Rounded boulders on Itokawa as clues to geological processes in the early solar system

Large rounded boulders on Itokawa are a surprising find and may be evidence of forceful inter-boulder collisions occurring over protracted periods of time. Surface textures of some boulders are reminiscent of those on terrestrial aeolian sand grains despite five orders of magnitude difference in scale. Using Hertzian analysis and fracture strength data, we calculate that the maximum collisional velocities involved in the comminution process are ~6-7 m/s. We hypothesise that boulder rounding could be a product of collisions in a gravitationally stable orbiting debris field in which boulders acquire collisional energy from YORP spin. Collisional paths may be instigated by Yarkovsky drift and gyroscopic effects of rotation. Collisional energy is dissipated by elastic damping, but rapidly renewed by YORP spinup that takes only hundreds to thousands of years to regenerate comminution-strength collisions. The rounded boulders on Itokawa are found amongst angular, unworn material which suggests a mixed origin for Itokawa׳s regolith.