Download Water for the rock: Did Earth`s oceans come from the heavens?

WATER FOR THE ROCK
Did Earth‘s oceans come from the heavens?
BY
BEN HARDER
The distribution of hydrogen and water beneath Earth’s surface
suggeststo many geochemiststhat 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 cometsin
the outer solar system, says Armand H. Delsemme, an astrophysicist now retired from the University of Toledo in Ohio. As
Jupiter formed,its growinggravitationaltug would have sentmany
system, including the distribution of water. Close to the nebula’s icy comets hurtlingfrom the range ofthe giant planets to all reaches
of the solar system.
center, high temperatures and pressures vaporOver a billion years, at least hundreds
izedice crystals and the light elementsand
of millions of comets collided with
compounds called volatiles. The
Earth, Delsemme says. The
action blew these materials
bombardment would have
toward the outskirts of the
been especially heavyjust
nebula, leaving mainly
after Earth formed.
grains of rock behind to
Attributingwater on
form the inner planets.
Earth to these lateFarther out, debris
comer comets neatly
coalesced in meteorites
explains a couple of
called carbonaceous
things: first, how water
chondrites,which carry
that originated at the
up to 10percent of their
outer edges of the solar
mass in ice. The giant
system got to at least one of
outer planets, such as Satits inner planets, and second,
urn and Jupiter, that arose in
how water arrived late enough in
this neighborhood also contain
Earth’s formation for the planet to
some ice. Beyond these planets, water
have sufficientgravity to retain it.
condensed in large quantities and formed
SPLASHDOWN
“The front-runner [hypothesis]
comets, which are about halfice.
Comparedwith these icy objects, Earth containslittlewater. Only
comets, belng half
until about 5 years ago was that
about 0.02 percent of its mass is in its oceans, and somewhatmore
water, were once widely
water came from comets and came
water sitsbeneaththe surface. Nevertheless, Earth has substantially
held to be the source of
in late,”says Kevin Righter, a planEarth‘s oceans.
etary geochemist at the University
more water than scientists would expect to h d at a mere 93 milofArizona in Tucson. “Onegroup of
lion miles from the sun. How did Earth come to possess its seas?
Over the years, planetary scientists have proposed severalpos- measurements changed that.”
Those measurements were spectral analyses of the chemical
sible answers to that question, but until recently they’ve had little data for testing their hypotheses. As research in the field pro- compositions of three comets-Halley, Hyakutake, and Halegresses, however, the picture is getting more complicated-not less. Bopp-during near-Earth passes they made in 1986,1996, and
Analyses of the geochemical properties of various bodies in 1997, respectively. These analyses, the first that examined the
the solar system and computer modeling of the dynamics of hydrogen in water on bodies from a remote region, revealed a
ancientplanetary interactionshave undermined a formerlypop- crucial chemical difference between the hydrogen in cometary
ular theory, which attributes Earth‘s water to a bombardment by ice and that in Earth’s water.
Most hydrogen atoms possess a nucleus made up of a sole procomets late in the planet’s formation.
New hypotheses are emerging as that theory’s plausibility ton. Rarer forms also contain a neutron or two. The one-protonfades, and planetary scientists are struggling to reconcile data one-neutron version, called deuterium, behaves chemically like
with these alternative scenarios.There’s one thing on which most hydrogen and can form water and other compounds. However,
geochemistsand astronomers agree: The celestial pantry is now the resultingmolecules are distinctlyheavier than those containempty of a key ingredient in the recipe for Earth.
ing the more common form, or isotope, of hydrogen.
Deuteriumis exceedinglyrare on Earth. Barely one such isotope 0
0
JUST ADD WATER Because comets contain a greater propor- exists for every 7,000 atoms of standard hydrogen. In contrast, 2
tion of water than other known celestial objects do, they make the deuterium-to-hydrogen ratios in the three comets, according 4
w
natural candidates as a source of Earth’s rivers, lakes, and oceans. to the new observations,were all twice that in Earth’s water.
ore 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
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The discovery gave researchers some pause. Assuming that the couldn’t have made a substantial contribution to the late veneer,
compositions of Halley, Hyakutake, and Hale-Bopp are represen- the researchers note in the March 7 Nature.
