17:15 〜 18:30
[MIS14-P07] No effective oxychloride formations on Europa's surface inferred from UV/electron irradiation experiments onto salt-ice mixtures
キーワード：エウロパ、照射実験、過塩素酸塩
Recent observations of Europa's surface suggest that chlorides (e.g., NaCl, MgCl2·nH2O) exist on Europa's geologically active chaos regions (Fischer et al., 2015; Ligier et al. 2016). Europa's surface is irradiated by high-energy particles, such as electrons, and solar UV light. These irradiations would cause the oxidation of surface materials due to dissociation of H2O ice and subsequent escape of H2 into space. Through the irradiation, chlorides on the surface could be oxidized to form oxychlorides, such as chlorates (e.g., Mg(ClO3)2·6H2O) or perchlorates (e.g., NaClO4, Mg(ClO4)2·6H2O) (e.g., Johnson et al., 2019). However, the oxidation reactions have been poorly investigated based on laboratory experiments.
Here, we perform laboratory experiments to investigate formation of oxychlorides on Europa. We irradiated ~10-keV electron or UV light onto mixtures of H2O ice and chlorides at temperatures of ~120 K using a newly-developed irradiation experimental system. In addition, we irradiated MeV electron onto mixture of H2O ice and chlorides using an electron-beam accelerator, ETIGO-III (Tokuchi et al., 1998). Our results show no significant formations of ClO4 and ClO3 from chlorides regardless of the experimental conditions. The upper limits of conversion efficiency from NaCl to NaClO4 are estimated in our experiments. The estimated upper limits of conversion efficiency are 10-19−10-20 /electron. Considering the upper limits of conversion efficiency, together with the fluxes of electrons or photons on Europa (Paranicas et al., 2009; Clare et al., 2012), an upper limit abundance of NaClO4 are estimated as less than ~2 mol% relative to H2O ice over Europa's surface age (30−100 Myr). Our experimental results and estimations suggest that oxychlorides are minor components as salts and oxidants on Europa's surface.
Here, we perform laboratory experiments to investigate formation of oxychlorides on Europa. We irradiated ~10-keV electron or UV light onto mixtures of H2O ice and chlorides at temperatures of ~120 K using a newly-developed irradiation experimental system. In addition, we irradiated MeV electron onto mixture of H2O ice and chlorides using an electron-beam accelerator, ETIGO-III (Tokuchi et al., 1998). Our results show no significant formations of ClO4 and ClO3 from chlorides regardless of the experimental conditions. The upper limits of conversion efficiency from NaCl to NaClO4 are estimated in our experiments. The estimated upper limits of conversion efficiency are 10-19−10-20 /electron. Considering the upper limits of conversion efficiency, together with the fluxes of electrons or photons on Europa (Paranicas et al., 2009; Clare et al., 2012), an upper limit abundance of NaClO4 are estimated as less than ~2 mol% relative to H2O ice over Europa's surface age (30−100 Myr). Our experimental results and estimations suggest that oxychlorides are minor components as salts and oxidants on Europa's surface.