Thermodynamics of Clay Minerals on Mars: Insight into the Geochemical Environment of Early Mars

Introduction: The Mars Express OMEGA imaging spectrometer recently identified clay minerals (nontronite and montmorillonite) in several outcrops of Noachian age [1]. Clay minerals usually result from long term alteration of primary minerals by liquid water and on Earth are usually the products of weathering processes. The presence of clay minerals suggests a primary environment completely different from those responsible for the formation of sulfate outcrops such those observed at Meridiani Planum [1]. While these also result from aqueous processes, sulfate minerals suggest a very different geodynamical environment to those responsible for the clay minerals, involving limited action of water [2]. Here we consider the thermodynamics of clay minerals in order to help elucidate the conditions of their formation and the geochemical environment on the surface of Mars during the Noachian period. Methods: The nontronite stability field has been determined using the water composition data in Table 1. This composition reflects possible primary solutions on Mars, with mainly a high Fe content and relatively lower Mg and Ca compared to solutions commonly found on Earth [3]. The Al has been set up at a low value, because of its low abundances in Martian rocks, and its generally low solubility. All the other diagrams are made by varying these parameters. The thermodynamic values come from [3,4]. They reflect “standard” values for clay minerals, which have variable chemical compositions, inducing changes in Gibbs free energy of formation. However, these changes are relatively small [4] and do not strongly influence the stability field of the phases.