"Hot" Rocks Found in Icy Comet

John Roach
for National Geographic News
March 14, 2006
Comet Wild 2 has spent most of its life in the most frigid reaches of our solar system. But at the comet, NASA's Stardust spacecraft has found minerals born of intense heat near the sun or other stars, scientists announced yesterday afternoon.

The surprising finding may alter our understanding of how comets form, they said.

(Watch video: Stardust Probe Mission Overview.)

Scientists have long thought of comets as cold, billowing clouds of ice, dust, and gases that formed on the edge of our solar system. The new discovery suggests that comets may have more complex histories.

"In the coldest part of the solar system, we have found samples that formed at extremely high temperatures—so the hottest samples [are in] the coldest place," said Donald Brownlee, an astronomy professor at the University of Washington in Seattle.

"It is quite exciting to find these things at the edge of the solar system," added Brownlee, who spoke at a briefing with reporters Monday at NASA's Johnson Space Center in Houston, Texas.

Brownlee is the principal investigator for the Stardust mission. The spacecraft launched February 7, 1999, to collect dust swirling off Wild 2 (pronounced Vilt 2). The capsule containing the dust samples landed in the Utah desert on January 15.

Findings from preliminary analysis of these samples were presented yesterday in Houston at the Lunar and Planetary Institute's annual science conference.

Star Origins

Many of the mineral samples examined could not have formed in or around the icy comet, Brownlee said. Rather, they were either shot out to the region near Pluto from the innermost and hottest regions of our solar system, or they formed in hot regions around other stars (interactive map of our solar system).

Further analysis of the samples will reveal whether the minerals came from our solar system or formed elsewhere in the universe, he added.

They key will be in the chemical signatures of the minerals, Brownlee said. Minerals from our solar system are chemically distinct from those found around other stars.

Michael Zolensky is Stardust curator and a co-investigator at Johnson Space Center. He explained that our solar system is broken up into several zones.

The system includes regions where minerals form in high heat, near the sun, and areas where they form in coldness, on the fringes out by Pluto.

Finding such hot materials in a comet born so far out in the solar system is a surprise.

"It suggests that, if these are really from our own sun, they've been ejected out—ballistically out—all the way across the entire solar system and landed out there," he said.

Stardust Collection

The Stardust spacecraft collected the samples using a tennis-racket-shaped device filled with cubes of a light, porous material called aerogel. The gel is 99.8 percent air and can trap delicate dust particles without damaging them.

Once the capsule was recovered from the Utah desert, scientists shipped it to Johnson Space Center. There, six of the aerogel cubes were sliced up and sent to about 150 researchers around the world.

Most of the particles are smaller than the width of a hair. Thousands are embedded in the aerogel.

Among the minerals returned to Earth is olivine—"probably one of the more common minerals in the universe," Brownlee said.

For example, it is the primary component of green sand found on some Hawaiian beaches. Scientists believe olivine forms in great heat close to stars, Brownlee added.

The samples also include high-temperature minerals rich in calcium, aluminum, and titanium.

The question facing scientists is why these minerals were part of Wild 2, a comet that formed beyond the orbit of Neptune when our solar system began taking shape some 4.6 billion years ago.

"When these minerals formed they were either red-hot or white-hot grains, and yet we collected them at a comet [from] the Siberia of the solar system, out by the orbit of Pluto," Brownlee said.

Further analysis over the coming months and years may yield answers to the new questions the findings raise about comet formation.

"We can't give you all the answers right now. It's just great we have new mysteries to worry about now," Zolensky said.

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