Early Martian Surface Conditions from Thermodynamics of Phyllosilicates

Introduction: The Mars Express OMEGA and Mars Reconnaissance Orbiter CRISM imaging spectrometers have identified phyllosilicates (Fe, Mg, Ca-smectites, kaolinite and chlorite) in Noachian aged terrains [1-3], often associated with lacustrine or fluvial deposits [4,5]. Carbonates have also been recently identified in similar areas [6]. Clay minerals usually result from long term weathering of primary minerals by liquid water at neutral to alkaline pH [7]. Their presence and association with carbonates [6] suggests an early environment completely different from the acidic conditions responsible for the formation of sulfate outcrops widely observed on Mars [8]. Starting from the hypothesis that phyllosilicates are formed by liquid water induced weathering of the primary basaltic outcrops, thermodynamic models are used to study the conditions on the surface during the Noachian. Focus was placed on the effect of CO2 partial pressure and solution temperature on the precipitating parageneses. Methods: The water composition data presented in Table 1 was used as input of the models. This composition reflects possible primary solutions on Mars [9]. Al and SiO2 have been set up at typical terrestrial values, being generally driven by their low solubility. The Geochemical Workbench software package was used to model thermodynamic equilibria, with the thermo.com.v8.r6+ database, which contains about 350 common silicates. This database uses Debye–Huckel theory for ionic activities, which is perfectly accurate to describe behavior of solutions at relatively to very low concentrations.