Tasting the air and testing the temperature, NASA's Mars rover Curiosity has shown that there could potentially be salty water currently beneath the Red Planet's surface.
If the brine does exist, there's not much, it occurs only fleetingly and, even when it does exist in liquid form, it's far too, well, briny to be of any use to living things. But the findings, published in the journal Nature Geoscience, could offer explanations for mysterious dark streaks that appear and then vanish on many Martian slopes.
The findings are also a reminder that not all water is good water when it comes to the search for life.
"Perhaps the statement 'Follow the water' as a clue ... should reexamined," lead author F. Javier Martin-Torres, a planetary scientist at Lulea University of Technology in Sweden, said in an email.
Finding signs of the brines involved a little serendipity, Martin-Torres added.
"Well, it was not that we were looking for them," he said. He and colleague Maria-Paz Zorzano of the Centro de Astrobiologia in Spain had been plotting temperature and relative humidity using the Rover Environmental Monitoring Station instrument when they noticed that, under certain conditions, there could be a thin skin of perchlorate-filled water just beneath the surface.
"We realized that we were seeing conditions where brines should be formed … and since then we have been studying the process in collaboration with other [Curiosity] team members," he said.
The scientists think these brines form on a few nights per year when the relative humidity is very high and the temperature is very low; they lie beneath the surface in the top layer of soil and quickly dry out when the sun rises.
These brines are made possible mainly by the presence of perchlorates, which are found all over Mars. Perchlorates, made of a chlorine and four oxygen atoms, have two talents that make them ideal for trapping liquid water. Like other salts, perchlorates lower the freezing temperature of water, keeping it liquid even in ice-cold climes. But they also have this ability to pull water molecules out of the air and render them liquid. It's a process with a strangely beautiful name: deliquescence.
"It's a dual property of the perchlorates that they can absorb large amounts of water vapor and they can release it in a way that forms a liquid — with the salt acting like an antifreeze," study coauthor Ashwin Vasavada, Curiosity's project scientist at the Jet Propulsion Laboratory, said in an interview.
However, perchlorates, which allow the water to remain liquid near the planet's surface, are also the same substance that makes the water utterly unusable for microbes.
"If you were able to sample these little films of water, they would have so much salt dissolved in them that it would be impossible for life to use that water," Vasavada said.
Martin-Torres agreed; after all, such harsh brines would wreck microbes' metabolism and ability to reproduce.
"Nature and extremophiles always surprise us," he said, "but in our paper we mention that the combination of relative humidity and temperature when at brine conditions are not compatible with [our knowledge of] current terrestrial microorganisms."
Curiosity doesn't have the tools to sample any brines, if they are indeed there, but it could be done by a future Mars mission, Vasavada said.
In any case, the fact that brines might be able to appear in Gale Crater tells scientists that they're even more likely to exist – and to last even longer – in much colder areas that are closer to the poles.
Even though they don't have astrobiological potential, these brines might help explain some geological phenomena occurring on the surface, Vasavada said. For one thing, it could explain the strange dark flows spotted by NASA's Mars Reconnaissance Orbiter that appear and lengthen on slopes during warmer seasons. These recurring slope lines could potentially be caused by brines rising all the way out to the surface.
"They haven't been able to definitely find liquid at those places either, so brine is still a candidate for those," Vasavada said. "But I think it helps us understand better what the chances are of forming brines on Mars."