Visible-near infrared spectral behavior of Mars-analog clays, sulfate, and basalt mixtures

Abstract

Understanding Mars’ past environmental and climate characteristics greatly relies on the orbital detection of the numerous hydrous minerals present on the planet’s surface. These include clay minerals and sulfates, especially when they are found in close proximity to each other. However, remote sensing observations pose several challenges and limits to quantitative mineral observations. In addition, these minerals are often likely mixed with basaltic regolith originating from the planet’s volcanic crust, which affects their spectral signature. In this framework, measurements on analogs in a controlled laboratory environment are essential support to remote sensing data to perform quantitative spectral analysis. We conduct visible and near-infrared reflectance spectroscopy on binary and ternary intimate mixtures among (a) basalt, (b) Fe/Mg-clay minerals (nontronite, saponite), and (c) polyhydrated sulfate (hexahydrite) powders. Binary mixtures include combinations of (a)-(b) and (a)-(c), while ternary mixtures combine all three: (a)-(b)-(c). Absorption feature variations are assessed with measurements of band center, band area, and band depth. The results of binary mixtures indicate that basalt does not generally interfere with the position of diagnostic OH- and H${}_2$O absorption features in the selected clays and sulfate samples but systematically reduces their band depth/area, leading to a possible underestimating of the hydrous component. Ternary mixing experiments highlight a strong and complex interaction between clay and sulfate endmembers, with variation in relative abundance causing the minima of their $1.4\mu m$ and $1.9\mu m$ OH- and H${}_2$O absorption features to shift. Such shifts are significantly larger than the possible basalt-induced effect and show a step-like behavior, with minimum values clustering between two groups separated by $\ge$ 30 nm. The gap typically corresponds to 1:2 clay-to-sulfate ratio. This characteristic places important constraints on the relative abundance of clays and sulfates in mixed settings, independently of the basalt abundance. The results presented here provide substantial support in studying orbital detections of mixed clay/sulfate signatures. Moreover, they offer a more realistic interpretation framework in which the effects of Mars-like basaltic regolith are directly assessed.