Topography on the Crust-mantle Boundary in Lunar Basins Due to Both

Introduction: The dynamics of the crust-mantle interface (Moho) underlying lunar impact basins has received considerable study, particularly those with strong positive free-air gravity anomalies associated with the locations of large basins (" mascons ") [e.g. 1-8]. These gravity anomalies, detected via orbital spacecraft observation, indicate that after surface topography and mare loads are accounted for, many lunar basin subsurfaces exist in a wide range of topo-graphic configurations. These states range from a bulls-eye pattern of concentric positive and negative gravity anomalies concentrated in the basin center, to a singular broad and subdued positive anomaly occupying most of the basin interior [10-15]. The subsurface basin topographies are of particular interest because they are used to infer a preserved state of isostatic (dis)equilibrium, and they have been qualitatively observed to correlate with basin age via the structural evolution of a topographically relaxing basin [e.g. 7-8,11]. The physical mechanisms of structural relaxation that operate to drive vertical isostatic adjustment also act laterally to remove surface and subsurface topography, albeit over dramatically different time scales [e.g. 7,16]. In particular, lateral flow of crustal or mantle material is strongly affected by temperature, stress, and strain rate [e.g. 17-19]. Thus, interpretation of the above Moho features as evolutionary requires developing models that promote topographic relaxation over geologic time (i.e. enhanced thermal environment , weaker rheology, volatile content, higher driving stresses, etc). However, recent hydrocode simulations of basin-forming impacts have revealed significant dependence of basin structure on initial target conditions, including background temperature and crustal thickness [20]. This leads to the question of whether lunar basin Moho topography might be at least in part, genetic rather than evolutionary. Process: We first generated a catalog of profiles of lunar impact basin Moho, including vertical magnitude and total width of the central uplift. The total width was determined by the distance from the basin center to the first major positive inflection in the topography. The vertical magnitude is the elevation difference between this inflection and the height of the Moho at the basin center. To obtain these measurements , we used a single-layer, spherical harmonic crustal thickness model complete through degree and