[meteorite-list] Asteroid Sample Nails Meteorite Source (Hayabusa/Itokawa)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 25 Aug 2011 14:35:13 -0700 (PDT)
Message-ID: <201108252135.p7PLZD3d001800_at_zagami.jpl.nasa.gov>

http://www.sciencenews.org/view/generic/id/333756/title/Asteroid_sample_nails_meteorite_source

Asteroid sample nails meteorite source
Dust shows where most space rocks come from
By Nadia Drake
Science News
August 25, 2011

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

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?' 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."
Received on Thu 25 Aug 2011 05:35:13 PM PDT