Habitability of the phoenix landing site

The Phoenix mission's key objective was to search for a habitable zone. Mission results are used to evaluate habitability where Phoenix landed. A habitability probability (HI) is defined as the product of probabilities for the presence of liquid water (Plw), energy (P_e), nutrients (P _ch), and a benign environment (P_b). Observational evidence for the presence of liquid water (past or present) includes clean ice at a polygon boundary, chemical etching of soil grains, and carbonate minerals. The presence of surface and near subsurface ice, along with thermodynamic conditions that support melting, suggest that liquid water is theoretically possible. Presently, unfrozen water can form only in adsorbed films or saline brines but more clement conditions recur periodically due to variations in orbital parameters. Energy to drive metabolism is available from sunlight, when semitransparent soil grains provide shielding from UV radiation and chemical energy from the redox couple of perchlorate and reduced iron. Nutrient sources including C, H, N, O, P, and S compounds are supplied by known atmospheric sources or global dust. Environmental conditions are within growth tolerance for terrestrial microbes. Surface soil temperatures currently reach 260 K and are periodically much higher, the pH is 7.8 and is well buffered, and the water activity is high enough to allow growth when sufficient water is available. Computation of HI for the sites visited by landers yields Phoenix, 0.47; Meridiani, 0.23; Gusev, 0.22; Pathfinder, 0.05; Viking 1, 0.01; Viking 2, 0.07. HI for the Phoenix site is the largest of any site explored, but dissimilar measurements limit the comparisons' confidence.