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August 03, 2010

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Asteroid “crime family” blamed in dinosaur wipeout

Sept. 5, 2007
Courtesy Southwest Research Institute
and World Science staff

As­tro­no­mers have long be­lieved that some sort of as­ter­oid or com­et im­pact killed the di­no­saurs about 65 mil­lion years ago. Now they say they have likely iden­ti­fied what the ob­ject was—or at least, which fam­i­ly it came from.

The di­no­saurs fell vic­tim to one of many broken-up chunks of a once-bigger as­ter­oid, a group known as Bap­tis­tina family as­ter­oids, re­search­ers say. In fact, they add, frag­ments of that same rock have been pelt­ing Earth for eons, and we’re only about now at the bom­bard­ment’s end.

The Ty­cho crat­er on the Moon is clear­ly vis­i­ble at the top of this im­age. Rays ap­pear­ing to spread out­ward mark the re­mains of fly­ing de­bris from the vi­o­lent im­pact. (Cour­te­sy NA­SA)

Astronomers have long believed that some sort of asteroid or comet killed the dinosaurs about 65 million years ago. Now they say they have likely identified what the object was—or at least, which “family” it came from. The dinosaurs fell victim to one of many broken-up chunks of a once-bigger asteroid, a group known as Baptistina family asteroids, researchers say. In fact, they add, fragments of that same rock have been pelting Earth for eons, and we’re only about now at the tail end of the bombardment. The U.S.-Czech research team combined observations with various computer simulations to reach the conclusion. They estimated that the parent body of the dinosaur-killer was some 170 kilometers (106 miles) wide. Some 100 million years before the dinosaurs’ catastrophic end, this colossus was floating through space deep inside the Solar System’s main asteroid belt between Mars and Jupiter, the scientists said. That’s when it slammed into another asteroid about one-third its diameter, creating thousands of large chunks. One of these eventually found its way here and wiped out the great reptiles, the scientists continued. With 90 percent certainty it left the giant pockmark now called Chicxulub crater on Mexico’s Yucatan Peninsula, they said. But that fragment wasn’t the only one to disrupt Earth or its neighborhood, the scientists added: a huge Moon crater called Tycho also has 70 percent likelihood of being caused a Baptistina family member. When the parent body broke up, its offspring continued moving on similar orbits to its own, the researchers explained. But these orbits gradually changed due to forces produced when they absorbed sunlight and re-emitted the energy as heat. The family spread out, and some members drifted into a nearby “dynamical superhighway,” a zone from which they could escape the main asteroid belt and slip into orbits that cross Earth’s path. The computations suggest that about 20 percent of surviving multi-kilometer- sized fragments in the Baptistina family were lost in this fashion, with some 2 percent of those going on to strike Earth. The result: a pronounced increase in the number of large asteroids striking Earth, the research team said. Both Earth and Moon show evidence of a two-fold increase in the formation rate of large craters over the last 100 to 150 million years, they continued. “The Baptistina bombardment produced a prolonged surge in the impact [rate] that peaked roughly 100 million years ago,” said David Nesvorny of the Southwest Research Institute in San Antonio, Texas, one of the researchers. “We are in the tail end of this shower now. Our simulations suggest that about 20 percent of the present-day, near-Earth asteroid population can be traced back to the Baptistina family,” said the institute’s William Bottke, another collaborator. Further evidence implicating the Baptistinas comes from the composition of the 180-kilometer wide Chicxulub crater, long believed to be associated with the dinosaurs’ misfortune, researchers added. Samples from the crater reveal a chemical composition consistent with that of the Baptistina asteroids, which are of a type known as carbonaceous chondrites. These are of great interest to scientists because of their primitive makeup: they’re believed to consist of pristine material similar to that of the cloud from which the Solar System formed. The findings are to appear in the Sept. 6 issue of the research journal Nature.