The possibility of a lake of liquid water buried beneath Mars’ southern ice cap has been cast in doubt by new computer simulations, which suggest that densely packed ice layers could produce the same radar reflections as liquid water.
In 2018, the European Space Agency Mars Express orbiter used its MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument to identify what appeared to be a 20-kilometer-wide (12.4 mi) more of liquid water buried deep under 1.5 km of ice in a region called Planum Australe, in the southern polar plain on Mars. Similar evidence later came to light potentially dozens of lakesbut some are so close to the surface that it seemed impossible for water to be liquid there.
That’s because the surface of Mars is too cold and the atmospheric pressure too low to allow liquid water to remain too close to the surface. However, at the base of the Antarctic ice cap, temperature and pressure conditions can, with the help of a little natural antifreeze, allow the formation of briny lakes.
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This antifreeze could be in the form of calcium and magnesium perchlorate, a chemical compound found by NASA on the surface of Mars. Phoenix mission in 2008. Magnesium and calcium perchlorate, when dissolved in water, would lower the freezing point to a minimum of minus 68 degrees and minus 75 degrees Celsius (minus 92 and minus 103 degrees Fahrenheit) respectively – very close to the predicted temperature of minus 68 degrees Celsius . degrees C (minus 90 degrees F) at the base of the ice sheet. It’s not too difficult, then, to imagine local conditions of temperature, pressure, and the concentration of perchlorate conspiring to make large pools of liquid water possible on Mars.
Further evidence for such lakes came from measuring the undulations of the surface ice; liquid water reduces the amount of friction between an ice sheet and the rock beneath it, allowing the ice sheet to flow more quickly over the rock. This increase in flow rate results in troughs and peaks in the surface ice exactly what is seen in Planum Australia.
However, despite all this evidence, many in the planetary science community are skeptical; the presence of liquid water on Mars would be an extraordinary find and require extraordinary evidence. Now a team of scientists from Cornell University has fanned the flames of this skepticism with new findings that offer an alternative explanation for the radar echoes.
“I can’t say it’s impossible that there is liquid water down there, but we show that there are much simpler ways to make the same observations without having to stretch as far, using mechanisms and materials that we already know.” know they exist there,” Cornell’s Daniel Lalich said in an rack. Lalich is the lead author of new research suggesting that compressed ice layers can send back a strong radar signal that looks exactly like the radar echo of a layer of liquid.
A large body of water can reflect the radar back to the source because of how flat a lake is, and beyond Soil bright radar reflections of the type detected by MARSIS would almost certainly indicate liquid water, similar to areas of water under Antarctica, such as Lake Vostok. However, planetary scientists should be wary of assuming that what is true for Earth is also true for other planets, where conditions are not the same.
Lalich’s group ran thousands of simulations to test whether multiple tightly packed layers of ice could mimic a lake’s radar signal. Each simulation varied both the thickness of the ice layers and their composition (that is, how dirty they were). They found that densely packed ice layers deposited long ago and crushed under the weight of the ice sheet can, in several cases, produce bright radar reflections just like those detected by MARSIS.
The trick is ‘constructive interference’ of the radar waves. The spatial resolution on MARSIS is limited, and if the ice layers are too thin, the radar instrument cannot distinguish them. Each layer would reflect back part of the radar beam, and because the layers are pressed so closely together, the radar echoes overlap and combine, amplifying their power and making them appear brighter.
“This is the first time we have a hypothesis that explains the entire population of observations under the ice sheet without having to introduce anything unique or strange,” says Lalich. “This result, where we get bright reflections spread everywhere, is exactly what you would expect from thin-layer interference in radar.”
For now, the question of whether there is a salt lake under the southern polar cap remains unanswered, but Lalich argues that the simulations at least offer a much simpler and, in his view, more likely explanation than a lake.
“The idea that there was liquid water even remotely on the surface would have been very exciting,” Lalich said. “I just don’t think it’s there.”
Lalich’s team’s findings were published June 7 in the journal Scientific progress.