Slopes that lighten and darken could mean ice or a damp patch of dirt. (so they say).
While writing my novel, all my research indicated that water would evaporate immediately whenever exposed to the Mars atmosphere. The freezing temperatures, lack of atmosphere, lower gravity, etc. So this report caught my attention. Somehow, they feel salt water has a better chance of not evaporating so easily and perhaps in the spring it lasts longer. My guess is it's coming from underground when the temperatures warm up, leaving a damp spot, and when it evaporates it may leave some salt behind.
I'm now thinking of doing a scene with water underground in the form of a stream for the sequel to Vallar. I think it's possible as long as the area is sealed, which wouldn't be hard to explain.
Click here for an article about salt water on Mars.