New Constraints on Early Mars Weathering Conditions From an Experimental Approach on Crust Simulants

A denser CO2 atmosphere and higher temperatures than present-day conditions are frequently invoked as prevailing conditions for the formation of some ancient hydrous mineralogical associations present at the surface of Mars. The environmental conditions are of particular interest to better understand and constrain the weathering processes of the early Martian crust. For this purpose, 6-month-long batch weathering experiments on Martian crust simulants and individual Martian mineral analogs were performed at low temperature (45 degrees C) under a dense CO2 atmosphere (1 atm). Constraints on the weathering conditions are deduced from the solution properties and thermodynamic calculations, as well as mass balance calculations. Experimental solutions vary from mildly acidic to near neutral (4.75-6.48 pH). The Eh-pH conditions (Eh from 0.189-0.416 V/standard hydrogen electrode) suggest favorable conditions for the formation of ferric minerals despite an anoxic CO2 atmosphere. The chemical weathering appears to be 4 times more intense for Martian simulants under a CO2 atmosphere than under Earth ambient air. The weathering trend under a CO2 atmosphere involves leaching of alkali and alkaline earth elements (Mg, Ca, Na, and K) and Si and enrichments of the solid phases in Al, Fe, and to a lesser extent Si compared to the initial chemical composition of the starting minerals. This geochemical partitioning between solution and solids resembles those deduced from weathering profiles on Earth. Our results strongly support the idea that carbonates could not have extensively formed at the surface of early Mars despite a dense CO2 atmosphere.