This Mars Reconnaissance Orbiter HiRISE image cutout shows Recurring Slope Lineae in Tivat crater on Mars in enhanced color. The narrow, dark flows descend downhill (towards the upper left). Analysis shows that the flows all end at approximately the same slope, which is similar to the angle of repose for sand.
Dark features previously proposed as evidence for significant liquid water flowing on Mars have now been identified as granular flows, where sand and dust move rather than liquid water, according to a new article published in Nature Geoscience by the U.S. Geological Survey.
Credit: NASA/JPL/University of Arizona/USGS

There appears to be a cascading truism about Recurring Slope Lineae, or RSL on Mars. That is, what causes these features is a recurring controversy.

On one hand, the appearance and growth of RSL resemble seeping liquid water. RSL features grow incrementally, fade when inactive and recur annually during the warmest time of year on Mars.

RSL are mostly found on steep rocky slopes in dark regions of Mars, such as the southern mid-latitudes, Valles Marineris near the equator, and in Acidalia Planitia on the northern plains.

But new research focused on these dark features have now been identified as granular flows, where sand and dust move rather than liquid water, according to a new article published in Nature Geoscience by the U.S. Geological Survey (USGS). This study was done in cooperation with the NASA Mars Reconnaissance Orbiter project.

Dry sand

If RSL are actually dry grain flow phenomena, this suggests that recent Mars has not had significant volumes of liquid water, which could preclude the presence of microbial life thriving at these sites. This finding would alleviate planetary protection concerns about habitable environments.

“We’ve thought of RSL as possible liquid water flows, but the slopes are more like what we expect for dry sand,” said USGS scientist and lead author of the published findings, Colin Dundas. “This new understanding of RSL supports other evidence that shows that Mars today is very dry.”

In a press statement, Dundas said that the terminal end of the RSL slopes are identical to the slopes of sand dunes where movement is caused by dry granular flows. Water almost certainly is not responsible for this behavior, which would require the volume of liquid to correspond to the length of slope available, producing more liquid on longer slopes. Instead, the 151 RSL examined all end on similar slopes despite very different lengths. Additionally, water is unlikely to be produced only near the tops of slopes at these angles and if it were, it should be able to flow onto lower slopes.

Flow physics

Planetary Science Institute (PSI) Senior Scientist Jim McElwaine is a co-author on the paper. PSI is headquartered in Tucson, Arizona.

“The RSL on Mars behave in a similar way to laboratory experiments on Earth,” said McElwaine who contributed expertise on the physics of granular flow and fluid dynamics to the research.

“What is still not understood is where the supply of fresh material comes from, though we do have some speculative ideas,” McElwaine said in a PSI statement.

Triggering mechanism

David Stillman, Senior Research Scientist at the Southwest Research Institute (SwRI) in Boulder, Colorado is an RSL investigator. He notes that McElwaine’s quote is correct.

“There is no known way to resupply/reset the slopes year after year after year. Also no known triggering mechanism that always triggers just below the bedrock-regolith interface and then for some reason slowly triggers further downslope as the RSL lengthen,” says Stillman.

NASA Mars Reconnaissance Orbiter’s HiRISE image of recurring slope lineae in Melas Chasma, Valles Marineris. Arrows point out tops and bottoms of a few lineae.
Credit: NASA/JPL-Caltech/University of Arizona

Furthermore, that is no explanation why dust can so quickly fade RSL away in low-dust areas, Stillman points out, compared to the decades needed to fade away slope streaks in dusty areas.

“Overall, it’s a great observation that will influence future RSL models, but many more things need to be understood before this theory is confirmed,” Stillman concludes.

Total speculation

“It is an important observation that RSL appear near the angle of repose, “but it is not a final, conclusive result,” responds Robert Grimm, a geophysicist in the Department of Space Studies at SwRI.

Everything is about the material volume budget, Grimm adds.

“Water can be replaced by an aquifer even if it is hard to explain the presence of that aquifer. How the sand would get replaced is total speculation at this

point. And this is related to the question of how RSL advance incrementally…easy to explain by flow of water in a porous medium, but sand must repeatedly trigger throughout the season – it’s not like flow down a dune face. “So the controversy continues,” concludes Grimm.

Resources

To access the new research in Nature Geoscience, “Granular flows at recurring slope lineae on Mars indicate a limited role for liquid water,” by Colin M. Dundas, Alfred S. McEwen, Matthew Chojnacki, Moses P. Milazzo, Shane Byrne, Jim N. McElwaine & Anna Urso, go to:

https://www.nature.com/articles/s41561-017-0012-5

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