Extended Data Fig. 2 — Limits for Martian brine chemistries.

As per Figure 1, the maximum water activity of a brine that is thermodynamically stable on Mars is 0.66. This would imply that a brine with a eutectic water activity higher than this value would not readily form on present-day Mars. Another way of seeing this is in the phase diagram. Following the typical phase diagram (relative humidity vs temperature), here we show the ice line in blue (i.e., RH_ice = 100%), the temperature-dependent deliquescence relative humidity (DRH) for calcium and magnesium perchlorate, as well as the sodium chlorate hydrate line in shades of green, from light to dark respectively. In dashed black lines, we plot two isobars for water vapor pressure, showing the typical maximum water vapor pressure measured on Mars by the Mars Science Laboratory rover and Phoenix lander, as well as the maximum surface water vapor pressure predicted by the MarsWRF model. The hyperarid conditions on Mars would not permit a salt with a eutectic water activity higher than 0.66 to form (i.e., eutectic temperature >230 K). For example, at a eutectic temperature of 236 K, sodium chlorate would not form a brine because there is insufficient water vapor in the Martian atmosphere