Space weathering of planetary surfaces is a complex process involving many mechanisms that work independently over different timescales. This research aims to address outstanding questions related to solar wind rim formation on space weathered regolith and tests a new hypothesis that dielectric breakdown plays an important role in the optical maturation of lunar regolith. The purpose of this work is to highlight the limitations imposed by laboratory equipment to accurately simulate the solar wind’s effects on regolith and to provide physical context for the possible contributions of dielectric breakdown to space weathering. Terrestrial and lunar samples were experimentally irradiated and damage was characterized using electron microscopy techniques. Low-fluence proton irradiation produced differential weathering in a lunar mare basalt, with radiation damage on some phases being inconsistent with that found in the natural lunar environment. Dielectric breakdown of silicates revealed two electrical processes that produce characteristic surface and subsurface damage, in addition to amorphous rims. The results of this research highlight experimental parameters that if ignored, can significantly affect the results and interpretations of simulated solar wind weathering, and provides a framework for advancing space weathering research through experimental studies.

The presence of ices (H2O and CO2) and liquid water is key to the evolution ofmartian geology, with implications for the potential for past or extant life, and the future of robotic and human exploration on Mars. In this dissertation, I present the first direct evidence that the smooth deposits covering mid-latitude, martian pole-facing slopes are composed of shallow dusty H2O ice covered by desiccated material. To analyze this H2O ice, I developed the first validated radiative transfer model for dusty martian snow and glacier ice. I found that these ice exposures have < 1% dust in them, and discovered the lowest latitude detection of H2O ice on Mars, at 32.9°S. After observing the ice disappear, and new gully channels form, I proposed a model for gully formation. In this model, dusty ice gets exposed by slumping, leading to melting in the subsurface and channels eroding within the ice and the wall rock beneath. Access to liquid water within this ice could provide potential abodes for any extant life. Next, I developed novel methodology to search for CO2 frosts within the entire Thermal Emission Imaging System (THEMIS) infrared dataset and found that about half of all gullies overlap with CO2 frost detections. I also used the Thermal Emission Spectrometer (TES) water vapor retrievals to assess the formation and distribution of H2O frosts on Mars. Additionally, I used radar data from the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument to investigate Mars’ ice-rich South Polar Layered Deposits (SPLD). I discovered radar signals similar to those proposed to be caused by a subglacial lake throughout the martian SPLD. Finally, I mapped martian polygonal ridge networks thought to represent fossilized remnants of ancient groundwater near the Perseverance rover landing site with the help of citizen scientists across a fifth of Mars’ total surface area and analyzed their thermophysical properties. All these studies highlight the key role that ices and liquid water have played in shaping Mars’ landscape through time, and provide an intriguing path forward in martian exploration and the search for alien life.

Understanding the history of water on Mars is one of the highest priority goals of the international Mars exploration community. Water would have played a key role in any potential abiogenesis in the past and will play a key role in the future human exploration of the planet. Chloride salts are an indicator of past hydrologic activity in the Martian geologic record and have the potential to preserve fluid inclusions and organic material within their crystal structure over geologic timescales. This dissertation will describe an innovative method for identifying chloride salts on the Martian surface, explore the implication of their distribution within Early Noachian terrains, and document important opportunistic discoveries made in the process. Decorrelation stretched Thermal Emission Imaging System (THEMIS) infrared images have long been used to identify chloride salts on Mars, but the process has been time-consuming, subjective, and qualitative. By analyzing the entire THEMIS dataset, acquired over more than twenty years at Mars, a globally-applicable covariance matrix was calculated that describes the geologic diversity of the Martian surface. This covariance matrix allows all THEMIS daytime infrared images to be translated into globally-consistent decorrelation stretch and principal component images, enabling an automatic, objective, and quantitative method for identifying chloride salts. A new global survey located 1,605 chloride salt deposits across the Martian surface, a significant increase over previous surveys. In particular, the 257 deposits in Early Noachian terrains have characteristics that indicate they formed contemporaneously with the surrounding terrain. In addition, a chloride salt formation was identified on the floor of Ares Vallis with a unique three-dimensional structure that has been interpreted as an exposed chloride salt diapir, which would indicate the presence of a significant subsurface chloride salt layer. By improving our understanding of the distribution and diversity of chloride salts on the Martian surface, this work has provided future investigators with new tools and avenues of research to explore the history of water on Mars.

