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https://www.wsj.com/articles/scientists-find-evidence-of-hidden-lake-on-mars-1532527200Deep under the ice of the Martian South Pole researchers have discovered signs of a hidden lake that could hold liquid water year-round, a crucial condition needed for life to develop.
An advanced satellite radar has found a massive anomaly—a 12-mile-long lake that researchers believe must be at least a meter thick. Previous discoveries on Mars have indicated liquid water that existed only seasonally.
“This is the first potential habitat,” said researcher Roberto Orosei at the Italian National Institute for Astrophysics. “We don’t know it is inhabited, but if you’re looking for life on Mars, this is one of the prime places to look.”
https://www.theatlantic.com/science/archive/2018/07/lake-mars-water/565988/New evidence suggests the presence of a lake one mile beneath the ice-coated surface of Mars’s south pole, according to new research published Wednesday in Science. Scientists say the lake stretches 20 kilometers, or about 12 miles, across, and is one meter, or more than 3 feet, in depth. The findings, if confirmed, would mark the detection of the largest body of liquid water on Mars today.
https://www.sciencealert.com/vast-liquid-water-reservoir-found-on-mars-radar-planum-australe-subglacial-lakeOther explanations, such as very cold and pure water ice, or carbon dioxide ice, could also explain a brightly reflective subsurface anomaly like this, but the research team ran simulations and found that the reflectivity profile did not match their results as well as liquid water.
But there's one other big problem: the temperature of the body is estimated to sit at around 205 Kelvin (-68.15 Celsius, or -90.67 Fahrenheit).
That is far below the point of freezing, even for hypersaline Antarctic lakes, which remain liquid above 260 Kelvin (-13 Celsius, or 8.6 Fahrenheit) thanks to their salt content.
But despite this, the water could still hypothetically remain liquid. We know that salts of sodium, magnesium, and calcium are abundant on Mars - they've been found on the surface.
http://science.sciencemag.org/content/early/2018/07/24/science.aar7268Radar profiles collected between May 2012 and December 2015 contain evidence of liquid water trapped below the ice of the South Polar Layered Deposits. Anomalously bright subsurface reflections are evident within a well-defined, 20-kilometer-wide zone centered at 193°E, 81°S, which is surrounded by much less reflective areas. Quantitative analysis of the radar signals shows that this bright feature has high relative dielectric permittivity (>15), matching that of water-bearing materials. We interpret this feature as a stable body of liquid water on Mars.