Three underground lakes have been detected near the south pole of Mars.
Scientists also confirmed the existence of a fourth lake – the presence of which was suggested in 2018.
Liquid water is vital for biology, so this discovery will be of interest to researchers studying the potential for life elsewhere in the solar system.
But the lakes are also believed to be extremely salty, which could make it difficult for any microbial life to survive.
The weak atmosphere of Mars means that the presence of liquid water on the surface is a near impossibility. But the water could remain liquid under the ground.
The latest discovery was made using data from a radar instrument on the European Space Agency’s (Esa) Mars Express spacecraft, which has been orbiting the Red Planet since December 2003.
In 2018, researchers used data from the Marsis radar to report signs of a 20 km-wide underground lake located 1.5 km below the southern polar layered deposits of Mars, a thick polar cap formed by layers of ice and dust.
However, this discovery was based on 29 observations collected by Marsis between 2012 and 2015. Now, a team including many of the same scientists from the 2018 study has analyzed a much larger dataset of 134 radar profiles gathered between 2010 and 2019 .
“Not only did we confirm the position, extent and strength of the reflector in our 2018 study, but we found three new light domains,” said co-author Elena Pettinelli from 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 conclusively determine whether 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 to analyze data from Marsis.
“The interpretation that best reconciles all of the available evidence is that the high intensity reflections (of Mars) come from large pools of liquid water,” said co-author Sebastian Lauro, also of Roma Tre University.
There isn’t enough heat at these depths to melt the ice, so scientists believe that liquid water must contain high concentrations of dissolved salts. These chemical salts (different from the ones we sprinkle on our chips) can drastically reduce the freezing point of water.
In fact, recent experiments have shown that water with dissolved salts of magnesium and calcium perchlorate (a chemical compound containing chlorine bound to four oxygenes) can remain liquid at temperatures of -123 ° C.
“These experiments have shown that brines can persist for geologically significant periods, even at temperatures typical of the Martian polar regions (significantly lower than the freezing temperature of pure water),” said co-author Graziella Caprarelli. from the University of South Queensland. , Australia.
“Therefore, we believe that any process of subglacial water formation and persistence under the ice polar caps requires the liquid to have high salinity. ”
Whether life can survive in such conditions depends on the salinity of these Martian pools. On Earth, only very specific types of microbes, called halophiles, can survive in the most salty bodies of water.
Roberto Orosei, chief scientist of the Marsis experiment, said: “While the existence of a single subglacial lake can be attributed to exceptional conditions such as the presence of a volcano under the ice cap, the The discovery of a whole system of lakes implies that their formation process is relatively simple and common, and that these lakes have probably existed for much of the history of Mars.
“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 early Earth. “