Sodium chloride hydrates provide an archive of ocean fluid freezing rates at icy worlds

Ocean-bearing icy worlds may harbour the necessary conditions for life, but we do not yet understand the processes that deliver ocean fluids to the surface where they are accessible to spacecraft. We demonstrate that hydrates of sodium-chloride (NaCl), a constituent of icy world oceans, record the thermal conditions experienced by NaCl rich fluids as they freeze. Through laboratory experiments, we identify four distinct NaCl-H 2 O assemblages, comprising crystalline and amorphous phases. The specific phase assemblage produced by freezing of brine depends on both the cooling rate and the initial brine salinity, with the production of metastable phases favoured at lower salinities and faster cooling rates. Different NaCl–H₂O assemblages exhibit characteristic near-infrared signatures that may provide a valuable tool for interpreting data collected by upcoming orbital missions to icy worlds. These results establish that the NaCl-H 2 O solid phase composition could be used as a new diagnostic probe of cryogenic processes on icy worlds, providing a means to reconstruct geological history of ocean-derived surface material and enabling space missions to assess the evolution of icy worlds across the outer Solar System