A network of strange features discovered underground at the south pole of Mars may not be lakes of liquid salty water after all.

According to a new analysis, the strange shiny patches in radar data collected from the Mars Express orbiting probe could be resulting from frozen clay - specifically, hydrous aluminium silicates, or smectite minerals.

The saga began when a team of scientists noticed something odd in data collected from MARSIS, the radar sounder attached to Mars Express.

Which, in turn, left a giant question mark hanging over the red planet: If the shiny patches are not liquid water, what the heck are they.

"All three agreed that smectites can make the reflections and that smectites are present at the south pole of Mars.

There's also abundant evidence for liquid water having been present at the Martian south pole in ages past, greater than 100 million years ago.

A simpler answer is that a material we now know exists at the south pole of Mars explains the anomalous observations better than an extraordinary claim of bodies of liquid water," Smith said.