Download An Argument for the Cometary Origin of the Biosphere

An Argument for the Cometary Origin
of the Biosphere
The evidence suggests that a rain of comets brought the Earth its water, its
organic molecules and its atmosphere-key ingredients for l$e’s beginnings
Armand H. Delsemme
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American Scientist, Volume 89
Figure 1. The young Earth appears to have
been bombarded by comets for several hundred million years shortly after it was
formed. This onslaught, perhaps involving
hundreds of millions of comet impacts, is
currently the best explanation for the origin
of the Earth’s oceans, atmosphere and organic molecules. Although historically a controversial idea, there is now a considerable
amount of physical and chemical evidence
supporting the theory. Here an artist’s
impression of an impending collision
between a fragmenting comet nucleus and
the newly formed Earth offers a glimpse of
that ancient era.
Armand H. Delsemme is an internationally YCCognized comet scientist who until 2988 was a
professor ofastrophysics at the University uf
Toledo, Ohio. He has made fundamental contributions to OILI understanding of the origins and
chemistry of comets, and has received many honorsfoy his work, including the Kuiper Prizefrom
the Division of Planetary Sciences of tke AmeYican Astronomical Society in 1999. A Sigma Xi
member and an avid skier, he celebrated his 80th
birthday in 1998 atop Vail Mountain in Colorado nt an elevation of 11,400 feet (which
brought him just a little closer to the comets).
Address for Delsemme: 2509 Meadowwood
Drive, Toledo, Ohio 43606.
he first 600 million years of our
planet’s history have been erased
from its surface. Between the time it
was formed about 4.6 billion years
ago and the formation of the oldest
known sedimentary rocks, which are
about 4 billion years old, the Earth
changed from a hot, dry little rock to a
world with an ocean and an atmosphere-a planet that was primed for
the origin of life. Those missing years
hold the key to everything we hold
dear on the surface of our planet. Remarkably, we are still not sure exactly
T
what took place during that time.
The subsequent evolution appears to
have been unique in our solar system.
None of the other planets is covered
with an ocean of liquid water, and
none of their atmospheres is especially
receptive for the evolution of life. Most
of the water on Mars is locked in its polar ice caps, and only recently has any
water at all been found on the Moonfrozen on its poles. Mercury and Venus
are both much too hot, and the outer
planets and moons are much too cold.
There is reason to believe that at least
one of Jupiter’s icy moons, Europa,
may harbor an ocean of water, but it
must be at some depth below the surface where the pressure and temperature could accommodate the liquid
state. All told, the Earth appears to
have been in just the right spot. But in
the right spot for what, exactly?
The question bears on the larger issue of whether life might exist on planets in other stellar systems as well.
Dozens of extrasolar planets have been
discovered so far, and it’s becoming
clear that their formation around other
2001 September-October 433
stars may be quite common. But
whether the formation of euvtklike planets is common, is another question.
Would an Earth-sized planet, formed
at just the right distance from a Sunlike star, always develop an ocean and
an atmosphere friendly to life? What
else might be necessary?
Planetologists have been struggling
with such questions for decades, and I
now believe we have found some answers. There is little doubt in my mind
that our oceans and our atmosphere
were delivered on the backs of comets
that bombarded the newly formed Earth
in its first few hundred million years.
What is more, the comets also appear to
have brought prebiotic molecules-organic building blocks that could be used
to get life started. These ideas have a
fairly long history but have been resisted
for various reasons over the decades. I
have been studying the chemistry of
comets for more than 50 years, and I admit that early in my career I too was reluctant to accept the possibility that
comets had played such a crucial role in
our planet’s history. But the evidence
has continued to accumulate over the
decades, and it now seems irrefutable.
Here I provide an overview of the reasoning behind this extraordinary idea.
A World Without Water
How do we know that the infant Earth
did not come fully formed with its continents, oceans and atmosphere? Surprisingly, the answer lies within small
rocks that fall from the sky. Certain meteorites, the variety known as ckond&es, are relics from the era when the
solar system was first formed. Examine a chondrite under a microscope
and you’ll see that it consists of very
fine dust grains of various compositions, gently compressed together to
form a “sedimentary” rock. The sedimentation of chondrites did not take
place in a river or an ocean, however.
Rather, these rocks were formed in the
The general course of events in the
formation of an accretion disk in the early solar nebula is now reasonably well
understood (see Protostaus, July-August,
and How Were the Comets Made?
