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

[PCG21-P03] Lifetime of Europa Surface chloride salts evaluated by plasma irradiation experiments on Magnesium chloride

*Miyu Okumoto¹, Tomoki Kimura¹, Misako Otsuki¹, Ryo Hoshino¹, Ayumi Kobayashi¹, 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, one of Jupiter's icy moons, is considered to be one of the celestial bodies where life is suggested to exist in its internal water ocean. Understanding the chemical composition of surface materials might lead to an understanding of the chemical composition and habitability of the internal ocean because the interior ocean material is transported out to the surface [Schubert et al.2004]. In Otsuki et al. (in prep), the hypothesis of an internal oceanic origin was verified based on plasma irradiation experiments for sulfate salt MgSO₄. They reproduced the chemical cycles caused by space weathering and obtained a depletion time of MgSO₄. The obtained lifetime of MgSO₄ is shorter than the average surface age of Europa [Zahnle et al.2009], which is consistent with surface spectral observations suggesting the presence of MgSO₄ in regions with possible ocean water eruptions [McCord et al.1998]. Thus the sulfate salt was concluded to be a candidate for the internal ocean material. On the other hand, Liger et al. (2016) conducted a numerical simulation using spectra obtained from laboratory experiments and observations in the near-infrared wavelength region with the VLT large ground-based telescope. Chloride salts including MgCl2 are also considered as candidates for internal oceanic material since the spectra of chloride salts including MgCl2 were identified in the region where the eruption of an internal ocean is expected. However, although the existence of chloride salts is suggested, there is no precedent for reproducing the depletion process, and the chemical cycle and depletion process due to space weathering is unknown, so the concentration of chloride salts in the interior ocean and other factors remain unresolved.
Here we irradiated magnesium chloride (MgCl₂), a type of chloride salt, with energetic electrons and oxygen ions under the same conditions as Otsuki et al.(in prep.) conducted for sulfate salts, aiming to reproduce the chemical cycle of chloride salts. We confirmed degassing from the chloride salt sample, of which pressure increased from 3.07E-04 Pa to 3.45E-03 Pa during 3-hour electron irradiation at a flux of 2.95E+14 /cm²/s. The pressure increase from 3.29E-04 Pa to 9.00E-04 Pa was observed during 3.5-hour oxygen irradiation at a flux of 9.10E+13 /cm²/s. In the mid-infrared spectrum, unlike the spectral change of sulfate (sulfur allotropic peak in the 7.6 μm band, Otsuki et al. in prep.), three new peaks appeared in the relative reflection spectrum after irradiation relative to non-irradiation near 13,6,4μm. The peaks decreased by 30%, 30%, and increased by 48% for electron irradiation, and increased by 478%, 187%, and 78% for oxygen irradiation, respectively. We plan to identify the gaseous and solid materials produced by irradiation and to conduct irradiation experiments on mixed sulfate and chloride salts to clarify the concentration change, surface lifetime, and mixing ratio. This presentation reports the current status of our study.