes of the planets are magnified by a factor of 1000; otherwise, they could not be seen at the scale of
planetary orbits.)
planetesimals and planets. Ice and carbon/nitrogen-rich solids were too
volatile and had no means of efficiently forming solids in the warm inner regions
of the nebula. Thus Earth has only trace amounts of these volatile components,
compared to bodies that formed farther from the sun. An excellent
example is the case of the carbonaceous meteorites, thought to be samples of
typical asteroids formed between Mars and Jupiter. These bodies contain up
to 20% water (in hydrous minerals similar to talc) and up to 4% carbon. The
bulk of Earth, by comparison, is only 0.1% water and 0.05% carbon.
Building a Habitable Earth
47
Had Earth formed from materials similar those in the asteroid belt, farther
from the Sun, its ocean could have been hundreds of kilometers deep,
and its carbon content would have been higher by many orders of magnitude.
Both of these aspects would have resulted in a planet totally covered by water
and with vast amounts of CO2 in its atmosphere. The resulting greenhouse
heating would have produced Venus-like surface temperatures of hundreds of
degrees Celsius, and the surface would have been too hot for the complex organic
molecules used by living organisms to survive. Such a planet could have
developed more Earth-like conditions only if cataclysmic changes had resulted
in the loss to space of most of its oceans and most of its carbon dioxide,
and this seems highly unlikely. With even twice as much water, Earth
would have ended up as an abyssal planet entirely covered with deep blue
water—a true “water world”—and very few nutrients would have been available
in the energy-rich surface waters of the ocean.
If natural processes in the nebula had acted in a different way, a radically
different Earth might have resulted. For example, the reason why Earth
is so carbon-poor is that most of the carbon in the inner parts of the nebula
was in the form of carbon monoxide gas. Like hydrogen and helium, gaseous
components could not be incorporated. If a way had existed to convert
gaseous carbon into solids, then enormous amounts of carbon could have
been accreted, and carbon would have been the dominant Earth element. In
the cosmic abundance distribution, carbon is half as abundant as oxygen and
ten times as abundant as iron, magnesium, and silicon. A genuinely carbonrich
planet would be entirely different from Earth. Imagine a planet with
graphite on its surface and diamond and silicon carbide in its interior. Neither
of these compounds would allow volcanism or even chemical weathering.
Carbon-rich planets are presumably rare, but they probably do occur in exotic
planetary systems where oxygen was less abundant than carbon in the
planet-forming nebula.
The arrival of the “biogenic elements” on Earth is a matter of considerable
speculation, but it is likely that most of them came from the outer regions.
In the coldest outer regions of the nebula, water and nitrogen and carbon
compounds could condense to form solids. Presolar interstellar solids
R A R E E A R T H
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carrying the light elements were also preserved in this region. Although most
of these materials stayed in the outer solar system, some would ultimately
have reached Earth by scattering. When they passed near an outer planet,
their orbits about the sun could have been significantly altered, sometimes
sending them toward the sun, where they might collide with terrestrial planets.
Such gravitational effects from encounters with planets can cause asteroidal
and cometary debris, rich in light elements, to assume earth-impacting
orbits. This “cross-talk” caused some degree of mixing between different
feeding zones and provided a means of bringing the building blocks of life to
what might otherwise have been a lifeless planet lacking in many biogenic elements
because it formed too close to the sun.
The formation of the giant outer planets is thought to have been particularly
effective in scattering volatile-rich planetesimals from the outer regions
of the solar system into the inner solar system, the realm of the terrestrial
planets. Even today, material from the outer solar system impacts Earth.
Most of the mass is in particles a quarter-millimeter in diameter that are derived
from comets and asteroids. These materials carry not only carbon, nitrogen,
and water but also relatively large amounts of organic material, as
was first proved when extraterrestrial amino acids were discovered in the
Murchison meteorite that fell in Australia in 1969. Life on Earth formed
from organic compounds, and it is possible that prebiotic compounds from
the outer solar system stimulated the first steps toward the origin of life on
Earth. Thus the outer solar system not only provided the essential elements
for life but also may have given the complex organization of the chemical
processes of life a critical head start. (In the context of Rare Earth, this “seeding”
would not have been unusual for a terrestrial planet. It is reasonable to
expect that inner planets in all planetary systems are exposed to organic-rich
“manna” from the distant comet cloud systems that invariably surround their
central star.)
