Spectroscopic measures of hydrated sulfate as a relevant planetary analogue for Jupiter’s icy moons

 Introduction:  On the surface of Jupiter's icy moons Europa and Ganymede, non-water ice materials are mixed with water ice at different proportions. Hydrated minerals can mimic the 1.5 and 2.0 μm water ice absorption bands. In particular, the presence of hydrated magnesium sulfate, such as hexahydrite (MgSO₄∙6H₂O), was first suggested on the surface of these icy bodies based on Galileo/NIMS data, which could be a remnant of past extrusion of liquid water from an ocean below [1]. Although other compounds, particularly chloride salts, have subsequently been proposed to explain spectral signatures observed by NIMS, the key interest for future close exploration is to understand whether these non-water ice materials are distinguishable in mixtures with water ice, and whether they are of exogenic or endogenic origins. Characterizing such minerals under environmental conditions similar to those found on the icy moons' surfaces is therefore crucial, especially considering the link between the presence of hydrated salts on Solar System bodies and geological processes that occurred in the presence of liquid water, with potential implications for the establishment of a habitable environment. 

This work is also important to support the future observations of the MAJIS (Moons and Jupiter Imaging Spectrometer) instrument [2,3] onboard the ESA JUICE mission. Although similar work on hexahydrite was carried out previously [4,5,6], the MAJIS spectral sampling, which is 3.6 nm from 0.50 to 2.35 μm and 6.5 nm from 2.25 to 5.54 μm, motivates performing new laboratory measurements at higher spectral resolution.

 

Experimental procedure and results:  For this objective, we prepared powders of hexahydrite at different grain sizes, ranging from 50 to 500 μm, to study the variation of the spectral features in the infrared and visible spectral ranges depending on the environmental conditions. The set of measures described in this abstract was performed as a deeper investigation following the measures taken with a different setup, CAPSULA [7]. We acquire both the reflectance and the visible image of the sample with a microscope coupled with an FTIR spectrometer and equipped with a cryogenic cell [Figure 1] that contains the sample in a controlled environment, which can go down to a pressure of 10^-4 mbar, and a temperature of 40 K. Different grain sizes of the sample were put inside the cryogenic cell thanks to a custom sample holder which allows the acquisition of different grain sizes during one single data taking [Figure 2], and brought to a pressure of 10^-4 mbar while acquiring spectra during the process. The reflectance spectra and the computed spectral parameters show that there is more than one critical pressure at which the sample starts to change part of its lattice structure. Moreover, different grain sizes respond differently to the pressure variation, as shown in Figures 3 and 4.

Figure 1: Experimental setup composed of a cryogenic cell that allows a controlled environment where we put the sample, and a microscope coupled with an FTIR spectrometer that enables both the acquisition of the spectral radiance and the visible image of the samples.

Figure 2: Custom sample holder placed inside the cryogenic cell. It contains 4 different grain sizes of hexahydrite.

Figure 3: Infrared reflectance spectra of different grain sizes of hexahydrite powder at room pressure and room temperature.

Figure 4: Close-up of the 2 microns water diagnostic absorption band in the continuum-removed reflectance spectra of a hexahydrite powder with a grain size between 50 and 75 microns at different pressures and room temperature. The sample shows a first spectral variation at around 100 mbar and a second one between 5 and 10^-1 mbar.

Conclusions:  The results shown here may constrain the correlation between the spectral features of this kind of material, planetary analogs for the icy satellites, and their physical properties. One of the most interesting aspects we came across is a change in the lattice structure of this sample, which seems incompatible in its crystalline and hydrated form (hexahydrite) at the extremely low pressure on the surface of the icy moons, that is, in the range 10^-8 -10^-12 mbar. In the laboratory, the process of amorphization and dehydration with vacuum occurs in a timescale of tens ofminutes at room temperature. Therefore, if hexahydrite was present on these bodies, it should be continuously replenishedor ephemeral, and this may sustain the subsurface liquid water as a possible source. On thesurface of Europa and Ganymede, this type of hydrated salt could eventually bepreserved thanks to some mechanism that involves the rapid emplacement in simultaneoussurface conditions of low temperatures and ultra-high vacuum. In this regard, a deeper laboratory investigation is required. 

These results are extremely important to avoid any ambiguity in the determination of the surface composition of the icy moons of Jupiter once the data MAJIS will acquire is available, and could also allow some constraints on what could be present underneath the surface.

References:

[1] T.B. McCord et al., Icarus, 209, 639-650 (2010)

[2] G. Piccioni et al., IEEE 5th International Workshop on Metrology for AeroSpace, 318-323 (2019)

[3] F. Poulet et al., Space Sci Rev, 27, 220 (2024)

[4] T.B. McCord et al., JGR, 104, 11827-11851 (1999)

[5] J.B. Dalton et al., Icarus, 177, 472-490 (2005)

[6] S. DeAngelis et al., Icarus, 281, 444-458 (2017)

[7] De Angelis S. et al. (IN PRESS) Mem. S.A.It., 75, 282.

 

Acknowledgements: This work has been developed under the ASI-INAF agreement n. 2023-6-HH.0. This work is supported by EU and Regione Campania with FESR 2007/2013 O.O.2.1