A New Twist To Dating An Asteroid

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

In the Nature article, Thomas and Robinson show that the asteroid'ssmooth patches can be explained by a seismic disturbance that occurredwhen a meteoroid crashed into Eros, shaking the asteroid and creatingShoemaker crater. The shaking caused loose surface material to fillsome small craters, essentially erasing craters from approximately 40percent of Eros' surface and making the asteroid appear younger thanits actual age.

The fact that seismic waves were carried through the center of theasteroid after the impact shows that the asteroid's interior iscohesive enough to transmit such waves, say the authors. And thesmoothing-out effect within a radius of up to 5.6 miles from the4.7-mile Shoemaker crater -- even on the opposite side of the asteroid-- indicates that Eros' surface is loose enough to get shaken down bythe impact.

Asteroids are small, planet-like bodies that date back to thebeginning of the solar system, so studying them can give astronomersinsight into the solar system's formation. And while no asteroidscurrently threaten Earth, knowing more about their composition couldhelp prepare for a possible future encounter. Eros is the mostcarefully studied asteroid, in part because its orbit brings it closeto earth.

Thomas and Robinson considered various theories for the regions ofsmoothness, including the idea that ejecta from another impact hadblanketed the areas. But they rejected the ejecta hypothesis whencalculations showed an impact Shoemaker's size wouldn't create enoughmaterial to cover the surface indicated. And even if it did, they add,the asteroid's irregular shape and motion would cause the ejecta to bedistributed differently. In contrast, the shaking-down hypothesis fitsthe evidence neatly.