Polar Subsurface “Lakes” May Not Be Water After All

Jun 30, 2021 | Daily Space, Mars

Polar Subsurface “Lakes” May Not Be Water After All
IMAGE: Examples of radar slice images of the Martian south polar region from Mars Express and the Mars Reconnaissance Orbiter. CREDIT: NASA/ESA/JPL- Caltech/University of Rome/Washington University in St. Louis

We talk a lot about water on Mars here on the Daily Space. It’s an important topic. With plans underway to send humans to the red planet, scientists and engineers are concerned about how to provide the necessary resources for a prolonged mission. Water is heavy, and fuel is expensive, making it better for everyone involved if water is already on Mars, accessible, and relatively easy to make drinkable.

Recently, there was research that came out that said the south polar ice caps had liquid lakes of water underneath the surface. They were discovered with bright radar reflections taken using the Mars Advanced Radar for Subsurface and Ionosphere Sounding, or MARSIS, instrument about the ESA’s Mars Express orbiter. These lakes likely have a lot of salt dissolved in them, making them more briny than clear and not easily drinkable. Still, subsurface liquid water would be fantastic for crewed missions to be able to access all that necessary, life-sustaining fluid, even if it had to be purified first.

But if there is one thing we know about space anywhere off our planet, it is not friendly to biological life. So before we send people off to a distant world, even one in our own solar system, we want to make absolutely certain that what we think is water is actually water. And in new research published this week in Geophysical Research Letters, a team of scientists led by Carver Bierson and including Planetary Science Institute’s Than Putzig examined the data previously collected by MARSIS. Using a different analysis method than the previous paper, they concluded that there are other materials that could explain that bright reflection.

IMAGE: Blue spots in the Martian south polar region earlier thought to show signs of possible subsurface water could now mark other subsurface materials. CREDIT: USGS Astrogeology Science Center, Arizona State University, INAF.

Put simply, the previous method involved how a material responds to an electric field. That led to the conclusion that liquid water caused the bright reflection. The new paper looked instead at how much electrical current the material could carry, using ice sheets here on Earth as analogs for Mars’ polar ice. And the results showed that the reflection could be due to clays, metallic minerals, or even salty ice.

These results do not rule out the possibility of liquid water, mind you. They just mean that we cannot always make a confident decision based on one possible analytic method if there are other methods available. Think of it this way: Just because you see the glint off a rock in the distance doesn’t mean you’ve found gold, so before you bring all your mining equipment over, you want to make sure you have found the right mineral.

Or as Than reiterates: Because water — particularly in a liquid form — is so important to sustaining life, seeking out where it may exist on Mars today or in the past is of paramount importance to astrobiological studies. Ensuring that we fully consider other possibilities for reported detections of liquid water is crucial to the scientific process.

More Information

Arizona State University press release

PSI press release

Strong MARSIS Radar Reflections from the Base of Martian South Polar Cap may be due to Conductive Ice or Minerals,” C. J. Bierson, S. Tulaczyk, S. W. Courville, and N. E. Putzig, 2021 June 28, Geophysical Research Letters 


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