In a new paper published in Nature Astronomy, Sebastian Lauro from the University of Rome and colleagues report the discovery of stable bodies of liquid water beneath layered ice in the south polar region of Mars.
To reach that conclusion, the authors analyzed 134 radar profiles from the MARSIS instrument onboard the European Mars Express spacecraft currently in orbit around the planet. The data were collected from 2010 to 2019 in an area called Ultimi Scopuli.
The method used in this new study differs from the one used in a previous discovery of a subglacial lake on Mars. Lauro adapted a signal processing technique commonly used in radar sounding studies of Greenland and Antarctica to discriminate between wet and dry conditions at the base of glaciers.
Intriguingly, not only did the new study independently confirm the existence of the previously discovered underground lake, it found more patches of liquid water surrounding it. This means that the investigated region, which is about 250 by 300 kilometers in area, is a more complex and varied environment than previously recognized, with many ponds and smaller bodies of water that could potentially be habitable.
On Mars, however, the water would have to be very salty to stay below the freezing point. Or, recent magma flows or geothermal activity could in principle keep temperatures in that region higher than expected.
Still, the concentration and even the type of salt in the water would be important factors in determining whether the lakes or patches of water are habitable. The authors infer that these brines would likely contain perchlorates, a water-soluble salt that has been previously detected on Mars. The highest tolerance of Earth microbes to sodium perchlorate has recently been reported by my research group. Surprisingly, we found a fungus that was able to withstand water with a concentration of about 23 percent sodium perchlorate, while the best performing bacterium was only able to handle 12 percent sodium perchlorate.
While these tolerances are impressive and many more microbial species have yet to be tested, they’re still far from what would be needed to survive in underground bodies of water on Mars. It takes a 50 percent concentration of sodium perchlorate to keep water liquid at a temperature of -30o C. At the even colder temperatures likely to exist in the small ponds under Ultimi Scopuli, it would require another type of salt, calcium perchlorate, which can keep water liquid down to temperatures of -77o C. So far, though, experiments have shown that microbes have a harder time tolerating perchlorate solutions with calcium as compared to sodium.
The bottom line: Any of the Earth microbes tested so far would not be able to survive in conditions likely to exist in these newly discovered Martian lakes. However, we need to be cautious with our assessment. Microbes on Mars could have evolved better adaptation mechanisms, having been exposed to perchlorates for long (geological) time periods. Microbes on Earth never evolved these mechanisms because they didn’t need to. There are only a couple of restricted environments on Earth—in Chile’s hyperarid Atacama Desert and in Antarctica—where we find naturally occurring perchlorates.