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.
To test their idea they experimented by sprinkling sand (representing pebbles) onto a ceramic plate (representing boulders), and then onto a layer of sand (representing the pebble seas). They found that the sand grains scattered far and wide when they hit the ceramic plate, but stacked into a neat pile when they landed on a layer of sand. Simulations that take into account the reduced gravity on Itokawa also supported this previously unexplored effect. Their findings are published in Physical Review Letters.
Shinbrot and his colleagues suggest that the particles that continually bombard Itokawa tend to bounce off when they encounter a boulder, but stick when they land in a pebble sea. The same mechanism may also explain surface features seen on other small asteroids.
