Recent hubbub about subsurface lakes detected on Mars has a new twist, as new research argues that the underground structures aren’t lakes at all. The researchers behind the study say that, rather than liquid water, the Martian south pole contains smectites, a class of clays that have been misinterpreted in the data.
Water on Mars is interesting to planetary scientists and astrobiologists because of its importance to life. The planet held water in its ancient history, like in the dried-up lakebed currently being explored by the Perseverance rover, but many have held out hope that liquid water still exists in significant amounts on the Red Planet. Some research had identified what appeared to be subsurface lakes, but now a paper published this month in the Geophysical Research Letters argues the conditions around the south pole aren’t right for liquid water and that smectites are a more likely culprit for signals in the radar data.
“I really don’t believe that the lake idea holds water, so an alternative was needed … smectites are abundant on Mars and heavily studied by spectroscopists, but they’ve been mostly neglected by the radar community. My hope is that we consider them more fully in the future and even revisit some of our previous work in light of these new results,” said Isaac Smith, a planetary scientist at York University and lead author of the new paper, in an email.
The data in question is from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) onboard the European Space Agency’s Mars Express spacecraft. The radar sounding equipment detected very bright regions that indicated a subsurface substance with greater electrical conductivity than Martian rock or ice, both of which are abundant in the planet’s south pole. Some researchers believed it was evidence of water below the frozen surface; others felt the conditions weren’t right.
The new research follows up on another paper published in the Geophysical Research Letters earlier this month, which identified a larger number of these subsurface bright spots than previously found. This finding indicated that the south pole could be peppered with subsurface lakes; as study co-author and research scientist at NASA Jeffrey Plaut put it in a NASA press release at the time, “Either liquid water is common beneath Mars’ south pole or these signals are indicative of something else.” The recent paper suggests the latter.
Aditya Khuller, a planetary scientist at Arizona State University and that paper’s other author, said in an email that the radar-bright spots “could actually be regions containing clays or similar materials that are causing the bright reflections previously interpreted to be indicative of a liquid water component … The work by Smith et al. (2021) provides experimental and theoretical framework in support of this scenario.”
There were immediately two problems with the idea that the subsurface structures could be liquid water lakes, as the new paper discussed. The first problem was salt: It lowers the melting point of water, but a lot more salt than is expected in Mars would be needed in the south pole to help melt the ice. The second problem was heat. Mars is very cold; temperatures average -81° Fahrenheit, well below the freezing point of water.
A 2019 paper by a different team posited that local heat anomalies would be necessary to warm up Martian ice enough to form water in the area, with magmatism being the most likely answer if water did indeed lie below the pole. Michael Sori, a planetary scientist at Purdue University who authored that paper and is unaffiliated with the new one, said in an email that “one thing that would be nice to see in the future is for their lab experiments to be performed at cooler temperatures. They performed the experiments at 230 K [-46°F], but as the authors admit, this temperature is probably much too warm for the bottom of the [south pole’s layered] ice.”
“Ultimately, I don’t think the liquid water hypothesis has been completely ‘disproven,’ but these authors and others in the community have nicely shown that there are possible alternative explanations that need to be taken very seriously,” Sori added.
The proof will be in the pudding (those bright areas around the south pole), but unfortunately we don’t have instruments that can dip into them right now. The Mars Express orbiter has been collecting data from above for nearly 20 years, but we may require more direct examination to know for sure what underground chemistry is going on.
“I would never rule out liquids in the Martian subsurface, but the MARSIS instrument was sent to find aquifers, and this was the best candidate in 18 years,” Smith said. “Everyone would love to find liquid water, but unfortunately, I don’t think we’re going to find any with current instrumentation.”