Mars’ South Polar Subsurface Is Mostly Smectite Clay, Not Liquid Water

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Mars’ enigmatic south pole — the one with the water ice surface deposits that seem to swirl like cappuccino froth — likely has a subsurface composed of smectite clays rather than liquid water lakes, researchers now say. 

Since 2018, data from the European Space Agency’s (ESA) Mars Express orbiter had indicated what researchers initially thought might be liquid water lakes as much as three miles under the Martian south pole. Specifically, they were intrigued by bright reflectance spectra detected by the orbiter’s MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument. 

But an international team of Mars polar climate researchers used MARSIS’ ground-penetrating radar and a diagnostic technique known as dielectric permittivity to measure this substructure’s ability to store electric energy. Isaac B. Smith, a planetary scientist at Canada’s York University, and co-authors of a paper just published in the journal Geophysical Research Letters used the radar’s reflection strength to estimate the permittivity difference between the ice and the base of the polar cap. This enabled the team to make comparisons that led to the conclusions that smectite clay best fit their lab models. 

The beauty of this result is it answers all questions related to the “lakes” idea without raising new ones, Smith, the paper’s lead author, told me. We actually see the clays there, and they can explain the radar observations, so at this point it feels like an open-and-shut case, he says.

As for smectites?

They are a class of clay that covers about half of the Martian surface, particularly in Mars’ southern hemisphere, reports York University. Formed when basalt (the volcanic rock that comprises most of Mars’ surface) breaks down chemically in the presence of liquid water, when frozen to cryogenic temperatures these clays can make the kind of bright radar reflection the team reports.

The south polar cap sits on the very old southern highlands, while the northern ice cap sits on the northern low plains, which are relatively new in comparison, Daniel Lalich, a research associate at the Cornell Center for Astrophysics and Planetary Science and a co-author on the paper, told me.

In contrast, Smith says Mars’ north polar cap is much younger than the south, and it sits on top of large piles of sand.  The south polar cap sits on ancient terrain that exhibits clear evidence of water activity in the past, but more than three billion years ago, he says.

But given what the researchers know about the conditions on Mars, any ice in the subsurface wouldn’t be warm enough to melt and form lakes.  However, the discovery of this clay beneath the surface does indicate that this south pole area did have liquid water at some point in Mars’ history.

Smectites, cover a large portion of Mars’ surface, mostly in the southern hemisphere, says Smith. Therefore, it’s not at all surprising to find clays at the south pole.  Clays form in the presence of water, he says, so we now have a better idea of the distribution of liquid water in the past, and that helps us understand Mars’ evolution through time. 

What does this result mean for the total water budget on Mars?

The majority of the water on the planet is locked up in the polar ice caps, with the next biggest reservoirs being mid latitude glaciers and icy mantling deposits, says Lalich. In terms of Mars astrobiology, this means we’re still searching for any currently habitable environments, he says.

“I’m sad to say that there is no known liquid water on Mars at the moment, it’s all frozen,” said Smith.

Although the team was disappointed that the radar measurements didn’t actually indicate liquid lakes deep beneath the surface of the Martian south pole, they take satisfaction that they think they’ve solved at least one of Mars’ mysteries.

“We know so little about the base of the polar cap, it just seemed like this would have to be some big complex problem,” said Lalich. “Then it turns out the whole thing can be explained by this relatively common type of clay. That was a pretty nice surprise.”

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