Dust to dust, boulders to boulders

An experiment shows how particles of varying sizes sort themselves out on the surface of a small asteroid

Back in 2005 a small asteroid, known as 25143 Itokawa, was visited by the unmanned Japanese spacecraft, Hayabusa. Close up images of the asteroid - which measures approximately 540m by 250m - revealed that the "lowlands" were covered by dust and centimetre-sized small pebbles, whilst the "highlands" were made up from larger boulders (5 to 40m diameter). But how did this segregation come about?

Initially researchers thought that the size sorting on Itokawa was most likely due to the Brazil Nut Effect, whereby smaller particles rattle downwards when something is shaken. But the force of gravity is weak on Itokawa, meaning that the Brazil Nut Effect would be unlikely to create such extreme sorting. Instead Troy Shinbrot, from Rutgers University in New Jersey, USA, and his colleagues suggest that for the high-speed particles that bombard the asteroid, pebbly regions are "stickier" than boulder fields.