Readings indicate the lake was about twelve and a half miles across and almost a mile below the planet's surface. Some of it was likely lost to space, due to Mars's diminutive gravitational field, but a significant fraction of the planet's aqueous inventory never really left, instead just freezing belowground.
"This took us long years of data analysis and struggles to find a good method to be sure that what we were observing was unambiguously liquid water", said study co-author Enrico Flamini, chief scientist at the Italian Space Agency during the research.
While it's always been known that the planet has substantial ice deposits, this is the first time scientists have detected a stable body of water in liquid form on the planet.
Lead researcher Roberto Orosei said it's the closest his team can get to confirming that the lake is truly water.
The pulses that came back created the aforementioned "well-defined, 20-kilometer-wide zone" and found that the radar reflected the brightness of the liquid water. The "lake" is about 12.5 miles wide, at least 3 feet deep and is located under about a mile of ice. Researchers have also likened the discovery to Antarctica's Lake Vostok.
The Italian team of researchers suspect the water is salty, or briny, which keeps it in liquid form.
The reason for the reflections at these boundaries has to do with the electrical properties of the materials, and according to Stuurman, liquid water has radically different electrical properties than rock or ice.
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He said: "I have a contract with Arsenal and the club is still counting on me so far". Hopefully his situation will be sorted out as quickly as possible.
"It's plausible that the water may be an extremely cold, concentrated brine, which would be pretty challenging for life", explained Dr Claire Cousins, an astrobiologist from the University of St Andrews, UK.
The research also determined that the specific region studied is not particularly unique.
Prof Orosei said: "It's a very promising place to look for life on Mars". "Based on analogy with Earth, if water still exists in the subsurface, there is no reason to believe that life which arose on Mars and evolved for underground conditions could not persist there into the present day...." While these were later shown to be optical illusions, they were mistranslated and popularized as canals.
In recent years, planetary scientists and astrobiologists have been scouring the Red Planet for water, one of the requirements for life as we know it.
When he and his team drilled into a sub-surface lake there a few years ago, they found microbes. To give one example, they consider a layer of carbon dioxide that was liquified by the pressure. Proposals for follow-up missions were nearly immediately submitted to NASA. "Europa and Enceladus are cold, but they're potentially good habitats; this makes Mars sound like an even better habitat".
Those early science fiction visions were dashed when the first spacecraft photos of the planet revealed a dry, cratered and lifeless looking surface - a seemingly dead planet. The end of the Apollo Program in 1973 signaled the end of the momentum given to space exploration generated by the US-Soviet space race. Image credit: NASA / Viking / JPL-Caltech / Arizona State University / ESA / ASI / University of Rome / R. Orosei et al. Scientists in the past have discovered ice on the planet, pointing to the logical conclusion that life could, if not did, exist there. Has matter evolved enough elsewhere in our Solar System to become as complex as it is on Earth?