Mars has a large saltwater lake at its south pole under a layer of one and a half kilometers of ice, as demonstrated by the observations of the Mars Express spacecraft of the European Space Agency (ESA). The discovery, published online in the journal Science, provides the first proof that Mars contains liquid water today and suggests that similar underground lakes could be found in other regions of the planet.

Both the lake of the south pole and those that may exist in other latitudes of Mars could harbor life forms, just as there are on Earth in the darkness of Lake Vostok, buried under 4,000 meters of ice in Antarctica.

Of course, this discovery opens up new possibilities for the search for extremophile microorganisms in the Martian environment, "Elena Pettinelli, co-author of the research, from the University of Rome Tres (Italy), said in an e-mail. Beyond the search for life, the presence of liquid water today provides valuable information to reconstruct the history of the climate and the geology of Mars.

The lake measures about 150 square kilometers

The search for water has been in the last two decades the storyline of NASA's Mars Exploration program, which has followed the slogan "Follow the water (follow the water)". In these two decades it has been discovered that Mars had rivers and lakes in the past, and that it still retains a part of that water in the form of ice on the surface and in the form of gas in the atmosphere.

It has been the Italian Marsis radar of the European ship Mars Express that has finally found the liquid water that Mars has today. He has done so based on observations made over three and a half years, from May 2012 to December 2015.

Researchers have analyzed how, when the spacecraft flew over a specific region of the south pole, the signal that captured the radar changed. The region is located the Planum Australe, a vast ice-covered plain, where until now nothing of special interest has been found.

When passing over this plain, the radar registered the signal reflected by the surface of the ice, as well as a second weaker signal reflected by the bedrock located under the ice. But over an area about 20 kilometers in diameter, a more intense reflection appeared. After considering different hypotheses, researchers have concluded that the only plausible explanation for this stronger signal is that the reflection is caused by liquid water.

This discovery opens new possibilities for the search for extremophile microorganisms

A detailed analysis of the data indicates that the lake has an approximately triangular shape, although the Marsis radar does not have enough resolution to accurately map it. The images published in Science allow estimating an area of about 150 square kilometers.

Its depth is unknown. You can not know why the reflection of the lake bed is not seen in the radar data, but, seeing the signal, the thickness of the body of water should be at least several meters.

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The temperature at the base of the polar ice cap has been estimated at 68 degrees below zero, a value too low to allow the existence of pure water in a liquid state. It is known that the melting temperature is reduced when the pressure increases, which allows the existence of lakes such as the Vostok in Antarctica at negative temperatures. Even so, the 1,500 meters of ice covering the lake at the south pole of Mars are not enough to explain that it contains liquid water.

On the other hand, also the presence of salts in the water reduces the melting temperature - which is the reason why salt is thrown on the roads when it snows, since it lowers the temperature at which ice forms. In the south pole of Mars, at 1,500 meters depth, magnesium perchlorate would allow liquid water at 69 ° C below zero and calcium perchlorate would allow it to have it up to 75 ° C below zero.

The liquid water has been discovered with the radar of the Mars Express spacecraft, of the European Space Agency

At this point, the researchers provide two data to explain the discovery of liquid water at the south pole of Mars. Remember that the NASA Phoenix mission found in 2008 large amounts of sodium, calcium and magnesium perchlorates near the north pole of Mars, so it is plausible that there are also in the south pole. And they point out that there are lakes in Antarctica that contain concentrations of salts much higher than those of seawater. With this background, the Marsis instrument team proposes that, if liquid water under the polar ice of Mars, it is probably thanks to a very high concentration of salts.

Researchers have searched if there may also be water under the ice of the Martian north pole, but they have not found it. The northern polar cap "is very transparent to the Marsis radio waves because it contains less impurities than the southern one. Therefore, the base of the ice bin looks very good on the radar data.

Even so, the Marsis team does not rule out that there may be underground lakes in other regions of Mars. In the same south pole, indications of other possible masses of water under the ice have been detected, although the radar data does not have enough resolution to confirm it. It is also possible that there are water reserves not yet discovered in other latitudes.