The researchers believe that the idea of argillaceous polar sediments is more plausible than the idea of subglacial lakes.
Planetologists concluded that the subglacial lakes of liquid water under the south pole of Mars discovered several years ago may in fact be hydrated and cold deposits rich in clay, in particular smectite. Scientists reached this conclusion after reanalyzing Mars Express radar data which led to further laboratory experiments.
New possibilities about the subsurface lakes on Mars
Lakes on Mars
At the south pole of Mars, there are accumulations of layered sediments, rich in water ice, several kilometers thick. It is believed that they formed over the past 10-100 million years and can tell a lot about the climatic evolution of Mars.
In 2018, observations of the Mars Express orbiter MARSIS radar showed that there are several lakes below the South Plateau at a depth of 1.5 kilometers, which could have formed due to volcanic activity and are most likely composed of brine-based water.
However, the interpretation of bright areas in radar images at the interface between ice-rich sediment and substrate is still a matter of controversy.
Substances beneath the surface
Jeffrey J. Plaut of NASA’s Jet Propulsion Laboratory and Aditya R. Khuller of Arizona State University analyzed MARSIS observations from 44,000 points at the planet’s south pole over 15 years. Scientists wanted to understand the properties of substances that lie beneath the surface of the south polar region of Mars.
The thickness of the deposits
Scientists have created new radar and topographic maps of the south polar region of Mars and found that the thickness of the south polar deposits can reach 3.7 kilometers, and their total volume is about 1.6 × 10 6 cubic kilometers.
The bright areas they found on radar images are visible at different frequencies, and the processes that generate them may not be limited to the region of subglacial lakes.
However, the researchers considered it unlikely that the bright areas under the thickest areas of the sediment were associated with liquid water since the temperature there is too low for the brine to remain liquid.
Determining the brightness of areas in radar images
Carver Bierson of Arizona State University and his colleagues published a theoretical paper in which they determined that the brightness of an area in a radar image, in addition to the dielectric constant, can also be influenced by differences in the electrical conductivity of the substances that make up polar deposits. Scientists have determined that it could be clays, metal-containing minerals, and frozen brine.
Isaac Smith of York University and his colleagues conducted a series of laboratory experiments and found that hydrated and cold clay-rich sediments can create a pattern observed on radar images. In particular, smectites cooled to 230 Kelvin, even when mixed with other substances, can generate a bright area in the image.
In addition, signs of the presence of smectites were found in the spectra obtained from observations from orbit in the visible and near-infrared regions of the south pole of Mars. Thus, the researchers believe that the idea of argillaceous polar sediments is more plausible than the idea of subglacial lakes.
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• Bierson, C. J., Tulaczyk, S., Courville, S. W., & Putzig, N. E. (2021, July 6). Strong MARSIS radar reflections from the base of Martian south polar cap may be due to Conductive ice or minerals. AGU Journals.
• Choi, C. Q. (2021, August 2). Buried ‘lakes’ on Mars may just be FROZEN CLAY. Scientific American.
• Gamillo, E. (2021, August 3). Subsurface ‘lakes’ on Mars may actually be FROZEN clay deposits. Smithsonian.com.
• Khuller, A. R., & Plaut, J. J. (2021, July 3). Characteristics of the BASAL interface of the Martian south polar layered deposits. AGU Journals.
• NASA. (2021, July 29). Clays, not water, are likely source of Mars ‘LAKES’ – NASA’s Mars exploration program.
• Smith, I. B., Lalich, D. E., Rezza, C., Horgan, B. H. N., Whitten, J. L., Nerozzi, S., & Holt, J. W. (2021, July 29). A solid interpretation of Bright Radar Reflectors under the MARS south polar ice. AGU Journals.
• Starr, M. (n.d.). There could be a new explanation of UNDERGROUND ‘Lakes’ on Mars, but it’s not water. ScienceAlert.