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Hydrodynamic Escape of a Reduced Proto-Atmosphere on Early Mars

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Hydrodynamic Escape of a Reduced Proto-Atmosphere on Early Mars EPSC Abstracts Vol. 13, EPSC-DPS2019-1499-2, 2019 EPSC-DPS Joint Meeting 2019 c Author(s) 2019. CC Attribution 4.0 license. Hydrodynamic escape of a reduced proto-atmosphere on early Mars Tatsuya Yoshida (1) and Kiyoshi Kuramoto (1) (1) Faculty of Science, Hokkaido University, Hokkaido, Japan ([email protected]) Abstract CO exist in the atmosphere, they may reduce the atmo- spheric escape rate by radiative cooling. If the atmo- A one-dimensional hydrodynamic escape model spheric escape rate had been kept small, the reduced which includes radiative cooling processes and photo- environment would continue for a long time. Here, chemical processes for a multi-component atmosphere we develop a one-dimensional hydrodynamic escape is developed and applied to estimate the duration of model which include radiative cooling processes and early Martian atmosphere with H2-rich reduced chem- photochemical processes for a multi-component atmo- ical compositions in which the degassed component sphere. We calculate the escape rate of a reduced Mar- and solar nebula component are mixed. The mass es- tian proto-atmosphere and estimate the duration of the cape rate decreases with increasing the mixing fraction reduced environment on early Mars. of CH4 and CO mainly because of the energy loss by radiative cooling by these infrared active species. Our 2. Model result suggests that a reduced environment on early Mars may have continued more than 100 Myr, pos- We solved the fluid equations for multi-component sibly up to 1 Gyr, and played an important role in atmosphere assuming spherical symmetry consider- ∼ warming climate and serving organic matters on the ing radiative processes and photochemical processes. martian surface. These equations are solved by numerical integration about time until the physical quantities settle into 1. Introduction steady profiles. We use the UV spectrum 100 Myr after the birth of the Sun estimated by the observa- The short growth time of proto-Mars inferred from tions of solar-type G stars [5]. CH4 and CO are con- analyses of Martian meteorites implies that the proto- sidered as radiative coolant from their energy transi- Mars likely gravitationally maintained both the solar tions with high emissivity. We calculate the radiative nebula component and the impact degassed compo- cooling rate by using the photon escape probability. 19 nent as a proto-atmosphere [1]. Planetary building photodissociation reactions including photoionization blocks containing metallic iron should induce produc- are considered for 18 atmospheric components. H2, tion of reduced volatiles such as H2, CH4 and CO CH4 and CO are given as primary constituents of the because metallic iron acts as a reductant for the de- proto-atmosphere. gassed component [2]. If a reduced proto-atmosphere was formed and maintained on early Mars, it may have 3. Results and Discussion played an important role in warming climate and serv- ing organic matters on the martian surface. As the basal mixing ratios of CH4 and CO increase, However, a reduced proto-atmosphere once formed the escape mass flux decreases: the total escape mass on a rocky planet has been considered to be rapidly lost flux when CH /H 0.07 is one order of magnitude 4 2 ∼ by hydrodynamic escape. Previous studies estimating smaller than that of the pure hydrogen atmosphere (fig- the atmospheric escape rate of an oxidized Martian ure 1). Concurrently, the mass fractionation between proto-atmosphere suggest that the proto-atmosphere H2 and other heavier species occurs more remarkably. with the amount equivalent to 100 bar could have been This result indicates that more fraction of CH4 and CO lost per 10 Myr under the EUV flux 100 times the tend to be left behind as the degassed component be- present [3,4]. But, their result likely depends on the comes more dominant in the proto-atmosphere. assumption that molecules are fully dissociated into Figure 2 and 3 represent the surface pressure of atoms. If infrared active molecules such as CH4 and initial atmosphere and the timescale for all H2 loss from initial atmosphere assuming that CH4-CO atmo- sphere whose total number of C is equivalent to 0.1 Total ] r H2 - 10 bar of CO2 was left behind after hydrodynamic y M / CH4 g k CO [ H2 loss. The timescale for H2 loss may exceed 100 s s o l c i Myr when Mars had massive solar nebula component. r 1019 e h p s Moreover, the reduced atmosphere may further con- o m t a f o tinue until reduced carbon species are almost fully ox- t n u o idized through photolysis and hydrogen escape. Ac- m a 18 10 e h cordingly, a reduced environment may have continued T more than 100 Myr, possibly up to 1 Gyr, depending 0 5 10 15 20 25 30 35 CH /H ( = CO/H ) at r = r [×10 2] on the chemical composition and the amount of initial 4 2 2 0 atmosphere. Figure 1: Escape mass fluxes of main gas species per 1 Acknowledgements Myr as a function of the lower boundary mixing ratio CH4/H2(=CO/H2). The black line represents the total We thank EPnetFaN members for providing us com- mass flux, the blue line represents that of H2, the red putational environment and technical support. line represents that of CH4 and the orange line repre- sents that of CO. References Prem(CO2) = 10 bar 104 Prem(CO2) = 1 bar ] r P (CO ) = 0.1 bar [1] Saito, Hiroaki, and Kiyoshi Kuramoto. "Formation of a a rem 2 b [ 103 hybrid-type proto-atmosphere on Mars accreting in the e r u s s 2 solar nebula." Monthly Notices of the Royal Astronomi- e 10 r p cal Society 475.1 (2017): 1274-1287. e c 1 a 10 f r u s l [2] Kuramoto, Kiyoshi, and Takafumi Matsui. "Partition- a 0 i 10 t i n ing of H and C between the mantle and core during the I core formation in the Earth: Its implications for the at- 10 1 0 5 10 15 20 25 30 35 2 mospheric evolution and redox state of early mantle." CH4/H2( = CO/H2) at r = r0 [×10 ] Journal of Geophysical Research: Planets 101.E6 (1996): 14909-14932. Figure 2: Surface pressure of initial atmosphere as a [3] Lammer, Helmut, et al. "Outgassing history and escape function of the mixing fraction CH /H (=CO/H ) of of the Martian atmosphere and water inventory." Space 4 2 2 P (CO ) Science Reviews 174.1-4 (2013): 113-154. initial atmosphere. rem 2 is the surface pressure of CO2 after hydrodynamic H2 loss. [4] Erkaev, N. V., et al. "Escape of the martian protoatmo- sphere and initial water inventory." Planetary and space science 98 (2014): 106-119. Prem(CO2) = 10 bar ] Prem(CO2) = 1 bar r 3 y 10 Prem(CO2) = 0.1 bar [5] Claire, Mark W., et al. "The evolution of solar flux from M [ s s μ o 0.1 nm to 160 m: quantitative estimates for planetary 2 l 10 2 H studies." The Astrophysical Journal 757.1 (2012): 95. l l a 1 r 10 o f e l a c 100 s e m i T 10 1 0 5 10 15 20 25 30 35 2 CH4/H2( = CO/H2) at r = r0 [×10 ] Figure 3: Timescale for all H2 loss as a function of the mixing fraction CH4/H2(=CO/H2) of initial atmo- sphere..
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