Underground Lake Found on Mars? Get the Facts.

The 12-mile-wide reservoir could solve a Martian mystery—and offer a new target in the hunt for extraterrestrial life.

An orbital view shows the south polar cap on Mars, where scientists using radar may have found a buried watery reservoir.

Liquid water is refreshingly abundant on moons in the outer solar system, but it has proven surprisingly tough to find in reliable quantities on Mars—until now.

Radar scans of the red planet suggest that a stable reservoir of salty, liquid water measuring some 12 miles across lies nearly a mile beneath the planet’s south pole. What’s more, the underground lake is not likely to be alone.

“There are other areas that seem to be similar. There’s no reason to say this is the only one,” says Elena Pettinelli of Italy’s Roma Tre University, a coauthor of the paper reporting the discoverytoday in the journal Science.

If confirmed, the buried pocket of water could answer a few questions about where Mars’s ancient oceans went, as well as provide a resource for future human settlements. Even more thrilling for astrobiologists, such a feature may be an ideal habitat for extraterrestrial life-forms.

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A self-portrait of the Mars rover Curiosity.

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This field of dunes lies on the floor of an old crater in Noachis Terra, one of the oldest places on Mars.
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This enhanced color image shows several craters somewhere in the southern mid-latitudes of Mars. The bluish deposits are most likely iron-bearing minerals that have not been previously oxidized, or rusted.

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The impact site of the heat shield from NASA's Mars exploration rover Opportunity.

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Sand dunes are among the most widespread aeolian, or wind-blown, features on Mars. These areas provide clues to the sedimentary history of the surrounding terrain.

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Groups of dark brown streaks were photographed by the Mars Reconnaissance Orbiter on melting pinkish sand dunes covered with light frost.

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The panoramic camera on NASA's Mars exploration rover Opportunity captured this scene of the west rim of the Endeavour crater during the summer of 2014.

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Sand dunes litter the floor of Aram Chaos, an eroded impact crater east of Mars's Valles Marineris canyon range.

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Melas Chasma is the widest segment of Valles Marineris, the largest canyon in the solar system.

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The Opportunity rover spent four months perched on the northern slope of Greely Haven and snapped more than 800 images of its surroundings.

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The Russell crater dunes are a favorite target for the Mars Reconnaissance Orbiter's HiRISE camera, not only because of their incredible beauty, but for measuring the accumulation of frost each fall and its disappearance in the spring.

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The holes visible in this image are not impact craters, but rather material that was ejected from a large crater called Hale that does not appear here. Explosions formed moats, which have been partially covered over time by sand dunes (top).

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NASA's Curiosity rover snapped this 360-degree panorama as part of a long-term campaign to document the context and details of the geology and landforms along Curiosity's traverse since landing in 2012.

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Recurring slope lineae (RSL) are seasonal flows on warm slopes and are especially common in central and eastern Valles Marineris.

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Victoria crater has a distinctive "scalloped" shape to its rim, caused by erosion and downhill movement of crater wall material.

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Dust accumulation on a rover's solar panels reduces its power supply, and the rover's mobility is limited until winter is over or wind cleans the panels.

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The Curiosity rover captured the Bagnold Dunes inside Gale crater on September 25, 2015.

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Partially exposed bedrock is exposed within the Koval'sky impact basin.

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This dramatic, fresh impact crater spans approximately 100 feet (30 meters) in diameter and is surrounded by a large, rayed blast zone.

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The north polar layered deposits are layers of dusty ice up to three kilometers (two miles) thick and approximately 1,000 kilometers (620 miles) in diameter.

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Sand dunes are among the most widespread wind-formed features on Mars. Their distribution and shapes are affected by changes in wind direction and wind strength.

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This selfie from NASA's Curiosity rover shows the vehice at the site where it reached down to drill into a rock target called "Buckskin" on lower Mount Sharp.

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Researchers used the Curiosity rover in March 2015 to examine the structure and composition of the crisscrossing veins at the "Garden City" site in the center of this scene.

