Three studies published in the past month have cast doubt on the premise of subsurface lakes below the Martian south pole.

Where there’s water, there’s life. That’s the case on Earth, at least, and also why scientists remain tantalized by any evidence suggesting there’s liquid water on cold, dry Mars. The Red Planet is a difficult place to look for liquid water: While water ice is plentiful, any water warm enough to be liquid on the surface would last for only a few moments before turning into vapor in Mars’ wispy air.

Hence the interest generated in 2018, when a team led by Roberto orosei of Italy’s Istituto Nazionale di Astrofisica announced they had found evidence of subsurface lakes deep below the ice cap at Mars’ south pole. The evidence they cited came from a radar instrument aboard the ESA (European Space Agency) Mars Express orbiter.
Radar signals, which can penetrate rock and ice, change as they’re reflected off different materials. In this case, they produced especially bright signals beneath the polar cap that could be interpreted as liquid water. The possibility of a potentially habitable environment for microbes was exciting.

But after taking a closer look at the data, along with experiments in a cold laboratory here on Earth, some scientists now think clays, not water, might be creating the signals. In the past month, a trio of new papers have unraveled the mystery – and may have dried up the lakes hypothesis.
A Scientific Ecosystem
Martian polar scientists belong to a small, tight-knit community. Not long after the lakes paper was published, about 80 of those scientists met for the  INTERNATIONAL CONFERENCE ON MARS POLAR SCIENCE AND EXPLORATION.
in Ushuaia, a seaside village at the southern tip of Argentina.
Gatherings like these provide an opportunity to test new theories and challenge each other’s perspectives. “Communities can generate their own little scientific ecosystems,” said Jeffrey Plaut of NASA’s Jet Propulsion Laboratory, one of the scientists who traveled to the conference. He’s also the co-principal investigator, along with Orosei, of the instrument behind the intriguing radar signals, called MARSIS, or the Mars Advanced Radar for Subsurface and Ionospheric Sounding. “These communities can be self-sustaining,” he continued, “because you bounce a question off someone and maybe a year or two later they help you figure out an answer.”

Lots of talk centered on the subsurface lakes. How much heat would it take to keep water liquid under all that ice? Could brine be lowering the freezing point of the water enough to keep it liquid?Of course, it wouldn’t be the first time an exciting water-related hypothesis set off a flurry of investigations. In 2015, NASA’s Mars Reconnaissance Orbiter found what looked like streaks of damp sand running down slopes, a phenomenon called “recurring slope lineae.
 But repeated observations using the spacecraft’s HIRISE  – or High-Resolution Imaging Science Experiment – camera have since revealed this is more likely the result of sand flow  A paper released earlier this year found many recurring slope lineae after a global dust storm on Mars in 2018. The finding suggested that dust settling on slopes triggers sand flows, which, in turn, expose the darker subsurface materials that give the lineae their distinctive coloration.

As with the damp-sand hypothesis, several scientists began thinking up ways to test the subsurface-lakes hypothesis. “There was a feeling that we should try to address this,” said Isaac Smith of York University in Toronto, who organized the conference in Ushuaia and led the most recent study showing that clays can explain the observations.