Numerical Investigation of Lunar Basin Formation Constrained by Gravity Signature.

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. Plain Language Summary: Large basin structures testify to the lunar bombardment history by bodies several tens to hundreds of kilometers in diameter. To reconstruct how often the Moon, and thus also Earth, was hit by large cosmic bodies of a given size, it is essential to relate the size of basin structures with impactor size. However, since the surface expression of most known basins on the Moon is not pristine, the definition of the actual basin size is often ambiguous. As an alternative, we use the observed pronounced positive gravity excursion across lunar basins as a measure of basin size. In addition, we consider the thermal evolution of the Moon which is thought to affect the characteristics of the gravity signature of basins. Therefore, the timing when a given basin was formed in the cooling history of the Moon plays an important role. We use the gravity signature and the lunar thermal state as constraints for modeling the formation of the lunar basins as a function of impactor size. Our results allow for estimating the impactor size and thermal state of the Moon for 16 lunar farside basins. The predicted thermal states are in line with proposed absolute ages of the basins. Key Points: We provide relationships between impactor size, gravity signal, and farside basin size as a function of cooling history and formation timeGravity anomalies serve as a measure of basin size as impacts altered morphology and crustal thickness models simplify subsurface structureOur basin formation models agree with the assumed formation time based on crater chronology and the estimated cooling history of the Moon [ABSTRACT FROM AUTHOR]