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The frigid Martian poles have long fascinated scientists not just because of their ice caps, but for the potential of what may exist underneath them.
While there are currently no missions dedicated to exploring the poles on Mars, the fleet of orbiters around the red planet have been able to capture images and radar data that provide intriguing peeks beneath the surface.
Data from a radar instrument on the European Space Agency’s Mars Express orbiter suggested in 2018 that there may be deep subsurface lakes beneath the Martian south pole’s ice cap.
The search for water on Mars is intrinsically linked with the search for life on another planet. The surface of Mars was once warmer and wetter billions of years ago, but now it’s a frozen desert that won’t allow for the stable presence of liquid bodies of water.
Subsurface water on Mars may have provided a hub for microbial life in the past – and now, if it exists. That is why the 2018 result was so appealing.
New research released this year, however, suggests that the bright radar signals from beneath the ice cap weren’t a sign of liquid water. Instead, those signals may belong to clays.
The fact that lakes may not exist beneath the polar cap isn’t necessarily disappointing. Clays can contain evidence of the story of Mars – the history we don’t know. And it’s the scientific process at work as researchers aim to get closer to the truth about our planetary neighbor.
Investigating the Martian south pole
When the idea of subsurface lakes beneath the poles was introduced, other scientists questioned how the water remained liquid.
A study published this year in Geophysical Research Letters involved the analysis of thousands of radar echoes over 15 years’ worth of orbiter data. Among those were dozens of bright radar reflections, but they were traced close to the surface – much too cold for liquid water to exist, even if it was salty.
Aditya Khuller, a doctoral student at Arizona State University and an intern at NASA’s Jet Propulsion Laboratory, in Pasadena, California, worked on this research to analyze the radar reflections.
“For the first time we peeled back these layers to see what was underneath,” he said. “(Our findings) made it seem a little more implausible that there was actually liquid water in all of these places, because that would mean there were lakes all over the south pole.”
Isaac Smith, a scientist at Toronto’s York University, questioned if the signal could be produced by clays. Smectite – a type of clay – can be found all over Mars. These clay minerals are present on Earth as well. Although they look like rocks, smectite clays were formed by liquid water over time. Smith described the clay as in between lava rock and what is used in pottery making.
Smith conducted lab experiments to check how they might be picked up by radar, and he also froze them to a temperature similar to what is expected at the Martian south pole: negative 58 degrees Fahrenheit (negative 50 degrees Celsius).
His results showed that they matched orbiter data. Smith and his colleagues also used data from NASA’s Mars Reconnaissance Orbiter, which includes a mineral mapper instrument, and detected smectites near the south polar ice cap. Smith’s study published in July.
“We’re always trying to get closer to the truth,” Smith said. “So you have multiple teams coming from different angles. When you look at it from different perspectives and answers, and this is how we get closer.”
The constant refinement of how scientists think about and approach researching Mars 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 together these pieces of a puzzle. Sometimes you might think a certain piece fits for a while, but then you realize as everything gets clearer that maybe we’re going the wrong way.”
What’s in a clay?
The presence of smectite clays suggests that liquid water was present on Mars, but it wasn’t there long enough to create other kinds of clays that are more involved, Smith said.
It contributes more potential evidence to water’s periodicity on Mars.
“It’s just adding more data to the idea that Mars was wet, but it wasn’t always wet,” Smith said. “It was kind of these periodic or episodic times when you have water flowing.”
But the south pole on Mars hasn’t always been frozen. In fact, it was probably very wet billions of years ago.
“There’s been a lot of debate about how wet and how cold Mars was in the past,” Smith said. “If you have clay forming at the south pole, then that means it wasn’t totally frozen over.”
Smith’s findings also suggest that it’s worth taking a look at other past study results that produced strange observations and revisiting them with the idea that clay may be present.
But why should we care about clay on Mars? It may be the key to unlocking the planet’s mysterious history. And it’s a big part of NASA’s Perserverance rover mission, which is investigating an ancient lake bed and river d