tative of all comets, explaininghow a hail of the objects could proTaken together with the signatures of volatiles on Earth, these
duce oceanswith an earthly deuteriumdata suggest that no more than 50
to-hydrogen ratio is like trying to make
percent, and probably less than 15
a low-fat dessert from heavy cream.
percent, of Earths water could have
According to the new data, cometary
been added from space at the end of
bombardment could account for no
our planet’s formation, says Drake.
more than half of Earth’s inventory of
If existing objects in space couldn’t
water, says Francois Robert, a geohave combinedto make Earth‘s unique
chemist at the Museum ofNatural Hismix of water and other elements, the
tory in Paris and one of several
planet must have formed from-and
researchers who brought the paradox
entirelydepleted-an ancient supplyof
of the incompatible ratios to light.
water-rich material that has no modSuch numbers might stillfit a revised
ern analog, Drake and Righter argue.
version of the late-veneer theory, says
Because their hypothesis requires that
Leonid M. Ozernoy of George Mason
Earth arose from water-containing
University in Fairfax,Va. In addition to
materials already present in the inner
comets, asteroid-size planetesimals
solar system, it’s called the wet-accrecontaining water with less deuterium
tion hypothesis.
could have contributed to the late
“Mostof Earth’s water has an indigeveneer, says Ozernoy.
nous origin:’ says Drake. The most
Smallerversions ofthese meteorites,
probable source is a water-containing
the carbonaceouschondrites, hit Earth
inner solar system reservoir at about
today in modest numbers. According
the same distance away from the sun
to a computer model Ozernoy and his
as Earth is now.
George Mason University colleague
In the wet-accretion hypothesis,
Sergei Ipatov have built, greater quanEarth developed from silicate rocks
tities and larger chunks of such matewith water trapped inside. This
rial could have showered Earth toward
hydrous material coalescedwith other
the end of its formation.
objects occupying the same swath of
Ozernoy and Ipatov have estimated
CELESTIAL R A I N - Toward the end of Earth’s
space. In their Nature report, Drake
the number of planetesimalsthat were
and Righter suggest that the band of
formation. a steady bombardment or ice-containing
flung at the early Earth from reservoirs
meteorites couicl have provided much of the
the solar nebula was cooler than the
of such bodies following orbits inside
planet’s water
temperature other researchers have
Jupiter’s path or crossing it. These
inferred, thus allowing water ice to
planetesimals could have delivered much of Earth‘s water, Ozer- condense and become bound to the silicates.
noy argued in January at the American Astronomical Society
meeting in Washington, D.C.
ONE BIG SPLASH The role of chance in the solar system’sevolution represents a wildcard that could trump both the late-veneer
WET BIRTH Adding wet planetesimals to the equation of Earth’s and wet-accretion models. Or it could fold for lack of hard eviearly years puts a different face on the late-veneer theory, but it dence.
still doesn’t satisfy many ofthe geochemical constraints that have
Allessandro Morbidelliofthe Observatoryof the C6te d ‘ k r in
been recently described, says Tobias C. Owen of the Universityof Nice, France, accepts Drake and Righter’s hypothesis that Earth
Hawaii in Honolulu.
formed wet. However, he doubts that the planet evolved solely
Water isn’t the only matter on our planet today that seems from material within a tight band at a specifk distance from the
unlikely to have formed at Earth’s proximity to the sun. There are sun, as the Arizona researchers envision. Their scenario isn’t conalso compounds and elements that readily vaporize, including sistentwith computer simulationsof planetaryformation,he says.
chemically inert noble gases, such as argon, krypton, and xenon,
Morbidelli returns to the notion that bodies from the outer solar
and the elements nitrogen, oxygen, and hydrogen.
system brought water and volatiles to the inner solar system, but
The ratio of xenon to krypton differs between Earth‘s atmo- he hypothesizes that they made their contribution as the planets
sphere and typical carbonaceous chondrites today. By the same were forming rather than late in planetary development. If water
token, the argon-to-water ratios are dissimilar. Therefore,these wet came from millions of comets or s d asteroids, the same steady
meteors’larger kin,the planetesimals, probably didn’t provide a celestial rain would have bombarded Mercury, Venus, Earth, and
veneer of material for Earth, Owen’s analysis suggests.
