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Discovery of new NaCl hydrates stable at icy moon conditions using X-ray diffraction

X-ray diffraction was used to probe the structure of salt water subjected to the extremely high-pressure and low-temperature conditions of icy moons. The samples crystallised into two previously unknown forms of salt hydrate, increasing our knowledge of the surface mineralogy of icy satellites.

Sodium chloride (NaCl), also known as table salt, is the prototype for ionic solids and aqueous electrolytes. The H2O NaCl system is the reference binary system in thermodynamics, physical chemistry, geosciences, medicine and engineering applications. Aqueous salt systems have been explored extensively at ambient pressures and at high pressure and high temperature due to their relevance in geofluids and biology, but have remained poorly studied at < 300 K and at elevated pressures due to the lack of scientific incentives. This has changed with the discovery of deep, cold and salty extra-terrestrial oceans in the icy moons of Jupiter and Saturn. This has created a paradigm shift in planetary science as these oceans represent the most promising targets for habitable environments in our solar system. These moons and their oceans will be explored by ESA s JUpiter ICy moons Explorer (JUICE) launched in 2023, and NASA s Europa Clipper and Dragonfly robotic missions, launching in 2024 and 2027 respectively. Before this study, the only known hydrate in the H2O NaCl system was hydrohalite, NaCl 2H2O (SC2), forming below the eutectic at 252 K and 23.3wt% NaCl. Interestingly, despite

NaCl being predicted as one of the major solutes in icy worlds, hydrohalite has never been identified in surface spectral data from previous missions such as NASA Galileo or NASA/ESA Cassini-Huygens. This suggested that NaCl might exist in other unknown mineral species on the cold surface of icy worlds.

To explore and characterise the H2O NaCl system at icy world conditions, this study employed in-situ high- pressure diamond anvil cell apparatus coupled with a helium cryostat and single-crystal X-ray diffraction at beamline ID15B. Systematic identification and characterisation of phases were performed down to 150 K and up to 2.5 GPa, covering the entire range of conditions expected in hydrospheres of icy moons of the solar system [1]. This permitted the discovery of three new phases of NaCl hydrates and the determination of the refined crystallographic structure of two of them. The first is the disodium chloride decaheptahydrate, 2NaCl·17 H2O (SC8.5), and the second is the sodium chloride decatriahydrate, NaCl·13 H2O (SC13). The third phase has a monoclinic structure (C2/c) for which only lattice parameters were obtained.

Hydrohalite, and most alkali halindes hydrates, have a low number of waters of hydration (< 5) at ambient conditions, with ions still ionically associated in the structure (Figure 108, left). Structurally, SC8.5 and SC13 are remarkable due to their fully dissociated ions in the structure (Figure 108, middle and right), with Na forming Na(H2O)6 octahedrons more characteristic of hydrates of inorganic molecular salts such as Na2SO4, MgSO4, NaPO4, etc. These results suggest that pressure

Fig 108: Crystallographic structures of hydrohalite (left). The two newly discovered sodium chloride hydrates, SC8.5 (middle) and SC13 (right).