Formation of Simple Impact Craters in Layered Targets: Implications for Lunar Crater Morphology and Regolith Thickness

Impact crater morphologies vary significantly across the lunar maria. Craters with diameter less than 400m are closely related to variations in target properties (rock strength, porosity, and layering) as well as the impact velocity. Here we investigate target and impact conditions feasible for reproducing crater morphologies, such as normal, central-mound, flat-bottomed, and concentric craters, using numerical models of impact crater formation in two-layer targets under lunar conditions (i.e., average-impact velocity and gravity). Based on more than 1,000 numerical models, we observe that concentric craters can form with a strength contrast as low as factor of 2 between the layers as long as the difference in cohesion is larger than a value between 50 and 450kPa (for an impact velocity of 12.7km/s). Because of this small contrast, concentric craters do not serve as a good indication for the lunar regolith-mare interface. Crater morphology changes with crater diameter according to three different scenarios depending on layers' strengths and the impact velocity. For high-impact velocity or/and moderate material strength, normal crater morphology transitions directly to concentric morphology, while with large material strengths and/or low-impact velocity, craters change with size from normal to flat-bottomed and then to concentric morphology; only this latter pathway is consistent with previous laboratory results. Lunar regolith thicknesses estimated from crater morphologies can differ by up to 80% from previously inferred thicknesses. The transition from normal to flat-bottomed craters is found to be the most robust transition to infer the thickness of the surficial target layer.