In 2018, scientists made a discovery that could change our understanding of the dusty, dry red ball that is Mars.
Radar signals bounced from just below the planet's surface revealed a shining patch, consistent with nothing so much as an underground pool of liquid water. Subsequent searches turned up even more shiny patches, suggesting a whole network of underground lakes.
Groundbreaking stuff, right? Although Mars has water in the form of ice, to date not a single drop of the liquid stuff has ever been found on our red buddy.
There's just one problem. According to a new analysis, which has found dozens more of these shiny patches, some of them are in regions that are just too cold for liquid water, even a brine, which can have a lower freezing temperature than freshwater.
"We're not certain whether these signals are liquid water or not, but they appear to be much more widespread than what the original paper found," said planetary scientist Jeffrey Plaut of NASA's Jet Propulsion Laboratory.
"Either liquid water is common beneath Mars' south pole, or these signals are indicative of something else."
The first feature was discovered at the Martian south pole, under the ice cap, using the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument on the Mars Express orbiter.
A follow-up search of archived data revealed three more of these 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, from using such technology on Earth, which signals are indicative of certain materials.
"Some types of material reflect radar signals better than others, and liquid water is one of those 'materials'," planetary scientist Graziella Caparelli of the University of Southern Queensland in Australia told ScienceAlert last year.
"Therefore, when the signals coming from the subsurface are stronger than those reflected by the surface, we can confirm that we are in the presence of liquid water.
The signals coming from these subsurface patches were, indeed, stronger than the signal coming from the surface itself, but the region in which they were found was relatively small.
So Plaut and planetary scientist Aditya Khuller of Arizona State University expanded the search. They mapped out 44,000 measurements across 15 years of MARSIS data to cover the entire Martian south pole.
They found dozens more of the highly reflective patches, spread over a greater range than previously identified. But the surface of some of the new patches lay barely a kilometer or so (less than a mile) below the surface, at which point temperatures are estimated to sit at around 210 Kelvin (-63 degrees Celsius, or -81 degrees Fahrenheit).
Previous research has found that water imbued with salts of calcium and magnesium can remain liquid at temperatures as low as 150 Kelvin for very long periods of time. We also know that Mars is rich in salts of calcium and magnesium, as well as sodium. But a 2019 paper found that no amount of salt is sufficient to melt the ice at the base of the Martian south pole layered deposits.
They concluded that there would need to be some form of basal heating, perhaps in the form of geothermal activity: volcanism. However, while there is recent evidence of volcanic activity on Mars, it was located in the lower latitudes, not the poles.
"They found that it would take double the estimated Martian geothermal heat flow to keep this water liquid," Khuller explained.
"One possible way to get this amount of heat is through volcanism. However, we haven't really seen any strong evidence for recent volcanism at the south pole, so it seems unlikely that volcanic activity would allow subsurface liquid water to be present throughout this region."
So what the heck are these shiny patches? Well, we don't know. The team believes it is unlikely to be liquid water – but their mapping may help figure it out. We now know, for example, that whatever is causing them is widespread across the Martian south pole.
And, if the patches do turn out to be liquid water, then the work will, the researchers said, help better understand how it came to be there.
The research has been published in Geophysical Research Letters.