Since 1971, when the Mariner 9 probe explored Mars' surface, scientists have suspected that subsurface ice might exist beneath the southern polar Ice cap. The 2004 Mars Express orbiter of the ESA confirmed this theory with its Mars Advanced Radar for Subsurface and Ionosphere sounding (MARSIS), which detected water ice at a depth 3.7 km (2.3 miles) below the surface.These results were encouraging as they suggested that Mars could still have liquid water where life could survive. A team of researchers from Arizona State University (ASU), reviewed the MARSIS data and proposed an alternative explanation. In a recent study, they suggested that radar reflections could be caused by clays, metal-bearing mineral, or saline water beneath the surface.Carver J. Bierson, a postdoctoral researcher from ASU's School of Earth and Space Exploration, led the study. It was published in Geophysical Research Letters. He was joined by Earth and Planetary Sciences Professor Slawek Tilaczyk from UC Santa Cruz (UCSC), Samuel Courville, an ASU researcher, and Nathaniel Putzig, a senior scientist at the Planetary Science Institute.View of the southern polar plain on Mars, with the highlighted area scanned using MARSIS. Credit: USGS Astrogeology Science Center/ASU/ INAFThe MARSIS instrument uses ground-penetrating radar to direct a beam towards Mars and measure the reflected echo. A subsurface zone of liquid water has very different electrical properties than surrounding ice and rocks, and will reflect extremely strongly. The Mars Express was able to create a subsurface map for Mars at depths up to 5 km (3 miles).In 2018, a group of Italian researchers analysed subsurface radar reflections. Their focus was primarily on electrical permittivity. This is what controls the speed radio waves within a substance. The density of the material (water, ice and rock) is a factor that determines how fast it can transmit radio waves. The waves will travel slower and the power of the reflections will also be affected. This bright radar reflection was mistaken for a large area of liquid, briny water because of its brightness.Radar reflection can also be brightened by a high contrast in electric conductivity or dielectric permittivity. Putzig explained this in a PSI press release.To explain the MARSIS observations, it is not necessary that liquid water be invoked at the base the polar caps. Clays, some metallic minerals and salty ice are all possible alternatives. Astrobiological studies are crucial because water, especially in its liquid form, is essential for life sustainment. Therefore, it is important to find out where it might be found on Mars today and in the past. It is vital that we consider all other possible detections of liquid water in our scientific research.Artist's impression of water beneath the Martian surface. The implications of underground aquifers for human exploration and colonization on the red planet could be huge if they are indeed true. Credit: ESAThe new research, however, focused on electrical conductivity. This could explain why bright radar reflections can also be caused by differences in conductivity. Bierson said that the team wanted to look at other materials than liquid water, in order to see if they could also cause bright reflections. Although less striking than the bright reflections from the ice sheet, salty ice and conductive minerals are closer to the extreme cold temperatures at Mars poles.Although electric conductivity and dielectric permittivity are closely related, most previous assessments of subsurface radar reflections have tended to ignore the latter. Although it was not unreasonable to attribute them to briny water caches, this may have been partially influenced by wishful thinking. Scientists have known for decades that around. Scientists have known for decades that Mars was home to flowing water and possibly microbial life 4 billion years ago.There have been many lines of evidence that suggest that much of this water has retreated underground during the massive Martian climate transition. This transformed the environment into the extreme cold, dry, and highly irradiated environment that we see today. Future crewed missions would be greatly aided by the knowledge that there are many underground water caches, particularly in the mid-latitude areas. Putzig said:Prior workers concluded that liquid water was probably very salty to make it liquid at temperatures and pressures expected to reach 1.5 km below the surface of Mars' south polar Ice cap. This explanation is most plausible for high radar reflectivity. The paper shows that conductivity can play an important role in some materials and therefore, this paper offers a viable alternative explanation.Radar detection of water below the south pole on Mars. Credit: ESA/NASA/JPL/ASI/Univ. RomeThis may seem a little disappointing for those who are optimistic about finding life on Mars. However, this research is only exploring the possibilities. It is possible that there is more ice under the South Polar Layered deposits, but this is still good news for those who hope to see astronauts exploring the Martian South Pole in future years or dream of humans settling on Mars.The search continues for evidence of Mars' past and future evolution. The search for evidence of Mars' past and subsequent evolution has been ongoing. Scientists are now able to speed up their efforts, allowing them to test long-held hypotheses as well as revise old ones. Each clue is a piece of the puzzle.Additional Reading
