Natural Analogue Constraints on Europa's Non-ice surface Material

Non-icy material on the surface of Jupiter’s moon Europa is hypothesised to have originated from its subsurface ocean, and thus provide a record of ocean composition and habitability. The nature of this material is debated, but observations suggest that it comprises hydrated sulfate and chloride salts. Analogue spectroscopic studies have previously focused on single phase salts under controlled laboratory conditions. We investigated natural salts from perennially cold (<0 °C) hypersaline springs, and characterised their reflectance properties at 100 K, 253 K and 293 K. Despite similar major ion chemistry, these springs form mineralogically diverse deposits, which when measured at 100 K closely match reflectance spectra from Europa. In the most sulfate-rich samples, we find spectral features predicted from laboratory salts are obscured. Our data are consistent with sulfate-dominated europan non-icy material, and further, show that the emplacement of endogenic sulfates on Europa’s surface would not preclude a chloride-dominated ocean.