Image: LUANN BECKER University of Washington

Some 250 million years ago, our planet was rocked by an event that triggered the most severe mass extinction in history. Indeed, 90 percent of all marine animals and 70 percent of terrestrial vertebrates perished, leading some scientists to refer to the episode as the Great Dying. New research, reported today in the journal Science, provides the strongest evidence yet that the event behind this extinction was an asteroid or comet crashing into Earth.

Though the offending space rock is estimated to have been six to 12 kilometers across, the evidence for its impact comes from entities orders of magnitude smaller: complex carbon molecules known as buckminsterfullerenes, or buckyballs (right). Luann Becker of the University of Washington and her colleagues extracted the buckyballs from rocks retrieved from the boundary between the Permian and Triassic periods. Measurements of the different isotopes of gases trapped in the cagelike buckyballs revealed unusual ratios of helium and argon molecules, indicating that the molecules are extraterrestrial. "These things form in carbon stars," Becker explains. "That's what's exciting about finding fullerenes as a tracer."

As if this massive impact were not dramatic enough, extensive volcanic activity appears to have erupted at around the same time. "It was a proverbial blast from the double-barreled shotgun," says team member Robert Poreda of the University of Rochester. "We're not sure of all the environmental consequences, but with both the impact and with the volcanic activity, we do know that Earth was not a happy place."