A new clue emerges as to what might lie under the south pole of Mars

(CNN) – The icy Martian poles have long fascinated scientists not only for their ice sheets, but for the potential of what may exist beneath them.
Although there are currently no missions dedicated to exploring the poles of Mars, the fleet of orbiters encircling the red planet has been able to capture images and radar data that allow for an intriguing glimpse below the surface.

Data from a radar instrument on the European Space Agency’s Mars Express orbiter suggested in 2018 that there could be lakes deep underground under the ice sheet of the Martian South Pole.

The search for water on Mars is intrinsically linked to the search for life on another planet. The surface of Mars was warmer and more humid billions of years ago, but now it is an icy desert that does not allow the stable presence of bodies of liquid water.

The groundwater on Mars may have provided a center for microbial life in the past and now too, if it exists at all. That is why the 2018 result was so attractive.

However, new research published this year suggests that the bright radar signals from beneath the ice sheet were not a signal from liquid water. Instead, those signals could belong to clays.

The fact that there are no lakes under the ice cap is not necessarily disappointing. Clays may contain evidence for the history of Mars – history we do not know. And it’s the scientific process doing its job as researchers seek to get closer to the truth about our planetary neighbor.

Investigating the Martian South Pole

When the idea of ​​underground lakes under the poles was introduced, other scientists wondered how water was kept in a liquid state.

A study published this year in Geophysical Research Letters included the analysis of thousands of radar echoes over 15 years of orbiter data. Between them were dozens of bright radar reflections, but they were drawn close to the surface, too cold for liquid water to exist, even if it was salty.

Aditya Khuller, a doctoral student at Arizona State University and a fellow at NASA’s Jet Propulsion Laboratory, Pasadena, California, worked on this investigation to analyze radar reflections.

“For the first time we peeled off these layers to see what was underneath,” he said. “(Our findings) made it seem a little more unlikely that there was actually liquid water in all these places, because that would mean there were lakes all over the South Pole.”

Isaac Smith, a scientist at the University of York in Toronto, wondered if the signal could be produced by clays. Smectite, a type of clay, is found throughout Mars. These clay minerals are also present on Earth. Although they look like rocks, smectite clays were formed with liquid water over time. Smith described clay as something intermediate between lava rock and what is used in pottery making.

Smith conducted laboratory experiments to see how they could be captured by radar, and also froze them at a temperature similar to that expected at the Martian south pole: -50 ° C.

Their results showed that they matched the orbiter’s data. Smith and his colleagues also used data from NASA’s Mars Reconnaissance Orbiter, which includes a mineral mapping instrument, and detected smectites near the south polar cap. The study Smith’s was published in July.

“We always try to get closer to the truth,” Smith said. “So you have multiple teams that come from different angles. When you look at it from different perspectives and responses, this is how we approach.”

The constant refinement of the way scientists think about and approach Mars research is leading to more answers.

“It’s like being a space detective,” Khuller said. “You’re sitting here on Earth and you get these images, radar data, and temperature data, and you’re putting these pieces of a puzzle together. Sometimes you may think that a certain piece fits for a while, but then you realize, as that everything is clarified, that perhaps we are going the wrong way “.

What’s in a clay?

The presence of smectite clays suggests that liquid water was present on Mars, but was not there long enough to create other, more complex types of clays, Smith said.

Clays also provide further potential evidence for the periodicity of water on Mars.

“It just adds more data to the idea that Mars was wet, but it wasn’t always wet,” Smith said. “It was kind of periodic or episodic times where there was water flowing.”

But the south pole of Mars was not always frozen. In fact, it was probably very humid billions of years ago.

“There has been a lot of debate about how wet and how cold Mars was in the past,” Smith said. “If clay is forming at the south pole, that means it was not totally frozen.”

Smith’s findings also suggest that it is worth taking a look at the results of other earlier studies that produced strange observations and reexamining them with the idea that clay may be present.

But why should we care about clay on Mars? It may be the key to uncovering the mysterious history of the planet. And it is an important part of the NASA’s Perserverance rover mission, who is investigating an ancient lake bed and river delta for evidence of microbial life in the past, if ever, on the red planet.

The rover will collect rock and soil samples on Mars that will be returned to Earth by future missions, and those samples could include smectite clays.

“In many ways, clays are one of our best windows into what ancient Mars looked like and whether or not it was habitable,” said Briony Horgan, associate professor of planetary sciences at Purdue University and a mission scientist for the rover. “The actual minerals that form in these environments, things like clays, tell us about the details of that environment.”

Clay minerals can reveal how long water was present and trap organic material, he said. The more scientists learn about the minerals on Mars, the more our understanding of the red planet changes.

“Clays are at the center of what we are trying to do with the Perseverance and Mars sample return mission because they are probably one of the most important minerals that we can bring back,” Horgan said.

Looking at the future

Scientists studying the Martian poles want nothing more than to investigate their area of ​​interest in greater detail by landing a mission at one of the poles.

“It’s really the only other place in the solar system where we could do that kind of climate study,” Horgan said.

On Earth, glacier cores and ice sheets act as a record of our planet’s atmosphere and climate dating back millions of years. Martian ice caps can be millions or even hundreds of millions of years old. The cores of the Martian poles would be a “gold mine,” Khuller said.

“The polar caps, especially in the north, are the engine of the distribution of water on the planet. They are basically the lungs of the planet,” said Smith. “We want to know how the north pole drives everything.”

In theory, a polar version of the Ingenuity helicopter flying alongside Perseverance could measure the layers of ice cliffs at the poles.

Smith’s dream is that one day there will be a mission to sample those thousands of layers at the poles that record the history of Mars, allowing scientists to truly peek into the past.