Assessing the depth of the regolith around radar-dark halo craters on the Moon

Impact cratering is a ubiquitous geological process in our Solar System and plays an important role in shaping the landscape of many planetary bodies, including the Moon. Our study focuses on the unique radar-dark halo craters (RDHCs) on the Moon, which are surrounded by distinctive, ring-shaped structures (i.e., haloes) that have unusually low radar backscatter. Our aim is to understand the interaction between the ejecta emplacement around RDHCs and the lunar regolith by assessing the local depths of the regolith surrounding RDHCs. We use the morphology of small impact craters (∼10–250 m) to infer the depth of the regolith around these RDHCs. We examine small craters in images from the Lunar Reconnaissance Orbiter's (LRO) Narrow Angle Camera (NAC) within the continuous and discontinuous ejecta blankets of five RDHCs. We then compare these results with those of five non-RDHCs. Overall, we observe similar regolith characteristics around RDHCs and non-RDHCs. Interestingly, our results do not show a strong correlation between crater age and regolith depths. This suggests that despite continued bombardment, the regolith layer is not getting significantly deeper with age. In addition to impact gardening, there may be other factors contributing to regolith formation around impact craters on the Moon. Additionally, our study suggests that the flat-floor small crater morphology is more dominant for the RDHCs than non-RDHCs, which points to a deeper and more uniform mixture of regolith in these regions. Our study thus suggests that the emplacement of fine-grained RDH material over bedrock disrupts the ordinary regolith formation. We conclude that a careful re-examination of our understanding of regolith formation and evolution is necessary.