May-June). Although the gas and dust
that formed the protosun and its circumstellar disk were originally quite
cold, the process of accretion heats the
materials as they fall into the gravitational well of the newly forming solar
system. This heating was modeled in
1978 by Alastair Cameron of the Smithsonian Astrophysical Observatory,
who showed that the temperature of
the disk in the zones where the planets
are formed reaches a maximum just before the dust sediments into the midplane. This is because mass infall induces both heating and turbulence.
When mass infall stops, gas turbulence
subsides, and dust, which is no longer
carried by gas eddies, then sediments
onto the midplane.
Figure 2. Evolutionary highlights of the Earth’s biosphere can be described by a few crucial events in its 4.6 billion-year history. The process
begins with the settling of dust in the accretionary disk of the protosolar system (a). The dust accretes into ever larger pieces, eventually
forming a hot, but dry, rock-the protoearth-after 40 million years (b). When the system is merely 50 million years old, a grazing collision
between the protoearth and a Mars-sized body results in the Moon’s formation and the loss of all volatiles and water brought by an early
cometary bombardment Cc). The heavy bombardment continues for at least the next 600 million years, with comets bringing water, atmospheric gases and prebiotic organic molecules to our planet Cd). Some of the oldest known sedimentary rocks-indicating the presence of
water-are formed when the Earth is merely 800 million years old; the sediments contain the first chemical evidence for life (e). About 4.6
billion years after the solar system began to form, an intelligent life form appears ($I.
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American Scientist, Volume 89
h
0.8 1.0 1.3
2.0 2.6
4.0
6.0 8.0
distance from protosun (astronomical units; log scale)
Figure 3. Chrondritic meteorites, such as this ordinary chondrite from Bjurbiile, Finland (right), offer clues to the nature of the accretionary
pieces-the planetesimals-that formed the protoearth. The chemical composition of ordinary chondrites and carbonaceous chondrites allows
scientists to determine the temperatures at which they were formed. A curve Cleft showing the decrease in temperature with increasing distance from the protosun can then be derived from a knowledge of where the chondrites were formed in the accretionary disk. Such a curve
shows that the dust in the zone of the accretionary disk where the Earth was formed reached temperatures between 900 and 1,400 kelvins-and
so was completely devoid of volatile materials, such as water and organic molecules. Although the Bjurbole meteorite was formed at even
cooler temperatures (but no less than 450 kelvins), it is similarly free of volatile components. The water, gases and volatile organic molecules
on the Earth must have been delivered to the protoearth from the outer solar system. Comets from the zone where Jupiter was formed, about 5
astronomical units from the protosun, are likely sources. (One astronomical unit is the average distance between the Sun and the Earth.)
eventually coalesce to form the planetary bodies. The chondrites are remnants of the circumstellar disk that
were not incorporated into a planet,
and so escaped the forces (heat and
gravity) that would have destroyed
their “pristine” state. Because of this,
the composition of a chondrite tells scientists much about the processes that
must have taken place in the protosolar
accretion disk.
When we take a closer look at the
chondritic meteorites, we notice three
general varieties: enstatite chondrites,
ordinary chondrites and carbonaceous
chondrites. The enstatite chondrites are
not relevant here. Ordinary chondrites
and carbonaceous chondrites are identical except for the depletion of volatile
elements (such as carbon, lead and bismuth) in the ordinary chondrites. The
difference indicates that the ordinary
chondrites experienced some heating
that cooked away the more volatile
components. The identity of the depleted elements also serves as a temperature gauge. Edward Anders of the University of Chicago and his colleagues
have shown that these volatile elements
are vaporized at temperatures above
450 kelvins. This temperature separates
the ordinary chondrites from their cooler, carbonaceous cousins.
So where in the disk can we place
this 450-kelvin boundary line between
the two classes of chondrites? Fortunately, we have a good idea where the
chondrites came from before they
landed on the Earth. There’s little
doubt that they are pieces of asteroid
families that lie in the region between
Mars and Jupiter. Scientists have been
able to take this a step further and deduce that most of the carbonaceous
chondrites come from the dark, C-type,
asteroids, whereas the ordinary chondrites are pieces of the brighter, S-type,
asteroids. Using the orbital statistics of
C and S asteroids, I was able to show
that the two classes of chondrites were
originally separated at a distance of 2.6
astronomical units (AU) from the protosun. (One AU equals the average distance between the Earth and the Sun.)
This tell us that the protosolar disk had
a temperature of 450 kelvins about 2.6
AU from the protosun just as the dust
sedimented into the midplane.
