Inside the intense scientific debate over whether Mars has liquid water
According to a new study, unexpected radio bright spots discovered around Mars’ south pole could be caused by a layer of ice on volcanic rocks.
In 2018, a team of researchers spotted a region with unusually bright radio reflections on Mars’ south pole ice cap, using data from a radio instrument called MARSIS on the Space Agency’s Mars Express Orbiter spacecraft. European. This detection launched a scientific debate that has not slowed down since.
Pockets of liquid water below the surface could have explained the readings, says Cyril Grima, a planetary scientist at the University of Texas and lead author of the study published in the journal Geophysical Research Letters. But it was a headache for the scientific community because water, underground but not too deep, would generally require a lot of salt and a heat source to stay liquid on a world where surface temperatures hover around minus 80° f.
To study a problem like this, researchers typically substitute material on Earth for what they expect to exist on Mars. By testing the properties of this analogue, they will see if they can recreate the strong radio reflections recorded by MARSIS.
But this method has an important limitation. No matter how well researchers study a material on Earth, they cannot know if and how exactly the same thing exists on Mars. Instead, Grima and his team took a different approach, using years of radio data from MARSIS that spans the entire planet.
Exploring this data allowed them to ask the following question: would it be possible to produce equivalent radio regions outside the Martian south pole, if the polar ice sheet extended over the entire surface of the planet?
The team modified the MARSIS data to simulate how radio signals would change as they passed through a global layer of dirty ice. In their simulation, they discovered that this layer of ice could, in fact, produce similar radio bright spots, providing another plausible explanation. This makes it less likely that the readings were caused by liquid water, Grima says.
Unlike using analog materials on Earth, researchers have a pretty good idea of the contents of Mars’ polar ice, he says. Grima’s squad used the same roster estimate as the 2018 squad.
[Related: Sorry, there’s probably no water under the South Pole of Mars]
It’s a good study, though like all hypotheses surrounding bright spots, “it also requires its own assumptions to make it work,” says Gareth Morgan, a planetary geologist at the Planetary Science Institute who specializes in icy planetary terrains and volcanics and works with data from a similar radio instrument called SHARAD. He points out that what scientists know about the composition of polar ice is limited to the same type of measurements – radio – as underground measurements. He hopes that in the years to come, scientists will be able to determine the composition of the poles more precisely to answer these questions.
Only certain types of salt water could barely stay in liquid form at such low temperatures, Grima says. The researchers explored other materials that could produce similar signals. Some clays could produce radio signals, or possibly volcanic basalt slabs if they’re “really dense and really iron-rich,” Grima says. There could also be a combination of clay and basalt that explains this, he says.
It is also possible that the Martian bedrock is covered in layers of frozen carbon dioxide and frozen water. This would change the way they reflect light, possibly causing a bright spot, much like “when your glasses are coated with a very thin film, you change the reflectivity properties of it,” Grima says.
Despite the myriad of possible explanations, the debate over the cause of polar bright spots is far from settled.
“I think the approach is interesting,” says Elena Pettinelli, of Grima’s study. Pettinelli is a geophysicist from Roma Tre University in Italy who was part of the 2018 team that discovered the bright spots and offered liquid water as an explanation. Pettinelli is the author of a new study that tried to show why clays and salty ice cannot explain the bright spots. The authors used a combination of extensive laboratory materials measurements and theoretical models to examine these alternative explanations.
She also criticizes the idea that basalt can generate such strong radar signals and reflections, based on her team’s previous work. Basalt rock “would only explain 25% of the strong reflections detected by MARSIS,” she says. Salty liquid water could explain all the bright spots, in his opinion.
Pettinelli’s paper will be part of an ongoing back-and-forth between research teams, Morgan said. “The document is really important because what it shows is that it’s going to take a lot of work to get to the bottom of it.” But this debate is a good thing, he says, because of the “enormous implications” of the discovery of liquid water on Mars.
In fact, the field of planetary science hasn’t seen this kind of rush for new Mars papers in decades, Morgan says. Perhaps only the Allan Hills 84001 meteorite, discovered in 1984 and which some scientists believe contained evidence of Martian microbes, caused such a stir.
Grima’s study was based on three years of MARSIS data, which is now 10 years old. The instrument has continuously collected some in the meantime, Grima says, meaning there’s about 15 years of data that hasn’t been used yet. He hopes to dive into it in future research to answer these questions.