Impacts into Coarse-Grained Spheres at Moderate Impact Velocities: Implications for Cratering on Asteroids and Planets

The surfaces of many planets and asteroids contain coarsely fragmental material generated by impacts or other geologic processes. The presence of such pre-existing structures may affect subsequent impacts, particularly when the width of the shock is comparable to or smaller than the size of pre-existing structures. Reasonable theoretical predictions and low-speed (<300 m s−1) impact experiments suggest that in such targets the cratering process should be highly dissipative, which would reduce cratering efficiencies and cause a rapid decay in ejection speed as a function of distance from the impact point. In this study, we assess whether these results apply at impact speeds between 0.5 and 2.5 km s−1. This study shows little change in cratering efficiency when 3.18 mm diameter glass beads are launched into targets composed of these same beads. These impacts are very efficient, and ejection speed decays slowly as a function of distance from the impact point. This slow decay in ejection speed probably indicates a correspondingly slow decay of the shock stresses. However, these experiments reveal that initial interactions between projectile and target strongly influence the cratering process and lead to asymmetries in crater shape and ejection angles, as well as significant variations in ejection speed at a given launch position. Such effects of asymmetric coupling could be further enhanced by heterogeneity in the initial distribution of grains in the target and by mechanical collisions between grains during excavation. These experiments help to explain why so few craters are seen on the rubble-pile asteroid Itokawa: impacts into its coarsely fragmental surface by projectiles comparable to or smaller than the size of these fragments likely yield craters that are not easily recognizable.