Some fascinating papers
The Phoenix reports are interesting. Some of the papers are speculative or sheer guesswork, but they begin to describe the problems in interpretation of the soil behaviour and suggest roles for highly concentrated brines - at least seasonally.
I don't think these are the final answer, but there are some intriguing possibilities.
What is clear is that IF "water" has a role on Mars, it is far more likely in a near-polar and strongly seasonal environment than at the equator (for the current orbital parameters, of course).