'Impact' Comet Is Rich in Carbon

Collision Suggests Porous Structure

When NASA's Deep Impact projectile hit Comet Tempel 1, it produced a giant plume of gas and dust far richer than expected in carbon compounds, reinforcing the view that comets may have contributed the chemical raw materials that produced life on Earth.

Scientists said Tuesday they do not know how typical Tempel 1 is, but they expect eventually to put together the first-ever complete profile of the substances that comets may have brought to Earth 4.5 million years ago.

"This will be the biggest contribution we will make," said University of Maryland astronomer Michael A'Hearn, lead scientist for NASA's Deep Impact mission. "Many hydrocarbons are in higher abundance than one would expect, and there are many others we haven't identified yet."

A'Hearn spoke during a telephone news conference to announce the first research findings from the spectacular July 4 rendezvous in space between Tempel 1 and the 820-pound copper-tipped "impactor" that slammed into it just before 2 a.m. Eastern time on Independence Day. The findings are to be published Thursday in the journal Science.

A'Hearn said data transmitted from the "flyby" spacecraft that accompanied the impactor offered several mildly surprising results, but "nothing that knocked my socks off." The flyby spacecraft dropped the impactor into the path of the comet, then photographed the encounter from a safe distance.

Horst Uwe Keller of Germany's Max Planck Institute said the impact threw about 5,500 tons of water into space, along with other types of ice -- including carbon dioxide, carbon monoxide and ammonia ice. But the impact tossed up an even greater quantity of dust.

A'Hearn said analysis of the dust had found hydrogen cyanide, methyl cyanide, acetylene and formaldehyde, with many other organic compounds still to be identified, adding that the presence of such compounds in such large quantities suggested a closer relationship between comets and the origins of life on Earth, Ahearn said.

Keller, who supervised observations of the impact from the European Space Agency's Rosetta spacecraft about 50 million miles away, said the "density of the comet is very low and the porosity is very high," and described the texture as "crumbly."

A'Hearn said the impactor hit the comet at an oblique angle and plowed into it to a depth of "tens of meters," but could not be more specific because the plume of powdery ice and dust that spouted upward was so thick that scientists have not finished electronically manipulating the images to be able to view the crater.

A'Hearn said he had expected the plunging impactor eventually to "reach a layer" where the comet was denser, but that did not happen: "I'm not convinced there is a solid layer," A'Hearn said. "We think something like 75 to 80 percent" of the comet "is empty space."

Another as-yet-unexplained phenomenon was the apparent presence of craters on the surface of Tempel 1, as if the comet had been struck by meteors or other space rocks during its journeys. Neither of two other comets previously photographed by spacecraft show such craters, which A'Hearn said mimicked a typical impact pattern like that found on the moon.

Comets, composed principally of dust and ice, wander periodically into the inner solar system from deep space, and some scientists have suggested that they may have brought water and organic compounds to the early Earth. A'Hearn said nothing he had found out about Tempel 1 so far had led him to disagree with the usual characterization of comets as "dirty snowballs."

But though comets display a showy "tail" as they approach the sun, their cores are composed of icy material essentially undisturbed by earthquakes, volcanoes or erosion since the solar system's formation 4.5 million years ago.

Deep Impact was designed to punch through Tempel 1's surface to get a first-ever look at a comet's primordial core and begin analyzing what influence comets may have had on the Earth's evolution. Besides the fly-by spacecraft and Rosetta, about 80 Earth-based telescopes observed the collision and gathered data.