Three underground lakes have been found near the south pole of Mars.
Scientists also confirmed the existence of a fourth lake, the presence of which was hinted at in 2018.
Liquid water is vital to biology, so the discovery will be of interest to researchers studying the potential for life in other parts of the Solar System.
But the lakes are also believed to be extremely salty, which could make it difficult for any microbial life in them to survive.
The thin atmosphere of Mars means that the presence of liquid water on the surface is almost impossible. But the water could remain liquid underground.
The latest discovery was made using data from a radar instrument on the European Space Agency’s (ESA) Mars Express probe, which has been orbiting the Red Planet since December 2003.
In 2018, researchers used Marsis radar data to report signs of a 20km-wide underground lake located 1.5km below the stratified deposits of the south pole of Mars, a thick polar ice cap made up of layers of ice and dust. .
However, that discovery was based on 29 observations collected by Marsis between 2012 and 2015. Now, a team that includes many of the same scientists as the 2018 study has analyzed a much larger dataset of 134 radar profiles collected between 2010. and 2019.
“Not only have we confirmed the position, extent and strength of the reflector from our 2018 study, but we have found three new bright areas,” said co-author Elena Pettinelli of Roma Tre University in Italy.
“The main lake is surrounded by smaller bodies of liquid water, but due to the technical characteristics of the radar and its distance from the Martian surface, we cannot definitively determine if they are interconnected.”
The team borrowed a technique commonly used in radar sounder surveys of subglacial lakes in Antarctica, Canada and Greenland, adapting the method for analyzing data from Mars.
“The interpretation that best reconciles all the available evidence is that the high-intensity reflections (from Mars) come from vast pools of liquid water,” said co-author Sebastian Lauro, also from Roma Tre University.
There isn’t enough heat at these depths to melt the ice, so scientists believe liquid water must contain high concentrations of dissolved salts. These chemical salts (other than the ones we sprinkle on our chips) can significantly lower the freezing point of water.
Indeed, recent experiments have shown that water with dissolved salts of magnesium and calcium perchlorate (a chemical compound containing chlorine bound to four oxygens) can remain liquid at temperatures of -123 ° C.
“These experiments have shown that brines can persist for geologically significant periods of time even at temperatures typical of the Martian polar regions (significantly below the freezing temperature of pure water),” said co-author Graziella Caprarelli, of the University of Southern Queensland, Australia.
“Therefore we think that any process of formation and persistence of sub-ice water under the polar ice caps requires the liquid to have a high salinity.”
Whether life could survive in such conditions depends on how salty these Martian pools are. On Earth, only very specific types of microbes, known as halophiles, can survive in the saltiest bodies of water.
Roberto Orosei, chief scientist of the Marsis experiment, said: “While the existence of a single subglacial lake could be attributed to exceptional conditions such as the presence of a volcano under the ice sheet, the discovery of an entire system of lakes implies that their formation process is relatively simple and common and these lakes have probably existed for much of Mars’ history.
“Because of this, they could still retain traces of any life form that might have evolved when Mars had a dense atmosphere, a milder climate, and the presence of liquid water on the surface, similar to the early Earth.”