Cratering on Mars with almost no atmosphere or volatiles: Pangboche crater

Pangboche crater (17.2 degrees N, 226.7 degrees E; 10.4 km dia.) lies close to the summit of Olympus Mons volcano, Mars, at an elevation of similar to 20.9 km above the datum. Given a scale height of 11.1 km for the atmosphere, this relatively large fresh crater most likely formed at an atmospheric pressure <1 mbar in essentially volatile-free young lava flows. Detailed analysis of Pangboche crater from High Resolution Imaging Science Experiment (HiRISE) and Context Camera (CTX) images reveals that volatile-related features (e.g., fluidized ejecta layers and pitted floor material) are absent. In contrast, abundant impact melt occurs on the floor, inner walls, and rim of the crater, and there is an extensive field of secondary craters that extend up to approximately 45 km from the rim crest. All of these attributes argue that it was the absence of volatiles in the target rocks at the time of crater formation, rather than the thin atmosphere, which had a controlling influence on crater morphology. Digital elevation data derived from the CTX images reveal that Pangboche crater has a depth of about 954 m (depth/diameter = approximately 0.092) and that uplifted target rocks comprise about 58% of the relief of the 180 m-high north rim. As the target material comprised a sequence of layered lava flows, Pangboche crater may well represent the best crater on Mars for direct comparison with craters formed on the Moon (permitting variations in gravitational effects to be investigated) or on Mercury (allowing the role of the atmosphere to be studied).