Grains on the surfaces of airless bodies are continually being modified due to their exposure to interplanetary space, a phenomenon known as space weathering. This dissertation uses a multi-faceted approach to understanding space weathering of the lunar and asteroidal surfaces. Chapters 1 and 2 provide an introduction to space weathering and a discussion of the methods employed in this work, respectively. Chapter 3 focuses on the analysis of returned samples from near-Earth asteroid Itokawa using the transmission electron microscope (TEM) and contributes to the first-ever comparison of microstructural and chemical features of space weathering in returned samples from two different airless bodies. This research uses high-resolution imaging and quantitative energy-dispersive x-ray spectroscopy (EDS) measurements to analyze space weathering characteristics in an Itokawa soil grain. These analyses confirm that space weathering is operating on the surface of Itokawa, and that many of the resulting features have similarities to those observed in lunar soils. Results show that while there is evidence that both major constituent space weathering processes are operating on the surface of Itokawa, solar wind irradiation, not micrometeorite impacts, appears to be the dominant contributor to changes in the microstructure and chemistry of surface material. Chapter 4 presents a detailed study of nanophase Fe (npFe) particles in lunar soil samples. For the first-time, the oxidation state of individual npFe particles was directly measured using electron energy-loss spectroscopy (EELS) in the TEM. The results show that npFe particles are oxidizing over their time on the lunar surface, and that the amount of oxidized Fe in the nanoparticles is correlated with soil maturity. The EELS data are also coupled to atomic-resolution imaging, which is used to determine the structure of the nanoparticles, confirming their mineral phase. This work challenges the long-standing paradigm that all npFe particles are composed of metallic Fe and that the chemical composition of these features remains static after their formation. A theoretical modeling investigation of the influence that npFe particles of different oxidation states have on the spectral properties of the material is also presented. The model results show that varied Fe-oxidation states of the nanoparticles can produce subtle changes in the optical properties of the soils, including the degree of reddening and the attenuation of characteristic absorption bands. These findings should be accounted for in future modeling of reflectance spectra. Chapter 5 presents a novel technique for simulating space weathering processes inside the TEM. Using an in situ heating holder, lunar soils were subjected to both slow- (~minutes) and rapid-heating (<seconds) events to simulate micrometeorite impacts. The slow-heating experiments show that npFe forms at ~575 ºC, providing a temperature constraint on initial npFe formation. Lunar soil grains that were subjected to a single, rapid, thermal pulse show the development of npFe particles and vesiculated textures near the grain rim. The vesicles were imaged and the npFe particles were imaged and then mapped with EDS. The oxidation state of the npFe particles was confirmed to be Fe^0 using EELS. Several lunar soil grains were subjected to multiple thermal shocks to simulate longer exposure times on the lunar surface. With each heating cycle, the number and size distribution of the npFe particles changed. The average size of npFe particles increased, and the size distribution became more gaussian after multiple heating events, versus the asymmetric distribution present after only one heating event. These results provide insight into the particle growth dynamics for space weathered soils and could offer a new way to place relative age constraints on grains in lunar soil.Chapter 6 provides a summary of the work presented here, discusses its implications for understanding space weathering processes across the solar system, and presents a perspective on the future of space weathering studies.

In order to more accurately predict loads subjected to the EEV (Earth Entry Vehicle) upon impact with a variety of materials, finite element simulations of soil/EEV impact were created using the program LS-DYNA. Various modeling techniques were analyzed for accuracy through comparison with physical test data when available. Through variation of numerical methods, mesh density, and material definition, an accurate and numerically efficient representation of physical data has been created. The numerical methods, Lagrangian, arbitrary Lagrangian-Eulerian (ALE), and spherical particle hydrodynamics (SPH) are compared to determine their relative accuracy in modeling soil deformation and EEV acceleration. Experimentally validated soil material parameters and element formulations were then used in parametric studies to gain a perspective on effects of EEV mass and geometry on its maximum acceleration across varying soil moisture content. Additionally, the effects of EEV orientation, velocity, and impact material were explored. Multi-material arbitrary Lagrangian-Eulerian (MMALE) formulation possess the most effective compromise between its ability to: accurately display qualitative soil behavior, accurately recreate empirical test data, be easily utilized in parametric studies, and to maintain simulation stability. EEV acceleration can be minimized through increase of EEV mass (with constant geometry), allowing for maximum penetration depth, and longest deceleration time. A critical orientation was discovered at 30⁰ from normal, such that maximum EEV surface area impacts the soil surface instantaneously, resulting in maximum acceleration. Off-nominal impact with concrete is predicted to increase acceleration by up to 630% from impact with soil. / MS

abstract: The search for life on Mars is a major NASA priority. A Mars Sample Return (MSR) mission, Mars 2020, will be NASA's next step towards this goal, carrying an instrument suite that can identify samples containing potential biosignatures. Those samples will be later returned to Earth for detailed analysis. This dissertation is intended to inform strategies for fossil biosignature detection in Mars analog samples targeted for their high biosignature preservation potential (BPP) using in situ rover-based instruments. In chapter 2, I assessed the diagenesis and BPP of one relevant analog habitable Martian environment: a playa evaporite sequence within the Verde Formation, Arizona. Coupling outcrop-scale observations with laboratory analyses, results revealed four diagenetic pathways, each with distinct impacts on BPP. When MSR occurs, the sample mass returned will be restricted, highlighting the importance of developing instruments that can select the most promising samples for MSR. Raman spectroscopy is one favored technique for this purpose. Three Raman instruments will be sent onboard two upcoming Mars rover missions for the first time. In chapters 3-4, I investigated the challenges of Raman to identify samples for MSR. I examined two Raman systems, each optimized in a different way to mitigate a major problem commonly suffered by Raman instruments: background fluorescence. In Chapter 3, I focused on visible laser excitation wavelength (532 nm) gated (or time-resolved Raman, TRR) spectroscopy. Results showed occasional improvement over conventional Raman for mitigating fluorescence in samples. It was hypothesized that results were wavelength-dependent and that greater fluorescence reduction was possible with UV laser excitation. In Chapter 4, I tested this hypothesis with a time-resolved UV (266 nm) gated Raman and UV fluorescence spectroscopy capability. I acquired Raman and fluorescence data sets on samples and showed that the UV system enabled identifications of minerals and biosignatures in samples with high confidence. The results obtained in this dissertation may inform approaches for MSR by: (1) refining models for biosignature preservation in habitable Mars environments; (2) improving sample selection and caching strategies, which may increase the success of Earth-based biogenicity studies; and (3) informing the development of Raman instruments for upcoming rover-based missions. / Dissertation/Thesis / Doctoral Dissertation Geological Sciences 2016