It's known that Mars has both ice and water vapor, and that further, summertime temperatures are warm enough to sustain liquid water.
It's the atmosphere that makes this a bit surprising. Martian atmospheric pressure is less than 1% that of earth in the deepest valleys, and about .006 atm on average. If you look at a phase diagram for water [0], you can see that at .006 atm and below, water goes directly from solid to gas ("sublimation"). Even slightly above that pressure, there's only a tiny temperature range in which liquid water will exist.
I would guess that's why they've theorized very salty water. Its lower freezing point is going to allow for liquid water in a bigger temperature range at typical Martian atmospheric pressures.
IIRC, Mars's global atmospheric pressure is about 25% higher in the southern summer than in the southern winter, due to differential sublimation and condensation of carbon dioxide, from the southern ice cap as well as from the soil at lower latitudes, and southern hemisphere summer temperatures can range all the way above zero celsius -- so there's quite a range of weather conditions to which any briny water would be subjected, especially in the southern hemisphere.
Edit to add, the Martian southern hemisphere seasons are more extreme because the effects of its significant eccentricity and its axial tilt are additive there, while they counteract each other to moderate the seasons in the north.
It's the atmosphere that makes this a bit surprising. Martian atmospheric pressure is less than 1% that of earth in the deepest valleys, and about .006 atm on average. If you look at a phase diagram for water [0], you can see that at .006 atm and below, water goes directly from solid to gas ("sublimation"). Even slightly above that pressure, there's only a tiny temperature range in which liquid water will exist.
I would guess that's why they've theorized very salty water. Its lower freezing point is going to allow for liquid water in a bigger temperature range at typical Martian atmospheric pressures.
[0] http://www.cims.nyu.edu/~gladish/teaching/eao/week2.html - top of the page