Composition of Comet Samples Surprises NASA Scientists

HOUSTON, March 13 -- Dust samples from a comet formed in deep space unexpectedly contain high-temperature mineral particles that may have been ejected by the young sun at the dawn of the solar system, scientists said Monday.

First-sample results from NASA's Stardust mission suggest that scientists may have to modify the traditional view that comets are bodies of ice and dust composed largely of interstellar material on the outskirts of the solar system.

Instead, the sun, in a process not yet fully understood, may have catapulted material outward even as the "dust disk" that formed the solar system was swirling inward like a whirlpool with the sun at its center, said Stardust lead scientist Donald Brownlee of the University of Washington at Seattle. "We have found fire and ice," Brownlee told reporters at the Johnson Space Center here. "We have found extremely high-temperature minerals coming from the coldest place."

Early analysis revealed minerals that included magnesium iron silicate, known as olivine, or, in its gem-quality variety, peridot; magnesium aluminum oxide, also called spinel; and titanium nitride. Brownlee said all these form at temperatures of at least 2,000 degrees Fahrenheit.

Brownlee said images of young stars often showed not only an inward-swirling disk but also "vertical jets" leaping high above the disk plane. "Even though [solar systems] are formed by things falling in, there are also things flying out," he said.

But it was unclear whether the jets were the source of the comet's unusual minerals, and although Brownlee said that was one possible explanation, the minerals may have come from another star. He said researchers will be able to tell the difference after further chemical analysis.

Stardust, the first spacecraft to return samples from a comet, flew through the shroud of dust and gas surrounding comet Wild 2 on Jan. 2, 2004, trapping perhaps a million tiny particles in a tennis-racket-like collector. The collector had 132 small compartments filled with aerogel, a fine spun glass that is 99 percent empty space, but easily able to muffle the shock of tiny impacts and engulf the particles like a pillow.

The Stardust space capsule parachuted to a flawless landing at the U.S. Army's Dugway Proving Ground in Utah on Jan. 15. The sample canister was flown to a special lab at the Johnson Space Center in the same "clean" facility that houses the moon rocks collected in the Apollo program.

Although the samples were tiny -- the largest are about two-thousandths of an inch in diameter, half the width of a human hair -- researchers are easily able to slice them up for analysis.

The space center's Michael Zolensky, the sample curator, said the Stardust team had emptied only six of the collection cells, from which they had extracted "a couple dozen" of the largest particles.

"By this week, more than 200 samples will be circulating," Zolensky said. "Everyone who wants one, and who is qualified to do the analysis, will have one."

Zolensky said research on the Stardust particles would likely last decades and evolve as instruments improved.

"It won't be just us who look at them," he said. "It will be our grandchildren."

Comets were formed out beyond Neptune of leftover dust, gas and debris from the creation of the solar system. The majority orbit the sun in the icy reaches of deep space, essentially unchanged for 4.6 billion years.

Occasionally, however, a close encounter with one of the outer planets or a disturbance from a passing star alters a comet's orbit, and it comes swooping into the inner solar system.

Wild 2 made this change in 1974 because of a gravity push from Jupiter and has settled into a 6.39-year elliptical orbit around the sun between Mars and Jupiter. Stardust, launched in 1999, caught up with Wild 2 after a 2.1 billion-mile journey, looping around the sun three times to catch its quarry.

Unlike Earth or other planets whose surfaces have been dramatically altered by geological activity, pristine comets offer unusual snapshots of the solar system at its beginnings. Zolensky said most of the Stardust samples tested are minerals and organic compounds expected to be seen in deep space, but "three or four" were the high-temperature surprises.

Brownlee noted that some models suggest that dust migrating inward during formation of the solar system may have created powerful "shocks" as the dust approached the sun, perhaps causing explosions to send high-temperature particles outward.

"It's a mystery story," he said. "Stay tuned."