Soil rich in chlorate can help us find liquid water on Mars

If liquid water exists on the surface of Mars, it is probably in the form of a brackish mixture with salts of magnesium chlorate, according to new experiments based on discoveries previously made by the undercarriages Phoenix and Viking from NASA.

Scientists from the Department of Earth Sciences and Space at the University of Washington, Seattle, studied water mixtures with known salts on Mars, to determine which ones were most likely to be liquid on 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. Saline solutions, however, do not evaporate or freeze as easily, and therefore are 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 Phoenix Lander's Thermal Evolved Gas Analyzer (TEGA), which was part of its integrated chemistry laboratory, found perchlorates in soil samples from the north polar region of Mars at concentrations of 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 the Viking landing gear contained perchlorates at a concentration of 0.1% or less. Then, in 2013, Curiosity Rover's SAM (Sample Analysis at Mars) instrument detected calcium perchlorate in soil samples from Rocknest, a location in Gale Crater.

Most recently, the Mars-based Mars Reconnaissance Orbiter CRISM (Mars Reconnaissance Imaging Spectrometer) has detected magnesium perchlorate, magnesium chlorate, and sodium perchlorate in slope lineages. recurring. 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, of the University of Washington, had previously modeled data from Phoenix's wet chemistry lab to understand how different salts behave in Mars freezing temperatures. They found that the soil samples probably contained magnesium sulfate, magnesium perchlorate, sodium perchlorate, potassium perchlorate, sodium chloride and calcium carbonate.

In their largest study, Toner and Catling produced solutions from these salts. They found that, among all the salt water mixtures, the magnesium chlorate solution had the lowest vapor pressure. This means that it is the least likely to evaporate or freeze and is most likely to absorb low levels of moisture in the Martian atmosphere.

So, to find liquid water on Mars, should scientists search only on Mars sites rich in magnesium chlorate?

"All 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 LG Phoenix, Toner says that sodium and magnesium chloramixurates are the most likely, while it is unlikely that calcium chloramixurates are present.

"Chlorate salts could be much more abundant than perchlorates on Mars," adds Toner. "Our results indicate that chlorates may be more apt to form water than perchlorates through decay [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 to more than half [lowest melting] concentration of points, [which is] what we expected on Mars, "says Schneegurt." Although we have not shown that microbes can grow to the highest concentrations, when we started, it was not clear that microbes could even increase by 0.6%. We are over 25% at the moment and go higher. We have shown microbial growth in these salt conditions and these salts are common on Mars. "

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More information:
J. D. Toner et al. Chlorate brine on Mars: implications for the presence of liquid water and decay, Scientific letters of the Earth and planets (2018). DOI: 10.1016 / j.epsl.2018.06.011