The Missing Craters and Basin Rings Beneath the Lunar Maria

Evidence for a population of craters buried beneath the nearside lunar maria has been found in the gravity data returned from the Gravity Recovery and Interior Laboratory mission. Although the total population of buried and visible craters within maria is comparable to the crater population in non-mare regions at large diameters, a deficit was observed for craters less than ∼90 km in diameter. This deficit is surprising because the data can resolve craters down to 10 km in diameter. Similarly, the Imbrium basin only has a partially exposed ring system, with individual ring widths of up to ∼100 km, but where those rings should be buried beneath the mare surface, we find the gravitational signature mostly non-existent. In this study, we test a series of mechanisms and scenarios that may explain the observed deficits in the buried crater populations by comparing localized Bouguer gravity power spectra and recovered crater size-frequency distributions from models of a simulated volcanically flooded cratered surface to the observed data. Our results indicate that the observed crater deficit and missing rings of Imbrium are best explained by a smoothing of the pre-mare surface. We represent this smoothing as a diffusional process, as might occur with thermomechanical erosion during the earliest stages of the mare eruptions. The removal of the missing craters and Imbrium rings was a massive and unprecedented event that sheds light on the early evolution of the mare region, possibly supporting high temperature voluminous floods of lava early during mare formation.