Much of the discussion about potential water on Mars has focused on the possibility of substantial subsurface ice reserves, or on the possibility of mining ice at the Martian poles. The Mars Reconnaissance Orbiter has found evidence that this may not be the only form of water on the planet. Periodic dark features on the Martian surface, known as recurring slope lineae, or RSL, have been interpreted as possible evidence of liquid water. Unfortunately, a new analysis shows this water (if it is water) may not be in a form astronaut visitors could ever use.
First, some basics. These RSL periodically appear, lengthen, and disappear in the same fashion that water might as it leaked to the surface. They appear during the summer season on sun-facing slopes at times when the temperature is often high enough to allow water to exist in liquid form (estimated surface temperatures in these areas can be as high as 27C; Martian summer temperatures at noon on the equator can reach 20C). These features were first discovered in 2011, and are thought to be created by extremely salty brine-infused water bubbling to the surface. Brine can dramatically lower the freezing point of water, allowing it to flow at much lower temperatures.
The problem with the RSL is that many of them appear at the tops of geological formations that aren’t large enough to supply a steady, ongoing amount of water year after year. The tops of rocky crags aren’t known for being reservoirs of water and the RSL themselves don’t seem to be linked by any common pattern (for example, being fed by a single proposed underground aquifer or reservoir). Because these canyons are close to the equator and undergo regular heating, the chances of an enormous subsurface liquid reserve after millions and millions of years of regular leakage are remote — again, there’s not enough room in the rock formations to contain that much water. Either these remote features have somehow tapped into vast subsurface reserves (7-12 Olympic swimming pools worth of water per year doesn’t sound like much until you multiply it by several hundred million years) or they all draw from a common source (unlikely, given the phenomenon’s distribution).
Could water be coming from the atmosphere instead? Until now, scientists assumed the Martian atmosphere was too dry to hydrate salts in the soil, Stillman said. But Chojnacki and his team suspect that in these canyons, the air is surprisingly humid. Atmospheric water could be transferred to the ground at the bottom of the canyons through a process called deliquescence, in which the salty surfaces absorbed more and more atmospheric water until they became a briny solution, the research team suggests. If enough water were absorbed on the slopes, it would form RSL, they said. In addition, Stillman said, if RSL are somehow recharged via an atmospheric process, then astronauts could theoretically emulate the capture mechanism of RSL to gather water.
They say “theoretically,” because until we understand how the process works, there’s not much chance of emulating it. There’s still the possibility that the RSLs are caused by other methods — so-called “dry flows.” But there’s no real evidence to support the theory that we’re seeing some other form of subsurface layer being exposed by wind. The dark areas look almost exactly like what you’d see if you poured a glass of water on concrete or sand and left it out to dry. That doesn’t automatically mean they’re water, of course, but it makes sense to investigate the phenomenon based on the physical process it resembles.
If these flows are recharged by atmospheric phenomena, it would be an interesting insight into the Martian atmosphere. But it would also mean that the most likely reservoirs of Martian water remain at the poles. This, in turn, limits astronaut landing sites and opportunities for exploring the warm, balmy (often sub-freezing) Martian equator.