re: Some History...
The polar regions are not the most likely places to find liquid water.
But there will certainly be surface or near surface ice there to the depths Phoenix will dig down to. So let's make a hypothetical.
Suppose the temperature at the landing site does reach the 250K I cited. And suppose there are abundant salts on the surface as has been seen at every Mars landing site. And keep in mind the relatively low elevation of the northern hemisphere sites keeps the atmospheric pressure above the 6.1 mbar triple point for water there year round.
What do you think will happen to that ice mixed in with salt at that temperature at that pressure?
Bob Clark