Obliteration of ancient impact basins on the Moon by viscous relaxation

The widely accepted accretion scenario of planet formation suggests that the Moon experienced a violent bombardment in its early history. The accretion scenario predicts that a total of ~300 basins with sizes greater than 300 km formed throughout its bombardment history; however, only ~40 basins of this size are identified on the Moon. The cause for this notable discrepancy is unknown. Here we investigate the viscous relaxation of impact basins formed within ~150 Myr after the completion of lunar magma ocean (LMO) solidification, as only impacts that happened afterwards could be retained by the crust. We find that, owing to the high temperature of the lower crust, basins formed within ~100 Myr after the LMO solidification could have been sufficiently relaxed by lower crustal inflow to escape detection in gravitational and topographic data. By contrast, basins formed afterwards should have limited relaxation, as the cooler temperature of the lower crust inhibits the inflow. Our results show that, to have ~40 retained basins, the Moon would have had ~300–1,000 basin-forming impacts throughout its history and the LMO would have solidified ~4.3 Gyr ago. The temperature-dependent viscous relaxation of post-LMO basins provides a realistic explanation for the low number of basins observed on the Moon. The substantial relaxation of early basins suggests that terrestrial planets, which experienced crustal cooling after magma ocean solidification, may have suffered far more impacts than the basin records indicate.