Understanding the Composition and Evolution of the Lunar Surface Through Laboratory Space Weathering Simulations and Remote Sensing

Abstract

Without an atmosphere, the lunar surface is vulnerable to space weathering, a process by which solar wind, galactic rays, and micrometeoroids bombard a planetary body and alter the surface’s physical, chemical, and spectral properties. The most well studied spectral changes are in the visible to near-infrared wavelengths, where increasing exposures to space weathering results in decreased reflectance, spectral reddening, and subdued absorption bands. Thus, mineralogical and compositional analyses of planetary bodies via spectral reflectance measurements are complicated by the space weathering process. The first part of this dissertation investigates sites on the lunar surface that contain olivine, a mineral that is relatively sparse in the crust and is critical to understanding the magmatic history of the Moon. We examine both known and previously unidentified olivine exposures using hyperspectral visible to near-infrared data from the Moon Mineralogy Mapper. In an effort to determine the origins and transport mechanisms that led to these individual exposures, we estimate crustal thicknesses using the Gravity Recovery and Interior Laboratory, investigate geologic settings using images from the Lunar Reconnaissance Orbiter Camera, and estimate mineral abundances using radiative transfer modeling. Our combined geophysical and spectral investigation allows for the identification of both volcanic and mantle-derived olivine. The second part of this dissertation characterizes space weathering effects in order to improve compositional and mineralogical analyses of spectral data in the future. We simulate space weathering (i.e., micrometeoroid bombardment) in the laboratory via kinetic impact, laser irradiation, and a combination of the two methods on lunar analog material to understand the various physical and spectral changes produced by each method and determine which method produces the most accurate lunar space weathering effects. We conclude that a combination of the two methods best replicates lunar-like space weathering, in part because subsequent laser irradiation of the analog that was first weathered by kinetic impacts enhances darkening of the material and produces more submicroscopic iron. This effect is likely due to the lower melting/vaporization temperature of the glass relative to minerals, which allows iron to be extracted from the glass more easily than from a crystalline structure. We also examine how space weathering affects regoliths in cold environments. We perform laser irradiation when minerals are held at two different temperatures, 85 K and 295 K. With equal amounts of irradiation, we find that the olivine and pyroxene irradiated at 85 K have higher reflectance than those irradiated at 295 K. Analyses of our samples using Scanning Transmission Electron Microscopy suggest that space weathering of low-temperature regolith produces less melting and less submicroscopic iron. Our results help to quantify the effects of space weathering on low-temperature regolith, which needs to be considered when interpreting spectral measurements at different latitudes, geologic settings, and distance from the Sun.