Ejecta Features in Martian Flat-Central Pit Impact Craters as Indicators of Sub-Surface Water, A Component in Central-Pit Impact Crater Formation
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Flat-central-pit impact craters (flat-CPICs) are a central-pit impact crater (CPIC) subset that exhibits little or no positive relief at the pit rim within the crater. The formation of CPICs has been associated with the presence of sub-surface target volatiles. Some Martian CPICs exhibit ejecta features that may be attributed to flow-like deposition. From a pre-exiting Martian impact crater database, ten impact craters were selected for detailed study. Cross-referencing the locations of these ten flat-CPICs with water-mass fraction data suggests a preferred target water content range for flat-CPIC formation of 4 – 8 wt.%. An examination was performed on the continuous ejecta of one flat-CPIC (29.08° S, 98.29° E) for features that support the presence of target water as a component in CPIC formation. Ejecta ramparts, ejecta lobes and curvilinear trough features were identified. These features are consistent with the flow-like deposition of water-rich ejecta. The presence of 2 flow-like eject features, ejecta ramparts and ejecta lobes, suggests a heterogeneous porosity in the target materials. The asymmetrical distribution of these flow-like features around the crater is consistent with porosity variations that may be the result of localized, pre-impact modification(s) to the pre-impact target substrate.