Using this information, we can extrapolate a temperature gradient for
the accretion disk at various distances
from the protosun (Figure 3). It’s a negatively sloping curve, based on the
thermal equilibrium of the solar nebula, with the temperatures increasing as
one gets closer to the protosun and de-
creasing as one moves farther away. In
the region where the primordial Earth
formed (between 0.8 and 1.3 AU from
the protosun), the accreting grains of
dust reached temperatures between
900 and 1,400 kelvins. Such searing
temperatures would have thoroughly
degassed these materials, leaving dust
grains almost solely consisting_,of silicates and reduced iron. (Not surprisingly, iron and silicate-bearing rocks
are the primary components of the
deep Earth.) The volatiles in the zone
of the protoearth’s formation were effectively separated from the solid components. All of the carbon was in carbon monoxide, all nitrogen in gaseous
N, and all of the water in hot steam.
No organics could have been present
in the dust.
Could these volatiles and organics
have been incorporated into the protoearth during the planet-building
process? This seems unlikely. In the
earliest stages, as the dust grains came
together to form larger pieces (about a
kilometer in diameter), the sheer size
of the planetesimals would have prevented any gas absorption. At an intermediate stage of accretion when the
protoearth was about 10,000 kilometers
across-about 83 percent of its final
mass-growing gravitational interac2001 September-October 435
Moon formation
theoretical
impact rate
0
0.5
1 .o
1.5
2.0
solar system age (blllions of years)
2.5
lunar highlands (Zej?), offer evidence of an era when the inner solar system was subject to a heavy bombardment of small bodies. The impact rate (deduced from the density of the craters) and their age (based on
radioactive dating of lunar rocks) conform to a theoretical model of cometary bombardment (right, black cuwe), which is based on changing
rates of cometary flux to the inner solar system from the regions of the giant planets in the outer solar system (colored lines). The observational data (crosses) suggest that the first 600 million years of bombardment can be explained by a large flux of comets from Jupiter’s zone. After
one billion years the excess impacts (crosses above the black cuwe) indicate another source, possibly asteroids. (Image courtesy of NASA.)
tions between ever larger planetesimals would have begun to stir up the
contents of the protoplanetary disk.
Materials from as close in as 0.4 AU
and as far away as 2.6 AU could have
been mixed into the forming planet.
Yet even this radial mixing of components could not have contributed
volatiles to the protoearth. As we’ve
seen, materials from a distance of 2.6
AU, cooked to 450 kelvins, were devoid of water and gas, much as are ordinary chondrites.
We must, however, account for the
final 17 percent of the Earth’s mass,
which roughly corresponds to the upper mantle. This material represents the
last stages of the planet’s accretion.
Theoretical considerations suggest that
a few relatively large, asteroid-like objects may have collided with the protoearth during this time. Some of these
large projectiles could have come from
beyond 2.6 AU and would have contained volatile-rich carbonaceous materials. Could such objects have brought
the Earth its water?
The composition of the upper mantle
can help us answer this question. In
1983, Heinrich Wanke, of the Max
Planck Institut fiir Chemie in Mainz,
and his colleagues analyzed the composition of the mantle based on the material spewed up in a volcanic eruption.
They found that volcanic ejecta are typically enriched in heat-resistant ele436
American Scienhst, Volume 89
ments by a factor of 1.3, but depleted
of moderately volatile elements by factors of 0.1 to 0.2, and depleted of very
volatile elements by factors of 0.01 to
O.OOOl! This is effectively the opposite
of what one would expect if a large,
carbonaceous chondritic asteroid had
hit the protoearth gently enough not to
produce any ejecta. Such a depletion of
volatiles can only be explained by the
intense heating produced by the collision that formed the Moon.
Even if an asteroid had made a contribution to the Earth’s upper mantle, it
could not have brought the world its
oceans. In 1991, I calculated that a giant
asteroid-about l,OOO-kilometers across
(about twice as massive as Ceres, the
largest known asteroid)-could not
have brought more water than a few
percent of the oceans’ mass. All of these
studies suggest that the oceans and the
atmosphere must have come after the
primordial Earth had acquired most of
its mass.
A Heavy, Heavy Rain
Anyone who has looked at the Moon
through a telescope cannot help but be
impressed by its pockmarked appearance. Its surface is covered with craters
caused by the hammer-like blows of
impacting objects. Photographs from
the orbiting Apollo missions reveal
that the far side of the Moon is even
more heavily cratered. The Moon is
not unique: Spacecraft missions to
Mars and Mercury have shown that
these planets are also heavily cratered.
