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Meteorites: quick change in a freezer.

Meteorites: Quick change in a freezer

Scientists favor Antarctica as a hunting ground for meteorites, not because an unusual number fall there, but in part because meteorites are more easily seen against the barren, frozen terrain. These extraterrestrial chunks are prime examples of their kind, with valuable details preserved against the erosion of more temperate climes for thousands or millions of years in the arid chill at the bottom of the world.

Even there, meteories can rust or change in other ways, but it has been unclear just how rapidly such weathering takes place in Earth's deep-freezer -- how long the "space rocks" actually remain pristine. Now a group of scientists reports what James L. Gooding of NASA's Johnson Space Center in Houston calls "the first age-date for a weathering product" on an Antarctic meteorite. The rock itself, a 220-kilogram chunk recovered in an area called Lewis Cliff, fell to Earth at least 32,000 years ago. But the researchers have determined that it also bears a tiny patch of a powdery, white mineral that formed on it in the 1950s.

The substance, a hydrous magnesium carbonate called nesquehonite, appears from isotopic measurements to have resulted from a reaction between the meteorite's original minerals and terrestrial water and carbon dioxide. The dating was possible because the carbon in the carbon dioxide provided an isotope "tracer" caused by nuclear tests in the Earth's atmosphere.

"Most workers," says A.J. Timothy Jull of the University of Arizona in Tucson, writing with Gooding and other colleagues in the Oct. 21 SCIENCE, "have presumed that weathering proceeds slowly in the cold, dry Antarctic environment." The results, however, suggest that, "at least for salt formation [such as necquehonite], weathering may be sufficiently rapid that most observable effects can develop in tens of years rather than over thousands of years."

The fact that the nesquehonite formed so rapidly "suggests, but does not prove" that most weathering effects on Antarctic meteorites occur after they have been "exhumed" from beneath the ice, according to the researchers. Some scientists have expressed the hope that it may someday become possible to trace the movements of the Antarctic ice by comparing the time between a meteorite's descent through the ice on Earth and its reexposure to air, though much more work remains to be done. Another concern is that chemical changes in Antarctic meteorites on Earth could mislead scientists trying to identify the materials available in the early solar system when the rocks formed.
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Title Annotation:search for meteorites in Antarctica
Author:Eberhart, Jonathan
Publication:Science News
Date:Oct 22, 1988
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