The scattering process that bestowed on Earth life-giving material from
the outer solar system also has a dark side. We have noted that the accretion
process never really ended. The rate is many orders of magnitude less than it
Building a Habitable Earth
49
was 4.5 billion years ago, but, as in any solar system where planets form by
accretion of solids, the process still goes on. The annual influx of outer solar
system material falling to Earth is 40,000 tons per year. This is mostly in the
form of small particles, but larger objects occasionally hit. The small-particle
flux is one 10-micron particle per square meter per day and one 100-micron
particle per square meter per year. The diameter of a typical human hair is
just under 100 microns. Larger objects are increasingly rare, but on the average,
an outer solar system object 1 kilometer in diameter randomly impacts
Earth every 300,000 years. Collision with a body this size, traveling at a
speed of well over 10 kilometers per second results in a very energetic impact
event. Every 100 million years, on average, a 10-kilometer object strikes
Earth. Such an impact can produce a transient crater tens of kilometers deep
and over 200 kilometers in diameter. It can eject enough fine debris into the
air to block sunlight from the entire Earth for months. Just such an impact
event killed all the dinosaurs on Earth 65 million years ago.
Early in the history of the solar system, the impact rate of very large objects
was much higher, and objects struck Earth that were as big as Mars
(about half the diameter of Earth). During the first 600 million years of Earth’s
history, there were impacts of bodies 100 kilometers in size that individually
delivered enough energy to heat and sterilize Earth’s surface down to depths
of several kilometers. The larger impacts would have vaporized the ocean and
parts of the crust. The occurrence of impacts that could cause global sterilization
raises an intriguing possibility: There may have been occasions when
all life on Earth was destroyed by a single impact. The intervals between devastating
impacts might have been long enough for life to form and again be
annihilated. If life forms easily and quickly when the conditions are right,
then life might have formed and been destroyed several times before the era
of sterilizing 100-kilometer and larger bodies finally ended. This effect has
been called “the impact frustration of the origin of life,” because life could not
permanently exist on Earth until major impacts had ceased. The giant impacts
essentially ended 3.9 billion years ago because most of the large impactors
had been swept up by planets, ejected from the solar system, or stored in
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50
distant orbits. Over the past 3.9 billion years, impacts have continued, but
not by bodies as large as 100 kilometers. Current impactors are comets and
asteroids perturbed, by gravitational effects of planets, from their reservoirs
in the asteroid and comet belts. The largest of these bodies can have calamitous
effects (the impact of a 10-kilometer body probably caused the extinction
of the dinosaurs), but they are too small to pose a threat of sterilizing the
entire Earth.
The final stages of Earth’s assembly process included the impact of several
very large objects. In Earth’s feeding zone, many celestial bodies were all
struggling to grow. During accretion, a given body in a feeding zone would
meet one of the following fates:
Growth by assimilation of others
Destruction by high-speed collision
Assimilation into a larger body
Ejection out of the feeding zone
The process resembled a brutal biological competition, and in the end,
only a single body survived to become Earth. In the final assembly stages,
however, many large bodies orbited within the feeding zone, some as large as
the planet Mars. The dramatic collision of these large bodies with the young
Earth played a role in determining the initial tilt values of Earth’s spin axis,
the length of the planet’s day, the direction of its spin, and the thermal state
of its interior. It is widely believed that the impact of a Mars-sized body was
responsible for formation of the Moon, an oddly large satellite relative to the
size of its mother planet.
The final composition of Earth had several crucial structural effects.
First, enough metal was present in the early Earth to allow formation of an
iron- and nickel-rich innermost region, or core, that is partially liquid. This
enables Earth to maintain a magnetic field, a valuable property for a planet
sustaining life. Second, there were enough radioactive metals such as uranium
to make for a long period of radioactive heating of the inner regions of the
planet. This endowed Earth with a long-lived inner furnace, which has made
Building a Habitable Earth
51
possible a long history of mountain building and plate tectonics—also necessary,
we believe, to maintaining a suitable habitat for animals. Finally, the
early Earth was compositionally able to produce a very thin outer crust of
low-density material, a property that allows plate tectonics to operate. The
thicknesses and stability of Earth’s core, mantle, and crust, could have come
about only through the fortuitous assemblage of the correct elemental building
blocks.
