Comets, asteroids may share traits

New research is challenging the long-held position that comets and asteroids are as different as lions and lemurs.

Instead, the latest analysis of material from the comet 81P/Wild 2 shows that some comets are very asteroid-like and, presumably, vice versa.

“This is a wake-up call that small bodies in the solar system don’t necessarily come in two flavors,” said Hope Ishii, a physicist at the Lawrence Livermore National Laboratory in California. “Instead, it’s more of a continuum.”

The research, published Friday in the journal Science, contains the latest results from NASA’s Stardust spacecraft, which flew through the tail of the comet and landed in the Utah desert in 2004.

Stardust, managed by the Jet Propulsion Laboratory in Pasadena, Calif., was the first spacecraft to return to Earth carrying comet dust. Its initial results, released in late 2006, showed what scientists at the time called a “zoo” of materials, some of which came from the inner solar system where asteroids originated. As research into the comet dust samples has continued, Ishii said, a picture is emerging of a body that not only looks asteroid-like but is missing markers from the outer solar system, the home of most comets.

“We went to a comet, got a sample and brought it home,” said University of Washington astronomer Donald Brownlee, a principal investigator on the Stardust mission. “We all expected the picture that emerged to be simple. It’s not.”

Conventional scientific theory has long held that asteroids were cooked by the sun’s heat before winding up in the asteroid belt between Mars and Jupiter. Comets, it was thought, were never cooked. Instead, they were thought to contain the most primitive material in the solar system: dust from other stars and other ancient material, as well as the ice and gas that give comets their tails when their orbits take them close to the sun.

Wild 2 tells a different story. Among the compounds scooped up by Stardust are calcium aluminum inclusions. These are produced by some of the highest heating processes in the solar system, Ishii said.

“The material is a lot less primitive and more altered than materials we have gathered through high-altitude capture in our own stratosphere from a variety of comets,” Ishii said.

John Bradley, a coauthor with Ishii of the new research, said he wasn’t disappointed by the results.

“I think this is science in action,” he said. “It’s really exciting because it’s just not what we expected.”

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