On the discovery of hyper-hydrated sodium chloride hydrates, stable at icy moon conditions.

Sodium chloride is expected to be found on many of the surfaces of icy moons like Europa and Ganymede. However, spectral identification remains elusive as no known NaCl hydrates can match current observations. Working at relevant conditions for icy worlds, we discover three new "hyperhydrated" NaCl (SC) hydrates, and refined two crystal structures (2NaCl·17H2O (SC8.5) ; NaCl·13H2O (SC13)). At these conditions, the dissociation of Na+ and Cl- ions within these crystal lattices allows for the high incorporation of water molecules and their hyper-hydration. This suggest that a great diversity of new hydrated crystalline phases of common salts might be found at similar conditions. Thermodynamic constraints indicate that SC8.5 is stable at room pressure below 235 K, implying it should be the most abundant NaCl hydrate on icy surfaces of the outer solar system and beyond. This discovery enables to explain the mismatch between observations and data on NaCl hydrates and the urgent need for new spectral data on hyper-hydrates.