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

TL;DR

This study investigates how salty granular ice analogs evolve under Europa-like conditions, revealing changes in spectral features over thousands of years and the transient presence of hydrohalite due to irradiation effects.

Contribution

It introduces a novel calibration method for temperature measurement and provides insights into spectral evolution and hydrohalite stability on Europa's surface.

Findings

Spectral water absorption bands change significantly over a few thousand years.

Hydrohalite forms during sublimation but dehydrates rapidly under radiation.

Detection of hydrohalite indicates recent surface activity.

Abstract

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). Thus, a detection of hydrohalite would clearly indicate recent activity.