Far-Flung Crash May Have Created Dino-Killer Asteroid

Richard A. Lovett
for National Geographic News
September 5, 2007
A collision between two asteroids somewhere beyond the orbit of Mars set in motion the chain of events that led to the extermination of the dinosaurs, a new study suggests.

The crash showered the inner solar system with fragments of rock, doubling the rate of asteroid impacts on Earth, the moon, and Mars for many millions of years.

One of those giant fragments may have been the K-T impactor, which marks the boundary between the Cretaceous and Tertiary geologic periods.

That asteroid, which hit what is now Mexico's Yucatán Peninsula 65 million years ago, is widely believed to have wiped out the dinosaurs.

(Related: "Single Asteroid Impact, Not Two, Killed Dinos, Study Says" [November 30, 2006].)

Another fragment appears to have hit the moon, creating Tycho Crater, a dramatic feature that Apollo astronaut studies determined was formed 110 million years ago.

The study will appear in tomorrow's edition of the journal Nature.

Asteroid "Family"

The new discovery was an accident, said study leader William Bottke of the Southwest Research Institute in Boulder, Colorado.

"We didn't actually set about to track down the K-T impactor," Bottke said.

Rather, his team of U.S. and Czech scientists stumbled onto the find while studying a group of asteroids called the Baptistina family.

By studying the Baptistina asteroids' orbits, Bottke's team concluded that the group was formed in a collision about 160 million years ago.

Computer modeling then revealed that the breakup was caused by two large crashing asteroids, one about 100 miles (170 kilometers) in diameter and the other nearly 40 miles (60 kilometers) in diameter.

In total, the scientists calculated, the collision produced 140,000 fragments larger than 0.6 miles (a kilometer) in diameter, including 300 pieces larger than 6 miles (10 kilometers)—the estimated size of the K-T impactor.

The collision also occurred near a part of the asteroid belt where gravitational tugs from Mars and Jupiter create an "escape hatch" that can eject asteroids from the asteroid belt and send them plunging into the inner solar system.

"Our modeling suggests that for a time, a lot of asteroids would have been flying around the inner solar system," Bottke said.

Collision Course

Most of the fragments fell into the sun, the models show, or were diverted onto orbits that pose no risk to Earth. But enough hit Earth to double the normal rate of asteroid impacts.

That means that there was "a high likelihood of one ten-kilometer asteroid and a lot of smaller ones," Bottke said.

Bolstering this theory, the K-T impactor seems to have been a "carbonaceous chondrite" asteroid, a type that appears to include the Baptistina family.

Overall, Bottke said, "there's a 90 percent chance [the K-T asteroid was] a fragment of the Baptistina family."

Bottke's team also said there is a 70-percent chance that the moon's Tycho Crater was also created by a Baptistina remnant.

Luckily, Bottke said, the asteroid shower is waning. (Related: "'Killer Asteroid' Debate Pits Gravity Tractors Against Bombs, Projectiles [March 8, 2007].)

"We are in the tail end of this shower now," Bottke said in a statement. "Our simulations suggest that about 20 percent of the present-day, near-Earth asteroid population can be traced back to the Baptistina family."

Jay Melosh, a planetary scientist at the University of Arizona who was not involved in the study, finds the research "very plausible," but not definitive.

"It does depend on a fairly long sequence of inferences," he noted.

Also, he said, big impacts are infrequent enough that even a doubling of their rate is a relatively small effect.

"But what it tells us is that events in the asteroid belt affect events on Earth."

Dino Killer

Bottke's finding also fits a theory by Princeton University paleontologist Gerta Keller that the dinosaur extinction wasn't caused by a single asteroid but by multiple events over the course of several hundred thousand years.

(Related story: "'Dinosaur Killer' Asteroid Only One Part of New Quadruple-Whammy Theory [October 30, 2006].)

"I'm not an expert in astronomy, but their findings are exciting," Keller said by email.

But lead author Bottke noted that the vast majority of geologists disagree with Keller.

In fact, he said, the dinosaurs survived for millions of years despite a high rate of asteroid bombardment by the smaller Baptistina fragments—only dying out when the big one hit.

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