Small Radar-Bright Crater Populations as a Guide to Mega-Regolith Thickness in the Southern Lunar Highlands

The southern highlands of the Moon comprise superposed ejecta layers, individually up to a kilometer in thickness, from the major basins. The radar properties of smaller (1-16 km diameter) impact craters that penetrate into this layered mega-regolith and excavate material from depth provide insight into the variability of mega-regolith depth above a postulated basement of large crustal blocks. We observe a significant difference in the population of radar-bright craters, 1-16 km and larger in diameter, between regions of the southeastern nearside highlands north and south of approximately 48°S latitude. There are about 1/3 fewer radar-bright craters south of this line than to the north, broadly coincident with the mapped boundary between southern deposits mapped as pre-Nectarian age and those of Nectarian to Imbrian age to the north. This difference in small radar-bright crater population is consistent with a mega-regolith thickness of about 1.5 km in the north and 2.5 km in the south, a difference that we attribute to South-Pole-Aitken basin ejecta.