Earth, water, and comets.

Although comets contain an abundance of water-ice, they could not have been the main source of water for Earth's oceans. That's the conclusion of a group of French, Swiss, and U.S. researchers who measured the ratio of heavy hydrogen, or deuterium, to ordinary hydrogen in water in Comet Hale-Bopp.

Using the James Clerk Maxwell submillimeter radio telescope atop Hawaii's Mauna Kea, they found that Hale-Bopp's deuterium-to-hydrogen ratio--0.0003--was about the same as the ratios measured by other researchers in the well-known comets Halley and Hyakutake. All three have twice the deuterium-hydrogen ratio of seawater, note Roland Meier, Tobias C. Owen, and David C. Jewitt of the University of Hawaii in Honolulu and their colleagues.

This across-the-board mismatch suggests that these comets, which are several kilometers in diameter, and other comets of their size did not fill terrestrial oceans, says Owen.

"I agree with them perfectly," says Louis A. Frank of the University of Iowa in Iowa City. He adds that the measurements in no way discount his controversial assertion that much smaller, water-bearing bodies, which he classifies as house-size comets, pelt Earth's upper atmosphere by the thousands each day and could have delivered the bulk of the planet's water supply (SN: 5/31/97, p. 332).

Frank notes that no one has yet measured the deuterium-hydrogen ratio in these small bodies, whose presence he most recently deduced in images taken by NASA's Polar satellite. If their ratios don't match the value measured in the oceans, Frank says, he would readily agree that these small bodies did not supply Earth's seas. He told Science News that he now has evidence that the bodies differ in some respects from typical, kilometer-size comets. In a report scheduled for September publication in Geophysical Research Letters, Frank argues that the house-size bodies contain much less dust and have less sodium than typical comets.

If comets didn't provide most of our planet's water, what did? Carbonaceous meteorites might have carried water to Earth, Owen says. Water trapped in Earth rocks is another possibility.

The team also measured the deuterium-hydrogen ratio in hydrogen cyanide in Hale-Bopp and found a higher value in this molecule than in water. The difference between the two ratios in the comet resembles that found in the interstellar medium, the source of material from which our solar system formed. The similarity suggests that comets are truly pristine bodies that preserve much of the original chemical makeup of interstellar space, he asserts.

Owen theorizes that water from the interstellar medium warmed only slightly when it entered the outer, frigid reaches of the solar nebula--the disk of gas, dust, and ice that swaddled the infant sun. He suggests that the period between the entry of interstellar water into the outer solar system and the time when it recondensed and became incorporated into comets was too short for the water to have been chemically altered.