Spectral field study of the Makgadikgadi Salt Pans in Botswana as a planetary analog for ancient fluvio-lacustrine environments on Mars

Abstract

The Makgadikgadi Salt Pans in northern Botswana offer a unique opportunity to study the mineralogy of evaporates and clays derived from fluvio-lacustrine sediments in their geological context. During the rainy season the pans are usually filled with water, but the level never exceeds ~50 cm in height. During the dry season, from June to October, an expanse of salt deposits dries in the sun above a layer of clay and sand. The pans are completely flat. However, locally, some exposed rocks such as granite, dolerite, exist. A field campaign taking place in August 2022, funded by Europlanet 2024 RI (grant agreement No 871149) was performed in order to 1) obtain a horizontal profile of the mineralogical diversity throughout the Pans from the topographic center to the rim/shoreline; and (2) investigate variations in the mineralogical composition of the evaporates and clays due to the influence of neighboring and/or underlying (bedrock) units. Spectral measurements were performed directly in the field with a portable visible/near-infrared spectrometer that samples the surface in the visible and near-infrared (VIS-NIR) wavelength range between 0.35 and 2.5µm. This range is known to be best suited for mineralogic research and most commonly used on planetary spacecrafts.

The acquired spectra 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 possibly develop as a consequence of alteration of sedimentary deposits associated with low-oxygen conditions. The collected spectra in combination with the knowledge of their geologic context will be extremely useful for identifying and mapping similar environments on Mars by spectrometers working in the visible-near infrared (VNIR) wavelength range such as Mars Express OMEGA and MRO CRISM.

In addition, spectra acquired in the field provide the spectral endmembers, which are now used to classify the currently available data of the pans provided by the Environmental Mapping and Analysis Program (EnMAP) of the German hyperspectral satellite mission. EnMAP data cover the same wavelength range in the VIS-NIR as the field instrument and measured major portions of the pans at the same seasonal period of the year. We present the distribution of the minerals detected in the field across the observed areas.

Furthermore, samples collected in the field are now analyzed by additional types of spectroscopy such as 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., 2022). Results of this multi-instrument approach will be presented.