The search for liquid water on Mars is one that has been on-going for decades. It can’t exist for long on the surface, as it will quickly sublimate into the cold, thin atmosphere. Aquifers deep below the surface are still possible, but there is also another tantalizing possibility which scientists have been considering: brines. Such salty liquid water could theoretically last a bit longer on the surface or in the near-subsurface, and now the Curiosity rover has provided more evidence that this may indeed be happening at its location in Gale Crater, as well as elsewhere.
The rover hasn’t found the brine itself—not yet anyway—but has determined that the environmental conditions are ideally suited for brines to exist in the soil. Small quantities could form at night, and then evaporate after sunrise. At higher latitudes, conditions would be even more favorable, where colder temperatures and more water vapor can result in higher relative humidity more often.
“We have not detected brines, but calculating the possibility that they might exist in Gale Crater during some nights testifies to the value of the round-the-clock and year-round measurements REMS is providing,” said Curiosity Project Scientist Ashwin Vasavada of NASA’s Jet Propulsion Laboratory, Pasadena, Calif. Vasavada is a co-author of the new report.
According to Morten Bo Madsen, associate professor and head of the Mars Group at the Niels Bohr Institute at the University of Copenhagen: “We have discovered the substance calcium perchlorate in the soil and, under the right conditions, it absorbs water vapour from the atmosphere. Our measurements from the Curiosity rover’s weather monitoring station show that these conditions exist at night and just after sunrise in the winter. Based on measurements of humidity and the temperature at a height of 1.6 meters and at the surface of the planet, we can estimate the amount of water that is absorbed. When night falls, some of the water vapour in the atmosphere condenses on the planet surface as frost, but calcium perchlorate is very absorbent and it forms a brine with the water, so the freezing point is lowered and the frost can turn into a liquid. The soil is porous, so what we are seeing is that the water seeps down through the soil. Over time, other salts may also dissolve in the soil and now that they are liquid, they can move and precipitate elsewhere under the surface,”
As to whether these brines could support microbial life, the jury is still out. Present conditions on the planet’s surface are very hostile for even the simplest of microorganisms.
“Liquid water is a requirement for life as we know it, and a target for Mars exploration missions,” said the report’s lead author, Javier Martin-Torres of the Spanish Research Council, Spain, and Lulea University of Technology, Sweden, and a member of Curiosity’s science team. “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.”
It might be a different story deeper down though, where there might be more water available and warmer temperatures. On Earth, many hardy microorganisms can exist just fine in very briny conditions.
The results come from a study of weather data from the Rover Environmental Monitoring Station (REMS) on Curiosity, including a relative-humidity sensor and a ground-temperature sensor. The Dynamic Albedo of Neutrons (DAN) instrument was also used to measure hydrogen abundance in the soil. The findings were published yesterday in Nature Geoscience.
If brines can form in Gale Crater, according to the report, they should be able to do so even more easily elsewhere on the planet.
“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. So if brines can exist there, that strengthens the case they could form and persist even longer at many other locations, perhaps enough to explain RSL activity,” said HiRISE Principal Investigator Alfred McEwen of the University of Arizona, Tucson, also a co-author of the new report.
Curiosity is the first mission to measure relative humidity in the Martian atmosphere close to the surface and ground temperature during all times of day and all seasons of the Martian year. Curiosity’s measurements of relative humidity range from about 5 percent on summer afternoons to 100 percent on autumn and winter nights.
The brines are possible due to salts in the soil, which Curiosity and other rovers and landers have found in many locations, specifically perchlorates. First, air filling pores in the soil interacts with air just above the ground. Then, when the relative humidity gets above a certain threshold, the salts can absorb enough water molecules to become dissolved in liquid, a process called deliquescence. As reported previously on AmericaSpace, various studies have shown how salts could allow water to remain liquid for brief periods of time on the Martian surface.
The Spirit rover actually found an abundance of salts in the soil in Gusev Crater by accident, when a broken wheel which was being dragged through the soil churned up the bright white salt deposits. This was definitive evidence of past water activity in this region.
Salty water brines are also considered to be a leading explanation for the Recurring Slope Lineae (RSL) phenomenon, where narrow dark streaks run down the faces of steep, equator-facing slopes in the mid-latitudes of Mars. The flows are known to repeatedly occur during the warmer seasons; they could result from pockets of brine or melting ice during those times.
Such brines may have been photographed already several years ago, by the Mars Phoenix Lander, near the north pole—what appear to be droplets of briny water formed on one of the landing legs during landing. It is theorized that ice in the soil was heated by the landing rockets, and droplets splashed onto the landing leg, before freezing again. There is still some debate, but they certainly look like liquid water droplets, and the soil at the landing site is known to contain both water ice and perchlorate salts.
Brines are not the most ideal form of liquid water as far as life is concerned, but it does show, along with other evidence, that water can still be found on the otherwise bone-dry Red Planet (along with the massive amounts of ice, of course). Coupled with evidence for more alkaline (non-briny) water as well in the planet’s past, it provides another vital clue to the habitability of Mars a long time ago and perhaps still today.
Curiosity has already found evidence of a former lakebed in Gale Crater as well as streambeds which once drained into that lake. Although those are now long gone, the presence of brines would indicate that there is still an ongoing hydrological cycle, even if it is much less active now than it used to be. Gale Crater was once a more habitable place which could have supported microbial life. Confirmation of present-day brines would certainly add another interesting chapter to that story.