Moreover, the craters on all three bodies are remarkably similar with respect to their sizes and densities, suggesting that all experienced the same
heavy bombardment at some point in
their histories. Despite the relative absence of impact craters on the Earth,
its proximity to these other planets
leaves little doubt that it too must
have faced such a period of bombardment. The question is, when did these
impacts take place, and what could
have caused them?
Fortunately, we needn’t rely solely on
theory to assess the age of these impacts. Lunar rocks, brought back by the
Apollo and Luna missions, have been
dated by their radioactive isotopes-a
measure of when the rocks solidified.
The ages of the rocks, taken from various regions of the Moon, correspond to
the time when these areas were covered
with molten lava, and so had obliterated any trace of previous craters. By
counting the density of the craters in
these regions, we can assess the intensity of the bombardment and its age.
These types of data suggest that the lunar impact rate was most intense during the first 600 million years of the
Moon‘s history (Figuue 4). Since the
Moon was formed merely 50 million
years after the Earth’s formation (4.56
billion years ago), this 600-million-year
period also corresponds to the missing
era in the primordial Earth’s history.
The intensity of the bombardment
during this period is simply too large
to be explained by planetesimals from
the inner solar system-most of these
would have been already incorporated into the planets themselves. The
long duration of the impact flux also
implies that the bombarding projectiles had to come from farther out, and
a clue to their identity comes from the
work of the Russian astronomer
Viktor Safronov.
In the late 196Os, Safronov demonstrated that in the final stages of their
growth, the giant planets, especially
Jupiter, tugged on the planetesimals
in the outer parts of the solar system,
sending them on hyperbolic trajectories into interstellar space. Since the
planetesimals were ejected in random
directions, a fair number of them
must have passed through the inner
solar system. In doing so, some of
them inevitably would have collided
with the inner planets, including the
primordial Earth.
The nature of these planetesimals
can be deduced by their location in the
accretion disk. At Jupiter’s orbital distance (and beyond), the temperature
was so low that the dust grains never
lost their frosty cover of water ice and
organic volatiles, originally acquired in
interstellar space. When they accreted
into larger planetesimals they formed
the icy bodies that we now call comets.
Indeed, a remnant population of ejected comets still exists in the form of the
Oort Cloud, an extended sphere of
comets that encloses our solar system
at a distance as far as 1,000 times the
orbit of Pluto. All of the Oort Cloud
comets, still gravitationallly bound by
the Sun, originally came from the zone
of the accreting giant planets. Their existence is a testimony to that era.
So comets must have peppered the
Earth early in its history, but could they
have brought enough water to fill the
world’s oceans? Safronov’s model predicts the mass of the scattered comets to
be about 6 or 7 times the masses of the
giant planets’ solid cores. This equates
to nearly 400 Earth masses’ worth of
comets in the region between Jupiter
and Neptune. Given that roughly half
the mass of a typical comet is water,
there must have been thousands of
frozen oceans in orbit beyond Jupiter.
One estimate holds that it would have
2 percent
Figure 5. The evolving cores of the four giant planets scattered many of the comets in the
early solar system with their gravitational energy. Theoretical considerations suggest that the
bulk (about 80 percent) of the comets that hit the protoearth originated in Jupiter’s zone,
whereas 12 percent came from Saturn’s zone, 6 percent came from Uranus’s zone and only 2
percent came from Neptune’s zone (see also Figure 4). The vast majority of the comets that
came within Jupiter’s influence where ejected from the solar system, whereas most of the
comets between Saturn and Neptune were launched into a distant repository-the Oort
Cloud (not shown)-that surrounds our solar system.
fold deuterium enrichment
.
‘$
; . *
”
.
q*
. .
l
l
Jupiter zone
*
extra
coating
of ice
sixteenfold
deuterium enrichment
du
grain
ice
“outer zone”
Figure 6. Cometary ice grains formed in Jupiter’s zone (left) contain less deuterated water
(HDO) than do ice grains formed in the “outer zone” (vi&t) between Saturn and Neptune.
This can be explained by the temperature differences in the two zones, which change the
kinetics of the neutral-isotope reaction that forms deuterated water. The Jupiter-zone ice
grains received an extra coating of ice, formed from the condensation of deuterium-depleted water from the inner solar system. The snow formed from the condensing steam
approaches a sixfold enrichment of deuterium (compared to the levels of deuterium in the
solar nebula) by way of a neutral-isotope reaction. The outer-zone comets contain water
with a sixteenfold enrichment of deuterium, formed by way of ion-molecular reactions in
interstellar frost. Most of the Earth’s seawater came from Jupiter-zone comets, and so has
proportionally less deuterium than comets originating in the outer solar system.
