Laboratory simulation of planetesimal collision: 2. Ejecta velocity distribution

Abstract

Velocity distributions of fragments produced by low‐velocity impact against rock were obtained from high frame rate photographs. Cylindrical projectiles of mild steel (S15CK) and two kinds of rocks (tuff and basalt) were impacted against spherical rock targets at velocities from 50 to 400 m/s. Target destruction initiates with longitudinal splitting, which is analogous to destructive uniaxial static compression. The ejecta velocity component normal to the incident direction (“lateral component”) differs for impacts into basalt (∼20 m/s) and tuff (∼5 m/s) targets. The lateral component of kinetic energy of the fragments is shown to be controlled by the total strain energy stored before fracturing begins. For equivalent impact energy per unit target mass, ejection velocities of larger fragments from low‐velocity impacts are slightly higher than velocities from high‐velocity impacts; in addition, ejection velocities relative to the impact velocity for low‐velocity impacts are much higher than those for high‐velocity impacts. Therefore, impact velocity and not the imparted energy density is suggested to be an important parameter for classifying these low‐velocity collisional events. The ejecta velocity distribution is also greatly influenced by differences in relative mechanical properties between the projectile and target. Ejecta velocities from rock‐rock collisions are much higher than velocities from steel‐rock collisions even for equivalent impact velocity. This implies that the mechanical properties of planetesimals have played an important role in the early stages of planetary formation.