The Salty Subglacial Lakes of Mars

For thousands of years, humans have looked up at the fiery wanderer in the sky - noting its brightness and redness as it traversed across the constellations, year after year. The fury of its light earning it the title of the Greek mythological God of War, coming second only to the mighty Jupiter. 

Mars. 

Its very name conjures images in our mind of us standing alone, panoramically scanning the red, desolate wasteland with nothing in sight but the soft sound of high pitched wind flowing over dunes and rusted stones. 

No other planet has fascinated us more. From the advent of the telescope, we’ve looked upon its surface in bewilderment - noting dark and light patterns and features, which at a time resembled canals and structures that could have been created by an alien civilization. 

These days, the planet is occupied by an array of spacecraft in orbit, meticulously observing its behaviour and systems from above - whilst an army of robots (both past and present - the first true Martians), analyse surface features and mineralogy looking for any sign of life.

What is it about Mars that has captivated our imagination for so long? In 1877, Italian astronomer Giovanni Schiaparelli reported observing canali (channels) on the surface. Towards the end of that century, H.G. Wells published the first edition of the popular science fiction writing, the War of the Worlds. And soon thereafter, even American astronomer Percival Lowell, made claims that the red planet was hosting huge vegetation crops with irrigation systems (Schiaparelli’s canali, erroneously translated as “canals” by Lowell) running besides them. 

In fact, due to the lack of resolving power of telescopes and effects of Earth’s turbulent atmosphere - no one knew exactly what these structures could have been until the arrival (at Mars) of the Mariner spacecraft in the mid to late 60s, when close up images of the surface finally put the conversation to rest - showcasing a lifeless, barren environment littered with craters, canyons, volcanoes and other features. 

And though the question of channels might have been resolved (with no evidence suggesting they were non-natural structures) the orbiting spacecraft and surface rovers started to give us a different view of Mars, which raised even more interesting questions. 

Enormous canyons once shaped by flowing water; craters which exhibit the drying of lakes over time; fluid-like landslides which have appeared during our observation period of the last several decades. The conclusion that many scientists believe from this growing set of data is equally as fascinating as Schiaparelli canali.

Mars may have once been a waterworld, with flowing water, lakes and even thought to have the majority of the northern hemisphere submerged beneath the Oceaunus Borealis. It is from this data - the evidence that Mars was once wetter and probably warmer than it is now - that numerous space agencies have driven a large volume of resources and investment in the long-term studies of Mars, using in-situ spacecraft and robotic explorers.

Though the evidence indicates that Mars once had flowing surface water, today almost all of the water content exists as ice found in locations like the polar ice caps, inside deep craters and below the surface, with some traces found in the atmosphere as water vapour.

In fact, the volume of water ice detected on Mars (roughly 21 million km³) through spacecraft images, remote sensing techniques (such as spectroscopic and radar analysis), and by rovers investigating the surface, would be able to completely cover the planet surface in a global ocean down to a depth of 35 metres. It’s obvious that this abundance of water has played a major contributing role in the geological history of this planet. 

In some areas, ice is located at shallow depths within Mars’s most surficial layers, with further patches around specific regions on the planet - like the Elysium volcanoes. At higher latitudes (> 60 degrees), ice concentrations become more abundant, ranging from approximately 25% to 100% at the poles, where ice caps cover the surface.

The existence of water on other bodies is important in our understanding of how life developed and evolved in the universe, because life on Earth requires liquid water to survive. Should liquid water be found on other planets, then there might be a chance that life could have the conditions to evolve (or have evolved) elsewhere. 

Whilst the majority of Martian water reserves are locked up as ice, in 2018, Italian scientists provided the first evidence that a subglacial lake (liquid water) existed 1.5 km below the southern polar ice cap, measuring a vast 20 km in length. 

The conditions on Mars are too hard for liquid water to exist on the surface (atmospheric pressure is too low, causing water to effectively boil away, and its temperatures are well below the freezing point of water), but subglacial lakes can exist in liquid form with the presence of high concentrations of salt within the mixture, or relatively close to a subterranean heat source. 

This was the first evidence of a stable body of liquid water on the red planet, which resembled a similar model to Lake Vostok under the Antarctic ice sheet here on Earth.

Now, two years after the 2018 discovery of a lake beneath the Martian south polar cap, an international collaboration of scientists have found further evidence of the existence of multiple bodies of water trapped below the same region of the red planet. 

The new study, published in the journal Nature Astronomy, features Australia-based scientist Dr. Graziella Caprarelli - adjunct research fellow with the Centre of Astrophysics at the University of Southern Queensland, who discusses how these lakes could remain in existence, even under such extreme conditions.

“Any process of formation and persistence of sub-ice water beneath the ice polar caps requires the liquid to have high salinity” Dr. Caprarelli, who was not involved in the 2018 study, says. 

