Number of models meeting given ﹩\frac{{{\rm{H}}}_{2}}{{{\rm{CO}}}_{2}}﹩ outgassing flux ratio as a function of fO₂ at a surface temperature of 1573 K. All other parameters (Table 3) are marginalized over. The red horizontal line shows modern Earth ﹩\frac{{{\rm{H}}}_{2}}{{{\rm{CO}}}_{2}}﹩ outgassing flux ratio for context. Bulk redox states of Mercury (M. Y. Zolotov et al. 2013), Earth (D. C. Catling & J. F. Kasting 2017), and Mars (A. E. Doyle et al. 2020) are marked by dashed lines. Solid red lines present Earth outgassing ratios (R. Hu et al. 2023).