CO-dominated atmospheres during the magma ocean stage

Recent characterisations of exoplanet atmospheres with the JWST, including especially the potential detection of CO as a dominant gas in the atmosphere of a rocky exoplanet (55 Cnc e, Hu et al. 2024), are in contrast to theoretical predictions from volcanic outgassing studies. Here we focus on the diverse atmospheres that can theoretically form at the end of the magma ocean stage of rocky planets by assuming different volatile concentrations in the magma ocean as well as redox states.
By using a first-principles magma ocean crystallization model considering redox-dependent partitioning of volatiles between the magma ocean and the solidifying mantle, gas speciation of volatiles, their solubility in the magma, as well as atmospheric chemical equilibrium, we model the redox- and composition-dependent formation of the primary outgassed atmosphere towards the end of the magma ocean stage.
We find that due to the low solubility of carbon species in melts, atmospheres in equilibrium with a magma ocean under low oxygen fugacity conditions would quickly become dominated in CO (even without photochemistry), which is in contrast to what we find for secondary atmospheres forming by long-term volcanic outgassing (Brachmann et al., 2025). In addition, we show that the often discussed catastrophic outgassing phase of a solidifying magma ocean only appears for oxidized magma oceans, whereas under reducing conditions the atmospheric pressures gradually increase during the crystallization, affecting also the storage capacity of volatiles in the solid mantle.
We show, that it is not only possible to explain recent potential findings of CO-dominated atmospheres around rocky planets by a low oxygen fugacity of the magma ocean and later rocky mantle, but also that the primordial outgassed atmosphere can differ strongly from secondary outgassed atmospheres, in contrast to what is often assumed in the astronomical community. We also plan to study the formation and destruction of CO by photochemistry and chemical reactions in the atmosphere in more depth using the 1D-TERRA climate-chemistry coupled model.