Figure 1: Water activity of stable brines over Mars-relevant temperatures and water vapor pressures.

Over the range of possible global surface temperature and water vapor pressure on Mars, the resulting water activity of an arbitrary stable solution is shown here in color. The Mars-relevant pairings of surface temperature and water vapor are contained between the cyan lines (see Extended Data Figure 1 for details). The solid blue line is where saturation with respect to ice is achieved (i.e., where relative humidity with respect to ice is RH_ice =100%). However, depending on the eutectic temperature of the brine, a metastable liquid and ice solution may be present. Here we show the eutectic temperature of a calcium perchlorate brine (dashed blue line) as an example. Only ice will exist below the eutectic temperature of the brine. Additionally, the Special Region lower limit temperature² (T =255 K) is noted by the green dashed line.