From CO2- to H2O-dominated atmospheres and back -- How mixed outgassing changes the volatile distribution in magma oceans around M dwarf stars
Abstract
We investigate the impact of CO2 on TRAPPIST-1 e, f and g during the magma ocean stage. These potentially habitable rocky planets are currently the most accessible for astronomical observations. A constraint on the volatile budget during the magma ocean stage is a link to planet formation and also needed to judge their habitability. We perform simulations with 1-100 terrestrial oceans (TO) of H2O with and without CO2 and for albedos 0 and 0.75. The CO2 mass is scaled with initial H2O by a constant factor between 0.1 and 1. The magma ocean state of rocky planets begins with a CO2-dominated atmosphere but can evolve into a H2O dominated state, depending on initial conditions. For less than 10 TO initial H2O, the atmosphere tends to desiccate and the evolution may end with a CO2 dominated atmosphere. Otherwise, the final state is a thick (>1000 bar) H2O-CO2 atmosphere. Complete atmosphere desiccation with less than 10 TO initial H2O can be significantly delayed for TRAPPIST-1e and f, when H2O has to diffuse through a CO2 atmosphere to reach the upper atmosphere, where XUV photolysis occurs. As a consequence of CO2 diffusion-limited water loss, the time of mantle solidification for TRAPPIST-1 e, f, and g can be significantly extended compared to a pure H2O evolution by up to 40 Myrs for albedo 0.75 and by up to 200 Mrys for albedo 0. The addition of CO2 further results in a higher water content in the melt during the magma ocean stage. Our compositional model adjusted for the measured metallicity of TRAPPIST-1 yields for the dry inner planets (b, c, d) an iron fraction of 27 wt-%. For TRAPPIST-1 e, this iron fraction would be compatible with a (partly) desiccated evolution scenario and a CO2 atmosphere with surface pressures of a few 100 bar. A comparative study between TRAPPIST-1 e and the inner planets may yield the most insights about formation and evolution scenarios.
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