Dark streaks on Martian slopes called Recurrent Slope Lineae (RSL) have been seen from Mars orbit for years, and various theories have been proposed about how they form. The latest suggestion, published in Nature Geoscience, involves water boiling under the low atmospheric pressure at the Martian surface.
A research group led by Marion Massé from the University of Nantes in France conducted experiments to mimic environmental conditions on the Martian slopes. They let a block of ice melt at the top of a 30-degree slope covered with loose, fine-grained sand, both at Earth atmospheric and Mars atmospheric conditions. The atmospheric pressure on Mars is less than one percent that of Earth—low enough to cause water to boil at the interface between wet sediment and the atmosphere. In the lab experiments, the force of the boiling water propelled sand grains into the air, and they accumulated over time (as seen in the video below by the Massé group). That process led to the formation of linear, dark-toned, downslope flows, similar to the RSL seen on Mars.
Other mechanisms have been proposed for how these odd features could come to exist on the Martian landscape: transient flow of brines (salty water) just below the surface, dry flows of sand grains, or processes involving carbon dioxide. The brines theory has been the most popular, due to observations of the spectral signature of salts associated with the streaks. However, one process doesn’t necessarily rule out the other. The Massé group also saw the boiling water effect in salty water, although it wasn’t as pronounced.
If the RSL are formed by boiling water, the Martian streaks may not be as important for astrobiology as previously thought. However, the formation process is still not clear, because other observations show that the streaks appear to grow during the Martian warm season and fade in the cold season. Deliquescence, the absorption of water from the atmosphere by salts in the RSL, would be another possibility. Those processes may also be occurring at the same time. If so, microorganisms that can tolerate high salt concentrations might be able to survive within the dark streaks after all.
There are reports that the slopes of Mount Sharp in Gale Crater, where NASA’s Curiosity rover is exploring, show some of these streaks as well. If confirmed, and if planetary protection regulations allow, sending Curiosity to take a look at the RSLs would be the best way to resolve whether these curious streaks are a potential habitat for Martian life.