A few weeks ago, we brought you the story that bright radar reflections from under Mars’ south polar ice caps were probably not water but more likely salty ice or some other conductive mineral. We even interviewed the Planetary Science Institute’s Than Putzig, who was an author on that paper.

Now, a new paper in Geophysical Research Letters, which is apparently having a banner week for newsworthy publications, provides multiple lines of evidence that show the mineral under the ice is a specific type of clay called smectite. The lead author on this paper is PSI’s own Isaac Smith. Not only did the team use experimental work and some modeling to demonstrate their hypothesis, but they also found evidence of smectites around the edges of that polar ice cap.

Let’s revisit the history of this ongoing controversy. Back in 2018, radar observations were made using the MARSIS instrument on board Mars Express, and scientists found a bright reflection coming from underneath the south polar ice cap. The hypothesis at the time was that liquid water was causing the reflection, and the scientific community ramped up the excitement because who doesn’t want to find liquid water on Mars?

So another group worked on trying to figure out if there could be enough heat under the polar ice caps to even keep water in a liquid state. They found even more bright radar reflections in the Mars Express data, but a lot of the areas were definitely too cold for liquid water. It was hypothesized that maybe there was recent volcanic activity under the surface that kept the water from freezing, but this paper calculated that the energy needed was at least double the martian geothermal heat flow.

Then came Putzig’s paper in June, which poked holes in the liquid water theory and presented alternatives such as salty ice or other conductive minerals, both of which could have caused the bright reflection. Neither paper gave any definitive answer, leaving water still on the table as a possibility. Honestly, it’s going to be difficult to prove anyone correct without actually going to Mars and digging through the thick ice.

Enter Smith and his work, which employed a variety of techniques in its analysis. As Smith explains: Our study combined theoretical modeling with laboratory measurements and remote sensing observations from The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument on NASA’s Mars Reconnaissance Orbiter. All three agreed that smectites can make the reflections and that smectites are present at the south pole of Mars. It’s the trifecta: measure the material properties, show that the material properties can explain the observation, and demonstrate that the materials are present at the site of the observation.

Smith goes on to put clays on Mars in perspective: Smectites are a type of clay that is extremely abundant on Mars, covering nearly 50% of the surface, especially focused in the southern hemisphere. I call them solid-state to reinforce the idea that these materials are solid. There is no unbound water. Further, our experiments show that when the clays are frozen to cryogenic temperatures, they become brittle, rather than a soft clay like you might use for pottery. Recent theoretical work had suggested that clays could make bright reflections, but no one had frozen them to temperatures we would see on Mars – namely 40 to 50 degrees below freezing – and measured them, nor had they identified these minerals at the south pole.

Does this put the entire question to rest? Possibly. It surely makes the liquid water hypothesis difficult to support. Personally, I think we’re done here, but who knows what next week’s press releases will bring?

More Information

NASA JPL press release

York University press release

“A Solid Interpretation of Bright Radar Reflectors Under the Mars South Polar Ice,” I. B. Smith et al., 2021 July 15, Geophysical Research Letters