HEMATITE BEARING RIDGE AS EVIDENCE FOR ANOXIC WATER DISCHARGE IN GALE CRATER

Mars Reconnaissance Orbiter CRISM hyperspectral imaging data covering Mount Sharp’s northern flank and acquired with pixels spatially oversampled have been processed to retrieve high signal to noise single scattering albedo spectra from 0.45 to 2.6 μm. The spectral characteristics of a ~200 m wide and 6.5 km long ridge at the northern end of a large channel system emanating from Mount Sharp were examined using this data. The ridge is composed of finely layered strata dipping to the north and exhibits a strong spectral signature of crystalline hematite. To the west, the ridge transitions to an escarpment overlain by younger strata and occasionally exposed through erosional windows; to the east, debris flows from Mount Sharp cover and obscure the ridge. The ridge is stratigraphically directly above the clay bearing strata. We interpret the ridge to be formed by erosion associated with outflow from the channel that preferentially stripped away flanking strata that were not as well cemented by hematite. We hypothesize that the hematite accumulated where anoxic groundwater discharged and came into contact with an oxidizing Martian atmosphere. Anoxic water may have originated as precipitation that percolated through the mound, leaching Fe²⁺ from the overlying sulfate layers until encountering the clay acquiclude. Alternatively, anoxic Fe²⁺-rich waters at Gale could have originated by groundwater discharge near the base of the mound. In this scenario, dissolved Fe²⁺ could be leached from an underlying basaltic aquifer or possibly the clay layer. Given that iron oxidation reactions on Earth may be mediated by chemolithotrophic microorganisms, the ridge provides a compelling site for a detailed measurement campaign by the Mars Science Laboratory Curiosity rover.