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.