Extended Data Fig. 2: Limits for Martian brine chemistries.
From: Distribution and habitability of (meta)stable brines on present-day Mars
As per Fig. 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 (that is, RHice = 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 vapour pressure, showing the typical maximum water vapour pressure measured on Mars by the Mars Science Laboratory rover and Phoenix lander, as well as the maximum surface water vapour 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 (that is, 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 vapour in the Martian atmosphere.
