Frozen Smectite Clays, Not Water, Detected Beneath Mars’ South Polar Region

Mars South Polar Layered Deposits on top of Martian Smectites: Top, the multi-kilometer thick south polar ice cap has a base that is composed, at least partially, of a common type of clays. These clays are found over nearly half of the planet's surface and now at the edges of the ice cap. Bottom, radar measurements of the clays from a lab led by Smith show that they can explain the bright reflections observed by MARSIS, a simpler explanation than bodies of liquid water.

Credits: ESA/DRL/FU Berlin (top), NASA (bottom).

What lies beneath the frozen surface of Mars’ south polar region has been a recent hot topic among researchers, and a new paper by Planetary Science Institute Research Scientist Isaac Smith refines the answer, pouring cold water on the subglacial lake theory. 

Instead of the bright radar reflectors detected at the Martian south pole being liquid water, they are clays, specifically frozen-solid smectite minerals, said Smith, the lead author of “A Solid Interpretation of Bright Radar Reflectors Under the Mars South Polar Ice” that appears in Geophysical Research Letters. 

Previous work, using the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) radar instrument aboard the European Space Agency’s Mars Express orbiter, detected areas of high radar reflectivity deep beneath Martian south polar ice deposits. That team said the bright reflections indicated that several bodies of water, commonly reported as lakes, were found. But a recent flurry of journal papers has made the presence of underground lakes in the Martian south polar region less likely, including a recent paper on which PSI Senior Scientist Nathanial Putzig was a co-author. 

“To date, all previous papers were only able to suggest holes in the lakes argument. We're the first paper to demonstrate that another material is the most likely cause of the observations,” said Smith, who is also affiliated with York University, Toronto, Canada. “Now, our paper offers the first plausible, and considerably more likely, alternative hypothesis to explain the MARSIS observations. Specifically, solid clays frozen to cryogenic temperatures can make the reflections. Considering the recent work on this topic finding faults with the lake theory, this is like a 1-2-3 punch combination that puts big holes in the lake interpretation and then solves the riddle. In my opinion, it's a knockout.” 

Sub-glacial lakes were first reported in 2018 and caused a big stir because of the potential for habitability on Mars. Astrobiologists and non-scientists were equally attracted to the exciting news. Now, the solution to this question, with great import to the planetary science community, may be much more mundane than bodies of water on Mars. 

The strength of this new study is the diversity of techniques employed. “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 said. 

Smith puts the clays 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.” 

The paper could put to rest the question of what lies beneath Mars’ south polar region. “Lakes under the ice leave more questions unanswered than answered. A simpler answer is that a material we now know exists at the south pole of Mars explains the anomalous observations better than an extraordinary claim of bodies of liquid water,” Smith said. “In my opinion, the liquid water interpretation is hard to support at this point.”

Spectral color map from the CRISM instrument on Mars Reconnaissance Orbiter draped over HiRISE imagery at the edge of the south polar ice cap. Specific colors from this map indicate the presence of smectite clays, an important discovery that helps to explain the MARSIS radar observations.

Credit: NASA/JPL/UA.

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