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There may not be a huge lake of liquid water at Mars’s south pole after all. In 2018, the European Space Agency’s Mars Express spacecraft spotted bright radar reflections under the ice cap there that seemed to indicate a lake of liquid water 20 kilometres across. But a new study shows that the signal could simply indicate iron-rich volcanic rocks under the ice.

The original signal was promising, but it was difficult to understand how the Martian climate could support a long-lived lake, even under the ice cap. “We do not understand how liquid water could be there, because we wouldn’t expect to have enough energy and pressure to melt water there, even if the water is salty,” says at the University of Texas at Austin.

To dig into what else the signal may be, Grima and his colleagues performed a simulation of what the entire surface of Mars would look like if, like the south pole, it were buried under 1.4 kilometres of ice. They found bright reflections like the ones that Mars Express spotted scattered everywhere across the planet, covering up to 2 per cent of its surface.

These bright areas tended to match up with the locations of volcanic plains, terrain created when iron-rich lava flowed across the surface of Mars early in its history. That indicates that the signal from beneath the ice may have come from volcanic rock, not liquid water.

“Mars is known to have these terrains all over the planet, so it’s far more likely to have this terrain under the ice than liquid water,” says Grima. “We aren’t ruling out this water, but it’s lowering by far the likelihood that it’s there.”

The best way to find out for sure would be to visit the south pole of Mars and take measurements from the surface, he says.

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