"We've thought of RSL [recurring slope lineae] as possible liquid water flows, but the slopes are more like what we expect for dry sand".
"This new understanding of RSL supports other evidence that shows that Mars today is very dry", Dundas added.
The research team led by Colin Dundas created 3-D models of the steep slopes using the HiRISE images of 151 RSL features from 10 different sites.
"What we found in this paper is that they end on slopes that match the "angle of repose" for dry sand, which is the slope that you get if you heap up dry sand and it slides under its own weight", he explains.
The discovery of the seasonal flowing feature also known as "recurring slope lineae" or RSL in 2011 had evoked fascination and controversy and had been believed as a marker for liquid water flow or brine in a planet which was otherwise considered as dry.
But researchers at the USGS say these features look identical to certain types of slopes found on sand dunes here on Earth.
Three years ago, scientists found evidence suggesting that the dark streaks on Mars' surface were formed by liquid water, leading some to theorize that the planet may have been home to microbial life. Plus, the dark streaks seem to flow out of the tops of the hills, but water probably wouldn't sprout out of the the tops of slopes at these angles, he says. It supports earlier theories that surface of modern Mars which had been exposed to cold, thin atmosphere, lacks flowing water.
However, RSL remain puzzling.
Darkening and fading might result from changes in hydration.
"RSL probably form by some mechanism that is unique to the environment of Mars", McEwen says, "so they represent an opportunity to learn about how Mars behaves, which is important for future surface exploration".
On top of that, if water were present it would surely appear on the lower slopes, rather than just near the top. That's bad news in the hunt for microbes, unfortunately. Small amounts of water could still be involved in their initiation in some fashion, as hydrated minerals have been detected at some RSL locations. "In particular, a full explanation of how these enigmatic features darken and fade still eludes us". JPL, a division of Caltech in Pasadena, California, manages the MRO project for the NASA Science Mission Directorate in Washington.