In the long hunt for water on Mars, researchers may have finally caught sight of flowing liquid.

High-resolution photographs from NASA's Mars Reconnaissance Orbiter (MRO) show dark, transient features on slopes in several midlatitude locations in the southern hemisphere. The features have appeared in Mars's southern spring across multiple years since the probe entered orbit in 2006, grow in length as they extend downhill, and then fade in late summer or early fall. The new features, which carry the purposefully uncontroversial moniker of recurring slope lineae, or RSL, were announced in a study in the August 5 issue of Science.

The cause of the RSL is unknown, but liquid brines just below the surface could well fit the bill. "We're not ruling out any reasonable explanation at this point," says lead study author Alfred McEwen, a planetary scientist at the University of Arizona in Tucson. The researchers have explored nonliquid processes such as rock slides, but McEwen notes that the timing of the surface markings' appearance and growth seems incompatible with dry phenomena. "There may be someone more clever than us that does come up with something," he says.

Until that happens, however, flowing water is a compelling explanation—and one that opens the door to speculation of life in the Martian soil. "This is the most promising place for water [on Mars] in some place that isn't too cold for life as we know it—and for water that is flowing," McEwen says. The southern slopes where the RSL occur are relatively temperate for the Red Planet, with peak temperatures that rise above 0 degrees Celsius in the summer.

Mars has no shortage of water in the form of ice buried below the surface. And planetary scientists think that the Martian landscape had liquid water billions of years ago. Several lines of evidence for more recent or even extant water on Mars have arisen in the past few years, including a controversial claim that NASA's Phoenix lander photographed droplets of water on its own struts in 2008. Phoenix briefly explored the frigid Martian arctic, so any water it may have encountered must have contained salts and chemicals called perchlorates to enable a very low freezing point.

If the slope streaks photographed by MRO, which occur in warmer locales, are indeed caused by water, that liquid would also be salty. "Mars is just a very salty place," McEwen says. "Any water that flows at the surface or subsurface gets salty." Brines of magnesium chloride, sodium chloride, calcium chloride or iron sulfates all have freezing points that match the MRO observations.

A spectrometer on board MRO was unable to identify the spectral signature of water from the RSL regions, so McEwen and his colleagues suspect that the ground itself is dry and that any liquid flows occur just below the surface.

James Head, a planetary scientist at Brown University who did not contribute to the new study, notes that the question of whether changing surface features on Mars are associated with water or watery brines has beleaguered planetary science for decades. The new discovery, he says, "brings us much closer to understanding the role of such liquids in shaping Mars, even in the current environment."

The potential signature of liquid water may have gone undetected in the orbiter's voluminous catalogue of data and imagery had it not been for the work of a University of Arizona undergraduate. Lujendra Ojha was working to identify changes in images taken by MRO of the same locations at different times when he noticed the transient markings on southern slopes.

"We had them hiding in our images for years without our noticing them," McEwen says. "It's hard to appreciate their significance until you see the images separated by a large amount of time."