Thickness of Lunar Mare Basalts: New Results Based on Modeling the Degradation of Partially Buried Craters

International audience; Partially buried craters on the Moon are those craters whose distal ejecta are covered by lava flows and where the crater rim crest still protrudes above the mare plain. Based on the difference in rim heights between a partially buried crater and an unburied crater, previous studies estimated the thicknesses of the lunar mare basalts. However, these studies did not consider the erosion of the crater rim height, which can result in an overestimate in the derived thickness. By using recent high-resolution topographic data, we report a basalt thickness estimation method based on numerically modeling the topographic degradation of partially buried craters. We identified 661 buried craters over the lunar surface, and their spatial distribution suggests a preferential occurrence along the mare-highland boundaries. An elevation model of fresh lunar craters was derived, and the topographic diffusion equation was used to model crater degradation. By modeling the formation, degradation, and flooding of partially buried craters, basalt thicknesses were estimated for 41 mare craters whose rims are completely exposed. The resulting mare basalt thicknesses vary from 33 to 455 m, with a median value of 105 m that is 95 m smaller than that derived when not considering crater degradation. The estimated eruption rate of lunar mare basalts is found to have peaked at 3.4 Ga and then decreased with time, indicating a progressive cooling of the lunar interior. As a by-product from the crater degradation model, our results suggest that the topographic diffusivity of lunar craters increases with diameter. Plain Language Summary The thickness of mare basalts that partially cover a lunar crater can be estimated from the crater shape. To simplify this problem, previous studies used the shape of a fresh lunar crater, which had been well-characterized beforehand. In reality, however, the lunar crater degrades with time, which results in a more complicated, time-dependent crater shape. In this study, a crater degradation model is used to better estimate mare basalt thicknesses. Our results show that the mare basalt thickness could be overestimated by a factor of two when crater degradation is not considered. Our results constrain the volcanic eruption rate with time and imply that the crater degradation rate scales with crater size.