Researchers report that millimeter- to pea-size rocks have pooled in three troughs on the asteroid's surface, suggesting that the grains flowed downhill like potato chip crumbs falling to the bottom of the bag. Although the true cause of the pooling is unclear, repeated shaking can make smaller rocks slide by allowing them to work their way around larger ones.
The finding implies that "things are happening even on this little cluster of rock," says space researcher Daniel Scheeres of the University of Michigan at Ann Arbor. "There are geological processes at work."
The observation comes from close-up photos of Itokawa snapped by the Japanese space probe Hayabusa, which touched down on the asteroid twice in November 2005. Researchers are scrutinizing those images as they await Hayabusa's return with hoped-for samples of Itokawan dirt.
The asteroid is essentially an oblong clod of rocks less than 500 meters across. Its gravity is weaker than Earth's by a factor of more than 5,000, so even collisions from centimeter-size rocks would be enough to rattle the whole asteroid and cause the rocks on its surface to jump, according to a paper by Scheeres and co-workers published online this week in Science.
"Itokawa is a natural laboratory for granules to be shaken under microgravity," says the report's lead author, researcher Hideaki Miyamoto of the University of Tokyo and the Planetary Science Institute in Tucson, Ariz.
But Scheeres notes that other effects could be dragging rocks downhill, including variations in the asteroid's gravity as it passed near Earth or during a possible earlier phase in which the asteroid consisted of two pieces orbiting one another.
In other signs of downhill movement, the pea-size grains tended to point sideways, as though they had been rolling, and boulders had clusters of the small ones piled behind them, implying that big rocks blocked the movement of smaller ones.
Given the asteroid's weak gravity, the apparent movement of its rocks is "pretty striking," says granular materials researcher Robert Behringer of Duke University.
"The method of shaking is rather novel," he says, compared with earthly phenomena such as the so-called "Brazil nut effect," in which larger nuts rise to the top of a shaken can of mixed nuts. "You don't imagine shaking the whole Earth when you're shaking your can of nuts."