Pervasive sedimentary diapirism in Acidalia Planitia, Mars: Implications and astrobiological perspective

Tens of thousands of relatively high-albedo, Amazonian-aged mounds extend across southern Acidalia Planitia. The distribution of these features and their internal structures,surface morphologies, associated flows, low thermal inertias, and mineralogy are most consistent with an origin from a sedimentary diapiric process. The profusion of these mounds is likely a consequence of the fact that Acidalia was distal to the circum-Chryse outflows and was a depocenter for accumulation of mud and fluid. Mud diapirs and mud volcanoes are the closest terrestrial analogues, but the diapirism in Acidalia may well have had distinctly martian attributes. The abundance and widespread occurrence of these mounds could reflect basin-scale events that triggered the diapirism. Examples of such events could include loss of overburden through sublimation of a frozen ocean, hydrothermal or compressional pulses from Tharsis, or destabilization of clathrates. The formation of the mounds in Acidalia appears to have been one of the last events in this part of the lowlands that involved basinwide transport of subsurface fluids to the surface. Terrestrial mud volcanoes are important sources of atmospheric methane, as could be their martian equivalents. On Mars, it has been suggested that significant amounts of methane (from either non-biological or biological sources) may exist in the subsurface as free gas and gas hydrates. It is possible that widespread sedimentary diapirism may have released substantial quantities of gas to the martian atmosphere, especially if the diapirism were triggered by, or resulted in, destabilization of clathrates. Mud diapirs and mud volcanoes on Earth often carry physical and chemical biomarkers from depth, making their martian equivalents key sites for astrobiological investigation. The geologic setting of Acidalia would have been a favored location for the concentration of any organic materials entrained in the outflow sediments. Consequently, the subsurface in Acidalia could contain organic remnants of possible microbial life that may have existed in its large catchment area. Sedimentary diapirism in Acidalia could have transported minimally-altered samples to the surface from depths of meters to even kilometers, providing access to materials of potential astrobiological importance from strata that might otherwise be unreachable. We thus propose that the high-albedo mounds in Acidalia offer an important new class of exploration targets for Mars.