Dark streaks on Mars created by flowing sand, not flowing water

These sites likely do not contain sufficient water to host microbial life.
By Laurel Kornfeld | Nov 21, 2017
Dark, linear streaks on the Martian surface initially thought to have been created by flowing water were actually produced by flowing sand, according to a new study of the features conducted with the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter (MRO).

First discovered in 2011, the steep, dark patterns, which appear during the spring and summer and fade away during the fall and winter, were viewed as having been created by flowing liquid water, much the way similar features are created on Earth.

Labeled "recurring slope lineae" or RSL, the dark streaks, which extend downhill, have been found in more than 50 rocky-slope areas on Mars between the equator and halfway toward each of the poles.

Almost all RSLs are seen in areaswith slopes of 27 degrees or higher, and their flow patterns resemble those of granular objects, such as grains of sand and dust, rather than those of liquid water.

"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," said Colin Dundas of the US Geological Survey's Astrogeology Science Center in Flagstaff, Arizona.

HiRISE Principal Investigator Alfred McEwen of the University of Arizona at Tucson noted, "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."

Angles of repose are the deepest areas of descent that granular material can make relative to the horizontal plane without sliding or slumping.

In contrast to granular materials, liquid water would flow into less steep slopes.

While a previous study called into question the notion that subsurface water is present at RSL sites, much about these features remains unknown, including their seasonal appearance and disappearance and the fact that they contain hydrated salts whose crystal structures contain bound molecules of water.

According to a study on the findings published in the journal Nature Geoscience, salts can pull water from the thin atmosphere and become hydrated. Changing hydration levels in these salts may trigger the expansion and/or contraction of RSL features as well as their darkening and fading.

A major conclusion drawn from this study is that RSL sites do not have sufficient water for microbial life to survive.

"Full understanding of RSL is likely to depend upon on-site investigation of these features," emphasized MRO Project Scientist Rich Zurek of NASA's Jet Propulsion Laboratory (JPL) in 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."

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