Evolution of granular salty ice analogs for Europa: Sublimation and irradiation

We study the evolution of the Vis-NIR reflectance spectrum of salty granular ice analog samples in a simulation chamber under conditions relevant to the surface of Europa. A novel application and custom calibration of a thermopile sensor enabled the measurement of the surface temperature of the samples in far infrared emission. This allows the kinetics of the observed changes to be scaled to equivalent timescales on Europa. We observed significant changes in the depth and shape of the broad water absorption bands for all samples on timescales of a few thousand years of equatorial conditions on Europa. This effect should be taken into account if quantitative predictions about bulk composition are made based on remote-sensing data. A narrow absorption feature attributed to hydrohalite formed during the sublimation of the sodium chloride sample. We used near-infrared spectroscopy in an irradiation chamber to assess the stability of this narrow feature under electron irradiation. We find that the radiation environment present on Europa dehydrates the hydrohalite on short timescales. Therefore, we expect hydrohalite not to be present on the surface, unless erupted very recently ( < 10 yr) or located in thermal anomalies ( > 145 K), so a detection of hydrohalite clearly indicates recent activity. • A novel application of thermopile sensors to measure cryogenic surface temperatures of a porous ice analog in vacuum is presented. • The surface temperature measurement allows the scaling of the sublimation kinetics to equatorial conditions on Europa. • All salty granular ice samples show a significant increase in the broad near-infrared water absorption bands under sublimation at a temperature between 160 K and 185 K over a few days, which can be translated to a timescale of a few thousand years at Europa’s surface conditions ( < 132 K). • Flash frozen water ice particles, produced from a sodium chloride solution, form the hydrated mineral hydrohalite under sublimation. • The strength of the 1.98 µm absorption feature of hydrohalite is weakened under electron irradiation, a process more efficient than the formation through sublimation of ice on Europa. • Hydrohalite is not expected to be present on the surface, unless recently erupted or located in hotspots; therefore, any detection is a clear indication of recent activity. • Flash frozen water ice particles containing magnesium sulfate do not form narrow hydration features under sublimation.