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Mars could be teeming with life after huge lakes of water were discovered under its south pole, scientists say.
The complex system has been likened to an underground "patchwork" in Antarctica filled with weird jellyfish and shrimp-like creatures.
Researchers say the four massive reservoirs of saltwater could have harboured alien life for more than four billion years.
Co-lead author Professor Elena Pettinelli said: "The presence of sub-glacial lakes could have important consequences for astrobiology – and the presence of habitable niches on Mars."
They are rich in salt – which lowers water's freezing point. This would keep it liquid at temperatures as low as -74 C.
The international team detected three reservoirs of various sizes a mile down under a thick layer of ice – and confirmed the existence of a much larger fourth.
They may have been there for more than four billion years – and be home to extremophiles that can survive the harshest conditions.
It's also possible aerobes or anaerobes are there – organisms that thrive with and without oxygen, respectively.
The study published in Nature Astronomy is based on an analysis of data from the Mars Express orbiter.
Two years ago its MARSIS radar instrument picked up signs of a briny, 12 mile wide underground lake.
Prof Pettinelli, of Roma Tre University in Rome, and colleagues have now taken a closer look – and verified its existence.
What's more, they identified a number of others across a region spanning more than 170 square miles – separated from the main body by strips of dry material.
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Prof Pettinelli said: "Not only did we confirm the position, extent and strength of the reflector from our 2018 study – but we found three new bright areas."
They contain extremely high concentrations of salt dissolved in water – enabling them to remain in liquid form despite the freezing temperature.
The main lake is surrounded by smaller bodies of water. It's not yet known if they are inter-connected.
Co-author Prof Roberto Orosei, of Italy's National Institute of Astrophysics in Bologna, said: "The existence of a single sub-glacial lake could be attributed to exceptional conditions such as the presence of a volcano under the ice sheet.
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"But the discovery of an entire system of lakes implies their formation process is relatively simple and common – and these lakes have probably existed for much of Mars' history.
"For this reason, they could still retain traces of any life forms that could have evolved when Mars had a dense atmosphere, a milder climate and the presence of liquid water on the surface, similar to the early Earth."
Mars is about 4.5 billion years old. The researchers used a signal reflecting technique with which satellites have identified lakes beneath the polar ice sheets on Earth.
Co-lead author Dr Sebastian Lauro, also of Roma Tre, said: "We borrowed a methodology commonly used in radar sounder investigations of sub-glacial lakes in Antarctica, Canada and Greenland.
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"We adapted the method to analyse old and new MARSIS data. The interpretation that best reconciles all the available evidence is the high intensity reflections are coming from extended pools of liquid water."
The frozen continent of Antarctica has over 400 underground lakes. A 2013 drilling expedition to Lake Whillans – half a mile down – found it's awash with organisms.
The Dead Sea on the borders of Jordan and Israel is also rich in microbes – despite being ten times saltier than seawater.
In other words, the best Earth-based analogues for the Martian lakes described in Nature Astronomy are inhabited.
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Prof Pettinelli said: "The possibility of extended hyper-saline water bodies on Mars is particularly exciting because of the potential for the existence of microbial life, such as extremophiles, anaerobes or even aerobes."
The solubility of oxygen in brines is up to six times the minimum level required for microbial respiration, she explained.
Prof Pettinelli said: "Future missions to Mars should target this region to acquire experimental data in relation to the basal hydrologic system, its chemistry and traces of astrobiological activity."
NASA's mantra for answering the question of 'are we alone?' is to "follow the water". On Earth, where you find water, you find life.
Prof Pettinelli said: "To establish the extent of sub-glacial water in this region we acquired new data, achieving extended radar coverage over the study area.
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"Our results strengthen the claim of the detection of a liquid water body at Ultimi Scopuli and indicate the presence of other wet areas nearby.
"We suggest the waters are very salty brines, known to form at Martian polar regions and thought to survive for an extended period of time on a geological scale at extremely low temperatures."
Lab experiments have shown brines can persist for millions or even billions of years at temperatures found at the Martian poles – way below water's freezing point.
Dr Enrico Flamini, of Chieti-Pescara University in Italy and former science mission programs manager with the Italian Space Agency, said: "To state these new results make me happy is not enough.
"The biggest unanswered question from our earlier paper was: is this the only evidence of sub-ice liquid water?
"At the time we did not have enough evidence to address this question, but this new research demonstrates the 2018 discovery was only the first piece of evidence of a widespread system of liquid water bodies in the Martian subsurface.
"It's exactly what I would have hoped: a great result, indeed."
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