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

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).