Researchers have found evidence of an existing body of liquid water on Mars.
Scientists believe there could be a 20km-long lake siting under Mar's south polar ice cap. Photo: Supplied / NASA/JPL/Malin Space Science Systems
What they believe to be a lake sits under the planet's south polar ice cap, and is about 20km across.
Previous research found possible signs of intermittent liquid water flowing on the martian surface, but this is the first sign of a persistent body of water on the planet in the present day.
Lake beds like those explored by Nasa's Curiosity rover show water was present on the surface of Mars in the past.
However, the planet's climate has since cooled due to its thin atmosphere, leaving most of its water locked up in ice.
Mars Photo: Supplied / NASA/JPL/Malin Space Science Systems
The result is exciting because scientists have long searched for signs of present-day liquid water on Mars, but these have come up empty or yielded ambiguous findings. It will also interest those studying the possibilities for life beyond Earth - though it does not yet raise the stakes in the search for biology.
The discovery was made using Marsis, a radar instrument on board the European Space Agency's (Esa) Mars Express orbiter.
"It's probably not a very large lake," Professor Roberto Orosei, who led the study, said.
Marsis was not able to determine how thick the layer of water might be, but the research team estimated that it was a minimum of one metre.
"This really qualifies this as a body of water. A lake, not some kind of meltwater filling some space between rock and ice, as happens in certain glaciers on Earth," Prof Orosei said.
Radar instruments like Marsis examine the surface and immediate subsurface of the planet by sending out a signal and examining what is bounced back.
The continuous white line at the top of the radar results above marks the beginning of the South Polar Layered Deposit; a filo pastry-like accumulation of water ice and dust.
Beneath this, researchers spotted something unusual 1.5km beneath the ice.
"In light blue you can see where the reflections from the bottom are stronger than surface reflection. This is something that is to us the tell tale sign of the presence of water," Prof Orosei said.
What does this mean for life?
Nothing definitive. Yet.
"We have long since known that the surface of Mars is inhospitable to life as we know it, so the search for life on Mars is now in the subsurface," Manish Patel from the Open University said.
"This is where we get sufficient protection from harmful radiation, and the pressure and temperature rise to more favourable levels. Most importantly, this allows liquid water, essential for life."
This principle of following the water is key to astrobiology - the study of potential life beyond Earth.
So while the findings suggest water is present, they do not confirm anything further.
"We are not closer to actually detecting life," Dr Patel told BBC News, "but what this finding does is give us the location of where to look on Mars. It is like a treasure map - except in this case, there will be lots of 'X's marking the spots."
The water's temperature and chemistry could also pose a problem for any potential martian organisms.
In order to remain liquid in such cold conditions (the research team estimate between -10°C and -30° where it meets the ice above), the water likely has a great many salts dissolved in it.
"It's plausible that the water may be an extremely cold, concentrated brine, which would be pretty challenging for life," Claire Cousins, an astrobiologist from the University of St Andrews, said.
-BBC