Constraining Sulfate and Hydrated Silica Abundances on Mars with Laboratory Mixtures

Visible-near infrared (VNIR) reflectance spectroscopy is highly sensitive to minerals resulting from water-rock interaction, and therefore can be used to investigate the environmental history of Mars. Orbital spectrometers, such as OMEGA and CRISM, have detected hydrated silica and sulfate mineral deposits in Noachian outcrops of the Mawrth Vallis region and elsewhere on Mars [e.g. 1], suggesting the presence of water during its early history. While these deposits are usually spatially and temporally distinct, they do occur together. In Mawrth Vallis, these minerals may be contributing to the unique "doublet" type spectra observed as two overlapping features near 2.21-2.22 and 2.25-2.27 µm [2]. Identification of specific mineral phases and their relative abundances would help determine the distribution and timing of hydrated mineral formation on Mars. For multi-component particulate mixtures, the nature of spectral mixing at visible and near-infrared wavelengths is highly nonlinear. We prepared laboratory mixtures of the sulfate mineral gypsum with two different hydrated silica phases (white and pink opal) to constrain their detection limits on Mars. Samples were ground and dry sieved to a particle size of 45 to 125 μm, and binary mixtures from 5% gypsum to 50% gypsum were prepared. VNIR reflectance spectra were measured using an ASD spectrometer under ambient lab conditions. Band depth calculations were conducted for gypsum and hydrated silica to quantify the strengths of their characteristic absorptions in the mixture spectra. Spectra collected for these mixtures demonstrate a strong signature for gypsum, even at weight percentages as low as 5%, suggesting its presence on the Martian surface is much easier to detect than that of hydrated silica. Therefore, if spectral signatures for hydrated silica are detected in mixture spectra, its abundance is likely to be significant. Quantification of component mineral influences on mixture spectra can be used to improve band depth parameter maps of Mawrth Vallis, and to better understand spatial and temporal relationships between sulfate and hydrated silica mineral deposits and their environments of formation.References:[1] Murchie et al. (2009), J. Geophys. Res., 114, E00D06, doi:10.1029/2009JE003342.[2] Bishop, J. L. et al. (2016), LPSC 47, Abstract #1332.