The hydrous alteration of ultramafic rocks, known as serpentinization, produces some of the most reduced (H2 >1 mmolal) and alkaline (pH >11) fluids on Earth. Serpentinization can proceed even at the low-temperature conditions (<50°C) characteristic of most of Earth’s continental aquifers, raising questions on the limits of life deep in the subsurface and the magnitude in the flux of reduced volatiles to the surface. In this work, I explored the compositions and consequences of fluids and volatiles found in three low-temperature serpentinizing environments: (1) active hyperalkaline springs in ophiolites, (2) modern shallow and deep peridotite aquifers, and (3) komatiitic aquifers during the Archean.

Around 140 fluids were sampled from the Oman ophiolite and analyzed for their compositions. Fluid compositions can be accounted for by thermodynamic simulations of reactions accompanying incipient to advanced stages of serpentinization, as well as by simulations of mass transport processes such as fluid mixing and mineral leaching. Thermodynamic calculations were also used to predict compositions of end-member fluids representative of the shallow and deep peridotite aquifers that were ultimately used to quantify energy available to various subsurface chemolithotrophs. Calculations showed that sufficient energy and power supply can be available to support deep-seated methanogens. An additional and a more diverse energy supply can be available when surfacing deep-seated fluids mix with shallow groundwater in discharge zones of the subsurface fluid pathway. Finally, the consequence of the evolving continental composition during the Archean for the global supply of H2 generated through komatiite serpentinization was quantified. Results show that the flux of serpentinization-generated H2 could have been a significant sink for O2 during most of the Archean. This O2 sink diminished greatly towards the end of the Archean as komatiites became less common and helped set the stage for the Great Oxidation Event. Overall, this study provides a framework for exploring the origins of fluid and volatile compositions, including their redox state, that can result from various low-temperature serpentinizing environments in the present and past Earth and in other rocky bodies in the solar system.

Establishing the timing of impact crater formation is essential to exploring the relationship between bolide impact and biological evolution, and constraining the tempo of planetary surface evolution. Unfortunately, precise and accurate impact geochronology can be challenging. Many of the rock products of impact (impactites) contain relict, pre-impact phases that may have had their isotopic systematics completely reset during the impact event, only partially reset, or not reset at all. Of the many isotopic chronometers that have been used to date impactites, the U/Pb zircon chronometer (ZrnPb) seems least susceptible to post-impact disturbances, and ZrnPb dates are typically much more precise than those obtained using other chronometers. However, the ZrnPb system is so resistant to resetting that relict zircons in impactites often yield dates that reflect the igneous or metamorphic ages of the target rocks rather than the age of the impact itself. The present study was designed to answer a simple question: is there a straightforward sample collection and analysis strategy for high-accuracy ZrnPb dating of an impact structure if the impactites collected from it may contain inherited zircons? To study this, ZrnPb dates were determined for impactites from a single crater with a well-constrained impact age: the West Clearwater Lake impact structure, located at Lake Wiyâshâkimî, Québec, Canada.

The amount of ZrnPb resetting and the mechanisms responsible for resetting varied amongst the samples. Each sample characteristically contained either: newly crystallized zircons from the impact melt ("neocrystalline"), relict zircons ~50-100% reset, or, relict zircons ~0-50% reset. The variably reset relict zircons define a discordia line from ~2700 Ma to ~286 Ma – consistent with the ages of the target rock and the impact, respectively (Schmieder et al., 2015a; Simard, 2004). ZrnPb measurements from the neocrystalline zircons provided a new preferred impact age of 286.64 ± 0.35 Ma (2σ), a ~10x improvement in precision. The characteristics of the West Clearwater ZrnPb data vary between samples yet become easily interpretable as a whole, showing that efforts to measure robust, precise impact ages benefit from strategies that prioritize applying multiple analytical techniques to multiple types of impactite from the same crater.

