Maybe the polar 'lakes' on Mars are just frozen clay.

According to a new study, the light reflections detected by the radar below the south pole of Mars are perhaps not underground lakes as previously thought, but rather clay deposits.

For decades, scientists have suspected that water lies below the polar ice caps of Mars, just as it does here on Earth. In 2018, researchers using the MARSIS radar sounder instrument on the European Space Agency's Mars Express spacecraft detected evidence for a lake hidden beneath the Red Planet's south polar ice cap, and in 2020, they found signs of several super-salty lakes there. If these lakes were remnants of water that was once on the surface, these reservoirs might have sheltered life and could again, the scientists noted.

However, to form and maintain liquid water at this spot on Mars, an implausible amount of heat and salt may be needed, given what is currently known about the Red Planet, according to the lead author of the new study, Isaac Smith, a planetary scientist at York University in Toronto, and his colleagues.

York and her team say that clays known to exist in the southern polar region of Mars can explain these radar reflections without invoking water lakes.

"Within the Martian community there was a certain skepticism about the interpretation of the lake, but no one had offered a really plausible alternative," Smith told Space.com. "It is therefore exciting to be able to demonstrate that something else can explain the radar observations and to demonstrate that the equipment is present where it should be. I love to solve puzzles, and Mars has endless puzzles."

Scientists have concentrated on minerals called smectites, a kind of clay whose chemical composition is closer to volcanic rock than with other types of clay. Smectites occur when eroded volcanic rocks undergo light chemical changes following interaction with water. These clays can contain significant amounts of water, they point out.

Smectite is extremely abundant on Mars, concentrated mainly in its southern uplands. "On Earth, they are often found close to volcanoes in Alaska or Central America, but they can be found on every continent," Smith said.

In the laboratory, the scientists cooled the smectites to minus 45 degrees Fahrenheit (minus 43 degrees Celsius), the type of cold that could be found on Mars. They found that water-laden smectites could generate the kind of bright radar reflections detected by MARSIS (short for "Mars Advanced Radar for Subsurface and Ionospheric Sounding"), even when mixed with other materials.

When Smith and his colleagues analyzed the visible and near-infrared data collected at the Southern Pole of Mars, they also found traces of smectites. They suggested that smectites formed in the South Pole of Mars during heat periods when the area was covered by water. These water-laden clays were then buried below the water ice.

"Looking back in time, when Mars was much wetter, it proves that liquid water was present on a greater area than we had expected,' said Mr. Smith. "Because these clays are below the southern polar ice sheet, there must have been sufficient heat long ago to support the liquids."

Overall, the researchers suggested that smectites are a more viable explanation for the brilliant radar glare observed there instead of super-salty lakes.

"Science is a process, and scientists are always working towards truth," said Mr. Smith. "Showing that materials other than liquid water can make radar observations does not mean that it was wrong to release the initial results in 2018. This provided a lot of ideas from people for new experiments, modeling and observations. Those ideas have translated into more Mars investigations and are already part of my team."

In the future, "I would like to repeat the measurements at even colder temperatures and with a more diverse array of clay," Smith said. "There are other types of clay that are found on Mars and I suspect they can also produce these insights, and it would be good to follow them."

The scientists presented their findings today (29 July) in the Geophysical Review Letters magazine.