NASA/JPL-Caltech/MSSS

Mars' Planum Australe.

Scientists have long known that Mars has water in the form of ice and certain minerals. Now, new data might settle the 30-year debate over whether the red planet holds liquid water too.

Radar data by the European Space Agency's Mars Express spacecraft has revealed a 12-mile-wide lake of liquid water one mile below Mars's south polar ice. 

The findings appeared in the journal Science.

Such radar reflects brightly where ice and water meet, a method scientist have used to detect sub-glacial lakes on Earth.

Here and on Mars, these lakes stay liquid because dissolved salts, and pressure from above, lower the water's freezing point.

Arizona State University's Jim Bell, who works on NASA's Mars missions, says it's the strongest evidence for liquid water so far.

"It's a really provocative result, if true, and so I think it really begs for additional data, additional radar data," he said.