Remote sensing in visible to near-infrared wavelengths is an important tool for identifying and understanding compositional differences on planetary surfaces. Electronic transitions produce broad absorption bands that are often due to the presence of iron cations in crystalline mineral structures or amorphous phases. Mars’ iron-rich and variably oxidized surface provides an ideal environment for detecting spectral variations that can be related to differences in surface dust cover or the composition of the underlying bedrock. Several imaging cameras sent to Mars include the capability to selectively filter incoming light to discriminate between surface materials.

At the coarse spatial resolution provided by the wide-angle Mars Color Imager (MARCI) camera aboard the Mars Reconnaissance Orbiter (MRO), regional scale differences in reflectance at all wavelengths are dominated by the presence or absence of Fe3+-rich dust. The dust cover in many regions is highly variable, often with strong seasonal dependence although major storm events can redistribute dust in ways that significantly alter the albedo of large-scale regions outside of the normal annual cycle. Surface dust reservoirs represent an important part of the martian climate system and may play a critical role in the growth of regional dust storms to planet-wide scales. Detailed investigation of seasonal and secular changes permitted by repeated MARCI imaging coverage have allowed the surface dust coverage of the planet at large to be described and have revealed multiannual replenishing of regions historically associated with the growth of storms.

From the ground, rover-based multispectral imaging acquired by the Mastcam cameras allows compositional discrimination between bedrock units and float material encountered along the Curiosity rover’s traverse across crater floor and lower Mt. Sharp units. Mastcam spectra indicate differences in primary mineralogy, the presence of iron-bearing alteration phases, and variations in iron oxidation state, which occur at specific locations along the rover’s traverse. These changes represent differences in the primary depositional environment and the action of later alteration by fluids circulating through fractures in the bedrock. Loose float rocks sample materials brought into the crater by fluvial or other processes. Mastcam observations provide important constraints on the geologic history of the Gale Crater site.

The present work covers two distinct microanalytical studies that address issues in planetary materials: (1) Genesis Na and K solar wind (SW) measurements, and (2) the effect of water on high-pressure olivine phase transformations.

NASA’s Genesis mission collected SW samples for terrestrial analysis to create a baseline of solar chemical abundances based on direct measurement of solar material. Traditionally, solar abundances are estimated using spectroscopic or meteoritic data. This study measured bulk SW Na and K in two different Genesis SW collector materials (diamond-like carbon (DlC) and silicon) for comparison with these other solar references. Novel techniques were developed for Genesis DlC analysis. Solar wind Na fluence measurements derived from backside depth profiling are generally lower in DlC than Si, despite the use of internal standards. Nevertheless, relative to Mg, the average SW Na and K abundances measured in Genesis wafers are in agreement with solar photospheric and CI chondrite abundances, and with other SW elements with low first ionization potential (within error). The average Genesis SW Na and K fluences are 1.01e11 (+9e09, -2e10) atoms/cm2 and 5.1e09 (+8e08, -8e08) atoms/cm2, respectively. The errors reflect average systematic errors. Results have implications for (1) SW formation models, (2) cosmochemistry based on solar material rather than photospheric measurements or meteorites, and (3) the accurate measurement of solar wind ion abundances in Genesis collectors, particularly DlC and Si.

Deep focus earthquakes have been attributed to rapid transformation of metastable olivine within the mantle transition zone (MTZ). However, the presence of H2O acts to overcome metastability, promoting phase transformation in olivine, so olivine must be relatively anhydrous (<75 ppmw) to remain metastable to depth. A microtextural analysis of olivine phase transformation products was conducted to test the feasibility for subducting olivine to persist metastably to the MTZ. Transformation (as intracrystalline or rim nucleation) shifts from ringwoodite to ringwoodite-wadsleyite nucleation with decreasing H2O content within olivine grains. To provide accurate predictions for olivine metastability at depth, olivine transformation models must reflect how changing H2O distributions lead to complex changes in strain and reaction rates within different parts of a transforming olivine grain.

