Earth's water probably didn't come from comets, Caltech researchers say

PASADENA—A new Caltech study of comet Hale-Bopp suggests that comets did not give Earth its water, buttressing other recent studies but contrary to the longstanding belief of many planetary scientists.

In the March 18 issue of Nature, cosmochemist Geoff Blake and his team show that Hale-Bopp contains sizable amounts of "heavy water," which contains a heavier isotope of hydrogen called deuterium.

Thus, if Hale-Bopp is a typical comet, and if comets indeed gave Earth its water supply billions of years ago, then the oceans should have roughly the same amount of deuterium as comets. In fact, the oceans have significantly less.

"An important question has been whether comets provided most of the water in Earth's oceans," says Blake, professor of cosmochemistry and planetary science at Caltech. "From the lunar cratering record, we know that, shortly after they were made, both the moon and Earth were bombarded by large numbers of asteroids or comets.

"Did one or the other dominate?"

The answer lies in the Blake team's measurement of a form of heavy water called HDO, which can be measured both in Earth's oceans using mass spectrometers and in comets with Caltech's Owens Valley Radio Observatory (OVRO) Millimeter Array. Just as radio waves go through clouds, millimeter waves easily penetrate the coma of a comet.

This is where cosmochemists can get a view of the makings of the comet billions of years ago, before the sun had even coalesced from an interstellar cloud. In fact, the millimeter-wave study of deuterium in water and in organic molecules in the jets emitted from the surface of the nucleus shows that Hale-Bopp is composed of 15 to 40 percent primordial material that existed before the sun formed.

The jets are quite small in extent, so the image clarity provided by the OVRO Millimeter Array was crucial in the current study. "Hale-Bopp came along at just the right time for our work," Blake says. "We didn't have all six telescopes in the array when Halley's comet passed by, and Hyakutake was a very small comet. Hale-Bopp was quite large, and so it was the first comet that could be imaged at high spatial and spectral resolution at millimeter wavelengths."

One other question that the current study indirectly addresses is the possibility that comets supplied Earth with the organic materials that contributed to the origin of life. While the study does not resolve the issue, neither does it eliminate the possibility.

Also involved in the Nature study are Charlie Qi, a graduate student in planetary science at Caltech; Michiel Hogerheijde of the UC Berkeley department of astronomy; Mark Gurwell of the Harvard-Smithsonian Center for Astrophysics, and Duane Muhleman, professor emeritus of planetary science at Caltech.