Comet’s Solid Makeup Wows the Scientists

The first analysis of data from NASA’s Stardust probe has shown that comet Wild-2 is not a loose ball of icy rubble, as scientists had expected, but a solid body pockmarked by craters and venting surprisingly patchy jets of gas and dust.

The analyses, reported today in the journal Science, reveal a hard but very brittle surface covered with remarkably debris-free craters, 300-foot-high mesas and pinnacles, as well as an unexpectedly large number of narrow jets spewing gas and debris into space.

The results have shattered the idea that comets are conglomerations of space debris, forcing scientists to revise their notions about what comets actually are.

“We were totally stunned by what we saw,” said Donald Brownlee of the University of Washington in Seattle, the principal investigator for the Stardust mission. “We expected to see a tired old surface like someone had dumped old charcoal on it.”

Launched in 1999, the Stardust probe passed within about 150 miles of Wild-2 in January, sending back data on the gas and dust in the tail, as well as the most detailed pictures of a comet’s surface to date. The NASA mission is managed by the Jet Propulsion Laboratory in Pasadena.

Scientists have been keenly awaiting the analyses because comets are some of the oldest members of the solar system. Understanding their makeup could change our understanding of the solar system’s formation and possibly the beginnings of life on Earth.

The researchers were “astonished and intrigued” by the unexpected terrain of Wild-2, Brownlee said.

Unlike craters on the moon, those on Wild-2’s surface are sharply defined, many having nearly vertical, cliff-like walls and lacking much dust or debris around them. Some even appear to have gouges down the walls, apparently from falling rocks, but the rocks that should be at the bottom are mysteriously gone. The debris could be missing because the comet does not have enough gravity to keep material on its surface, Brownlee said.

The sharp edges of the craters, two of which have been named “Left Foot” and “Right Foot,” due to their uncanny resemblance to footprints, indicate that Wild-2 consists of a porous, brittle material, like freeze-dried astronaut ice cream, Brownlee said.

Some craters bear a remarkable resemblance to lunar microcraters -- craters less than half an inch across that are often seen on small moon rocks. Even though the Wild-2 craters are much larger -- as much as 1,500 feet across -- scientists think that similar processes were involved in their creation.

The mesas and pinnacles, in contrast, show that some of the comet’s surface is eroding as it warms during its pass through the inner solar system, leaving behind the towering remnants of harder rock.

Solar heating also seems to be responsible for the narrow jets emanating from the surface. Like a geyser, heated gas collects below the surface of the comet and eventually breaks through, carrying dust and occasionally chunks of the comet with it, said Claudia Alexander, a JPL scientist on the Rosetta comet mission that launched this year.

These chunks disintegrate in the comet’s tail, leaving behind the dust swarms that buffeted the probe as it passed through them. Those bumps, like clear air turbulence, surprised investigators.

“I would have told you it wasn’t going to be like that,” Alexander said. “A bumpy ride -- Oh, heavens, no!”

Some of the debris the probe encountered in the tail was fairly large -- almost grape-sized -- and traveling at nearly 13,000 mph. “We were basically flying through a cloud of bullets,” Brownlee said.

Fortunately, none of the larger pieces hit the craft, and it caught thousands of smaller particles a few hundredths of an inch across or smaller. Stardust is now bringing those samples back to Earth for analysis. They will parachute into the Utah desert in 2006.

Because the comet is so ancient, Brownlee said, “we were actually collecting the true building blocks of the solar system.”

An instrument aboard Stardust called the Cometary and Interstellar Dust Analyzer has transmitted back a preliminary look at the dust in Wild-2’s tail. Like Halley’s comet, Wild-2 is filled with organic molecules, particularly compounds containing carbon and nitrogen that are fundamental to forming the building blocks of life.

Compounds from comets such as this have played a major role in the origin of life on Earth, which was nearly devoid of organic materials at its birth, said Benton C. Clark, a Stardust team member from Lockheed Martin Astronautics in Denver.