Why the lunar South Pole-Aitken Basin is not a mascon

Lunar mascon basins exist for only a range of observed diameters. We modeled the full formation of South-Pole Aitken basin using a sequential two-code (hydrocode and Finite Element Model) approach to understand why this range does not extend to the largest lunar basin. Similar to previous work, we found that the best-fit hydrocode had impact parameters of a 170 km in diameter dunite projectile striking at 10 km/s, with a pre-impact lunar thermal gradient of 50 K/km (until a depth of 28 km at which an adiabat is reached), and pre-impact crustal thickness value of 40 km. Unlike previous work, we matched the crustal distribution of the inner basin by utilizing a weaker melt rheology in our model. The crust in the models with a weaker melt rheology flowed inward after crater collapse to cover the basin center; therefore, not requiring an additional step of melt differentiation. Given the large diameter of South-Pole Aitken and the high geothermal gradient at impact, the steady state configuration after relaxation and thermal cooling was driven by isostatic forces without a significant contribution from lithospheric rigidity. Without lithospheric rigidity, South-Pole Aitken relaxed to a slightly negative free-air gravity signature, not forming a mascon, in agreement with GRAIL observations.