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Struck from above

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10:30am, December 27, 2007

From San Francisco, at a meeting of the American Geophysical Union

Evidence of an extraterrestrial object striking Earth at the height of the last ice age comes not from a crater in the ground, but from the micrometeorites embedded in the tusks of creatures grazing the Alaskan tundra when the event occurred.

Richard B. Firestone, a nuclear physicist at the Lawrence Berkeley (Calif.) National Laboratory, and his colleagues have examined thousands of fossils collected in Arctic regions. Among those relics, the researchers found eight mammoth tusks—representing about 0.1 percent of the fossils—that have small metallic particles embedded in them. X-ray images reveal zones of shattered material around each particle. When multiple particles are found on a single fossil, all of them appear on the same side of the tusk—as if they came from the same direction, says Firestone.

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