2001 September-October 437
taken a million comet impacts to account for all the seawater on our planet.
Could there have been that many?
More, actually. My calculations, based
on Safronov’s model, suggest that the
comets brought 5.8 times the amount
of seawater now in our oceans and 680
times the gas in our atmosphere!
From a dearth of volatiles to an overabundance. How do we account for
the “missing” water and gases? The
mystery is solved if one considers the
velocity of the impacting objects.
Hurled by the gravitational energy of
the giant planets, and accelerated by
their fall toward the sun, the comets
would have hit the Earth with an average speed of more than 42 kilometers
per second. This greatly exceeds the escape velocity of our planet-about 11.2
kilometers per second-so that the
whole process must have thrown several oceans and hundreds of atmospheres back into space.
Chemical Evidence
If comets did bring the Earth its oceans
and its atmosphere, then we might expect some chemical evidence of their
contribution on the surface of our planet. Our seawater, the air we breathe
and even the ground we walk on may
well have some trace of the massive
bombardment-even though it occurred more than 4 billion years ago.
The trick lies in determining what it is
that we should find. It turns out to be
less straightforward than we might
have hoped.
Figure 7. Comets are dirty snowballs-rich
in water, organic molecules and silicate
rocks-precisely the components needed to
build a biosphere.
Consider the first piece of evidence,
which involves the simple hydrogen
(H) atom. Garden-variety hydrogen
merely contains a single proton in its
nucleus. But its more exotic cousin,
deuterium (D), holds one proton and
one neutron-forming an unstable
isotope that burns into helium at stellar temperatures. The deuterium atom
is interesting because every natural bit
of it was created in the Big Bangabout 20 to 30 parts per million (ppm)
hydrogen atoms. It’s not easy to make
deuterium any more, and its abundance has been diminishing over the
eons to the point where the local interstellar clouds only have about 5 to 15
seawater
I
atmosphere
Jupiter-zone comets
chondritic
silicates
0.2 kilometers
ppm deuterium. When the solar system was formed 4.56 billion years ago,
the deuterium abundance was still
about 20 ppm, as demonstrated by its
presence in the atmospheres of Jupiter
and Saturn, which were captured
from the gas of the solar nebula before
it dissipated. Earthly seawater contains 160 ppm of deuterium, an enrichment of eight times relative to the
deuterium in the solar nebula. So we
would expect the comets to be similarly enriched, no?
No. As it happens, the water in the
three bright comets-Halley, Hyakutake and Hale-Bopp-that recently
swung past the Earth averaged about
320 ppm deuterium, an enrichment of
16 times over the deuterium in the solar nebula, and twice that found in
seawater! How do we account for
these differences?
We must again return to Safronov’s
model of how the newly forming giant planets scattered the comets. Because of their differing locations and
masses, the planets flung the comets
in their vicinity in different directions
and speeds. My calculations show
that the largest planet, Jupiter, accelerated the nearby comets to more than
35 kilometers per second, effectively
sending most of them out to interstellar space, beyond the grasp of the
Sun’s gravity. The comets accelerated
by the other giant planets-Saturn,
Neptune and Uranus-did not attain
sufficient velocities to escape the Sun
and so became trapped in the Oort
0.5 kilometers
carbon
compounds
1.21 kilometers
0.1 kilometers
Figure 8. Cometary contributions to the protoearth would have covered the planet with a thick layer of materials if much of it hadn’t been
ejected back into space. Comets from Jupiter’s zone provided the bulk of our seawater and our atmosphere, whereas the outer-zone comets
made much smaller contributions to these components of the biosphere.
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American Scientist, Volume 89
oxygen
29.0
26.0
30
31
carbon
6.4
10.5
13
IO
nitrogen
1.4
2.4
1
2.7
sulfur
0.06
0 13
0.8
0.3
phosphorus
0.12
0.13
calcium
0.23
(0.08)
-
Figure 9. Earth’s biosphere (left, is rich in the elements present in interstellar frost and comets-suggesting a cosmic origin. The proportions
of the various elements in living organisms and cosmic sources are remarkably similar, except for those of calcium, which is a recent innovation of “highly evolved” animals. Phosphorus has not been observed in comets, but should be present in cometary snows at cosmic abundance levels (shown here). The aquatic biosphere is represented by the concentration of chlorophyll a (red at maximum to violet at minimum),
whereas the land’s biosphere is represented by a “vegetation index ” (blue-green at maximum to orange at minimum). (Image courtesy of
SeaWiFS Project, NASA/Goddard Space Flight Center and ORBIMAGE.)
