A formation mechanism for concentric ridges in ejecta surrounding impact craters in a layer of fine glass beads

Ejecta patterns are experimentally examined around craters formed in a layer of glass beads by vertical impacts at low velocities. The diameters of the constituent glass beads of three different targets range 53–63 μm, 90–106 μm, and 355–500 μm. The impact velocities and ambient pressures range from a few to 240 m s−1 and from 500 Pa to the atmospheric pressure, respectively. Various ejecta patterns are observed around craters and are classified into two major classes based on whether they have concentric ridges or not. We propose a possible formation model for the ridges in which the wake created by a projectile as it passes through the atmosphere causes the crater rim to collapse: The model can explain the observation that the degree of collapse of the resultant crater rim depends on the impact velocity and ambient pressure. Using the ratio between the hydrodynamic drag of the airflow induced by the wake and the gravitational force of the degraded part of the rim, we calculate the critical conditions of the impact velocity and ambient pressure necessary for the wake to erode the rim. The conditions turn out to be roughly consistent with the boundary between the two morphological classes. As a result, it is possible that the projectile wake triggers the collapse of the crater rim, leading to a ground-hugging flow that settles to form the distal ridge observed in this study. This mechanism may play a role in producing ejecta morphologies on planetary bodies with atmosphere.