Mean molecular weight (μ) of the outgassed atmosphere (in amu) as a function of varying surface pressure, oxygen fugacity (f_O2), and graphite activity (a_C). Each panel isolates the effect of one parameter by holding it fixed while allowing the other two to vary, as indicated in each panel. The value of μ shown corresponds to mean molecular weight of the volatile partition at a given degassing rate. This figure illustrates how redox state, pressure, and carbon content influence the dominant molecular species and therefore control the overall atmospheric composition.