掲載種別：研究論文（学術雑誌）   出版者・発行元：American Astronomical Society  

Abstract

Terrestrial planets currently in the habitable zone around M dwarfs are estimated to have been in runaway greenhouse conditions for up to ∼1 Gyr due to the long-term pre-main-sequence phase of M dwarfs. These planets likely lose a significant portion of water during the pre-main-sequence phase owing to H₂O photolysis followed by hydrogen and oxygen loss to space. However, the effects of H₂O reproduction reactions and UV shielding by chemical products that reduce photolysis-induced water loss have yet to be estimated. Here, we apply a 1D photochemical model to a H₂O-dominated atmosphere of an Earth-like planet around a pre-main-sequence M dwarf to estimate these effects. We find that water loss is suppressed by efficient H₂O reproduction reactions and by UV shielding due to O₂. The water loss rate decreases by several to several hundred times compared to that in previous studies, with the assumption that the water loss rate is limited by stellar X-ray and extreme-ultraviolet flux or hydrogen diffusion through the atmosphere. Our results imply that terrestrial planets currently in the habitable zone around M dwarfs are more likely to retain surface water than previously estimated.

DOI： 10.3847/1538-4357/ad3e7e

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その他リンク： https://iopscience.iop.org/article/10.3847/1538-4357/ad3e7e/pdf

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Springer Science and Business Media LLC  

Abstract

Formaldehyde (H₂CO) is a critical precursor for the abiotic formation of biomolecules, including amino acids and sugars, which are the building blocks of proteins and RNA. Geomorphological and geochemical evidence on Mars indicates a temperate environment compatible with the existence of surface liquid water during its early history at 3.8–3.6 billion years ago (Ga), which was maintained by the warming effect of reducing gases, such as H₂. However, it remains uncertain whether such a temperate and weakly reducing surface environment on early Mars was suitable for producing H₂CO. In this study, we investigated the atmospheric production of H₂CO on early Mars using a 1-D photochemical model assuming a thick CO₂-dominated atmosphere with H₂ and CO. Our results show that a continuous supply of atmospheric H₂CO can be used to form various organic compounds, including amino acids and sugars. This could be a possible origin for the organic matter observed on the Martian surface. Given the previously reported conversion rate from H₂CO into ribose, the calculated H₂CO deposition flux suggests a continuous supply of bio-important sugars on early Mars, particularly during the Noachian and early Hesperian periods.

DOI： 10.1038/s41598-024-52718-9

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その他リンク： https://www.nature.com/articles/s41598-024-52718-9

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：American Astronomical Society  

Abstract

Atmospheric escape is crucial to understanding the evolution of planets in and out of the solar system and to interpreting atmospheric observations. While hydrodynamic escape simulations have been actively developed incorporating detailed processes such as UV heating, chemical reactions, and radiative cooling, the radiative cooling by molecules has been treated as emission from selected lines or rotational/vibrational bands to reduce its numerical cost. However, ad hoc selections of radiative lines would risk estimating inaccurate cooling rates because important lines or wavelengths for atmospheric cooling depend on emitting conditions such as temperature and optical thickness. In this study, we apply the correlated-k distribution (CKD) method to cooling rate calculations for H₂-dominant transonic atmospheres containing H₂O or CO as radiative species, to investigate its numerical performance and the importance of considering all lines of the molecules. Our simulations demonstrate that the sum of weak lines, which provides only 1% of the line emission energy in total at optically thin conditions, can become the primary source of radiative cooling in optically thick regions, especially for H₂O-containing atmospheres. Also, in our hydrodynamic simulations, the CKD method with a wavelength resolution of 1000 is found to be effective, allowing the calculation of escape rate and temperature profiles with acceptable numerical cost. Our results show the importance of treating all radiative lines and the usefulness of the CKD method in hydrodynamic escape simulations. It is particularly practical for heavy-element-enriched atmospheres considered in small exoplanets, including super-Earths, without any prior selections for effective lines.