There is no direct information about Earth’s early history because no
rocks older than 3.9 billion years have survived. We can say with confidence,
however, that the period included episodes of great violence due to the effect
of giant impacts. The largest high-velocity collisions would cause heating
and actually resurface the planet. Cratering events of the magnitude of those
that created the major basins on the Moon (the large circular regions seen
without a telescope, including the eye of the “Man in the Moon”) may have
blown parts of the atmosphere into space. These events may have produced
truly horrific environments. Impacts that vaporize large amounts of water and
liberate carbon dioxide from surface rocks can lead to phenomenal greenhouse
effects. After the direct heating effects due to the kinetic energy of impact
have dissipated, the greenhouse gasses linger in the atmosphere and retard
the escape of infrared radiation. With its main cooling process blocked,
the atmosphere heats up. The greenhouse effect in the dense carbon dioxide
atmosphere on modern Venus produces surface temperatures of 450°C; it has
been estimated that the vast amount of gas injected into Earth’s early atmosphere
by giant impacts may have produced surface temperatures hot enough
to melt surface rocks!
It is the heritage of terrestrial life that violent events and truly hostile
environments preceded it. The violent events of these times may have determined
the final abundance of water and carbon dioxide, two compounds that
play crucial roles in the ability of Earth to maintain an environment where
life can survive. It is interesting to speculate about what would have happened
if the final abundance of these had varied. If Earth had had just a little
more water, continents would not extend above sea level. Had there been
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52
more CO2, Earth would probably have remained too hot to host life, much
like Venus.
F I N I S H WO R K
A process that strongly influenced the ultimate evolution of life on Earth was
the creation of the atmosphere, oceans, and land. Events involved in the formation
of all three were highly intertwined.
Without an atmosphere there would be no life on Earth. Its composition
over Earth history is one of the reasons why our planet has remained a
life-supporting habitat for so long. Today the atmosphere is highly controlled
by biological processes, and it differs greatly from those of other terrestrial
planets, which range from essentially no atmosphere (Mercury) to a
CO2 atmosphere a hundred times denser (Venus) and a CO2 atmosphere a
hundred times less dense (Mars). Even viewed from a great distance, Earth’s
strange atmospheric composition would provide a strong clue that life is present.
Composed of nitrogen, oxygen, water vapor, and carbon dioxide (in descending
order of abundance), it is not an atmosphere that could be maintained
by chemistry alone. Without life, free oxygen would rapidly diminish
in the atmosphere. Some of the O2 molecules would oxidize surface materials,
and others would react with nitrogen, ultimately forming nitric acid. Without
life, the CO2 abundance would probably rise, resulting in a nitrogen and CO2
atmosphere. To an alien astronomer, Earth’s atmospheric composition would
be clearly out of “chemical equilibrium.” This situation would provide convincing
evidence of life and a vigorous ecosystem capable of controlling the
chemical composition of the atmosphere. Telescopic detection of such peculiar
atmospheres is the basis of a strategy for detection of life outside the solar
system by the “Terrestrial Planet Finder”; we will discuss it in Chapter 10.
The atmosphere was formed by outgassing from the interior, a process
that released volatiles originally carried to Earth in planetesimal bodies as
well as by delivery from impacting comets. The composition and density of
the atmosphere are influenced by the amount and nature of the original acBuilding
a Habitable Earth
53
creted materials, but in Earth’s case they are most strongly affected by processes
that recycle atmospheric components in and out of the atmosphere.
The oceans are a by-product of outgassing and the formation of the atmosphere.
When the atmosphere was very hot, a great deal of it was composed
of steam. Gradually, as the early Earth cooled, the steam condensed as
water and formed the vast oceans we still see today. Although they were originally
fresh, the oceans became salty through chemical interactions with
Earth’s crust.
Land provides a home for nonaquatic life, and the vast regions of shallow
water that surround land offer crucial and complex habitats where
oceanic life can flourish. Shallow water is also a setting where interactions
between ocean and atmosphere alter the composition of the atmosphere.
