In 2018, researchers found evidence of lakes of liquid water underneath Mars's south pole. Using radar imaging of the ground beneath the ice, scientists saw bright radar reflections that they found consistent with large bodies of water. A new study now argues that those detections were nothing but a "dusty mirage".

In Geophysical Research Letters, an international team of researchers compared the south pole observations to the rest of the Red Planet. By adding an imaginary mile-thick ice sheet across the whole of Mars, the radar observations picked up reflecting regions all over the place, particularly in the location of volcanic plains. 

The researchers argue that the reflective surfaces seen under the ice of the south pole only looked like water, and that these structures are either ancient lava deposits rich in iron or possibly dried-up river beds, now covered in mineral deposits. They think their findings are a more plausible explanation for the observations of the lakes without the questions of how they survived for millions of years under the poles.  

“For water to be sustained this close to the surface, you need both a very salty environment and a strong, locally generated heat source, but that doesn’t match what we know of this region,” lead author Cyril Grima, planetary scientist at the University of Texas Institute for Geophysics, said in a statement.

This is not the first time the existing liquid lake scenario has been brought into question. Last summer, there was a paper in the same journal that argued that clays, which are abundant on Mars, might explain the bright reflections seen under the ice.

“Science isn’t foolproof on the first try,” explained the lead author of that study, Issac Smith, who was not involved in this latest study. “That’s especially true in planetary science where we’re looking at places no one’s ever visited and relying on instruments that sense everything remotely.”

“I think the beauty of Grima’s finding is that while it knocks down the idea there might be liquid water under the planet’s south pole today, it also gives us really precise places to go look for evidence of ancient lakes and riverbeds and test hypotheses about the wider drying out of Mars’ climate over billions of years.”

Recent research has suggested that Mars was a frigid wet world around 3 billion years ago, with a vast northern ocean as well as extensive ice caps and glacial valleys. The authors of the two papers are now investigating missions that might use radar to find water on Mars.