When searching for life beyond Earth, scientists commonly “follow the water.” One of the best places to do that could be the Martian south pole, where new research reveals that—beneath a mile of ice—multiple small ponds appear to surround a large lake.
In 2018, scientists announced the discovery of the larger subglacial lake, presumed to be a body of salt water some 12 miles across. Now, additional observations from the same team have identified at least three more watery patches, smaller than the first, in the same region. Those salty deposits—perhaps leftover water from an ancient ocean—could be oases where life may have taken hold.
“The system was probably existing a long time ago, when the planet was very different, and this is maybe the remnant of that,” Pettinelli says.
But not everyone who studies Mars is convinced the underground pockets contain liquid water. That interpretation, some scientists say, is inconsistent with other observations from the same area. Instead, the patches might be soggy puddles of sludge rather than Martian swimming pools. Scientists also don’t know exactly how water could stay liquid in a region where temperatures are only rarely warmer than minus 150°F.
“Patchy sludge at best, maybe?” the University of Arizona’s Jack Holt, who studies Mars using similar techniques, writes in an email. “I’m not really sure what to make of it.”
Probing the poles for hidden lakes
More than a decade ago, Pettinelli and her colleagues turned their attention to a region on Mars called the South Pole Layered Deposits, where radar observations had uncovered bright, reflective areas beneath the frozen glaciers.
“We weren’t looking for water,” she says. “We were looking for bright things and trying to understand what they were.”
To peer beneath the ice, the team aimed a radar called MARSIS, which rides aboard the European Space Agency’s Mars Express spacecraft, at the deposits. MARSIS sends radio waves at the Martian terrain, which travel through surface layers until they meet a change in density or composition and are reflected back to the spacecraft. By deciphering the patterns in reflected radio waves, scientists can figure out what’s beneath a planet’s surface, whether liquid, rock, or mud.
Data collected from 2012 to 2015 originally indicated that a large, salty lake might be buried beneath a region called Ultimi Scopuli. On Mars, the salts are as alien as the planet—instead of being familiar sodium chloride, they’re toxic perchlorates, derived from the planet’s ruddy surface. Based on 29 observations, the team determined that the lake measured roughly 12 miles across. But scientists were skeptical, even those on the discovery team, and were hesitant to say for sure that the lake was liquid rather than sludge.
“We can’t choose between one or the other,” Pettinelli said at the time. “We don’t have enough information to say this is a lake or saturated sediment like an aquifer.”
Now, 105 additional observations, collected between 2015 and 2019 and covering a larger area, have reinforced the original detection. Pettinelli and her colleagues also processed the new data using techniques that are commonly employed to search for lakes beneath the ice sheets near Earth’s poles. In addition to returning evidence that the reservoir was water, the data pointed to at least three smaller patches around it, separated by dry soil.
While the team is confident that the larger reservoir is salt water, Pettinelli says those smaller patches could easily be soggy sediments—an environment where life could still exist.
“I agree with the authors that the most plausible explanation for the MARSIS observations is the presence of a localized body of liquid water, surrounded by either smaller bodies of liquid water or patches of water-saturated sediment,” Steve Clifford of the Planetary Science Institute writes in an email.
Mars’s disappearing water
Although liquid water is abundant on Earth and among the outer solar system’s icy moons, it’s been surprisingly tough to find on Mars. Its signatures are all over the planet’s surface, in the form of carved river beds, fanning deltas, and ancient shorelines, so scientists know that early Mars was a wetter world than the planet we see today—perhaps even temperate and hospitable.
The exact climate conditions of that ancient world are still up for debate. But clearly the climate shifted early in the planet’s history, and Mars transformed from a more watery world into the desiccated planet we see today. Now, scientists are asking: Where did all that water go?
Some of it is locked into ice caps that crown the planet’s poles—permanent icy deposits that seasonally shrink and expand. Shining brightly in telescope observations, the polar caps have been a subject of study for decades as scientists try to better understand the historical record that might be preserved in their accumulated layers.
The bright reflections underneath the ice caps could hint at some of that history. But even so, some Mars experts still question the true nature of the material under the glaciers.
A mystery lurking below
Holt says the new data are more convincing than the original detection—but he’s not yet sure the team has interpreted the observations correctly. Another radar instrument on the Mars Reconnaissance Orbiter has been unable to detect the bright spots. This could be because that radar operates at different frequencies and can’t see all the way to the base of the deposits, but a lake is such a bright reflector that it should still show up, he says.
The interpretation is also inconsistent with MARSIS observations in nearby regions, where similarly bright patches extend to the very edge of the ice sheets. So far, Holt says, no one has explained what those bright patches are—but buried brines are not the answer, because if they were, brines in those areas would seep to the surface.
“If we apply their logic we should see springs along the [glaciers’] edge,” he says. “This is obviously not observed.”
Given that salt water isn’t seeping out from the ice sheet margins, Holt suggests one way to solve the mystery of the supposed underground lakes is to apply the same analytical techniques to a broader data set that includes these other areas of bright reflections. He also notes that the liquid’s observed dielectric permittivity—a measure of its ability to hold an electric charge, determined by radar—is too low for salt water.
“If there was a lake, or lots of liquid, the value would be much higher than what they see,” he says. And even if there is some explanation for those measurements that involves water, “you have to explain the persistence of brine under current conditions.”
Water in a frozen wasteland?
Ice does not melt easily at the frigid Martian poles. At the surface, temperatures hover around minus 270°F, and even though it’s slightly warmer beneath the ice, it’s likely nowhere near warm enough for water to remain liquid.
“Quite chilly, it’s not a nice place,” Pettinelli says.
In 2019, after considering the team’s initial discovery, a pair of scientists suggested that recent magmatic activity could have created a hotspot beneath the south pole. Perhaps, they said, a young magma chamber opened up and is producing enough heat to keep the otherwise frigid region warm enough for liquid salt water to persist. But in the absence of an underground heat source, it’s tough to account for the water’s presence.
“I’m cautiously optimistic that it’s liquid water,” Purdue University’s Ali Bramson, one of the two study authors, writes in an email. “My cautiousness is mostly just related to the extremely cold temperatures here.”
Pettinelli and her team argue that relying on recent volcanic activity to explain the liquid pockets requires an improbable set of circumstances. Instead, they suggest the chemical makeup of the Martian brine could keep water liquid at low temperatures. That might be true, Bramson says, but scientists don’t yet know whether the chemistry and conditions on Mars could create and maintain liquid brines.
“I’m convinced something funky is going on at this site to cause a spike in the reflection,” Bramson says. “Certainly if there is some weird, super-cooled, sludgy salt solution at the base of the polar cap, that’s super cool.”
Perhaps, Clifford says, the new observations could help explain the fate of Mars’s lost ocean. “As the early Martian climate cooled, such an ocean would have frozen and eventually sublimed away,” meaning it evaporated without melting first, Clifford says. Once in the atmosphere, the water vapor would have been transported and then deposited as ice at the planet’s colder poles. Clifford notes that the polar caps were once more expansive, and the planet’s internal heat flow was much higher, meaning water near the base of the ice caps would gradually melt and seep into the subsurface.
Over time, the planet’s vanished ocean may have ended up stored as groundwater or permafrost. The patches of salt water that survive today could be the final remnants of this process—and a place protected from harmful radiation where, over billions of years, life might have found a way to take hold.
“It’s obviously exciting if it is liquid water,” Bramson says. “I think we all want it to be liquid water.”