The boulder-strewn surface of the asteroid Eros was likely shaped by a single collision, scientists studying data from a recent NASA mission reported Thursday.

In a set of papers published in the latest issue of the journal Nature, scientists involved with the Near Earth Asteroid Rendezvous (NEAR) mission concluded that a collision between Eros and another body a billion years ago distributed most of the small rocks seen on the asteroid's surface.

Images of Eros taken from the orbiting NEAR-Shoemaker spacecraft: (a), scale bar, lower right, measures 2 kilometers (1.2 miles), crater Shoemaker as viewed from the south; (b), scale bar 1 kilometer, crater Psyche; (c), scale bar 200 meters, interior of crater Shoemaker; (d), scale bar 100 meters, large rock inside Psyche; (e), scale bar 50 meters, and (f), scale bar 100 meters, large rocks ejected from crater Shoemaker that were deposited in older craters; (g), scale bar 10 meters, and (h), scale bar 20 meters, the range of shapes of large rocks on Eros -- from angular to falling apart. Photo: NEAR imaging team SEE LARGER IMAGE

That conclusion came after a detailed analysis of images recorded by NEAR during its year-long orbit of Eros, as well as additional high-resolution images collected during the spacecraft's controlled descent to the surface of Eros on February 12. Scientists identified 6,760 rocks at least 15 meters in diameter on the surface in those images and found that a majority of them likely came from the vicinity of a crater provisionally named Shoemaker at one end of the asteroid.

"We know they came from Shoemaker because the mapping of the geography of the pattern [of the rocks] on the surface closely matches the predicted paths from the one impact event that made Shoemaker," said Cornell University senior researcher Peter Thomas. "One big impact spread all this debris."

About 44 percent of the boulders found on Eros were within Shoemaker itself, likely pieces that "went straight up and down" according to Thomas. The rest traveled in arcs that took hours to land, carrying debris across the rest of the surface of the asteroid.

Thomas and others believe that the Shoemaker crater, eight kilometers wide, was formed when a small asteroid or comet collided with Eros about one billion years ago. That collision and the resulting rain of debris may have erased similar boulders ejected from two other, older, impact craters also seen on the asteroid.

Eros's surface is not completely rocky, however. NEAR images also turned up a number of regions where the surface is very smooth and flat, so smooth that no sign of roughness could be seen in NEAR's sharpest images, at just 1.2 centimeters per pixel. Such "ponded" deposits appear to be associated with craters and other depressions.

NEAR itself may have landed on or near one such ponded deposit, based on the final images transmitted by the spacecraft prior to touchdown, according to Joseph Veverka, a Cornell University professor and head of NEAR's imaging team. That may explain why the spacecraft survived the landing and was able to continue to operate for several more days: had it landed in a rockier region it could have tipped over on a rock, damaging or disabling itself.

How such ponded deposits -- not yet seen on other asteroids or planetary bodies -- formed is an open question. Mark Robinson of Northwestern University speculates in another Nature paper that a photoelectric effect caused by exposure to sunlight could allow the finest particles on Eros's surface to separate out and flow by gravity into craters and depressions, forming the deposits. However, he notes, "This requires a lot of assumptions, and does not explain all the mechanisms."

Veverka cautioned against drawing too many conclusions about Eros from these observations. "It is difficult to compareŠ Eros with observations of other asteroids owing to the lack of images of a comparable resolution," he wrote in a Nature paper. Galileo images of the main-belt asteroids Gaspra and Ida, which, like Eros, are S-class bodies, are of much lower resolution and thus cannot show the fine surface details seen in the NEAR images.