Asteroid sample nails meteorite source

Dust shows where most space rocks come from

Scientists have scraped the heavens and seen the solar system in a speck of dust.

DUST IN THE WIND Dust recovered from the asteroid Itokawa, pictured here, holds clues to the early solar system and a record of the space rock’s life.

Well, more than 1,500 specks, most less than 50 millionths of a meter in diameter. Plucked from the surface of the asteroid Itokawa by Japan’s Hayabusa spacecraft, the tiny grains carry a record of the solar system’s early days. Now, scientists have decoded the particles and read in them a tale of the asteroid’s history, a story that spans billions of years, from the asteroid’s birth to its future demise.

Teams of scientists in Japan and elsewhere report their findings in six papers published in the Aug. 26 Science.

“I think they have done a stupendous job of characterizing and classifying this asteroid based on a pittance of material,” says planetary scientist Hap McSween of the University of Tennessee in Knoxville.

For starters, the dust reveals which type of asteroid is responsible for the meteorites that litter Earth, says planetary scientist Bill Bottke of the Southwest Research Institute in Boulder, Colo. Until Hayabusa returned with a sample of Itokawa’s surface, scientists couldn’t prove that stony S-type asteroids like Itokawa were the source of the most common space rocks hitting Earth, called ordinary chondrites.

“Most of the time, when you see a shooting star, in all likelihood if it reached the ground it would be a chondrite,” McSween says.

The problem was that gazing at asteroids through a telescope produced a different set of spectral colors than the chunks that had fallen to Earth, says MIT planetary astronomer Richard Binzel.  But Tomoki Nakamura of Tohoku University in Sendai, Japan and colleagues were able to link S-type asteroids with chondrites.

The differences in the spectra are due to the effects of space weather on Itokawa’s surface. Recovered dust grains have been altered by the solar wind and a rain of tiny micrometeorites, the scientists report.

Other analyses of the dust retrieved from Itokawa’s surface show that the material had been exposed for only around 8 million years. Keisuke Nagao of the University of Tokyo figured this out by looking for the effects of high-energy cosmic rays on the particles and levels of solar wind–derived noble gases. The data suggest that charged solar particles and micrometeorites are eroding the asteroid by tens of centimeters per million years. That may not sound like much, but Itokawa is small — only 500 by 300 by 200 meters — and losing material at such a rate will erase the little rocky body entirely within a billion years, Nagao says.

Bottke says he’s not entirely convinced that Itokawa might be sandblasted to death. “I’m not ready to go there yet,” he says. “But I’m not saying they’re wrong.”

Either way, it seems that being a small asteroid in a big solar system isn’t easy. “It’s a hard-knock life,” Binzel says. “These small asteroids don’t live forever. They encounter a planet and get flung out of the solar system or they crash into the sun.”

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