BLOIS, FRANCE--Many planetary scientists have surmised that most of the water on Earth's surface could have originated in comets, which are made largely of ice and have hit Earth in vast numbers over geologic history. But a close look at the water in two recent comets, described at a recent planetary science meeting here, now challenges that conclusion.
Tobias Owen and Roland Meier of the University of Hawaii's Institute for Astronomy and their colleagues measured the ratio of ordinary water molecules to molecules containing deuterium, a heavy isotope of hydrogen, in this spring's spectacular comet, Hale-Bopp. They used the James Clerk Maxwell Telescope on Mauna Kea to pick up radio emissions from the deuterium-containing molecules. The intensity of the emissions indicated that Hale-Bopp contains about three atoms of deuterium--a heavy isotope of hydrogen--for every 10,000 atoms of ordinary hydrogen. That's about twice the ratio in seawater. But it agrees with measurements of last year's comet Hyakutake, made by a French-American collaboration and also announced at a meeting here last month.
"These results show that you can't make the bulk of Earth's oceans with water from these sorts of comets," which come from the so-called Oort cloud in the farthest reaches of the solar system, says Owen. Other planetary scientists tend to agree. Says David Stevenson of the California Institute of Technology's Geological and Planetary Science Division, "I think the hypothesis is in trouble, and comets are perhaps less important than we thought in making the oceans."
But not all experts are ready to fall in line. University of Iowa physicist Lou Frank, author of a controversial proposal that miniature comets might be quietly showering our planet (Science, 30 May, p. 1333), thinks his theory is unscathed by the new measurements. "Large comets [such as Hale-Bopp] have nothing to do with the water in our oceans," says Frank, who believes that the water arrived in swarms of tiny, fluffy objects whose existence he has inferred from ultraviolet emissions observed in Earth's upper atmosphere. These comets, he says, have markedly different compositions from the Comet Hale-Bopp and its ilk, and perhaps different deuterium-to-hydrogen ratios as well. Then again, no one has yet made a definitive sighting of a tiny comet, let alone measured its composition.