Mention "asteroid" or "comet," and the fire-and-brimstone fantasy of an earth-shattering collision will pop into many people's minds. Two thirds of the planet's species, including the dinosaurs, died in the aftermath of one such impact 65 million years ago. But that was a minor tragedy compared with the catastrophic extinction that swept the globe 185 million years earlier. At that time, 95 percent of life in the oceans vanished forever¿and surprising new evidence points to a similar cosmic killer.

Researchers long assumed that gradual changes in climate or sea level prolonged that mass death, which marks the boundary between the Permian and Triassic periods, over half a million years or more. But last year paleontologists who examined marine fossils from Austria and China reported that the doomed Permian creatures disappeared in 8,000 years or less¿a sudden death in geologic terms. No compelling culprit turned up until early March, when the news of possible extraterrestrial involvement appeared in Science. Luann Becker of the University of Washington, Robert J. Poreda of the University of Rochester and their colleagues extracted strange traces of helium and argon from rocks at the site in China and at a third locale in Japan. Helium and argon, both noble gases, exist naturally inside the earth and its atmosphere, but the isotopic signatures of the gases in these particular rocks require a cosmic origin.

   
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