This latest epic discovery
was achieved using a radar instrument on a Mars orbiter, with Italian
scientists finding a huge liquid reservoir hidden 1.5 kilometres (0.93 miles)
under the southern polar ice cap, extending 20 kilometres (12.4 miles) across.
The
researchers say it's a lot like the subglacial lakes trapped beneath the ice of
the Arctic and Antarctica here on Earth. And, like our terrestrial subglacial
lakes, it might be where we find surprising life.
A subglacial
lake has long been hypothesised as a likely place to find water on the Red Planet,
but probing beneath glacial regions is not an easy task even here on Earth.
It's only been in recent years that scientists have used satellites equipped
with radar to uncover the mysteries of the hidden waters of our own planet.
For the Mars
mission, the research team, from a number of institutions in Italy, used
similar technology to study Mars' polar ice caps. An instrument designed
specifically for subsurface surveys is on board the European Space Agency's
Mars Express probe - in orbit around Mars since 2003.
The Mars
Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) uses radar to
look for features under the surface of Mars, and has been looking for signs of
subsurface liquid water for over 12 years.
It was
between May 2012 and December 2015 that the research team seriously
investigated a 200-kilometre-wide section of the southern ice cap, in a
location called the Planum Australe. They took 29 radar profiles of the region,
bouncing radio waves deep beneath the surface of Mars, and collecting the
return signal on a receiver.
It's by
measuring changes between the transmitted signal and what returns that
scientists study subsurface features. Radar returning through water is returned
more strongly, or 'brightly', than radar returning through rock or sediment.
This is what the research team found in their radar results: an anomalously
bright region in the Planum Australe.
Other
explanations, such as very cold and pure water ice, or carbon dioxide ice,
could also explain a brightly reflective subsurface anomaly like this, but the
research team ran simulations and found that the reflectivity profile did not
match their results as well as liquid water.
But there's
one other big problem: the temperature of the body is estimated to sit at
around 205 Kelvin (-68.15 Celsius, or -90.67 Fahrenheit). That is far below the
point of freezing, even for hypersaline Antarctic lakes, which remain liquid
above 260 Kelvin (-13 Celsius, or 8.6 Fahrenheit) thanks to their salt content.
But despite
this, the water could still hypothetically remain in liquid state. We know that
salts of sodium, magnesium, and calcium are abundant on Mars - they've been
found on the surface.
If dissolved
into the water, and combined with the pressure of the ice cap on top, they
could drop the freezing point to below 200 Kelvin (-74 celsius and -101
Fahrenheit). Life has been found in subglacial Earth lakes. It's been
previously proposed that a subglacial Martian lake might therefore also harbour
life. This discovery reopens that possibility more prominently than ever
before.
"There is evidence on Earth of substantial microbial life in the waters below the poles - and even microbes that can survive within ice veins," said astrobiologist Brendan Burns of the University of New South Wales, who was not involved with this research. "Whether similar scenarios are occurring on Mars remain to be experimentally established, but this finding of potential liquid water beneath the surface of Mars opens up fascinating areas of space exploration."
It is still
important to keep our cool on this one. It's extremely possible that the sheer
concentration of salt required to keep the water liquid is absolutely hostile
to life.
We also have
absolutely no means of sampling the water at this point, or any point in the
near future. But it's a lot more accessible than Europa and Enceladus, the
other Solar System candidates in the search for life; and the water itself
could yield clues about the climate history of Mars, its hydrosphere, and what
may have happened to its long-disappeared ocean.
In addition,
now that the team has demonstrated its technique, other researchers know how to
look for more such reservoirs on the Red Planet. But we're also going to take a
moment to squee about the possibility of Martian microbes.
"This is an amazing discovery from the Mars Express," said astrophysicist Brad Tucker of the Australian National University, who was not involved in the research. "For decades, we've been finding evidence of either ice or past flows. Now, we know that liquid water currently exists on Mars, and just as subsurface lakes exist in Antarctica here on Earth, we now have that on Mars."
"Every
month, new discoveries are being made that is getting us closer to answering
the fundamental question - does life exist somewhere beyond Earth." The
team's research has been published in the journal Science.
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