Fragment shapes in impact experiments ranging from cratering to catastrophic disruption

Abstract

Laboratory impact experiments have found that impact fragments tend to be elongated. Their shapes, as defined by axes a, b and c, these being the maximum dimensions of the fragment in three mutually orthogonal planes (a⩾b⩾c), are distributed around mean values of the axial ratios b/a∼0.7 and c/a∼0.5. This corresponds to a:b:c in the simple proportion 2:√2:1. The shape distributions of some boulders on Asteroid Eros, the small- and fast-rotating asteroids (diameter <200m and rotation period <1h), and asteroids in young families, are similar to those of laboratory fragments created in catastrophic disruptions. Catastrophic disruption is, however, a process that is different from impact cratering. In order to systematically investigate the shapes of fragments in the range from impact cratering to catastrophic disruption, impact experiments for basalt targets 5–15cm in size were performed. A total of 28 impact experiments were carried out by firing a spherical nylon projectile (diameter 7.14mm) perpendicularly into the target surface at velocities of 1.60–7.13km/s. More than 12,700 fragments with b⩾4mm generated in the impact experiments were measured. We found that the mean value of c/a in each impact decreases with decreasing impact energy per unit target mass. For instance, the mean value of c/a in an impact cratering event is nearly 0.2, which is considerably smaller than c/a in a catastrophic disruption (∼0.5). The data presented here can provide important evidence to interpret the shapes of asteroids and boulders on asteroid surfaces, and can constrain current interpretations of asteroid formation. As an example, by applying our experimental results to the boulder shapes on Asteroid Itokawa’s surface, we can infer that Itokawa’s parent body must have experienced a catastrophic disruption.