Icy Mars lakes

Scientists using ground-penetrating radar have discovered a cluster of lakes beneath the Martian south polar ice cap.

The radar is mounted on the European Space Agency’s Mars Express spacecraft, which has been orbiting the Red Planet since 2003. Called MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding), this instrument can ping the ice cap as it flies overhead with radar capable of penetrating through more than 1000 metres of ice to see what lies beneath.

Mostly it finds rock (as well as layers within the ice marking different epochs in the ice cap’s formation). But in a study published in Nature Astronomy, scientists found an unusual signal that appears to be a cluster of at least four lakes, looking like a clutch of eggs in a nest, hidden beneath 1.5 kilometres of ice.

The largest, about 20–30 km wide, had been spotted by the same team in 2018. That’s what prompted them to take a closer look at the area to see if it was one off, or if subglacial lakes on Mars are actually more common.

The newly discovered lakes are smaller than the central one, but are definitely distinct lakes, separated by dry strips, says Graziella Caprarelli, a geoscientist at the University of Southern Queensland, in Toowoomba, who was part of the study team.

“We could think of them as ‘satellites’ of the principal water body,” she says. It is possible, she adds, that they are connected by channels too small to be detected from space.

Similar lakes are known to exist on Earth, beneath the Antarctic and Greenlandic ice caps and in the Canadian Arctic. But Mars is a lot colder. This, Caprarelli says, means that the lakes probably contain a great deal of salt, which can act as antifreeze and keep them liquid even under Martian conditions.

A good analogy, she says, might be the Don Juan Pond in Antarctica, which never freezes, even though it sits on the surface. Its salinity, she says, is about 40%, “even higher than the Dead Sea”.

That, however, doesn’t mean there couldn’t be life in the Martian lakes.

“Salty lakes and salt plains exist on every continent on Earth,” Caprarelli says. “Inevitably, regardless of the specific chemistry of the waters and the oxygen content in the environments, microbiologists have found living bacteria and other microorganisms in them.”

But don’t expect to find an answer to whether similar life exists in the Martian lakes anytime soon.

Caprarelli can’t think of “any technology currently available to us” that could be sent to Mars and used to drill through that much ice to take a sample.

But, she says, the mere existence of the lakes is revealing, because they had to have formed somehow.

At the base of the ice caps, she says, the temperature is warmer than on the surface, but not by a lot. It’s still about -70°C.

“Under these conditions, pure water ice does not melt,” she says. “We therefore think that liquid bodies of water beneath the south polar cap are linked to the climate history of Mars.”

She means that they formed under conditions different from those under the ice today, and are relics of the conditions under which they formed.

Meanwhile, the search is on for more lakes.

Do they also exist under the Martian north polar ice cap? So far, none have been detected, but that may just mean they haven’t yet been seen.

Peering through that much ice isn’t easy, Caprarelli says. Not only must the radar signal penetrate to the bottom of the ice, but its reflection must come back up strongly enough to be detectable. Differences in the composition of the ice, including the amount of dust it contains, could greatly affect the process.

At the moment, she says, the search for lakes beneath the north polar ice has just begun. But she says, if there really prove to be none there, that itself could be important information, helping us understand not only how the lakes formed at the south pole, but revealing important clues about the Red Planet’s glacial, climate and geological history.