(Image: Okayama University/JAXA)
It’s like a cosmic version of Russian nesting dolls. Tiny dust grains collected from the surface of the asteroid Itokawa harbour even smaller particles and craters of their own - the scars of high-speed collisions with nanometre-sized impactors.
In 2010, Japan’s Hayabusa probe delivered more than 1500 rocky particles from Itokawa to Earth. Studies of some of the grains last year suggested that the 500-metre-long asteroid probably began its life as a much larger rock, measuring at least 20 kilometres across. Indeed, Itokawa has a very low density, suggesting it is a rocky rubble pile that probably coalesced after its parent body was smashed in an impact.
Now researchers led by Eizo Nakamura of Okayama University in Misasa, Japan, have revealed violence on an even smaller scale. Scanning electron microscope images of five dust grains, ranging from 0.04 to 0.11 millimetres wide, show even smaller particles stuck to the grains.
These are thought to be mostly shrapnel from neighbouring grains that were struck by high-speed space dust, and occasionally bits of the extra-asteroidal impactors themselves. Some grains also show miniature craters, where nanometre-sized impactors lodged into the grains, sometimes melting their immediate surroundings.
Understanding these processes sheds light on how the space environment “weathers” asteroids’ surfaces, which in turn may help researchers better estimate asteroids’ ages.