Sluggish hydrodynamic escape of early Martian atmosphere with reduced chemical compositions

Abstract Proto-Mars growing within the solar nebula may have maintained both the hydrogen-dominated solar component and the impact degassed component enriched in H2, CH4 and CO as a proto-atmosphere. Such a proto-atmosphere would experience hydrodynamic escape early in the history of Mars but its flux and duration of reduced composition remain highly uncertain. Here we develop a one-dimensional hydrodynamic escape model which includes radiative cooling processes and photochemical processes for a multi-component atmosphere and estimate the amount of atmosphere lost by hydrodynamic escape and the duration of early Martian atmosphere with reduced chemical compositions. The mass escape rate decreases more than one order of magnitude with increasing the mixing fraction of CH4 and CO ≥ 10% mainly because of the energy loss by radiative cooling by these infrared active chemical molecules. Concurrently, the mass fractionation between H2 and other heavier species occurs more remarkably. Assuming that carbon species equivalent to 1 bar of CO2 was left behind when most H2 completed its hydrodynamic escape, it possibly takes as long as 30 Myr for most H2 to be lost by hydrodynamic escape. The total amount of CH4 and CO lost by hydrodynamic escape exceeds 10 bar, equivalent to 20 bar of CO2 when the initial atmosphere is hydrogen-rich, which may explain the paucity of CO2 on Mars compared to Earth and Venus. The isotopic fractionation during hydrodynamic escape is not enough to elevate the D/H ratio from the proto-solar value to the present value, but it can explain the 13C/12C ratio of present Mars. The reduced atmosphere with enriched in CH4 and CO may further continue until these species are almost fully oxidized through photolysis and hydrogen escape. Our result suggests that a reduced environment may have been kept and played an important role in producing warm and wet climate and serving organic matters on Mars.