The occurrence of exogenic, meteoritic materials on the surface of any world presents opportunities to explore a variety of significant problems in the planetary sciences. In the case of Mars, meteorites found on its surface may help to 1) constrain atmospheric conditions during their time of arrival; 2) provide insights into possible variabilities in meteoroid type sampling between Mars and Earth space environments; 3) aid in our understanding of soil, dust, and sedimentary rock chemistry; 4) assist with the calibration of crater-age dating techniques; and 5) provide witness samples for chemical and mechanical weathering processes. The presence of reduced metallic iron in approximately 88 percent of meteorite falls renders the majority of meteorites particularly sensitive to oxidation by H2O interaction. This makes them excellent markers for H2O occurrence. Several large meteorites have been discovered at Gusev Crater and Meridiani Planum by the Mars Exploration Rovers (MERs). Significant morphologic characteristics interpretable as weathering features in the Meridiani suite of iron meteorites include a 1) large pit lined with delicate iron protrusions suggestive of inclusion removal by corrosive interaction; 2) differentially eroded kamacite and taenite lamellae on three of the meteorites, providing relative timing through cross-cutting relationships with deposition of 3) an iron oxide-rich dark coating; and 4) regmaglypted surfaces testifying to regions of minimal surface modification; with other regions in the same meteorites exhibiting 5) large-scale, cavernous weathering. Iron meteorites found by Mini-TES at both Meridiani Planum and Gusev Crater have prompted laboratory experiments designed to explore elements of reflectivity, dust cover, and potential oxide coatings on their surfaces in the thermal infrared using analog samples. Results show that dust thickness on an iron substrate need be only one tenth as great as that on a silicate rock to obscure its infrared signal. In addition, a database of thermal emission spectra for 46 meteorites was prepared to aid in the on-going detection and interpretation of these valuable rocks on Mars using Mini-TES instruments on both MER spacecraft. Applications to the asteroidal sciences are also relevant and intended for this database.

Chemical and mineralogical data from Mars shows that the surface has been chemically weathered on local to regional scales. Chemical trends and the types of chemical weathering products present on the surface and their abundances can elucidate information about past aqueous processes. Thermal-infrared (TIR) data and their respective models are essential for interpreting Martian mineralogy and geologic history. However, previous studies have shown that chemical weathering and the precipitation of fine-grained secondary silicates can adversely affect the accuracy of TIR spectral models. Furthermore, spectral libraries used to identify minerals on the Martian surface lack some important weathering products, including poorly-crystalline aluminosilicates like allophane, thus eliminating their identification in TIR spectral models. It is essential to accurately interpret TIR spectral data from chemically weathered surfaces to understand the evolution of aqueous processes on Mars. Laboratory experiments were performed to improve interpretations of TIR data from weathered surfaces. To test the accuracy of deriving chemistry of weathered rocks from TIR spectroscopy, chemistry was derived from TIR models of weathered basalts from Baynton, Australia and compared to actual weathering rind chemistry. To determine how specific secondary silicates affect the TIR spectroscopy of weathered basalts, mixtures of basaltic minerals and small amounts of secondary silicates were modeled. Poorly-crystalline aluminosilicates were synthesized and their TIR spectra were added to spectral libraries. Regional Thermal Emission Spectrometer (TES) data were modeled using libraries containing these poorly-crystalline aluminosilicates to test for their presence on the Mars. Chemistry derived from models of weathered Baynton basalts is not accurate, but broad chemical weathering trends can be interpreted from the data. TIR models of mineral mixtures show that small amounts of crystalline and amorphous silicate weathering products (2.5-5 wt.%) can be detected in TIR models and can adversely affect modeled plagioclase abundances. Poorly-crystalline aluminosilicates are identified in Northern Acidalia, Solis Planum, and Meridiani. Previous studies have suggested that acid sulfate weathering was the dominant surface alteration process for the past 3.5 billion years; however, the identification of allophane indicates that alteration at near-neutral pH occurred on regional scales and that acid sulfate weathering is not the only weathering process on Mars.