Depths, Diameters, and Profiles of Small Lunar Craters From LROC NAC Stereo Images

Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) images (pixel scale ~0.5 m) provide new 3-D views of small craters (40m>D>200m). We extracted topographic profiles from 85 of these craters in mare and highland terrains between 18.1-19.1°N and 5.2-5.4°E to investigate relationships among crater shape, age, and target. Obvious secondary craters (e.g., clustered) and moderately- to heavily-degraded craters were excluded. The freshest craters included in the study have crisp rims, bright ejecta, and no superposed craters. The depth, diameter, and profiles of each crater were determined from a NAC-derived DTM (M119808916/M119815703) tied to LOLA topography with better than 1 m vertical resolution (see [1]). Depth/diameter ratios for the selected craters are generally between 0.12 and 0.2. Crater profiles were classified into one of 3 categories: V-shaped, U-shaped, or intermediate (craters on steep slopes were excluded). Craters were then morphologically classified according to [2], where crater shape is determined by changes in material strength between subsurface layers, resulting in bowl-shaped, flat-bottomed, concentric, or central-mound crater forms. In this study, craters with U-shaped profiles tend to be small (<60 m) and flat-bottomed, while V-shaped craters have steep slopes (~20°), little to no floor, and a range of diameters. Both fresh and relatively degraded craters display the full range of profile shapes (from U to V and all stages in between). We found it difficult to differentiate U-shaped craters from V-shaped craters without the DTM, and we saw no clear correlation between morphologic and profile classification. Further study is still needed to increase our crater statistics and expand on the relatively small population of craters included here. For the craters in this study, we found that block abundances correlate with relative crater degradation state as defined by [3], where abundant blocks signal fresher craters; however, block abundances do not correlate with U- or V-shaped profiles. The craters examined here show that profile shape cannot be used to determine the relative age or degradation state as might be inferred from [4, for example]. The observed variability in crater profiles may be explained by local variations in regolith thickness [e.g., 2, 5], impactor velocity, and/or possibly bolide density. Ongoing efforts will quantify the possible effects of solitary secondary craters and investigate whether or not depth/diameter ratios and crater profiles vary between different regions of the Moon (thick vs thin regolith, highlands vs mare, and old vs young mare). References: [1] Tran T. et al. (2010) LPSC XXXXI, Abstract 2515. [2] Quaide W. L. and V. R. Oberbeck (1968) JGR, 73: 5247-5270. [3] Basilevsky A. T. (1976) Proc LPSC 7th, p. 1005-1020. [4] Soderblom L. A. and L. A. Lebofsky (1972) JGR, 77: 279-296. [5] Wilcox B. B. et al. (2005) Met. Planet. Sci., 40: 695-710.