Regionally compartmented groundwater flow on Mars

Groundwater flow on Mars likely contributed to the formation of several types of morphologic and mineralogic features, including chaotic terrains, valley networks, Meridiani Planum geologic units and, potentially, sulfate and phyllosilicate deposits. A central issue for these features is the spatial scale of groundwater flow required for their formation. For groundwater simulation purposes, a global Martian aquifer has frequently been assumed, but the validity of this assumption has not been tested. Chaotic terrains, thought to have formed owing to the disruption of a cryosphere under high aquifer pore pressures, provide the basis for such a test. Specifically, we use groundwater models to predict regions of cryosphere disruption due to recharge-driven pore pressure increases, and we compare these regions to observed Martian chaotic terrains. Our results suggest that a globally connected aquifer cannot give rise to cryosphere disruption at the two locations where large chaotic terrains are observed (the circum-Chryse region and the eastern Hellas Planitia). Conversely, modeled cryosphere disruption occurs in locations such as Amazonis Planitia and west Hellas Planitia where no supporting evidence is present, suggesting again that groundwater flow was likely regionally compartmented. Furthermore, the consistent occurrence of modeled breakouts in the Valles Marineris canyon system suggests that large-scale fractures there likely discharged most of the groundwater required for circum-Chryse outflow channel formation, with only minor contributions from chaotic terrains. The fractures are close to a likely source of recharge over Tharsis, and their low elevations lead to high pore pressures even if groundwater flow is regionally compartmented.