RATES OF OXIDATIVE WEATHERING ON THE SURFACE OF MARS

The presence of sulfates and ferric-bearing assemblages in the Martian regolith implies that oxidative weathering of iron- and sulfur-bearing minerals has occurred through time on the surface of Mars. A model of acid weathering has been proposed for the iron-rich basalts on Mars. In this model, aqueous oxidation of iron sulfides released SO42- and H+ ions that initiated the dissolution of basaltic ferromagnesian silicates and released Fe2+ ions. The Fe2+ ions eventually underwent ferrolysis reactions and produced insoluble hydrous ferric oxide phases. Measurements of the time-dependence of acid weathering reactions have shown that pyrrhotite (Fe7S8) is rapidly converted to pyrite (FeS2) plus dissolved ferrous iron, the rate of pyrite formation decreasing with rising pH and lower temperatures. The rate of oxidation of aqueous Fe2+ ions derived from weathering of iron sulfides by dissolved atmospheric oxygen is known to be pH-dependent; rates are fast above pH 4.5 at ambient temperatures on the Earth's surface and much slower in acidic groundwater. On Mars, oxidation rates of dissolved Fe2+ ions in equatorial melt-waters in contact with the atmosphere (P(O2) = 10(-5) atm) are estimated to lie in the range 0.3-3.0 ppb Fe yr-1 over the pH range 2 to 6. These oxidation rates are more than 10(5) times slower than corresponding reactions near the freezing point of water on Earth. Oxidation of Fe2+ ions is estimated to be extremely slow in brine eutectic solutions that might be present on Mars and to be negligible in the frozen regolith. Such slow oxidation rates of aqueous Fe2+ ions in acidic groundwater (now permafrost) during the evolution of the Martian regolith would aid the stability, solubility and transport of iron in solution and other dissolved species over the surface of Mars.