Mars's northern ice cap.

Scientists have detected a series of saltwater lakes beneath the glaciers of Mars' southern ice cap. The researchers believe the liquid in these lakes doesn't freeze and become solid, despite the low temperatures of Mars' glaciers, due to their extremely high concentrations of salt.

The Mars Express spacecraft, which has been surveying the planet since 2005, had previously detected signs of a subglacial lake basin on Mars' south pole, but it was unclear whether the lake was liquid or what it contained.

The southern ice cap of Mars, April 17, 2000.

To find out, a group of Italian, German, and Australian researchers applied a radio-echo technique that Earth satellites use to detect subsurface lakes in Antarctica. They scanned the area multiple times from 2010 to 2019, then published their results in the journal Nature Astronomy on Monday.

The analysis confirmed the liquid-water nature of Mars' underground lake, as well as its extreme saltiness. What's more, the researchers say, they uncovered "a more extensive, complex scenario with ubiquitous water patches surrounding the subglacial lake."

The discovery offers yet another possible habitat for life to persist on Mars.

Ancient life may have retreated to underground lakes 

Scientists think the Martian surface was once rich with rivers, lakes, and seas, but all the surface water evaporated as a flow of particles from the sun stripped away the planet's atmosphere. Earth's strong magnetic field, by contrast, has allowed it to hold onto its atmosphere and its surface water.

On Mars, any microbial life that might have existed on the surface could have migrated underground as water disappeared — perhaps to lakes like the ones beneath the red planet's south pole.

In July, NASA launched its nuclear-powered Mars Perseverance rover, which is set to search for signs of ancient life on the planet's surface and prepare Martian rock samples for a future mission that would return them to Earth. The rover is expected to land on Mars on February 18, 2021.

An artist's illustration shows NASA's Perseverance rover and Ingenuity helicopter on Mars.

"Is there life out there? We have, for 20 years, learned about the environment of Mars and are ready to ask that," NASA Administrator Thomas Zurbuchen said during a broadcast of the rover's launch. "For the first time in decades, [an] astrobiology mission? We're ready for it."