by Ben Harder
March 23, 2002
More than 4.5 billion years ago, the sun and its planets were taking
shape from a rotating disk of ice, gas, and dust.
This protosolar nebula was hotter and
denser toward its center and cooler and less dense farther out.
These gradients profoundly influenced the chemical composition of
different regions of the early solar system, including the
distribution of water. Close to the nebula's center, high
temperatures and pressures vaporized ice crystals and the light
elements and compounds called volatiles.
The action blew these materials toward
the outskirts of the nebula, leaving mainly grains of rock behind to
form the inner planets.
Farther out, debris coalesced in meteorites called carbonaceous
chondrites, which carry up to 10
percent of their mass in ice. The giant outer planets, such as
Saturn and Jupiter, that arose in this neighborhood also
contain some ice. Beyond these planets, water condensed in large
quantities and formed comets, which are about half ice.
Compared with these icy objects, Earth contains little water. Only
about 0.02 percent of its mass is in its oceans, and somewhat more
water sits beneath the surface. Nevertheless, Earth has
substantially more water than scientists would expect to find at a
mere 93 million miles from the sun.
How did Earth come to possess its seas?
Over the years, planetary scientists have proposed several possible
answers to that question, but until recently they've had little data
for testing their hypotheses. As research in the field progresses,
however, the picture is getting more complicated - not less.
Analyses of the geochemical properties of various bodies in the
solar system and computer modeling of the dynamics of ancient
planetary interactions have undermined a formerly popular theory,
which attributes Earth's water to a bombardment by comets late in
the planet's formation.
New hypotheses are emerging as that theory's plausibility fades, and
planetary scientists are struggling to reconcile data with these
There's one thing on which most
geochemists and astronomers agree:
The celestial pantry is now empty of
a key ingredient in the recipe for Earth.
JUST ADD WATER
Because comets contain a greater
proportion of water than other known celestial objects do, they
make natural candidates as a source of Earth's rivers, lakes, and
The distribution of hydrogen and water
beneath Earth's surface suggests to many geochemists that water
hasn't mixed deep into the planet, so they thought that the cometary
bombardment applied a veneer of water to the dry planet relatively
late in its formative period.
One attraction of this late-veneer scenario has been that it fits
well with the early movements of planets and the many comets in the
outer solar system, says Armand H. Delsemme, an
astrophysicist now retired from the University of Toledo in Ohio.
As Jupiter formed, its growing
gravitational tug would have sent many icy comets hurtling from the
range of the giant planets to all reaches of the solar system.
Over a billion years, at least hundreds of millions of comets
collided with Earth, Delsemme says. The bombardment would have been
especially heavy just after Earth formed.
Attributing water on Earth to these latecomer comets neatly explains
a couple of things: first, how water that originated at the outer
edges of the solar system got to at least one of its inner planets,
and second, how water arrived late enough in Earth's formation for
the planet to have sufficient gravity to retain it.
"The front-runner [hypothesis] until
about 5 years ago was that water came from comets and came in
late," says Kevin Righter, a planetary geochemist at the
University of Arizona in Tucson. "One group of measurements
Those measurements were spectral
analyses of the chemical compositions of three
comets - Halley, Hyakutake, and
Hale-Bopp - during near-Earth passes they made in 1986,
1996, and 1997, respectively.
These analyses, the first that examined
the hydrogen in water on bodies from a remote region, revealed a
crucial chemical difference between the hydrogen in cometary ice and
that in Earth's water.
Most hydrogen atoms possess a nucleus made up of a sole proton.
Rarer forms also contain a neutron or
two. The one-proton/one-neutron version, called
deuterium, behaves chemically like
hydrogen and can form water and other compounds. However, the
resulting molecules are distinctly heavier than those containing the
more common form, or isotope, of hydrogen.
Deuterium is exceedingly rare on Earth. Barely one such isotope
exists for every 7,000 atoms of standard hydrogen. In contrast, the
deuterium-to-hydrogen ratios in the three comets, according to the
new observations, were all twice that in Earth's water.
