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Researchers have discovered an underground, liquid water lake on Mars! What might it be like, and why did it take us so long to find it?
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Host: Caitlin Hofmeister

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We literally stopped the presses, or at least the video equivalent, to make this episode, because today, Wednesday, July 25, a team of scientists in Italy published a paper in the journal Science announcing they found a whole lake of liquid water underneath Mars' surface.  Not some tiny trickling streams or frozen polar ice caps, we're talking a 20km wide, super salty liquid lake.  That's just a little bit smaller than Flathead Lake in Montana where we're based, so it's really big, and that's way more water than we've ever seen evidence for on Mars, unless you count water that's been gone for the last few billion years.  

But once we're done celebrating, the next steps to studying it are going to be a little tricky.  We know Mars was a watery planet billions of years ago but these days, that water is mostly floating in the atmosphere or frozen in ice.  We haven't been able to find a conclusive source of liquid water anywhere.  The closest we've gotten was in 2015, when scientists dropped the news that streaks on the planet's surface called recurring slope lineae might contain salty liquid water, but since then, other papers have suggested those streams might just be sand instead.

This time, though, things seem different.  From May 2012 to December 2015, this team of researchers studied a region on Mars' South Pole about 200km wide.  They used an instrument on the European Space Agency's Mars Express orbiter called marsis, which has been studying the planet since 2005.  To do that, marsis uses radar, sending out radio waves that bounce off the planet's surface.  Based on the sorts of radar reflections or echoes that return, scientists can get an idea of what's on the ground and underneath it. 

The region this team surveyed isn't super remarkable at first glance, and the echoes confirmed it's mostly made of stuff like volcanic rock, except for a patch about 20km wide.  There, about 1.5km under the surface, the echoes marsis picked up were weirdly bright.  Such strong signals usually only appear in areas where liquid water meets rock, and on Earth we see them on places like Greenland where there's water under sheets of ice.  

After doing a follow-up analysis, the team concluded these signals are probably exactly what they look like: a sub-surface liquid water lake.  Now, Mars Express has been studying this planet for almost 15 years, so it might seem weird that we've just figured this out.  Like, we're pretty confident about sub-surface oceans on Europa, but we didn't realize our next-door planet had a lake, but it makes more sense once you look at how our knowledge of Mars has grown.  

This actually isn't the first time we've seen strange echoes in this part of Mars' south pole.  In 2007, another team using marsis saw basically the same thing.  They even admitted the signal kind of looked like liquid water except since Mars' poles are way below freezing, they thought that was really unlikely.  But today, we know it's more reasonable.  In the last 10 years, researchers have discovered a class of salt on Mars called perchlorates that can dramatically lower the freezing point of water.  Depending on the exact compound, they can lower it from 0 degrees Celsius all the way to -75, and the temperature in that region of Mars is only about -68 degrees.  

But don't get out your scuba gear just yet!  Liquid water under the ice doesn't necessarily mean it's like a lake on Earth.  It could be a runny goop of water-saturated sediment, which honestly sounds kind of awesome.  I mean, it's a lake on Mars, and according to the team, there could be more of them.  The radar measurements don't have great resolution, so there could be smaller patches of water we just can't see.  There's always a chance something else is going on, but none of the other hypotheses the researchers looked into have panned out.

For example, they pointed out that subsurface carbon dioxide ice could also cause weird radar echoes, but there's no evidence for it in the specific region where they think this lake is.  To confirm, they checked their measurements against regions where there is confirmed CO2 ice underground and the signals were different. 

Another possibility was that the region could be made of super cold, extra pure water ice, but turns out it's the wrong density for that.  Of course, what would really close the case is a sample from underneath the ice, but we're unlikely to get that anytime soon.  To keep us from spreading Earth microbes around the universe and potentially contaminating sources of alien life, spacecraft designed to study water can only have 30 microbial spores on them, according to NASA's planetary protection laws.  

We are capable of building instruments that clean, but it's an expensive process.  A more realistic next step might be to build another radar equipped orbiter, one even more sensitive than marsis, to look for cracks in the ice.  When we researched out to Nilton Renno, an astrobiologist at the University of Michigan, he suggested that studying these cracks could be a way to look for evidence of life without having to take samples ourselves.  It's like how we're able to study a sub-surface lake in Antarctica, by observing a crack in the glacier known as Blood Falls, a lake that, for the record, is also pretty cold and salty, but contains life.

A more sophisticated orbiter could also look for smaller bodies of sub-surface water or the connections running between them.  If there was a 20km-wide lake we didn't know about, chances are, there's still a lot more to discover.  If you'd like to learn more about whether or not life could exist in that briny cold lake and what this means for astrobiology, we did a whole separate episode about it over on the main SciShow channel with Hank, and thanks for watching this special episode of SciShow Space News, and thanks especially to our Patreon Patrons, who make this show and everything we do possible.