Pockets of drifting dust have cropped up on the airless surface of an asteroid called Eros. But with no wind to push the dust around, researchers weren't sure how it gathered into smooth patches. Now, thanks to stunning images from a craft that landed on the asteroid, scientists have proposed an exotic explanation: electrostatic levitation, triggered by sunlight.
Until recently, planetary geologists thought asteroids were barren rock, because, they reasoned, jolting collisions with other objects would propel any soil or dust into space. But images from the NEAR-Shoemaker spacecraft are dispelling that notion. The craft, which began orbiting Eros in February 2000, spied a jumble of deposits on the asteroid's surface, including oddly smooth "ponds" inside craters. On 12 February 2001, controllers guided the explorer to land on one such pond. Photos revealed details as small as 1.2 centimeters across--by far the closest scrutiny of an asteroid.
NEAR researchers examined these and thousands of other images to explain the ponds. Their report--part of a suite of papers on Eros in the 27 September issue of Nature--points to sunlight as the driving force. Sunlit dust particles near the asteroid's equator build up an electrostatic charge compared to their shadowed neighbors. Spectral analysis suggests that many grains are much less than 50 micrometers wide; that's light enough for the charges to briefly levitate the grains in the weak gravity of Eros. Each time they are aloft, the particles gradually migrate to the low parts of craters to form ponds in a process that may take millions of years to unfold, says planetary geologist and NEAR team member Mark Robinson of Northwestern University in Evanston, Illinois.
A similar suspension probably occurs on the moon, where astronauts saw dust wafting above sunlit zones, says planetary scientist Erik Asphaug of the University of California, Santa Cruz. "It's exciting to see these fine, loose soils on an asteroid," Asphaug says. "Levitation is a great possibility," although impacts also may shake dust into motion, he notes. One consequence, Asphaug adds, is that future asteroid landers may need to cope with a problem faced by astronauts on the moon: coatings of charged dust on their instruments.