Two years ago, a team of Italian scientists claimed to have discovered a subglacial lake near the Martian south pole. The same team has collected further evidence to bolster this claim, including the apparent discovery of even more buried bodies of liquid water. The new research speaks to the potential for life on Mars, but not everyone is convinced by the evidence.
More liquid water may exist beneath the Martian surface than we realized, according to new research published today in Nature Astronomy. The new paper describes several newly detected subglacial water bodies beneath the Martian south pole, in addition to the described water body found two years ago.
Liquid water existed on Mars billions of years ago, but it’s mostly gone now, for reasons that aren’t fully understood. The prospect of liquid water beneath the surface is sure to attract the attention of astrobiologists, as similar subglacial lakes on Earth, including Lake Vostok in Antarctica, are known to harbor microbial life.
To be fair, this liquid water, if it does exist—still a big if—is super salty and probably very slushy. The hypersaline aqueous solution stays in liquid form despite temperatures much lower than the freezing point of water, according to Elena Pettinelli, the lead author of the study and an associate professor at the Università degli Studi Roma Tre in Rome.
“Something interesting is happening here, but there’s a really high bar for proof when it comes to talking about liquid water on Mars,” Cassie Stuurman, a radar scientist with NASA’s Jet Propulsion Laboratory, said in an email. “To be really convincing, most scientists would want to see this corroborated by other lines of data and evidence,” said Stuurman, who’s not involved with the new study.
In 2018, Pettinelli and her colleagues detected signs of a subglacial lake in a region called Ultimi Scopuli using the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument on board the Mars Express satellite. The radar data pointed to a 12-mile-wide (20-kilometer) aqueous region around a mile beneath the surface, which the researchers interpreted as a subglacial lake or a patch of liquid water.
It was an intriguing discovery, but several questions remained unanswered, such as the origin of the supposed lake, the accompanying hydrodynamics, and even the plausibility of the whole thing, as Stuurman explained. For Pettinelli and her team, an important unresolved question was whether this body of water represented a single occurrence or one of many.
“The discovery of multiple lakes answers that question, showing that subglacial liquid water may be common,” wrote Pettinelli in an email. “This forces us to consider global processes of formation and stabilization of the liquid water, which are essential pieces of the puzzle for our understanding of the past and present climate, geology, and possible habitability conditions on Mars.”
For the new study, the team used MARSIS again, but this time they scanned a much larger area than before, bouncing radar onto a region measuring 190 miles (300 km) wide. The team then applied a similar method used by scientists to detect subglacial lakes on Earth, namely those in Antarctica, Greenland, and northern Canada.
“We wanted to establish the extent of subglacial water in this region, therefore we acquired new data, achieving unprecedented radar coverage over the study area,” said Pettinelli. “We used a new method of analysis of the complete MARSIS dataset, based on signal processing procedures usually applied to terrestrial polar ice sheets. Finally we compared new results with the previous method finding very similar results.” The new method “gave us confidence about the validity and reliability of the results,” she added.
Stuurman said the new paper addresses the smoothness of the land under the polar cap and the physical differences between this region and the outlying areas. She said it’s “really hard to argue with how unique this region looks in the radar data.”
The new paper, in addition to claiming further confirmation of the liquid nature of the lake found in 2019, also resulted in the discovery of three other, smaller patches of water nearby. As the new analysis shows, the water is clustered in the same general area but is surrounded by dry bedrock.
“Our results strengthen the claim of the detection of a liquid water body at Ultimi Scopuli and indicate the presence of other wet areas nearby,” said Pettinelli. “We suggest that the waters are hypersaline perchlorate brines, known to form at Martian polar regions and shown to survive for geologically significant periods of time at temperatures well below the freezing point.”
Stuurman said the new paper carries important implications for the likelihood of microbial life existing on Mars, but she did express some reservations about the study.
“To be quite honest, I expect this paper to create a lot of debate,” she said. “The idea that there could be large amounts of liquid water on present-day Mars is not a very popular hypothesis.”
Stuurman was concerned, for example, that the team used a technique for detecting subglacial lakes on Earth and that they did so “without any cross-validation” to show that it also works for “an entirely different planet.” Also, the criteria used by the team to determine whether a radar signal corresponds to a subglacial lake—also borrowed from Earth science—are “not fulfilled by this study,” she said. Using the criteria cited in their study, “there are four different properties that must be met before a lake is definite,” said Stuurman, but “given the available data, it’s impossible to meet any more than three of these, and that means—by the authors own definition—that the lake is not ‘definite’ in this case.”
Looking ahead to future research, Pettinelli said her team would like to better characterize the topography of these features, as the shape of the topography is a key parameter that allows scientists to identify the presence of a subglacial lake on Earth. Given the limitations of MARSIS, however, the team could not “determine the topographic variations of the bedrock,” leaving some important questions unanswered. In addition to this, the team will look for similar areas on Mars, and, assuming they exist, will perform similar scans, Pettinelli said.
That salty liquid water exists beneath the Martian south pole may be a controversial proposition, but it’s not as outlandish as it might seem. As other research has shown, our solar system is a surprisingly wet place. It’s just rarely in plain sight.