Effect of Topographic Degradation on Small Lunar Craters: Implications for Regolith Thickness Estimation

Small crater morphology method has been used extensively in lunar regolith thickness estimation. However, topographic degradation can change crater morphology and thus bias regolith thickness estimation. In this study, we first developed a shape model for small fresh craters with normal, central mound, flat‐bottomed, and concentric geometry. We then simulated their degradation processes by using a topographic diffusion model. Simulation results show that as a small crater degrades, its morphology changes from concentric/central mound to flat‐bottomed, from flat‐bottomed to normal, and from normal to invisible, depending on its initial morphology. Upon the time a crater becomes invisible, its diameter can be enlarged by a factor of ∼70%. We proposed a revised small crater morphology method and applied it to the Apollo 11 and 14 landing sites. Our revised method permits a more accurate estimate of regolith thickness, and our results are helpful in understanding the evolution of the Moon's surface. Plain Language Summary On the Moon's surface, small impact craters generally exhibit four typical morphologic types: normal, central mound, flat‐bottomed, and concentric. Laboratory impact experiments show that morphology of small craters depends primarily on the thickness of the regolith layer. With the high‐resolution optical images acquired from recent missions, the small crater morphology method has been used extensively to estimate regolith thickness, which can provide critical information for the evolution of lunar surface. However, topography erosion by subsequent micrometeorite bombardments can change the morphology of impact craters, especially for small ones, preventing an accurate estimation of regolith thickness. In this study, we investigated the degradation effect on lunar regolith thickness estimation using small crater morphology method. We first designed an elevation profile model for small fresh craters with different geometry, and then simulated crater degradation processes by using a topographic diffusion model. These simulations give a quantitative measure on the changes in diameter and morphology of a small crater as it degrades. We further proposed a revised small crater morphology method for lunar regolith thickness estimation and applied it to the Apollo 11 and 14 landing sites, and the results are in good agreement with the estimates from small fresh craters only. Key Points A new shape model of small fresh lunar craters with normal, centr