Water flows on present-day Mars

Enlarge / 3D perspective showing some of the seasonal features that appear in the Hale Crater on Mars. (credit: NASA/JPL/University of Arizona)

About five years ago, scientists noticed something unusual on Mars. Images taken at different times of the Martian year by the Mars Reconnaissance Orbiter's HiRISE camera showed dark areas developing on steep slopes in the summer and then vanishing during the Martian winter. The pattern of these dark areas looked like what you'd expect from a liquid flowing downhill.

At the time, water seemed like an obvious explanation for the dark areas, which would make this the first evidence of liquid water in the present of Mars. But to confirm this theory, scientists needed to get a reading on the chemical composition of the dark streaks. Now, researchers have overcome some major technical hurdles to get these readings, and the results indicate that the streaks contain water-rich salts.

The dark features have picked up the name "recurring slope lineae," or RSL. They appear on steep slopes, such as crater walls, and form braided patters that look like water flows. RSL are absent in the winter and spring and only appear near the Martian equator during the summer, at which point the temperatures in the area can often climb above the freezing point of water. Any liquid water should evaporate into the sparse Martian atmosphere quickly, but dissolved salts will inhibit evaporation and can lower the freezing point of water by as much as 80K.

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