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The high perchlorate (ClO 4  − ) to chloride (Cl − ) ratios observed at the Phoenix landing site, northern polar region of Mars, have been puzzling since detection. However, a lack of understanding of perchlorate-chloride-water systems under cryogenic conditions makes it difficult to assess ClO 4  − /Cl −  ratios during deliquescence-related processes. Here we quantitatively evaluate ClO 4  − /Cl −  fractionation in deliquescence-induced brines of magnesium- and calcium-perchlorate-chloride salt mixtures under subzero conditions, by measuring solubility data and constructing temperature-dependent thermodynamic models. We find that under specific relative humidity (RH) and temperature ( T ) conditions, deliquescence of perchlorate-chloride mixtures may form brines with fractionated ClO 4  − /Cl −  signatures. Appropriate RH -T , water-limited conditions, and aeolian processes are required to produce and preserve the elevated ClO 4  − /Cl −  signatures in soils. Under the present climate, the north polar region can support ClO 4  − /Cl −  fractionation and potentially enrich perchlorate for longer periods on global Mars. This highlights the uniqueness of Mars’ arctic environment and its implications for modern habitability.

Cryogenic origin of fractionation between perchlorate and chloride under modern martian climate