Asteroids implicated in dino deaths not guilty after all

Maggie McKee, space news editor

(Image: NASA/JPL-Caltech)

Honour has been restored to the Baptistina family of asteroids. In 2007, astronomers accused a member of the family of taking out the dinosaurs 65 million years ago, but new observations seem to have cleared them of any wrongdoing.

Dozens of asteroid families have been identified, each chips off the old block of a parent that broke apart after an impact. Studying the distribution of family members can reveal when the break-ups happened.

In 2007, a team led by David Nesvorny of the Southwest Research Institute in Boulder, Colorado, named a newly identified group Baptistina after the 40-kilometre-wide space rock that is its largest known member. As New Scientist reported at the time, the team concluded that:

The dinosaur-killer was probably a lost member of this family, formed by a collision in the inner part of the asteroid belt 160 million years ago. Nesvorny's team has calculated that a 10-kilometre asteroid, one of roughly 300 chunks of the original 170-kilometre 'mother rock', would have collided with the Earth.

Now infrared observations with NASA's WISE satellite suggest the family formed only 80 million years ago, leaving too little time for one of its members to reach Earth and wipe out the dinosaurs.

The new timeline is based on better measurements of the sizes of Baptistina asteroids. Visible-light observations merely reveal how much sunlight they reflect, not their actual size. For example, a very large, but inherently dark asteroid may reflect as much light as a small but pale space rock. Infrared observations, which show how much heat is emanating from the bodies, provide a much better gauge of physical size.

Larger asteroids do not tend to move as much after a collision as smaller ones. So WISE's new and improved estimates of the sizes of Baptistina asteroids allowed astronomers to "rewind" the asteroids' paths from their current positions to reveal they were part of a parent that broke apart 80 million years ago.

That is just half as much time as was thought previously, and means that after the break-up, one of the Baptistina asteroids would have had only 15 million years to travel to Earth after reaching a "resonance" spot in the asteroid belt where gravitational tugs from Jupiter and Saturn would have kicked it out of the belt.

"This doesn't give the remnants from the collision very much time to move into a resonance spot and get flung down to Earth," Amy Mainzer of NASA's Jet Propulsion Laboratory in
Pasadena, California, said in a statement. "This process is thought to normally take many tens of millions of years." She is one of the authors of a paper on the result that is to appear in The Astrophysical Journal.

"As a result of the WISE science team's investigation, the demise of the dinosaurs remains in the cold case files," Lindley Johnson of NASA's Near Earth Object Observation Program said in the statement.

Source: http://feeds.newscientist.com/c/749/f/10897/s/18a699b1/l/0L0Snewscientist0N0Cblogs0Cshortsharpscience0C20A110C0A90Casteroids0Eimplicated0Ein0Edino0Ed0Bhtml0DDCMP0FOTC0Erss0Gnsref0Fonline0Enews/story01.htm

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