Soaked in… sand? NASA/JPL/University of Arizona

Dust to dust. The mysterious dark flows on Mars may not be water after all. Instead, they could be rivulets of sand, set in motion by sunlight on the Martian surface.

The dark streaks form on Mars’s slopes during warm seasons, and are known as recurring slope lineae. While there is no direct evidence of water near these areas, the leading theory is that they are caused by briny water streaming down the sides of craters and hills.

“These effects happen at the hottest times in the hottest locations, so there’s part of your brain that immediately tells you that it should be ice melting,” says Sylvain Piqueux at NASA’s Jet Propulsion Lab in Pasadena, California. “The problem is, it’s really hard to melt ice on Mars.” It’s easier for the ice to turn directly into water vapour, he says.


Some models suggest that recurring slope lineae could be made of water condensing out of the atmosphere, but Mars’s atmosphere isn’t humid enough to account for what we see.

No liquid required

In the absence of a wet explanation, Frédéric Schmidt at the University of Paris-South and his colleagues branched out into ideas that did not require liquids. “We thought that if it’s purely dry, there should be no seasonal effects,” Schmidt says. “But here we suggest that there’s a dry process that is linked to [seasons].”

Schmidt and his colleagues say the features could be sand avalanches, similar to the ones we might see on a dune on a windy day. But instead of wind, these flows are caused by sunlight and shadow.

When sunlight hits the sand, it heats up the top layer while leaving deeper layers cool. This temperature gradient causes a corresponding change in the pressure of tiny pockets of gas surrounding the sand particles, shifting the gas upwards. That in turn jostles grains of sand and soil, causing them to slip down the Martian slopes.

This effect should be most pronounced in afternoon shadows cast by boulders or outcrops. Then, the contrast between the cooling top of the sand and the still-warm layers just below creates a second pressure gradient, shifting the gas and sand even more.

The recurring slope lineae that we see originate on sloping, rocky landscapes, matching the predictions of this new model. “It doesn’t necessarily explain all of the recurring slope lineae, but I think they have the right idea in that there is some unique Martian mechanism going on here,” says Alfred McEwen at the University of Arizona in Tucson. “But there are observations that do not fit.” Some streaks are in shadow-free or surprisingly cool areas, for instance.

If recurring slope lineae are created without liquid, it could dismantle our hopes that they might make life easier, both for organisms native to Mars and eventual human explorers. “We can’t think of Mars as a friendly planet,” says Schmidt. “It’s a very hard transition to go there, and even harder if these flows don’t have liquid water.”

Journal reference: Nature Geoscience, DOI: 10.1038/ngeo2917