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"SPACE WEATHERING" CEMENTS ASTEROID-METEORITE LINK

SKY & TELESCOPE'S NEWS BULLETIN - SEPTEMBER 1, 2000

For decades planetary scientists have struggled to learn which asteroids are the source of ordinary-chondrite meteorites, which make up more than 80 percent of all known falls on Earth. The most likely "parents" were thought to be the rocky bodies called S-types that dominate the inner asteroid belt. But there's a compositional mismatch: the S asteroids' slightly red color and weak spectral lines implied that they contained too much metal to be the parent bodies for the chondrites. Even Galileo's flybys of the S asteroids 243 Ida and 951 Gaspra failed to solve the mystery. So some researchers proposed that some kind of "space weathering" was masking the asteroids' true optical properties. Others posited the existence of an entire population of undiscovered asteroids that were peppering Earth with their chondritic castoffs.

The solution, it turns out, had unknowingly been discovered 25 years ago. In 1975, while probing why the lunar regolith becomes darker and redder with time, Bruce Hapke (now at the University of Pittsburgh) and two colleagues proposed that the solar wind slowly evaporated the lunar soil, causing individual grains to be coated with a microscopic film of iron droplets just a few nanometers (billionths of a meter) across. "Our suggestion was completely ignored at the time," recalls Hapke, in part because no one could find any trace of the vapor deposits.

But at this week's meeting of the Meteoritical Society, held in Chicago, meteorite specialists at last convinced themselves that space weathering really does happen, and that "nanophase iron" is primarily responsible for it. "It's a pretty tight case," notes Carle Pieters (Brown University). "Observations, experiments, and theory are now all telling us the same thing." The stage for this consensus was set in 1993, when electron microscopy by NASA-Johnson Space Center researchers Lindsay P. Keller and David S. McKay revealed the tiny iron droplets in lunar samples from the Apollo 16 and 17 landing sites. They theorized that the iron is deposited from the puff of vapor created when micrometeorites slam into the lunar soil.

"I think everyone now agrees that ordinary chondrites come from the S types," observes Beth Clark (Cornell University). She has found evidence of space weathering in detailed spectra of the asteroid 433 Eros gathered by the NEAR-Shoemaker spacecraft. In Chicago Clark reported that Psyche, the largest crater on Eros, exhibits patches of soil that are much brighter and slightly redder than elsewhere. She believes the bright patches were exposed by tiny landslides. Once a fresh surface is exposed to space, suggests Clark Chapman (Southwest Research Institute), the microscopic film of iron begins to accumulate, first reddening then darkening over time.

Despite the newfound optimism, the case for space weathering won't be considered ironclad until the tiny beads can be found in ordinary-chondrite meteorites themselves. And that, Clark and Pieters agree, may prove difficult even with state-of-the-art microscopic techniques.


Copyright 2000 Sky Publishing Corporation. S&T's Weekly News Bulletin and Sky at a Glance stargazing calendar are provided as a service to the astronomical community by the editors of SKY & TELESCOPE magazine. Widespread electronic distribution is encouraged as long as these paragraphs are included. But the text of the bulletin and calendar may not be published in any other form without permission from Sky Publishing (contact permissions@skypub.com or phone 617-864-7360). Updates of astronomical news, including active links to related Internet resources, are available via SKY & TELESCOPE's site on the World Wide Web at http://www.skypub.com/.

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