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. Plain Language Summary Bright radar reflections from beneath the ice cap on the south pole of Mars provided tantalizing evidence for liquid water bodies beneath the ice. However, three recent papers present new challenges for this interpretation. The first shows that similar bright reflections are widespread across much of the ice cap. The environment on Mars already makes it difficult for water to remain stable and these echoes mean that it would have to do so across an even wider range of challenging temperatures and pressures. The two other papers show that other materials, like salty ice and clay, could also produce the bright reflections. Although these new papers do not rule out the possibility of very salty bodies of liquid water beneath the ice cap, they do raise the possibility of alternative sources for the bright reflections.

Automated summary

Bright radar reflections beneath Mars' south polar ice cap initially suggested the presence of liquid water. However, recent research indicates that similar reflections are widespread and other materials may also produce the bright signals, raising the possibility of alternative sources for the observations.