Impact of atmospheric cooling (thermochemical cooling) and photochemistry on the relative molecular abundances, namely, CO₂/CO and CO₂/CH₄, used as atmospheric tracers for the interior fo₂. (Top) Relative molecular abundances CO₂/CO and CO₂/CH₄ at the quench point for different end-member cases of P_s and fo₂ (see Table 2). The black marker shows the prediction by outgassing alone and the colored markers show the prediction including atmospheric chemistry. (Bottom) Schematic summarizing the main atmospheric processes affecting CO₂/CO and methane buildup. The terms “thick” and “thin” atmospheres are used respectively to mark the transition from a regime dominated by thermochemistry toward a regime dominated by photochemistry. This transition is not quantified using pressure ranges as other variables must be considered including orbital distance and O/H (see Sect. 4.2).