The Near-Infrared Spectral Characteristics of Water Ice, Epsomite, and Halite Mixtures Relevant to Europa

Abstract

We publish the first NIR spectra of grain particulate mixtures of water ice, epsomite, and halite at cryogenic temperatures. Furthermore, we perform a quantitative assessment of the ability of both intimately- and linearly-mixed models to reproduce laboratory data of different grain mixtures of water ice, as well as water ice mixed with epsomite. We find that smaller grains of water ice impart a stronger influence than larger grains of water ice on the 2.0 {\mu}m spectral feature in epsomite, and grain size signatures for both halite and epsomite are challenging to discern for larger grain sizes as a result of the saturated absorption features. These findings may indicate that an observation bias toward smaller grain sizes of ice could exist, and that quantitative assessments provided by spectral mixture analyses will be the most reliable method for determining compositions and abundances of materials. We also find that the linearly-mixed and intimately-mixed models of water ice appear to match the laboratory spectra as expected, though still display some inconsistencies, often either in the continuum or the absorption features. When modeling pure water ice and water ice mixed with epsomite, no discernible difference is observed between the fits of the linearly- and intimately-mixed models. Future spectral mixture analyses that use epsomite should be aware of a potential error in the published epsomite optical constant data, in which the cryogenic data appear to be taken at ambient conditions.