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This sounds like the premise for an Arnold Schwartzenegger sci-fi movie, but it isn’t.  The European Space Agency’s Mars Express orbiter, circling the Red Planet since 2003, has discovered what they believe to be a lake under the planet’s south polar ice cap, and it is about 20km (12 miles) across. The discovery was made using Marsis, a radar instrument aboard the orbiter.

The researchers say it’s a lot like the subglacial lakes trapped beneath the ice of the Arctic and Antarctic here on Earth. Moreover, like our own terrestrial subglacial lakes, it might be a place to look for life. Mars was once apparently carved by intermittent liquid water flowing on its surface, but this is the first sign of a persistent body of water on the planet in the present day.

Nasa’s Curiosity rover has found liquid water just under the surface of Mars, though without its protective sheath of atmosphere, hard cosmic radiation penetrates up to a meter into the Martian soil. The hardiest of Earth’s extremavores would not survive such an onslaught. Most of the planet’s water, whatever is left of it, is mostly locked up in the form of ice at its poles.

The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) uses radar to look for features under the surface of Mars, and has been looking for signs of subsurface liquid water for over 12 years. A careful investigation of a 200-kilometer wide section of the southern ice cap between May of 2012 and December of 2015, in a region called the Planum Australe, yielded some surprising telemetry. They took 29 radar profiles of the region, bouncing radio waves deep beneath the surface of Mars, and collecting the return signal on a receiver.

Measuring the changes between the transmitted signal and what comes back gives scientists information on what might be down there hiding beneath the surface. Radar returning through water is returned more strongly, or ‘brightly’, than radar returning through rock or sediment. This is what the research team found in their radar results: an anomalously bright region in the Planum Australe.

It could be very cold, very pure water ice. It could also be frozen carbon dioxide, but the reflection profile didn’t look as good for carbon dioxide as it did for plain old liquid water. There is one problem, though: if there’s water down there, it’s at a temperature of about -90 degrees Fahrenheit (-68.15 Celsius). That’s far, far below the freezing point of water, even if you add salt. Our hypersaline Antarctic lakes hover around 8.6 degrees Fahrenheit (-13 Celsius), so this is far colder and frankly doesn’t make a lot of sense. Despite this, there are ways it could still remain liquid. Salts of sodium, magnesium, and calcium are abundant on Mars — they’ve been found on the surface. If there’s any of that down there, dissolving it into the water and then putting it under pressure from the ice cap on top could drop the freezing point that low.

Life has been found in subglacial Earth lakes. It’s been previously proposed that a subglacial Martian lake might therefore also harbour life. This discovery reopens that possibility more prominently than ever before.

“There is evidence on Earth of substantial microbial life in the waters below the poles – and even microbes that can survive within ice veins,” said astrobiologist Brendan Burns of the University of New South Wales, who was not involved with this research.

“Whether similar scenarios are occurring on Mars remain to be experimentally established, but this finding of potential liquid water beneath the surface of Mars opens up fascinating areas of space exploration.”

Can we get down underneath the icecaps to find out? With our current technology, probably not. We’d need a pretty long borehole to take a sample, and that would probably require humans on the surface to drill it. Still, the idea of gigantic masses of liquid water on Mars does raise the potential that there may yet be life there.

To read more about it, visit the research team’s writeup of it on Science Magazine.

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