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Date Permissions Signed
Date of Award
Department or Program Affiliation
Master of Science (MS)
Rice, Melissa S.
Kraft, Michael David
Rock weathering products are important clues for understanding past environmental conditions on Mars. They can be identified using reflectance spectroscopy because the formation of new minerals and textures on a rock surface will change its spectral signature. Previous studies demonstrate that the spectral signature of coated rock surfaces can vary with viewing geometry (the angle between incident and emitted light); however, these photometric effects have not been extensively characterized for naturally weathered rocks. My goal in this study is to quantify how both weathering and viewing geometry affect visible to near-infrared (VNIR) reflectance spectra of a subset of naturally weathered igneous rocks. Improving our understanding of how weathering changes the spectral signature of terrestrial samples can be applied to Martian rocks to make inferences about their original compositions and the environmental conditions that formed any weathering products present. I compared weathered surface compositions and textures to their unweathered rock interiors using powder X-ray diffraction (XRD), petrographic analysis of thin sections, and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS). Differences between the surfaces and interiors are primarily textural with minor compositional variations. Weathering rind mineralogy is made up of majority primary minerals, with small volumes of secondary clays and Fe-oxides. Weathered surfaces show increased microfracturing and porosity in their rinds, especially within 1-2 mm of surfaces and macroscale fractures. I used Western Washington University's hemispheric spectrogoniometer to collect reflectance spectroscopy data from natural and cut rock surfaces at many viewing geometries covering the full scattering hemisphere. Additionally, I convolved lab spectra to Mastcam-Z wavelengths and compared them to Mastcam-Z spectra from olivine-bearing rocks in the South Séítah region within Jezero crater on Mars. Viewing geometry findings indicate that spectra taken outside a lab standard geometry (e.g. i=30, e=0, az=0) allow for accurate mineral identification. Using band depth, magnitude of reflectance, or near-infrared slope to interpret differences between spectral observations is not recommended, however, if the spectra were collected at widely different phase angles. Band depths decrease and near-infrared slopes are more positive (red sloped) with increasing phase angle regardless of a sample's surface weathering or scattering behavior. Absorption band center positions can shift slightly with phase angle due to noise and sample heterogeneity, but changes are small with no correlation to viewing geometry. Minor compositional differences between samples create strong signals in the spectral data; thin, intermittent Fe-oxide coatings on a surface begin to mask the sample's primary mineralogy. Relatively unweathered samples tend to be more forward scattering, while weathered samples tend to be more backscattering. This is likely due to surface roughness and near-surface microporosity differences. Direct applications of these findings include making surface texture inferences based on remote phase angle experiments, especially for the olivine-bearing Séítah rocks.
reflectance spectroscopy, photometry, weathering, Mars, Twin Sisters dunite, Mastcam-Z, analog
Western Washington University
Subject – LCSH
Reflectance spectroscopy; Weathering; Igneous rocks; Geometric analysis
Mars (Planet)--Environmental conditions
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Curtis, Sabrina, "Spectral Variability in Naturally Weathered Rock Surfaces and Implications for Mars" (2022). WWU Graduate School Collection. 1118.