DOI： 10.3847/1538-4357/ad187f

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その他リンク： https://iopscience.iop.org/article/10.3847/1538-4357/ad187f/pdf

掲載種別：研究論文（学術雑誌）   出版者・発行元：Springer Science and Business Media LLC  

Abstract

We introduce a new flexible one-dimensional photochemical model named Photochemical and RadiatiOn Transport model for Extensive USe (PROTEUS), which consists of a Python graphical user interface (GUI) program and Fortran 90 modules. PROTEUS is designed for adaptability to many planetary atmospheres, for flexibility to deal with thousands of or more chemical reactions with high efficiency, and for intuitive operation with GUI. Chemical reactions can be easily implemented into the Python GUI program in a simple string format, and users can intuitively select a planet and chemical reactions on GUI. Chemical reactions selected on GUI are automatically analyzed by string parsing functions in the Python GUI program, then applied to the Fortran 90 modules to simulate with the selected chemical reactions on a selected planet. We performed a benchmark test of PROTEUS to confirm its validity, by applying it to the Martian atmosphere and the Jovian ionosphere. PROTEUS can significantly save the time for those who need to develop a new photochemical model; users just need to write chemical reactions in the Python GUI program and just select them on GUI to run a new photochemical model.

Graphical Abstract

DOI： 10.1186/s40623-023-01881-w

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その他リンク： https://link.springer.com/article/10.1186/s40623-023-01881-w/fulltext.html

掲載種別：研究論文（学術雑誌）   出版者・発行元：American Astronomical Society  

Abstract

The atmosphere of Mars is mainly composed by carbon dioxide (CO₂). It has been predicted that photodissociation of CO₂ depletes ¹³C in carbon monoxide (CO). We present the carbon ¹³C/¹²C isotopic ratio in CO at 30–50 km altitude from the analysis of the solar occultation measurements taken by the instrument Nadir and Occultation for Mars Discovery on board the ExoMars Trace Gas Orbiter (ExoMars-TGO). We retrieve ¹²C¹⁶O, ¹³C¹⁶O, and ¹²C¹⁸O volume mixing ratios from the spectra taken at 4112–4213 cm⁻¹, where multiple CO isotope lines with similar intensities are available. The intensities of the ¹²C¹⁶O lines in this spectral range are particularly sensitive to temperature, thus we derive the atmospheric temperature by retrieving CO₂ density with simultaneously measured spectra at 2966–2990 cm⁻¹. The mean δ¹³C value obtained from the ¹³C¹⁶O/¹²C¹⁶O ratios is −263‰, and the standard deviation and standard error of the mean are 132‰ and 4‰, respectively. The relatively large standard deviation is due to the strong temperature dependences in the ¹²C¹⁶O lines. We also examine the ¹³C¹⁶O/¹²C¹⁸O ratio, whose lines are less sensitive to temperature. The mean δ value obtained with ¹²C¹⁸O instead of ¹²C¹⁶O is −82‰ with smaller standard deviation, 60‰. These results suggest that CO is depleted in ¹³C when compared to CO₂ in the Martian atmosphere as measured by the Curiosity rover. This depletion of ¹³C in CO is consistent with the CO₂ photolysis-induced fractionation, which might support a CO-based photochemical origin of organics in Martian sediments.

DOI： 10.3847/psj/acd32f

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その他リンク： https://iopscience.iop.org/article/10.3847/PSJ/acd32f/pdf

担当区分：筆頭著者   掲載種別：研究論文（学術雑誌）   出版者・発行元：American Astronomical Society  

Abstract

The isotopic signature of atmospheric carbon offers a unique tracer for the history of the Martian atmosphere and the origin of organic matter on Mars. The photolysis of CO₂ is known to induce strong isotopic fractionation of the carbon between CO₂ and CO. However, its effects on the carbon isotopic compositions in the Martian atmosphere remain uncertain. Here, we develop a 1D photochemical model to consider the isotopic fractionation via photolysis of CO₂, to estimate the vertical profiles of the carbon isotopic compositions of CO and CO₂ in the Martian atmosphere. We find that CO is depleted in ¹³C compared with CO₂ at each altitude, due to the fractionation via CO₂ photolysis: the minimum value of the δ¹³C in CO is about −170‰ under the standard eddy diffusion setting. This result supports the hypothesis that fractionated atmospheric CO is responsible for the production of the ¹³C-depleted organic carbon in the Martian sediments detected by the Curiosity Rover, through the conversion of CO into organic materials and their deposition on the surface. The photolysis and transport-induced fractionation of CO that we report here leads to a ∼15% decrease in the amount of inferred atmospheric loss when combined with the present-day fractionation of the atmosphere and previous studies of carbon escape to space. The fractionated isotopic composition of CO in the Martian atmosphere may be observed by ExoMars Trace Gas Orbiter and ground-based telescopes, and the escaping ion species produced by the fractionated carbon-bearing species may be detected by the Martian Moons eXploration mission in the future.

