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An artist impression of the Phoenix lander on Mars, who discovered perchlorate in the Martian regolith. Image Credit: NASA / JPL – Caltech / University of Arizona.
If liquid water exists on the surface of Mars, it is probably in the form of a brackish mixture with magnesium chlorate salts, according to new experiments based on NASA's previous discoveries. Phoenix and viking landers, as well as Rover Curiosity.
Scientists from the Department of Earth Sciences and Space at the University of Washington, Seattle, have studied mixtures of water with known salts on Mars, to determine which ones were most likely to be liquid to the surface of Mars. The experiments compared the vapor pressure and the water absorption capacity of saline solutions. The results suggest that water mixed with magnesium chlorate would be less likely to evaporate or freeze on Mars than in water mixed with sodium or potassium chlorate. The results will be presented in the September 2018 issue of Scientific letters of the Earth and planets.
Gases, liquids and solids
The "triple point" of a substance is the temperature and pressure at which it can coexist in the three phases: gas, liquid and solid. For water, the triple point is 0.01 degrees Celsius (32 degrees Fahrenheit) and 6.12 millibar, or 0.6 percent of the atmospheric pressure on the Earth's surface. In other words, one could imagine a bucket of water at the triple point, where the water exists in the form of ice floating on a layer of liquid water, with steam of water. 39 water just above the ice that has sublimated or evaporated. Steam in contact with the ice puts pressure on the ice, called vapor pressure.
Although in some equatorial regions of Mars, the optimal conditions may be just below the triple point of water, for the rest of the planet, temperatures are generally well below the triple point. When the atmospheric pressure is lower than the vapor pressure of a liquid, the liquid evaporates. The pure water would evaporate quickly under the fine Martian atmosphere, which represents about 1% of that of the Earth. Salt solutionshowever, do not evaporate or freeze as easily, and therefore more likely to remain liquid on Mars.
Understanding which combinations of salt and water are most likely to remain liquid could help us find traces of liquid water, and thus possibly life, on the red planet.
Saline solutions are hygroscopic, which means that they are good for absorbing water. Some salts can absorb even small amounts of water available on Mars. Many types of salts are present on Mars, such as chlorides, sulphates, chlorates and perchlorates. These can be found on the entire Martian surface, according to data from various landing and rover missions.
In 2008, the Phoenix lander"S Thermal Evolution Gas Analyzer (TEGA), which was part of its edge Laboratory of wet chemistry, found perchlorates in soil samples from the northern polar region of Mars, at concentrations from 0.4 to 0.6%. This encouraged scientists to re-analyze the soil sample data from Viking lander missions, which occurred in the 1970s.
The new analysis suggested that the soil found at Chryse and Utopia Planitiae by Viking landers contained perchlorates at a concentration less than or equal to 0.1%. Then, in 2013, the Sample Analysis at Mars (SAM) of the Curiosity roverinstrument found calcium perchlorate in Rocknest soil samples, which is a place in the Gale crater.
Viking 2 looks through Utopia Planitia. A recent analysis of Viking data showed that he had detected perchlorates. Image credit: NASA / JPL.
More recently, the Compact Recognition Imaging Spectrometer for Mars (CRISM) instrument aboard NASA's Mars Reconnaissance Orbiter Magnesium perchlorate, magnesium chlorate and sodium perchlorate have been detected in recurrent slope lines. These are descending streaks during the warmer Martian season and, for a time, it was strongly believed that they were produced by running water. Due to the salts, the weak atmosphere and the freezing temperatures, such water would probably be brackish rather than pure. Although the results now suggest that dry matter flows produce some streaks, it remains possible that water may still exist as a liquid somewhere on the surface of Mars.
Mix of salts
Which of these chlorates and perchlorates would be most likely to be dissolved in water under Martian conditions?
Jonathan Toner and David Catling, from the University of Washington, had previously modeled data from the Phoenix wet chemistry lab, to understand how different salts behave in the freezing temperatures of Mars. They found that soil samples likely containedmagnesium sulphate, magnesium perchlorate, sodium perchlorate, potassium perchlorate, sodium chloride and calcium carbonate.
In theirlateStudy, Toner and Catling made solutions from these salts. They found that, among all the salt water mixtures, the magnesium chlorate solution had the lowest vapor pressure. It means that Is theis likely to evaporate or freeze and is most likely to absorb low levels of moisture present in the Martian atmosphere.
So, to find liquid water on Mars, should scientists search only on Mars sites rich in magnesium chlorate?
"All the salts in Mars soils will probably be a mixture of salts, so it's important to measure the properties of these mixtures," says Toner. Based on soil chemistry measured by the Phoenix lander, Toner says that sodium chlorate and magnesiummixturesare the most likely, while calcium chlorateit is unlikely that mixtures will be found.
"Chlorate salts could be much more abundant than perchlorates on Mars," adds Toner. "Our results indicate that chlorates can have an even greater capacity to form water than perchlorates via a deliquescence [i.e. absorbing moisture and dissolving in it] and melting ice.
Water for life
Would there be enough water in these brines to support microbial life? Extrophilophilic studies cultured in perchlorate and chlorate solutions suggest that the microbes could survive in brines that may exist on Mars. A group of scientists led by Mark Schneegurt, a professor of biological sciences at Wichita State University in Kansas, discovered that several species of halotolerant bacteria, that is, salt-tolerant, could grow in high concentrations. chlorate salts.
"Until now, we have grown bacteria in chlorates at more than half of their production. [lowest melting] concentration of points, [which is] what we expected on Mars, "says Schneegurt. "Although we have not shown that microbes can grow at the highest concentrations, when we started, it was unclear that microbes could even grow in the 0.6% found by Phoenix. We are over 25% at the moment and go higher. We have shown microbial growth under these salt conditions and these salts are common on Mars.
Toner and Catling research was supported through NASA Living worlds Program. Astrobiology of NASA provides resources for livable worlds and other research and analysis programs within The NASA Directorate of Scientific Missions (SMD) who solicit proposals for research in astrobiology. Schneegurt's research is funded by NASA Planetary Protection Research in collaboration with NASA / Jet Propulsion Laboratory.
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