There's plenty to suggest that water exists in various non-liquid forms on Mars, as well as signs that liquid water existed in the past, but now scientists have unearthed the first evidence that microscopic pools of water are developing and evaporating just below the surface of the Red Planet on a regular basis.
The latest development can be traced back to the work of NASA's Phoenix Mars Lander which touched down in 2008. Among the spacecraft's discoveries was the presence of perchlorate salts in the planet's soil. Long posited as an indication of transient liquids on Mars, these salts have now been identified as agents for water vapor absorption from the atmosphere, during certain seasons.
Scientists arrived at this conclusion after observing more than a Martian year's (687 Earth days) worth of temperature and humidity measurements. This data suggests that under the right conditions, the water vapor is absorbed by the salts, which lowers its freezing temperature and ultimately turns it into a liquid, before evaporating after sunrise.
This process is known as deliquescence and hinges on the levels of relative humidity in the air just above the ground. When this relative humidity passes a certain threshold, the perchlorate salts absorb enough water molecules to become liquid.
These super-salty microscopic puddles are said to exist within the Gale Crater, which the Curiosity rover has been exploring since 2012. The researchers say that even with liquid brines underneath the surface, the chilly environment here almost rules out the possibility of microbial life, but their presence could be indicative of something equally as valuable.
"Conditions near the surface of present-day Mars are hardly favorable for microbial life as we know it, but the possibility for liquid brines on Mars has wider implications for habitability and geological water-related processes," says Javier Martin-Torres of the Spanish Research Council, member of Curiosity's science team and the paper's lead author.
The Gale Crater is located near the Martian equator and scientists believe that colder conditions at higher latitudes, where perchlorate salts have also been detected, could better lend themselves to the deliquescence process.
"Gale Crater is one of the least likely places on Mars to have conditions for brines to form, compared to sites at higher latitudes or with more shading," Alfred McEwen, co-author of the paper. "So if brines can exist there, that strengthens the case they could form and persist even longer at many other locations."
The findings were published in the journal Nature Geoscience.