Those underground 'lakes' on Mars are getting more and more mysterious, study shows

In 2018, scientists made a discovery that could change our understanding of Mars, the dusty, dry red ball.

Radar signals reflected from just below the planet's surface showed a shiny patch, exactly the same as a subsurface pool of liquid water. Subsequent searches uncovered more shiny patches, indicative of an entire network of subterranean lakes.

Groundbreaking stuff, right? Despite water in ice form on Mars, we haven't found a single drop of liquid material on our red buddy so far.

There is only one problem. According to a new analysis that has found more of these shiny patches, some are located in regions that are too cold for liquid water, even salt water, which may freeze at cooler temperatures than fresh water.

"We're not sure if these signals are liquid water, but they seem to be much broader than what the original paper found," said Jeffrey Prout, a planetary scientist at NASA's Jet Propulsion Laboratory, "or liquid water at the south pole of Mars. Downside is common, or these signals indicate something else."

The first feature was discovered under the Martian South Pole Ice Sheet using the Mars Advanced Radar for Subsurface and Ionosphere Exploration (MARSIS) instrument on the Mars Express orbiter.

Subsequent searches of archived data revealed three additional lake-like features. MARSIS uses radar signals to probe beneath the Martian ice cap, which consists of alternating layers of carbon dioxide and water ice.

We know what signals can indicate certain substances by using this technology on Earth.

"Certain types of matter reflect radar signals better than others, and liquid water is one of those 'materials,'" says planetary scientist Graziera Caparelli of the University of Southern Queensland in Australia. When the signal from the ground is stronger than the signal reflected from the surface, we can confirm that we have liquid water.

The signals from these subsurface patches were indeed stronger than those from the surface itself, but they were found in relatively small areas.

So Jeffrey Prout and planetary scientist Aditya Cooler of Arizona State University expanded their search. They mapped 44,000 measurements over 15 years of MARSIS data, covering the entire south pole of Mars.

They found dozens of highly reflective patches spread over a larger area than previously identified. But the surface of some of the new patches is only about a kilometer (less than a mile) below the surface, at an estimated temperature of about 210 Kelvin (-63 degrees Celsius, or -81 degrees Fahrenheit).

Previous research has found that water containing calcium and magnesium salts can remain liquid for long periods of time at temperatures as low as 150 Kelvin. We also know that Mars is rich in calcium and magnesium salts and sodium. But a 2019 paper found that no amount of salt was enough to melt the ice at the bottom of the layered sediments at Mars' south pole.

They concluded that some form of basal heating was needed, perhaps in the form of geothermal activity: volcanic activity. However, while there is recent evidence of volcanic activity on Mars, it is located at lower latitudes, not the poles.

"They found that to keep this water liquid requires twice the estimated Martian geothermal flow," Kuehler explained.

"One possible way to get this much heat is through volcanism. However, we haven't seen any strong evidence of recent volcanism in Antarctica, so it seems unlikely that volcanism has allowed subsurface liquid water to exist throughout the region. "

So what exactly are these shiny patches? Well, we don't know. The team thinks it's unlikely to be liquid water -- but their mapping might help clarify. For example, we now know that the causes of them are all over the south pole of Mars.

And, if the patches are indeed liquid water, the researchers say the work will help to better understand how it formed.