Acid-sulfate Weathering of Synthetic Martian Basalt: the Acid Fog Model Revisited

Introduction: One of several models that has been proposed to explain the present chemical and minera-logical nature of the Martian surface is referred to as the " acid-fog " model. This model involves the recent (up to 1Ga), slow and continuous interaction of acidic volatiles, presumably of volcanic origin, with the exposed Martian basaltic crust [1, 2]. As a result of this interaction, mineral dissolution occurs and subsequent evaporation of the fluid results in formation of evaporite salts. Banin et al. [2] have tested the acid-fog model experimentally by interacting terrestrial analog basalt with sulfuric, hydrochloric and nitric acid mixtures and found that upon fluid evaporation, the Ca and Al-sulfate salts gypsum and alunogen precipitated as secondary mineral phases with no other phases detected by X-ray diffraction. The study presented here revisits this model using more relevant Martian basaltic analogs synthesized in the laboratory. Results show that the chemical and mineralogical composition of the basaltic analog material is critical in determining fluid chemistry and the subsequent secondary products that are formed. Different chemical compositions of basaltic analogs are used in this study to provide a thorough evaluation of fluid-rock interaction as a function of host rock composition. Experimental and Analytical Procedures: The compositions of synthetic basaltic analogs used in this study include those most relevant to one of the most important sources of chemical data for the Martian surface-the Mars Pathfinder landing site. As an attempt to encompass the chemical spectrum that the Pathfinder data may reflect, two " end member " compositions are used as analogs. These compositions include a lowS Pathfinder rock composition representative of a basaltic andesite Pathfinder surface rock composition and averaged Martian soil (recalculated on a Sand Cl-free basis), which may represent a typical upper crustal composition. The basaltic analog materials are synthesized following the procedure outlined by Tosca et al. [3] to achieve the desired compositions. The products are then lightly crushed and sieved to constrain the particle size between 700 and 70 µm. The analog material is then ultrasonically rinsed in acetone to remove ultra-fine particles and dried overnight at 120 o C. The dried material is then divided and added to 5 different sulfu-ric/hydrochloric acid mixtures with a S:Cl molar ratio of 4, equal to that observed in the Pathfinder soils. Acid mixture concentrations range from a 1M