Exploration for Standing Bodies of Water on Mars: When Were They There, Where did They go, and What are the Implications for Astrobiology?

Mars Global Surveyor altimetry data (MOLA) have shown that the major topographic features of Mars formed early and have changed little over geologic time. Thus, analysis of topography, slopes and roughness data can test previous hypotheses for standing bodies of water, provide important new information, and explore astrobiological implications of these results. 1) Noachian lakes and oceans: Valley networks, enhanced degradation rates, smooth planer topography, candidate buried channels in the northern lowlands, higher heat flux and thinner cryosphere, all provide evidence for local to regional standing bodies of water; key to their nature and fate is the degree to which Mars was `warm and wet' in the Noachian. 2) South circumpolar deposit meltback: The meltback of an extensive circumpolar ice sheet-like unit in the Hesperian, the Dorsa Argentea Formation, formed drainage channels and extensive lakes in adjacent craters and depressions; water ultimately drained into the Argyre and Hellas basins. 3) Outflow channels and the northern lowlands: Evidence exists for large ocean-scale standing bodies of water formed by outflow channel effluent that likely underwent geologically rapid freezing and sublimation; the sublimation residue is preserved as the Vastitas Borealis Formation. 4) Cryospheric seal penetration and outflow: By the Amazonian Period, a global cryosphere apparently existed and communication with subsurface groundwater occurred only locally (e.g., Utopia, Elysium and Amazonis Planitiae) through melting of this global aquitard by magmatic events, such as dikes and sills; outflow events in Amazonis Planitia are interpreted to have occurred as recently as the last several percent of the history of Mars. These geological settings for water suggest liquid water environments changed from surface to dominantly subsurface by the Early Hesperian, and that the following astrobiological environments were potentially important: 1) fluvial, 2) lacustrine, 3) ice (polar, ice sheets, glaciers, layers on standing bodies of water, etc.), 4) cryospheric (frozen ground, with and without water), and 5) subsurface groundwater. Important is the linkage between early environments perhaps more favorable to life on the surface, and later environments in which liquid water existed almost wholly in the subsurface. If life evolved early in the history of Mars, extensive ice deposits may provide an environment and record. Water-related deposits abound on the surface throughout the geological record and provide a wide range of astrobiological exploration strategies.