Equatorial Layered Deposits in the Firsoff crater area: process variability and habitability potential

The depositional processes proposed to explain the formation of the Equatorial Layered Deposits (ELDs) on Mars invoke very different systems such as sub-glacial volcanism, aeolian/airfall, lacustrine, lacustrine/volcanic and spring-fed deposition. We performed a detailed geological analysis of Firsoff crater and surrounding areas, where ELDs are present within and outside craters, using all the available datasets. ELDs stay on top of the Noachian Plateau Sequence and are covered by the Hesperian Ridged Plains Material. Within Firsoff crater, ELDs form a central bulge - estimated more than one kilometer thick - while, outside the craters, ELDs form flat-lying deposits probably not thicker than few tens of meters. Inside craters, terraced-like morphologies made of ELDs are found to mark the topographically highest deposits toward the rim. Although locally heavily eroded by wind, ELDs within Firsoff crater and within the other craters of the area show several morphologies that appear to be depositional, which would exclude that the craters were originally filled by ELDs. Within craters, ELDs consist of two facies: a roughly meter thick high albedo layered unit which drape and onlap the Plateau Sequence and hundreds meters large / tens of meters thick cone-shaped mounds made of layered breccia embedded in a matrix. The layered unit is disrupted in a post-depositional polygonal pattern and show no evidence of sedimentary structures. Mounds sometimes display an orifice at the top of their structure. Both facies appear to be linked to and possibly sourced from tectonically controlled fissure ridges. The two facies appear to be stratigraphically in heteropy thus suggesting a temporal as well as a genetic association. Polyhydrated sulfates have been detected in correspondence of ELDs. Outside the craters, ELDs show large-scale cross stratification associated to duneforms. On the base of these morphological, sedimentological and stratigraphic elements, we distinguish between fluid expulsion ELDs forming inside craters and aeolian ELDs outside and the craters. Fluid expulsion ELDs would result from groundwater upwelling and subsequent evaporitic precipitation, whereas aeolian ELDs from reworking and redeposition of the spring ELDs. While the climate supporting such a cycle was likely arid by terrestrial standards, these surface conditions in a terrestrial environment would have been conducive for microbial colonization. The potential for habitable conditions of evaporite settings as shown on analogues on Earth coupled with the high preservation potential within sulfates, make these deposits good candidate to understand the potential past habitability of Mars.