Caveats for the Water-Loss Limits at the Inner Edge of the Habitable Zone

The moist greenhouse water-loss limit" delineates the pessimistic inner edge of the habitable zone, and depends on the abundance of water vapor in the stratosphere. Previous numerical modeling work has shown that the incident stellar flux needed to reach this limit depends on the surface water distribution, global volatile inventory, atmospheric mean molecular weight, and model dimensionality. Here, we show that these limits are also strongly modulated by up to 20% by the oxygenation state of the planet. Using a 3D chemistry climate model, we conducted simulations of rapidly-rotating dry planets and slowly-rotating wet planets at the IHZ edge. We find a positive correlation between the specified O2 and resultant H2O mixing ratios, as the CH4-OH reaction becomes a strong source of stratospheric water vapor in strongly-oxygenated dry atmospheres. On wet and slowly-rotating planets however, we find that the stratospheric water vapor and thermospheric hydrogen mixing ratios change non-monotonically with each increase in oxygenation level. The reduction in O2 and O absorption of UV fluxes leads to rapid water vapor photodissociation. For these moist greenhouse climates the most oxygen-depleted simulation has the highest rate of atomic hydrogen production and escape, a result that has strong implications for long-term liquid water surface stability.