This image from NASA’s Mars Reconnaissance Orbiter shows ice sheets at the south pole of Mars. The spacecraft discovered clays near this ice; Scientists have suggested that such clays are the source of radar reflections previously interpreted as liquid water.Credits: NASA / JPL-Caltech / University of Arizona / JHU
Three studies published last month cast doubt on the premise of subterranean lakes below Mars’ South Pole.
Where there is water, there is life. That is at least the case on Earth, and it is also the reason why scientists are still irritated by any evidence that suggests that there is liquid water on cold, dry Mars. The Red Planet is a difficult place to look for liquid water: while water ice is abundant, any water warm enough to be liquid on the surface would only take moments before it settles in the delicate air of Mars turned into steam.
Hence the interest that was aroused in 2018 when a team led by Roberto Orosei from the Italian Istituto Nazionale di Astrofisica announced that they had found evidence of underground lakes deep under the ice cap at the south pole of Mars. The evidence they cite comes from a radar instrument aboard the ESA (European Space Agency) Mars Express orbiter.
The colored dots represent locations where bright radar reflections were detected by the ESA orbiter Mars Express at the south polar cap of Mars. Such reflections were previously interpreted as subterranean liquid water, but their prevalence and proximity to the cold surface suggest it may be something else.Credits: ESA / NASA / JPL-Caltech
Radar signals that can penetrate rocks and ice change when reflected from different materials. In this case, they generated particularly bright signals under the polar cap that could be interpreted as liquid water. The possibility of a potentially habitable environment for microbes was exciting.
But after a closer look at the data and experiments in a cold laboratory here on Earth, some scientists now think that sound, not water, could be generating the signals. In the past month, a trio of new papers solved the mystery – and possibly dried up the lake hypothesis.
Isaac Smith of York University in Toronto teamed up while working in a laboratory to freeze smectite clays with liquid nitrogen to test how they respond to radar signals. The results have challenged the hypothesis that underground lakes are found at the South Pole of Mars.Credits: York University / Craig Rezza
A scientific ecosystem
Meetings like this offer an opportunity to test new theories and question one another. “Communities can generate their own small scientific ecosystems,” said Jeffrey Plaut of NASA’s Jet Propulsion Laboratory, one of the scientists who traveled to the conference. He is also, together with Orosei, the chief investigator of the instrument behind the fascinating radar signals called MARSIS or the Mars Advanced Radar for Subsurface and Ionosphere Sounding. “These communities can be self-sustaining,” he continued, “because you bounce a question off someone and maybe a year or two later you help find an answer.”
Polar explorers on Mars belong to a small, tight-knit community. Not long after the Lake Paper was published, around 80 of these scientists met for the International Conference on the Science and Exploration of Mars in Ushuaia, a coastal village on the southern tip of Argentina.
Much of the conversation revolved around the underground lakes. How much heat would it take to keep the water flowing under all the ice? Could brine lower the freezing point of water enough to keep it fluid?
Of course, it wouldn’t be the first time an exciting water-related hypothesis has sparked a spate of research. In 2015, NASA’s Mars Reconnaissance Orbiter found what looked like strips of wet sand running down the slopes, a phenomenon known as the “recurring slope lineae”. But repeated observations with the spacecraft’s HiRISE – or High-Resolution Imaging Science Experiment – camera have since shown that this is more the result of sand currents. A paper published earlier this year found many recurring slope lineae after a global dust storm on Mars in 2018. The results suggested that dust settling on slopes triggers sand currents, which in turn expose the darker subsurface materials that make up the lineae give characteristic coloring.
As with the wet sand hypothesis, several scientists began thinking about how to test the subterranean lakes hypothesis. “We felt we should try to address this,” said Isaac Smith of York University in Toronto, who organized the Ushuaia conference and led the latest study showing that clay can explain the observations.
Too cold for lakes
Plaut was among these scientists. He and Aditya Khuller, an Arizona State University PhD student doing an internship at JPL, analyzed 44,000 radar echoes from the base of the polar cap from 15 years of MARSIS data. They showed dozens more bright reflections like those in the 2018 study. But in their recent article in Geophysical Research Letters, they found many of these signals in near-surface areas where it should be too cold to remain fluid, even with water Perchlorates, a type of salt commonly found on Mars that can lower the freezing temperature of water.
Two separate teams of scientists then analyzed the radar signals to see if something else could be generating the signals.
ASU’s Carver Bierson completed a theoretical study that suggested several possible materials that could cause the signals, including clays, metal-containing minerals, and salt ice. But Isaac Smith of York University, who knew that a group of clays called smectites existed all over Mars, went even further in a separate third article: he measured smectite properties in a laboratory.
Smectites look like ordinary rock, but were formed a long time ago by liquid water. Smith placed several samples of smectite in a cylinder designed to measure how radar signals interact with them. He also doused them with liquid nitrogen and froze them to minus 58 degrees Fahrenheit (minus 50 degrees Celsius) – close to what they would be at the south pole of Mars.
“The lab was cold,” said Smith. “It was winter in Canada at the time, and pumping liquid nitrogen into the room made it colder. Because of COVID-19, I was wrapped in a hat, jacket, gloves, scarf and mask. It was pretty uncomfortable. “
After freezing the sound samples, Smith found that their response matched the MARSIS radar observations almost perfectly. Then he and his team searched for sound on Mars near these radar observations. They relied on data from MROs carrying a mineral mapper called the Compact Reconnaissance Imaging Spectrometer, or CRISM.
Bingo. While CRISM cannot see through ice, Smith found smectites scattered near the ice cap of the South Pole. Smith’s team showed that frozen smectite can create the reflections – no unusual amounts of salt or heat are required – and that they are present at the South Pole.
There is no way to confirm the bright radar signals without landing at the south pole of Mars and digging through miles of ice. But recent publications have given plausible explanations that are more logical than liquid water.
“In planetary research, we often only approach the truth slowly,” says Plaut. “The original paper didn’t prove it was water, and these new papers don’t prove it. But we try to narrow the possibilities as much as possible in order to reach a consensus. “
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