A Complex Fluviolacustrine Environment on Early Mars and Its Astrobiological Potentials

Chloride-bearing deposits and phyllosilicates-bearing units are widely distributed in the southern highlands of Mars, but these phases are rarely found together in fluviolacustrine environments. The study of the coexistence of these minerals can provide important insights into geochemistry, water activity, and ultimately the climate and habitability of early Mars. Here we use high-resolution compositional and morphological orbiter data to identify and characterize the context of diverse minerals in a Noachian fluviolacustrine environment west of Knobel crater (6.7°S, 226.8°W). The chlorides in this region are likely formed through the evaporation of brines in a closed topographic basin. The formation age of chlorides is older than 3.7 Ga, based on stratigraphic relationships identified and previously obtained crater retention ages. The timing of the alteration of basaltic materials to iron-magnesium smectites in relation to the chloride formation in this location is enigmatic and is unable to be resolved with currently available remote sensing data. Importantly, we find that this close relationship between these key minerals revealed by the currently available data details a complex and intimate history of aqueous activity in the region. Of critical importance are the evaporitic deposits as analogous terrestrial deposits have been shown to preserve ancient biosignatures and possibly even sustain microbial communities for hundreds of millions of years. These salts could have protected organic matter from ultraviolet radiation, or even allow modern habitable microenvironments in the shallow subsurface through periodic deliquescence. The high astrobiology potential of this site makes it a good candidate for future landed and sample return missions (e.g., the Chinese 2020 Mars mission).