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

The lunar regolith is a layer of unconsolidated material covering the lunar surface. The physical properties of the regolith play a significant role in understanding many aspects of lunar geology, including its history and evolution over time. Moreover, impact craters on the Moon are largely responsible for creating and revealing this regolith layer which can inform us about the possible source depth of surface rocks. This study aims to understand the properties of regolith around some unique impact craters on the Moon. These craters are surrounded by distinctive, ring-shaped structures having unusually low radar return and are called radar-dark halo craters (RDHCs). To infer the depth of the regolith around these RDHCs, we used the unique morphological features found in small impact craters (~10-200 m). These features include central mounds, concentric rings, and flat bottoms. They are believed to have formed when impacts happen in a substrate covered with fragmental, less cohesive material (i.e., regolith). Previous studies have used the dimensions of such craters and their morphological features to estimate the lunar regolith depth, which in our case, may be helpful in better understanding the formation of the dark haloes. We used Lunar Reconnaissance Orbiter’s (LRO) Narrow Angle Camera (NAC) images to examine such small craters in three different regions: (i) continuous ejecta around RDHCs, (ii) the dark haloes and (iii) a “typical” region beyond the halo. Preliminary observations for Copernicus crater show higher crater density and greater regolith thickness in the dark halo region (5.94 ± 0.32 m) compared to the continuous ejecta (2.85 ± 0.24 m). The typical region south of the dark halo shows average regolith thickness of 9.34 ± 1.73 m. These observations compare favourably with a previous study indicating thinner regolith depths correlating with lower crater densities. Moreover, our regolith depth estimations compare well with the average regolith thickness of the near side lunar mare regions (~ 2-4 m), however it is interesting to note that the dark halo regolith depth is deeper than the average.