Something's going on under the frozen south polar ice cap on Mars, but scientists can't agree on what. A new study revisits the idea that Mars is hiding liquid water under all that ice. It's an exciting concept, though it's far from settled.
If liquid water is proven to exist beneath the cap, that sparks hope for the existence of hardy microbial life on the red planet.
Buckle up, there's a lot of history to this Martian liquid water debate. Excitement kicked up in 2018 when scientists interpreted radar data from the European Space Agency's Mars Express spacecraft as pointing to a salty subglacial lake tucked in under the thick ice layer. A 2019 study expanded that idea to suggest there could be even more brackish ponds than previously thought.
But wait, there's more. In 2021, NASA looked into the bright radar reflections and questioned how the spots could contain liquid water in places where any water would be expected to remain frozen. Extremely salty water or warming by volcanic activity could explain that mystery, but the researchers said recent volcanism at the pole was unlikely.
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Yet another study came out earlier this week that points to geological layering as a possible cause of the reflections. That study, in Nature Astronomy, is aptly titled "Explaining Bright Radar Reflections Below the South Pole of Mars Without Liquid Water." Stay tuned for the lead author's thoughts on the newest liquid-water study.
Revisiting liquid water
Now we're all caught up to Thursday and the release of a different study in Nature Astronomy that circles back to the potential for a hidden expanse of liquid water. An international team led by a researcher at the University of Cambridge looked beyond the Mars Express reflections to deliver what the university called "the first independent line of evidence, using data other than radar, that there is liquid water beneath Mars' south polar ice cap."
This team used topographic measurements of the south pole surface and computer modeling to back up the case for liquid water. The study found undulations on the Martian surface that look and behave like similar undulations over subglacial lakes on Earth. This is where the computer models came in. The team ran simulations of ice flow to determine what would happen on the surface if liquid water was present underneath. The researchers also simulated geothermal heat coming from inside the planet.
"The combination of the new topographic evidence, our computer model results, and the radar data make it much more likely that at least one area of subglacial liquid water exists on Mars today, and that Mars must still be geothermally active in order to keep the water beneath the ice cap liquid," said study lead Neil Arnold from Cambridge's Scott Polar Research Institute.
Questioning liquid water
Research associate Dan Lalich of Cornell is the lead author of the study from earlier this week that points to geological layering as the source of the radar reflections. Lalich told me he isn't an expert in ice flow modeling, but he said there could be explanations other than liquid water for the surface undulations. He doubts Mars is putting out the geothermal heat needed to back up the liquid water idea, saying telltale signs of such activity -- such as flexing of the planet's crust or indications of ice flow -- haven't been seen.
What we have are a lot of different explanations for what ESA's spacecraft saw at the Martian south pole. Who's right? Expect that question to linger as scientists continue to poke and prod the data.
"So far, no line of evidence can conclusively rule out any of the possibilities put forward. My work doesn't disprove the existence of liquid water below the ice, so in that sense yes the two ideas can coexist," Lalich said. "I just think there are more-likely explanations that are more consistent with the observations."
Maybe we really need an absolute unit of a Mars drill to get to the bottom of the matter, but that's more in the realm of sci-fi than reality for now.