DOI： 10.3847/psj/acc030

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その他リンク： https://iopscience.iop.org/article/10.3847/PSJ/acc030/pdf

担当区分：筆頭著者   掲載種別：研究論文（学術雑誌）   出版者・発行元：American Astronomical Society  

Abstract

Terrestrial planets currently in the habitable zones around M dwarfs likely experienced a long-term runaway-greenhouse condition because of a slow decline in host-star luminosity in its pre-main-sequence phase. Accordingly, they might have lost significant portions of their atmospheres including water vapor at high concentration by hydrodynamic escape induced by the strong stellar X-ray and extreme ultraviolet (XUV) irradiation. However, the atmospheric escape rates remain highly uncertain due partly to a lack of understanding of the effect of radiative cooling in the escape outflows. Here we carry out 1D hydrodynamic escape simulations for an H₂–H₂O atmosphere on a planet with mass of 1M_⊕ considering radiative and chemical processes to estimate the atmospheric escape rate and follow the atmospheric evolution during the early runaway-greenhouse phase. We find that the atmospheric escape rate decreases with the basal H₂O/H₂ ratio due to the energy loss by the radiative cooling of H₂O and chemical products such as OH and OH⁺: the escape rate of H₂ becomes one order of magnitude smaller when the basal H₂O/H₂ = 0.1 than that of the pure hydrogen atmosphere. The timescale for H₂ escape exceeds the duration of the early runaway-greenhouse phase, depending on the initial atmospheric amount and composition, indicating that H₂ and H₂O could be left behind after the end of the runaway-greenhouse phase. Our results suggest that temperate and reducing environments with oceans could be formed on some terrestrial planets around M dwarfs.

DOI： 10.3847/1538-4357/ac7be7

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その他リンク： https://iopscience.iop.org/article/10.3847/1538-4357/ac7be7/pdf

担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Oxford University Press (OUP)  

ABSTRACT

Recent cosmochemical studies have shown that most of Earth’s building blocks were close to enstatite meteorites in isotopic compositions. This implies the formation of an impact-induced proto-atmosphere enriched in H2 and CH4 on accreting Earth. Such a reduced proto-atmosphere would have been largely lost by hydrodynamic escape, but its flux and time-scale for hydrogen depletion remain highly uncertain. Here we carry out 1D hydrodynamic escape simulations for such an H2–CH4 proto-atmosphere by incorporating expanded chemical networks and radiative cooling processes for estimation of the duration of the H2-rich surface environment on early Earth. In the escape outflow, CH4 is dissociated effectively by direct photolysis and chemical reactions with photochemically produced ion species. On the other hand, radiative cooling by photochemical products such as H$_{3}^{+}$, CH, and CH3 significantly suppresses atmospheric escape. Even though CH4 and their concentrations are small, the heating efficiency decreases to $\sim 5\, { { \ \rm per\ cent } }$ when CH4/H2 = 0.007 in the lower atmosphere and CH4 would suffer negligible escape when CH4/H2≳ 0.01. The time-scale for H2 escape consistent with the constraints of the isotopic compositions and the amount of C and N on the present Earth is possibly more than several hundred million years. Our results suggest that a long-lived hydrogen-rich reduced environment played important roles in climate warming and the generation of organic matters linked to the emergence of living organisms during the first several hundred million years of Earth.

DOI： 10.1093/mnras/stab1471

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その他リンク： http://academic.oup.com/mnras/article-pdf/505/2/2941/38626766/stab1471.pdf

担当区分：筆頭著者   記述言語：日本語   掲載種別：研究論文（学術雑誌）  

DOI： 10.14909/yuseijin.30.2_52

J-GLOBAL

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Elsevier BV  

DOI： 10.1016/j.icarus.2020.113740

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