A new study suggests that dark streaks on Mars are signs of flowing sand — not water.
Monday's news throws cold water on 2015 research that indicated these recurring slope lines were signs of water currently on Mars. Instead, Arizona scientists said these lines — called "recurrent slope lineae" (RSL) — appear more like dry, steep flows of sand, rather than water trickling downhill, at or near the surface.
Using a camera on NASA's Mars Reconnaissance Orbiter (MRO), researchers concluded in their study, published Monday in Nature GeoScience, that the dark streaks are only found on slopes steep enough for dry grains to descend in the manner they have on active dunes.
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"We've thought of RSL as possible liquid water flows, but the slopes are more like what we expect for dry sand," said Colin Dundas of the U.S. Geological Survey's Astrogeology Science Center, in a statement. "This new understanding of RSL supports other evidence that shows that Mars today is very dry."
Thousands of RSL have been found across 50 different locations on the Red Planet, mainly around rocky slopes.
The scientists say if water is present, it's likely a small amount — and not conducive to life.
NASA, meanwhile, says the jury is still out.
The space agency's top Mars scientist, Michael Meyer, says the latest study does not rule out the presence of water. But he acknowledges it's not as exciting as "the idea of rivers going down the sides of cliffs."