Compaction as the origin of the unusual craters on the asteroid Mathilde

The asteroid Mathilde has suffered at least five giant impacts. Previous studies suggest that Mathilde's giant craters should be surrounded by blankets of ejecta that are kilometres deep(1,2), yet the craters show no evidence of filling by material excavated during later nearby impacts(1,3). Computer simulations of impacts have been used to suggest that the absence of ejecta arises because the impact energy is deposited in a small volume, due to Mathilde's unusually high porosity(4), which produces ejecta velocities so high that nearly all of the material escapes Mathilde's gravitational field(5). Here we report laboratory measurements of high-velocity impacts into porous material, which support an alternative explanation(3): the crater is formed mainly by compaction, not excavation. The small amount of ejecta lofted in our experiments have velocities sufficiently low that nearly all of the material is redeposited within the crater bowl. The crater itself results from material being compressed, rather than ejected. This type of cratering implies that highly porous asteroids are minor contributors of meteorites, because essentially no material escapes the asteroids.