NASA discovers another mystery based on Stardust comet mission

SPACE CENTER, Houston -- NASA scientists have a new mystery to solve: How did materials formed by fire end up on the outermost reaches of the solar system, where temperatures are the coldest?

The materials were contained in dust samples captured when the robotic Stardust spacecraft flew past the comet Wild 2 in 2004. A 100-pound capsule tied to a parachute returned the samples to Earth in January.

The samples include minerals such as anorthite, which is made up of calcium, sodium, aluminum and silicate; and diopside, made of calcium magnesium and silicate. Such minerals only form in high temperatures.

"That's a big surprise. People thought comets would just be cold stuff that formed out ... where things are very cold," said NASA curator Michael Zolensky. "It was kind of a shock to not just find one but several of these, which implies they are pretty common in the comet."

The discovery raises questions about where the materials in comets form, he added.

One theory is that particles from the outer reaches of the solar system slowly move toward the sun, where they are set ablaze and shot back out. A scientific model once suggested that might be a natural occurrence, but it wasn't accepted because materials tend to cluster in zones the farther they are from the sun, Zolensky said.

If the model were true, materials would mix more, the NASA scientist said.

"It raises a question of why we still see zoning in the asteroid belt. It is a big mystery now," Zolensky said. "It's kind of really exciting."

He said it is also possible that the comet particles could have been formed in another solar system and catapulted into our solar system.

To determine where the particles originated, scientists are now studying their isotopic makeup. About 150 scientists worldwide have been studying the dust since it arrived.

During the $212 million mission, the Stardust spacecraft looped around the sun three times to capture the interstellar and comet dust. The comet dust was captured in a silicone-based material contained in a tennis racket-sized collector mitt.

The mother ship, which has traveled nearly 3 billion miles, remains in permanent orbit around the sun. The next time it flies by Earth will be in January 2009.

Don Brownlee, a University of Washington astronomer who is the mission's principal scientist, said in a few weeks or months he and his colleagues hope to know more.

"It depends on whether the isotopic composition indicates these grains are from our solar system or from another star," he said. "It's a real exciting mystery story. So stay tuned."

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On the Net:

www.nasa.gov/stardust