[P-CG21] Planetary Magnetosphere, Ionosphere, and Atmosphere

[PCG21-P02] Comparison of Titan's atmospheric escape processes by Saturn and Solar winds using 3D magnetohydrodynamic simulations

*Ryoma Takada¹, Tomoki Kimura¹, Shotaro Sakai², Yuki Maeda³, Eitaro Nakada⁴, Kusano Yuri¹, Minami Tokushige¹, Naoki Terada⁵ (1. Tokyo University of Science, 2.Department of Geophysics, Graduate School of Science, Tohoku University, 3.Atmosphere and Ocean Research Institute, The University of Tokyo, 4.Hokkaido University, 5.Graduate School of Science, Tohoku University)

Keywords:Titan, atmospheric escape, Saturn

Titan is the only body in our solar system with an abundant atmosphere of 1 atm pressure on the surface. Titan's atmosphere is nitrogen-dominated (nitrogen: 94%, methane: 5%, hydrogen: 1% Magee et al, 2009), like the atmospheric composition of past Earth just before biotic oxygen began to increase. Therefore, the evolutionary process of Titan's atmosphere is essential for the elucidation of the atmospheric evolution of the past Earth. The energy deposition to atmospheric particles by ion pickup and sputtering is likely more dominant than EUV in the upper atmosphere (Micheal et al, 2005). These non-thermal escape processes are important for elucidating atmospheric evolution. Titan is normally located in Saturn's magnetosphere and is blown by the mainly oxygen ions O⁺ in the magnetospheric plasma (Saturn wind). However, the atmosphere should be significantly affected by solar wind protons H⁺ when the magnetosphere shrinks as the solar wind dynamic pressure increases, and Titan moves out of Saturn's magnetosphere (Bertucci et al, 2008). Since the full extent of nonthermal escape by the external plasma flows, the Saturn and solar winds, is difficult to elucidate only with spacecraft observations, numerical simulations have been used in comparison with observations (Modolo et al, 2008, Strobel,2009, H.Gu et al, 2019). However, the previous studies did not treat non-thermal escape for hydrogen and also not the Saturn and solar winds by the same method, thus the differences in the escape rates for the two external plasma flows were not discussed.
In this study, we simulated the global non-thermal escape of nitrogen and hydrogen, which are the main components of the atmosphere, using a 3D multi-component ion magnetohydrodynamic simulation by Terada et al, (2009), and evaluated the escape rate by the Saturn wind and compared it with that by the solar wind for the first time. Based on Cassini in-situ observation data(Sitter et al, 2009), the escape rate was estimated under the Saturn wind conditions of 0. 2[/cm³] density, 120 [km/s] velocity, and 7.0 [nT] magnetic flux density, resulting in an escape rate of nitrogen-associated ions are 2.4×10²³ [/s] hydrogen-associated ions are 4.3×10²³ [/s]. On the other hand, the escape rate was estimated under the solar wind conditions (Bertucci et al, 2015) with a density of 0.35 [/cm³], a velocity of 360[km/s], and 0.5 [nT] magnetic flux density, resulting in nitrogen-associated ions are 1.4×10²³ [/s] and hydrogen-associated ions are 8.5×10²⁴ [/s]. These results indicate that the Saturn wind suppresses the escape of hydrogen-associated ions and enhances the escape of nitrogen-associated ions more compared to the solar wind. This suggests that Saturn and solar winds control the abundance of hydrogen in Titan's atmosphere. The Saturn wind promotes a more hydrogen-rich reduced atmospheric composition, and the solar wind promotes oxidation.
In this study, we estimated the total non-thermal escape rates for nitrogen and hydrogen, the major components of Titan's atmosphere. However, the quantitative evaluation of the escape rates for each ion species and the escape mechanisms still need to be evaluated. We plan to investigate them closely in the future. In this presentation, we report the current status of the above.