It is known that Mars has frozen water at its poles and water vapour in its atmosphere. Today researchers associated with NASA's Mars Reconnaissance Orbiter have identified flowing water on Mars' surface.
Imagery of Mars' surface shows dark streaks that reappear periodically on slopes. These streaks, which appear and grow in the downslope direction during warm seasons when temperatures reach about 250–300 K, are called recurring slope lineae (RSL). By analyzing spectral data from the Compact Reconnaissance Imaging Spectrometer for Mars instrument onboard the Mars Reconnaissance Orbiter researchers have found evidence for hydrated salts at four separate locations on Mars' surface. They have found that the hydrated salts occur during warm seasons when RSLs are most extensive. This supports the hypothesis that the RSLs represent flowing briny water. The salts in the water prevent it from freezing at temperatures of 250–300 degrees K.
Source: Spectral evidence for hydrated salts in recurring slope lineae on Mars, Lujendra Ojha, Mary Beth Wilhelm, Scott L. Murchie, Alfred S. McEwen, James J. Wray, Jennifer Hanley, Marion Massé & Matt Chojnacki, Nature Geoscience(2015) doi:10.1038/ngeo2546, Published online 28 September 2015.