Asteroid studies reveal new puzzles

Nearly half the largest rocks strewn across the asteroid 433 Eros come from material blasted from a single impact, a new analysis reveals. A detailed study of Eros-the first asteroid that a spacecraft has landed on (SN: 7/21/01, p. 38: Landing data confirm Eros’ primitive nature)–reveals that among 6,760 rocks larger than 15 meters across, 44 percent originated from what is now a crater called Shoemaker. This 7.6-kilometer-wide depression lies at one end of the asteroid.

Peter C. Thomas of Cornell University and his colleagues found that the placement of the large rocks matched the predicted paths of debris ejected by the impact that created the Shoemaker crater. Although some of the debris “went straight up and straight down,” other chunks traveled as far as two-thirds of the way around the rotating, 34-km-long asteroid, Thomas notes.

He and his collaborators, who include Joseph Veverka of Cornell and Mark S. Robinson of Northwestern University in Evanston, Ill., describe several new findings about Eros in a trio of reports in the Sept. 27 Nature.

One puzzle, notes Thomas, is that debris from two other large craters, Himeros and Psyche, don’t seem to have made a significant contribution to the larger rocks observed on the asteroid’s surface. Both craters are older than Shoemaker, and it could be that rocky material excavated by these impacts has since been buried or eroded, says Thomas. Alternatively, because of differences in rock strength and composition at various locations, the impacts that gouged Himeros and Psyche may simply have made fewer large rocks.

Detailed maps showing features on Eros as small as 1.2 centimeters across reveal another mystery. The asteroid’s surface is coated with fine material, some of which settled out as flat deposits–so-called dust ponds–on the floors of craters and smaller depressions. “We’ve never seen such a feature on any other object” in the solar system, notes Thomas.

The coatings of dust belie the view of asteroids as bare rocks. The ponds appear in the last images taken by the Near Earth Asteroid Rendezvous-Shoemaker craft before it hit Eros. That suggests the craft landed within meters of a pond, says Veverka.

With no wind to lift and spread the dust on Eros, researchers are seeking other explanations for the ponds. Robinson suggests that electrostatic levitation could be the solution. He notes that sunlit portions of Eros’ equator tend to lie next to highly shadowed regions. As a result, the charge acquired when sunlight strikes some of the smallest equatorial dust particles–those no bigger than a few tens of micrometers–would allow them to briefly lift off the surface.

A fog of such particles might then migrate and settle down as deposits on crater floors, Robinson speculates. Once they settle in these depressions, out of reach of sunlight, the particles would eventually lose their charge and stay put.

To support his theory, Robinson notes that the largest dust ponds lie along Eros’ equator, where exposure to sunlight is highest. He also notes that on Earth’s moon, astronomers have detected fine dust suspended along the terminator–the dividing line between night and day.

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