The Link Between Subsurface Rheology and Ejecta Mobility: The Case of Small New Impacts on Mars

The dynamics of crater ejecta are sensitive to the material properties of the target, much like the crater size and morphology. We isolate and quantify the effect of target properties on the ejecta mobility (EM) - the maximum radial extent of ejecta scaled by the crater radius. We compile geologically motivated subsurface structures based on data gathered by orbiters and landers. Those structures arise from varying properties of materials in single layers (strength, composition, porosity); the thickness of top regolith cover; and the sequence and thicknesses of 3–4 stacked layers. We realize 2D simulations with the iSALE shock physics code which result in a 50 m diameter crater (an analog of new craters formed in the period of spacecraft observation). We find that varied subsurface rheologies result in EM numbers with a wide range of values between 7 and 19. Some subsurface models can result in a similar EM, and some have distinct EMs, which shows potential for using this quantity as a new diagnostic of target properties. We also show that ejecta dynamics are sensitive not only to the material in the excavation zone but also at much greater depths than commonly assumed (at least 1–2 crater radii). EM also depends on both material properties and layering: the impedance contrast governs the nature of wave propagation, while the layer depth controls the timing of the shock wave reflection. Detailed studies of EM thus have promise for unveiling shallow subsurface rheologies on many Solar System bodies in the future.