An artist's impression of the Mars Express spacecraft probing the southern hemisphere of the planet. Image / USGS Astrogeology Science Center/Arizona State University/ESA/INAF
Using a satellite to peer beneath layers of dust and ice at Mars' south pole, scientists have detected a 20km-wide span of briny water, a large, stable reservoir akin to lakes buried beneath the Antarctic ice sheet on Earth.
The long-sought discovery, the largest detection of liquid water on the Red Planet yet, raises the tantalising possibility of a very cold, very salty niche where life might have once existed - or even persisted.
"This could be, perhaps, the first habitat we find on Mars," said planetary scientist Roberto Orosei of the National Institute of Astrophysics in Italy, who led the study published in the journal Science.
To be clear, there's no sign of any actual Martian microbes swimming around, and the environment is not obviously hospitable - the water at the base of the polar cap is estimated to be minus-68 degrees C, far below the typical freezing point of water. Scientists believe the water is kept in liquid form by a salty brine that Orosei and colleagues speculatively describe as a "sludge."
Scientists aren't even exactly sure what to call the body of water, which they detected by analysing radar echoes gathered over three years by the orbiting Mars Express spacecraft. They cannot see the bottom with existing equipment, but they estimate it is at least 1m deep, otherwise they would not have detected it at all. It could be a sub-glacial lake, an aquifer, or a layer of sediment saturated with water.
Evidence of water has been seen on Mars many times, but it is usually ancient, fleeting or frozen. The detection of a long-standing reservoir of liquid water is thrilling evidence of an idea that first began to be debated three decades ago: that there could be water at the base of Mars' ice caps, similar to what's present on Earth.
The finding, if confirmed, will provide new insight into Mars' climate history, spur a search for other subsurface water sources, provide a possible resource if people ever travel to the planet - and help focus the ever-hopeful search for signs of Martian life, past or present.
One of the ingredients that scientists look for in the search for life is water - not just trace amounts of humidity or ice that freezes and vaporizes, but stable sources of water - such as an underground lake or aquifer.
"When extremes occur, life moves into the rocks. That's a fundamental aspect of astrobiology," said Jim Green, chief scientist of Nasa, who was not involved in the study. "The concept of liquid water somewhere on Mars leads one to believe that there may be an environment that would harbor extent life."
Orosei and colleagues used a radar instrument called Marsis (Mars Advanced Radar for Subsurface and Ionosphere Sounding) aboard the Mars Express spacecraft to make their discovery. Marsis sends electromagnetic pulses down to the planet and measures how they echo back - and Orosei and colleagues discovered especially bright reflections from a broad region spanning about 12 miles, about a mile below the ice.
The measurements were taken over three years. Then, concerned that their hope the bright spots might be water could blind them to other explanations, Orosei and colleagues spent almost as long trying to demolish their own data. Mars, like many frontier areas of science, has a history of exciting findings - such as the discovery of flowing water on the surface in 2015 - that turn out to be explained mostly by something far more mundane, flowing dust.
Jeffrey Plaut of Nasa's Jet Propulsion Laboratory said in an email that the interpretation that it is liquid water is "certainly plausible, but it's not quite a slam dunk yet."
"On Earth, nobody would be surprised by such a discovery. The normal interpretation would be we have discovered a subglacial lake and people would drill and find if that's true," Orosei said. "On Mars, that's much more difficult, of course, because we can't really drill into the ice."
Green said that a lander mission called InSight that's already on its way to Mars might be able to help provide corroborating evidence. The lander, set to touch down in November, has a heat probe instrument that will drill five meters into the ground and take temperature measurements. That will allow scientists to create models of the heat flowing out of the planet, like a cake that's cooling off after it has been baked - and should give insight into whether it's plausible that the temperature could be high enough to keep water in liquid form at that depth.
Several researchers said it would be crucial to figure out whether this body of water is the only one, or part of an interconnecting body of underground aquifers - in part because a network increases the possibility it could have harbored life.
Stephen Clifford, a scientist who works today at the Planetary Science Institute, first laid out the idea that there could be bodies of water beneath Mars' polar caps three decades ago.
"It is rewarding, in the sense that the work that I did was purely theoretical, and I think it's always a good thing when someone finds some evidence your theoretical work has some relation to reality," Clifford said. "This result has made the effort worth it - and the wait for the evidence worth it."