Two years ago, planetary researchers reported the discovery of a large saltwater lake under the ice at the south pole of Mars, a result that met with excitement and skepticism. Now researchers say they have confirmed the presence of this lake ̵
The discovery reported on September 28 in Natural astronomy1was created using radar data from the European Space Agency’s (ESA) Mars Express spacecraft. This is followed by the discovery of a single subterranean lake in the same region in 2018 – which, if confirmed, would be the first liquid body of water ever discovered on the red planet and a potential habitat for life. However, that finding was based on only 29 observations from 2012 to 2015, and many researchers said they needed more evidence to support the claim. The most recent study used a broader data set with 134 observations between 2012 and 2019.
“We identified the same body of water, but we also found three other bodies of water around the main body of water,” says planetary scientist Elena Pettinelli from the University of Rome, one of the co-authors of the paper. “It’s a complex system.”
The team used a radar instrument on Mars Express, Mars Advanced Radar for Underground and Ionospheric Probing (MARSIS), to survey the planet’s southern polar region. MARSIS emits radio waves that bounce off layers of material on and below the surface of the planet. The way in which the signal is reflected back indicates the type of material that is present in a given location – for example, rock, ice or water. A similar method is used to identify subterranean glacial lakes on Earth. The team discovered some areas of high reflectivity that indicate liquid water trapped under more than a kilometer of Martian ice.
The lakes extend over around 75,000 square kilometers – an area around a fifth the size of Germany. The largest central lake has a diameter of 30 kilometers and is surrounded by three smaller lakes, each a few kilometers wide.
On the surface of Mars, the low pressure resulting from the lack of a substantial atmosphere on the planet makes liquid water impossible. But scientists have long thought that water might be trapped beneath the surface of Mars, perhaps a holdover from when the planet had seas and lakes billions of years ago. If such reservoirs exist, they could be potential habitats for life on Mars. On earth, life can survive in subglacial lakes like Antarctica.
However, the amount of salt present could cause problems. It is believed that all of the underground lakes on Mars must have appropriately high salinity levels in order for the water to remain fluid. Although a small amount of heat escapes from inside Mars so far below the surface, that alone would not be enough to melt the ice into water. “From a thermal point of view, it has to be salty,” says Pettinelli.
Lakes with salinity about five times that of seawater can support life, but when you get close to 20 times that of seawater it is gone, says John Priscu, environmental scientist at Montana State University.
“There isn’t a lot of active life in those saltwater pools in Antarctica,” says Priscu, whose group studies microbiology in icy environments. “You’re just pickled. And that could be the case [on Mars]. ”
The presence of the Mars lakes themselves is also still being discussed. Following the 2018 discovery, researchers raised concerns such as the lack of a sufficient source of heat to turn the ice into water. And while the latest evidence supports the 2018 observation and includes much more data, not everyone is convinced that the regions identified are liquid water.
“If the bright material is really liquid water, I think it’s more of a kind of mud or mud,” says Mike Sori, a planetary geophysicist at Purdue University in West Lafayette, Indiana.
Jack Holt, a planet party scientist at the University of Arizona, says that while he believes the latest data is okay, he is not sure about the interpretation. “I don’t think there are lakes,” says Holt, who is on the Mars Shallow Radar Sonar (SHARAD) science team on NASA’s Mars Reconnaissance Orbiter (MRO). “There’s not enough heat flux to support a brine here, even under the ice cap.”
A Chinese mission en route to Mars could provide an opportunity to verify the claims. The Tianwen-1 mission will enter orbit in February 2021. In addition to using a rover on the surface, the orbiter will carry a number of scientific instruments. This includes radars that can be used to make similar observations. “His skills are similar to those of MARSIS and SHARAD,” says David Flannery of the Queensland University of Technology.
The prospect that these lakes are remnants of Mars’ humid past remains an exciting possibility for now. “There may have been a lot of water on Mars,” says Pettinelli. “And if there was water, there was the possibility of life.”