A Method of Estimating Transient-Cavity Diameters for Impact Craters Formed in Dry Sand

Analyses of impact craters formed in laboratory experiments historically have been the source of many fundamental observations and interpretations of the impact-cratering process itself. Due to its ready availability, ease of handling, and lack of strength, dry sand of various types has been the target material of choice in the majority of such experiments. A consequence of its lack of intrinsic strength, however, is dry sand's inability to maintain slopes above its angle of repose. Evidence from field observations of simple terrestrial craters and laboratory craters formed in more cohesive granular media suggests that transient cavities are similar to paraboloids in shape. Cross-sections of craters formed in dry sand, however, are nearly conical with the wall slopes at or near the angle of repose, indicating that the original crater form has been modified by one or more processes, among which is simple slope failure. Because the dimensions and shape of the transient cavity reflect the detailed conditions of a given impact event, its characterization has long been a desired goal in experimentation. A means of estimating the position of the transient cavity's rim is suggested below, relying on determination of velocities of material ejected from the growing cavity.