An asteroid or comet similar to the one that wiped out the dinosaurs smacked
into Earth 251 million years ago, triggering the biggest extinction in Earth
history. The findings by scientists from the University of Washington,
University of Rochester, NASA, and New York University are published in the
Feb. 23 issue of Science and provide the strongest evidence yet that an
impact played a role in the extinction known as “the Great Dying.”

The impact of the asteroid or comet, estimated to be anywhere from six to 12
kilometers wide, would have released unimaginable fury. “The impact of a
bolide of this size releases an amount of energy that is basically about 1
million times the largest earthquake recorded during the last century. It
was like a magnitude 12.0 earthquake on the Richter scale,” says Robert
Poreda, associate professor of earth and environmental sciences at the
University of Rochester and one of the authors.

The impact and rapid extinction occurred simultaneously with some of the
most extensive volcanic activity the world has ever seen: More than 1.6
million cubic kilometers of lava, enough to cover the entire planet with 10
feet of lava had it spread evenly around the globe, oozed out of the ground
in Siberia in a relatively short amount of time, less than 1 million years.

“It was the proverbial blast from the double-barreled shotgun,” Poreda says.
“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. It may be that the combined effects of impact and volcanism are
necessary to cause such a tremendous extinction.”

The evidence for the impact comes in the form of cosmic stowaways, helium
and argon molecules formed elsewhere in the universe that survived a journey
through space and crashed into Earth as part of the impact. The molecules
were encased in carbon molecules known as buckyballs, which are big enough
to hem in small gas molecules and hardy enough to survive a massive impact
between a bolide (asteroid or comet) and Earth.

By making sensitive measurements of different forms or isotopes of the gases
locked within the carbon cages, the scientists determined that the ratios of
helium and argon molecules are characteristic of meteorites and comets and
must have been formed in space.

The gas measurements were made by Poreda and Rochester post-doctoral
associate Andrew Hunt, who were part of a team headed by Luann Becker of the
University of Washington. Becker says that it’s unlikely that the collision
was directly responsible for the extinction; rather, it may have triggered
a series of events, such as the volcanic activity and changes in sea level
and climate, that wiped out more than 90 percent of marine animals and about
70 percent of land vertebrates.

“If the species cannot adjust, they perish. It’s a survival-of-the-fittest
sort of thing,” Becker says. “To knock out 90 percent of organisms, you’ve
got to attack them on more than one front.”

Several scientists have suggested how extensive volcanic activity could
contribute to a large extinction. Volcanoes release tons of sediment and
ash, as well as massive amounts of carbon dioxide, a greenhouse gas, into
the atmosphere. The dust could have blocked out sunlight across the Earth,
preventing plant photosynthesis and causing food chains to collapse. Or,
carbon dioxide could have trapped the sun’s heat, sending temperatures on
Earth soaring.

The coupling of a massive impact and widespread volcanic activity also
occurred in the more widely publicized extinction event that wiped out the
dinosaurs 65 million years ago, at the K/T (Cretaceous/Tertiary) boundary.
Scientists generally agree that an impact on Mexico’s Yucatan Peninsula
played a role in that extinction. Simultaneously, there was a dramatic
outpouring of lava in present-day India, in a complex known as the Deccan
Traps. Geologists have shown that both that eruption and the Siberian Traps,
both known as flood basalt volcanism, originated as a plume from the Earth’s
mantle deep beneath the Earth’s crust.

“These two extinctions are like bookends for the age of the dinosaurs,”
Poreda says. “The P/T boundary helped to usher in the age of the dinosaurs,
and the K/T boundary snuffed it out.

“There has been lots of flood basalt volcanism over time, and many impacts,
but these impact events caused major extinctions. Both coincided with
periods of heavy volcanic activity. It’s possible that you need both the
impact trigger and the major eruption of flood basalts to tip the Earth
over the edge, to really put the ecosystem under stress,” says Poreda.

The paper in Science is the latest in a series by Becker, Poreda, and
colleagues that exploits buckyballs, or fullerenes, to learn more about our
universe. Buckyballs are molecules made of carbon atoms that link together
in the shape of a soccer ball, forming a tiny cavity where molecules of
such light elements as helium can nestle. Becker has developed methods to
extract the molecules from rock samples, then sends the fullerenes —
which usually form a small blob akin to ear wax — to Poreda for analysis.
He and his colleagues in the Rare Gas Laboratory use a sophisticated gas
spectrometer to measure different isotopes of elements like helium, argon,
and xenon. The samples cited in the Science paper came from the Permian-
Triassic (P/T) boundary in China, Japan, and Hungary.

In 1996 Becker, Poreda and colleagues discovered that fullerenes found
in a huge impact crater near Sudbury, Ontario came from space nearly two
billion years ago and arrived on Earth intact. Last year they showed that
even more complex carbon molecules, with as many as 200 atoms, had survived
an impact from space at the same time as an impact wiped out the dinosaurs
at the K/T boundary.

Both papers showed that it’s possible for comets and meteorites to deliver
organic compounds to Earth, adding credence to the theory that early life
on Earth was somehow seeded from space with complex carbon compounds.
Scientists say it’s possible that carbon cages might provide a template or
skeleton for other molecules, and the gases they carried to Earth might
somehow make up part of our atmosphere.

The project was funded by NASA and the National Science Foundation. Also
contributing to the research were Theodore Bunch of NASA Ames Research
Center in California, and Michael Rampino of New York University.