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The lakes of liquid water in the southern ice sheet of Mars can only be mirrors

Maybe hold on to that Martian ice fishing trip. Two new studies sprinkle cold water on the idea that there are potentially habitable liquid water lakes under the southern polar ice sheet of the Red Planet.

The possibility of a lake about 20 kilometers wide first arose in 2018, when the European Space Agency's Mars Express spacecraft probed the planet's southern polar layer with its Mars Advanced Radar for Subsurface and Ionosphere Sounding, or MARSIS instrument. The orbiter detected bright spots in the radar measurements, hinting at a large mass of liquid water under 1.5 kilometers of solid ice that could be inhabited by living organisms (SN: 25/07/18). Subsequent work found evidence of additional swimming pools surrounding the main lake basin (SN: 28/09/20).

But the planetary scientific community has always maintained some skepticism about the existence of lakes, which would require some kind of continuous geothermal warming to maintain subglacial conditions (SN: 19/02/19). Below the ice, temperatures average -68 ° Celsius, far from the freezing point of water, although lakes are a brine that contains a healthy amount of salt, which reduces the freezing point of water. An underground magma pool would be needed to keep the area liquid, an unlikely scenario given the current lack of volcanism on Mars.

"If it's not liquid water, is there anything else that can explain the bright radar reflections we're seeing?" asks planetary scientist Carver Bierson of Arizona State University in Tempe.

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In a study published in the July 16 geophysical research letters, Bierson and colleagues describe other substances that could explain the reflections. The reflectivity of the radar depends on the electrical conductivity of the material passing through the radar signal. Liquid water has a rather distinct radar signature, but when examining the electrical properties of clay minerals and frozen brine, it was revealed that such materials could mimic this signal.

A study by an independent team, published in the same issue of Geophysical Research Letters, adds weight to the non-lake explanation. The initial aqueous discoveries of 2018 were based on MARSIS data centered on a small section of the southern ice sheet, but the instrument has now built three-dimensional maps of the entire South Pole, where hundreds to thousands of additional bright spots appear.

“We find them literally all over the region,” says planetary scientist Aditya Khuller, also of Arizona State University. "These signatures are not unique. We see them in places where we expect it to be very cold."

Creating plausible scenarios to keep liquid water in all these places would be a difficult exercise. Both Khuller and Bierson think it is much more likely that MARSIS points to some kind of widespread geophysical process that created frozen minerals or brines.

Although previous work has already raised doubts about the interpretation of the lake, these additional data could represent the rattle of the pools. “Putting these two works together with other existing literature, I would say this tells us 85 percent that this is not a lake,” says Edgard Rivera-Valentin, a planetary scientist at Houston’s Lunar and Planetary Institute who was not involved in any of the two studies.

Lakes, if any, are likely to be extremely cold and contain up to 50 percent salt, conditions in which no known organism on Earth can survive. Given this, swimming pools would not make especially strong astrobiological targets anyway, Rivera-Valentin says. (SN: 5/11/20).

Laboratory work exploring how substances react to conditions on the southern polar ice sheet of Mars could help restrict what generates bright radar spots, Bierson says.

Meanwhile, Khuller already has his eye on other areas of potential habitability on the Red Planet, such as the warmer mid-latitude regions where satellites have seen evidence of melted ice in the sun. “I think there are places where liquid water could be on Mars today,” he says. "But I don't think it's at the South Pole."

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