[P-CG19] Planetary Magnetosphere, Ionosphere, and Atmosphere

[PCG19-09] Verification of the endogenic hypothesis for Europa's surface materials by oxygen ion and electron irradiation experiments

*Misako Otsuki¹, Tomoki Kimura¹, Tomohiro Kitano¹, Ryo Hoshino¹, Yusuke Nakauchi², Fuminori Tsuchiya³, Jun Kimura⁴, Shuya Tan⁵ (1.Tokyo University of Science, 2.Japan Aerospace Exploration Agency, 3.Planet. Plasma Atmos. Res. Cent., Tohoku Univ., 4.Osaka University, 5.Earth-Life Science Institute, Tokyo Institute of Technology)

Keywords:Europa, surface composition

Jupiter's icy moon Europa likely has the interior water ocean beneath the icy shell. The chemical composition of the interior ocean is the most important problem to be solved for assessing the interior ocean's habitability. Sulfur is one of the possible major constituent elements of Europa's surface materials. Two hypotheses have been proposed for the source of sulfur: the sulfates plumed from the interior ocean [Kargel et al., 2000], or the sulfur ion originating from the volcanic gasses plumed from the moon Io [Alvarellos et al., 2008]. The surface materials are suggested to be chemically altered continuously by irradiation of high-energy plasma from Jupiter's magnetosphere, which drives the radiolytic cycles between several sulfur compounds [Carlson et al., 2002]. Because such cycles prohibit understanding the source of surface sulfur, neither the endogenic nor exogenic hypotheses have been demonstrated yet. This study verifies the endogenic hypothesis for Europa's surface sulfur by modeling the radiolytic sulfur cycles based on the laboratory experiment. We irradiated the magnesium sulfate (MgSO4) sample, which is a possible candidate for the constituents of the surface and interior ocean of Europa, with oxygen ions and electrons. It was confirmed that some sulfur compounds such as octasulfur (S8), sulfur dioxide (SO2), hydrogen sulfide (H2S), sulfuric acid (H2SO4), and sulfur tetroxide (SO4) were newly synthesized from MgSO4. This result successfully demonstrates the radiolytic sulfur cycle on Europa's surface. Numerical simulation for time variation of surface chemical composition was made with the production rates of each sulfur compound estimated by our experiments. The depletion time of MgSO4 was found to be 1.3e+5 years for the oxygen irradiation and 1.7e+3 years for the electron irradiation. The depletion cross sections were estimated to be 1.2e-18cm² and 7.1e-19cm², respectively. Assuming the simultaneous irradiation of oxygen ions and electrons to model the actual Europa’s surface the environment, the total depletion time of MgSO4 is estimated to be 1.7e+3 years, and the total amount of S8, SO2, H2S, H2SO4, and SO4 produced in the depletion time to be 14.2%, 30.1%, 37.4%, 2.21%, and 16.2% of the original MgSO4, respectively. The estimated lifetime of 1.7e+3 years is sufficiently shorter than Europa's average surface age of ~1e+7 years estimated by the creator chronology. Therefore the endogenic sulfate is suggested to be depleted by the plasma irradiation before it is turned over by the geological processes. In the previous studies, the infrared observations suggested the sulfate and sulfuric acid hydrates in the geological units that likely have active plumes [Carlson et al., 2009]. From the standpoint of the endogenic hypothesis, if the sulfate is present in the units, it suggests that the seawater from the interior ocean containing the endogenic sulfate was supplied to Europa's surface within the last ~2e+3 years, and the supplied sulfate has survived to the present without depletion while been involved in the radiolytic sulfur cycles. We are going to conduct the irradiation experiment for sulfur allotrope and H2O icy samples and the numerical simulation to verify the endogenic hypothesis. The exogenic hypothesis is going to be verified by the sulfur ion irradiation experiment.