Collisional Disruption of Ice by High-Velocity Impact

Abstract

High-velocity impact among icy planetesimals is a physical phenomenon important to the planetary evolution process in the outer Solar System. In order to study this phenomenon, impact experiments on water ice were made by using a two-stage light gas gun installed in a cold room (−10°C) to clarify the elementary processes of collisional disruption and to study the reaccumulation and the escape conditions of the impact fragments. Cubic ice targets ranging in size from 15 to 100 mm were impacted by a nylon projectile of 7 mg with an impact velocity ( v i) from 2.3 to 4.7 km/s. The corresponding mass ratio of the projectile to the target ( m p/ M t) ranged from 10 −3 to 10 −6, which is two orders of magnitude lower than that used in previous studies (Arakawa et al. 1995, Icarus 118, 341–354). As a result, we obtained data on elementary processes such as attenuation of the shock wave and fragmentation dynamics. We found that the shock pressure attenuates in the ice target according to the relation of P∝( L p/ r 2, irrespective of the mass ratio between 10 −3 and 10 −5, where L p is the projectile size and r is a propagation distance. The largest fragment mass ( m l) normalized by the original target mass has a good relationship to a nondimensional impact stress ( P I, NDIS) defined as the ratio of the antipodal pressure to the material strength. This relationship is described as m l/ M t ∝ P I −1.7 for a wide range of impact conditions (50 m/s< v i<4 km/s and 10 −1< m l/ M t<10 −6), and shows the utility of NDIS. Using a measured shock wave decay constant of 2, the reaccumulation and the escape conditions of icy bodies in high-velocity collisions were estimated. As a result, it was clarified that a rubble pile could be formed when large icy bodies (radius>20 km) reaccumulated. On the other hand, when smaller icy bodies (radius<2 km) disrupted catastrophically, all fragments escaped and a rubble pile was never formed.