Water trickles down gullies and crater walls on the surface of Mars, raising intriguing questions about whether life could exist on the red planet

Water trickles down gullies and crater walls on the surface of Mars, raising intriguing questions about whether life could exist on the red planet

The findings are the strongest evidence yet that liquid water darkens the Martian surface today; we've known for years that water flowed on Mars long ago

Described Monday in Nature Geoscience, the observations suggest that this water appears seasonally, forming dark lines as it tumbles down steep Martian slopes. But scientists still don’t know where the water is coming from, or if the chemistry is even right for supporting life

For now, the find solves the mystery of dark streaks that appear and disappear seasonally, called recurring slope lineae

The streaks were first spotted in 2010 by Lujendra Ojha, then an undergraduate at the University of Arizona. Ojha was studying images returned to Earth from the HiRISE camera, aboard NASA’s Mars Reconnaissance Orbiter. At the time, he says, he had no clue how important his observation would be

A big question swirls around the origin of that water: Where is it coming from? One possibility is that the seeps are fueled by an aquifer or melting subsurface ice. These scenarios would have Mars essentially sweating, with saltwater seeping from its pores and trickling down slopes as the planet warms

The water might also be atmospheric in origin, which is the hypothesis the team seems to favor. In this scenario, surface salts absorb water vapor in the Martian atmosphere

But until now, evidence has been scarce for flowing water at the surface today. What it means, in the bigger picture of planetary exploration and the search for life beyond Earth, is still a mystery