New findings provide evidence that Earth’s most severe
mass extinction — an event 250 million years ago that wiped
out 90 percent of the life on Earth — was triggered by a
collision with a comet or asteroid.

Over 90 percent of all marine species and 70 percent of land
vertebrates perished as a result, according to the NASA-funded
research team, led by Dr. Luann Becker of the University of
Washington (UW), Seattle. The team’s findings will be
published tomorrow in the journal Science.

The collision wasn’t directly responsible for the extinction
but rather triggered a series of events, such as massive
volcanism, and changes in ocean oxygen, sea level and climate.
That in turn led to species extinction on a wholesale level,
according to the team.

“If the species cannot adjust, they perish. It’s a survival-
of-the-fittest sort of thing,” said Becker, UW acting
assistant professor of Earth and Space Sciences. “To knock out
90 percent of organisms, you’ve got to attack them on more
than one front.”

The scientists do not know the site of the impact 250 million
years ago, when all Earth’s land formed a supercontinent
called Pangea. However, the space body left a calling card —
complex carbon molecules called buckminsterfullerenes, or
Buckyballs, with the noble gases helium and argon trapped
inside the caged structure. Fullerenes, which contain at least
60 carbon atoms and have a structure resembling a soccer ball
or a geodesic dome, are named for Buckminster Fuller, inventor
of the geodesic dome.

The researchers know these particular Buckyballs are
extraterrestrial because the noble gases trapped inside have
an unusual ratio of isotopes, atoms whose nuclei have the same
number of protons but different numbers of neutrons.
Terrestrial helium is mostly helium-4, while extraterrestrial
helium is mostly helium-3.

“These things form in carbon stars. That’s what’s exciting
about finding fullerenes as a tracer,” Becker said. The
extreme temperatures and gas pressures in carbon stars are
perhaps the only way extraterrestrial noble gases could be
forced inside a fullerene, she said.

These gas-laden fullerenes were formed outside the Solar
System, and their concentration in the sedimentary layer at
the boundary of the Permian and Triassic periods means they
were delivered by comets or asteroids. The researchers
estimate the comet or asteroid was roughly 3 3/4 to 7 1/2
miles (6 to 12 kilometers) across, or about the same size as
the asteroid believed responsible for the extinction of the
dinosaurs 65 million years ago.

The telltale fullerenes containing helium and argon were
extracted from sites where the Permian-Triassic boundary layer
had been exposed in Japan, China and Hungary. The evidence was
not as strong from the Hungary site, but the China and Japan
samples bear strong evidence, Becker said.

The team’s work was made more difficult because there are few
250 million-year-old rocks left on Earth since most rocks of
that age have been recycled through the planet’s tectonic
processes. “It took us two years to do this research, to try
to narrow it down enough so that we could see this fullerene
signature,” Becker said.

Scientists have long known of the mass extinction 250 million
years ago, since many fossils below the boundary — such as
trilobites, which once numbered more than 15,000 species —
diminish sharply close to the boundary and are not found above
it. There also is strong evidence suggesting the extinction
happened very rapidly, on the order of 8,000 to 100,000 years,
which the latest research supports.

Previously, it was thought that any asteroid or comet
collision would leave strong evidence of the element iridium,
the signal found in the sedimentary layer from the time of the
dinosaur extinction. Iridium was found at the Permian-Triassic
boundary, but not nearly in the concentration as from the
dinosaur extinction. Becker believes that difference is
because the two space bodies that slammed into Earth had
different compositions.

Members of the research team are Becker; Robert Poreda and
Andrew Hunt from the University of Rochester, NY; Ted Bunch of
the NASA’s Ames Research Center, Moffett Field, CA; and
Michael Rampino of New York University and NASA’s Goddard
Institute of Space Sciences, New York. Funding for the
research was provided by NASA’s Astrobiology and
Cosmochemistry programs and the National Science Foundation.