EXPERIMENTAL IMPACTS INTO LAYERED TARGETS: IMPLICATIONS FOR THE MOON (Invited Presentation)

Impact craters provide clues about planetary surface properties. In the 1960s, for example, Oberbeck and Quaide experimentally demonstrated that crater morphology in the lunar maria reflected the thickness of a weaker regolith layer on top of its more competent, parent basalts. Small craters in the mare regions of the Moon commonly show a morphological progression from simple bowl-shapes to those with flat floors or central mounds, and finally to concentric structures as crater diameter increases. Using their experimental data and the transition diameters between these various crater forms in a given mare unit, Oberbeck and Quaide could then estimate the regolith thickness. This technique continues to be used today with even higher resolution images of the lunar surface (e.g., Bart et al., 2011; Bart, 2014).

The work presented here expands upon Oberbeck and Quaide’s pioneering experiments by examining laboratory-scale impacts into layered targets consisting of a loose sand over a bonded substrate. New analytical techniques allow us to derive quantitative data not only regarding the morphologies and morphometries of the craters formed in these targets, but also for the excavation-stage flow in the form of their ejecta curtains as these craters grow. In this study, we scan the resultant craters in three dimensions to obtain high-resolution topography; we also track ejected particles in flight to compare the excavation of craters in a layered target to those in a completely cohesionless one.

To apply our results to the Moon, co-author Bart was given the crater profiles from these experiments without knowing the true "regolith" thickness used for each target. She then applied her expertise in measuring regolith thicknesses on the Moon to determine the "regolith" thickness for each experiment. We will discuss her results and their implications for using this technique to estimate lunar regolith thicknesses. We will also examine the two different excavation-stage components observed during these experiments —central ejecta and curtain ejecta — and discuss their potential contributions to self-secondaries and boulders within and around small craters on the Moon.