Multiple Salty Lakes Discovered On Mars Underneath Its South Pole

The Martian south pole as seen by Mars Express on 17 December 2012, in infrared, green and blue light, using its High Resolution Stereo Camera. ESA/DLR/FU Berlin / Bill Dunford

Two years ago, researchers announced the likely discovery of a large lake of water on Mars, underneath its South Pole. New observations have now confirmed that the lake really is there, and in fact, it's not alone.

As reported in Nature Astronomy, multiple bodies of water have been discovered around the main lake, which is roughly 20 kilometers (12 miles) across. The bodies are separated from each other by strips of dry land and are all located roughly 1.5 kilometers (0.9 miles) below Mars's surface, in a region called Planum Australe.

“[In this work,] we have confirmed the existence of the big body of water independently and we also find the other patches, so it means that it is not an isolated, casual discovery. It is a system. And this changes things,” corresponding author Professor Elena Pettinelli, from University of Rome 3, told IFLScience.

How this water remains liquid in these lakes is a big puzzle. Their temperatures are expected to be about -68°C (-90°F). On Earth, subglacial lakes in Antarctica remain liquid thanks to pressure from ice above. For water to remain liquid under the frigid temperature of Mars, pressure from the above ice is not enough. The researchers think that they must be briny lakes with high concentrations of salt. The new observations also suggest that they are not new or temporary features; they've been there for a geologically long time.

This relative dielectric permittivity map obtained from the radar observations allowed the detection of several bodies of water on Mars. Pettinelli et al. 2020 Nature Astronomy

“We now think [the lake system] has probably survived a very long time. We are thinking for millions of years for sure. It probably got progressively covered by ice when the climate changed,” Professor Pettinelli explained.

After the 2018 discovery, there were discussions of a potential geothermal source underneath the lake to keep it liquid, but the discovery of the other three bodies of water makes this scenario less likely.

The discovery was made using the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) onboard the Mars Express spacecraft. Back in 2018, they discovered the lake using 29 observations of the areas and doing some clever data processing. The spacecraft releases radar pulses that are reflected by underground material.

This region of Mars is particularly ideal for this work, as it is very flat, which makes it easier to process the data. In the new work, the team changed their approach and had a lot more data to work with: 134 observations covering an area of 250 by 300 kilometers (155 by 186 miles).

They used techniques that have previously been employed to study subglacial lakes beneath the ice sheets of Antarctica and Greenland. However, studying lakes on another planet is a different ballgame. Observations on Earth are done by plane, 500 meters (1,640 feet) above the ice, while MARSIS operates at an average 400 kilometers (248 miles) altitude.

Further study and potential discovery of more lakes will be difficult with current orbiters around Mars, but the team believes that it's very likely more trapped water exists below the ice at the South Pole of the Red Planet.

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