JAROSITE ON MARS: AN INDICATOR OF WATER-LIMITED CHEMICAL WEATHERING

Thermodynamic reaction-path modeling of chemical weathering at Mars-relevant conditions indicates that ferric sulfates represent water-limited, short-term aqueous alteration of basalts. Reaction of basalt with an acidic (pH=1) H₂O-SO₄^2--SiO₂-Al³⁺-Fe²⁺-Mg²⁺-Ca²⁺-Na⁺-K⁺ fluid produces an alteration assemblage consisting of jarosite, an SiO₂ phase, an iron hydroxide phase, kaolinite, and gypsum early in the chemical weathering process. As weathering proceeds, the pH of the fluid increases and leaves the jarosite stability field, resulting in the replacement of jarosite by ferric hydroxide. Weathering models calculated over a wide range of water:rock ratios follow identical paths through Eh-pH space, varying only in their endpoint along this path (i.e., the final equilibrium assemblage). High water:rock ratios (100:1) include jarosite in the final equilibrium assemblage, while the final pH of lower water:rock ratios is outside of the stability field of jarosite. This indicates that jarosite will be preserved in the weathering assemblage if (1) a large volume of water reacts completely with a thin veneer of rock or (2) a small volume of water partially weathers a large volume of rock producing a weathering rind. In either case, the chemical weathering process must be water limited, with water removed from the system after a relatively short period of alteration, likely through the formation of hydrous phases and evaporation. Aqueous surface environments on early Mars may not have differed greatly from early environments on Earth where life began and evolved. However, due to the water-limited nature of early Mars environments, the liquid water necessary for biological activity would have been present for only a relatively short period of time, perhaps preserving the precursors of life or early products of evolution.