Crater shake: tremors erased asteroid's topography.

The equivalent of a gigantic shiver might have reduced the number of small craters found on the surface of the asteroid Eros. Astronomers analyzing the asteroid's surprisingly smooth complexion say that seismic shock waves from a large meteor strike appear to have turned the craters into flat fields of rubble.

When astronomers have considered the pattern of craters on Eros, something has seemed off, says Peter Thompson of Cornell University. Although most of the asteroid is heavily cratered, about 40 percent of Eros' surface appears devoid of craters smaller than 0.5 kilometer in radius. Thompson and others had speculated that debris thrown up during an impact that left behind a large crater named Shoemaker settled back into and filled many of Eros' other craters (SN: 10/27//01, p. 264).

Using high-resolution images from NASA's Near Earth Asteroid Rendezvous Shoemaker spacecraft, Thompson calculated crater densities over the 33-km-long, banana-shaped asteroid. He found that any site on Eros within a 9-km radius of Shoemaker's center, including some sites on the other side of the asteroid, had far fewer craters than did areas beyond this radius. In the July 21 Nature, Thompson and Mark Robinson of Northwestern University in Evanston, Ill., suggest that this pattern doesn't match that of models of material sprayed from a meteor impact.

Instead, the researchers argue, seismic shock waves from the Shoemaker impact propagated straight through the asteroid, shaking the craters into formless rubble. Because Eros is irregularly shaped, a 9-km line through the asteroid from the impact site can reach some locations that would be farther away for a surface traveler. Seismic shock waves can explain the uneven pattern of crater obliteration on the asteroid's surface, the scientists say.

Andy Cheng of the Johns Hopkins Applied Physics Lab in Laurel, Md., and others had previously theorized that seismic shaking from large-meteor impacts could fill in craters on asteroids such as Eros. "The significance of this study to me is they're finding observational evidence that the seismic shaking idea is correct," Cheng says.

The finding also adds to a debate about asteroid structure. Scientists had proposed that repeated meteor impacts gradually smash asteroids as small as Eros into loosely cemented aggregates of rubble. "You can imagine that it would be so beat up, so fractured, that the seismic waves wouldn't be able to propagate from one side to the other," says Cheng. Thompson and Robinson's study suggests, however, that the asteroid's interior remains somewhat cohesive.

In the event that an asteroid someday happens to be on a collision course with Earth, Thompson speculates, information about asteroid structure could become uncommonly useful for deflecting or destroying the threat.