"So, if there's a boundary between air and ice, or ice and dust, or ice and water, we get a reflection back", Stuurman said.
Using observations spanning a period of four years, a team of researchers from Italy found evidence of a large lake of salty water, buried 1.5km beneath Mars's southern polar cap.
But until now evidence from the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument, MARSIS, the first radar sounder ever to orbit another planet, remained inconclusive.
One of the exciting things about this discovery is that there could be other liquid water pockets like this one - the place these scientists studied is one of the few places where there are now enough measurements to be able to detect liquid water underneath the surface.
MARSIS was created to find subsurface water by sending radar pulses that penetrate the surface and ice caps. This drastic change, as mentioned above, is similar to radar signal profiles acquired from glacial regions on Earth where liquid water is known to exist beneath the ice.
"It's an exciting discovery, if it's true", said Tanya Harrison, Professional Martian and Director of Research for Arizona State University's Space Technology and Science Initiative, who was not involved in this discovery, "since we know that life can survive in subglacial lakes, and brine pockets inside glaciers, here on Earth". If there is indeed a persistent body of water there, it could have huge implications for future Mars missions. It was home to plenty of liquid water and lakes at least 3.6 billion years ago.
He suspects Mars may contain other hidden bodies of water, waiting to be discovered.
Scientists believe there could be a 20km-long lake siting under Mar's south polar ice cap.
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The European Space Agency's Mars Express orbiter, shown in this artist's conception, has been circling Mars since 2003. Although they pass relatively unscathed through most substances, these pulses reflect back up to the spacecraft each time they encounter boundaries between different materials, such as the interface of ice and bedrock.
Between 2012 and 2015, the team obtained 29 radar samples and used them to map the subsurface almost one mile deep in the area and about a dozen miles wide. It's situated near the planet's South Pole, where thick layers of ice have been observed in the past. Researchers are keenly interested in such reservoirs since they are reminiscent of subglacial lakes in Antarctica, which are teeming with microbial life.
Several researchers said it would be crucial to figure out whether this body of water is the only one, or part of an interconnecting body of underground aquifers - in part because a network increases the possibility it could have harbored life.
There is no reason to conclude that the presence of subsurface water on Mars is limited to a single location.
Lastly, Mars won't be this close again to Earth until the year 2035. This strong radar reflection was interpreted by the study's authors as liquid water - one of the most important ingredients for life in the Universe. They would have to drill through the ice first to sample the water below. "I see cells in it, but it's so briny that the cells-I can't get them to metabolize, " he said, referring to the chemical processes that all organisms undertake to survive.
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Even if this new discovery is validated, experts are lukewarm about whether this body of water would be suitable for life. Calculations have suggested that these conditions may be met at the Martian poles, where large ice caps composed of both water and frozen carbon dioxide exist.