Water for the rock; did Earth's oceans come from the heavens? - research into the origin of the Earth's seas

Science News, March 23, 2002 by Ben Harder

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 alternative scenarios. 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 oceans. 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 changed that."

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.