That ‘Lake’ at Mars’ South Pole is Probably Just a Giant Hunk of Rock

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Sadly, after further analysis, it appears that the briny subsurface ‘lake’ researchers reported at Mars’ south pole is much more likely to be a huge mass of volcanic rock.

In 2018, European Space Agency (ESA) researchers reported signs suggesting the presence of a mass of liquid water at the planet’s south pole. There was a bright patch on the Mars Express orbiter’s radar that didn’t seem to belong; the researchers interpreted it as a twenty-kilometer-wide lake. But the reports weren’t a slam dunk. It’s tough to square the presence of liquid water with what we know about conditions on Mars. For one thing, in the extremely thin Martian atmosphere, it’s easy for ice to sublimate away. So easy, in fact, that Mars’ north polar ice cap is seasonal — it’s formed and destroyed each year by sublimation. Furthermore, at Martian temperatures, fresh water would long since have frozen. But if the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) readings didn’t indicate water, what did they indicate?

There may not be a lake at Mars' south pole, but this image shows what the planet's primordial oceans might have looked like.

Ancient Mars may have had enough water to form oceans that covered a third of its surface. Based on geological data, this artist’s impression shows what those oceans might have looked like. Image by Ittiz, CC BY-SA 3.0

Even if we had a rover down there, we wouldn’t be able to take samples to confirm or correct the lake hypothesis, because the bright patch at Mars’ south pole is buried under almost a mile of ice. So, Dr. Cyril Grima and colleagues ran a simulation of what the surface of Mars would look like entirely buried under the same amount of ice. Lo and behold, a bunch of the same bright patches popped up — but many of them were directly atop known volcanic plains, a kind of terrain created by lava flows.

Water Isn’t Always Wet

“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,” said Grima, corresponding author of a new analysis of the 2018 findings. There’s water ice on Mars, and water sequestered in Martian bedrock. But in the ice-sheet simulation, neither produced the right radar signature. What did? Volcanic plains. These broad, flat, iron-rich Martian lava flows occur all over the planet’s surface.

“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.”

We’ve been hunting for water on Mars for decades, with varying success. Mostly, we’ve found ice. Finding a lake at Mars’ south pole would have been incredible. But lake or no lake, scientists aren’t giving up hope. NASA’s Perseverance rover is in Mars’ Jezero Crater, hunting for signs of ancient life. Water is life, here on Earth — so we’re looking for it on Mars.

Clay at the bottom of the crater indicates that Jezero once held a crater lake. That lake was fed by a river, whose delta opens up into a formation called Three Forks, on the northwestern edge of the crater. From orbit, it looks like the delta is full of boulders. How did they end up there? On Earth, it takes a decent-sized flood to lift and carry rocks that big. Forthcoming images and data from Perseverance’s RIMFAX, SHERLOC and WATSON instruments will help us learn more about just how wet ancient Mars might have been.

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