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鈥檚 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.
鈥淭hese effects happen at the hottest times in the hottest locations, so there鈥檚 part of your brain that immediately tells you that it should be ice melting,鈥� says at NASA鈥檚 Jet Propulsion Lab in Pasadena, California. 鈥淭he problem is, it鈥檚 really hard to melt ice on Mars.鈥� It鈥檚 easier for the ice to turn directly into water vapour, he says.
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Some models suggest that recurring slope lineae could be made of water condensing out of the atmosphere, but Mars鈥檚 atmosphere isn鈥檛 humid enough to account for what we see.
No liquid required
In the absence of a wet explanation, at the University of Paris-South and his colleagues branched out into ideas that did not require liquids. 鈥淲e thought that if it鈥檚 purely dry, there should be no seasonal effects,鈥� Schmidt says. 鈥淏ut here we suggest that there鈥檚 a dry process that is linked to [seasons].鈥�
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. 鈥淚t doesn鈥檛 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,鈥� says at the University of Arizona in Tucson. 鈥淏ut there are observations that do not fit.鈥� 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. 鈥淲e can鈥檛 think of Mars as a friendly planet,鈥� says Schmidt. 鈥淚t鈥檚 a very hard transition to go there, and even harder if these flows don鈥檛 have liquid water.鈥�
Nature Geoscience
