4:45 PM - 5:00 PM
[PPS06-12] Hydrodynamic escape of reduced proto-atmospheres: different consequences for surface volatile inventories between Mars and Earth
Keywords:hydrodynamic escape, radiative cooling, photochemistry, proto-atmosphere, Mars, Earth
The present Martian surface is significantly depleted in volatiles compared to the Earth’s one. On the other hand, Mars may have obtained a massive proto-atmosphere because Mars was likely formed from volatile-rich building blocks due to its large distance from the Sun (Dreibus and Wanke, 1985; Saito and Kuramoto, 2018). H, C, N, and noble gases on Mars are enriched in heavier isotopes, indicating that the paucity of the volatiles on Mars may be largely due to atmospheric escape.
Hydrodynamic escape is one of the few mechanisms that can change the composition of a planetary atmosphere irreversibly (Hunten, 1990). The X-ray and extreme ultraviolet (XUV) radiation from the young Sun may have induced hydrodynamic escape of early atmospheres. Proto-atmospheres on Mars and Earth are expected to have been composed of reduced species such as H2 and CH4 considering the chemical reduction of volatiles by reactions with metallic iron and the gravitationally capture of surrounding solar nebula gas (Kuramoto and Matsui, 1996; Saito and Kuramoto, 2018). Such hydrogen-rich atmospheres on low-gravity rocky planets are susceptible to hydrodynamic escape with rates possibly very large compared to those of other atmospheric escape processes (Catling and Kasting, 2017).
Considering the difference in the planetary masses, the hydrodynamic escape rate on Mars is expected to be much larger than that on Earth, which may largely explain the paucity of the volatiles on the present Martian surface. The difference in the planetary gravity may lead to the difference in the atmospheric temperature, radiative cooling efficiency, and thereby atmospheric escape rate. Therefore, to clarify the difference in the hydrodynamic escape of the proto-atmosphere, it is needed to investigate the radiative and photochemical processes and the energy balance in each escaping outflow. However, previous studies simplified the radiative and photochemical processes, and therefore the difference in hydrodynamic escape rates was uncertain.
Here we carry out one-dimensional hydrodynamic escape simulations considering radiative and photochemical processes for proto-atmospheres on Mars and Earth. Then, we clarify the difference in the hydrodynamic escape and its effect on the amount of surface volatiles between Mars and Earth. In this modeling, fluid equations for a multi-component gas assuming spherical symmetry are solved by numerical integration about time until the physical quantities settle into steady profiles. We apply the XUV spectrum 100 Myr after the birth of the Sun from 0.1 to 165 nm provided by Claire et al. (2012), whose total flux is about 100 times the present. We consider radiative cooling by thermal line emissions of CH4, CO, CH, CH3 and H3+. 157 photochemical reactions are considered for 23 atmospheric components. We assume that the atmospheres at their low altitudes are composed of H2 and CH4 for Earth, and composed of H2, CH4 and CO for Mars referring to the thermodynamic analysis on accreting planets (Kuramoto, 1997).
The suppression of the hydrodynamic escape by the radiative cooling on Earth’s atmospheres is more drastic than that on Mars. The difference is mainly due to the differences in the planetary mass and gravity. Under smaller gravity, adiabatic cooling of expanding atmosphere is effective on Mars, leading to the lower atmospheric temperature and inefficient radiative cooling. The total amount of carbon species lost by hydrodynamic escape on Mars exceeds 10 bar equivalent to 20 bar of CO2 when the proto-Mars obtained >10 bar of H2 assuming that carbon species equivalent to 1 bar of CO2 was left behind. On the other hand, CH4 on Earth escapes hardly even when the mixing ratio of H2 is as high as ~99 % due to efficient radiative cooling by photochemical products from CH4 and H2. The difference in the efficiency of hydrodynamic escape may have resulted in a significant difference in the amount s on the surfaces of Mars and Earth.
Hydrodynamic escape is one of the few mechanisms that can change the composition of a planetary atmosphere irreversibly (Hunten, 1990). The X-ray and extreme ultraviolet (XUV) radiation from the young Sun may have induced hydrodynamic escape of early atmospheres. Proto-atmospheres on Mars and Earth are expected to have been composed of reduced species such as H2 and CH4 considering the chemical reduction of volatiles by reactions with metallic iron and the gravitationally capture of surrounding solar nebula gas (Kuramoto and Matsui, 1996; Saito and Kuramoto, 2018). Such hydrogen-rich atmospheres on low-gravity rocky planets are susceptible to hydrodynamic escape with rates possibly very large compared to those of other atmospheric escape processes (Catling and Kasting, 2017).
Considering the difference in the planetary masses, the hydrodynamic escape rate on Mars is expected to be much larger than that on Earth, which may largely explain the paucity of the volatiles on the present Martian surface. The difference in the planetary gravity may lead to the difference in the atmospheric temperature, radiative cooling efficiency, and thereby atmospheric escape rate. Therefore, to clarify the difference in the hydrodynamic escape of the proto-atmosphere, it is needed to investigate the radiative and photochemical processes and the energy balance in each escaping outflow. However, previous studies simplified the radiative and photochemical processes, and therefore the difference in hydrodynamic escape rates was uncertain.
Here we carry out one-dimensional hydrodynamic escape simulations considering radiative and photochemical processes for proto-atmospheres on Mars and Earth. Then, we clarify the difference in the hydrodynamic escape and its effect on the amount of surface volatiles between Mars and Earth. In this modeling, fluid equations for a multi-component gas assuming spherical symmetry are solved by numerical integration about time until the physical quantities settle into steady profiles. We apply the XUV spectrum 100 Myr after the birth of the Sun from 0.1 to 165 nm provided by Claire et al. (2012), whose total flux is about 100 times the present. We consider radiative cooling by thermal line emissions of CH4, CO, CH, CH3 and H3+. 157 photochemical reactions are considered for 23 atmospheric components. We assume that the atmospheres at their low altitudes are composed of H2 and CH4 for Earth, and composed of H2, CH4 and CO for Mars referring to the thermodynamic analysis on accreting planets (Kuramoto, 1997).
The suppression of the hydrodynamic escape by the radiative cooling on Earth’s atmospheres is more drastic than that on Mars. The difference is mainly due to the differences in the planetary mass and gravity. Under smaller gravity, adiabatic cooling of expanding atmosphere is effective on Mars, leading to the lower atmospheric temperature and inefficient radiative cooling. The total amount of carbon species lost by hydrodynamic escape on Mars exceeds 10 bar equivalent to 20 bar of CO2 when the proto-Mars obtained >10 bar of H2 assuming that carbon species equivalent to 1 bar of CO2 was left behind. On the other hand, CH4 on Earth escapes hardly even when the mixing ratio of H2 is as high as ~99 % due to efficient radiative cooling by photochemical products from CH4 and H2. The difference in the efficiency of hydrodynamic escape may have resulted in a significant difference in the amount s on the surfaces of Mars and Earth.