Alternatives to Liquid Water Beneath the South Polar Ice Cap of Mars

Radar sounding of Mars' south polar ice cap has revealed portions of its base with stronger radar returns than its surface. These have been used to estimate high real permittivity values consistent with liquid water. However, the thermal, chemical, and geological conditions required to sustain such water bodies are challenging to achieve in the contemporary Martian environment. Three recent papers address this conflict. Khuller and Plaut (2021, https://doi.org/10.1029/2021GL093631) find similar radar signatures across the ice cap, expanding the range conditions associated with water candidates. Bierson et al. (2021, https://doi.org/10.1029/2021GL093880) show that, for non-zero conductivity, a wider range of materials could produce the observed echoes. Similarly, Smith et al., (2021, https://doi.org/10.1029/2021GL093618) use modeling, laboratory tests, and remote sensing data to propose smectite deposits as their specific source. While not incompatible with bodies of liquid brines, these papers provide potential alternative sources for the bright radar observations.