Chunk of Death-Dealing Asteroid Found.

Sifting through seafloor muck from the bottom of the Pacific, a geological detective has recovered what appears to be a piece of the murder weapon that silenced two-thirds of Earth's species 65 million years ago and brought to a close the reign of the dinosaurs.

The tiny extraterrestrial rock--only twice the thickness of a dime--holds the potential to solve a mystery that has nagged scientists for nearly 2 decades: What kind of object smacked the planet and unleashed the mayhem? "There is this significant question whether it was an asteroid or a comet," says the discoverer, Frank T. Kyte of the University of California, Los Angeles. "The [new] data would support an asteroid."

Kyte found the geological gem in a cylindrical core of sediments pulled up from the middle of the North Pacific. Amid the monotonous chocolate-brown clay, Kyte spied a small speck of pale clay containing an angular rock, 2.5 millimeters across. From previous chemical analysis of the core, Kyte knew that the rock sat in sediment contemporaneous with the mass extinction, which marks the boundary between the Cretaceous (K) and Tertiary (T) periods.

Geologists have spent nearly 20 years pursuing the killer from the K-T boundary. In the early 1990s, researchers identified on the Yucatan peninsula a giant, buried crater that had formed at the time of the extinctions. But the planet-wrenching energy unleashed by the impact had seared away all signs in the crater that could pinpoint the responsible body.

Kyte reports that the newfound rock has characteristics similar to a class of meteorites known as carbonaceous chondrites, fine-grained carbon-rich rocks peppered with little balls of silica-rich minerals, such as olivine. The silicate minerals in these meteorites often contain grains of nickel-iron metal.

Within the Pacific meteorite, Kyte found clay with a texture similar to olivine and with iron oxides inside. He interprets these materials as relicts of the original meteorite, transformed over the millennia.

Because Kyte's meteorite dates to the same time as the Yucatan crater, the chances are high they are related, he says. "This is really the first thing we can say is a piece of a meteorite from the K-T boundary," says Kyte, who described his discovery in the Nov. 19 Nature.

"He has made a pretty good circumstantial argument that this was a piece of the meteorite that was the culprit for this havoc," says Harry Y. McSween Jr., a meteorite scientist at the University of Tennessee in Knoxville. The discovery should prompt others to look for corroborating evidence from different sites, he says.

From the meteorite's composition, Kyte links it to several classes of carbonaceous chondrites, which are thought to come from asteroids. Occasionally, the orbits of asteroids become unstable and they veer across Earth's path.

Asteroids hit the planet at about half the speed of comets, and calculations suggest that some fraction of an incoming asteroid would survive the cataclysm,

says H. Jay Melosh of the University of Arizona in Tucson. In the Yucatan crash, he says, the shattered backside of the asteroid could have been lofted back into space and then sprayed the Pacific with tiny meteorites. Geologists have postulated that the impact filled Earth's atmosphere with poisonous gases, knocked the climate out of whack, sparked global conflagrations, blocked out the sun, and triggered other unpleasant side effects.