Transitional impact craters on the Moon: Insight into the effect of target lithology on the impact cratering process

Abstract

We studied a data set of 28 well‐preserved lunar craters in the transitional (simple‐to‐complex) regime with the aim of investigating the underlying cause(s) for morphological differences of these craters in mare versus highland terrains. These transitional craters range from 15 to 42 km in diameter, demonstrating that the transition from simple to complex craters is not abrupt and occurs over a broad diameter range. We examined and measured the following crater attributes: depth (d), diameter (D), floor diameter (Df), rim height (h), and wall width (w), as well as the number and onset of terraces and rock slides. The number of terraces increases with increasing crater size and, in general, mare craters possess more terraces than highland craters of the same diameter. There are also clear differences in the d/D ratio of mare versus highland craters, with transitional craters in mare targets being noticeably shallower than similarly sized highland craters. We propose that layering in mare targets is a major driver for these differences. Layering provides pre‐existing planes of weakness that facilitate crater collapse, thus explaining the overall shallower depths of mare craters and the onset of crater collapse (i.e., the transition from simple to complex crater morphology) at smaller diameters as compared to highland craters. This suggests that layering and its interplay with target strength and porosity may play a more significant role than previously considered.

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

“…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, ). Simple craters have been observed, experimented, and modeled to form with a d/D of ~0.2 on targets that have bulk properties such that they can be considered cohesionless, largely homogeneous, and not unusually porous (Kenkmann et al, ; Melosh, ; Melosh & Ivanov, ; Pike, , , ; Salamunićcar et al, ; Wood & Anderson, ).…”

“…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.…”

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

“…This massive crust has been battered by impacts ever since its formation and is weaker owing to fracturing of rocks by shock waves and accumulation of impact ejecta and regolith over time. Though the highland material is less coherent than the younger mare, the homogeneity in its strength relative to the layered mare seems to be the primary factor in stabilizing the transient cavity, resulting in simple craters at larger diameters than in the mare (Krüger et al, ; Osinski et al, ).…”

“…At the position of the localized slumped material, their profiles show a gradual decrease in wall slopes to values that cover a wide range and can also be as low as~5°F ( Figures 7a-7d). Such a morphology qualifies for the definition of a transitional crater (Cintala et al, 1977;Cintala & Grieve, 1998;Howard, 1974;Kalynn et al, 2013;Osinski et al, 2018;Pike, 1974Pike, , 1977bPike, , 1988Plescia, 2015b;Robbins & Hynek, 2012;Smith & Sanchez, 1973). The craters whose wall slopes exhibit a change due to accumulated material that was primarily contributed by ejecta of impact craters on or adjoining the rims would be classified as simple craters.…”

“…We seek to determine the causes of the morphological diversity of lunar craters measuring 15–20 km in diameter. Inferred target and/or impactor properties will advance our understanding of simple and complex cratering mechanics along with providing additional information on the regional crustal variations beyond the well‐established mare‐highlands dichotomy on the Moon (Kalynn et al, ; Osinski et al, ; Pike, , ; Smith & Hartnell, ). In general terms, a strong correlation of crater morphology with geologic settings, based on target parameters such as lithology, topography, strength, and porosity, would be an indicator that some property of the lunar surface (the target) is causing the observed morphological differences.…”