The discovery of chlorides and perchlorates has profoundly influenced our view of liquid water on Mars (M. H. Hecht et al. 2009). These salts attract much attention because their hygroscopic nature and low eutectic temperature allow for the possibility of liquid water on the surface of Mars today (D. L. Nuding et al, 2014; D. Li et al, 2022). Recent studies showed that confined space significantly affects the phase boundary of salt solution, particularly in the freezing and deliquescence process (J. Meissner et al 2016; T. Talreja-Muthreja et al 2022). As the natural porous media, Martian soil produces such a confined environment for the brines, and influences the water adsorption and freezing behavior (A. O. Shumway et al, 2023).
In this work, we utilized silica porous materials (e.g. SBA-15, MCM-41) to mimic the soil environments, and experimentally investigate the eutectic temperature and deliquescence humidity of Mars-relevant salts (CaCl2, MgCl2, Ca(ClO4)2, and Mg(ClO4)2) in nanoconfinement. The results showed that the confinement effect on the melting points and deliquescence humidity of brines is more significant than in pure water, and the first-order phase transition of brines in narrow pores can even be suppressed completely. An ion interaction model (based on Pitzer theory and Kelvin equation) was developed to calculate the phase boundary shift of the brines in nanopores. These findings enrich our understanding of the water cycle among gas, liquid, and solid phases under Martian conditions.

Mars,Brine,Phase equilibrium,Nanoconfinement