It’s awfully hard to kill a planet—and Mars should know, because Mars ought to be dead by now. Long ago, perhaps 4.3 billion years back, the Red Planet was a place not unlike Earth. It had a thick atmosphere and abundant water, much of which might have been concentrated in a vast ocean in its northern hemisphere. All over the rest of the planet were lakes, smaller oceans and rivers.
But Mars’s interior soon cooled. That snuffed out its protective magnetic field, which in turn allowed charged particles streaming from the sun to claw away Mars’ atmosphere. Once the air was gone, the water sputtered into space. Without water, life, at least as we know it, is impossible.
And yet, Mars is hanging on. In a study published Wednesday in Science, researchers working with the European Space Agency’s Mars Express spacecraft report the existence of a 12.5-mile-wide liquid water lake just beneath a layer of ice at Mars’s south pole. On Earth, life got its start in stable bodies of water, and the same could be true on Mars. What’s more, where there’s one lake, there could easily be several.
“There is no reason to conclude,” the study’s authors wrote, “that the presence of subsurface water is limited to a single location.”
It’s been clear for a while that at least some Martian water was able to survive the great drying that hit the planet so long ago. In 2008, the robotic arm on NASA’s Phoenix lander scraped away the Martian topsoil and found a layer of water ice. More remarkably, in 2011, NASA’s Mars Reconnaissance Orbiter discovered streaks on Martian slopes that occur during the planet’s spring, likely caused by underground ice deposits thawing and running as the temperature grows warmer.
Until now, however, standing water had not been spotted on Mars. But the researchers who made the new discovery—led by astrophysicist Roberto Orosei, of the National Institute of Astrophysics in Bologna, Italy—knew that the place to look was in the polar regions. In Greenland and Antarctica, geologists have discovered lakes beneath surface ice sheets, kept liquid by the pressure of the ice and by salts and other minerals, which lower the melting point of water, preventing it from freezing.
Mars has ice caps too, and there is plenty of sodium perchlorate, magnesium and calcium in the planet’s soil, which could have similar anti-freeze effects. Oreosei and his colleagues thus decided to request time with the radar system onboard Mars Express, to see if they couldn’t go prospecting for water in a southern polar region known as Planum Australe, where earlier radar soundings had produced reflections that at least suggested the presence of water.
Over the course of three and a half years, from May 2012 to December 2015, the team conducted 29 soundings as the spacecraft swung by the targeted region in the course of its orbits of Mars. What they were looking for were slight differences in the two-way travel time of the radar pulses as they were beamed to the surface and bounced back to the spacecraft. That can reveal the density and make-up of whatever type of surface or subsurface material the radar pulses struck.
On each pass, the spacecraft sampled a slightly different patch of ground, producing a series of radar footprints that overlapped in varying degrees. Consistently, a single 12.5-mile patch returned the distinctive signature of liquid water, lying beneath a relatively thin 0.9-mile-thick layer of ice. The water is not pristine; it’s briny and at points perhaps sludgy, mixed with subsurface soils. But it’s unmistakably water and it’s unmistakably liquid.
Whether life could survive in Mars’ south polar lake—which is yet unnamed—is a decidedly iffier matter. On Earth, salty ocean water can remain liquid down to temperatures of just over 28º F (-2º C). In the Martian lake, with its overlying pressure and its rich mineral mixture, the water could be as cold as -103º F (-75º C). That’s an awfully frigid environment in which to try to make a biological go of things.
Still, life on Earth has been found in some pretty punishing places: in the interior of desert rocks, in the scalding hot springs of Yellowstone National Park, in Siberian permafrost. Compared to that, a little Martian ice water doesn’t seem so bad. Ultimately, Mars may well turn out to be an entirely dead world. But for now, the question remains very much an open one—and thanks to the new study, researchers now have more places than ever to look for a hint of a Martian pulse.
Correction July 25
The original version of this story misstated the potential freezing temperature of ocean saltwater on Earth. It is about -2º C, not 2º C.