December 1, 2005

New twist to dating an asteroid

By Megan Fellman

It turns out you can’t judge an asteroid by its cover, according to a study in the journal Nature. Or at least you can’t accurately date a certain asteroid called 433 Eros by counting the impact craters on its surface — the traditional method for determining an asteroid’s age.

Peter Thomas, a senior research associate in astronomy at Cornell University and lead author on the paper, and Mark Robinson, research associate professor of geological sciences at Northwestern, analyzed images of Eros gathered four years ago by the Near Earth Asteroid Rendezvous mission. The mission mapped the 20-mile-long, potato-shaped asteroid and its thousands of craters in detail. The two researchers focused on a large impact crater, known as the Shoemaker crater, and a few unusual crater-free areas.

In the Nature article, Thomas and Robinson show that the asteroid’s smooth patches can be explained by a seismic disturbance that occurred when a meteoroid crashed into Eros, shaking the asteroid and creating Shoemaker crater. The shaking caused loose surface material to fill some small craters, essentially erasing craters from approximately 40 percent of Eros’ surface and making the asteroid appear younger than its actual age.

The fact that seismic waves were carried through the center of the asteroid after the impact shows that the asteroid’s interior is cohesive enough to transmit such waves, say the authors. And the smoothing-out effect within a radius of up to 5.6 miles from the 4.7-mile Shoemaker crater — even on the opposite side of the asteroid — indicates that Eros’ surface is loose enough to get shaken down by the impact.

Asteroids are small, planet-like bodies that date back to the beginning of the solar system, so studying them can give astronomers insight into the solar system’s formation. And while no asteroids currently threaten earth, knowing more about their composition could help prepare for a possible future encounter. Eros is the most carefully studied asteroid, in part because its orbit brings it close to earth.