The discovery of perchlorate (or more broadly oxychlorine species) is of great interest on the exploration of Mars. The strong hygroscopicity of perchlorate makes it possible to absorb water from extremely dry atmosphere and produce brines. The very low freezing point of these perchlorate brines (< -70 ℃) allows the presence of liquid water on modern Martian surface. Accordingly, perchlorates are seen as the most important salts controlling the properties of martian liquid water, which are of vital importance for understanding the habitability, bioavailability and bio-matter detection. In 2009, the Phoenix Lander launched by the US first detected high levels of perchlorate (ClO₄^-; 0.62–0.67 wt%) and high ClO₄^-/Cl^- ratios of 6.13 in three topsoil samples from the Mars arctic region (68.2188°N, 125.7492°W). Subsequent in situ investigations at other landing sites further highlighted the uniqueness of the Phoenix soils, and how the high ClO₄^- and particularly high ClO₄^-/Cl^- ratios in the topsoil of Phoenix occurred is still a mystery.

        To figure out the mystery, scientists in the Qinghai Institute of Salt Lakes, CAS promoted a project on the cryogenic phase diagrams of Martian chlorine (perchlorate and chloride) brine, and collaborated with planetary scientists from Center for Lunar and Planetary Sciences, Institute of Geochemistry, CAS, Institut de Recherche en Astrophysique et Planétologie (IRAP), Université de Toulouse, OMP-CNRS-UPS, and Institute of Space Sciences, Shandong University. Recently, their advances have been published on Communications Earth & Environment entitled “Cryogenic origin of fractionation between perchlorate and chloride under modern martian climate” (https://www.nature.com/articles/s43247-022-00345-5). The story BEHIND THE PAPER can be found at Nature Portfolio Chemistry Community.

        The work highlights the unique environment of the modern Martian arctic region, and suggested that cryogenic brine phase diagrams are essential knowledge for constraining the aqueous activities on Mars. For Mars, the study indicated that the high ClO₄^-/Cl^- ratios found in Phoenix soil are not universal on Mars, and chloride salts should be the major and primary phase of total chlorine. Moreover, a high ClO₄^-/Cl^- signature may be used to indicate deliquescence processes in cold and arid environments on Mars.

Fig. 1. Deliquescence phase diagrams of mixed hydrated Mg^- and Ca^- perchlorate/ chloride salt mixtures.

Fig. 2. Global distribution of the fractional duration in a Martian year of RH-T conditions leading to the formation of ClO₄^-/Cl^- > 0.2 during deliquescence on the surface of modern Mars.