From CO₂- to H₂O-dominated atmospheres and back

Aims . We investigate the impact of CO 2 on the distribution of water 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 key link to planet formation and also to judging their habitability. Methods . We expanded the MagmOc module of the VPLanet environment to perform simulations with 1-100 terrestrial oceans (TOs) of H 2 O with and without CO 2 and for albedos 0 and 0.75. The CO 2 mass was scaled with initial H 2 O by a constant factor between 0.1 and 1. Results . The magma ocean state of rocky planets begins with a CO 2 -dominated atmosphere but can evolve into a H 2 O dominated state, depending on initial conditions. For less than 10 TO initial H 2 O, the atmosphere tends to desiccate and the evolution can end with a CO 2 dominated atmosphere. Otherwise, the final state is a thick (>1000 bar) H 2 O-CO 2 atmosphere. Complete atmosphere desiccation with less than 10 TO initial H 2 O can be significantly delayed for TRAPPIST-1 e and f, when H 2 O has to diffuse through a CO 2 atmosphere to reach the upper atmosphere, where photolysis due to extreme ultra violet irradiation occurs. As a consequence of CO 2 diffusion-limited water loss, the time of mantle solidification for TRAPPIST-1 e, f, and g can be significantly extended compared to a pure H 2 O evolution by up to 40 Myrs for an albedo of 0.75 and by up to 200 Mys for an albedo of 0. The addition of CO 2 further results in a higher water content in the melt during the magma ocean stage. Thus, more water can be sequestered in the solid mantle. However, only up to 6% of the initial water mass can be stored in the mantle at the end of 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 (partially) desiccated evolution scenario and a CO 2 atmosphere with surface pressures of a few 100 bar. Conclusions . A comparative study between TRAPPIST-1 e and the inner planets may yield the most insights about formation and evolution scenarios by confronting, respectively, a scenario with a desiccated evolution due to volatile-poor formation and a volatile-rich scenario with extended atmospheric erosion.