Mineralogy in the Martian Hydrosphere

Introduction: Mineral equilibria provide predic-tors of properties important in aqueous geochemistry (solubility, freezing, precipitation, dissolution, etc.). Prediction works well where groundwater is well above freezing, but less so where only thin films of cold water and ice are present. Calculations are also limited where water/rock ratios are small, the general case on Mars. On Mars many models of aqueous systems call on concentrated brines [1]. Recent low-temperature thermodynamic modeling has improved calculations [see presentations at this meeting], but much remains to be done in extending thermodynamic data to cold conditions, especially where mineral-H 2 O reactions are solid-vapor rather than solid-liquid. Dissolution and precipitation: High solubilities of salts and their abundance on Mars [2] make them a point of focus for studies of the martian hydrosphere. Sulfates and halides are a significant component of the martian regolith [3] and are common vein components in martian meteorites [4]. Lander chemical data from Viking, Pathfinder, and MER indicate a dominant sul-fate component with less abundant halogen salts (Cl, Br). Orbital data support widespread distribution of sulfates of Mg, Ca, and Fe; carbonates are much rarer than expected. Recent results from Phoenix show per-chlorate (ClO 4) salts at high latitude; associated salts include anhydrite and Ca-carbonate [5,6]. The surprising abundance of perchlorate at the Phoenix site shows that our ability to predict salt assemblages from available data is limited. Identification of Ca-carbonate at Phoenix shows that mineralogy seen from orbit may be insensitive to salt mixtures in low abundance or covered by soil that masks underlying material. Silica is a common precipitate in the terrestrial hy-drosphere; authigenic quartz coats and cements detrital grains in many sandstones and cristobalite and opaline silica are common terrestrial precipitates from silica-saturated fluids. These precipitates can greatly reduce porosity, permeability, and specific storage of aquifer rocks, especially clastic sediments. Simulated acid weathering of basalt under martian condition produces opal [7]. Recent spectral data from Mars suggest surface opaline deposits [8]. Silica leach rates from sili-cate glasses are greater than those from crystalline silicates; volcanic and impact glasses on Mars provide abundant material for silica in solution. Slow recycling of impact products on Mars is likely to leave a large inventory of shocked silicate materials susceptible to aqueous leaching. Special disordered materials such as maskelynite, rare on Earth, may be widely distributed