Redox state and interior structure control on the long-term habitability of stagnant-lid planets

A major goal in exoplanet science is the search for planets with the right conditions to support liquid water. The habitability of a planet depends strongly on the composition of its atmosphere. Interiors and atmospheres of rocky planets are linked through feedback processes and evolve as a coupled system. In particular, volcanic outgassing shapes the atmospheric composition, but the exact composition of outgassed species not only depends on the volatile content and redox state of the mantle, but also on the current state of the atmosphere.

In an extensive parameter study of rocky exoplanets, we investigated the emergence of habitable surface conditions for a wide range of initial conditions, including the planet mass, interior structure, volatile content and redox state. The model accounts for the main mechanisms controlling the long-term evolution of stagnant-lid rocky planets (i.e. bodies without plate tectonics). It includes a large number of atmosphere-interior feedback processes, such as a CO2 weathering cycle, volcanic outgassing, a water cycle between ocean and atmosphere, greenhouse heating, as well as escape processes of H2.

We find that only a narrow range of the mantle redox state around the iron-wüstite buffer allows forming atmospheres that lead to long-term habitable conditions. At more oxidizing conditions, most planets instead end up in a hothouse greenhouse state (akin to Venus) due to strong CO2 outgassing. On the other hand, on planets with more reducing mantles, the amount of outgassed greenhouse gasses is often too low to keep the surface above the freezing point of water.