Here is how it works. Adding a solute reduces the vapor pressure of water. In a saturated solutoin the vapor pressure reduction may be very great. In addition, the solute greatly lowers the freezing point. Therefore the liquid can be cooled well below the normal freezing point of liquid water before freezing. This further lowers the vapor pressure of water in the solution. At equilibrium the vapor pressure of water from the solution is exactly equal to the vapor pressure of water in the air. There are humid places on mars, where there are clouds of water droplets or ice crystals. In such places, the vapor pressure of water MUST exceed the vapor pressure of pure water at the ambient temperature or the clouds would not form. Because the vapor pressure of the saturated solution is LESS THAN the vapor pressure of pure liquid water or ice at the same temperature, it follows that such a solution would not suffer evaporation in such conditions. Instead it would persist indefinitely.