Formation of Central Pits in Impact Craters on Mars: A Statistical Investigation of Proposed Mechanisms

Central pit craters (CPCs) are complex craters that exhibit depressions within their floors or central uplifts. Some proposed models for central pit formation can be grouped by similar mechanism: (A) explosive volatile release, (B) central peak collapse, and (C) drainage of subsurface melted volatiles. We test these groups of mechanisms for central pit formation on Mars based on morphometries expected from each mechanism. Volatiles in the target (A and C) would be expected to result in a correlation with layered ejecta, also formed from volatiles, for CPCs over non-CPCs. Central peak collapse (B) should result in larger diameters for pit rims than central peaks due to outward flow. Explosive volatile release (A) should yield smaller volumes for pit rims than pits due to mass loss. Data were collected on Context Camera images and digital elevation models for random samples of CPCs to assess the presence of these expected morphologies. Results of a Fisher exact test showed no preference in layered ejecta with CPCs over non-CPCs, inconsistent with volatiles in the target. Results of an independent t test showed that central pit rim diameters are larger than central peak diameters, supporting some component of central peak collapse, although measurement uncertainty makes this interpretation tentative. Central pit rim volumes were not found to be statistically smaller than their pits, weighing against formation by explosive volatile release. Thus, our findings do not support any single group of formation mechanism tested here, although they allow for formation by some combination of these (or other) processes.