Numerical Investigation of Lunar Basin Formation Constrained by Gravity Signature

Abstract

Impact basins on the Moon can serve as a benchmark for timing and intensity of the impact flux in the inner solar system. The basin morphology and morphometry depend on impactor size, mass, and velocity as well as the thermal state of the lunar lithosphere which is a function of the cooling history. Erosion by superimposed impact bombardment has altered the surface expression of basin structures over time, making it difficult to determine the size unequivocally solely based on topographic expression. The gravity signature of basins is thought to be a less altered measure of the size of impact structures. By a systematic study of basin formation using the iSALE-2D shock-physics code, we investigate the influence of the lunar thermal state and different impactor properties on the transient crater and final basin size and on the resulting gravity anomaly. As constraints we use gravity data of 16 farside basins and their assumed formation ages to estimate the subsurface temperature related to the cooling history of the Moon. Our modeling results confirm that the thermal state affects the basin formation process and the basin sizes significantly. We provide quantitative relationships between the observed gravity signal, the different basin sizes, and the impactor diameter considering the thermal state of the Moon upon impact, which correlates with the formation ages or periods in the literature. Our study allows for estimating the impactor size from the observed gravity field if the formation age and, thus, the thermal state of the lithosphere is approximately known.