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Water once flowed freely on the Red Planet’s surface, but it is not stable today. Discovering liquid water buried underground is essential for understanding the evolution of Mars.Credit: ESA/Roscosmos/CaSSIS, CC BY-SA 3.0 IGO

Bright radar reflections from the south pole of Mars point to the existence of subglacial lakes, according to researchers in Italy. These lakes may remain liquid because of extremely high concentrations of toxic perchlorate salts.

Proving that there is liquid water on Mars would be momentous because it raises the possibility of life surviving on the planet. This was a major motivation for sending the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS), which has been orbiting onboard the Mars Express spacecraft since 2005, bombarding the surface with radar pulses and collecting the reflections.

“The MARSIS radar sounder was designed in Italy and built by a collaboration between NASA-JPL and the Italian Space Agency,” says Elena Pettinelli at Roma Tre University, who led the team analysing a region of the Mars southern ice cap, called Ultimi Scopuli. Italian research groups have vast experience in radar development, signal processing and applied electromagnetics, Pettinelli explains.

Similar radio echo sounding methods are used on Earth to search for liquid water beneath Antarctica and Greenland, although the challenges are very different.

“On Earth, we can use high resolution radar from low-flying aircrafts, so it is relatively easy to follow the shape of the bedrock below the ice and identify depressions where water might collect,” explains Pettinelli. “But MARSIS data does not allow us to define the basal topography because the spacecraft orbits at altitudes above 250 kilometres. Nevertheless, we can still use some methodologies applied on Earth to discriminate between wet and dry subglacial areas, based on the features of the reflected signal.”

The radar ‘footprints’ on the surface are represented in the image and are colour-coded corresponding to the ‘power’ of the signal reflected from features below the surface.Credit: ESA/NASA/JPL/ASI/Univ. Rome; R. Orosei et al 2018

Pettinelli’s team selected the most cutting-edge methods recently tested in Greenland to confirm that a previously discovered, large, brightly reflecting patch in Ultimi Scopuli is probably liquid water. They also discovered several smaller patches of water nearby.

“On Mars the very low temperature and moderate pressure at the base of the ice require some anti-freeze agent for water to remain in the liquid state,” says Pettinelli. “We suggest that perchlorates, being ubiquitous on the Martian surface, could have inhibited freezing.”

There is some precedence for this theory on Earth. Pettinelli notes that extremely salty subglacial lakes, at very cold temperatures, were recently discovered in the Canadian Arctic.

Neil Ross, an expert in Antarctic subglacial lakes based at Newcastle University, UK, commented on the results. “Even on Earth, characterising basal environments from ice-penetrating radar requires large assumptions, with knock-on uncertainties that can lead to misidentification of subglacial lakes,” he says. “However, given the data available to them, Pettinelli’s team is certainly pushing forward our understanding of these enigmatic, potentially watery, Martian subglacial environments.”

Pettinelli acknowledges the limitations on her teams’ conclusions, but believes there are currently no other explanations for the strong echoes received by MARSIS.

“The ultimate evidence would be to get to the water by drilling through 1.5 kilometres of ice, but that is extremely unlikely in the near future,” she says. “For now, other measurements could be obtained by active seismic prospecting through landing platforms on the south polar ice cap.”