Doctor of Philosophy
Planetary Science and Exploration
Dr. Gordon Osinski
Dr. Livio Tornabene
Geological processes, including impact cratering, are fundamental throughout rocky bodies in the solar system. Studies of terrestrial impact structures, like the Ries impact structure, Germany, have informed on impact cratering processes – e.g., early hot, hydrous degassing, autometamorphism, and recrystallization/devitrification of impact glass – and products – e.g., impact melt rocks and breccias comprised of clay minerals. Yet, clay minerals of authigenic impact origin remain understudied and their formation processes poorly-understood. This thesis details the characterization of impact-generated clay minerals at Ries, showing that compositionally diverse, abundant Al/Fe/Mg smectite clays formed through these processes in thin melt-bearing breccia deposits of the ejecta, as well as at depth. The inherent complexity of smectites – their formation, type, structure, and composition – makes their provenance difficult to discern; these factors may explain why impact-generated and altered materials, which comprise an appreciable volume and extent of Mars’ ancient Noachian crust, are not generally recognized as a source of the enigmatic clays that are ubiquitous in those regions. Clay minerals can provide a defining window into a planet’s geologic and climatic past as an indicator of water availability and geochemistry; the presence of clay minerals on Mars has been used to support the hypothesis of climatically “warm, wet” ancient conditions. Yet, climate modeling of Early Mars suggests that the likely nature of the climate was not conducive to long-term aqueous activity. We suggest that the omission of impact-generated materials in current models of Mars clay mineral formation leaves a fundamental gap in our understanding of Noachian crustal materials – materials that were continually recycled and completely transformed on a global scale over millennia on Mars. The opportunity to investigate clay-bearing impactites and strata-bound clay minerals will be presented to the upcoming NASA Mars 2020 and ESA ExoMars rovers; this thesis offers caution in assigning clay mineral provenance until samples are returned to Earth from these missions. We furthermore suggest a methodological approach to augment current rover-based exploration frameworks to characterize potential impact-origin. Discerning clay species and provenance – and acknowledging the implications of that provenance – is central to understanding the geologic context of Mars, and thus its past climatic conditions and habitability potential.
Summary for Lay Audience
Impact cratering is a fundamental geologic process throughout the solar system, one that serves as an external force to cycle and recycle crustal materials and produce new materials known as impactites. Impactites from terrestrial impact structures are known to comprise smectites (swelling, hydrated clay minerals). The heavily cratered, ancient Noachian region of Mars holds a diversity of alteration minerals, including widespread smectites. The presence of smectites on Mars is generally considered to indicate Earth-like paleoclimate conditions that supported long-term atmospheric stability and surface-stable liquid water. However, climate modeling of Early Mars suggests that the likely nature of the climate was not conducive to long-term aqueous activity. The work of this thesis may address the disconnect between the widespread clay minerals observed on Mars and the predictions of a “cold, dry” climate through the study of clay-bearing impactites. Impact processes can produce clay minerals even in the absence of an active hydrosphere and under very cold conditions; however, the production of clay minerals in impact craters is poorly-understood. Impact-generated clay minerals are not typically represented in current models of Mars clay mineral formation, and we suggest that this leaves a fundamental gap in our understanding of Noachian crustal materials that were continually recycled and completely transformed on a global scale over millennia. This thesis begins with extensive mapping of impactite deposits of Bakhuysen Crater, Mars. Next, two in-depth laboratory studies of clay-bearing impactites at the Ries impact structure provide a reference for impact-related clay minerals when investigating the provenance of materials on Mars. A main goal of this thesis is to incite new ways of thinking about the variety of complex processes – including impact cratering – that produced materials observed on the surface of Mars. Intellectual and exploration frameworks – i.e., science questions and resultant Mars exploration strategies – were investigated through a simulated Mars rover mission: CanMars. The difficulties in properly characterizing geologic context and the inherent difficulty in identifying and adequately characterizing smectite clay minerals (relevant for upcoming Mars rover missions) calls for a methodological approach. This thesis augments current exploration frameworks with the broader goal of expanding current knowledge on the larger geologic context of Mars.
Caudill, Christy M., "Characterization of Impactite Clay Minerals with Implications for Mars Geologic Context and Mars Sample Return" (2020). Electronic Thesis and Dissertation Repository. 6935.