Mars, so they would all have begun with the same water characThe isotope profiles of nitrogen and oxygen on meteorites and teristics, he says. However, the waters of those four planets now
Earth also argueagainstthesebodiespraidingmuchofawetveneer. have dissimilarprofiles, Owen and other geochemistshave found.
Michael J. Drake of the University of Arizona, who works with
If, on the other hand, a relatively small number of planetary
Righter, agreesthat alate veneer didn‘t provide Earth‘swater. While building blocks brought water into the inner solar system, chance
he and Righter don’t dispute that a veneer accounts for some of would dictate whether any one of them glommed onto an embryEarth‘s material, it couldn’t have been wet. Certain metals, such as onic planet. Achance encounter-literally an accident in spaceosmium, would have been pulled into Earth‘s central core ifthey couldhaveessentiallyflooded a planet in one big splash,but accordhad been present before the planet got wet. Therefore, all osmium ing to the luck of the draw, other planets could have spared. This
in Earth‘s upper layers must have come in as a late veneer.
could explainthe current planets’differences in water content and
Drake and Righter have determined that the isotope profile of why no exkting objects appearto have been in the recipe for Earth.
near-surface osmium Closely matches that in ordinary chondritesTo ciuny so much water, the impactor that doused Earth must
a type of meteorite that’sbone-dry. And since carbonaceouschon- have come from between Mars and Jupiter, Morbidelli says. Comdrites don’t have the right proportion of osmium isotopes, they puter models thathe and his colleagues describedin the Oct. 1,2001
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Icarus show how this might have happened.
a planetary scientist at the University of Arizona in Tucson. “If
The researchers began with the premise that early in the solar Earth got its water lodlf’-as Drake and Righter suggest-“then
system’s formation, scores of planetary embryos about the sue Mars [too] should have been swimming in water,”Lunine says.
of Earth’s moon were scattered around the sun to a distance four
Preliminary data from the Mars Global Surveyormission sugtimes that between Earth and the sun now (4 AU). The embryos’ gest that the Red Planet has large deposits of water (SN: 3/7/02,
gravitational interactions with
p. 149).Further analysis of Mars
each other and with a growing
could indicate how much water
Jupiter would have caused their
the inner planets received from
orbits to begin crossing.
common sources,such as comets
Someofthesebodieswouldhave
and meteorites. It could also help
collided with each other, building
scientists characterize the
into ever-larger embryonic plansources of the remainder of
ets. Eventually, the researchers’
Earth‘s original water budget.
simulations show, “out of a hunScientistsare also countingon
dred or more embryos, just a few
data from future comet encounterrestrial planets form between
ters. Contour, an unmanned
0.5 and 2 AU“ from the sun, says
NASA probe scheduled for
Morbidelli. Each planet’s unique
launch in July, will rendezvous
mix of building blocks includes
with at least two poorly studied
some embryosfrom outsideits jinal
comets. It will pass Encke in
orbit. In some cases, one or more
November 2003 and then
embryos hail from far enough out
Schwassmann-Wachmann 3 in
BEFORE THE DELUGE - Dozens of dry moon-sizeobjects could
that they would have been wet.
June 2006. Then, NASA may
have been fusing together to form Earth when, by chance, a cosmic
The weak point in Morbidelli’s water balloon barreled in.
park the probe in a distant orbit
model is that there’s no way to
to observe any other cometsthat
test whether a chance water delivery occurred in the case of come by. Data from the close encounters willgive scientistsbetEarth, Drake says. The carrier’s elemental and isotopic charac- ter information on the noble gases in comets and could indicate
teristics would have to have been unlike those of any object that how much cometary material ended up on Earth.
researchers have yet found in the solar system. ‘You can’t rule
If any comets are found to have Earthlike deuterium-hydrogen
out a [planetary building block] crashing in at 4.5 billion years ratios, they could add power to the late-veneer theory. Delsemme
ago, but it. . . doesn’t seem geochemically plausible,”he says.
maintainsthat the comets responsible for the late veneer formed
Only more data, especiallymore information about the amount closer to the sun thari the bulk of those left today-and thus had
and composition of water on Mars, will resolve the mysterious unique isotopic signatures. If he’s right, then perhaps our oceans
history of the inner solar system’swater, says Jonathan I. Lunine, aren’t a product of a rare celestial accident aRer all. rn
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