Earth’s topography and the total amount of water determine what fraction of
Earth’s surface is land. The oceans contain enough water to cover a spherical
Earth to a depth of about 4000 meters. If the surface of the planet varied
only a few kilometers in elevation, Earth would be devoid of land. It is easy
to imagine an Earth covered by water, but it is difficult to imagine that, with
its present water supply, it could ever be dominated by land. To make more
land or even produce an Earth dominated by land, the oceans would have to
be deeper to accommodate the same volume of water in spite of having less
total surface area. Thus the planet’s remarkable mixture of land and oceans is
a balancing act.
Land formation on Earth has throughout its history occurred by two
principal means: simple volcanism creating mountains and the more complex
processes related to plate tectonics. Simple volcanism leads to the formation
of small islands such as Hawaii and the Galapagos archipelagos. Volcanic islands
similar to Hawaii were probably the predominant landform on the early
Earth. These were lifeless islands with no plant roots to slow the ravages of
erosion. Low islands would have been bleak and desert-like, sterile surfaces
bombarded by intense ultraviolet radiation from the sun unfiltered by Earth’s
early atmosphere. If climatic conditions were anything like today, the higher
islands would have had copious rainfall leading to extensive erosion. Although
Earth evolved beyond the stage where its only land consisted of eroding and
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54
doomed islands, many of the water-covered planets elsewhere probably only
have transient basaltic islands, at best. At worst they have no land at all.
In the case of our planet, Earth managed to form continents that could
endure for billions of years. This required the formation of land masses made
of relatively low-density materials that could permanently “float” on the
denser underlying mantle while parts of them extended above the sea.
How did the first continents form? Early continental land masses may
have formed when the impact of large comets and asteroids melted the outer
region of Earth to form a “magma ocean,” a planet-smothering layer of molten
rock. The concept of a global magma ocean grew out of studies of the Moon.
The heat generated from the rapid accretion of many planetesimals into our
solid Earth appears have melted the upper 400 kilometers of the Moon’s surface.
In the lunar case, as the magma ocean cooled, myriad small crystals of a
mineral called plagioclase feldspar (a low-density mineral rich in calcium, aluminum,
and silicon) formed and floated upward to create a low-density crust
nearly 100 kilometers thick. This ancient crust is still preserved and can even
be seen with the naked eye as the bright, mountainous lunar “highlands.” In
like fashion, a magma ocean on Earth, may have led to formation of the first
continents. Alternatively, the processes leading to the formation of the first
continental land may have occurred beneath large volcanic structures. The
initial land mass was small and was not until half way through its history that
land covered more than 10% of the Earth’s surface. In any event, the outcome
was a planet with both land and sea. This fortuitous combination may be the
most important factor that ultimately made life possible.
By about 4.5 billion years ago Earth was built. The next step was to
populate it—the subject of the next chapter.
Life’s
First Appearance
on Earth
One amino acid does not a protein make—let alone a being.
—Preston Cloud, Oasis in Space
Once life evolves, it tends to cover its tracks.
—John Delaney
The discovery of extremophilic microbes has radically changed our conception
of where life might be able to exist in the Universe—it causes
us to reassess the concept of habitable zones. Scientists now realize
that habitats suitable for microbial life are far more widely distributed in our
solar system, and surely in the Universe as well, than was considered possible
even in the most optimistic views of the 1980s and before. On the other
hand, these same studies are showing that complex life—such as higher animals
and plants—may have fewer suitable habitats than was previously
thought. But just because life could exist in a place doesn’t mean it is actually
there. Life can be widely distributed in the Universe only if it can come into
55
4
R A R E E A R T H
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being easily. In this chapter we will examine current knowledge and hypotheses
about how life may have first formed on Earth and in what type of
environment this may have taken place.
HOW DI D L I F E B E G I N ?
What really is life? And how do we recognize its formation? These questions
seem simple, but the answers are dauntingly complex. In its most commonsense
definition, life is able to grow, reproduce, and respond to changes in the
environment. By this definition, extremophiles are obviously alive, for example.
Yet many crystals can do as much, and they are clearly not life. The great
British biologist J.B.S. Haldane pointed out that there are about as many living
cells in a human being as there are atoms in a cell. Individual atoms themselves
are not alive, though. “The line between living and dead matter is
therefore somewhere between a cell and an atom,” Haldane concluded.
