[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
Atmospheric losses including escapes to space and depositions to the surface play an essential role in the evolution of the Martian surface environment. Especially, a ratio of total losses of hydrogen and oxygen from the atmosphere is crucial to determine its atmospheric redox state. In the condition that H and O losses originate from H2O, an atmospheric redox state remains the same if the ratio is 2:1, otherwise it is driven into oxidizing or reducing. It was shown by McElroy. (1972) that Jeans escape flux of H and H2 and nonthermal escape flux of O were regulated to be in the ratio of 2:1 in a converged model of present-day Mars, which is called “self-regulation”. Whether or not the self-regulation works in real atmospheres depends on its timescales, but time responses of the self-regulation are not well understood in different atmospheric conditions.
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.
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.