The Formation of O2-dominated Atmosphere under High EUV Flux on Early Mars

Abstract:

The atmosphere on early Mars was quite different than the modern dry and thin atmosphere. The solar EUV flux associated with the magnetic activity in the solar upper atmosphere has diminished over time (Ribas et al., 2005). Tian et al. (2009) calculated the carbon thermal escape flux under the conditions of 3, 10 and 20 times higher EUV flux than the present level. They implied that Mars should have gained O₂ in the atmosphere over time at a rate proportional to the excess of C escape over that of O in early Noachian(~4Ga) because C is lighter than O. The thermal escape of C could have been stronger than that of atomic O in that time period. Curiosity rover found Mn (hydr)oxides at Gale crater, which was later analyzed and highly likely to be MnO₂ (Noda et al., 2019). This finding suggests a highly oxidized atmosphere could have existed in the Hesperian. However, reproducing such a highly oxidized atmosphere is still challenging by a photochemical model of Mars.

Here we investigate whether the highly oxidized atmosphere can be formed under the strong EUV flux in the Noachian and Hesperian by taking into account C thermal escape in addition to H. We use a 1 D time-dependent photochemical model basically with the boundary condition calculated by Tian et al. (2009).

We find that whether O₂-dominated atmospheres are formed under the strong EUV flux depends on volcanic CO2 outgassing rate at ~4 Ga. In the condition of CO₂ outgassing rate of <2e10 cm^-2s^-1, O₂-main atmospheres are formed and maintained for ~0.1Gyr. In the condition of higher CO₂ outgassing, however, the CO₂-dominated atmosphere does not shift to oxidized even under the condition of high EUV flux. After 3.95 Ga, C thermal escape dramatically decreases and is not a dominant C loss process from the atmosphere anymore. In either high or low CO₂ outgassing rate case, CO₂-dominated atmospheres are maintained after 3.95 Ga. However, the time when O₂-dominated atmosphere formed in our calculation is not consistent with the time period after Gale crater was probably formed (Le Deit et al., 2013). This discrepancy might be resolved by the different solar EUV flux evolution (Amerstorfer et al., 2017). Our results also suggest a new false positive of abiotic O₂-rich atmosphere for future exoplanetary observations. Finally, the scenarios of the atmospheric evolution of early Mars will be discussed.