Three studies have cast doubt on the subterranean lake hypothesis below the south pole of Mars. Scientists have proposed as an alternative that clays they are the true source of radar reflections that have previously been interpreted as liquid water.
In 2018, a team led by Roberto Orosei from Italy’s Istituto Nazionale di Astrofisica announced that it had found evidence of underground lakes far below the ice sheet at the south pole of Mars. The evidence they cited came from a radar instrument aboard the ESA (European Space Agency) Mars Express orbiter.
Radar signals, which can penetrate rock and ice, change as they reflect off different materials. In this case, they produced especially bright signals below the polar cap that could be interpreted as liquid water. The possibility of a potentially habitable environment for microbes was exciting. But after taking a closer look at the data, along with experiments in a cold laboratory here on Earth, some scientists now think that clays, not water, could be creating the signals. In the past month, a trio of new articles have unraveled the mystery and may have ruled out the lakes hypothesis.
Jeffrey Plaut of NASA’s Jet Propulsion Laboratory and Aditya Khuller, an Arizona State University doctoral student doing an internship at JPL, analyzed 44,000 radar echoes from the base of the ice cap along the 15 years of MARSIS data, the same Mars Express instrument from which the lakes hypothesis was made. Dozens of brighter highlights appeared like the ones in the 2018 study. But in their recent article published in Geophysical Research Letters, They found many of these signals in areas near the surface, where it should be too cold for the water to remain liquid, even when mixed with perchlorates, a type of salt commonly found on Mars that can lower the freezing temperature of water. .
Then two separate teams of scientists analyzed the radar signals to determine if something else could be producing those signals.
Carver Bierson of Arizona State University completed a theoretical study suggesting several possible materials that could cause the signals, including clays, metal-containing minerals, and saline ice. But Isaac Smith of York University, knowing that a group of clays called smectites were present all over Mars, he went further in a third separate article: he measured the properties of smectite in a laboratory.
Smectites look like ordinary rocks, but they were formed long ago by liquid water. Smith placed several smectite samples in a cylinder designed to measure how radar signals would interact with them. He also doused them with liquid nitrogen, freezing them to minus 58 degrees Fahrenheit (minus 50 degrees Celsius), close to what they would be at the Martian south pole.
“The lab was cold,” Smith said. it’s a statement. “It was winter in Canada at the time, and pumping liquid nitrogen into the room made it colder. He was wrapped in a hat, jacket, gloves, scarf, and a mask due to COVID-19. It was quite uncomfortable.”
After freezing the clay samples, Smith found that his response matched almost perfectly with the MARSIS radar observations. Then he and his team searched for clays present on Mars near those radar observations. They were based on data from MRO, which carries a mineral mapper called CRISM.
Bingo. While CRISM cannot look through the ice, Smith found smectites scattered in the vicinity of the south pole ice sheet. Smith’s team showed that frozen smectite can make 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 what the bright radar signals are without landing on the south pole of Mars and digging up miles of ice. But recent articles have offered plausible explanations that are more logical than liquid water.
“In planetary science, we often get closer to the truth little by little,” Plaut said. “The original document did not prove that it was water, and these new documents do not prove that it is not. But we try to narrow the chances as much as possible to reach a consensus“.