Since 1971, when the Mariner 9 probe surveyed the surface of Mars, scientists have theorized that there may be ice beneath the surface beneath the southern polar ice cap on Mars. In 2004, ESA’s Mars Express orbiter further confirmed this theory when its Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument detected something at a depth of 3.7 km (2.3 miles) below the surface, that looked like water ice.
These results were very encouraging as they suggested that there might still be sources of liquid water on Mars where life could survive. Unfortunately, after reviewing the MARSIS data, a team of researchers led by Arizona State University (ASU) suggested an alternate explanation. As they showed in a recent study, the radar reflections could be the result of clays, metal-containing minerals, or salty ice beneath the surface.
The study, which recently appeared in the Geophysical Research Letters, was led by Carver J. Bierson – a postdoctoral fellow at ASU’s School of Earth and Space Exploration (SESE). He was supported by Professor Slawek Tulaczyk of UC Santa Cruz (UCSC), ASU research fellow Samuel Courville, and Nathaniel Putzig – a senior scientist at the Planetary Science Institute (PSI) – in geo and planetary sciences.
A view of the southern polar plane of Mars with the area scanned by MARSIS highlighted. Photo credit: USGS Astrogeology Science Center / ASU / INAF
The MARSIS instrument works by directing a ground penetrating radar beam onto the Martian surface and then measuring the reflected echo. An underground zone of liquid water has completely different electrical properties than surrounding ice or rock and will reflect very strongly. This technique enabled the Mars Express to create an underground map of Mars to a depth of 3 miles.
In 2018, an analysis of subsurface radar reflections by a team of Italian researchers focused primarily on electrical permittivity, which controls the speed of radio waves within a material. The denser the material in question (water, ice, rock, etc.), the slower the waves propagate and the power of the reflected waves is also affected. Due to its brightness, this bright radar reflection was interpreted as a large patch of liquid, salty water.
However, the radar reflection may be bright due to a large contrast in either dielectric permittivity or electrical conductivity. As Putzig explains in a PSI press release:
“It is not necessary to conjure up liquid water at the base of the polar cap to explain the results of the MARSIS observations. Alternatives are clays, some metallic minerals, and salty ice. Because water – especially in liquid form – is so important to the maintenance of life, it is of the utmost importance for astrobiological studies to find out where it could be on Mars today or in the past. It is critical to the scientific process to ensure that we fully consider other options for reported evidence of liquid water. “
Artist’s impression of water beneath the surface of Mars. If there were indeed underground aquifers, the effects on human exploration and the eventual settlement of the Red Planet would be far-reaching. Photo credit: ESA
In contrast, the new research focused on electrical conductivity, where conductivity contrasts between materials could also explain the bright radar reflection. “Our team wanted to take a step back and ask if there were any other materials besides liquid water that could be causing these bright reflections,” said Bierson. “Salt ice or conductive minerals at the base of the ice sheet are less noticeable, but they correspond more to the extremely cold temperatures at the Martian poles.”
Dielectric constant or electrical conductivity are related, but most previous evaluations of subsurface radar reflections have chosen to focus on the former and neglect the latter. Attributing them to salty water supplies was hardly unreasonable and may have been little influenced by wishful thinking. Scientists have known for decades that about 4 billion years ago there was running water and possibly microbial life on the surface of Mars.
Since then, various pieces of evidence have surfaced suggesting that much of this water retreated underground as the Martian climate underwent a massive transition and became the extremely cold, arid, and highly irradiated environment we see there today. Knowing that there are abundant water supplies underground, especially in the mid-latitudes, would also be a boon for future manned missions to the Red Planet. Said Putzig:
“This decision led previous workers to conclude that liquid water – likely very salty to be liquid at the temperatures and pressures expected 1.5 kilometers below the surface of Mars’ south polar ice cap – the most reasonable explanation for that The reflectivity of the radar is high. However, the work presented in this paper shows that conductivity can be an important factor for some materials, and therefore such materials offer a viable alternative explanation for the data. “
Radar detection of water under the south pole of Mars. Photo credit: ESA / NASA / JPL / ASI / Univ. Rome
This may seem a little daunting to those who believe we will find life on Mars, but it is important to remember that this research is just exploring all possibilities. Assuming there is simply more ice beneath the south polar layered deposits, this is still good news for those who hope that astronauts will explore Mars’ South Pole in the years to come (or dream of some day settling humans on Mars see).
In short, the search for evidence of the past and later evolution of Mars continues. This search is ongoing and is picking up speed, giving scientists the opportunity to test long-held hypotheses and revise previous ideas. Every clue is another piece of the puzzle!
Further reading: PSI