[P-CG19] 惑星大気圏・電磁圏

[PCG19-01] Exospheric retrieval from energetic ions for isotope ratio measurements by numerical simulations: Implications for future MSA observations onboard MMX

*堺 正太朗^1,2、寺田 直樹¹、益永 圭³、中川 広務¹、横田 勝一郎⁴、笠羽 康正^1,2 (1.東北大学大学院理学研究科地球物理学専攻、2.東北大学大学院理学研究科 惑星プラズマ・大気研究センター、3.宇宙航空研究開発機構 宇宙科学研究所、4.大阪大学大学院理学研究科宇宙地球科学先行)

キーワード：火星、同位体比測定、外圏リトリーバル、火星衛星探査計画MMX、数値実験

Mars has experienced drastic climate changes over the past 4.6 Gyr due to a significant atmospheric escape. How much carbon dioxide (CO₂), which is a major component of the Martian atmosphere, has escaped into space is important for understanding the climate change on Mars. The isotope ratio is particularly one of the key parameters in revealing the Martian atmospheric evolution. Jakosky et al. (2017) suggested that more than 1 bar of oxygen (O) was lost to space from measurements of argon isotope ratios (³⁸Ar/³⁶Ar) based on observations by the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, assuming that the all lost O primarily originated from CO₂, but these estimations were not derived from direct observations of CO₂. The evolution of O and carbon (C) isotope ratios (¹⁶O/¹⁸O and ¹²C/¹³C) between the surface and upper atmosphere is relevant to understanding the CO₂ loss process, but there have been no observational constraints on these isotope ratios in the upper atmosphere. Note that Curiosity identified the isotope ratios of ¹⁶O/¹⁸O ~ 476 and ¹²C/¹³C ~ 85 near the surface (e.g., Mahaffy et al., 2013), and the Atmospheric Chemistry Suite onboard Trace Gas Orbiter (TGO) found the isotope ratio of ¹⁶O/¹⁸O ~ 420 in the middle atmosphere below 60 km altitude (Alday et al., 2019). Japanese future sample return mission "Martian Moons eXploration (MMX)" is a candidate for this observation. Mass Spectrum Analyzer (MSA) onboard MMX can measure escaping ions with a high mass resolution of M/dM > 100. MSA allows us to measure the isotope ratios in the escaping atmosphere for the first time, and furthermore, it is possible to estimate the isotope ratios in the exosphere by retrieving the neutral atmosphere of each isotope from the ion observations. To prepare for future MMX observations, this study investigates the energetic oxygen ions seen at midnight around the Phobos' orbit and their sources using test particle simulations under electric and magnetic fields obtained from magnetohydrodynamic simulations (e.g., Sakai et al., 2021). The particle simulations are conducted under certain interplanetary magnetic field conditions. The ¹⁶O⁺ flux from the simulations is consistent with the previous study (Curry et al., 2013). The energetic ions that reach Phobos' orbit at midnight are picked up in the magnetosheath or the solar wind region. The ions with energies between ~1 and ~5 keV come from the magnetosheath, while those with energies greater than ~10 keV, which cannot be measured by MSA, come from the solar wind. The ions coming from the magnetosheath have two sources: the flank region of the induced magnetosphere and just below the equator, which are determined by the electric field and potential in the magnetosheath. Establishing a retrieval method would enable us to determine the pickup position from the ion energy around the Phobos' orbit in MMX. Multiple measurements of isotope ratios at different altitudes by MMX, Curiosity, and TGO would lead to a better understanding of atmospheric evolution on Mars, and this simulation aims to help future measurements by MMX.