[P-PS01] Outer Solar System Exploration Today, and Tomorrow

[PPS01-P05] Lifetime of Europa’s Surface Chloride Salts Estimated by Laboratory Plasma Irradiation Experiments

*Miyu Okumoto¹, Tomoki Kimura¹, Yusuke Nakauchi², Fuminori Tsuchiya³, Shuya Tan⁴, Jun Kimura⁵ (1.Tokyo University of Science, 2.Ritsumeikan University, 3.Planetary Plasma and Atmospheric Research Center, Graduate School of Science, Tohoku University, 4.Japan Agency for Marine-Earth Science and Technology, 5.Osaka University)

Keywords:Icy moons, Irradiation experiment

Europa, an icy satellite of Jupiter, is one of the objects suggested to have a habitable environment because of its interior ocean composed of liquid water. Comparison of reflectance spectra obtained with the Near-Infrared Mapping Spectrometer (NIMS) by the Galileo spacecraft with those obtained from laboratory experiments (Carlson et al. 2001) suggested the transport of material from the interior ocean to the surface. (Carlson et al. 2001, Dalton et al. 2007). The understanding of the chemical composition of surface materials is thought to indirectly lead to an understanding of the chemical composition and habitability of the interior ocean (Schubert et al. 2004). However, surface materials experience space weathering, in which composition is degenerated by irradiation such as plasma around planets (Cooper et al. 2001). This weathering results in a chemical cycle consisting of multiple species of materials, which makes the composition of Europa's surface materials before weathering uncertain. Chlorides and sulfates are currently candidates as the surface materials originating in the interior ocean (Cooper et al. 2001). Otsuki (master's thesis, Tokyo University of Science, 2024) verified the hypothesis that sulfates originated from the interior ocean based on plasma irradiation experiments on MgSO4. The chemical cycle caused by space weathering was reproduced, and its detailed process was estimated. The experiments show a depletion of MgSO4. The experimentally estimated MgSO4 lifetime of 1.1E+3 yr is significantly shorter compared to the globally averaged Europa surface age of ~1E+7 yr (Zahnle et al. 2009), which is rather consistent with local regions with possible ocean water eruptions (McCord et al. 1998) where the observations suggested the presence of MgSO4. Otsuki (master's thesis, Tokyo University of Science, 2024) concluded that MgSO4 likely originates in the interior ocean. On the other hand, Ligier et al. (2016) conducted a numerical spectral simulation with "constraints" by the near-infrared spectroscopy with the ground-based telescope, the Very Large Telescope (VLT), and spectra obtained from laboratory experiments. They considered the chloride salts including MgCl2 as candidates for the interior oceanic material since the spectra of chloride salts were identified in the region of the possible water eruption from the interior ocean. However, although the presence of chloride salts is suggested, there is no study reproducing the depletion process, and the chemical cycle and depletion process due to space weathering are unknown. Therefore, the chloride salts in the surface layer have not been demonstrated, which makes the chemical composition of the interior ocean still unknown. In this study, magnesium chloride (MgCl2), was irradiated with electrons, oxygen ions, and hydrogen ions at fluxes of 3.71E+14(/cm2/s), 1.85E+14(/cm2/s), and 5.05E+14(/cm2/s), respectively, under the same conditions as the previous study for the sulfate (Otsuki, master's thesis, Tokyo University of Science, 2024). As a result of irradiation for 3 hours each, the degassing of such as Cl, Cl2, and HCl was observed in the chloride salt samples. In the relative reflection spectra before and after irradiation in the mid-infrared region, new peaks were observed around 13 μm, 6 μm, and 4 μm, which are different from the sulfate experiment results (sulfur allotrope peak in the 7.6 μm band, Otsuki, Master's thesis, Tokyo University of Science, 2024). These results indicate that irradiation produced new materials and changed the crystal structures in the sample. In the future, we plan to estimate the depletion time of MgCl2 based on numerical simulation with the reaction cross-section evaluated from the production rate of degassing species in the experiment. The depletion process of chloride salts on Europa’s surface will be discussed by comparing our results with a previous study on sulfate salts (Otsuki, Master's thesis, Tokyo University of Science, 2024). This presentation will report the current status of our study.