Suspected Mars Water Really Is Water

Life as we know it requires liquid water, which is why finding areas of liquid water on Mars is an ongoing objective of scientists. Finding areas of liquid water on Mars would indicate places we might look for potential extant life or even create settlements.

One place of great interest is Mars’ South Pole. The South Pole has a permanent ice cap of water ice with a layer of dry ice (frozen CO2) covering it. More intriguingly though, a bright radar signal was detected using The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) - a signal that looks a lot like an underground lake of liquid water.  

There have been many interpretations of the signals since they were observed. In 2021, the theory that these signals might be from clays rather than liquid water was shown to be inconsistent with the obtained data. While other studies have claimed that topography might be the cause of the signal, another theory was that dry ice layered with water ice may have caused the unusually bright reflectance signal.

“Several interpretations explaining the basal reflections have been proposed since bright radar reflections were detected in a region of Mars known as Ultimi Scopuli,” said Dr Graziella Caprarelli.

An international team including the University of Southern Queensland’s Dr Graziella Caprarelli has been studying whether layered dry ice and water ice deposits could indeed cause this signal. The team’s most recent paper shows that dry ice couldn’t cause the observed signal, ruling out another possible cause. 

It was originally theorised that briny water was the cause of the bright signal. Water with a high concentration of dissolved perchlorates or chlorides (types of salts) would have a much lower freezing temperature than plain water. Given the average surface temperature of Mars is estimated at 162 K (about -111 degrees Celsius) and the atmosphere is very thin, it is incredibly difficult for pure liquid water to remain on Mars for long periods of time.

“Just like our work published earlier this year in Earth and Planetary Science Letters – where we demonstrated that clays are not an alternative to water to explain the bright signals from the MARSIS radar – this latest paper demonstrates through simulations that layers of dry ice (CO2 ice) cannot produce the radar signals observed,” said Dr Caprarelli.

“Both these papers reinforce and further corroborate our earlier interpretation (published in Science in 2018, and in Nature Astronomy in 2021), that liquid water exists on the Red Planet.”

This latest study ran over one million simulations, varying only the thickness of each layer in a dry ice - water ice - dry ice sandwich.

These papers continue to demonstrate that briny water is the most likely cause of the bright reflectance signal seen at the South Pole of Mars. This has implications for many different fields - including bioburden limits for landers and rovers going to the South Pole, searching for extant life, and theorising where humans might live on Mars.

"The scientific community continues to test materials that are likely to be at the base of the south polar cap, to check whether any of those can be the source of bright basal reflections, apart from liquid brines," added Dr Caprarelli. "Based on laboratory experiments, the brines remain thus far the best (and only) candidates in this investigation."

"Therefore, the next steps (for our team) are going to be directed toward figuring out the conditions at which salty water can still be liquid at temperatures such as those hypothesized for the base of the deposits (-70 deg C, or even lower, depending on models). This direction of investigation will involve geological, climatological, cosmochemical and astrophysical science, and therefore we might also be able to break new ground and generate new knowledge in this field."

"We need more “eyes” on the polar regions, and specifically active instrumentation experiments that can probe the subsurface. The Mars Express mission has been extended, which will allow our team to fly again over the polar regions, so we can acquire additional data using new sophisticated processing methodologies."

"It would be nice if (one day) we could put a seismometer array on the ground, but given the current technology, I do not think that is an option any time soon. However, more orbiting radars would be nice, and perhaps we could try and land a GPR (ground penetrating radar) somewhere on the surface," she said.

"Liquid brines at the base of the SPLD can be viewed as a new microenvironment. We know that high salinity of water correlates with more oxygen dissolved in the water, so there could be enough oxygen for certain types of organisms to survive. I am not a biologist, and therefore cannot say much more about this topic, because I simply do not know enough."

"However, salty ocean waters probably present under the icy shell of Europa (one of the moons of Jupiter) are the next big target of investigation in the search for life in the solar system. Thus, basal liquid brines buried under an ice sheet, such as those on Mars, are important analogs also for future studies about life elsewhere," concluded Dr Caprarelli.