Cloud. In fact, we can calculate that
the three outer giants contributed
about 96 percent of the Oort Cloud’s
cometary mass, whereas comets from
the vicinity of Jupiter only account
for the remaining 4 percent. The
same model predicts that 80 percent
of the comets that hit the Earth (after
the Moon was formed) came from
Jupiter’s orbit, whereas only 20 percent came from the orbits of the three
outer planets (Figure 5).
These differences help to explain
the paradoxical values of deuterium
in seawater and the three bright
comets. Simply put, the comets that
contributed the bulk of the Earth’s
seawater are different. They were
formed at a higher temperature, in the
zone of Jupiter’s orbit, where they acquired more water but less deuterium
than the comets formed at much lower temperatures in the zones of the
outer giant planets. The Jupiter-zone
comets are enriched in water because
here the icy grains create a “cold
front” that condenses any steam vaporized in the hotter zones of the inner solar system. This steam carries
much less deuterium than the interstellar frost and so dilutes the relative
proportion of this isotope in the
Jupiter-zone comets. The extra steam
also adds an extra layer of ice to the
dust grains before they accrete onto
the comets, and so increases their total
content of water (Figure 6).
There are several ways in which
water can be enriched in deteurium
relative to interstellar hydrogen. One
reaction involves the exchange of
neutral isotopes:
H,O+HD=HDO+H,
This reaction is extremely sluggish
at very cold temperatures. However,
there are many other ways to produce
deuterated water, which involve ionmolecular reactions. Charge exchanges
between ions and molecules easily
cross the thermal-energy barriers, and
so easily exchange deuterium with hydrogen, as for instance in:
HD’ + H,O = HDO+ + H,
Reactions of this type are fast even
at temperatures close to absolute zero,
hence they prevail in interstellar space
and help to explain why the frost that
covers interstellar dust grains is so enriched in deuterium. Although the degree of enrichment depends on the local interstellar temperature, it is
generally about 10 to 20 times the deuterium levels of the solar nebula. This
is consistent with the sixteenfold enrichment seen in the three bright
comets, and it suggests that they must
have formed at very low temperatures.
It also suggests that the outer zones
never experienced temperatures that
would have altered the relative enrichment of the interstellar grains.
Near the zone of Jupiter’s formation,
however, the temperatures are considerably warmer, about 225 kelvins. This
zone corresponds to thefvost line of the
accretion disk, where steam condenses
to form snow. Laboratory experiments
at this temperature show that deuterated snow approaches a sixfold enrichment in the presence of hydrogen. The
comets formed in Jupiter’s zone would
have had less than half the deuterium
of the outermost comets.
Now, if we consider the proportions
of comets that came from Jupiter’s
zone (80 percent, with a sixfold enrichment) and from the outermost planets
(20 percent, with a sixteenfold enrichment), the eightfold enrichment of
deuterium in our seawater is entirely
consistent with a cometary origin. The
paradox is not only solved, but theory
actually predicts it!
Among the comets we observe
nowadays, only four percent of the
long-period comets and none of the
short-period comets originated in
Jupiter’s zone. However, a Jupiterzone comet may have passed by the
Earth recently. Mike Mumma of the
NASA Goddard Space Flight Center
and his colleagues have shown that
Comet C/1999 S4 LINEAR contains
four to five times more water (in proportion to cometary volatiles) than
most other comets, and the authors argue that its unusual composition may
indicate its formation in Jupiter’s zone.
Unfortunately, C/LINEAR was very
fragile, breaking into fragments that
vaporized in just a few days-before
its deuterium to hydrogen ratio could
be measured.
The comet-bombardment scenario
also helps to explain the abundances of
2001 September-October
439
two other chemical markers-krypton
and xenon-in our atmosphere. Because these inert gases don’t combine
with the other elements, and their large
atomic masses prevent them from escaping the Earth’s gravity, their atmospheric abundances reflect a primordial
composition. Compared to solar abundances, however, their isotopic proportions are anomalous, suggesting a fractionation process in which some
isotopes are enriched relative to others.
