PRELIMINARY LABORATORY INVESTIGATIONS OF EJECTA EMPLACEMENT DYNAMICS AND MORPHOLOGY WITH PLANETARY APPLICATIONS (Invited Presentation)

The preponderance of impact craters and the associated crater ejecta facies are leading agents of geomorphic change across the Solar System. Interpretation of planetary landscape evolution, sample provenance, and regolith gardening all benefit from a thorough understanding of ejecta emplacement dynamics. Constraining the type and range of these dynamics has received little attention even as the effects of primary impacts have become well-constrained from experiments and numerical simulations. To address the knowledge gap surrounding ballistic ejecta emplacement and interaction with the substrate, we built and characterized a novel ejecta emplacement catapult and demonstrated it to accurately reproduce the ejecta mass and velocity profiles predicted for in-flight natural ejecta curtains. Based on this dynamic similarity to larger, natural systems, we proceeded with a preliminary exploratory suite of experiments to constrain runout and erosion efficiencies of flowing ejecta. Our quantitative results suggest a new set of scaling rules for granular ejecta systems that are emplaced more slowly than hypervelocity impacts. Our results also show significant ejecta runout efficiencies of ~1–2 % (similar to terrestrial debris flows 12 decades more voluminous) with important erosive efficiencies of ~2–4%. Our qualitative results reveal a repeatable but somewhat stochastic system that include ejecta “saltation” and implantation, regolith exhumation, erosion, and deep shearing. These observations indicate that ejecta behaves locally with some similarities to the secondary cratering model proposed by Oberbeck (1975), who treated the bulk mechanics of ejecta in terms of its individual impacting constituents. Our findings show, however, that each impacting grain of ejecta works as an amalgam resulting in a continuum flow more akin to a debris flow. These initial laboratory ejecta emplacement investigations provide a framework to the role ejecta plays in controlling its deposit morphology and topography, and how ejecta emplacement influences interpretations of sample provenance, ejecta stratigraphy, and geochemical boundaries.