Comet Tied to a Mass Extinction

By Guy Gugliotta
Washington Post Staff Writer
Friday, February 23, 2001; Page A03

Scientists presented strong evidence yesterday that the worst cataclysm ever to befall the planet was triggered by a comet that collided with Earth in a spectacular explosion 250 million years ago.

Researchers found molecules of "stardust," containing trapped gases that could have come only from outer space, in ancient sediments laid down at various locations around the planet at the time of the catastrophe.

The molecules were carried to Earth by a comet whose impact caused oceans to die and killed 90 percent of all marine species, the researchers said. Seventy percent of land species also vanished, perhaps starving to death because there were no plants left to eat.

The mass extinction marks the boundary between the Permian and Triassic periods of geologic time and is the worst of five similar catastrophes during Earth's prehistory, the most recent of which scientists believe led to the extinction of dinosaurs 65 million years ago.

The impact may have triggered or exacerbated volcanic eruptions that spewed sheets of lava across an area two-thirds the size of the United States. Clouds of dust and grit probably flew into the atmosphere, provoking massive global warming and downpours of acid rain or wreathing Earth in a chill twilight like an ancient version of nuclear winter.

On land, thousands of species of insects, reptiles and amphibians died, along with the evergreens that formed their habitat. In the ocean, coral formations vanished, taking with them snails, urchins, sea lilies and some fish. The event destroyed every species of trilobite, the cockroach-like creature whose fossils are familiar to schoolchildren.

Scientists have long speculated that the Permian-Triassic extinction began with the impact of a comet or asteroid, but little evidence existed until a team led by University of Washington geochemist Luann Becker found soccer ball-shaped carbon molecules in the Permian sediments. Known as fullerenes, or "Buckyballs," the molecules contained the distinctive mix of the
gases helium and argon that is found only in space. These molecules probably came from a comet.

"Pressures need to be very high for this [formation of Buckyballs] to happen, and stars appear to be a good place," Becker said. "The asteroid or comet was a messenger from a collapsing star."

Becker said the space invader was four to eight miles across, "big enough to spread the fullerenes globally," but not in large concentrations. The team examined shale and chert in China, Japan and Hungary and reported its findings in today's issue of the journal Science. The results were presented at a news conference sponsored by NASA, which helped fund the research.

Stanford University life scientist Christopher Chyba said the research suggests that Earth may be ripe for a potentially apocalyptic encounter with a space object about every 100 million years and that a civilization "only has a finite period of time to evolve a technological means to avoid the impact threat."

But he emphasized that scientists believe they have identified all of the objects that could threaten Earth on a cataclysmic scale, and about 40 percent of the medium-size objects with serious damage potential. "If we're on a fuse, it's a very long fuse," he said. Humans would have decades "or even centuries" to prepare for the next impact.

Becker's estimate of the Permian-Triassic object's size was about the same as that of the asteroid that punched an enormous crater in Mexico's Yucatan Peninsula, the event scientists regard as the trigger for dinosaurs' extinction.

No crater has been found for the Permian-Triassic impact, although some research has suggested a touchdown in Western Australia. Becker said the event occurred so long ago that any impact crater may have been absorbed back into Earth's crust or erased by millions of years of plate tectonics.

Still, the fossil record leaves no doubt that a mass extinction occurred 251.4 million years ago, ending the Permian Period and emptying the world of most of its life forms before the onset of the Mesozoic Era and the age of dinosaurs.

It took about 5 million years before the sea began to evolve the rich mixture of species that would lead to the modern ocean. On land, the dawn of the Triassic Period was dominated by Lystrosaurus, a tusked, pig-sized mammalian reptile that ruled Earth until other, and ultimately much larger, species displaced it.

"It's a far more significant event" than the Yucatan explosion, said paleontologist Douglas Erwin of the National Museum of Natural History. "In the ocean, it is the transition from the dawn ecosystem to modern organisms, wiping out 300 million years of history."

Erwin stressed, however, that "this was one of the most complicated events in the history of the planet" and that the new research "isn't the end of the story." Instead, he added, "it's a step along the way."

"We have a number of things going on, and it's not clear they are all tied together," Erwin said.

Erwin noted that the extinction occurred contemporaneously with an enormous outpouring of "flood basalts" from volcanoes in what is now northern Siberia, a possible source of airborne ash and sulfates that could cause catastrophic acid rain and global warming.

Scientists have not linked the volcanic activity directly to an extraterrestrial impact, but Becker team member Robert Poreda of the University of Rochester suggested that the impact could have converted ongoing volcanic activity into a "catastrophic eruption," leading to the extinction.

                                 © 2001 The Washington Post Company