The Missing Craters and Basin Rings Beneath the Lunar Maria.

Liang, Weigang; Andrews‐Hanna, Jeffrey C.; Evans, Alexander J.

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. Plain Language Summary: A visual comparison of the nearside mare and the highlands shows a clear deficit in crater density in the nearside mare regions, as expected due to the mare covering up the past craters. However, gravity data should reveal the buried craters. While gravity data show signatures of the larger buried craters, we do not see much signal from buried craters that are less than ∼90 km in diameter, even though the resolution of the gravity data should allow buried craters of 10 km in diameter to be detected. In addition, very large impacts produce ring structures of 100 km in width around the basin center, as seen in the Orientale basin on the farside. In contrast, only a fraction of the Imbrium ring system is exposed above the mare surface, but the gravitational signatures of ring structures buried beneath the maria are largely absent in both topography and gravity data. In this work, we test different hypotheses of mechanisms that could have resulted in the missing buried craters and the missing Imbrium rings. We find that smoothing or erosional processes, such as the sustained thermomechanical erosion due to the hot mare floodings, best explain the observed data. The removal of so many buried craters as well as a structure as large as the Imbrium rings may imply past mare floodings of unprecedented scale, intensity, and duration. Key Points: A deficit in the nearside mare buried crater population and a lack of buried ring structures around the Imbrium impact basin are observedWe use gravity data to test different hypotheses that may explain the missing buried craters and the missing rings of ImbriumA smoothing or erosional process, possibly the sustained thermomechanical erosion due to the mare floodings, is our preferred hypothesis

IMPACT craters; LUNAR craters; LUNAR maria; POWER spectra; VOLCANIC eruptions; HIGH temperatures; FLOODS

Journal of Geophysical Research. Planets, 2023, Vol 128, Issue 12, p1

2169-9097

Academic Journal

10.1029/2023JE007876