Abstract

R-mode factor analysis of Fourier harmonics that describe the shape-in-plan-view of 716 large (diameter > 15 km) nearside lunar craters shows that two factors explain 84.3% of shape variance observed in the sample. Factor I accounts for 68.2% of the sample variance and describes moderate-scale roughness defined by harmonics 7 through 10. Shape variation described by these harmonics is related to surficial lunar processes of degradation that modify crater shape-in-plan. Dominant among these processes are ejecta scour from large impact events and ongoing aging. Factor II accounts for 16.1% of the observed shape variance and describes polygonal shape elements related to harmonics 2, 3, 4, and 6. Variation in these harmonics is tied to variables that distort the spherical symmetry of crater-forming processes. The dominant contributor among these variables is the nature of geologic structural patterns in impacted material. Unlike transient features described by factor I, polygonal shape elements described by factor II do not change appreciably with time. The permanence of these features and their relation to lunar geologic structure suggest that the shape of old craters carries the imprint of geologic structural relationships present in early lunar crust.

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