9:15 AM - 9:30 AM
[PPS07-02] The effect of radiative cooling on the hydrodynamic escape of a Martian proto-atmosphere
Keywords:Martian proto-atmosphere, hydrodynamic escape, radiative cooling
If the Martian proto-atmosphere was originally so massive, a large fraction of the atmospheric mass should have escaped to space to be consistent with the thin atmosphere on present Mars. One of the candidate mechanisms to induce such massive escape is hydrodynamic escape. Hydrodynamic escape occurs when radiative heating of an atmosphere accelerates a radial outflow of an atmosphere against the planetary gravity. From observations of young solar proxies, the extreme ultraviolet (EUV) flux of the young Sun is estimated to be as strong as ~100 times the present mean flux (Ribas et al., 2005). This would be powerful enough for the initial Martian atmosphere to be largely lost by hydrodynamic escape.
Previous numerical studies (Lammer et al., 2013; Erkaev et al., 2014) estimate that a proto-atmosphere with the amount equivalent to ~100 bar could have been lost from early Mars per 10 Myr under the EUV flux 100 times the present. If such a EUV flux intensity was kept over ~100 Myr after the formation of planes as estimated from the observation of young solar proxies, their result suggests that a proto-atmosphere with an amount equivalent to ~1000 bar could have been eventually lost.
These previous studies assumed that all of molecules were dissociated into atoms in the upper atmosphere supposing that high EUV flux may dissociate molecules such as H2, H2O and CO2. However, it is likely that a significant fraction of molecules stays undissociated at least in the lower part of EUV absorption region. If infrared active molecules were included in the atmosphere, they may reduce the amount of atmospheric loss due to the radiative cooling, but its efficiency remains poorly understood. Here, we develop a 1D radiative hydrocode which includes molecular species and analyze the effect of radiative cooling on the hydrodynamic escape of a proto-Martian atmosphere. First, we apply the model including chemical processes to a pure hydrogen atmosphere. Second, we add CO to a hydrogen atmosphere and consider the effect of radiative cooling on the hydrodynamic escape. The solar EUV flux was given to be 100 times the present mean flux with the time-averaged present solar EUV spectrum.
In the calculation of a pure hydrogen atmosphere, we consider 15 chemical reactions and assume that atmospheric components are H, H2, H+, H2+, H3+. We assume that only H2 exists at the lower boundary. Model simulations show that molecular H2 survives and dominate the escaping atmosphere in all calculated altitude. This result suggests that the radiative cooling by molecules may be important.
Next, we consider a H2-CO atmosphere and the effect of radiative cooling by CO. In the calculation of a H2-CO calculation, we neglect the chemical processes. As the mixing ratio of CO increases, the escape mass flux decreases due to the radiative cooling by CO. Simultaneously, the crossover mass, i.e. the largest molecular mass of atmospheric constituents that can be dragged up by H2, decreases. The crossover mass may become smaller than the mass of CO when the mixing ratio of CO becomes greater than ~15%. In such a case, only H2 may escape leaving behind CO in the proto-atmosphere. Our results indicate that molecular species are likely dominant even in a proto-atmosphere exposed to intense EUV flux, and radiative cooling by molecules may significantly decrease the total amount of atmospheric loss from early Mars.