GRAIL Investigation of the Subsurface Structure of South Pole-Aitken Basin

The GRAIL mission [1] has revealed a lunar crust that is globally less dense and presumably more porous [2] 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 [3] and spatial variations in the density structure [4] have been determined. Here we used an admittance approach [4] 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 [7]. The pre-existing crust was probably completely removed [8]; 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 [2]. 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 [4]. 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 [5] 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.

[1] Zuber et al. 2013 [2] Wieczorek et al. 2013 [3] Han et al. 2013 [4] Besserer et al. 2014 [5] Hurwitz and Kring 2014 [6] Vaughan and Head 2013 [7] Petro and Pieters 2004 [8] Potter et al. 2012