Flowing Sand, Not Water, Source of Some Recurring Dark Martian Surface Streaks

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 Nature Geoscience paper.
The dark streaks found on some Martian slopes 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.”

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 lineae) 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.”

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 at these slope areas, which could preclude the presence of microbial life thriving at these sites.

Above: This image shows seasonal dark surface flow lines found in Horowitz Crater on Mars.

Credit: NASA/JPL/University of Arizona