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Science goes deeper with Antarctic meteorites

Press Release From: National Science Foundation
Posted: Saturday, February 14, 2004

image By Brien Barnett, The Antarctic Sun

As the Antarctic Search for Meteorites team returns this week with newfound treasures from the ice, scientists in other parts of the world are studying previously collected samples in new ways.

The United States meteorite collection has tripled in the past 10 years thanks to ANSMET, said Timothy McCoy, a geologist and curator of the Smithsonian Institution's meteorites.

"They've gotten more from Antarctica in 25 years than the previous 500." McCoy said. "It's an enormous boon to science to get this material from the Ice. And it's pristine material."

Of the 15,000 meteorites in the U.S. collection, Antarctic meteorites number about 10,000. As a comparison, the largest Antarctic collection in the world is at Japan's National Institute of Polar Research and numbers about 16,700 meteorites, according to the institute's Naoya Imae.

Until recently, techniques to conduct analysis other than visual and microscopic inspections of the surface were tightly controlled and rarely performed. The rocks were rare and the potential for different discoveries was too valuable to risk a sample being destroyed.

However, today the science of studying early solar history is advancing because of the sheer number of specimens and newly developed techniques that obtain and analyze minute samples of the sub-surface parts of meteorite.

Before the ANSMET team returned to the field late last year, Ralph Harvey, who heads the team, said the U.S. collection had now reached the point where "destructive analysis" is more acceptable as a way to learn the characteristics of a meteorite much more quickly.

McCoy said the Meteorite Working Group, the organization that oversees the U.S. collection, receives about 100 requests a year to do destructive analysis. He said about 90 percent of those requests are approved after careful study.

Kevin Righter, the new Antarctic Meteorite Curator at Johnson Space Center in Houston, said destructive analysis is actually less destructive than the phrase implies.

One of the new techniques involves firing a laser at a sample to generate a circular crater of about 50 microns in size. A micron is a scientific unit of measurement. There are about 1 million microns in a meter.

Another method is to check the sample with an ion microprobe, which generates small pits in the surface of the sample of about 15 to 20 microns in size.

With each technique, scientists analyze subsurface material and, using a mass spectrometer, measure isotope ratios that yield information about the age and origin of meteorites.

Some people study meteorites because they are basaltic and represent volcanism on another planet or asteroid, Righter said. Those scientists study elements that will tell them about volcanic processes similar to people who study volcanoes on earth. Others try to determine the age of the sample from its radioactive isotopes.

Righter said the new techniques for studying the specimens aid that quest.

"They're of great interest because they allow the determination of new isotope ratios and new elements that haven't been able to be analyzed in the past," Righter said.

In addition, the ANSMET team has recovered 12 lunar and 10 Martian meteorites. Those are particularly interesting to scientists because they are like getting a peek at those celestial bodies without having to go there. And sometimes, the meteorites spur exploration such as the current rover missions to Mars.

"We can link those missions directly to the arguments about life in an Antarctic meteorite," McCoy said. "That resulted in a billion-dollar mission to Mars."

Righter oversees the initial characterization of meteorites received from the field in Antarctica and tracks scientists studying them. The specimens themselves are first classified, then stored, some at a giant Smithsonian Institution warehouse in Suiteland, Md.

Each field season, the ANSMET team recovers several dozen to hundreds of samples. One year, about 1,000 meteorites were recovered. All of them are processed the same, careful way. Righter said his group of three researchers has a backlog of about 1,000 samples at any given time, each of which takes about a half-hour to process. His team publishes a semi-annual newsletter, Antarctic Meteorite, to update the scientific community as samples are processed.

The initial characterization and classification of a type of meteorite called an ordinary chondrite has relied on taking a small sample of the specimen, crushing it up and analyzing its composition under a microscope. For other specimens, thin sections are created and analyzed using a petrographic microscope and an electron microprobe.

"It's been working well for over 25 years," Righter said.

The advanced techniques have also led to new discoveries on previously analyzed rocks, McCoy said.

"Over the last 25 years we've gone from not knowing they were there to knowing to being able to isolate them," McCoy said. "It's like burning down the haystack to find the needle."

NSF funded research featured in this story:
Ralph P. Harvey, Case Western Reserve University, http://www.cwru.edu/affil/ansmet

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