The discovery gave researchers some pause. Assuming that the
compositions of Halley, Hyakutake, and Hale-Bopp are representative
of all comets, explaining how a hail of the objects could produce
oceans with an earthly deuterium-to-hydrogen ratio is like trying to
make a low-fat dessert from heavy cream.
According to the new data, cometary bombardment could account for no
more than half of Earth's inventory of water, says Francois
Robert, a geochemist at the Museum of Natural History in
Paris and one of several researchers who brought the paradox of the
incompatible ratios to light.
Such numbers might still fit a revised version of the late-veneer
theory, says Leonid M. Ozernoy of George Mason University
in Fairfax, Va. In addition to comets, asteroid-size planetesimals
containing water with less deuterium could have contributed to the
late veneer, says Ozernoy.
Smaller versions of these meteorites, the carbonaceous
chondrites, hit Earth today in
modest numbers. According to a computer model Ozernoy and his
George Mason University colleague Sergei Ipatov have
built, greater quantities and larger chunks of such material could
have showered Earth toward the end of its formation.
Ozernoy and Ipatov have estimated the number of planetesimals that
were flung at the early Earth from reservoirs of such bodies
following orbits inside Jupiter's path or crossing it.
planetesimals could have delivered
much of Earth's water, Ozernoy argued in January at the American
Astronomical Society meeting in Washington, D.C.
Adding wet planetesimals to the equation
of Earth's early years puts a different face on the late-veneer
theory, but it still doesn't satisfy many of the geochemical
constraints that have been recently described, says Tobias C.
Owen of the University of Hawaii in Honolulu.
Water isn't the only matter on our planet today that seems unlikely
to have formed at Earth's proximity to the sun.
There are also compounds and elements
that readily vaporize, including chemically inert noble gases, such
as argon, krypton, and xenon, and the elements nitrogen, oxygen, and
The ratio of xenon to krypton differs between Earth's atmosphere and
typical carbonaceous chondrites today. By the same token, the
argon-to-water ratios are dissimilar. Therefore, these wet meteors'
larger kin, the planetesimals, probably didn't provide a veneer of
material for Earth, Owen's analysis suggests.
The isotope profiles of nitrogen and oxygen on meteorites and Earth
also argue against these bodies providing much of a wet veneer.
Michael J. Drake of the University of Arizona, who works with
Righter, agrees that a late veneer didn't provide Earth's
water. While he and Righter don't dispute that a veneer accounts for
some of Earth's material, it couldn't have been wet. Certain metals,
such as osmium, would have been pulled into Earth's central core if
they had been present before the planet got wet.
osmium in Earth's upper layers must
have come in as a late veneer.
Drake and Righter have determined that the isotope profile of
near-surface osmium closely matches that in ordinary chondrites - a
type of meteorite that's bone-dry. And since carbonaceous
chonidrites don't have the right proportion of osmium isotopes, they
couldn't have made a substantial contribution to the late veneer,
the researchers note in the March 7 Nature.
Taken together with the signatures of volatiles on Earth, these data
suggest that no more than 50 percent, and probably less than 15
percent, of Earth's water could have been added from space at the
end of our planet's formation, says Drake.
If existing objects in space couldn't have combined to make Earth's
unique mix of water and other elements, the planet must have formed
from - and entirely depleted - an ancient supply of water-rich
material that has no modern analog, Drake and Righter argue.
Because their hypothesis requires that
Earth arose from water-containing materials already present in the
inner solar system, it's called the wet-accretion hypothesis.
"Most of Earth's water has an
indigenous origin," says Drake.
The most probable source is a
water-containing inner solar system reservoir at about the same
distance away from the sun as Earth is now.
In the wet-accretion hypothesis, Earth
developed from silicate rocks with water trapped inside. This
hydrous material coalesced with other objects occupying the same
swath of space.
In their Nature report, Drake and
Righter suggest that the band of the solar nebula was cooler than
the temperature other researchers have inferred, thus allowing water
ice to condense and become bound to the silicates.
