Stardust's Comet Clues Reveal Early Solar System

Richard A. Lovett
for National Geographic News
December 15, 2006
The first ever analyses of comet dust are in, and they stand to revolutionize our understanding of the early solar system, scientists say.

The particles were collected by Stardust, a NASA probe that passed within 150 miles (240 kilometers) of comet Wild 2 (pronounced Vilt 2) to scoop up escaping debris.

(Watch a video overview of the Stardust mission.)

The probe encountered the comet between the orbits of Mars and Jupiter in January 2004 and returned to Earth two years later.

It netted approximately 10,000 dust grains—the first such samples ever retrieved from a distance beyond the moon.

One of the most interesting findings is that 10 percent of the dust appears to have originated near the sun, according to scientists.

Conventional theory has held that comets form from dust grains that had never been close to the sun.

But many of the grains brought back by Stardust contain minerals that only crystallize under extreme heat, scientists said yesterday.

"They formed in the hottest possible places in the solar system," the project's lead scientist, David Brownlee of the University of Washington, said at a meeting of the American Geophysical Union in San Francisco, California.

"It was quite stunning to find this in a body that formed in the coldest place in the solar system."

Brownlee's colleague, Michael Zolensky of NASA Johnson Space Center in Houston, Texas, likened the comet to "a big vacuum cleaner" that swept up dust from all over the solar system as it formed far beyond the orbit of Pluto.

How the inner-system dust got so far from the sun, however, is still a mystery.

The team reports its findings in a series of studies appearing in this week's issue of the journal Science.

Surprise Differences

Wild 2, which is billions of years old, has spent most of its lifetime circling far beyond Pluto—preserving it in frozen isolation from the rest of the solar system.

(Explore a virtual solar system.)

But although the comet contained a great deal of ice, none of the comet's minerals appear to have interacted with liquid water, the scientists said.

That means the ice was always frozen, the researchers explained.

This was not the case for comet Tempel 1, which scientists examined in 2005 when the probe Deep Impact was deliberately crashed into it.

Materials analyzed after that collision showed that Tempel 1 appeared to contain clay and other minerals that can only form in conjunction with liquid water.

(Read "'Deep Impact' Comet Spewed Tons of Water, Study Finds" [April 4, 2006].)

"But we've not seen any evidence of any of those [minerals] in this comet," Zolensky said.

The scientists cannot yet explain this disparity between the two comets.

"Probably some comets are different from other comets," Zolensky said.

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