Salty SeaFreeze: new experimental efforts toward a comprehensive thermodynamic representation for icy moons hydrospheres.

The water-rich layer of icy moons of the outer solar system contains a variety of organic (e.g. CH₄, NH₃, CO₂) and inorganic (e.g. NaCl, MgSO₄, Na₂SO₄, MgCl₂) solutes detected by robotic mission like NASA Cassini and Galileo probes. Many of the geological processes on these moons, like cryovolcanisms, tectonics, oceanic evolution, involves processes of partial melting and fractional crystallization that are controlled by  the stability of solid phases (ices and hydrates) and their eutectic. The ices and pure water thermodynamic are now accurately constrained at the high pressure and low temperature found inside these moons by the SeaFreeze numerical representation. However, the thermodynamics and equilibria in salty aqueous systems relevant for icy moons remains underexplored. SeaFreeze is used broadly in the planetary science community and beyond, the next logical step is the representation of the effect of salts (e.g. anti-freeze effects, eutectics, density, formation of hydrates, water activity), that requires important experimental effort. We will discuss new results from sound speed measurements, isochoric freezing and in-situ single crystal X-Ray diffraction experiments at high pressure and low temperatures that allowed us to constrain the equilibria (eutectics and phases stability) as well as solution thermodynamics in the H₂O-(Na^-,Cl⁺,Mg²⁺,SO₄^2-) systems. We will present how these new data are implemented into SeaFreeze to allow for a comprehensive representation of water-salts systems from the cryogenic conditions at the surface to the incredible pressures found in icy worlds. We will discuss the implication for chemical physics/mineralogy as well as for the upcoming exploration of icy worlds of our solar system by NASA and ESA robotic space missions.