2002-01-28 | SCIENCE
The Great Dying

The Permian-Triassic (P-T) extinction 250 million years ago was the most severe of several mass extinctions in Earth history. Over 90 percent of species in the oceans were wiped out, along with 70 percent of species on land, making the P-T far worse than the famous Cretacious-Tertiary extinction, which ended the reign of the dinosaurs.

Scientists have proposed several possible causes of the P-T extinction. To identify the culprit, NASA researchers led expeditions to rocky areas in Hungary, Japan and China where the geologic boundary between the Permian and Triassic periods is exposed. The team examined fullerenes—soccer-ball-shaped carbon molecules also known as "buckyballs"—taken from the boundary region of the rock. Trapped inside the cage-like fullerenes were helium-3 and argon-36 atoms, isotopes much more common in space than on Earth. The findings are evidence that an asteroid or comet struck the Earth at the time of the P-T extinction.

But other factors may have contributed to the catastrophe. A massive volcanic fissure in what is now Siberia discharged a million and a half times more lava than Mt. St. Helens did in 1980. What's more, the continents had drifted together to form the supercontinent Pangea, thereby altering global climate patterns. Life on Earth was already under assualt; the impact from space may have simply been the straw that broke the camel's back. Understanding how events originating in space affect life on Earth is a significant focus of astrobiology.

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