Researchers working with the European Space Agency’s Mars Express spacecraft have found strong evidence for a sub-glacial lake of liquid water under Mars’ southern polar ice cap. The 20-kilometer (12.5-mile) long salty lake raises hopes for finding existing lifeforms on the Red Planet, now that it may have joined the not-so-exclusive club of worlds in our solar system that have substantial subsurface bodies of water, such as Enceladus, Europa, and Ceres.

In 2008, the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) sensor aboard the Mars Express orbiter discovered a bright radar return from under the ice near Mars’ south pole. In 2012, the Italian research team studying the data began a new, 3-year survey of the area, collecting new radar data over 29 orbits. They then spent the next two years dissecting the data, trying to account for other potential sources for the bright radar reflections — but in the end their conclusions pointed to water.

"Progressively, we were narrowing the possibilities," explains paper co-author Elena Pettinelli, an associate professor with the department of math and physics at Rome’s Universita degli Studi Roma Tre. "In some ways, we didn’t want to think it was water… but really there’s no other explanation."

"When we arrived at that point, we said: ‘OK, this is something that everybody was looking for over 12 years.’ Because this radar was sent to Mars to actually look for water — that was the main idea," Pettinelli said. "And when we did, we were excited and surprised and a little bit worried."

The triangular-shaped lake is buried under 1.5 kilometers (0.9 miles) of ice, found in a region called Planum Australe, and is roughly 20 kilometers (12.5 miles) long. The researchers estimate that the lake’s temperature is around -68°C (-90.4°F), well below the freezing point of water, but the presence of magnesium, calcium, and sodium salts could lower the water’s freezing point enough to keep it liquid at those temperatures.

"This is a really exciting discovery," exclaims Tanya Harrison, a planetary scientist and director of research for Arizona State University’s Space Technology and Science Initiative. Although not involved with the ESA team’s research, Harrison is involved with NASA’s Mars Opportunity rover.

"I think this has massive astrobiological potential. On Earth, anywhere we find water, we find life. So if there is this 20-kilometre-wide thing of liquid brine just hanging out under the ice on Mars, maybe there’s some kind of little bug living in there."

If the subsurface feature at Planum Australe does indeed turn out to be a liquid-water lake, this also raises the question of whether or not it is a lone anomaly, or are there other subsurface lakes — and perhaps even oceans — found across the Martian landscape?

"If this lake is a single occurrence, if there is no other liquid water anywhere else, then the implication would be that we are seeing a quirk of nature—an effect of residual decay, a hydrothermal vent, some thermal irregularity in the crust," explains team lead Roberto Orosei, a radioastronomer at the National Institute for Astrophysics.

"But, if we were to find that Mars possesses not one subglacial lake, but several, that would change the game." 

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