Ejection Velocity of Ice Impact Fragments

Abstract

Laboratory experiments on the impact disruption of ice were carried out to investigate the collisional phenomena of an icy planet. Ice projectiles were impacted on ice targets at impact velocities of 30 to 530 m/sec, and mass ratios of the projectile to the target of 0.1 to 0.0035. An ejection velocity at an antipodal point (Va) and a largest-fragment mass normalized by the target mass (ml/Mt) were used to examine two scaling parameters, an energy density (Q) and a nondimensional impact stress (Pl). The largest-fragment mass and the antipodal velocity have good correlations to Q but show apparent differences from a basalt target obtained by previous studies. The ice target begins to break up at 83 J/kg, which is two to five times smaller than for basalt. The ejection velocity is about two times higher than for basalt at the same Q. A modified nondimensional impact stress, P*l , that involves a variable decay constant depending on the impact pressure is suggested. This eliminates the size dependence in the relation between Pl and the largest fragment. If we assume a simple relation to the antipodal velocity, Va /V* = 2P*l, where V* is a characteristic velocity, the decay constant decreases with increasing impact velocity. The revised P*l improved the relation between ml /Mt and the nondimensional impact stress. Ejection velocity in a center-of-mass frame was used to estimate a collisional condition for the icy planet to reaccumulate the disrupted fragments by planetary gravity.