Thermal and Phase Behavior of Chloride–Perchlorate Brines on Mars: Insights from the MgCl2–Mg(ClO4)2–H2O System

Cl-bearing brines (Cl–, ClO4–, ClO3–) are critical to the Martian water cycle because they can extend the liquid water stability under present-day conditions. Natural Martian brines are likely multicomponent mixtures, yet their low-temperature behavior remains poorly constrained. Here, we investigate the binary MgCl2–H2O and Mg(ClO4)2–H2O and ternary MgCl2–Mg(ClO4)2–H2O systems to constrain cryogenic phase equilibria relevant to Mars. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) experiments are used to characterize phase behavior during controlled cooling–heating cycles and establish a low-temperature phase framework for the ternary system. All systems exhibit pronounced supercooling and metastability. In the ternary system, liquids persist metastably to ∼201 K prior to ice nucleation, while complete crystallization, encompassing ice formation and subsequent crystallization of the freeze-concentrated solution, is delayed to ∼173 K or avoided entirely through vitrification, depending on the Mg(ClO4)2 composition. Cold crystallization (crystallization of previously amorphous or partially crystalline phases upon reheating above the glass transition temperature) is commonly observed. The ternary eutectic temperature is 212 ± 0.8 K. XRD data show two final crystalline assemblages: (i) a stable assemblage of hexagonal ice+MgCl2·12H2O+Mg(ClO4)2·6H2O and (ii) a metastable assemblage comprising MgCl2·8H2O+MgCl2·6H2O+Mg(ClO4)2·4H2O. These results show that the ternary brines often produce nonequilibrium phase assemblages during cooling, where kinetic factors strongly dictate phase selection and stabilization. This study implies that without laboratory constraints, the true complexity of Martian brines, including phase assemblages and transformation pathways, may be underestimated, as equilibrium models cannot capture kinetically controlled brine evolution.