Struck from above

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.

The particles, which are strongly attracted to a magnet, are mostly made of iron, a common component of many meteorites. Chemical analyses of the particles revealed an abnormally high proportion of nickel and a lower-than-average concentration of titanium—both of which hint at an extraterrestrial origin, Firestone notes. Carbon-dating techniques suggest that most of the iron-peppered tusks are between 30,000 and 34,000 years old.

Previous studies had suggested that several animal populations living on the land bridge connecting Alaska to eastern Siberia at that time—including bison, bears, horses, and mammoths—declined significantly at some point less than 36,000 years ago. The impact of an extraterrestrial object, such as the one purported to have plagued these Alaskan mammoths, could have rendered much of northern Alaska inhospitable for decades, if not longer, says Firestone.

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