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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.