Geometry-induced variations and effects on the remote-sensing identification of re-hydrated magnesium sulfates on the Galilean Icy-Moons

Introduction. Hydrated salts such as magnesium sulfates are expected to be found at the surface of Europa [1], possibly originating from hydrothermal activity in the moon’s subsurface ocean. To support the identification of these compounds from remote-sensing instruments data of the icy moons, such as from MAJIS [2,3] onboard the ESA JUICE mission [4], it is of paramount importance to investigate the spectral response of these compounds across different experimental conditions [5]. Here, we focus on the spectral changes of hexahydrite (MgSO4 · 6H2O) upon dehydration via vacuum processing and subsequent re-hydration, investigated at various optical configurations to optimize re-hydrated magnesium salt detection via remote sensing.

Materials and Methods. We produced a 13-mm diameter pellet of approximately 1-mm thickness by pressing 300 mg of hexahydrite powder with grain size between 75 and 100 µm for 1 minute under a pressure of 10 tons (fig.1). We acquired spectral reflectance infrared data from 0.8 to 15 microns of the pellet’s surface at various geometric configurations using a Bruker FTIR-spectrometer coupled to a goniometer. A set of 15 different geometric configurations have been acquired with illumination angle i = 0°, 30°, 40°, 60, and a variety of emission angles. The pellet was put in a vacuum chamber, and the pressure was decreased progressively for 2 hours, down to P ~ 2.3 mBar. Ambient air was then progressively reintroduced in the chamber until the pressure in the vacuum cell reached 1 Bar. The pellet was then exposed to the air for several days. The spectral evolution of the pellet’s surface during the pressure decrease and its re-exposure to ambient air was followed via a Bruker Hyperion FTIR microscope. After this period of exposure, the pellet underwent the same spectroscopic geometric characterization described before to assess if and how the spectroscopic features of hexahydrite behave differently at various geometric configurations before and after the vacuum processing.  

Figure 1. hexahydrite pellet before vacuum processing.

Preliminary results and discussion. Results indicate that the hexahydrite pellet does not fully rehydrate after exposure to the ambient air for multiple days. Moreover, the changes in the Reststrahlen bands upon vacuum-driven dehydration suggest that the dehydration was coupled with crystal lattice amorphization (fig.2). The optimal geometric configuration to detect this amorphization, as well as other spectroscopic changes related to the vacuum processing, is still to be determined and data analysis is still ongoing. 

 

Figure 2. The S-O stretching and S-O bending features decrease in intensity and widen after vacuum processing, which is compatible with lattice amorphization.

Acknowledgments. This work has been developed under the ASI-INAF agreement n.2023-6-HH.0.

Bibliography. [1] Dalton J. B. et al. 2005. Icarus 177 (2): 472–90. [2] Poulet F. et al. 2024. Space Sci Rev 220. [3] Piccioni G. et al. 2019. IEEE. pp. 318–323. [4] Grasset O. et al. 2013. Planet Space Sci. 78: 1–21. [5] De Angelis S. et al. 2017. Icarus 281 (January):444–58.