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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. Those in turn led to species extinction on a wholesale level, said Luann Becker, UW acting assistant professor of Earth and Space Sciences.
"If the species cannot adjust, they perish. It's a survival-of-the-fittest sort of thing," Becker said. "To knock out 90 percent of organisms, you've got to attack them on more than one front."
Becker; Robert Poreda and Andrew Hunt from the University of Rochester, N.Y.; Ted Bunch of the National Aeronautics and Space Administration's Ames Research Center at Moffett Field, Calif.; and Michael Rampino of New York University and the Goddard Institute of Space Sciences, present their findings in the Feb. 23 edition of the journal Science.Funding for the research came from NASA's Astrobiology and Cosmochemistry programs and the National Science Foundation.
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 - a much higher level of complex carbon molecules called buckminsterfullerenes, or Buckyballs, with the noble (or chemically nonreactive) gases helium and argon trapped inside their cage structures. 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, who invented the geodesic dome.
 Luann Becker holds a rock from the Permian-Triassic mass extinction 250 million years ago. Carbon molecules called fulerenes found in this rock are considered to be the sign of comet or asteroid collision with Earth. (UW Photo by Kathy Sauber) |
The researchers know these particular Buckyballs are
extraterrestrial because the noble gases trapped inside have
an unusual ratio of isotopes. For instance, terrestrial
helium is mostly helium-4 and contains only a small amount of
helium-3, while extraterrestrial helium - the kind found in
these fullerenes - 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 at the Permian-Triassic boundary means they were delivered by a comet or asteroid. The researchers estimate the comet or asteroid was 6 to 12 kilometers across, or about the size of the asteroid that left the huge Chicxulub crater near what is now the town of Progresso on Mexico's Yucatan Peninsula 65 million years ago. That impact is believed responsible for the extinction of the dinosaurs. The scientists determined the size based on two factors - if the body were smaller than 6 kilometers the effects wouldn't be seen globally, as they appear to have been; if it were larger than 12 kilometers there would have to be more gas-laden fullerenes distributed globally. |
| 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, in as few as
8,000 to 100,000 years, which the latest research supports.
"That's a microsecond in geologic time," Becker
said. 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 at the concentration found in sediments from the time of the dinosaur extinction. Becker believes that difference might be because the two space bodies that slammed into Earth had different compositions. While the findings illustrate that impact with large space bodies can be detrimental to life on Earth, Becker noted that there also is evidence they might have been key to life starting here in the first place. Some scientists believe the first life-forming chemicals were deposited on Earth in collisions with comets or asteroids, and some believe comets carried virtually all of the water that exists on the planet today. |
 This image depicts a fullerene with noble gas trapped inside. (University of Washington) |
"It took us two years to do this research, to try to narrow it down enough so that we could see this fullerene signature," she said.
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For more information, contact Becker at (206) 221-6749 or lbeck00@u.washington.edu; Poreda at (716) 275-0051 or poreda@earth.rochester.edu; Hunt at (716) 275-8691 or hunt@earth.rochester.edu; Bunch at (520) 717-1916 or tbear1@primenet.com; or Rampino at (212) 998-3743 or mrr1@nyu.edu
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