The xenon in our atmosphere is enriched in the heavier isotopes, but the
krypton isotopes are not similarly enriched. This decoupled fractionation
cannot be explained by any evolutionary history on the Earth.
There is, however, a cosmic explanation. Laboratory studies by Akiva
Barn-Nun and his coworkers, of Tel
Aviv University, show that krypton
and xenon can be trapped within crystalline clathrate hydrates or, at very
low temperatures (with practically the
same results), within amorphous ice.
These studies suggest that the krypton
and xenon in the Earth’s atmosphere
rigure 10. Some ot the oldest known sedimentary rocks (lower lejt) can be found on Akilia
island (top) of West Greenland. The very same sediments hold apatite crystals (lower right)
containing organic carbon that appears to have been produced by living organisms. The
sediments are at least 3.83 billion years old. (Images courtesy of Stephen Mojzsis,
University of Colorado.)
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were trapped within amorphous ice
formed at about 30 to 75 kelvins-temperatures found in the comet-forming
zones of Uranus and Neptune. On the
other hand, the temperatures near the
comet-forming zones of Jupiter (225
kelvins) and Saturn (130 kelvins) were
not cold enough to trap xenon and
krypton. The relative paucity of comets
from Uranus and Neptune thus helps
to explain the very low abundance of
xenon and krypton in our atmosphere. Accurate measures of the isotopic gas fractions in comets of different origins should eventually resolve
any lasting doubts.
The last piece of chemical evidence
I’ll consider here lies in the Earth’s
crust and upper mantle. Much as oil
and vinegar separate according to their
densities in a bottle of salad dressing
(with the lighter oil on top), the core
and mantle of the Earth are layered according to their densities. The separation of these layers dates to the earliest
period of the Earths formation, when
it was still accumulating mass by the
accretion of planetesimals. The energy
of the accretionary impacts was transformed into a heat so intense that
Earth’s surface was covered with a
thick layer of molten lava, perhaps to
very great depths. This liquid state provided a medium for differentiation:
The heaviest elements-notably ironfell towards the nascent planet’s core.
As the iron sank into the planet’s
depths, it would have carried siderophile (“iron-loving”) metals, such as
nickel, cobalt and molybdenum, with
it. The crust and the upper mantle
should be fairly depleted of these elements, but they are not. In fact, the
siderophile metals are present in nearly
solar proportions, suggesting that they
must have been brought to the planet
after its core had formed.
The Primeval Biosphere
The primordial surface of the comet-barraged Earth would surely have been unrecognizable to us. It would have looked
rather like the lunar highlands, where
the density of impact craters is so great
that they overlap one another other. This
helps to explain why there are almost no
geological traces of sediments dating
from this period: They were obliterated.
In fact, as two studies reported earlier
this year, all that may remain of the oldest known continental crust are grains
of zircon--existing as detritus in much
younger rocks!
dropping from 30 atmospheres
to 5 atmospheres)
ejections ot
water and gas
of intermittent comet impacts, a steaming-hot ocean, a very thick atmosphere and torrential acid rains. Giant comet impacts would have ejected large amounts of material into space and spun off violent hurricanes and tornadoes. Although the atmosphere was originally rich in carbon dioxide (CO,), photodissociation would have gradually
increased the proportion of nitrogen in the atmosphere, while releasing hydrogen into space. Silicate rocks, dissolved by the acidic rain,
would have been gradually replaced by carbonate rocks. Prebiotic organic molecules, delivered by the comets, would have provided the
“seed” for the evolution oithe first life.
Simon Wilde, of Curtin University
of Technology in Australia, and his
colleagues have dated grains of zircon
from Western Australia to 4.4 billion
years. And Stephen Mojzsis, now at
the University of Colorado, and his
coworkers dated zircon grains from
the same region to 4.3 billion years
ago. Both studies suggest that a continental crust and an ocean existed
within 200 to 300 million years after
the Earth was formed.
The comet flux would have been
very heavy when these early continents arose, but the rate decreased exponentially over the course of the
bombardment (see Figure 4). After 400
million years, the comet flux fell to
one percent of its initial value, and
then to merely 0.1 percent after 600
million years.
All told, about lOI8 tons of cometary
material crashed onto the Earth’s surface
during this era. This is equivalent to one
billion small cometary nuclei (about one
kilometer across) or one million large
nuclei (about 10 kilometers across). The
average rate of bombardment equates to
a few dozen medium-sized comets hitting the Earth every century.
