New research finds that a lake beneath the Martian ice cap is unlikely

Cornell University researchers have provided a simple and comprehensive—if less dramatic—explanation for bright radar reflections originally interpreted as liquid water beneath the ice cap at the south pole of Mars.

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 can produce reflections whose intensity and variability match what has been observed so far—not only in the region suggested as liquid water, but across the so-called southern polar layers.

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

Lalich is first author of “Small Variations in Ice Composition and Layer Thickness Explain Bright Reflections Under Martian Polar Cap Without Liquid Water,” published June 7 in Scientific advances.

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

The implications were huge: 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 had previously shown that clear radar signals could be produced in the absence of liquid water, but said assumptions about layers of frozen carbon dioxide beneath the ice cap were probably incorrect.

The new research tells a more complete story, he said, closing the gaps in the radar interference hypothesis with more realistic modeling. Thousands of randomly generated layering scenarios were based only on conditions known to exist at the Martian poles and varied the composition and distribution of ice sheets in ways that would be expected over distances of tens or hundreds of miles.

These minor adjustments sometimes produced clear subsurface signals consistent with observations on each of the three frequencies used by the Mars Express orbiter’s MARSIS radar instrument, a partnership between NASA and the Italian Space Agency. Probably for a simple reason, Lalich argues: Radar waves bouncing off layers too close for the instrument to detect can combine to amplify their peaks and troughs.

“This is the first time we have a hypothesis that explains the entire population of observations under the ice cap without introducing anything unique or special,” Lalich said. “This result, where we get bright reflections scattered all over the place, is exactly what you would expect from thin layer interference in radar.”

While not ruling out the potential for some future detection by more capable instruments, Lalich said he suspects the story of liquid water and potential life on the red planet is long over.

“The idea of ​​liquid water even a little bit near the surface would be really exciting,” Lalich said. “I just don’t think it’s there.