Fingers of dark material running down steep slopes in the warmest regions of Mars could be the result of salty water flows, say scientists interpreting repeat observations snapped by Mars Reconnaissance Orbiter's (MRO) powerful HiRISE camera.

The dark stripes, which are on the order of a few metres wide and several hundred metres long, were identified on several steep equator-facing slopes in the middle latitudes of the red planet's southern hemisphere. The features are different to the much wider gullies already seen on many cold, pole-facing slopes, and appear to fade during the winter months, reappearing in springtime. In some locations more than 1,000 individual flows have been identified, some of which had grown by more than 200 metres in just two Earth months.

“But the sites themselves are rare,” says Alfred McEwen of the University of Arizona's Lunar and Planetary Laboratory, principal investigator of HiRISE and lead author of a paper reporting the results in the latest edition of the journal Science. “We have seven individual sites confirmed, with 20 candidates waiting to be confirmed in repeat imaging.”

In one location, the pattern is repeated over three martian summers. “A volatile material is likely involved in the activity,” says McEwen. The scientists suspect the flows might be briny, which is consistent with the detection of salt deposits in many locations over Mars’ surface. Salt lowers the freezing temperature of water, so although the temperatures observed would not melt pure water ice, the salt content of this fluid – saltier than Earth's oceans – is enough to keep the water liquid. Yet MRO's Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) has, to date, not found any indication of water at these sites.

"It's a mystery now, but I think it's a solvable mystery with further observations and laboratory experiments," says McEwen. “It’s the first time we’ve seen the potential for water; it might be salty, but it’s liquid.”

On Earth, where there’s water, there’s life, and NASA’s mantra of “follow the water” to find life has stepped up a gear with this discovery. Evidence for ancient rivers and lakes exist all over Mars, and sites which potentially host liquid brines will help in the quest to determine whether life exists, or once existed, on the planet. Frozen water is widespread, and ice has been detected in freshly formed shallow impact craters and seen in trenches dug by the Phoenix Mars Lander near the planet's north pole, for example. Furthermore, beads of droplets seen on Phoenix's landing struts were argued to be globules of brine-rich liquid. Landslides seen in gullies are also thought to have been facilitated by flowing water.

“The mid-latitudes are turning out to be the place on Mars where a lot of the action is,” says Philip Christensen, a geophysicist at Arizona State University. “The new discovery provides evidence that water is closer to the equator than previously thought, which has the potential to be liquid at various times throughout the year. We’re now at the beginning of a scientific process to probe the details.”

Until that crucial direct detection is made, this is surely the best evidence yet of liquid water flowing on Mars.