|As the oldest and largest well-preserved basin on the Moon, the South Pole – Aitken Basin (SPA) is relevant to a broad range of lunar science topics including the composition, structure, and evolution of the lunar crust and upper mantle, the nearside-farside dichotomy, the absolute ages of lunar impact basins and the late heavy bombardment, and the mechanics of large impact processes. SPA is a high-priority target for future sample return missions, which could address each of these issues. Several focused efforts are underway in order to identify appropriate landing sites. We are currently working with Moon Mineralogy Mapper (M3) hyperspectral data in order to assess mineralogical diversity across the basin and evaluate the diverse local stratigraphy arising from both the SPA impact event and eons of subsequent cratering and space weathering processes.
SPA exhibits a mafic-bearing composition with optical properties dominated by pyroxenes. Compositional variations among these pyroxenes are captured in the 1 μm and 2 μm spectral absorption bands as measured by M3. We have developed an approach to identify and characterize the compositions of the most pristine pyroxene-bearing exposures in the basin by measuring the depths and centers of these absorption bands in geologic context. The approach has been validated using a wide variety of laboratory spectroscopic data.
Most of the well-defined mafic exposures in SPA are associated with crater structures of varying sizes. We use impact crater scaling laws (specifically regarding excavation, impact melting, and central peak formation) to estimate the origin depth of the exposed materials. By considering several exposures in and around several SPA craters, the local stratigraphy is evaluated. Due to the large size of SPA, the basin-wide stratigraphy can only be evaluated by integrating local geologic context including the regional history of impact cratering and magmatism.
From an initial analysis of SPA subregions, several stratigraphic trends are beginning to emerge. Several (but not all) central peaks (which represent the deepest material exposed in a given crater) exhibit a narrow range in pyroxene composition relatively rich in Mg. Wall material (originating from shallower depths) is often of a different composition. The apparent uniformity of the central peak composition suggests the presence of a relatively homogenous Mg-rich zone tapped by these diverse craters. Material with distinctively more Fe- and Ca-rich pyroxenes occurs principally near the center of the basin (e.g., mafic mound). The observed relationships arise from some combination of SPA impact melt heterogeneity, diversity of crust/mantle clasts in basin floor materials, impact melt differentiation, redistribution of materials in subsequent impacts, mare basalt emplacements, and soil development. Analysis of the composition across additional local areas will further constrain the character and origin of the diverse stratigraphy observed in SPA.