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 appear more like dry, steep flows of sand, rather than water trickling downhill, at or near the surface.
The scientists say if water is present, it's likely a small amount -- and not conducive to life.
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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."
Researchers have studied imagery provided by a powerful camera on NASA's Mars Reconnaissance Orbiter. The new study supports the possibility that the dark streaks seen in the images are caused by sand and dust slipping downhill.
The Red Planet features were discovered in 2011 and appear to descend gradually downhill in warm seasons, fade in winter, then reappear the next warm season. How these "recurring slope lineae" (RSL) form remains a mystery to researchers.
"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 Flagstaff, Arizona. "This new understanding of RSL supports other evidence that shows that Mars today is very dry."
Dundas is the new study's lead author. His team examined 10 sites with a total of about 150 RSLs.
"Full understanding of RSL is likely to depend upon on-site investigation of these features," said scientist Rich Zurek of NASA's Jet Propulsion Laboratory in Pasadena, California. "While the new report suggests that RSL are not wet enough to favor microbial life, it is likely that on-site investigation of these sites will still require special procedures to guard against introducing microbes from Earth, at least until they are definitively characterized. In particular, a full explanation of how these enigmatic features darken and fade still eludes us. Remote sensing at different times of day could provide important clues."