Fig. 3: Deliquescence-induced ClO4−/Cl− fractionation at the Phoenix landing site with Mg(ClO4)2•6H2O + MgCl2•6H2O and Ca(ClO4)2•6H2O + CaCl2•6H2O salt mixtures.
From: Cryogenic origin of fractionation between perchlorate and chloride under modern martian climate
a For the Mg(ClO4)2•6H2O + MgCl2•6H2O salt mixture, the deliquescence surface (i.e., color surface represented by the legend) intersect with the RH-T conditions (i.e., cyan surface: the best fit line of the scattered RH-T data in situ determined by Phoenix lander and independent of ClO4−/Cltotal) in a very small region (i.e., the cyan dot), with a ClO4−/Cl− molar ratio of 2.5. The RH is calculated based on RH% = 100•aw by the thermodynamic model. b For the Ca(ClO4)2•6H2O + CaCl2•6H2O salt mixture, the deliquescence surface (i.e., color surface represented by the legend) intersect with the RH-T conditions (cyan surface) in a relatively large region (i.e., cyan trapezoid), with ClO4−/Cl− molar ratios ranging from 9.0 to 13.3. Starting with Mg- and Ca- salt mixtures that are initially poor in perchlorate (e.g., yellow dots as ClO4−/Cl− molar ratio = 0.11), deliquescence would simultaneously produce a nearly pure chloride salt (on the left arrow) and a ClO4−-rich brine (on the right arrow to the dark cyan intersection). The maximum ClO4−/Cl− molar ratios of deliquescence-induced brine are determined by the types of parent salt mixtures, regardless of the initial ClO4−/Cl− ratios in the salt mixtures. See Supplementary Fig. 16 for additional discussion on the ternary phase diagram and the projections.
