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

Abstract

The diameter range of 15 to 20 km 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.

Geologic Investigation of Deep Simple Craters in the Lunar Simple‐to‐Complex Transition

“…The morphological differences in similar‐sized craters can be governed by variations in target properties and/or impactor parameters. To determine the factors behind these morphological differences, Chandnani et al () performed a geologic investigation of 244 well‐preserved lunar craters in the 15‐ to 20‐km size range. Craters with sharp rims, distinctly visible features and no apparent post impact degradation (class 1 craters defined by Arthur et al, ) were selected.…”

“…Craters with sharp rims, distinctly visible features and no apparent post impact degradation (class 1 craters defined by Arthur et al, ) were selected. Using high resolution Lunar Reconnaissance Orbiter Camera (Robinson et al, ) wide angle camera (LROC WAC), LROC narrow angle camera (NAC) images and Lunar Orbiter Laser Altimeter (LOLA; Smith et al, ) gridded topography (Gridded Data Records, GDR) data, Chandnani et al () classified craters based on cavity shape and its components (e.g., presence of localized slumped material terraces central uplift, floor fractures) into seven morphologies: simple crater; crater with localized slumps; crater with localized slumps and terraces; crater with localized slumps and central uplift; crater with localized slumps, terraces, and central uplift; concentric crater; and floor‐fractured crater.…”

“…From the group of 244 craters, Chandnani et al () identified 117 simple craters based on uniform wall slopes and roughly bowl‐shaped cavities and observed that they are confined to flat or gradually sloping surfaces of the highlands and complex craters (the ones with central uplift) are more abundant in the mare. The study suggested that formation of simple craters is favored by a substrate whose attributes (strength, lithology, and topography) are spatially and vertically homogeneous (weaker but nonlayered highlands), whereas heterogeneity in terrain properties (layering in mare) facilitates cavity collapse, hence formation of shallower, complex craters (also previously reported by Cintala et al, , Cooper, , Dence, , Osinski et al, , Pike, , Quaide & Oberbeck, , Roddy, , Senft & Stewart, , Smith & Hartnell, , and Stewart & Valiant, ).…”

“…The mean is 0.191 with a standard deviation of 0.023; about 35% of highland simple craters have a d/D ratio larger than 0.2. Based on the historical citations of 0.2 as a solar‐system wide mean for fresh craters and the recent work of Krüger et al (), we set the threshold for a “deep” crater as one with a d/D that exceeds 0.200 by more than the standard deviation in the d/D value; six of the 117 simple craters in Chandnani et al () met this criterion. These craters occur in the highlands in proximity to the mare‐highlands boundaries or within the ejecta blanket of the mare basins.…”

“…The mare terrain is more coherent and stronger than the more porous regolith‐dominated highlands (Kiefer et al, ; Soderblom et al, ; Wieczorek et al, ) and therefore is expected to be more resistant to cavity collapse thereby supporting deeper craters. The mare themselves, which are composed of laterally extensive layers of basalt flows, likely interleaved with regolith layers (Head, ; Hiesinger et al, ; Philpotts & Schnetzler, ; Shoemaker & Hackman, ; Smith et al, ; Taylor, ), and this layering produces strength heterogeneities with depth that results in slumping/terracing in this diameter range (Chandnani et al, ; Cooper, ; Kalynn et al, ; Osinski et al, ; Pike, ; Quaide & Oberbeck, ; Roddy, ; Smith & Hartnell, ). So, the second hypothesis may not work for the mare.…”