Chemistry, mineralogy, and grain properties at Namib and High dunes, Bagnold dune field, Gale crater, Mars: A synthesis of Curiosity rover observations

Abstract

The Mars Science Laboratory Curiosity rover performed coordinated measurements to examine the textures and compositions of aeolian sands in the active Bagnold dune field. The Bagnold sands are rounded to subrounded, very fine to medium sized (~45–500 μm) with ≥6 distinct grain colors. In contrast to sands examined by Curiosity in a dust‐covered, inactive bedform called Rocknest and soils at other landing sites, Bagnold sands are darker, less red, better sorted, have fewer silt‐sized or smaller grains, and show no evidence for cohesion. Nevertheless, Bagnold mineralogy and Rocknest mineralogy are similar with plagioclase, olivine, and pyroxenes in similar proportions comprising >90% of crystalline phases, along with a substantial amorphous component (35% ± 15%). Yet Bagnold and Rocknest bulk chemistry differ. Bagnold sands are Si enriched relative to other soils at Gale crater, and H 2 O, S, and Cl are lower relative to all previously measured Martian soils and most Gale crater rocks. Mg, Ni, Fe, and Mn are enriched in the coarse‐sieved fraction of Bagnold sands, corroborated by visible/near‐infrared spectra that suggest enrichment of olivine. Collectively, patterns in major element chemistry and volatile release data indicate two distinctive volatile reservoirs in Martian soils: (1) amorphous components in the sand‐sized fraction (represented by Bagnold) that are Si‐enriched, hydroxylated alteration products and/or H 2 O‐ or OH‐bearing impact or volcanic glasses and (2) amorphous components in the fine fraction (<40 μm; represented by Rocknest and other bright soils) that are Fe, S, and Cl enriched with low Si and adsorbed and structural H 2 O.

Geochemistry of the Bagnold dune field as observed by ChemCam and comparison with other aeolian deposits at Gale Crater

“…The lower H, S, and Cl signals observed in the Bagnold dunes are thus consistent with this interpretation. This is also consistent with the measurements made by the DAN instrument, which indicate a water content of ~0.8 wt % in the dunes, which corresponds to at least 50% less than the average value of water abundance measured along the Curiosity traverse by this instrument (between 2 and 3%) [ Ehlmann et al ., ]. SAM also found a low water content in the dunes (~1 wt %), which is about half the content measured at Rocknest sand shadow [ Sutter et al ., ].…”

“…Finally, dump D (from the >1 mm sieve) is similar to dump C. In fact, a discrete dump pile was not created at the expected dump D location, likely due to the fact that no grains >1 mm in size are present [ Ehlmann et al ., ]. At the dump D location, the average grain size (likely derived from dump C sample) is ~176 μm (ranging from 70 to 350 μm; Figure d).…”

“…The investigation of the MAHLI images for the dump piles has shown a distribution between 70 and 250 μm on average, which is similar to the results of Ehlmann et al . []. Thus, ChemCam LIBS data are expected to sample multiple grains with each laser shot.…”

Visible/near‐infrared spectral diversity from in situ observations of the Bagnold Dune Field sands in Gale Crater, Mars

“…These results are somewhat similar to the Rocknest bed form results [ Blake et al ., ], but Gobabeb contains ~5% more olivine, ~3% more augite, and 4% less plagioclase. However, the Rocknest sieved materials (<150 µm) were much finer grained than the corresponding sieved samples from Gobabeb [ Ehlmann et al ., ; Minitti et al ., ].…”

“…King and Ridley , ], although the 150–1000 µm fraction is brighter than the >150 µm fraction, likely owing to the presence of darker >1000 µm grains in the >150 µm fraction. Although this is opposite to the trend observed in the ChemCam spectra (Figure ), it is consistent with the conclusions from MAHLI images that Gobabeb sands have few grains >1000 µm [ Ehlmann et al ., ].…”