Near-Infrared Reflectance (NIR)/Raman combined-spectra of NaCl and MgSO4 brines at airless ocean worlds surface conditions

Europa and Ceres are among the main ocean worlds considered by ESA and NASA as astrobiological targets [1]. Past space missions such as Galileo and Dawn [2, 3], and more recently telescope observations (e.g., [4]), are uncovering evidence for the existence of interior salty oceans and/or extensive aqueous reservoirs, which act as a source of cryomagmatic activity. Among other substances, NaCl and MgSO₄ have been suggested as important cryomagmas constituents (e.g., [5, 6]).  Depending on the cryomagma source and eruption style, cryomagma may be released either explosively or more slowly through progressive ascent and flow at the surface [7]. The final mineral associations will depend on the ascent cryomagma from the interior and on the cooling rate once exposed at the cold space. Once at the surface, the environmental change to ultra-low pressure (in airless worlds), change in temperature and radiation, which can vary spatially, will make the final mineral associations continue to evolve over time.

In the present study, we investigate the Infrared and Raman characteristics of frozen cryolava analogues, considering fast or slow cooling rate and post-eruption sublimation processes.  Our main goal is to generate new spectroscopic databases to help with the interpretation of data from space missions; including those of the coming JUICE and Europa Clipper missions. Here we report the behavior of NaCl and MgSO₄ frozen brines, both in low (≤ 5 wt.%) and eutectic concentrations, under low pressure conditions (10^-5 bar) and at changing temperature (80 - 200 K).  Different preparation method (with different cooling rates), resulting in different grain size distribution (>100 microns, ~ 40 microns, and ~ 25 microns). The samples have been then monitored by macro-NIR (1-5 microns) and micro-Raman spectroscopy, with the objective of associating a NIR spectrum at a resolution similar to flight instruments with a detailed mineralogical evolution.  For NaCl icy samples, we showed the Infrared and Raman spectral signatures of hydrohalite significantly vary depending on the preparation methods and after a sublimation cycle, while for MgSO₄ icy samples, much less variability is observed. For NaCl icy samples, we identified new absorption features in the 2.5-5 microns range, which was not investigated by previous studies [8, 9, 10].  The combination of both techniques has allowed us to potentially detect metastable glass and crystalline phases (e.g., [11, 12]), monitor alteration/amorphization processes, and address the effect of salts on ice grain size decrease after partial sublimation.

ACKNOWLEDGEMENTS

The authors acknowledge support from the Marie Curie Postdoctoral Fellowship program (HORIZON-MSCA-2022-PF-01), grant nº 101105979 - SECRECY.

REFERENCES

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