Liquid water "lakes" on Mars may just be frozen clay, say new studies
A few years ago astronomers announced the incredible discovery of lakes of liquid water on Mars, buried beneath the ice cap at the south pole. But sadly, a trio of new papers refutes the claim, with new experiments suggesting that the “water” signal was more likely produced by frozen clay instead.
While there’s plenty of evidence pointing to a very wet ancient past for the Red Planet, finding liquid water still pooling there today was a bit of a Holy Grail. And in 2018, scientists finally found it – the Mars Express orbiter returned radar signals consistent with a water interface. The team concluded that a lake of liquid water lay beneath 1.5 km (0.9 miles) of solid ice, and extended as far as 20 km (12.4 miles). Further observations turned up a whole series of them.
But right away their apparent existence raised more questions. For one – how could water remain liquid at such cold temperatures? The scientists hypothesized that a generous mix of salts, plus the pressure of the ice from above, would have lowered its freezing point. Other teams later suggested that the lake was heated from below, which implies that Mars is more volcanically active than we thought.
Now, it seems like Occam’s razor has cut the idea down in three new papers. The first study analyzed 44,000 radar echoes measured in the area over 15 years, and identified dozens more of the same type of reflections. Many of these were detected too close to the surface for liquid water to exist, even considering the factors that might lower the freezing point.
With the lake hypothesis looking less likely, the other studies investigated what else could be creating the bright signals. They found that sub-surface clays, metal-bearing minerals and saline ice could all produce similar signals.
So in the third study, researchers put one of these ideas to the test. They focused on a type of clay called smectites, which were formed in the presence of liquid water long since dried up. Samples of smectites were frozen with liquid nitrogen to -50 °C (-58 °F) – around the temperature they’d be subjected to at the Martian south pole – and placed in an instrument that measured their radar reflections.
Sure enough, it was an almost perfect match to the orbiter’s observations. Further evidence came from data from the Mars Reconnaissance Orbiter (MRO), which has found that such smectites are common across the surface in the region.
Of course, the studies can’t confirm that smectites are definitively the source of the strange signals. But they do pour cold water (pun intended) on the theory, offering a more plausible explanation than subsurface lakes – even if it isn’t as exciting.
“In planetary science, we often are just inching our way closer to the truth,” says Jeffrey Plaut, co-principal investigator of one of the studies. “The original paper didn’t prove it was water, and these new papers don’t prove it isn’t. But we try to narrow down the possibilities as much as possible in order to reach consensus.”
Source: NASA JPL