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

From a group of well‐preserved lunar simple craters in the 15‐ to 20‐km diameter range and with the help of Lunar Orbiter Laser Altimeter topography data, we identified a subset of eight deep craters (depth/diameter ratio > 0.20). These craters are in the regions around the mare‐highlands boundaries, which are characterized as having the highest porosity on the lunar surface. To understand the cratering mechanics behind the formation of these craters, a geologic investigation of the terrains of these craters was performed. We evaluated the depth/diameter ratios of smaller simple craters surrounding several 15‐ to 20‐km diameter craters, analyzed the morphometry of the craters, and visually examined the cavities using multiple data sets. We conclude that deep transient cavities were formed from compaction of porous target material. The result was a deeper than normal simple crater without an identifiable increase in the volume of excavated material, as inferred from the craters' rim heights and shapes. While all of these craters formed in areas of high porosity, not all craters in high‐porosity regions are deep. It may be that some unusual impactor property is also required to produce a deep crater, such as a high velocity impact, a near‐vertical impact, or a dense impactor that yielded a large penetration depth.