Title:Fates of hydrous materials during planetesimal collisions

Abstract:Hydrous minerals are found on the surfaces of asteroids, but their origin is not clear. If their origin is endogenic, the hydrous minerals that were formed in the inner part of a planetesimal (or parent body) should come out on to the surface without dehydration. If their origin is exogenic, the source of hydrous minerals accreting onto asteroids is needed. Collisions in the asteroid belt would be related to both origins because collisions excavate the surface and eject the materials. However, the fate of hydrous minerals in large planetesimals during the collisional process has not been well investigated. Here, we explore planetesimal collisions by using the iSALE-2D code, and investigate the effect of an impact for the target planetesimal containing hydrous minerals. Our numerical results for the fiducial case (5 km/s of the impact velocity) show that hydrous minerals are slightly heated during the collisions. This moderate heating indicates that they can avoid the dehydration reaction and keep their original composition. Some hydrous minerals have larger velocity than the escape velocity of the collision system. This means that hydrous minerals can escape from the planetesimal and support the theory of exogenic origin for the hydrous minerals on asteroids. Meanwhile, the velocity of other hydrous minerals is smaller than the escape velocity of the system. This also indicates the possibility of an endogenic origin for the hydrous minerals on asteroids. Our results suggest that hydrous minerals on asteroids can be provided by planetesimal collisions.