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W. M. Murphy, D. S. Bass (Southwest Research Institute)
Consideration of multiphase equilibria is required to make realistic speculations of conditions on early Mars and can provide indications of modern exploration targets to aid characterization of that remote environment. Greenhouse gases are proposed to sustain elevated surface temperatures compatible with geomorphologic evidence for the stability of liquid water on early Mars. CO2 is commonly invoked as a greenhouse gas at pressures up to 5 bars or more, e.g., [1]. However, solid-gas equilibria show that CO2 pressure is limited by formation of CO2 ice in the upper atmosphere for surface pressure of 2 bars [2]. Recently, SO2 at 10-7 bar in a 2 bar CO2 atmosphere (0.1 ppmv SO2) has been postulated to augment warming in the upper atmosphere on early Mars [3], which could inhibit CO2 precipitation. However, oxidation of SO2 would produce sulfuric acid which dissolves in liquid water and attacks rock components, producing metal sulfate solutions and precipitating gypsum (CaSO4•2H2O). Occurrence of gypsum in SNC meteorites demonstrates conditions permitting oxidation of SO2 on Mars [4]. Equilibrium aqueous speciation calculations show that at low O2 pressure, 10-11 bar of SO2 (i.e., 4 orders of magnitude less than proposed) at equilibrium with water generates sulfuric acid of pH 0.5. Reaction path calculations indicate that silicate minerals would dissolve rapidly in this solution with precipitation of a silica phase, clay minerals, and gypsum. Precipitation of gypsum would deplete atmospheric SO2. These multiphase interactions demonstrate that greenhouse stabilization of liquid water due to an SO2 pressure of 10-7 bar is incompatible with a realistic water-rock system. In the absence of elevated concentrations of atmospheric SO2, at elevated CO2 pressure, and at near neutral pH, geochemical equilibrium models predict that the mineral nahcolite (NaHCO3) may precipitate as a consequence of gas-water-rock interactions. If observed, this mineral could be used together with other constraints as a paleo-CO2 barometer for Mars. 1. Pollack et al., 1987 Icarus 71, p. 203-224. 2. Kasting, 1991 Icarus 94, p. 1-13. 3. Yung et al., 1997. Icarus 130, p. 222-224. 4. Gooding et al., 1991. Meteoritics 26, p. 135-143.