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Hydrodynamic escape of reduced proto-atmospheres on Mars and Earth
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| Title: | Hydrodynamic escape of reduced proto-atmospheres on Mars and Earth |
| Other Titles: | 火星・地球における還元型原始大気の流体力学的散逸 |
| Authors: | 吉田, 辰哉1 Browse this author |
| Authors(alt): | Yoshida, Tatsuya1 |
| Issue Date: | 25-Mar-2021 |
| Publisher: | Hokkaido University |
| Abstract: | Mars and Earth may have obtained reduced proto-atmospheres enriched in H2 and CH4 during accretion. Such reduced proto-atmospheres would have been lost by hydrodynamic escape, but their fluxes and timescale for hydrogen depletion remain highly uncertain. The largest ambiguity in the previous numerical studies for hydrodynamic escape is the radiative loss of energy from the upper atmosphere where the outflow accelerates due to the solar XUV heating. The presence of infrared active species such as CH4 would induce significant loss of thermal energy to space and thereby decrease escape flux. However, previous studies treated the degree of energy loss as a free parameter. On the other hand, chemical species in an escaping atmosphere would suffer photolysis under the intentive XUV environment. To estimate the evolution of proto-atmospheres of Mars and Earth correctly, precise modeling including exact radiative balance and chemical processes is required. Here we develop a one-dimensional hydrodynamic escape model which includes radiative processes and photochemical processes for a multi-component atmosphere and applied to reduced proto-atmospheres on Mars and Earth. The escape rate of the reduced Martian atmosphere decreases more than one order of magnitude and the mass fractionation occurs more remarkably as the mixing ratio of CH4 and CO increases primarily because of the radiative cooling by CH4 and CO. The total amount of carbon species lost by hydrodynamic escape 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 when most H2 completed its hydrodynamic escape. The timescale for H2 escape from H2-CH4-CO atmospheres becomes about one order of magnitude longer than that from pure hydrogen atmospheres especially when the CH4/CO ratio is high. If the proto-Mars obtained > 100 bar of H2, the timescale for H2 escape exceeds ∼ 100 Myr, which implies that a reduced environment allowing the production of organic matter deposit may have been kept on early Mars traceable from geologic records. Our result also implies that atmospheres on Mars-sized protoplanets may have survived during the giant impact phase and contribute as a source of atmospheres on the larger terrestrial planets that have experienced proto-planet collisions. In escaping outflow on Earth which has the deeper gravitational well, CH4 is dissociated rapidly by direct photolysis and chemical reactions with ions in escaping outflow, whereas radiative cooling by photochemical products such as H+3 , CH and CH3 suppresses atmospheric escape significantly even though their concentrations are small: the heating efficiency decreases to ∼ 0.05 at CH4/H2=0.007 and species heavier than H2 cease to escape at CH4/H2≳ 0.01. The maximum timescale for H2 escape to satisfy the constraints of the isotopic compositions and the amount of volatiles on the present Earth is more than several hundred million years. Our results suggest that a hydrogen-rich reduced environment had been kept and played an important role in producing warm climate and organic matters linked to the emergence of life organisms on early Earth. The difference between hydrodynamic escape rates on Earth and that on Mars results mainly from the difference of the planetary mass and the gravity. The escape rate is larger for Mars even taking into account the weaker XUV flux due to the larger distance from the Sun. On Mars, the weak gravity allows relatively fast escape flow that makes adiabatic cooling effective, leading to the lower atmospheric temperature and inefficient radiative cooling. The more efficient hydrodynamic escape on Mars may have resulted in the observed paucity of volatiles on Mars compared to those on Earth. |
| Conffering University: | 北海道大学 |
| Degree Report Number: | 甲第14361号 |
| Degree Level: | 博士 |
| Degree Discipline: | 理学 |
| Examination Committee Members: | (主査) 教授 倉本 圭, 教授 石渡 正樹, 教授 佐藤 光輝, 准教授 鎌田 俊一 |
| Degree Affiliation: | 理学院（宇宙理学専攻） |
| Type: | theses (doctoral) |
| URI: | http://hdl.handle.net/2115/91661 |
| Appears in Collections: | 課程博士 (Doctorate by way of Advanced Course) > 理学院(Graduate School of Science)
学位論文 (Theses) > 博士 （理学）
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