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

Kurzfassung

A major goal in exoplanet science is the search for planets with the right conditions to support liquid water (1). The habitability of a planet depends strongly on the composition of its atmosphere. Meanwhile, the interior and atmosphere of rocky planets are intricately linked through feedback processes and consequently evolve as a coupled system. In particular, volcanic outgassing of volatile species from the planet's silicate mantle shapes the atmospheric composition, temperature, and pressure, 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 (2, 3). This means that the interior dynamics of planets can not be neglected, especially since much of the surface water on terrestrial planets originates from the planetary mantle. 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, as well as the distance of the planet to its host star. The model accounts for the main mechanisms controlling the global-scale, long-term evolution of stagnant-lid rocky planets (i.e. bodies without plate tectonics), and 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 runaway 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. References: (1) Noack, L., Snellen, I. & Rauer, H. Water in Extrasolar Planets and Implications for Habitability. Space Sci Rev 212, 877-898 (2017). (2) Ortenzi, G. et al. Mantle redox state drives outgassing chemistry and atmospheric composition of rocky planets. Sci Rep 10, 10907 (2020). (3) Gaillard, F. & Scaillet, B. A theoretical framework for volcanic degassing chemistry in a comparative planetology perspective and implications for planetary atmospheres. Earth and Planetary Science Letters 403, 307-316 (2014).