At left, the animation which serves as evidence, two photos of a trench dug by the lander, taken four days apart. Note the little bits of material in the shadowy lower left trench which disappear; this is behavior consistent with frozen water ice sublimating to vapor in the low-pressure Martian atmosphere. The temperature is too far above the boiling point of CO2 at this time of the Martian year, and clods of salt would not disappear like that on a clear day without wind. More study is needed, of course, and Phoenix will continue to dig and test soil samples for water and organic chemicals, but the Phoenix team seems to be pretty sure.
If that is indeed water, then it has massive implications for Mars’ past and future — a liquid water past may have facilitated the evolution of life similar to Earth’s, and easily accessible water will be important to future manned missions to Mars. This has been theorized, and its secondhand effects observed in the past, but now there is evidence — literal solid evidence — that there is water underground at Mars’ polar regions.