Frontier Adventure

Unfortunately, There are Other Viable Explanations for the Subsurface Lakes on Mars

Ever since 1971, when the Mariner 9 probe surveyed the surface of Mars, scientists have theorized that there might be subsurface ice beneath the southern polar ice cap on Mars. In 2004, the ESA’s Mars Express orbiter further confirmed this theory when its Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument detected what looked like water ice at a depth of 3.7 km (2.3 mi) beneath the surface.

These findings were very encouraging since they indicated that there could still be sources of liquid water on Mars where life could survive. Unfortunately, after reviewing the MARSIS data, a team of researchers led from Arizona State University (ASU) has proposed an alternative explanation. As they indicated in a recent study, the radar reflections could be the result of clays, metal-bearing minerals, or saline ice beneath the surface.

The study, which recently appeared in the Geophysical Research Letters, was led by Carver J. Bierson – a postdoctoral researcher at ASU’s School of Earth and Space Exploration (SESE). He was joined by Earth and Planetary Sciences Professor Slawek Tulaczyk of UC Santa Cruz (UCSC), ASU research associate Samuel Courville, and Nathaniel Putzig – a senior scientist with the Planetary Science Institute (PSI).

A view of the southern polar plain of Mars with the area scanned by MARSIS highlighted. Credit: USGS Astrogeology Science Center/ASU/ INAF

The MARSIS instrument works by directing a ground-penetrating radar beam towards the surface of Mars, then measuring the reflected echo. An underground zone of liquid water will have very different electrical properties from surrounding ice or rocks and will reflect very strongly. This technique allowed the Mars Express to create a subsurface map of Mars up to depths of 5 km (3 mi).

Back in 2018, an analysis of the 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 will travel, and the power of the reflected waves will be affected as well. Due to its brightness, this bright radar reflection was interpreted as a large patch of liquid, briny water.

However, radar reflection can be bright due to a large contrast in either dielectric permittivity or electric conductivity. As Putzig explained in a PSI press release:

“It is not necessary to invoke liquid water at the base of the polar cap to explain the results of the MARSIS observations. Alternatives include clays, some metallic minerals, and salty ice. Because water — particularly in a liquid form — is so important to sustaining life, seeking out where it may exist on Mars today or in the past is of paramount importance to astrobiological studies. Ensuring that we fully consider other possibilities for reported detections of liquid water is crucial to the scientific process.”

Artist’s impression of water under the Martian surface. If underground aquifers really do exist, the implications for human exploration and eventual colonization of the red planet would be far-reaching. Credit: ESA

In contrast, the new research focused on electrical conductivity, where contrasts in conductivity between materials could
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