Infrared reflectance spectra of Hexahydrite at typical Jupiter’s icy moons environmental conditions.

Hexahydrite is a hydrated magnesium sulfate (MgSO₄∙6H₂O), whose existence has been suggested on icy satellites like Europa and Ganymede [1]. Spectral characterization of minerals of this kind at the typical conditions found on the surface of the icy moons of Jupiter is essential to better constrain their existence and the implication of their detection, also in view of the future observations coming from the MAJIS (Moons and Jupiter Imaging Spectrometer) instrument [2,3] on board the ESA JUICE mission. We evaluate the variation of spectral features depending on environmental conditions by measuring the reflectance spectra of hydrated salts, in the infrared spectral range, at the representative low surface temperatures and pressures of the icy moons, with a spectral resolution comparable to the one of MAJIS. We used the experimental setup CAPSULA [4], which is a cylindric chamber that allows a controlled environment with pressure down to 10^-8 mbar, and temperature down to 40 K, coupled to a FTIR spectrometer equipped with an MCT detector, allowing the acquisition of spectral reflectance of samples. In the first set of measurements, the sample was brought to a pressure of 10^-6 mbar, acquiring spectra. Then, it was chilled to 40 K, and additional spectra were acquired during the warming up to room temperature. The sample shows a spectral variation due to dehydration and amorphization, as confirmed later by Raman spectra. During the warming up, the spectra show no variation with temperature, which is coherent with an almost anhydrous sulfate inert to temperature variation. The second set of measurements was performed using a slower vacuum pump to better characterize the pressure dependence. The spectra [Figure 1] show that at about 140 mbar the sample starts to change its structure. However, once exposed back to air, the sample returns to its initial structure, a crystalline hexahydrite, forming a crustal structure on its surface which swells, as shown in Figure 2.  

The results shown in this abstract are a starting point to better constrain the correlation between the spectral features of planetary analogs for the icy satellites and their physical properties. From these preliminary measurements, the change in the structure of this sample tends to suggest that it is improbable to find its crystalline and hydrated form (hexahydrite) at the extremely low pressure on the surface of the icy moons (10^-8-10^-12 mbar), at least not occasionally or transient. If it was present, it could come from a subsurface liquid reservoir or even the underground ocean and should be continuously replenished. With this regard, a deeper laboratory investigation is required.

Acknowledgments:  This work has been developed under the ASI-INAF agreement n. 2023-6-HH.0.   CAPSULA setup is funded in the frame of INAF Fundamental Research Grant 2022.

References: [1] McCord et al. (2010) Icarus, 209, 639-650. [2] Piccioni et al. (2019) IEEE 5^th IWMA, 318-323. [3] Poulet et al. (2024) SSR 220, 27. [4] DeAngelis et al. (IN PRESS) Mem. S.A.It., 75, 282.