We can tally their material contribution to the young planet based on the
average composition of a cometwhich is about 50 percent water, 13
percent volatile organics (carbon
monoxide, carbon dioxide and various
volatile organics bearing nitrogen), 15
percent refractory organic molecules
(relatively resistant to destruction) and
22 percent silicate rocks (Figwe 7). If
much of the material hadn’t been ejected back into space, it would have coated the Earth with a uniform layer of
cometary matter several kilometers
deep (Figwe 8).
A picture of the young Earth near
the end of the bombardment period
would show a cloudy atmosphere,
dozens of times thicker than our own
(Figure 12). Such an atmosphere would
protect the ground and prevent it from
cooling rapidly in an era when the
young Sun was about 30 percent less
bright than it is now. Even so, most of
the cometary water would have turned
into steam. As the ground temperature
fell below the boiling point (403 kelvins
at a pressure of 30 atmospheres), the
steam would have condensed to form
a hot ocean. In the residual atmos-
phere, the solar ultraviolet radiation
and the high temperatures would have
induced both photodissociations and
polymerizations of some cometary
molecules. Formaldehyde (CH,O)
would have polymerized to form a fog
of solid polyformaldehyde particles in
the stratosphere. Ammonia (NH,)
would have been photodissociated into
molecular nitrogen (NJ and hydrogen
(HJ, and the hydrocarbons would oxidize and lose their hydrogen. All of the
hydrogen would have escaped from
the top of the atmosphere and been
quickly lost to space.
At this point the atmosphere would
consist of 80 percent carbon dioxide
(CO,), 10 percent methane (CH,), 5 percent carbon monoxide (CO) and 5 percent nitrogen (N2). The atmosphere has
yet to reach a steady state, however, because it is still being fed by a (diminishing) cometary bombardment. It is turbulent in the extreme. Incessant torrential
rains of water acidified with CO, attack
the silicate rocks on the ground. The
ground temperature is soon low enough
(much below 373 kelvins) that the carbonates made by this reaction become
stable, forming solid sediments that
2001 September-October 441
bury the CO, in the ground, and so
cause the atmospheric pressure to fall
drastically, to 10 and then perhaps to 5
atmospheres and less. Indeed, major
sediments of limestone (calcium carbonate, CaCO,) and dolomite (a carbonate of calcium and magnesium) still
exist in what is now Greenland and are
3.8 billion years old.
At the same time, some of the organic molecules delivered by the comets
may have had a few interesting chemical interactions of their own-actually
giving a “jump start” to the first life on
our planet. Although some have questioned whether organics could survive
the heat of an impact, the issue now
seems to be resolved. The survival of
74 different amino acids (most of
which are not known on the Earth) on
carbonaceous chondrites, such as the
Murchison meteorite, suggest that organics could at least survive a minor
impact. And recent studies by Elisabetta Pierazzo, of the University of Arizona, and Christopher Chyba of the
SET1 Institute in Mountain View, California, suggest that some amino acids
could even survive the shock heating
of kilometer-sized cometary impacts.
In any case, Anders and I have, independently, argued that an extremely
large flux of interplanetary dust particles (derived from the tails of comets
that missed the Earth during its first
600 million years) could have salted
the young Earth with enormous quantities of prebiotic molecules. Indeed, in
1985 Don Brownlee of the University
of Washington, Seattle, showed that
cometary dust grains, captured in the
upper atmosphere, contain undamaged organic molecules.
About 3.5 billion years ago large
cometary impacts would have become
increasingly rare, but when they did
occur, they produced enormous cataclysms. The oceans would have boiled
near the impact site, causing hurricanes and gigantic waterspouts with
fantastic ejections of gas and water into
space. Under these chaotic and seemingly inhospitable conditions, a phenomenon occurs that is going to have
astonishing consequences: Bacteria begin to multiply in the hot waters of the
first oceans.
Conclusion
There is now considerable evidence to
support the idea that we owe the existence of our biosphere to a heavy
bombardment of comets in the very
442
American Saentist. Volume 89
early history of our planet. Indeed the
delivery of water and prebiotic molecules explains why life emerged so
soon after the conditions ceased to be
utterly hostile. The oldest fossil imprints of bacteria date to about 3.456
billion years ago (in Australian rocks),
and there is indirect evidence that life
was present 3.8 billion years ago in
the ancient sediments of Greenland
(Figure 10).
This process could have occurred
many times on rocky planets in other
stellar systems. And, if our solar system is any indication, the scattering of
comets by cold giant planets might be
a necessary condition for the emergence of life in the universe.
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