Small lunar craters around the rim of Shackleton crater at the lunar South Pole: Spatial distribution and size-frequency

Small lunar craters a few meters to 1 km in diameter are dominant on the lunar surface. They are important to understand local geology because their distribution and density provide us information about degradation or resurfacing processes. There are plenty of small craters on the surface of the lunar poles including permanently shadowed regions, and craters in these regions were studied in diameters larger than 200 m (e.g., Zuber et al., 2012, De Rosa et al., 2012, Tye et al., 2015). However, craters small than 200 m have not well investigated. In this research, we count craters 10 m to ~900 m in diameter which are superposed on the rim of Shackleton crater in order to characterize the spatial and size-frequency distributions of small lunar craters. For crater identification, we used 18 high-resolution (0.5-2.0 m/pixel) images obtained by the Narrow Angle Cameras onboard the Lunar Reconnaissance Orbiter. The central coordinates and the diameters of more than 27,000 craters were measured within the "crater wall" (Region C), the "crater rim crest" (Region D), the "inner rim annulus excluding steep region" (Region E), the "steep region within annulus" (Region F) defined by Zuber et al. (2012). For the Regions C, D, and E, the slopes of the cumulative size-frequency distributions (CSFDs) are almost -2 in a diameter range of 10 m to 200 m, which suggests the standard lunar equilibrium. However, for Region F, the CSFD in diameters 50-150 m suggests resurfacing (not crater equilibrium). Moreover, in diameters less than 200 m, the crater density of each region increases with a decrease in its surface topographic gradient. This could be explained by downslope movements of surface materials triggered by seismic shaking.