If you can cast your mind back through the mists of time to last week, you may remember we reported on a very sad (for the alien enthusiasts among us) new study that revealed our strongest evidence for liquid water on Mars was nothing more than a dusty mirage, shattering our most potent hope for discovering microbial life on the red planet.
Now, however, the researchers who provided the original evidence back in 2018 have come back with a vengeance, doubling down on their conclusions that there actually IS a sub-glacial liquid lake on Mars in a new study.
So, what’s the deal? IS there liquid water on Mars or not?
The original research
The original announcement sent ripples through the scientific community in 2018, when a team of Italian researchers announced evidence of liquid water beneath a glacier covering Mars’ south pole.
It wasn’t the first evidence of water on Mars. We know that Mars is covered in vast regions of ice, and that the planet used to host liquid water in the distant past, thanks to the unequivocal impressions that rivers and oceans carved on its rocky surface billions of years ago.
But this was the first time we had any strong evidence of liquid water on the planet in this day and age.
The glint of possible sub-glacial water was first glimpsed by the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS), an instrument on the European Space Agency’s Mars Express Orbiter. MARSIS beams radio waves down to the red planet, and listens back for reflections.
As MARSIS scientists began to sift through their observations, they noticed small, bright echoes beneath the southern polar ice cap – a reflection so bright it hinted at liquid water rather than rock.
This signal alone wouldn’t have been enough to truly start thinking in watery terms, so the team looked at another clue: the ability of the reflecting material to store energy in an electric field, also known as permittivity. Water has a higher permittivity than rock and ice.
The team found high relative permittivity, matching the kind you might see in Earth’s lakes. Moreover, the fact that the highly reflective, 20km-wide zone was braced on all sides by much less reflective zones led them to suspect a lake.
There were doubts even at the time from within MARSIS’ ranks: speaking to Science in 2018, one of MARSIS’ principal investigators Jeffrey Plaut noted, “I would say the interpretation is plausible, but it’s not quite a slam dunk yet.”
Nonetheless, the announcement fuelled hopes that we might, with a bit of focused exploration, one day find microbial life on Mars.
Then came late-January 2022, and a team of researchers from the University of Austin, in the US, published a paper in Geophysical Research Letters that refuted the Italian team’s original claims, and proffered an alternate explanation for the shiny-bright signal underneath Mars’ southern pole.
That team found evidence that those shimmering reflections were actually a mirage caused by volcanic rock buried under the ice.
How did they come to that conclusion? Well, the team wondered whether other geological features might show up this vibrant signal if they mapped a virtual ice-sheet over the entire surface of Mars in a computer program. The did so using MARSIS’ exceptionally detailed radar images of Mars, and then watched out for that shiny signal coming from any other well-understood areas.
The team found that the bright reflective signals the original researchers had taken for a sign of water could be found all over the planet’s surface, and mostly matched the locations of known volcanic plains.
By analogy with Earth, they reasoned that iron-rich lava flows often leave behind rocks that reflect radar in a similar way, and suggested this as an alternate explanation.
“Science isn’t foolproof on the first try,” said Isaac Smith, a Mars geophysicist not involved with the study, speaking in Cosmos last week. “That’s especially true in planetary science where we’re looking at places no one’s ever visited and relying on instruments that sense everything remotely.”
The debunking of the debunking
Meanwhile the original team of researchers were addressing alternate theories to produce their own new studydoubling down on their belief that there is still strong evidence for liquid water under the Martian south pole.
One of the major questions hanging over their original study was how liquid water could persist in the utterly frigid (-73 degrees Celsius) temperatures assumed to occur beneath the ice cap?
Combining their original data with new simulations and laboratory measurements, however, the Italian team found that alternate explanations for the shiny source – clay (like the volcanic clay described in the Austin University study), hydrated salts and saline ices have all been proposed – couldn’t produce the kind of strong signal MARSIS was picking up.
Instead, the team found that a group of salts commonly found in the Martian soil have anti-freeze properties, and so there could indeed be a briny, sub-glacial lake sloshing around underneath all that ice.
So, where do we stand?
At this point, we don’t yet know whether there is liquid water on Mars, but it’s exciting that this scientific back-and-forth inches us closer to the truth.
“The bottom line is that there’s been an ongoing debate for decades, actually for centuries, about whether there’s water on Mars or not,” says Gretchen Benedix, a mineralogist and astro-geologist at Curtin University.
“Back in the 1800s, there was a guy named Schiaparelli who looked at the surface and saw these lines on it,” Benedix says. “Because the telescopes were really, really bad at the time, he surmised that they were canals that were taking water from the warmer equatorial regions to the cooler polar regions, to help with agriculture.
“And then in the 60s, we actually sent some spacecraft that took pictures of the surface, and we found that nope, it’s a desert. Since then, we’ve noticed that there actually is evidence of water, and now we keep trying to see whether it’s liquid.”
Science, by its nature, is a process with twists and turns and, in some cases, questions left unanswered.
“But it means that science is working,” says Benedix. “Scientific debate is very helpful, because it helps us hone in on what is the most likely explanation for things.
“So, in our search for water throughout the solar system, we’re really coming up with more accurate questions about how we define what we’re looking for. Where can we go looking for it? What technological advances do we need to actually go there and learn more about this particular area?”
Perhaps one day in our lifetimes we’ll know the answer for sure. Until then, sit back, grab some popcorn, and enjoy the ride.