GRAIL Investigation of the Subsurface Structure of South Pole-Aitken Basin
Abstract:The GRAIL mission  has revealed a lunar crust that is globally less dense and presumably more porous  than anticipated. Measurements of the inferred (effective) density as a function of wavelength provide a way of probing the vertical density structure of the crust. Both the mean vertical density stratification  and spatial variations in the density structure  have been determined. Here we used an admittance approach  to investigate the subsurface structure of South Pole-Aikten (SPA) Basin.
The SPA-forming impact was large enough to generate enormous quantities of melt [5,6] and a global ejecta layer . The pre-existing crust was probably completely removed ; GRAIL data indicate a depth to the base of the crust (or other density interface) at a depth of about 13-20 km beneath the basin center . Cooling and crystallization of the melt pool will have generated a distinct stratigraphy and density structure [6,5]. This predicted density structure can then be compared to that inferred from the GRAIL observations.
We determined the effective density spectrum of SPA using a localized multitaper approach . The effective density decreases from 2.7 g/cc at spherical harmonic degree l=250 to 2.6 g/cc at l=550, indicating an increase in density with depth. Two model predictions from  yield effective densities in the range 3.0-3.2 g/cc. The discrepancy between the predictions and the observations could be explained by 1) incorporations of large volumes of crustal material into the recrystallizing melt-sheet; or 2) ~20% fracture porosity extending to depths of 10 km or more. The latter possibility is more likely given SPA early formation and subsequent reprocessing and delivery of low-density material by impacts.
 Zuber et al. 2013  Wieczorek et al. 2013  Han et al. 2013  Besserer et al. 2014  Hurwitz and Kring 2014  Vaughan and Head 2013  Petro and Pieters 2004  Potter et al. 2012