Separating pebbles from boulders on asteroid Itokawa

Boulders, pebbles, and grains of sand tend to collect in separate areas along a mountain slope or dried-out riverbed. That’s due to a combination of gravity, erosion, and movement of the wind or water in which the rocks were transported.

Strong size segregation between rocks and sand on the 540-m-long asteroid 25143 Itokawa (pictured), which lacks any of those Earthly influences, is therefore a surprise. Images of Itokawa taken by the Japanese spacecraft Hayabusa show boulders up to 40 m in diameter occupying raised plateaus; adjacent valleys seem to be filled with centimeter-sized pebbles and fine dust.

Asteroids form by the agglomeration of dust, pebbles, and larger rocks in the solar nebula. Physicist Troy Shinbrot at Rutgers University, along with Pinaki Chakraborty and Tapan Sabuwala at the Okinawa Institute of Science and Technology, proposed that incoming pebbles striking Itokawa during its formation may have rebounded off boulders and sank into the now-pebbly regions and thus led to the asteroid’s current surface morphology.

To test their idea, Shinbrot and colleagues sprinkled 1-mm-diameter glass beads onto landscapes of differently sized river stones. The researchers found that the beads tended to land far from their point of impact when they bounced off large stones, but they would travel shorter distances when they struck pebbles or other glass beads. As the sprinkling progressed, that difference in trajectory caused seas of beads to form between the stones. Inelastic collisions between incoming beads and deposited beads further intensified the segregation. Simulations confirmed that the ballistic sorting mechanism produces strong size segregation and reliably obeys an analytic formula. Based on the findings and images of Itokawa’s surface, the researchers deduced that the number of pebbles on the asteroid’s surface exceeds the number of boulders by nine orders of magnitude.

Ballistic sorting may help explain processes by which asteroids—and potentially other minor planets, comets, and moons—develop their geomorphology during formation. The mechanism may also play a role in forming the flat “ponds” of fine particles observed on larger, denser asteroids such as Vesta and Eros. (T. Shinbrot et al., Phys. Rev. Lett., in press .)