<p>The <i>Curiosity</i> and <i>Perseverance</i> rovers have both found overwhelming evidence of a long-lived history of complex rock-water interactions on Mars. Understanding how the mineralogy of these deposits is related to depositional and diagenetic environments is critical for evaluating past habitable environments and guiding the search for signs of life with the <i>Curiosity</i> and <i>Perseverance</i> rovers. However, the chemistry and timing of these aqueous environments are poorly constrained. In particular, it is unclear which secondary minerals in the rock record formed in primary lacustrine vs. later diagenetic events. Understanding the origin of alteration minerals is crucial for studying habitability because they provide constraints on the timing and types of environments that existed. The goal of my thesis research is to better constrain the history of diagenetic processes in Gale and Jezero craters using the morphology, sedimentology, and mineralogy of features from rover and orbiter observations and comparisons to Earth analogs to understand their formation mechanisms. This research contributes to building a framework of the history of water in Gale and Jezero craters and will help us better understand past climate, habitability, and sources of water on Mars.</p><p>The <i>Curiosity</i> rover on the Mars Science Laboratory (MSL) mission has found extensive evidence that Gale crater once hosted a habitable lacustrine environment; however, there are remaining questions about the chemistry and duration of the lake and the nature of the climate at the time. In Chapter 2 of this thesis, I use Mastcam multispectral data to investigate the mineralogy of the Sutton Island member of the Murray formation, a part of the basal layers of Mt. Sharp, which consists of heterolithic mudstone and sandstone that are distinct from the finely laminated mudstones that dominate much of the Murray. Sutton Island includes at least one instance of desiccation cracks, indicative of subaerial exposure, and uniquely irregular diagenetic features that may be related to local bedrock permeability. These features suggest that Sutton Island experienced a complex history of deposition and diagenesis which may be crucial for understanding changing water-rock interactions within Gale. I find that most Mastcam bedrock spectra in this region lack the absorptions associated with hematite found throughout the Murray, and instead show deeper absorptions shifted toward longer wavelengths that are more consistent with Fe-smectites such as nontronite. Elemental chemistry from ChemCam supports this interpretation, as SiO, MgO, Li, and the chemical index of alteration are elevated in this region. Combined with observations of bedrock sedimentology, this suggests that Sutton Island was deposited in a nearshore or low stand environment, and we hypothesize that the clay minerals were produced in this region due to sub-aerial exposure and weathering in a semi-arid climate.</p><p>In Chapter 3, I use the Middle Jurassic Carmel Formation from Utah as a terrestrial analog to understand how the history of rock-water interactions is expressed in the rock record on Mars and how we can interpret this history of deposition and diagenesis using visible/near-infrared/short wave-infrared reflectance spectroscopy at rover scales. The Carmel Formation consists of carbonate- and sulfate-rich heterolithic strata deposited in a range of environments from fluvial, aeolian, and coastal sabkha to shallow marine settings. The alteration mineralogy, variable sedimentology, and diagenetic features present makes this formation a good analog for parts of the Murray formation in Gale crater and rocks from the Jezero crater delta front. In this thesis, we find that changes in lake level and climate manifest themselves in diagenetic features and mineralogy in the Carmel Formation with increased carbonate content in marine strata and increased evaporite/clay mineral content in near-shore/playa deposits. These results generally correspond to correlations with sedimentology and bedrock composition observed in Gale and Jezero craters and allows us to better interpret evidence of complex rock-water interactions on Mars using reflectance spectroscopy.</p><p>Although NASA’s <i>Curiosity</i> rover has found evidence of diagenesis, at a variety of scales, the broader extent of diagenesis in Gale crater is poorly constrained. <i>Curiosity</i> has observed extensive evidence of diagenesis at the unconformity between Mt. Sharp group fluvial/lacustrine mudstones and Siccar Point group (SPg) aeolian sandstones, which is part of the much larger Mound Skirting Unit (MSU) that mantles Mt. Sharp. This diagenetic horizon is visible as a light-toned tan, gray, or blue region in color images from both the ground and orbit. In Chapter 4 of this thesis, I use orbital color images and spectroscopy to look for possible evidence of alteration at the MSU unconformity elsewhere in Gale crater. I find that color variations appear at the MSU unconformity across Mt. Sharp and are co-located with detections of alteration minerals such as hydrated silica and phyllosilicates. This suggests that some of the diagenetic alteration observed by <i>Curiosity </i>below the MSU unconformity was extensive across Mt. Sharp. I hypothesize that this diagenesis was primarily driven by differences in permeability, where the more permeable SPg/MSU sandstones provided a conduit for diagenetic fluids that stagnated within and altered the upper few meters of less permeable clay bearing strata in the Mt. Sharp group below. The extensive diagenesis observed in Gale implies that subsurface fluids were long-lived and widespread in this region on Mars. Gaining a better understanding of what rock properties control and influence diagenetic fluid flow on Mars will help us improve the search for ancient aqueous environments, and possible biosignatures, on Mars.</p><p>The work included herein contributes to our understanding of rock-water interactions on Mars by demonstrating how bedrock properties, such as changes in permeability, can affect the flow of diagenetic fluids. These studies emphasize the importance of reflectance spectroscopy as a useful tool for constraining bedrock mineralogy and how it links to variable depositional and alteration environments. This will help guide current and future missions to search for past habitable environments and biosignatures on Mars.</p>