Was the 'Dinosaur Killer' Unfairly Charged?

Wanting to bring the master evildoer, not just a henchman, to justice is human enough. So when planetary scientists traced the asteroid that wiped out the dinosaurs 65 million years ago back to a rampaging rock named Baptistina in the asteroid belt there was palpable satisfaction that the ultimate culprit seemed to have been nailed.

But now a group of astronomers is challenging that claim. Baptistina did blast another asteroid to smithereens, sending a devastating shower of debris into the inner solar system, but that cataclysmic collision probably came too late to have sent the dino killer to Earth, they argue.

The original CSI-like case against Baptistina involved an odd link between an asteroid's size and the ability of sunlight to move it across the asteroid belt. In their 2007 Nature paper, planetary scientists William Bottke, David Vokrouhlický, and David Nesvorný of the Southwest Research Institute (SwRI) in Boulder, Colorado, identified asteroids whose similar orbits mean they are members of the "family" of asteroids formed in a collision between asteroids 170 and 60 kilometers in diameter, 40-kilometer-diameter Baptistina being the largest survivor.

By assuming how reflective Baptistina family members are, the SwRI group could estimate the size of each asteroid from the amount of visible light they reflected. Their sizes, in turn, determined how quickly sunlight could ease debris away from the collision. As each fragment absorbs solar energy, it radiates the heat away to give an ever-so-gentle rocketing effect. That nudging could have driven fragments toward a known orbital spot from which Jupiter's gravity could fling them toward Earth. Judging by how far from the collision Baptistina family members have gotten, the group estimated that the collision occurred about 160 million years ago, early enough for the solar-driven rocketing to drive a 5- or 10-kilometer fragment to the jumping-off point to Earth by 65 million years ago.

But now team members on the Wide-Field Infrared Survey Explorer (WISE) mission dispute this line of evidence in a paper just out in The Astrophysical Journal. Joseph Masiero of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, and colleagues report that WISE has returned estimates of the size of 100,000 asteroids, the Baptistina family among them. These estimates are more accurate than those the SwRI group had because WISE detects infrared light, not the visible spectrum. The SwRI group had to assume a reflectivity, but the WISE infrared measurements yielded actual measurements of reflectivity that are four times larger than the SwRI group had assumed. That in turn gave smaller sizes, faster moving fragments, and therefore a younger collision time—about 80 million years ago—than the SwRI group had calculated.

"This doesn't give the remnants from the collision very much time to ... get flung down to Earth 65 million years ago," says Amy Mainzer, a co-author and the principal investigator of the asteroid phase of WISE at JPL. So instead of the Baptistina collision being ultimately responsible, another, as-yet-undated collision may have been responsible. Or the dino killer was a random asteroid that happened to wander out of the asteroid belt then.

Bottke doesn't see a problem. Indeed, the WISE results "if anything ... make our story stronger," he writes in an e-mail. The SwRI group's 2007 calculations show that "lots of things escape from the Baptistina family right away ... decreasing the age of the Baptistina family is not a problem."

Before, the collision was so early that the dino killer would have had to have been among the sparse debris that reached Earth long after the collision, Bottke says; with a more recent collision, far more Baptistina fragments would have been raining toward Earth 65 million years ago. Bottke's argument "provides a good counterpoint to the conclusions reached by the WISE team," writes dynamicist Derek Richardson of the University of Maryland, College Park. Now perhaps the prosecution and the defense can work on a settlement.