A devastating impact from an asteroid or comet may have been responsible for the world's biggest mass extinction 251 million years ago, new research suggests.
After a decade-long search, geologists have uncovered telltale evidence of an impact in rocks from Japan and China. The rocks contain small amounts of spherical, cage-like carbon molecules called buckminsterfullerenes, or buckyballs, which are filled with the same blends of helium and argon isotopes found in meteorites.
The evidence is far from conclusive, however. Years of searching have failed to find other solid evidence of an impact at the end of the Permian period, such as enriched iridium levels and shocked quartz crystals-ones containing fractures.
"What is perplexing," says Peter Ward, a palaeontologist at the University of Washington, is that none of the rocks at the Permian boundary resemble the debris layers from the Cretaceous impact that wiped out the dinosaurs 65 million years ago. Yet, says Andrew Knoll, a Harvard University palaeontologist, "for the first time, one wants to take an impact very seriously."
A huge impact could have caused the Permian extinction, which wiped out most plants, 90 per cent of marine species, and 70 per cent of land animals in less than 100,000 years (see "Target Earth"). However, a lack of direct evidence led many researchers to blame the Permian mass extinction on massive volcanic eruptions that occurred in Siberia at the time.
Buckyballs from the end of the Cretaceous had previously been found, so Luann Becker of the University of Washington in Seattle and colleagues painstakingly analysed rocks from the end of the Permian. In samples from China and Japan, the concentrations of isotopes are 50 times higher in the layer of sediments that formed at the very end of the Permian than in nearby layers. The amounts are tiny. "We're finding only parts per billion of fullerenes," she says.
The abundance of helium and argon isotopes trapped inside the buckyballs matched the levels found in carbonaceous chondrite meteorites. "We think the fullerenes formed around a star outside our Solar System," says Becker. They were then carried into the Solar System and survived the impact to be scattered around the globe.