Sabkha Environments on Earth and Mars: Implications for the Martian Hydrological System

Sabkha (playa) lakes and their deposits are well known in arid zones on the Earth. Their presence indicates the existence of a water table and strong evaporative processes influencing the shallow subsurface. A large number of sediments and facies are produced by these environments and related processes. The main lithotype produced in these environments are evaporites, consisting of several salts such as halite, gypsum, carbonate. Their deposition may occur at the surface, but most time they are accumulated in the near subsurface at the contact between the water table and the overlying dry sediments. High-albedo areas have been recognised on the Martian surface. Several of them consists of superficial deposits, others have been identified in layered deposits within craters or basins. One of the interpretations for these high-albedo zones suggests they are the remnant of sabkha, because evaporitic deposits usually display a very high albedo. The presence of these purported Martian sabkha is also consistent with some hydrological models and they are the natural companion of the probable deep standing bodies of waters that have been possibly identifies in other location on the surface of Mars. However, the presence of sabkha on Mars would have profound implications in the geological analysis. A first requirement to have sabkha lakes is the presence of an active water table and consequently a large hydrological cycle. Sabkha are far to be simple sinks filled by superficial water which quickly evaporates. They are rather complex basins with a continuous, but variable, supply of water. The subsurface water should be continuously recharged to maintain the active aquifer. A second important implication is that most of these salts and mostly carbonate, occur below the sedimentary interface and these lithologies do not crop out at the surface. In arid and semi-arid zones on the Earth it is extremely common the deposition of carbonates in the subsurface, ranging from scattered nodules (caliche) to thick crust (cornestone). The carbonate deposited in this way will be not directly detectable from remote sensing instruments orbiting the planets.