Trumbo, et al., report that the potential habitability of Europa’s subsurface ocean depends on its chemical composition, which may be reflected in that of Europa’s geologically young surface. Investigations using Galileo Near-Infrared Mapping Spectrometer data led to the prevailing view that Europa’s endogenous units are rich in sulfate salts. However, recent ground-based infrared observations have suggested that, while regions experiencing sulfur radiolysis may contain sulfate salts, Europa’s more pristine endogenous material may reflect a chloride-dominated composition. Chlorides have no identifying spectral features at infrared wavelengths, but develop distinct visible-wavelength absorptions under irradiation, like that experienced on the surface of Europa. Using spectra obtained with the Hubble Space Telescope, we present the detection of a 450-nm absorption indicative of irradiated sodium chloride on the surface. The feature correlates with geologically disrupted chaos terrain, suggesting an interior source. The presence of endogenous sodium chloride on the surface of Europa has important implications for our understanding of its subsurface chemistry.
The presence of NaCl on Europa has important implications for our understanding of the internal chemistry and its geochemical evolution through time. Whereas aqueous differentiation of chondritic material and long-term leaching from a chondritic seafloor can result in a system rich in sulfates, more extensive hydrothermal circulation, as on Earth, may lead to an NaCl-rich ocean. The plume chemistry of Enceladus, which is perhaps the best analog to Europa, suggests an NaCl-dominated ocean and a hydrothermally active seafloor. However, the compositional relationship between Europa’s ocean and its endogenous material is unknown, and the surface may simply represent the end result of a compositional stratification within the ice shell. Regardless of whether the observed NaCl directly relates to the ocean composition, its presence warrants a reevaluation of our understanding of the geochemistry of Europa.
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