Visible-infrared Spectra of keV Electron Irradiated Cryogenic Salts with Implications for Europa Surface Composition

Abstract

Visible to near-infrared spectral coloring of cryogenic halides, sulfates, their frozen brines, and sulfuric acid hydrate formed through one to tens of keV electron radiation can inform on the composition of Europa’s surface. The visible wavelength spectra of some of these materials present potential matches to the color of Europa’s leading and trailing hemispheres, while others are clear mismatches, and the infrared reflectance provides additional insights. Consistent with previous studies, irradiated anhydrous NaCl uniquely exhibits a color center near 460 nm. Cryogenic saturated and dilute brines of NaCl do not contain color centers, although each reddens almost identically to each other when irradiated. Hydrated MgCl2 and MgSO4 minerals and brines also redden but differently from NaCl brines. Partially desiccated and irradiated sulfates and sulfuric acid hydrate offer potential matches to both the leading and trailing hemispheres based upon their visible spectra. The significant spectral mismatch of hydrohalite, magnesium chlorides, iron-bearing chlorides, and iron-bearing sulfates in the visible as well as the infrared imply that they are not present at more than trace abundances. The most likely explanation for the visible and infrared reflectance spectra of Europa’s surface is a small amount of desiccated NaCl with a larger quantity of hydrated magnesium sulfate and sulfuric acid. We infer the subsurface ocean of Europa has significant sulfates as well as some chlorides, with the relatively incompatible sulfates preferentially being dominating in the Europan crust.