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A network of strange features discovered underground at the south pole of Mars may not be lakes of liquid salty water after all.
According to a new analysis, the strange shiny patches in radar data collected from the Mars Express orbiting probe could be resulting from frozen clay – specifically, hydrous aluminium silicates, or smectite minerals.
“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 planetary scientist Isaac Smith of the Planetary Science Institute and York University in Canada.
“Now, our paper offers the first plausible, and considerably more likely, alternative hypothesis to explain the MARSIS observations.”
The saga began when a team of scientists noticed something odd in data collected from MARSIS, the radar sounder attached to Mars Express. Beneath the southern polar ice cap of Mars, there was a region that strongly reflected the radar signal. This, the team found, was consistent with a large pocket of liquid water – a subsurface lake.
Follow-up research revealed that the region was not alone. Another three really shiny patches were found in MARSIS data. This was huge, since it suggested a place in which Mars might be habitable to extremophile, chemosynthetic (living on chemical reactions rather than sunlight) microbial life.
But other scientists found a significant problem: Mars is really, really freaking cold. Too cold, a recent paper found, for large reservoirs of liquid water, even saturated with salt, which lowers the liquid’s freezing point. Which, in turn, left a giant question mark hanging over the red planet: If the shiny patches are not liquid water, what the heck are they?
After taking a look at the data, Smith believes he and colleagues have hit on a very plausible explanation.
“Solid clays frozen to cryogenic temperatures can make the reflections. Our study combined theoretical modeling with laboratory measurements and remote sensing observations,” he said.
“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.”
Smectite clay, he clarified, is present on nearly 50 percent of the Martian surface, with a larger concentration occurring in the southern hemisphere, particularly the southern highlands. The Curiosity rover has examined smectite deposits in the ancient, dried-up lakebed it explores.
There’s also abundant evidence for liquid water having been present at the Martian south pole in ages past, greater than 100 million years ago.
Smith and team believe that smectite clays may have formed at this time, and been subsequently buried beneath the southern polar ice cap. Any ice lost from the clay layer would be replenished from the ice cap above, or frozen soil beneath, so it remains to this day.
The team tested their hypothesis with samples of calcium-montmorillonite clay, which is known to be abundant on Mars, freezing it to 230 Kelvin (around -43 degrees Celsius, or -45 degrees Fahrenheit) and measuring its dielectric permittivity – the property picked up by ground-penetrating radar. They found it consistent with the MARSIS data.
They also used modelling to estimate the echo power that would be observed by MARSIS in different scenarios, and again found that a smectite clay detection was a plausible explanation for the signal.
Liquid water could still be explicable if Mars is heated from within – which may be possible, considering recent clues that the planet is geologically and volcanically active. But we don’t know that there’s any such activity below the south pole. Smectite clay provides an answer that doesn’t require an unverified factor.
“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.
“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.”
The research has been published in Geophysical Research Letters.