This inner slope of a Martian crater has several of the seasonal dark streaks called ‘recurrent slope lineae’ that a November 2017 report interprets as granular flows, rather than darkening due to flowing water. Credits: NASA / JPL-Caltech / UA / USGS

New research has revealed that dark features found on the surface of Mars might not indicate the subsurface flowing of water as previously believed by scientists, but could be the signs of granular flows, where grains of sand and dust slip downhill to create dark streaks.

Examination by Nasa’s Mars Reconnaissance Orbiter (MRO) has shown that these seasonal dark streaks exist only on slopes steep enough for dry grains to descend upon, similar to the way they do on faces of active dunes.

However, scientists are yet to explain the origin and evaluation of the numerous dark streaks on Mars, which are known as recurrent slope lineae (RSL).

“We’ve thought of RSL as possible liquid water flows, but the slopes are more like what we expect for dry sand.”

The newly published report suggests that small amounts of water could be involved in the formation of the RSL, as hydrated salts were observed at some of the flow sites.

Colin Dundas from the US Geological Survey’s Astrogeology Science Centre is the lead author of the report, which is based on observations made by the High Resolution Imaging Science Experiment (HiRISE) camera on the MRO.

Dundas said: “We’ve thought of RSL 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.”

RSL have currently been identified in more than 50 rocky-slope areas, from the equator to roughly halfway to the poles of the Martian land.

Nasa Jet Propulsion Laboratory MRO project scientist Rich Zurek said: “While the new report suggests that RSL are not wet enough to favour 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 characterised.

“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.”