Inevitability of low-latitude melting on Mars: implications for the sedimentary record

The recently published MOC-NA database of sedimentary rock locations shows an extraordinary concentration of sedimentary rocks near the equator - 64% at <10° latitude (59% when Valles Marineris is excluded). These rocks overwhelmingly date from the late Noachian to the middle Hesperian, when many sulfate-bearing deposits formed. With the reasonable assumption that liquid water is required for lithification, we hypothesize that liquid water only occurred near the equator during this era. As an initial test of this hypothesis, we model melting on Early Mars assuming a weak greenhouse effect similar to today. Combining the Laskar group's chaotic diffusion parameterization of orbital evolution with simple assumptions about ice stability, we show that melting under a weak greenhouse is most likely when (1) obliquity is high, (2) eccentricity is moderately high, (3) at equinox, (4) when the longitude of perihelion corresponds to equinox, and (5) at the equator. We compare discharge results from a snowpack Energy Balance Model to published discharge constraints at three Early Mars locations - SW Melas, Gale-Aeolis-Zephyria, and Meridiani. If these discharges cannot be reproduced under a weak greenhouse similar to today, then a stronger Early Mars greenhouse effect is required to explain these observations. We show how the fraction of a precession cycle during which melting occurs - the 'stratigraphic wet fraction' - can be used to set a lower bound on the strength of the Early Mars greenhouse effect. The stratigraphic wet fraction can be measured by MSL at Gale.