Is there really a huge underground lake near the south pole of Mars?

Lina Portman4 weeks ago

0 3 minutes read

Doubts have been cast on the possibility of a lake of liquid water buried beneath Mars' southern ice cap by new computer simulations, which suggest that tightly packed layers of ice could produce the same radar reflections as liquid water .

In 2018, the European Space Agency Mars-Express The 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-mile) field. liquid water lake buried deep beneath 1.5 km (0.93 miles) of ice in a region called Planum Australe, in the southern polar plain of Mars. Similar evidence was later revealed for potentially dozens of lakesbut some are so close to the surface that it seemed impossible for the water to be liquid there.

This is because the surface of March East too cold and the atmospheric pressure too low to allow liquid water to persist too close to the surface. However, at the base of the South Pole ice cap, temperature and pressure conditions, aided by some natural antifreeze, could allow for the existence of brackish lakes.

Related: Water on Mars: exploration and evidence

This image taken by NASA's Mars Reconnaissance Orbiter shows ice caps at Mars' south pole. (Image credit: NASA/JPL-Caltech/University of Arizona/JHU)

This antifreeze could be in the form of calcium and magnesium perchlorate, a chemical compound discovered on the surface of Mars by NASA. Mission Phoenix in 2008. Magnesium and calcium perchlorate, when dissolved in water, would lower its 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 C (minus 90 degrees F) at the base of the ice sheet. Therefore, it is not far-fetched to imagine localized conditions of temperature, pressure, and perchlorate concentration conspiring to allow large pools of liquid water on Mars.

Further evidence for the existence of such lakes came from measuring the undulations of surface ice; Liquid water reduces the amount of friction between an ice sheet and the bedrock below, allowing the ice sheet to flow more quickly over the bedrock. This increase in flow causes dips and peaks in the surface ice, which is exactly what we see in the Planum Australe.

Despite all this evidence, many in the planetary science community have remained skeptical; the presence of liquid water on Mars would be an extraordinary discovery and would require extraordinary evidence. Now, a team of scientists from Cornell University has stoked the flames of that skepticism with new findings that provide an alternative explanation for radar echoes.

“I can't say it's impossible that there is liquid water there, but we show that there are much simpler ways to get the same observations without having to go that far, using mechanisms and materials that we already know exist there.” said Daniel Lalich of Cornell in a statement. Lalich is the lead author of new research suggesting that compacted layers of ice could send back a powerful radar signal that resembles the radar echo from a layer of liquid.

A large body of water is capable of reflecting radar back to its source due to the flatness of a lake, and on Earth bright radar reflections of the type detected by MARSIS would almost certainly mean liquid water, similar to pockets of water under Antarctica such as Lake Vostok. However, planetary scientists should be careful not to assume 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 layers of tightly packed ice could mimic a lake's radar signal. Each simulation varied both the thickness of the ice layers and their composition (i.e. how dirty they were). They found that, in many cases, tight layers of ice deposited long ago and crushed under the weight of the ice sheet can produce bright radar reflections, just like those detected by MARSIS.

The trick lies in the “constructive interference” of radar waves. The spatial resolution of MARSIS is limited and if the ice layers are too thin, the radar instrument cannot distinguish them. Each layer would reflect part of the radar beam, and because the layers are squashed so closely together, the radar echoes overlap and combine, amplifying their strength and making them 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,” Lalich said. “This result where we get bright reflections scattered everywhere is exactly what one would expect from thin-film interference in radar.”

For now, the question of whether there is a brackish lake beneath the south polar cap remains unanswered, but Lalich says the simulations provide at least a much simpler and, in his view, more likely explanation than a lake.

“The idea of there being liquid water, even a little bit near the surface, would have been really exciting,” Lalich said. “I just don’t think it’s there.”

Lalich's team's findings were published June 7 in the journal Scientists progress.