Constraints on Evaporation Processes at Meridiani Planum: Combining Theoretical and Experimental Data

Introduction: The identification of sulfate minerals at Meridiani Planum and other regions identified by the OMEGA instrument has clearly demonstrated the importance of evaporation processes at the martian surface. The formation of saline mineral assemblages found at Meridiani Planum has been approached by both theoretical and experimental means. Such approaches serve to provide an independent test of the hypothesis that chemical weathering has generated dilute fluid compositions and evaporation has led to the formation of soluble materials found in reworked sediments. Theoretical approaches to evaporation often exploit the fact that equilibrium is typically reached between fluid and mineral phases over the course of evaporation. In addition, robust models have been built that adequately calculate thermodynamic concentrations (activities) in high ionic strength fluids. However, limitations of theoretical approaches to evaporation can be significant, especially in the unique and somewhat unfamiliar geochemical system typified by Meridiani Planum sediments [1]. Such limitations can indeed be responsible for the presence or absence of particular minerals of a modeled saline assemblage. Experimental data, however, can be used to improve and test existing geochemical models and strengthen their predictions by highlighting discrepancies between theoretical and experimental phenomena. The goal of this study is to combine experimental and theoretical results obtained from the evaporation of basaltic weathering derived fluids and provide more detailed knowledge of the controls on saline mineral formation at the martian surface. The results discussed below relate to the first portion of this effort – understanding the Meridiani Planum geochemical system. Theoretical Model, Methods and Input data: The theoretical model employed in this study is described in detail in [1]. The model traces reaction paths of a given system by iteratively calculating its equilibrium state. Pitzer's equations are used for activity coefficient calculations and include the components Ca, Mg, Na, K, Fe(II), Fe(III), Al, SO 4 , Cl and H 2 O for temperatures at 25 o C and pH less than or equal to 4. Al was added to the database presented in [1], largely using interaction parameters from [2] and solubility constants for Al-minerals from [2, 3]. Experimental data were obtained by conducting evaporation experiments with a fluid derived from weathering olivine-bearing basalt at pH ~2. The experimental approach and details of the data are discussed in [4]. Over 50 complete fluid analyses were obtained for the experiment and all major chemical components were analyzed [4]. As a check on the quality …