Gravitational signatures of lunar floor-fractured craters

Abstract

Lunar floor-fractured craters are impact craters characterized by distinctive shallow floors crossed by important networks of fractures. Different scenarios have been proposed to explain their formations but recent studies showed that the intrusion of magma at depth below the crater floor is the most plausible explanation. The intrusion of dense magma within the light upper-most part of the lunar crust should have left a positive signature in the gravity field. This study takes advantage of the unprecedented resolution of the lunar gravity field obtained from the NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission, in combination with topographic data obtained from the Lunar Orbiter Laser Altimeter (LOLA) instrument, to investigate the gravitational signatures of both normal and floor-fractured craters. Despite the large variability in their gravitational signatures, the floor-fractured and normal craters in the Highlands show significant differences: the gravitational anomalies are significantly larger at floor-fractured craters. The anomaly amplitudes for floor-fractured craters are in agreement with synthetic gravity anomalies based on the predicted intrusion shapes from a theoretical flow model. Our results are consistent with magmatic intrusions intruding a crust characterized by a 12% porosity and where the intrusion has no porosity. Similar studies have been carried out in the lunar maria and South Pole–Aikten basin. Although the average gravitational signature of floor-fractured craters is larger than at normal craters in these regions, they cannot be distinguished statistically due to the small number of craters and the large variability of the anomalies. In general, a better characterization of the signal due solely to the initial impact crater is needed to isolate the magmatic intrusion signal and characterize the density contrast between the magma and crust.