New research shows that a lake under Mars’ ice cap is unlikely

Researchers at Cornell University have provided a simple and comprehensive – albeit less dramatic – explanation for bright radar reflections that were initially interpreted as liquid water beneath the ice cap at Mars’ south pole.

Their simulations show that small variations in water ice layers – too subtle to be resolved by ground-penetrating radar instruments – can cause constructive interference between radar waves. Such interference could produce reflections whose intensity and variability match observations to date – not only in the region proposed as liquid water, but over the so-called south polar layered deposits.

“I can’t say it’s impossible that there is liquid water down there, but we show that there are much simpler ways to get the same observation without having to stretch so far, using mechanisms and materials that we already know.” know they exist there,” said Daniel Lalich, research associate at the Cornell Center for Astrophysics and Planetary Science. “Only by random chance can you create the same observed signal on radar.”

Lalich is the first author of “Small Variations in Ice Composition and Layer Thickness Explore Bright Reflections Below Mars Polar Cap Without Liquid Water,” published June 7 in Scientific progress.

Robotic explorers have provided extensive evidence that water flowed on the surface of ancient Mars, including in a former river delta now being explored by NASA’s Perseverance rover. Relying on a radar instrument that can probe beneath the surface to detect water ice and potentially hidden aquifers, members of the European Space Agency-led Mars Express orbiter team announced in 2018 that they had discovered a lake buried beneath the southern polar cap.

The implications were enormous: where there is liquid water, microbial life can exist.

But while the same bright radar reflections would likely indicate a subglacial lake on Earth, Lalich said, the temperature and pressure conditions on Mars are very different.

Using simpler models, Lalich previously showed that the bright radar signals could be created in the absence of liquid water, but he said assumptions about layers of frozen carbon dioxide beneath the ice sheet were likely incorrect.

The new research tells a more complete story, he said, filling in the gaps in the radar interference hypothesis with more realistic models. The thousands of randomly generated layering scenarios were based only on conditions known to exist at Mars’ poles, and varied the composition and spacing of the ice layers in ways that would be expected over tens or hundreds of kilometers.

Those small adjustments sometimes produced bright subsurface signals that were consistent with observations in each of the three frequencies used by the MARSIS radar instrument on the Mars Express orbiter, a partnership between NASA and the Italian Space Agency. Likely for a simple reason, Lalich argues: radar waves reflecting off layers too close together for the instrument to detect can combine, amplifying their peaks and valleys.

“This is the first time we have a hypothesis that explains the entire population of observations under the ice sheet, without having to introduce anything unique or strange,” Lalich said. “This result, where we get bright reflections spread everywhere, is exactly what you would expect from thin-layer interference in radar.”

While not ruling out the possibility of future detection by better instruments, Lalich said he suspects the story of liquid water and possible life on the red planet ended long ago.

“The idea that there was liquid water even remotely on the surface would have been very exciting,” Lalich said. “I just don’t think it’s there.”