Recurring streaks on Mars not flowing water, new study says

Using NASA's Mars Reconnaissance Orbiter (MRO), researchers found the seasonal streaks on Marsexist only on slopes steep enough for dry grains to fall the way they do on the faces of active dunes
By Jackie Flores | Nov 22, 2017
In a new study of the seasonal dark streaky features on Mars that some have interpreted as being signs of possible subsurface water, researchers conclude the markings likely are due to granular flows of sand and dust slipping downhill.

The findings are published in the journal Nature Geoscience.

Using NASA's Mars Reconnaissance Orbiter (MRO), the researchers found the seasonal streaks exist only on slopes steep enough for dry grains to fall the way they do on the faces of active dunes, according to a statement by the space agency's Jet Propulsion Laboratory in Pasadena, California.

The authors used observations from the High Resolution Imaging Science Experiment (HIRISE) camera on the MRO and looked at 151 streaky features at 10 sites.

But how these flows originated and why they gradually grow is still a mystery.

The dark streaks extend gradually downhill during warm seasons and disappear in cold weather. Thousands of these features, called "recurring slope lineae (RSL)," have been observed in more than 50 areas.

"We've thought of RSL as possible liquid water flows, but the slopes are more like what we expect for dry sand," said lead author Colin Dundras of the U.S. Geological Survey's Astrogeology Science Center in Flagstaff, Arizona, in the statement. "This new understanding of RSL supports other evidence that shows that Mars today is very dry."

Almost all the RSL are restricted to slopes steeper than 27 degrees.

"The RSL don't flow onto shallower slopes and the lengths of these are so closely correlated with the dynamic angle of repose, it can't be a coincidence," said co-author and HIRISE Principal Investigator Alfred McEwem at the University of Arizona.

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