| 26.07,18. 04:30 AM |
Mars has a vast liquid water lake beneath its southern pole, scientists believe
This simulated view shows Mars as it might have appeared during the height of an ice age. (Supplied: NASA)
In a massive shot in the arm for the search for life on Mars, a giant "lake" of liquid water seems to be buried beneath the ice near the red planet's south pole.
Using ground-penetrating radar on an orbiting spacecraft, an Italian team picked up signs of a 20-kilometre-wide body of liquid water, hidden 1.5 kilometres under the ice cap.
They published their observations in Science today.
Whether that body of water is a relic of past oceans or part of a bigger network of subterranean lakes is still a mystery, said Roberto Orosei, a planetary scientist at the Italian National Institute of Astrophysics and lead author of the paper.
"We need to determine if this discovery is unique or if it's something that exists elsewhere on Mars," he said.
Whether it's a one-off or not, he added, has important implications for finding life lurking on Mars today.
Water, water everywhere, but not a drop to drink
Life, as we know it, needs liquid water to survive.
While our nearest neighbour in the solar system is a prime suspect in the search for extra-terrestrial life, the hunt for liquid water on Mars has suffered a few false starts.
In 1877, Italian astronomer Giovanni Schiaparelli peered through his telescope and thought he saw channels — or "canali" — crisscrossing the red planet's surface.
His observations were misinterpreted by media at the time to mean that intelligent life had excavated "canals" to funnel water around the planet.
More recently, planetary scientists got really excited when they saw what appeared to be dark streaks of salty water that "flowed" down Martian dunes during warm seasons.
That theory had cold water thrown on it last year, with geologists suggesting those rivers could be nothing but avalanches of dry sand.
And when you think about it, the surface of Mars isn't the best place to store liquid water.
For one, it's cold — really cold. Surface temperatures average around minus 55 degrees Celsius.
This chilliness conspires with the planet's extremely low atmospheric pressure to ensure surface water exists as ice or vapour, but not liquid, said Gretchen Benedix, an astrogeologist at Curtin University who wasn't involved in the study.
"Based on what we know about how water behaves, it can't exist [as a liquid] on the surface of Mars," she said.
"If you're going to find it, it's going to be underground."
It's not a new concept: the idea of vast lakes of liquid water hidden under the Martian ice caps has been bandied around for more than 30 years.
We have them on Earth. Lake Vostok is a freshwater lake, some 250 kilometres long, buried nearly 4 kilometres beneath the ice of East Antarctica.
Why not Mars?
How to find water on Mars
Enter the MARSIS radar instrument, orbiting the red planet on the European Space Agency's Mars Express spacecraft.
Designed specifically to look for bodies of liquid water hidden underground, it pings pulses of radio waves down to the surface and measures their reflection.
The radio waves pass through rock and ice, but bounce off the point where liquid water, or watery sediments, butt up against overlying ice.
And, between May 2012 and December 2015, MARSIS mapped out a particularly bright reflective patch on a region called Planum Australe.
The echoes appear to show a layer of liquid water, a touch over 20 kilometres wide, trapped 1.5 kilometres below the surface.
"This [bright reflection] is what you see in places like Antarctica, over Lake Vostok and other subglacial lakes," Dr Orosei said.
But whether the Martian subsurface lake is mostly pure water or sludgy sediment remains to be seen.
The MARSIS instrument can't tell us how deep the water is or how much stuff is mixed with it.
It could be pure water, maybe a metre deep or so, or water-saturated sediments tens of metres thick, Dr Orosei said. Both scenarios would produce the same radar signal.
So what can we deduce about the lake?
To stay liquid in such frigid conditions, the lake must be salty.
Dissolved salts lower water's freezing point — seawater on Earth, for instance, freezes at minus 2 degrees Celsius.
Applying pressure, too, can lower water's freezing point further. Perhaps the weight of the ice cap overhead helps keep the lake in liquid form.
These conclusions hinge on a few crucial assumptions, Dr Benedix said.
For instance, even though we've been sending probes to Mars for decades, planetary scientists don't exactly know what Martian polar caps are made of.
"We assume [ice caps are made of] water ice with silicates in it, then maybe with a light dusting of something else over the top," Dr Benedix said.
But if the ice cap's composition is different — perhaps with less silicate and more salt, for instance — the bright radar reflection might not necessarily be produced by liquid water.
Can life find a way? It depends
If it does contain liquid water underground, then Mars has all the ingredients needed for life as we know it.
And organisms can be found in such extreme, seemingly inhospitable environments. Researchers have found signs of thousands of species of fungi and bacteria in Lake Vostok.
So, what are the odds of finding life thriving in a subsurface Martian lake today?
That depends if the underground lake is a one-off or part of a bigger network of interconnected reservoirs.
"It could be an isolated geyser, maybe the last surviving hydrothermal vent on Mars," Dr Orosei said.
"Or it could be we're underestimating the thermal conditions on Mars and in fact there are several lakes around the polar caps."
This is important because Mars undergoes a cycle of ice ages, where the ice that's usually clustered around the poles creeps towards the equator.
Without the crushing pressures of an ice cap above, the liquid water lake that MARSIS detected would freeze.
"But if there's an interconnected system of liquid water under the surface of Mars, life could survive," Dr Orosei said.
Dr Benedix agreed that life is much more likely to be around today if Mars contained a subsurface network of liquid water.
"The issue isn't just liquid water, but also radiation. There's no atmosphere on Mars to protect life from [cosmic] radiation," she said.
MARSIS hasn't mapped both polar ice caps, so whether there are more lakes remains to be seen.
Still, we might know more about Mars's underground water reservoirs within the next few years.
Even though MARSIS is likely reaching the end of its mission — it's been circling Mars since 2003 — a Chinese probe is scheduled to be launched to the red planet in 2020.
On board will be a ground-penetrating radar instrument similar to MARSIS that will explore Mars's underground using a slightly different frequency.
Dr Orosei hopes the new instrument will find more subsurface lakes: "It might be that they have a chance to finish the work MARSIS started."