Meteorites from the moon's lava plains.
Scientists have collected thousands of meteorites in Antarctica, where low temperatures keep the rocks from eroding away, a geological oddity concentrates some of them in certain places, and the often snowy and icy terrain makes them easy to spot. Researchers have identified about 10 of these meteorites as coming from the moon. All of the lunar meteorites found heretofore appear to have originated in the heavily cratered highlands that seem to constitute most of the moon's crust. But researchers now report finding three that may have arrived from the lunar lava plains, or maria.
One of these, weighing about 30.7 grams (a little more than an ounce), was gathered in an Antarctic region called the Elephant Moraine, say Jeremy S. Delaney of Rutgers University in New Brunswick, N.J., and Paul H. Warren of the University of California, Los Angeles. Designated MAC88105, this meteorite is a breccia -- a rock composed of numerous basaltic fragments and smaller grains held in a glassy matrix. Its basalt is a type with very low levels of titanium. Although remotely measured spectra have led some researchers to believe titanium is common on the moon, it is rare in basaltic moonrocks returned to Earth by space missions.
According to Delaney and Warren, researchers initially listed the Elephant Moraine as a eucrite--a piece of an asteroid. Later, however, studies of its composition revealed details (such as the ratio of different oxygen isotopes) that strongly suggest the meteorite came from a lunar mare.
The largest of the three apparent mare samples, weighing 442 grams, is one of the largest lunar meteorites ever identified. Researchers have temporarily named it Asuka 31, after Japan's newest Antarctic research station. This one, too, resembles a eucrite, but its ratio of iron to manganese resembles that of other lunar rock types, notes Keizo Yanai of the Institute of Polar Research in Tokyo.
The third meteorite, collected in Antarctica's Yamato Mountains and designated Y793169, weighs only 6.09 grams. Like the other two, it is composed of basalt with very little titanium, suggesting a consistent compositional difference between the moon's maria and its highlands.
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|Date:||Mar 24, 1990|
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