NEW YORK (NYTIMES, AFP, BLOOMBERG, XINHUA, WASHINGTON POST) - For the first time, scientists have found a large, watery lake beneath an ice cap on Mars.
Because water is essential to life, the discovery offers an exciting new place to search for life-forms beyond Earth.
Here's what we know about the discovery so far:
1. WHERE WAS WATER FOUND?
Scientists working on the European Space Agency's Mars Express mission said that a 19.3km wide underground liquid pool - not just the momentary damp spots seen in the past - had been detected by radar measurements near the Martian south pole.
The region corresponded to a basin, adding to speculation that liquid water had flowed into this spot. The patch of Mars that was under study is known as the Planum Australe.
The lake was beneath some 1.5 kilometres of ice near Mars' south pole. It is the largest body of liquid water ever found on the Red Planet.
The forces allowing water to pool under Antarctic ice explain how the same phenomenon can happen on Mars, according to a research paper, published on Wednesday (July 25) in the journal Science.
Enormous pressures, like those under Antarctica or over a kilometre under Mars, can lower water's freezing point.
Also, the Martian soil is filled with magnesium, calcium and sodium salts, a mix that would have the effect of lowering the freezing temperature of water further still, to perhaps a minimum -60 degrees Celsius.
Astronomers have previously discovered water-ice and seasonal liquid water on Mars.
2. HOW WAS IT DISCOVERED?
Ground-penetrating radar on Earth is used for identifying groundwater, tunnels, unexploded bombs, ancient human-built structures and more.
The principle is the same above Mars. A satellite broadcasts radar signals to the surface. The rate and strength at which the signals bounce off each stratum allows the team to try and reconstruct the spacing and composition of layers below the surface.
The findings were based on data collected by radar sounder MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding), which was designed by Italian researchers with a contribution from Nasa's Jet Propulsion Laboratory and the University of Iowa in the United States.
The instrument was positioned on board of the European Space Agency's Mars Express spacecraft in 2003, and started scanning Mars' surface in July 2005. The discovery of the lake, more specifically, was based on information collected by the instrument between May 2012 and December 2015.
MARSIS used low-frequency electromagnetic waves, transmitting its pulses toward the red planet, researchers explained. Such waves were able to penetrate the surface up to 4-5 kilometres, and reacted differently according to the various types of material they met.
Then, they were reflected back to the radar aboard the Mars Express, providing geological information on the portion of planet under examination.
A total of 29 sets of radar samplings showed a "very sharp change in its associated radar signal", allowing scientists to map the outlines of the lake.
"The radar profile of this area is similar to that of lakes of liquid water found beneath the Antarctic and Greenland ice sheets on Earth, suggesting that there is a sub-glacial lake at this location on Mars," according to the report on the discovery.
3. WHAT KIND OF WATER IS IT?
"It is liquid, and it's salty, and it's in contact with rocks," according to Dr Enrico Flamini, the former chief scientist of the Italian Space Agency, who oversaw the research.
Scientists could not measure the thickness of the lake, but said that it had to be around a metre or so thick for the radar pulses to bounce back.
A back-of-the-envelope calculation indicated several hundred million cubic metres of water, equivalent to tens of billions of litres.
Computer models indicate that temperatures would be about minus 67 degrees Celsius - far colder than the melting point of water. That suggests that the water is brim full of salts, allowing it to melt.
"The salts - which are likely similar to those found by Nasa's Phoenix spacecraft in the ice of the northern polar area (of Mars) - work like 'anti-freeze', helping maintain the water in a liquid state," the Italian Space Agency (ASI) said in the statement.
However, no such conclusions are technically conclusive until a probe drills through the Martian soil and rock and actually takes a sip. And while that would require "technological developments that at the moment are not available," the study's lead author, Dr Roberto Orosei of the National Institute for Astrophysics, said the researchers nonetheless have high confidence in their results.
4. WHAT DOES THE FINDING MEAN IN THE SEARCH FOR LIFE ON MARS?
In the history of Mars exploration ever since, the more we learn, the more we think it could have had a watery, perhaps life-sustaining past. The surface is scored by old gorges, canyons, beaches, ocean basins and giant volcanoes, whose eruptions could have kept things riled up on the planet.
Where this water went and how, taking most of Mars' atmosphere with it, is one of the great and ominous environmental mysteries of our time. If life did arise from those early, cosy conditions, it could have moved underground as the surface cooled and dried.
The body of water appears similar to underground lakes found on Earth in Greenland and Antarctica. On Earth, microbial life persists down in the dark, frigid waters of one such lake.
The ice on Mars would also shield the Martian lake from the damaging radiation that bombards the planet's surface.
Being able to access water sources could help humans survive on a future crewed mission to Earth's neighbouring planet.
This particular lake, however, would be neither swimmable nor drinkable. Whether microbial forms of life could lie within is a matter of debate.
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 harboured life.
Some experts are sceptical of the possibility since the lake is so cold and briny, and mixed with a heavy dose of dissolved Martian salts and minerals. The temperature is likely below the freezing point of pure water, but can remain liquid due to the presence of magnesium, calcium, and sodium.
Dr Jonathan Lunine, director of the Centre for Astrophysics and Planetary Science at Cornell University, noted, "If it is liquid water, the intense saltiness would make it hard for life, at least life as known on Earth, to survive in the lake. It may exceed the salt content that any terrestrial organisms that we know of can survive in."
Still, he said, "Having a stable body of liquid water today is very intriguing and worthy of study."
Dr John Priscu, a professor of ecology at Montana State University, has been studying Antarctica biology. There, as on Mars, the surface is barren, but is more hospitable farther down.
When he and his team drilled into a sub-surface lake there a few years ago, they found microbes.
"They haven't seen the light of day for hundreds of thousands of years," he said. "They're eating the rocks for energy."
If it were possible to drill over a kilometre into Mars into the newly discovered lake, he said he would bet there was life there too.
"I've been studying life in ice for 35 years," he said. "We've been finding life in places it shouldn't be according to our current thinking of life. But that's changing."