Somewhere in between atoms and the living cell there is the entity known
as a virus. Viruses are smaller than the smallest living cells and do not seem to be
alive when isolated (they cannot reproduce), yet they are capable of infecting
and then changing the internal chemistry of the cells they invade. Are they
alive? In isolation they do not seem to be, but in combination with the host they
very well may be. The boundary between living and nonliving is ambiguous at
these levels of organization. By the time we reach the level of organization
found in bacteria and archaea, however, we are sure that we have unambiguous
life. We are also sure that all life on Earth is based on the DNA molecule.
Deoxyribonucleic acid, or DNA, is predominantly composed of two
backbones that spiral around one another (the famous “double helix” described
by its discoverers, James Watson and Francis Crick). These two spirals
are bound together by a series of projections, much like steps on a ladder,
made up of the distinctive DNA bases adenine, cytosine, guanine, and
thymine. The term base pair comes from the fact that the bases always join up
in the same way: Cytosine always pairs with guanine, and thymine always
joins with adenine. The order of bases on each strand of DNA supplies the
Life’s First Appearance on Earth
57
language of life; these are the genes that code for all information about a particular
life form.
There might be many kinds of life elsewhere in the Universe, and there
is a great deal of speculation among scientists about whether DNA is the only
molecule on which life can be based or one of many. It is certainly the only
one capable of replication and evolution on Earth, and all life here contains
DNA. The fact is that all organisms on Earth share the same genetic code is
the strongest evidence that all life here derives from one common ancestor.
Was the rise of life inevitable on this planet? Let us perform a thought experiment:
If every environmental condition that ever existed on the Earth during
its 4.5-billion-year history were exactly reduplicated in the same order, would
life itself again evolve? And if it did, would it evolve with DNA as its crux?
The formation of this complex molecule is thus the starting point for
any discussion of life’s history on this planet—and perhaps on any other.
There may indeed be other ways to produce life; one would be a system
where ammonia, rather than water, is the solvent necessary for life. This route
may even have been followed, only to be erased later, probably because water
is a better solvent than ammonia. (Solvents are a rather humdrum yet essential
ingredient in life’s recipe. Many of the chemicals necessary for life can be
delivered into a cell only in solution, and for that a solvent is needed.) Thus
“DNA life” may be either the only type of life that formed or the only survivor.
Life seems to have appeared on this planet somewhere between 4.1 and
3.9 billion years ago, or some 0.5 to 0.7 billion years after Earth originated.
However, the fact that no fossils were preserved at this time in Earth’s history
clouds our understanding of life’s earliest incarnation. The oldest fossils that
we do find are from rocks about 3.6 billion years of age, and they look identical
to bacteria still on Earth today. There may have been earlier types of life
that are no longer represented on Earth, but our present knowledge suggests
that bacteria-like forms were the first to fossilize.
The Earth formed about 4.5 to 4.6 billion years ago from the accretion of
variously sized “planetesimals,” or small bodies of rock and frozen gases. For the
first several hundred million years of its existence, a heavy bombardment of
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58
meteors pelted the planet with lashing violence. Both the lava-like temperatures
of Earth’s forming surface and the energy released by the barrage of incoming
meteors during this heavy bombardment phase would surely have created
conditions inhospitable to life. As we recounted in the last chapter, this
constant rain of gigantic comets and asteroids would have driven temperatures
high enough to melt surface rock. No water would have formed as a liquid on
the surface. Clearly, there would have been no chance for life to form or survive
on the planet’s surface. It was hell on Earth.
As we showed earlier, the new planet began to change rapidly soon
after its initial coalescence. About 4.5 billion years ago, Earth began to differentiate
into different layers. The innermost region, a core composed
largely of iron and nickel, became surrounded by a lower-density region
called the mantle. A thin crust of still lesser density rapidly hardened over the
mantle, while a thick, roiling atmosphere of steam and carbon dioxide filled
the skies. In spite of its being waterless on the surface, great volumes of water
would have been locked up in Earth’s interior, and water would have been
present in the atmosphere as steam. As lighter elements bubbled upward and
heavier ones sank, water and other volatile compounds were expelled from
the interior and added to the atmosphere.
