September 6, 2005

Deep Impact probe shows a fragile, empty comet

By Deborah Zabarenko

WASHINGTON (Reuters) - Comet Tempel 1, the target of NASA's Deep Impact probe, turns out to be quite fragile, with no more substance than a snowbank, scientists said on Tuesday.

"The comet is mostly empty, mostly porous," said Michael A'Hearn, a comet specialist at the University of Maryland. "Probably all the way in, there is no bulk ice. The ice is all in the form of tiny grains."

The material on the comet's surface, down to a depth of several dozen yards (meters) is "unbelievably fragile, less strong than a snowbank," A'Hearn said in a telephone news briefing to release early findings from the mission.

The comet's dust and ice grains form a fluffy structure of fine particles held together loosely by a weak gravitational pull, the researchers said.

The surface of Tempel 1 is pocked with apparent impact craters, features that have not been detected before on close-up observation of two other comets.

Deep Impact collided purposely with Tempel 1 on July 4, freeing a plume of primordial material from its nucleus, the first time astronomers have been able to glimpse the interior of a comet.

The smash-up with Deep Impact's washing-machine-sized probe was monitored by another part of the NASA spacecraft that flew above the comet, along with a European spacecraft called Rosetta and more than 70 ground-based telescopes.

Scientists hope research into Tempel 1 will help unlock the secret of how life arrived on Earth. Variously described as dirty snowballs or snowy dirtballs, comets are prime candidates for seeding planets, including Earth, with water and organic material.

An analysis of material in the plume showed a huge increase in the amount of molecules that contain carbon. This suggests that comets like Tempel 1 contain a substantial amount of organic material, which means they might have brought such material to Earth early in the planet's history at a time when asteroid and meteor strikes were common.

The research on Tempel 1 will be published Thursday in the journal Science.