a,b, Maps of [O2]aq according to the best estimate for the annually averaged O2 solubility [O2]aq in Ca-perchlorate brines with supercooling (BE) (a) and without supercooling (BE/-SC) (b), under modern Martian conditions with a perennial south polar CO2 cap. Without supercooling, mainly latitudes equatorward of ~50° favour liquid brines. In a, the black dashed lines mark poleward solubility values above the aerobic respiration limit for sponges. c,d, Histogram and boxplots of [O2]aq values for Ca-perchlorate brines and BE with (red) and without (purple) supercooling, and biological breathing limits (dashed lines). In d, the central mark indicates the median, and the bottom (Q1) and top (Q3) edges of the box indicate the 25th and 75th percentiles, respectively. With IQR = Q3 − Q1, the whiskers extend to Q1 − 1.5 IQR or above Q3 + 1.5 IQR, and the outliers are plotted individually as dots. Comparable results for Mg-perchlorate.