An international team of researchers claim to have found a lake of liquid water beneath an ice cap on Mars.
The lake, which is circular and only about 20 kilometres wide, is the first time that liquid water has been found on the planet, and comes on top of evidence that Mars may have hosted seas and rivers in the past. The lake is about 1.6 kilometres below the surface and at least 900 centimetres deep - any less and it would not have been picked up.
Water is one of the essential elements required to support life and, hence, indicates not only that the planet could once have supported life, but could potentially support human settlement in the future.
However, with a temperature estimated at between -10 and -30 degrees Celsius, the assumption is that the water has an exceptionally high salt content and probably doesn't currently support life of any form.
"It's plausible that the water may be an extremely cold, concentrated brine, which would be pretty challenging for life," Dr Claire Cousins, an astrobiologist from the University of St Andrews, told the BBC.
The finding was made using the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument, a radar device on the Mars Express orbiter, a European Space Agency satellite orbiting the red planet.
Mars Express was launched in 2003 when the orbits of Earth and Mars were the closest they had been in 60,000 years. When it arrived, it dropped the Beagle 2 lander, which never made contact after it was jettisoned and declared lost in February 2004.
The satellite, though, has made more than 5,000 orbits of the red planet, taking images and other measurements with the various instruments on board and sending them back to Earth for analysis.
Mars Express was the first probe to discover ice on Mars back in January 2004 - at the same South Polar ice cap under which one of its devices has now been used to discover water.
The research is published today in the journal Science.
The presence of liquid water at the base of the Martian polar ice cap has long been suspected but not observed, explains the research paper, while NASA's Curiosity rover recently found evidence that the surface of the planet once had running water - evidence, too, that the planet had an atmosphere and a warmer surface at some point in the past.
The paper goes on to explain how the scientists came to their conclusion.
"Radar profiles collected between May 2012 and December 2015 contain evidence of liquid water trapped below the ice of the South Polar Layered Deposits. Anomalously bright subsurface reflections are evident within a well-defined, 20-kilometre-wide zone centred at 193°E, 81°S, which is surrounded by much less reflective areas."
There is a possibility that rather than water, the images are showing evidence of liquid CO2. However, the scientists rejected that possibility "either because of the very specific and unlikely physical conditions required, or because they do not cause sufficiently strong basal reflections".
The search for water on Mars has been ongoing since the first probes and rovers were landed on the planet. An investigation by the University of Arizona in 2008, however, poured cold water on earlier reports that some gulleys and troughs on the planet's surface might hold liquid H2O.
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