The heavy bombardment by comets and asteroids lasted more than half
a billion years and finally began to diminish around 3.8 billion years ago as
the majority of debris was incorporated into the planets and moons of our
solar system. During the period of heaviest impact, the steady bombardment
would have scarred our planet by craters in the same manner as the moon. Yet
the comets and asteroids raining in from space delivered an important cargo
with each blow. Some astronomers believe that much, or even most, of the
water now on our planet’s surface arrived with the incoming comets; others
think that only a minority of Earth’s water arrived in this fashion.
Comets are made up of dust and volatiles, such as water and frozen carbon
monoxide, and there is no doubt that a good many of them hit the early
Earth. These cargoes of water slamming into Earth would have turned instantly
to steam. The dense early atmosphere remained hot for hundreds of
millions of years. Perhaps 4.4 billion years ago, its surface temperatures might
Life’s First Appearance on Earth
59
have dropped sufficiently, and for the first time liquid water would have condensed
from steam onto our planet’s surface, successively forming ponds,
lakes, seas, and finally a planet-girdling ocean. The study of ancient sedimentation
suggests that by slightly less than 3.9 billion years ago, the amount of
oceanic water on Earth may have approached or attained its present-day
value. But these were not tranquil oceans or oceans even remotely similar to
those of today.
We have only to look at the Moon to be reminded of how peppered
Earth and its oceans were during the period of heavy impact, between 4.4 and
3.9 billion years ago. Each successive, large-impact event (caused by comets
larger than 100 kilometers in diameter) would have partially or even completely
vaporized the oceans. Imagine the scene if viewed from outer space:
the fall of the large comet or asteroid, the flash of energy, and the evaporation
of Earth’s planet-covering ocean, to be replaced by a planet-smothering
cloud of steam and vaporized rock heated (at least for some decades or centuries)
far above the boiling point of liquid water. It is difficult to conceive of
life—whatever its form—surviving anywhere on the planet during such
times, unless that survival occurred deep underground.
Scientists have made mathematical models of such ocean-evaporating impact
events. The collision with Earth of a body 500 kilometers in diameter results
in an almost unimaginable cataclysm. Huge regions of Earth’s rocky surface are
vaporized, creating a cloud of “rock-gas” several thousand degrees in temperature.
It is this superheated vapor, in the atmosphere, that causes the entire ocean
to evaporate into steam. Cooling by radiation into space would take place, but a
new ocean produced by condensing rain would not fully form for thousands of
years after the event. Much of the revolutionary detective work behind these
conclusions was described in 1989 by Stanford University scientist Norman
Sleep, who realized that the impact of such a large asteroid or comet could evaporate
an ocean 10,000 feet deep, sterilizing Earth’s surface in the process.
How ironic that the comets may have brought some of Earth’s life-giving,
liquid water—a prerequisite for life—and then snatched that gift away for a
time with each successive large-impact event. Yet it is not only water that these
comets may have brought. They could have played a role in determining the
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60
chemical evolution of Earth’s crust. And they may have contributed another ingredient
to the recipe for what we call life: They may have brought organic
molecules—or even life itself—onto our planet’s surface for the first time.
If some time machine made it possible to visit the Earth of about 3.8 billion
years ago, at the end of the period of heavy bombardment, our world
would surely still appear alien to us. Even though the worst barrage of meteor
impacts would have passed, there still would have been a much higher frequency
of these violent collisions than in more recent times. The length of
the day was shorter, because Earth was rotating far faster than it does now.
The sun was much dimmer, perhaps a red orb supplying little heat, for it not
only was burning with less energy than today but also had to penetrate a poisonous,
turbulent atmosphere composed of carbon dioxide, hydrogen sulfide,
steam, and methane. In such an environment, we would have had to wear
spacesuits of some sort, for only traces of oxygen were present. The sky itself
might have been orange to brick red in color, and the seas, which surely covered
all of the planet’s surface except for a few scattered, low islands, would
have been muddy brown and clogged with sediment. Yet perhaps the greatest
surprise to us would be the utter absence of life. No trees, no shrubs, no
seaweed or floating plankton in the sea; it would have seemed a sterile world.
Somehow, the fact that we have not yet detected life on Mars seems consistent
with its satellite images. A waterless world fits our picture of a lifeless
world. Even when the young Earth was covered with water, however, it was
still devoid of life. But not for long.
Life on other planets, a real possibility ? on Mon Oct 19, 2009 9:37 pm
