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Electron Irradiated Cryogenic Hydrated Materials as Spectral Matches to Europa

Presentation #114.06 in the session Laboratory Investigations (Oral Presentation)

Published onOct 23, 2023
Electron Irradiated Cryogenic Hydrated Materials as Spectral Matches to Europa

The surface of Europa contains hydrated material(s) that are continually altered by bombardment with Jovian magnetospheric ions and electrons. Several candidate salts and other hydrated materials have bene postulated to exist on their surfaces but so far only sodium chloride (NaCl) has been unequivocally identified through absorption bands near 460 nm 230 nm [Trumbo et al., 2019; 2022]. We have explored the visible – infrared bidirectional reflectance of irradiated hydrated forms of NaCl (hydrohalite and flash-frozen brines at saturation and at 3.5% concentration – similar to Earth’s ocean), magnesium sulfate (dodecahydrate, epsomite, and flash frozen brines at saturation) as well as other candidate hydrated materials. Frozen NaCl brine darkens and reddens under irradiation as do all forms of MgSO4 hydrates and brine; however, it is likely only desiccated NaCl forms a color center at 460 nm. In comparing laboratory spectra to that of Europa, we find that the visible spectrum of Europa’s leading hemisphere is somewhat similar to the spectrum of irradiated frozen brine of NaCl and to a greater extent to the spectrum of irradiated cryogenic epsomite (MgSO4*7H2O). However, the infrared spectrum of Europa’s leading hemisphere (which is interpreted as essentially waterice) is quite similar the spectrum of frozen dilute NaCl brine, only somewhat similar to cryogenic epsomite (the water-related IR absorptions features do not match), yet quite dissimilar from that of hydrohalite. This is consistent with previous hypotheses for the surface of the leading hemisphere of Europa reflecting contact with a subsurface ocean. We hypothesize this ocean composition may be a solution of both NaCl and MgSO4 ions. Irradiation of the frozen solution results in darkening with some desiccation to explain the visible color and shallow 460-nm band, while the nonice material at depth remains hydrated in order to explain the characteristics of the infrared hydration features. The trailing hemisphere of Europa may be the end result of this combined process of irradiation and desiccation with the added complication of sulfur implantation to create hydrated sulfuric acid. We would like to acknowledge the support of SSW Grant # 80NSSC20K1044.

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