Press Release From: Planetary Science Institute
Posted: Monday, November 20, 2017

Dark surface features previously considered evidence of subsurface water flow on Mars have now been interpreted as being the result of dry, granular flows according to a new Nature Geoscience paper.
The dark streaks are most likely the result of repeated avalanches of sand and dust, rather than the ground being darkened by seeping water as has previously been claimed, according to the paper “Granular flows at recurring slope lineae on Mars indicate a limited role for liquid water” published today.
Planetary Science Institute Senior Scientist Jim McElwaine is a co-author on the paper. U.S. Geological Survey scientist Colin Dundas is lead author.
“The RSL (recurring slope linae) 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.”
“We’ve thought of RSL as possible liquid water flows, but they seem to act more like dry sand,” said lead author Dundas. “This suggests that the surface of Mars is quite dry today.”
The RSL appear seasonally, and appear to exist only on Martian slopes steep enough for dry grains to descend as they do on faces of active dunes.
RSL, long bright or dark markings on the surface of a planet or moon, have long been thought of as evidence for significant liquid water on Mars. If they 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.

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