14:45 〜 15:00
[PCG19-05] 水素・重水素比分布とその時間変動から大気散逸の律速過程を解明する
キーワード：地球型惑星、大気、大気散逸、水素・重水素、水
Hydrogen escape is a driver of the long-term evolution of planetary atmospheres and surface environments. The rate-limiting process is a useful concept for evaluating how the escape rate has changed through time. For instance, hydrogen escape from Earth today is considered to be in the diffusion-limited regime, where diffusion from the homopause to the exobase is a bottleneck to determine the escape rate (Hunten 1973). Determining the rate-limiting process is also crucial to evaluate the net isotopic fractionation in hydrogen (H) and deuterium (D) escape, which is then used to estimate the integrated water loss from the geochemical record of surface-water D/H ratios. However, while the theoretical prediction on the rate-limiting process has been made and widely used (e.g., Catling and Kasting 2017), the observational test on the prediction is limited, mostly due to the lack of useful proxies. This study proposes that the upper atmospheric D/H profile and its time variation can be utilized to constrain whether hydrogen escape is in effusion- or diffusion-limited regime.
For this purpose, we conducted both analytic and numerical model calculations for the atmospheric D/H profile under different effusion velocities. The analytic model ignores photochemistry, and is used to show the dependence of the D/H profile in the upper atmosphere on the rate-limiting process and how different solar-system bodies compare with each other. The numerical model is used to see whether this concept holds when atmospheric photochemistry is considered. Chemistry is situation-dependent, and here we used a model developed to simulate current Mars (Cangi and Chaffin 2022; Chaffin et al. 2017; Cangi et al. 2020, 2023, 2024) as an example.
Results of our analytic model without photochemistry show that the atomic D/H ratio in the heterosphere show three different trends (Figure 1): Case I) decreasing with the altitude, Case II) increasing at the bottom and constant at the top, and Case III) constant at the bottom and increasing at the top, which correspond to three different cases where i) both H and D are in the effusion-limited, ii) H is in the diffusion-limited while D in the effusion-limited, and iii) both H and D are in the diffusion-limited regimes, respectively. These three cases show different time variation of the D/H profile with respect to the change of effusion velocities. Photochemical modeling for modern Mars as a test case shows that correlations with the atomic D/H profiles are seen for molecular D/H profiles for photochemically-short-lived species as well.
We classify the solar-system bodies under their averaged conditions into those three cases: Earth and Titan into Case III, Venus to Case I, and Mars into Case I/II boundary. We predict different D/H profiles and time variation in their upper atmospheres. We discuss the comparison with existing and future observations to observationally elucidate the rate-limiting processes of hydrogen escape.
For this purpose, we conducted both analytic and numerical model calculations for the atmospheric D/H profile under different effusion velocities. The analytic model ignores photochemistry, and is used to show the dependence of the D/H profile in the upper atmosphere on the rate-limiting process and how different solar-system bodies compare with each other. The numerical model is used to see whether this concept holds when atmospheric photochemistry is considered. Chemistry is situation-dependent, and here we used a model developed to simulate current Mars (Cangi and Chaffin 2022; Chaffin et al. 2017; Cangi et al. 2020, 2023, 2024) as an example.
Results of our analytic model without photochemistry show that the atomic D/H ratio in the heterosphere show three different trends (Figure 1): Case I) decreasing with the altitude, Case II) increasing at the bottom and constant at the top, and Case III) constant at the bottom and increasing at the top, which correspond to three different cases where i) both H and D are in the effusion-limited, ii) H is in the diffusion-limited while D in the effusion-limited, and iii) both H and D are in the diffusion-limited regimes, respectively. These three cases show different time variation of the D/H profile with respect to the change of effusion velocities. Photochemical modeling for modern Mars as a test case shows that correlations with the atomic D/H profiles are seen for molecular D/H profiles for photochemically-short-lived species as well.
We classify the solar-system bodies under their averaged conditions into those three cases: Earth and Titan into Case III, Venus to Case I, and Mars into Case I/II boundary. We predict different D/H profiles and time variation in their upper atmospheres. We discuss the comparison with existing and future observations to observationally elucidate the rate-limiting processes of hydrogen escape.