Two New Studies Discuss Presence of Martian Polar Lakes

For decades humans have looked to Mars and asked if anything could ever have lived there, and if, in the future, that could be us. Mars is our closest neighbour, and in many ways looks a lot like Earth. We see mountains and valleys carved by water, old volcanic remnants, sands and dust, the occasional cloud and the more frequent dust storm, and a northern and southern pole, complete with ice caps.

The ice caps are of particular interest as they indicate the presence of water. Both the northern and southern polar caps have a layer of dry ice (frozen CO2) over them, but underneath is water ice, just like the ice caps on Earth.

For the most part, we see Mars as a dry and dusty landscape. The images and videos provided by rovers and now the Ingenuity helicopter show pictures reminiscent of the deserts here on Earth - an idea not helped by the dust devils that whip past, or the recordings of eerie howling wind. 

However, the water ice of the polar ice caps dispels the idea that Mars has always been the desert we see today. For there to be water ice on Mars indicates the presence of water at some point in the planet's history, and the potential for some water to remain, not just locked up as water ice, but in underground lakes, in hydrous clay minerals, or as subglacial briny pools at the poles.

Where water might be on Mars, and how it is stored could be key to working out where life may have previously formed or where remnants of it may be, as well as where humans could create bases to study the planetary evolution of Mars and create settlements into the future if we ever decide to create permanent settlements on Mars.

A recent study conducted by scientists around the world has investigated possible causes of a bright reflectance signature coming from the South Pole of Mars. The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument aboard the European Space Agency’s Mars Express orbiter detected this anomalously bright radar signature approximately 1.4km below the South Polar Layered Deposits.

“It is generally assumed that the temperature at the base of the solar ice cap is less than -73℃, and we know that pure water exists only as solid ice at these subzero temperatures,” said Dr Caprarelli from the University of Southern Queensland, one of the authors of this study.

“However, our work suggests a group of salts found in Martian soil have an anti-freezing effect, and that brines formed by the liquefaction of these salts could be stable at these temperatures.”

The study uses Mars clay and salt analogues available on Earth to determine what could be causing this reflectance signature. Reflectance was tested on a variety of hydrous clay minerals with different hydration rates, as well as a number of brine solutions. These tests were conducted at a number of different temperatures that might occur on Mars or at Mars’s south pole. The team concluded that brines of salts such as magnesium perchlorate or calcium chloride could be causing this signature as opposed to hydrous clay minerals or frozen salty lakes.

“The team undertook extensive laboratory experiments on samples of clays, saline ices and brines in subzero temperatures,” said Dr Caprarelli.

“Our experiments show that brines alone have the properties consistent with the bright reflections measured by MARSIS (a scientific instrument on board the European Space Agency’s Mars Express spacecraft), supporting our interpretation that liquid water exists at the base of the south polar ice.”

This, however, is still contentious. Another recently published study attributes the reflectance signature to the topography, indicating that if it were covered in an ice sheet it may produce the signature seen by MARSIS.

Conducting studies such as this using observational data from space is extremely difficult, and so the team hopes to make a case to the European Space Agency to continue the Mars Express orbiter mission until 2025 so that more observations can be made and allow for further study.

“Our work has the potential to determine the geological and climatic processes that formed the Martian polar caps and help with the many unanswered questions about the planetary evolution of Mars,” said Dr Caprarelli.

“Ultimately, greater understanding of Mars could one day lead to humans being able to land and potentially live on the planet.”