Killer Collision: Dino demise traces to asteroid-family breakup

A huge chunk of rock hit Earth 65 million years ago, setting off events that wiped out the dinosaurs. That chunk, astronomers now say, was a wayward fragment from a collision between two giant asteroids in the inner part of the asteroid belt, which lies between Mars and Jupiter. The new study adds to the evidence that both Earth and moon have been bombarded by about twice the usual number of asteroid fragments during the past 200 million years.

Earth is now at the tail end of this asteroid shower, say Bill Bottke of the Southwest Research Institute in Boulder, Colo., and his colleagues in the Sept. 6 Nature.

The researchers began their study by pondering the pattern of craters on 951 Gaspra, a member of the Flora family of asteroids. Searching for objects that could have caused the impacts, the team examined a much darker, harder-to-detect group of asteroids, the Baptistina family, that resides close to the belt’s inner edge and near the Flora family.

Bottke and his colleagues were intrigued to find that the Baptistina family stretches over a region containing two gravitational-escape hatches, places where a gentle nudge could kick an asteroid out of the belt and into the inner solar system toward Earth. Once ejected, members of the Baptistina family could wallop a lot more objects than just Gaspra.

Tracing the paths of the Baptistina asteroids back in time, the researchers calculate that the objects originated as a single, 170-kilometer-wide body that was shattered by another big rock in the belt some 160 million years ago. About 20 percent of the Baptistina family escaped the belt, and about one-tenth of those asteroids would have continued on to Earth, doubling the number of objects striking the planet over the past 150 million years, Bottke and his collaborators say. Some 20 percent of the near-Earth asteroids are Baptistina-family escapees, they estimate.

“This is an amazing result and shows that recent events [in the asteroid belt] can strongly influence our impact history,” says planetary scientist Derek Richardson of the University of Maryland at College Park.

To link these findings to the death of the dinosaurs, Bottke’s team examined the composition of the 180-km-wide Chicxulub crater in Mexico. Most scientists accept the crater as proof that a space object collided with Earth 65 million years ago, causing the mass extinction at that time.

Sediments from the crater indicate that the impactor must have been a carbonaceous chondrite, an especially primitive meteorite. Such rocks have compositions matching that of the Baptistina asteroids but not that of several other candidates. Bottke’s team calculates a 90 percent chance that the dinosaur killer came from the Baptistina family.

“Dead is dead, no matter where the bullet came from,” notes asteroid researcher Alan W. Harris of the Space Science Institute in La Cañada, Calif. What’s most important, he says, is that the team, studying recent data on hundreds of thousands of asteroids, has dated the origin of this family and argued that it eventually sprayed an “asteroid shower” toward Earth.

Another escapee from the Baptistina family probably gouged the 85-km-wide Tycho crater on the moon, the team suggests.