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Comet Not Like Scientists Expected

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Los Angeles Times

Published: January 25, 2008

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 known as 81P/Wild 2 shows that some comets are very much like asteroids 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 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 like an asteroid but is missing markers from the outer solar system, the home of most comets.

Picture Isn't 'Simple'

"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 long has 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.

This assumption was bolstered by decades of studies of comet dust captured by high-altitude balloons and aircraft.

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-temperature heating processes in the solar system, Ishii said.

Evidence Possibly Destroyed

Meanwhile, the research team found that two primitive materials normally found in interplanetary dust particles, glass with embedded metals and sliverlike rock-forming minerals, have not been found in the expected quantities.

"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.

One explanation, according to Ishii, is that the capture process destroyed some evidence.

When the spacecraft flew through the comet's tail, its tennis racket-shaped capture grid was slammed by thousands of particles, some the size of BBs.

That doesn't explain why only inner solar system material would survive, however.

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