[meteorite-list] Asteroids and Meteorites Reveal Family Resemblance

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri Sep 8 20:10:41 2006
Message-ID: <200609090010.RAA22249_at_zagami.jpl.nasa.gov>

http://www.brown.edu/Administration/News_Bureau/2006-07/06-019.html

September 7, 2006
Contact: Marty Downs (Media_Relations_at_brown.edu)
(401) 863-2476

Asteroids and Meteorites Reveal Family Resemblance
Brown University

Using data collected by the Japanese space probe Hayabusa in a
rendezvous with the 550-meter asteroid Itokawa, researchers have
demonstrated that space weathering occurs even on small asteroids. The
new data, published in Nature, confirm that the mineral composition of
such asteroids is consistent with meteorites fallen on Earth.

------------------------------------------------------------------------

PROVIDENCE, R.I. - Asteroids and meteorites are supposed to be made of
the same stuff - at least that's what earth science teachers have been
telling their students for decades. But until recently, the data didn't
quite fit the story. When researchers compared the near-infrared
reflectance of asteroids (as measured from Earth) and meteorites
(collected on Earth) they found enough differences to raise doubts about
whether the asteroids really could be the source of Earth's meteorites.

A detailed new comparison of the near-Earth asteroid Itokawa with
existing meteorite samples confirms that the process of space-weathering
can explain the difference in reflectance pattern (spectrum) between
asteroids and ordinary chondrites, the most common class of meteorites.

"They [chondritic meteorites] are so abundant, there have to be many,
many asteroid sources," said Takahiro Hiroi, a senior research associate
at Brown University and the paper's lead author, "but we couldn't find
any that matched so clearly, until now. These observations really let us
see space weathering at work."

Over millions of years, the flow of high-energy ions and microscopic
particles vaporizes the surface of asteroids, depositing a thin film
that changes the asteroid's optical properties. Highly weathered areas
tend to appear dark and red. (The near infrared spectrum of such areas
is shifted toward the red end of the spectrum.)

Hiroi visited several museums and collected dozens of samples of fresh,
or newly fallen, meteorites. He rejected many samples because the
oxidation caused by rain and air on the Earth's surface changes the
rock's composition and interferes with the asteroid comparison. Together
with other researchers from the Hayabusa mission, Hiroi compared the
near-infrared reflectance spectra of meteorite samples with spectra
observed at specific locations on the asteroid.

One sample (from a meteorite dubbed Alta'ameem, for the area in Iraq
where it fell) resulted in a near-identical match after correction for
the changes that result from space weathering. Those changes include a
reduction in mean optical path length - usually a sign of smaller grain
size - and an increase in tiny iron particles known as nanophase
metallic iron or npFeo.

Hiroi was able to see the effects of space weathering by taking spectra
from one light and one dark area on the asteroid's surface. Matching the
observed spectra to that of the Alta'ameem meteorite, he estimated that
the highly weathered site contained about 0.069 percent nanophase
metallic iron and the less-weathered site contained about 0.031 percent.
Because Alta'ameem is an LL chondrite, a class that represents only 10
percent of ordinary chondrite meteorites, Hiroi suggests that there must
be many asteroids in near Earth orbit with compositions similar to the
more common L- and H-type meteorites.

Evidence of space weathering has been seen before on moons and larger
asteroids, but such clear evidence is new for smaller asteroids, such as
the 550-meter Itokawa. It had been thought that such bodies, with their
smaller gravitational fields, would quickly be stripped of the weathered
material. This new evidence shows that space weathered material does
accumulate on small asteroids, which probably are the source of most
meteorites.

######
Received on Fri 08 Sep 2006 08:10:37 PM PDT