If you’re among the many who did somersaults in 2015 when NASA announced it had found “the best evidence yet” for water flowing on the surface of Mars, you’ll probably want to sit down.
On Monday, a team of researchers from the U.S. Geological Survey published a study that suggests dark streaks spotted on the steep slopes of Mars probably aren’t moist patches that could harbor microbial life, but instead are left behind by flowing sand or dust.
The finding comes as a disappointment to scientists who had pointed to the “recurring slope lineae,” as they’re known, as a strong indicator for liquid water below the planet’s surface.
“This new understanding . . . supports other evidence that shows that Mars today is very dry,” lead author Colin Dundas said in a news release.
The study, published in the journal Nature Geoscience, is based on observations made by the Mars Reconnaissance Orbiter’s HiRISE camera. The new work rolls back earlier findings that the dark streaks were likely caused by liquid salty water that flowed when the seasons and temperatures were just right. U.S.G.S. scientists were joined in making the new analysis by researchers from the Planetary Science Institute, the University of Arizona, and Durham University in England.
"We've shown that RSL are likely granular flows, which changes our assessment of what they mean for flowing liquid water on Mars and points to formation processes with little or no liquid," wrote Dundas in an email.
All hope for liquid water—and, perhaps, microbes—isn’t lost, though. The Mars rovers have found numerous sites where liquid water likely covered the planet’s surface in the past, and the presence of perchlorates suggests that water, even if it’s not flowing, could still be playing a role in creating the streaks that have been observed.
"There's still plenty of other evidence for H2O on Mars," Dundas said. "There is a lot of ice in the subsurface and at the poles, and deliquescent salts can draw water out of the atmosphere and form liquid under some conditions. The rovers have made several discoveries pointing to a range of environments with liquid water in the past, and there are hydrated minerals bound in the rocks. But there may be little liquid at the surface today."
The best bet to solve the mystery will likely come when geologists and other scientists touch down on the planet’s equator, northern plains, or areas in southern mid-latitudes where the streaks have been observed.
"Full understanding of RSL is likely to depend upon on-site investigation of these features," Mars Reconnaissance Orbiter Project Scientist Rich Zurek of NASA's Jet Propulsion Laboratory said in a statement.
If you’re game to make the trip, there’s probably still time to get fitted for one of these.