Our fit to the oxygen fugacity over the magma ocean as a function of planetary mass using a fifth-order polynomial. We use a surface temperature of 2100 K (Deng et al. 2020). For reference, Figure 3 of Deng et al. (2020) gives the oxygen fugacity as a function of the magma ocean base pressure instead. Earth’s upper mantle surface redox state and Earth’s mass are given in dashed lines. The polynomial constants are given in Table 1.