Reflectance properties of analogues for the surfaces of icy moons

Spectrometers and colour imagers on past and future space missions, as well as ground-based telescopes, help us to improve our understanding of the composition of icy surfaces in the outer solar system. To help interpret these datasets, we study the VIS-NIR (0.4-2.5 µm) reflectance properties of granular (salty) ice particles exposed to simulated space environments.

We further developed an original experimental chamber (SCITEAS-2) to study the evolution of samples at temperatures representative of icy planetary surfaces in the outer solar system. We built a new cooling/heating stage to precisely control the sample’s temperature, allowing us to decouple the effects of temperature and time on the sublimation process. The surface temperature of the ice is monitored by measuring IR-emission using Thermopile sensors. To study the reflectance of the sample, we use a hyperspectral imaging system consisting of a Halogen light source, a monochromator, and two cameras (CCD and MCT). We produce granular ice particles with a broad size distribution (d≈1-400µm) by flash-freezing dispersed droplets in LN2. These particles can be made from pure water or salty solutions.

We observe that the VIS-spectrum of pure water ice is flatter than the one of the ice produced from a 10wt% NaCl solution, which has a blue slope. The most prominent feature of granular 10wt% NaCl-ice is a narrow absorption feature at 1.98 µm, attributed to hydrohalite (NaCl · 2H₂O), which is not present in the pure ice sample. However, it only appears after some sublimation of the sample. While the spectra of pure water ice and 10wt% NaCl ice match well for the pristine samples, sublimation strongly increases the albedo of salty ice. Sublimation forms a crust atop the sample, affecting the reflectance and strongly influencing other thermo-physical properties. Therefore, we propose that sublimation is an important ingredient in interpreting spectral data of the Jovian Moon Europa because the timescales of the effects of sublimation are smaller than surface renewal by micrometeorite gardening or sputtering.

These datasets will help to interpret high-resolution colour images and spectra acquired by the EIS and MISE instruments on Europa Clipper as well as similar instruments on JUICE.