[MIS07-04] Stability of atmospheric redox states of early Mars inferred from time response of the regulation of H and O losses
Keywords:Mars, Photochemistry model, Atmospheric escape, Habitability, Water loss process
Here we study time responses of the self-regulation in different atmospheric conditions and discuss the stability of atmospheric redox states. We use a 1D time-dependent photochemical model for various atmospheric conditions and parameters, such as atmospheric CO2 pressure, surface temperature and O escape rate.
We find that the self-regulation timescale is essentially controlled by the net redox balance (pOx [mbar] = 2pO2 – pCO – pH2) in a converged state. The timescale gets longer as |pOx| increases, which suggests that redox-neutral atmospheres have the shortest timescale. We also find that the self-regulation can be classified into two regimes. First regime is the same as the one explained by Liu and Donahue. (1976), which tends to work in oxidizing atmospheres (pOx > 0) including present-day Mars in a way that H escape changes to reach the regulated state following a change in H2 transportation from the lower to upper atmosphere. Second one is likely to work in thicker and reducing atmospheres (pOx < 0) over a relatively long timescale. The regulation occurs dominantly by changes in CO abundance in the lower atmosphere. These results imply that thicker atmospheres in early Mars are less redox-stable than present-day Mars. Our model calculations also indicate that CO-dominated atmosphere of about 100 mbar might be possible around 3 Ga. We finally discuss the redox stability of H2-rich CO2 atmosphere of early Mars.