Can Liquid Water exist on Mars? Outside of a specially crafts environment, could liquid water exist on Mars? Not some time in the distant past, but within the last few years. Mars' atmosphere is too thin for water to exist, right?

Perhaps if the water was mixed with salts or other minerals, its sublimation point might be altered enough to exist? Mars can get warm during its summer days, up to about 20 degrees Celsius at the equator. Perhaps, in certain areas and time periods, we may get a kind of liquid water?

Sorry for the speculative nature of this question, but after NASA talked about the possibility of liquid water, it got me wondering if it was possible.
Liquid water can in principle exist at many locations on present-day Mars, but there are a few interesting twists to the story.


At low elevation, the atmospheric pressure is high enough. The triple point pressure of H2O is 611 Pascal, which corresponds to about mid-elevation. At low elevations, such as the northern lowlands, the atmospheric pressure will exceed the triple point pressure, so the liquid phase of H2O becomes possible.
There will be substantial "sublimation cooling" or evaporative cooling. When the partial pressure of H2O is almost as high as the total pressure, vapor rapidly escapes into the atmosphere, which takes a lot of energy. If you put an ice cube at the equator of Mars, it will not melt, because the sublimation cooling is larger than the incoming solar energy, even at noon.
However, if that ice cube is covered with a thin layer of dust, or contains dust that will accumulate on top as the ice sublimates, vapor loss will be reduced. In this circumstance, ice can melt to form liquid water. 
That leaves the question of how the ice cube would have gotten there initially. I call it the "source problem". Places on Mars that get hot have already lost all their ice, and places on Mars that have ice do not get warm enough. So, it is difficult to get liquid water on Mars, but it is possible in principle.
With salts, which lower the melting point, everything becomes much easier, although supplies of ice/frost and salt are needed. When water with salt evaporates, the salt concentration increases, so the solution is driven toward the eutectic point, where the melting point suppression is the strongest.