Geologic Analyses of the Causes of Morphological Variations in Lunar Craters Within the Simple-to-Complex Transition

The diameter range of 15 to 20km is within the transition from simple to complex impact craters located on the Moon. This range spans roughly a factor of 3 in impact energy for the same impactor speed, composition, and trajectory angle. We analyzed the global population of well-preserved craters in this size range in order to assess the effects of target and impactor properties on crater shapes and morphologies. We observed that within this narrow diameter range, simple craters are confined to the highlands, and complex craters are more abundant in the mare. We found unusually deep craters around the highlands-mare boundaries and favor the hypothesis that they form by impact cratering on high-porosity terrain. We infer that target properties primarily contribute to the observed morphological variations in the craters. Simple crater formation is favored by a terrain that is more homogeneous in strength and topography, while transitional and complex crater formation is aided by heterogeneity in lithology, topography, or strength, or a combination of these parameters. Clearly visible impact melt deposits in a small percentage of simple craters, and two cases of craters differing in morphologies from their nearest neighbors in similar geologic settings, suggest that variation in impactor properties such as impact velocity and impactor size may have some role in causing morphological differences between similar-sized craters. Plain Language Summary We investigated the causes of variations in the appearances of impact craters on the Moon whose diameters are similar. The 15-20 km diameter range comprises simple craters (bowl-shaped depressions), transitional craters (craters in which material slumped down the wall and accumulated locally), and complex craters (in addition to the wall failure, subsurface bedrocks were uplifted to form a central mound at the center of the floor). We noted that simple craters preferentially formed in the highlands, and the complex craters were favored by the mare, while the transitional craters are scattered across the lunar surface. In the mare, weathered material accumulated during the time between lava flows. We hypothesize that this layering reduced target strength and thus caused the cavity formed upon impact to weaken and the material to mobilize and form transitional and complex craters. The highlands are non-layered and therefore can help stabilize the transient cavity to form simple craters. Transitional craters predominantly occur on highland terrains marked by abrupt increase in slopes. For two cases in which we did not find any differences between properties of the terrains around a simple crater and a transitional crater, we attributed morphological variations to the differences in impactor properties.