Chloride oxidation on Europa? Constraints from laboratory experiments and infrared observations using Subaru telescope

The surface of Europa is irradiated by high-energy particles, such as electron, accelerated by the Jovian magnetic field. The irradiations of high-energy particles cause dissociation of H₂O ice and formation of oxidants through hydrogen escape to space. This process may oxidize surface (hydro)chloride into chlorate and perchlorate. Formation of oxidized salts is important not only for surface material evolution but also for supply of oxidants to the subsurface ocean. However, chloride oxidation on Europa's surface have been poorly investigated both by laboratory experiments and telescope observations.

Here we report our results of laboratory experiments on electron irradiation onto (hydro)chloride and infrared observations of Europa using the Subaru telescope. We irradiated electron with ~5−10 keV onto mixtures of H₂O ice and (hydro)chloride at low temperatures (~100 K). After 10-hour irradiation, the irradiated samples were analyzed with an ion chromatograph and XAFS. In addition, we performed an observation of Europa's surface using the Subaru telescope and IRCS with a high-wavelength resolution of δλ = 0.0023 μm in the wavelength of 0.95 to 1.5 μm. The surface was observed with high-spatial resolution of 200−400 km/pixel around chaotic terrains. Based on these results, we discuss the possibility of oxidation of (hydro)chlorides on Europa.