An Italian team of scientists says it has strong evidence of a subsurface lake of liquid water on Mars. It's a discovery that adds to the speculation that there could once have been life on Mars, and raises the possibility that it might be there still today, since liquid water is an essential ingredient for life.

The evidence comes from an instrument called MARSIS aboard the European Space Agency Mars Express spacecraft in orbit around the Red Planet. MARSIS is a special kind of radar called ground penetrating radar.

"Ground penetrating radars use radio signals that are capable of penetrating into the ground and then get reflections from the material under the surface," says Roberto Orosei, principal investigator on MARSIS and a planetary scientist at the Italian National Institute for Astrophysics.

Such radars are useful when searching for liquid water, "because water is a very strong radar reflector," he says.

From May of 2012 through December of 2015 the Mars Express radar mapped an area near the south pole of Mars.

When Orosei and his colleagues analyzed the data, they saw what looked like a lake of water about 12 miles in diameter and a mile below a surface layer made up of water ice and dust. There are hundreds of this of subglacial lake on Earth, mostly in Antarctica.

But Orosei and his colleagues knew better than to run right out and announce to the world they'd found liquid water on Mars. Remarkable claims that seem too good to be true frequently are.

So instead, they tried to come up with as many other explanations for what they were seeing as possible.

"After years, literally a couple of years of...discussion, debate, and let's say general head scratching, we really felt confident that any other explanation would fail," Orosei says.

Or, he adds, paraphrasing Sherlock Holmes, "Once you exclude everything that is impossible, what remains necessarily has to be the truth."

A report on the discovery of the underground lake appears in the journal, Science.

Other scientists are impressed.

"I think this is extremely strong evidence that there is liquid water beneath the poles in this south polar layered terrain on Mars, which is extremely exciting," says Ellen Stofan, the John and Adrienne Mars Director at the Smithsonian's National Air and Space Museum. Before joining the museum, Stofan was a member of the MARSIS science team.

Stofan says finding liquid water is something scientists are extremely interested in scientifically, "because life here on Earth evolved in liquid water. It stayed in the oceans for over a billion years. And so as we go outward from the Earth, looking for evidence of life beyond Earth, we're always looking for liquid water."

Stofan says the water in the Martian underground lake is probably salty, otherwise if would freeze solid, even deep in the ground. And the salts are mostly likely made up of something called perchlorates, "which are very toxic to life here on Earth. But on Mars, who knows?"

It's not clear where the water the Italian team found is coming from, and it's not certain there are any other underground lakes to be found.

But you can bet the new results will encourage others to look.

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