The nature of the Mars glacial lakes was explained by volcanic deposits and wet brine

Planetologists have come up with new explanations for the nature of the mysterious luminous regions found in radar images of the deposits at Mars’ south pole. These can be both volcanic rocks and liquid solutions of perchlorates and chlorides, which can be found on the edge of ice or soil grains. Articles (1, 2) published in magazines Letters of Earth and Planetary Science and Letters of Geophysical Research.

In 2018, an analysis of observation data from the MARSIS multi-frequency radar instrument installed on the Mars Express orbit made it possible to detect unusually bright areas in sediment accumulations at the south pole of Mars. Such phenomena are inherent in materials with high dielectric constant, which is why scientists have decided that below the Southern Plateau at a depth of 1.4-1.5 km there are water deposits in the form of lakes or a layer saturated with water. Liquid water is unlikely to be stable for a long time in modern Mars conditions, so such reservoirs could form due to volcanic activity and consist of water-based brine. However, the interpretation of the areas found in the radar images has been the subject of controversy to this day. In particular, last year the idea was expressed that MARSIS did indeed find hydrated deposits rich in clay, especially smectite, and not in lakes.

Two teams of planetary scientists decided to figure out which substances or layers could have high reflective properties and explain the MARSIS data. Cyril Grima of the Institute of Geophysics at the University of Texas and his colleagues decided to determine how the relative baseline reflectivity (ratio of the strength of the superficial echo signal to the surface echo signal) of the modern surface of Mars would be different covered with a thick layer of ice, similar to the sediments of the South Pole at the site of the candidate lakes, and the ice content at the level of 10 percent by volume (similar to the ice in the South Polar sediments). Landscapes located at 192–194 ° east longitude and 80–82 ° south latitude were examined in the project.

It turned out that 0.3-2 percent of the surface of Mars, which is covered with ice, can give a radar image similar to the deposits of the South Pole. The surface ceases to have such reflective properties for values ​​of the tangent to the dielectric loss greater than 0.010. Most of the interesting sites of fiery origin found (for example, the Sun Plateau (Solis Planum), which represents a basalt plain) and belong to periods from the Late Neolithic to the Late Amazon. It is assumed that the refractory material can acquire high reflectivity due to the high ilmenite content.

David Stillman of the Southwestern Research Institute and colleagues conducted experiments in the laboratory to study the dielectric properties of Martian clay analogues at temperatures close to Martian and compared the results with the findings of other MARSIS work and observations. The scientists used six clay samples collected in Italy: Holocene river deposits, Pliocene-Early Pleistocene marine mudstones, and metamorphic deposits dating from the Jurassic to the Oligocene. The samples were sieved, then dried in an oven, rehydrated with distilled water until they returned to their original water content. In addition, experiments with Mg (ClO) solutions were performedfour)2 and CaCl2.

The researchers concluded that clay deposits at temperatures comparable to the base of the South Polar ice sheet (less than 200 Kelvin), even with very high clay content, could not produce bright areas in radar images. Frozen brine will not be able to create them either. According to scientists, liquid solutions of perchlorates and chlorides, which can be found at the boundaries of ice or soil granules, are better suited for the role of a material with high reflectivity. In this case, the volume fraction of the liquid brine can reach several tens of percent, which will be enough to achieve the desired dielectric constant.

Earlier, we talked about how scientists deemed salt water on Mars unfit for life and mapped the distribution of water ice in the planet’s northern hemisphere.

Alexander Voytyuk

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