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Date Permissions Signed

7-1-2022

Date of Award

Summer 2022

Document Type

Masters Thesis

Department or Program Affiliation

Geology

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

Rice, Melissa S.

Second Advisor

Kraft, Michael David

Third Advisor

Mulcahy, Sean

Abstract

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.

Type

Text

Keywords

reflectance spectroscopy, photometry, weathering, Mars, Twin Sisters dunite, Mastcam-Z, analog

Publisher

Western Washington University

OCLC Number

1338620850

Subject – LCSH

Reflectance spectroscopy; Weathering; Igneous rocks; Geometric analysis

Geographic Coverage

Mars (Planet)--Environmental conditions

Format

application/pdf

Genre/Form

masters theses

Language

English

Rights

Copying of this document in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this document for commercial purposes, or for financial gain, shall not be allowed without the author’s written permission.

Rights Statement

http://rightsstatements.org/vocab/NoC-NC/1.0/

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