“Laboratory experiments aimed at studying the stability of hypersaline aqueous solutions (brines), convincingly demonstrate that these can persist for geologically significant periods of time even at the temperatures typical of the Martian polar regions, considerably below the freezing temperature of pure water”.

Focusing on an area roughly equal to 250 km x 300 km around the location of the 2018 study, scientists used a technique known as radio-echo sounding (RES) to derive the results of this recent paper. RES employs bursts of radio waves to image subsurface geological structures and has proven useful in detecting subglacial lakes on Earth in Canada, Antarctica and Greenland.

As the radio waves pass through the ice and reflect off the sub-surface bedrock they can reveal features that are buried below the surface, including any layering that is presented in the reflectivity of the returning signal. Different features, such as bedrock vs. a body of liquid will reflect the signal in their own unique manner, allowing scientists to use this deep remote sensing technique to work out what is present.

Some types of material reflect radar signals better than others, and liquid water is one of those "materials". Therefore, when the signals coming from the subsurface are very strong, it is highly probable that they are being reflected by the surface of water bodies

To acquire data, scientists used the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument, which is a part of the Mars Express spacecraft. Launched in 2003 from Earth, MARSIS has been collecting data on Mars since 2005.

The team thus collected, from 2010 through to 2019, 134 radar profiles over Ultimi Scopuli, part of the southern polar region of Mars. Applying the RES methodology to process the signals, and an innovative probabilistic technique to analyse the data, the scientists confirmed that the intensity of reflections are likely coming from pools of liquid water. 

"While the existence of a single subglacial lake could be attributed to exceptional conditions such as the presence of a volcano under the ice sheet, the discovery of an entire system of lakes implies that their formation process is relatively simple and common, and that these lakes have probably existed for much of Mars' history. For this reason, they could still retain traces of any life forms that could have evolved when Mars had a dense atmosphere, a milder climate and the presence of liquid water on the surface, similar to the early Earth," said Principal Investigator of the MARSIS experiment, Roberto Orosei.

There are several reasons why we no longer observe water in abundance on the surface of Mars, which are linked to the planet gravity, atmosphere and orbital dynamics. 

Mars itself is just over half of Earth’s diameter, and only about 11% the mass of Earth. For this reason, its surface gravity is about 37% of that of Earth’s. With a lower surface gravity, elements with higher kinetic energy can effectively escape the planet gravitational hold and leak into space (this is known as the Jeans escape). This in turn reduces the atmospheric pressure on Mars, which then causes water to either freeze out as an ice or sublime into vapour. Data from another spacecraft (MAVEN) showed that even the early Martian atmosphere had a tough time remaining in place, being stripped from the planet due to the solar wind. 

Seasonal variations also impact the water content on Mars, with recent studies finding that during the warmest and stormiest part of the planet’s orbit, large volumes of water vapour reach the upper atmosphere. This is then subject to being broken down into hydrogen and oxygen components by ultraviolet radiation from the Sun, which then triggers the hydrogen to escape from the planet’s atmosphere. 

Surface liquid water during the past, before Mars lost most of its atmosphere, would have played an important role for the planet - geologically as evidenced with observations, but also for the opportunity for life to have had a chance to emerge and flourish. 

“Follow the water, in other words, the search for liquid water, has been one of the major goals of all planetary missions, as it is evident that life as we know it on Earth, is dependent on water,” said Dr. Caprarelli.

An interesting discussion and investigation will now continue about the nature of these hypersaline liquid bodies residing below the cold, ice caps of Mars. What is their exact composition? What, if any, is their heat source (potentially an active magma chamber below the surface)? And are the conditions too extreme for life (as we know it) to exist or have existed in these regions? 

With the study published in Nature Astronomy the team confirms that thick ice sheets, far from being uniformly structured wastelands, should be viewed as stratigraphically and physically complex geological formations, deserving to be fully explored in detail. 

At the conclusion of their paper the team suggests that, because brines have been shown to have potential to sustain microbial life in extreme conditions, renewed efforts should be made to explore the polar regions of Mars, with the specific purpose of finding reservoirs of subglacial water, and of determining their composition and astrobiological potential. 

Even though the current evidence doesn’t point to the multiple bodies discovered being connected to one another, should this evidence be presented one day in the future - it will be a nice tie back to Schiaparelli’s canali - a connected series of channels with flowing water below the surface. 

Even after all these years of exploration from Earth or in-situ by spacecraft and robots, the red planet continues to fascinate us by revealing ever more glimpses of the secrets it keeps below its cold, barren surface. 

This month, Mars will reach opposition to Earth in its orbit (closest point to our planet) where it will shine brightly in the sky. When looking up at the deep red, significant point of light, remind yourself that there stands another world - one we still have more to learn about. 

Video credit: ESA.