Mars' Hidden Lakes? The Rise and Fall of the Saltwater Lake Claim — and What Comes Next

Few headlines have fired the imagination quite like the one from 2020: a lake of liquid water, buried beneath the south pole of Mars. The discovery promised everything explorers dream of — water, and perhaps even life, hiding under the red planet's ice. But what began as a triumph of radar science turned into one of the most hotly contested debates in planetary geology. Six years on, the "lake" may be gone, but the story of how it was found — and how it was challenged — reveals how science really works.

The Discovery That Started It All

The hunt began with a European spacecraft called Mars Express and its radar instrument, MARSIS, which has been probing beneath the Martian surface since 2005. Radar works like an echo sounder: it sends radio waves into the ground and listens for the reflections. Liquid water reflects radar far more strongly than ice or dry rock, so a bright echo from deep below is a classic signature of buried water — the same trick scientists use to find subglacial lakes in Antarctica, like the famous Lake Vostok.

In 2018, a team led by Roberto Orosei of Italy's National Institute for Astrophysics reported a bright radar reflection about 1.5 kilometers beneath the layered ice deposits of Mars' south pole, near 81 degrees south latitude. The reflection was roughly 20 kilometers across — and on Earth, such a signature would almost certainly mean a lake of liquid water trapped between ice and bedrock.

Then came the bigger claim. In 2020, Elena Pettinelli of Roma Tre University and colleagues analyzed 134 MARSIS radar profiles collected between 2010 and 2019 over a region called Ultimi Scopuli. Their conclusion, published in Nature Astronomy, was striking: not just one lake, but a central body of water roughly 20 by 30 kilometers across, surrounded by smaller pools and wetlands, separated by strips of dry rock. The team used the same analytical method that had already located subglacial lakes on Greenland and in Antarctica.

Why a Liquid Lake on Mars Would Be So Strange

On the surface, Mars is brutally cold — average temperatures hover around minus 63 degrees Celsius, and the south polar ice cap is far colder still. Pure water could not stay liquid at the base of that ice. The researchers' answer: salt. Extremely salty water, loaded with perchlorate salts known to exist in Martian soil, can remain liquid at temperatures far below freezing. Laboratory work has shown that certain exotic brines can stay liquid down to around minus 75 degrees Celsius.

This is where the story gets scientifically interesting — and where the trouble began. For a lake to exist, you need not just salt, but enough heat from the planet's interior to keep the bottom of the ice cap from freezing solid. And Mars is a small, geologically quiet planet with a feeble internal heat flow compared to Earth.

The Skeptics Strike Back

Almost immediately, planetary scientists raised objections. In 2021, researchers including Ali Bramson and others published analyses showing that the amount of salt and geothermal heat required to keep a lake liquid at the base of the south polar cap was far more than Mars actually provides. The math simply didn't add up: Mars would need either an implausibly concentrated brine or a local heat source nobody could find.

Then came the alternative explanations. A team led by Isaac Smith of York University in Canada, published in Geophysical Research Letters, demonstrated that smectites — a common type of clay mineral — could explain all of the radar observations without any liquid water at all. The team backed this with laboratory experiments and modeling, and even found spectral evidence that smectites exist at the edges of the south polar cap. As Smith put it, the scientific community had been skeptical of the lake hypothesis from the start.

Meanwhile, researchers at Cornell University, led by Daniel Lalich, offered yet another explanation: constructive interference of radar waves. Their simulations showed that natural, random variations in the composition and thickness of the ice layers could produce exactly the bright reflections MARSIS saw — "through random chance," as Lalich explained — using only materials already known to exist at the Martian poles. No lakes, no brines, nothing exotic required.

The Decisive Test: A Second Radar Looks Closer

The most telling evidence came from a completely different instrument. NASA's Mars Reconnaissance Orbiter carries SHARAD, a shallower-penetrating radar that had never been able to see the base of the ice in the lake region at all — until engineers tried something creative. By rotating the spacecraft a full 120 degrees in space — a "very large roll" maneuver — they boosted SHARAD's signal enough to reach the bottom of the ice cap for the first time.

A team led by Gareth Morgan examined 91 SHARAD scans over the same high-reflectivity area. The result: only with the new maneuver did SHARAD detect any echo from the base of the ice, and even then the signal was extremely weak — roughly one-tenth of one percent of the surface reflection, and nearly identical to a basal reflection 80 kilometers away in a region with no suspected lakes. MARSIS, by contrast, had seen the spot reflecting more than three times brighter than the surface. That gap of about 30 decibels — a thousandfold difference in power — is not what liquid water would produce. The faint SHARAD echo, researchers suggested, is more consistent with a smooth, flat patch of ground under the ice that happens to reflect radar strongly.

The study, published in Geophysical Research Letters, didn't completely rule out liquid water. But it added a powerful piece of evidence making lakes much less likely at this location.

The Other Side: The Case for Brine Lives On

The story isn't entirely one-sided. A Southwest Research Institute geophysicist, David Stillman, conducted laboratory measurements of ice-brine mixtures at temperatures as low as minus 98 degrees Celsius, showing how salty water trapped between grains of ice or sediment could produce exactly the kind of bright reflections MARSIS detected. Rather than a big open lake, the bright signal might come from brine-soaked sediments — a kind of slushy, water-rich layer rather than a swimming pool.

This middle-ground view keeps the door open: even if there are no grand subglacial lakes, Mars may still harbor pockets of salty liquid water in its subsurface — environments that, on Earth, host microbial life in places like Antarctica's Blood Falls and deep-sea brine pools.

Why It Matters: The Search for Life

The stakes were always about more than geology. Liquid water is the single most important requirement for life as we know it. If stable lakes of brine existed under Mars' south pole, they would be prime targets in the search for extraterrestrial life — protected from radiation, insulated from the freezing surface, and potentially stable for millions of years. That's why the 2020 claim electrified astrobiologists, and why its unraveling is genuinely disappointing.

But the episode also taught planetary science a valuable lesson about how easily our hopes can color our interpretation of data. A bright radar echo is not a lake until every alternative has been eliminated — and on Mars, the alternatives turned out to be more plausible than the lake.

What Comes Next for Mars Water Science

The debate is not over, and future missions will keep digging — literally and figuratively:

• Ongoing radar mapping continues to refine our picture of the Martian subsurface, with both MARSIS and SHARAD still operating.
• The ExoMars Rosalind Franklin rover, Europe's long-delayed mission, will drill up to two meters into Martian soil searching for chemical signatures of past or present life.
• Sample return missions aim to bring Martian material to Earth laboratories, where far more sensitive analysis is possible than anything a rover can do on the surface.
• Groundwater science has shifted toward a broader question: even without south-polar lakes, Mars almost certainly has deep groundwater — possibly global — left over from its warmer, wetter past. Finding it is now one of the great challenges of the next decade.

Key Takeaways

• In 2018 and 2020, MARSIS radar data suggested liquid saltwater lakes beneath Mars' south polar ice cap.
• Follow-up studies showed the required salt and heat were implausibly high — and that clays, radar interference, or flat bedrock could explain the same signals.
• A 2025 SHARAD "very large roll" test found only a faint echo at the lake site, strongly weakening the lake hypothesis.
• Lab work keeps alive the possibility of brine-soaked sediments rather than open lakes — still interesting for the search for life.
• The episode shows science at its best: bold claims, rigorous challenge, and a more accurate picture of Mars emerging from the debate.