Chemistry and spectroscopy of Europa’s surface materials: Insights from laboratory experiments and infrared observations

Abstract:

Recent observations using large telescopes suggest chloride salts on Europa's surface. The surface of Europa is irradiated by high-energy particles, such as electron, accelerated by the Jovian magnetic field. Those irradiations of high-energy particles cause a darkening of sodium-chloride (NaCl) in the visible wavelength. Although the darkening of NaCl would also affect the slope of the reflectance in the near-infrared wavelength, it is poorly investigated both by laboratory experiments and telescope observations. Here we report our results of laboratory experiments on electron irradiation onto NaCl and near-infrared observations of Europa using the Subaru telescope. We irradiated electron with 10 keV onto NaCl crystals. The irradiated samples were analyzed with FTIR. In addition, we performed an observation of Europa’s surface using the Subaru telescope and IRCS in the wavelength of 1.1 to 1.8 μm. Our observational results show a dark reflectance compared with that of H₂O ice in 1.1‒1.5 μm. This would be explained by the darkening of NaCl. Based on these results, we discuss the abundance of NaCl on the surface.