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Moon Rock Tells All

29 July 2004 (All day)
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Slice of history. This lunar meteorite contains traces of four impacts, its birth, two subsequent impacts (thin capping layers) and its ejection (cracks).

Rocks from space rarely come with a return address as specific as that of Sayh al Uhaymir 169. Scientists now have traced this meteorite, found in the Sultanate of Oman in 2002, back to a particular patch of Moon and dated its departure for Earth. Geologists can even tell how many times it was roughed up by asteroid impacts before it was thrown into space. The meteorite has the most complete pedigree of any ever found on Earth.

If most rocks tell a story with their substance, Sayh al Uhaymir 169 records a soap opera. It was born when a gigantic asteroid smashed into the moon so hard it created an extensive crater called Mare Imbrium and melted underlying rock. Sayh al Uhaymir is mainly composed of that brew, infused with chunks of debris. Two more impacts occurred after this, each raising the rock through the rubble and glomming on extra debris.

The fourth impact, about 340,000 years ago, blasted Sayh al Uhaymir 169 off the surface. In the 30 July issue of Science, Edwin Gnos, a geologist at the University of Bern, Switzerland, and colleagues describe using four isotopic dating methods, each effective over a certain time period, to find the age of each impact. The rock's chemistry nailed its origin in Mare Imbrium.

It's very unusual to trace a lunar rock's history this precisely, says Randy Korotev, a lunar geochemist at Washington University in St. Louis, Missouri. Because the moon is covered in a thick layer of dust and debris, it's often difficult to tell where a rock came from--even rocks from the Apollo mission have uncertain origins. "If impacts on the moon are so efficient that we have rocks on Earth from the Moon, than any rock on the Moon could come from anyplace," he says. For now at least, Sayh al Uhaymir is unique.

Related Sites
Randy Korotev's site
Article about lunar meteorites

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