Image: NEAR IMAGING TEAM
The fruits of a controlled landing that the NEAR-Shoemaker spacecraft made last February on the asteroid Eros 433 are now ripe for the picking. A paper in today's issue of Nature¿one of three new reports¿reveals that after a careful analysis of the myriad pictures NEAR took, most of the rocks on Eros likely came from a single crater, currently referred to as Shoemaker (top right).
Peter Thomas and Joseph Veverka of Cornell University¿together with Mark Robinson of Northwestern University and Scott Murchie of Johns Hopkins University¿now propose that a single collision one billion years ago between Eros and a meteorite formed the crater, blasting large rocks across the asteroid's surface. "One big impact spread all this debris," Thomas says. "This observation is helping us start answering questions about how things work on the surface of an asteroid."
Thomas and colleagues surveyed NEAR's images of Eros's entire surface and counted 6,760 large rocks, each measuring more than 15 meters across (images g and h at right). Some 44 percent of these rocks resided inside the Shoemaker crater, which itself spans 7.6 kilometers (images e and f at right). When the scientists predicted the paths that rocks ejected from Shoemaker might have taken, they concluded that many other boulders along Eros's equator had also come from the crater. Curiously, the other craters on Eros¿dubbed Himeros and Psyche¿apparently did not spew rock in the same way, or if they did, those rocks later eroded or were buried.
Other mysterious features seen in the photos of Eros include flat so-called pond deposits of very fine dust, which seems to have sorted itself out from the coarser surface debris. The scientists suggest that an electrostatic effect, similar to that believed to exist on the moon's surface, may have helped to amass the collections of fine grains. But "this [theory] requires a lot of assumptions," Thomas says, "and does not explain all the mechanisms."
Finally, the researchers don't know whether the features on Eros are at all akin to those on other asteroids. "Nothing comparable to the flat pond deposits has been noted on Gaspra, Ida or Phobos," Veverka writes, "even though Phobos coverage is certainly adequate to show such features if they were present." Perhaps only future missions will find out.