Color centers in salts - Evidence for the presence of sulfates on Europa

Abstract

We have conducted experiments to better understand the extent to which radiation-induced color changes observed at near-ultraviolet through near-infrared wavelengths for various salts could provide insight into the composition of the nonice material(s) on the surface of Europa's trailing hemisphere. Salts of NaCl, KCl, Na2SO4, partially hydrated MgSO4, partially hydrated FeSO4, Na2CO3, and CaCO3 were irradiated with 1020 electrons/cm2 at room temperature; the electron energy was 40 keV. This is equivalent to ~100,000 years of exposure to electrons at Europa in this energy range, or to an equivalent few thousand years of exposure to electrons of at least ~100 eV, a conservative estimate for the lowest energy electrons capable of producing color centers. Each salt either browned, darkened, and/or developed color centers. Most salts also developed a probable colloidal band near 600 nm that would be expected to be much weaker or non-existent in materials at the temperature of Europa's surface. The ultraviolet through near-infrared telescopic spectra of Europa's trailing hemisphere is most consistent with a linear mixture of irradiated MgSO4·nH2O with a few percent of another salt that exhibits a color center near 600 nm (either Na2SO4·nH2O, Na2CO3, CaCO3, or KCl). Other irradiated salts exhibit spectral features that limit their abundances in the nonice material on the trailing hemisphere, with the abundance of NaCl, because of its strong color centers, limited to not more than approximately 10%.