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

Abstract

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.

Meter‐Scale Topographic Roughness of the Moon: The Effect of Small Impact Craters

“…For each crater, azimuthally averaged radial profile of elevation from crater center to three crater radii is extracted from NAC DTM data. Superposed features such as craters can cause a large variation in surface elevation, and such regions are excluded in topographic profile extraction (e.g., Du et al, 2019). Figure A1 shows the normalized topographic profiles of the 71 fresh craters, where the elevation is normalized by the crater diameter and the radial distance is rescaled by crater radius.…”

“…In LROC NAC optical images, we first selected 71 small fresh craters according to their morphologic prominence (e.g., Basilevsky, 1976), with diameters ranging from 36 to 1,016 m. For each individual crater, we extracted azimuthally averaged topographic profiles from NAC DTMs, using a similar procedure as in Du et al. (2019). With these topographic profiles, we then regressed size‐dependent morphologic relations for the rim height, depth/diameter ratio, and inner wall slopes.…”

Thickness of Lava Flows Within the Northern Smooth Plains on Mercury as Estimated by Partially Buried Craters

“…Each given initial elevation profile was input into a diffusion equation to model the topographic degradation as a function of time. For partially buried craters, the final modeled profile is dependent on six parameters, including the initial crater diameter, exterior and interior lava flow thicknesses, and two diffusivity‐time products before and after the lava flow emplacement (Du et al, 2019). By varying the parameters in the model space, the best fitting lava flow thickness is determined by the minimum misfit between the final modeled and observed topographic profiles.…”

“…For a given crater, the study region is defined as a circle that extends three crater radii from the crater center. We azimuthally averaged the data as a function of 10.1029/2020GL090578 distance from the crater center, excluding any atypical topographic features such as large craters and scarps (Du et al, 2019).…”

“…Similar to the northern smooth plains on Mercury, the lunar surface is also covered by lava flows, and their thicknesses have been estimated from partially buried craters (Du et al, 2019). A comparison of lava flow volumes between the two bodies provides insights into the mechanisms of the production and eruption of magmas.…”