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This crater near Sirenum Fossae has steep inner slopes carved by gullies.

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This image covers many shallow irregular pits with raised rims, but researchers aren't sure how these odd features formed.

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At this site in the lower mounter in Gale crater, orbiting instruments have detected signatures of both clay minerals and sulfate salts. This change in mineralogy may reflect a change in the ancient environment in Gale crater.

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Viscous, lobate flow features are commonly found at the bases of slopes in the mid-latitudes of Mars and are often associated with gullies.

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Many Martian landscapes contain features that are similar to ones we find on Earth, like river valleys, cliffs, glaciers, and volcanoes.

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Two of the raised treads, called grousers, on the left middle wheel of NASA's Curiosity rover broke during the first quarter of 2017, including the one seen partially detached at the top of the wheel in this image from the camera on the rover's arm.

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In the scenic Murray Buttes area, individual buttes and mesas were assigned numbers. This one is referred to as "M9a."

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This observation from NASA's Mars Reconnaissance Orbiter shows late summer in Mars's southern hemisphere. The sun is low in the sky, highlighting the subtle topography.

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NASA's Curiosity rover team has assessed the small bright object just below the center of this image and believe it is debris from the spacecraft.

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The High Dune, which is part of the Bagnold Dunes, was the first Martian sand dune ever studied up close. The dunes are active, migrating up to about one yard or meter per year.

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A towering dust devil casts a serpentine shadow over the Martian surface in this stunning late-springtime image of Amazonis Planitia.

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Landforms called "gullies" found on many large Martian sand dunes consist of an alcove, channel, and apron.

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NASA plans to launch another rover to Mars in 2020.

Tap images for captions

“In this kind of environment that we know of on Earth, in the Antarctic, we have bacteria,” Pettinelli says. “They can be deep in the ice.”

“Mars will come to fear my botany powers.”

Billions of years ago, Mars was likely warm and covered in seas like its larger, bluer neighbor. But the Mars of today is a parched, toxic desert, and for decades, scientists have been trying to decipher what happened to the water that once soaked, sloshed, and tumbled over its sands.

To date, scientists have found water on Mars multiple times, but it’s usually quite transient or inaccessible, either hovering in the atmosphere, locked into permafrostor polar caps, or perhaps seasonally seeping down crater slopes. And the amount we’ve found doesn’t quite fill those ancient Martian seas or make crop-growing particularly easy.

“We know there was a lot of water on Mars’s surface, and we can’t quite account for all of it today,” says Bobby Braun of the University of Colorado, Boulder. So, scientists hypothesized that some of the missing water could be trapped in subsurface aquifers containing vast stores of liquid.

“I'm going to have to science the s--t out of this.”

However, humans didn’t launch spacecraft capable of sniffing out buried water until earlier this millennium.

One of them, the European Space Agency’s Mars Express spacecraft, has been orbiting the fourth rock from the sun since 2003; riding on board is an instrument called MARSIS, which uses radar pulses to see into the Martian subsurface. It works by sending low-frequency radio waves into the planet, which burrow beneath the ground until they bounce off geologic structures and boundaries. By studying how those waves are reflected back to the spacecraft, scientists can infer what might lie beneath.

In 2008, the MARSIS team saw glimmers of what could be some very bright reflections near the planet’s south pole, in an area where frozen ice sheets are stacked atop one another. On Earth, the brightest radar reflectors are pools of briny water—and so the team decided to take a closer look at the region.

After a few years of gathering data that weren’t exactly useful, the team finally began collecting enough observations in 2012 to assemble a bigger picture. From there, it took another three years and 29 spacecraft passes before scientists had enough information to work with.

Mars 101

“We knew that there was something there, and we were curious to know what was under that area,” Pettinelli recalls. “And we were stubborn enough to do the data analysis.”

Sorting through the MARSIS data wasn’t easy. Over the next two years, the team compiled and processed the observations and worked very hard to rule out alternate possibilities, such as deeper layers of carbon dioxide ice.

