Distribution of boulders and the gravity potential on asteroid Itokawa

Abstract

The small asteroid Itokawa was visited in 2005 by the Japanese spacecraft Hayabusa. The images of the surface showed a scenario different to previously visited asteroids. Itokawa has a small number of large craters and many large boulders randomly distributed on most of the surface.We analyse images taken at different surface resolution and configurations corresponding to several regions on the asteroid’s surface. By overlapping visual images and maps of the total potential and surface gravity, we observe a correlation between the distribution of boulders and these parameters. The boulders on the surface were identified by visual inspection of several images. After fitting ellipses to every boulder, we computed their size and the size distribution from decimeters to several meters at different locations in the surface. We found that the size distribution is correlated with the total potential and the surface gravity. A steeper size distribution shifted towards the small objects is observed in the low negative total potential (high surface gravity), which corresponds to the Muses-C region. Meanwhile, in the “head” and “bottom” regions of high potential (low surface gravity), we obtain a shallower size distribution, shifted towards the large boulders.We confirm there is a size segregation that is correlated with the gravity field which can be explained under the action of the Brazil nut effect. There is a global relocation of boulders, with large ones going into the high potential regions and small ones into the low potential ones. A shape segregation is also observed on the location of the boulders: more rounded ones are found in the regions of high potential, while more elongated ones are frequent in regions of low potential.