Mineralogical characterization of the Makgadikgadi Salt Pans in Botswana as a Martian analog for ancient lacustrine environments

The Makgadikgadi Salt Pans (MSPs) in northern Botswana offer to study the mineralogy of evaporates and clays derived from fluvio-lacustrine sediments in their geological context. A field campaign taking place in August 2022, funded by Europlanet 2024 RI (grant agreement No 871149) was performed to investigate variations in the mineralogical composition of the pan materials with respect to neighboring and/or underlying (bedrock) units. Spectral measurements were performed directly in the field with a portable spectroradiometer (PSR) that samples the surface in the visible and near-infrared (VNIR) wavelength range between 0.35 and 2.5µm. In addition to VNIR spectroscopy, samples collected in the field were analyzed in the laboratory using laser-induced breakdown spectroscopy (LIBS) and Raman spectroscopy, a triple combination, which has proven to significantly enhance the scientific potential for studying the mineralogy of planetary analog materials (Stephan et al., 2025). Even more, VNIR spectra acquired in the field provide spectral endmembers, which are now used to classify the currently available data of the MSPs provided by the Environmental Mapping and Analysis Program (EnMAP) of the German hyperspectral satellite mission. EnMAP data cover the same wavelength range in the VNIR as the field instrument and covered major portions of the pans at the same seasonal period of the year.

The acquired spectral data reveal that salts dominate a more or less fresh, white to light brown, several mm-thick uppermost crust throughout the pans. They are particularly prominent where the salts themselves or at least the clays underneath this layer are still wet from the rainy season. The special shape of the water-related feature at 2 µm implies that sodium hydrogen carbonates such as trona dominate the salt layer. Although halite should be also present, its spectral signature might be masked by the signature of trona. In the wettest location, a thin greenish layer of organic material has been found, which causes a characteristic feature near 0.7 µm. In regions that have been dry for a prolonged period, clays such as montmorillonite dominate over salts. Bed rocks that are in direct contact with the pan deposits often show a distinct greenish color. Spectra of these rocks are dominated by glauconite (sometimes in combination with illite), which are known to develop as a consequence of slow sedimentation in a marine environment associated with low-oxygen conditions.

Intriguingly, lacustrine glauconitic clays could also be confirmed to exist in an ancient lake on Mars (Losa-Adams et al., 2021). Therefore, the collected spectra in combination with the knowledge of their geologic context will be extremely useful for identifying similar environments on Mars by spectrometers working in the visible-near infrared (VNIR) wavelength range (Mars Express OMEGA, MRO CRISM) and providing key parameters for characterizing aqueous Martian palaeoenvironments.

 

References:

Stephan et al. (2025). Multi-spectral field study of planetary analog material in extreme environments—alteration products of volcanic deposits of Vulcano/Italy. Earth and Space Science, 12, e2024EA004036. https://doi.org/10.1029/2024EA004036.

Losa-Adams et al. Long-lasting habitable periods in Gale crater constrained by glauconitic clays. Nat Astron 5, 936–942 (2021). https://doi.org/10.1038/s41550-021-01397-x.