Eventually, by comparing the reflectance patterns on Mars with those seen on Earth, scientists were convinced that they’d found a subglacial lake. Perhaps several feet deep, the lake contains various salts that help keep it liquid at extremely cold temperatures, the team speculates.

“This is space. It does not cooperate.”

The Mars Express team compares the discovery to lakes tucked beneath the ice sheets of Greenland and Antarctica, which are sometimes huge and certainly more than capable of hosting life as we know it.

But not everyone is convinced that the “lake” is actually a lake. Even according to the team, it could instead be a deposit of dampened sludge, more like muddy sediments than a pocket filled with liquid. Determining the exact nature of the structure will require a different instrument, Pettinelli says.

“We can’t choose between one or the other. We don’t have enough information to say this is a lake or saturated sediment like an aquifer,” Pettinelli says. “The lake will be more interesting.”

There is another ground-penetrating radar orbiting Mars right now, and it adds an interesting twist.

“We do not see this reflector,” says Bruce Campbell of Smithsonian’s National Air and Space Museum, who is a member of the team operating an instrument called SHARAD on NASA’s Mars Reconnaissance Orbiter.

MRO has orbited Mars since 2006, sweeping its radar over vast swaths of alien landscape, including many passes over the south pole’s layered deposits, and it hasn’t turned up anything resembling a reservoir.

This is likely because MRO’s radar uses different wavelengths that are scattered by the polar ice before they reach the potential reservoir’s depth, says Jack Holt of the University of Arizona. But anything as reflective as a liquid lake should be identifiable by SHARAD, he notes.

Dark basins and bright polar caps are among the most distinctive features on Mars.

“A brine is probably the strongest radar reflector you can come up with, aside from metal,” he says. “A lake would mean a smooth, mirror-like reflection that is more likely to show up in SHARAD [but] if it’s saturated sediments, it could be a rougher surface and therefore easier to miss with SHARAD.”

In all, scientists are eager to confirm the finding—including those on the MARSIS team.

“I think, we’ve done a good job in trying to kill this idea, in the sense that we have been trying to destroy the possibility that it was water many times,” Pettinelli says. “So we are quite convinced now, and we hope to be more convinced in the future with other data.”

“The good thing is, I know the recipe: You take hydrogen, you add oxygen, and you burn.”

Assuming it’s there, this little pocket of saltwater may help solve the mystery of Mars’s missing oceans. It’s also a clue about the planet’s hydrological cycle, which is theorized to involve buried aquifers charged by melting polar ice caps, with the bulk of the water running north from the southern highlands, says Nathalie Cabrol of the SETI Institute.

“You can intuit that there would be either very damp sediments at the poles of Mars, or a lens of liquid water,” she says. “This is where you expect this reservoir to be.”

No stranger to water in weird places, Cabrol’s work on Earth includes investigating Mars analog environments, which sometimes requires diving into lakes tucked among the peaks of the high Andes. Whether the MARSIS find is soggy sediments or an actual lake, she says it’s thrilling.

“What you see here is potentially the presence of water, of shelter ... and you’re going to produce nutrients out of the minerals,” she says. “What you need is a source of energy … and if there were recent volcanoes in the polar regions, then this is definitely a place that would become a high habitability and life target.”

On the other hand, she notes, “it would be a very problematic place to go, because it would be under the special regions for planetary protection,” she says, referring to the UN regulations aimed at preventing interplanetary contamination of habitable environments.

The new reservoir is also the kind of resource that humans interested in settling on Mars might want to take advantage of—though perhaps not right away.

“I think it’s very unlikely the first humans on Mars are going to drill down to several kilometers,” says Braun, an advisor for National Geographic’s MARS series and NASA’s former chief technologist.

“But I think it’s probably true that if this is a lake, there are other bodies of water like it that are perhaps closer to the surface,” he says, “and if we knew that there was a big body of water at tens of meters down, that would be something you’d certainly want to know about when you’re planning a base camp.” 

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