Researchers reveal how magnesium-chloride brines crystallize under icy-moon pressure

Researchers used in situ neutron diffraction — beaming neutrons at a sample to reveal its atomic structure — at the ISIS Neutron and Muon Source to crystallize a nearly saturated magnesium-chloride brine under pressures from ambient to 0.4 GPa, recreating conditions inside icy moons, and presented the work at the Europlanet Science Congress 2026 in The Hague, Netherlands, on 7–11 September 2026.1

The team found that pressure changes how the brine crystallizes. At ambient pressure, substantial supercooling — cooling below the freezing point without solidifying — drives magnesium-chloride dodecahydrate, MgCl2·12D2O, to form first. Raising pressure to just 0.1 GPa fundamentally changes the crystallization pathway, and from 0.1 to 0.4 GPa decahydrate-II, MgCl2·10D2O-II, becomes the dominant hydrate. A decahydrate-II phase also formed when the solution froze at 0.2, 0.3 and 0.4 GPa, identifying it as the stable phase across the tested pressure range at the concentration studied.2

The team found that the brine’s magnesium-chloride to heavy-water ratio was 1:10.3, close to the saturation limit at ambient pressure. Under substantial supercooling at that pressure, MgCl2·12D2O crystallized as the primary phase. The expected octahydrate phase was hard to obtain and appeared only briefly, during heating immediately before melting, pointing to kinetic complexity in the magnesium-chloride–water system. The team reported this ambient-pressure result together with the high-pressure shift.3

The team ran the experiments in titanium-zirconium gas-pressure cells with argon as the pressure medium at ISIS, reaching 0.4 GPa with accurate control during heating and cooling. Long acquisition times limited temperature resolution to about 10 degrees Celsius, yet the gas-cell approach still let the researchers follow the full crystallization sequence over a broad temperature range directly. As pressure rose, ice II crystallized at lower temperatures than the magnesium-chloride hydrate, and the eutectic temperature — the lowest temperature at which the mixture can remain liquid — increased, which the researchers link to the slight excess of water in the solution compositions they studied.4

The researchers also found a thermal equation of state for decahydrate-II: the crystal direction that compresses most easily under pressure shows the smallest expansion when warmed, a contrast in how the structure responds to pressure versus temperature. The straightforward isobaric — constant-pressure — method reliably produces decahydrate-II for later spectroscopic work that could support remote detection of this salt on icy moons.5

The team notes that magnesium chloride ranks second among the salts in Earth’s oceans and is considered highly relevant to icy-moon chemistry, while high-pressure polymorphism of its hydrates at interior pressures has received limited study. Hydrates with n equal to 1, 2, 4, 6, 8 and 12 have been reported at ambient pressure, and the first structure solved was bischofite, MgCl2·6H2O, in 1934.6

The results are relevant to missions heading to Jupiter’s moons. NASA’s Europa Clipper and ESA’s JUICE are both due to reach the Jovian system in the early 2030s, and the results help interpret geophysical data from those missions.7

The research was presented as abstract EPSC2026-510 at the Europlanet Science Congress 2026 in The Hague, Netherlands, on 7–11 September 2026 by M. Beaumont, J. Li, M. Novoveska, I. Collings, C. Bull and C. Salzmann, whose affiliations include University College London, the Natural History Museum in London, and the ISIS Neutron and Muon Facility at Rutherford Appleton Laboratory.8