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New Findings Surprise Scientists About Comet Composition - 2004-06-18

New photographs from a U.S. spacecraft have dramatically changed scientists' knowledge about the structure of comets. Researchers are studying comets because they are thought to be primitive remains from the time the solar system formed.

When the U.S. Stardust spacecraft flew close to comet Wild 2 in January, scientists expected its pictures to show a big chunk of rock and ice covered with black dust, obscuring any interesting features.

Instead, they got images rich with distinct formations like craters, canyons, ridges, flat areas, and tall columns.

"We were totally stunned by what we saw," said University of Washington astronomer Donald Brownlee.

"What we saw was a thrilling object," he added. "The body itself has incredible features."

Mr. Brownlee's paper in the journal Science describes lots of craters that were made when other space objects crashed into the comet over the ages. Some craters have a rounded central pit and rough surrounding terrain. Others have vertical walls and flat floors. Two are shaped like big footprints.

"We see different kinds of features we haven't even named because we don't understand [them]," said Mr. Brownlee. "There are irregular shaped depressions that differ from the other ones because the others are circular."

This cratered surface means Comet Wild 2 was strong enough to withstand space impacts. U.S. space agency researcher Claudia Alexander says if it had been a loose pile of rocks or a fragile ice ball, as some theories about comets suggest, it would not have held together.

"What's exciting about that from my perspective is that it doesn't look like a rubble pile," she said. "It has more cohesiveness to it. So we are astonished and intrigued."

Equally surprising are the comet's internal processes. It is spewing 20 narrow jets of gas and dust from many spots on its surface. Donald Brownlee says the jets form as the comet comes closer to the sun because the sun heats up water, carbon dioxide, and other volatile chemicals in the nucleus, building pressure to eject them. These were a surprising aspect of how a comet creates its long, wispy tail.

"It's telling us something fundamental about the way comets work," he said. "People have watched comets for as long as people have lived on the Earth, yet this is the first time we've been able to see how the gas comes off, how the dust comes off, and produces those tails."

Comets are of interest because they are thought to be repositories of primordial dust from the time the solar system formed 4.5 billion years ago from this material. The U.S. spacecraft collected thousands of these ejected particles in a canister, surviving a nightmarish cometary dust storm in the process. Scientists hope the particles and other data will help them better understand how the solar system came together.

Many researchers believe comets transferred the organic compounds necessary for life to Earth during a continuous series of collisions. This notion is supported by another finding from the U.S. probe. Space scientist Benton Clark of Lockheed Martin Astronautics in Denver says one of the instruments detected organic compounds in the comet's particles, including molecules containing a bond between the chemical elements carbon and nitrogen.

"This is important for the origin of life because very complex molecules, such as DNA, the proteins, and enzymes in all organisms must have this carbon-nitrogen combination," said Mr. Clark.

The Stardust mission is only one of several U.S. and European comet rendezvous flights. The European Space Agency launched its Rosetta craft in February to drop scientific instruments on a comet to study its composition. The most spectacular project is the U.S. Deep Impact mission. After its scheduled launch in December, it is to crash into a comet one year from now to get a peek below its surface.