ONE BIG SPLASH
The role of chance in the solar system's
evolution represents a wildcard that could trump both the
late-veneer and wet-accretion models. Or it could fold for lack of
Allessandro Morbidelli of the Observatory of the Cote
d'Azur in Nice, France, accepts Drake and Righter's hypothesis
that Earth formed wet. However, he doubts that the planet
evolved solely from material within a tight band at a specific
distance from the sun, as the Arizona researchers envision.
Their scenario isn't consistent with
computer simulations of planetary formation, he says.
Morbidelli returns to the notion that bodies from the outer solar
system brought water and volatiles to the inner solar system, but he
hypothesizes that they made their contribution as the planets were
forming rather than late in planetary development. If water came
from millions of comets or small asteroids, the same steady
celestial rain would have bombarded Mercury, Venus, Earth, and Mars,
so they would all have begun with the same water characteristics, he
However, the waters of those four
planets now have dissimilar profiles, Owen and other geochemists
If, on the other hand, a relatively small number of planetary
building blocks brought water into the inner solar system, chance
would dictate whether any one of them glommed onto an embryonic
planet. A chance encounter - literally an accident in space-could
have essentially flooded a planet in one big splash, but according
to the luck of the draw, other planets could have spared.
This could explain the current planets'
differences in water content and why no existing objects appear to
have been in the recipe for Earth.
To carry so much water, the impactor that doused Earth must have
come from between Mars and Jupiter, Morbidelli says. Computer models
that he and his colleagues described in the Oct. 1, 2001 Icarus show
how this might have happened.
The researchers began with the premise that early in the solar
system's formation, scores of planetary embryos about the size of
Earth's moon were scattered around the sun to a distance four times
that between Earth and the sun now (4
The embryos' gravitational interactions
with each other and with a growing Jupiter would have caused their
orbits to begin crossing.
Some of these bodies would have collided with each other, building
into ever-larger embryonic planets. Eventually, the researchers'
simulations show, "out of a hundred or more embryos, just a few
terrestrial planets form between 0.5 and 2 AU" from the sun, says
Morbidelli. Each planet's unique mix of building blocks includes
some embryos from outside its final orbit. In some cases, one or
more embryos hail from far enough out that they would have been wet.
The weak point in Morbidelli's model is that there's no way to test
whether a chance water delivery occurred in the case of Earth, Drake
The carrier's elemental and isotopic
characteristics would have to have been unlike those of any object
that researchers have yet found in the solar system.
"You can't rule out a [planetary
building block] crashing in at 4.5 billion years ago, but it...
doesn't seem geochemically plausible," he says.
Only more data, especially more
information about the amount and composition of
water on Mars, will resolve the
mysterious history of the inner solar system's water, says
Jonathan I. Lunine, a planetary scientist at the University of
Arizona in Tucson.
Earth got its water locally - as Drake
and Righter suggest,
"then Mars [too] should have been
swimming in water," Lunine says.
Preliminary data from the Mars Global
Surveyor mission suggest that
Red Planet has large deposits of water (SN:
3/7/02, p. 149).
Further analysis of Mars could indicate
how much water the inner planets received from common sources, such
as comets and meteorites. It could also help scientists characterize
the sources of the remainder of Earth's original water budget.
Scientists are also counting on data from future comet encounters.
Contour, an unmanned NASA probe scheduled for launch in July, will
rendezvous with at least two poorly studied comets.
It will pass Encke in November 2003 and
Schwassmann-Wachmann 3 in June
Then, NASA may park the probe in
a distant orbit to observe any other comets that come by. Data from
the close encounters will give scientists better information on the
noble gases in comets and could indicate how much cometary material
ended up on Earth.
If any comets are found to have Earthlike deuterium-hydrogen ratios,
they could add power to the late-veneer theory.
Delsemme maintains that the comets
responsible for the late veneer formed closer to the sun than the
bulk of those left today - and thus had unique isotopic signatures.
If he's right, then perhaps our oceans
aren't a product of a rare celestial accident after all.