The Origin of Peak-ring Basins: Observational Framework and Path Forward in Constraining Models of Impact-basin Formation

Introduction: Impact basins provide windows into a planetary body's crustal structure and stratigraphy; however , interpreting the origin of impact basin materials requires constraints on the processes controlling basin formation and morphology. Peak-ring basins (exhibiting a rim crest and single interior ring of peaks) provide important insight into the basin-formation process, as they are transitional between complex craters with central peaks and multi-ring basins. Recent remote sensing datasets from instruments onboard the Lunar Reconnaissance Orbiter (LRO), MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER), Chandrayaan-1, and Gravity Recovery and Interior Laboratory (GRAIL) spacecraft have permitted a reassessment of the origin of peak-ring basins [e.g., 1-6]. This reassessment has resulted in an observational framework for validating and refining current models of impact-basin formation. The following summarizes key observations of peak-ring basins on the Moon and Mercury, including comparisons with numerical models [e.g., 7-9] and paths for future model refinement. 1. Shape of Impact Basins: The final topography of peak-ring basins is unique compared with smaller complex craters and defined by five major features: rim crest, basin wall, floor annulus, peak ring, and central floor. These features have been qualitatively and quantitatively described in detail [1-4]. Due to the large vertical scale of impact-basin-forming events and limitations of model spatial resolution, most studies employing hydrocode models have not fo-cused on quantifying the surface topography of the final impact basin. However, overthrusting of the collapsed central peak over the transient crater wall in current numerical simulations often results in a peak ring configured as a broad mound with a steeper outward face and more gradually sloping inward face [7-9]. This is inconsistent with present observations. The discrepancies in topographic profile occur in the upper few kilometers of the simulated event, which are comparatively small in context of the kilometers scale translations that make up the bulk of numerical models. Improved resolution and focus on the final surface profile of impact basins in numerical models will improve quantitative comparisons and more rigorous testing of the final basin topography predicted in models with that characterized from orbital data. 2. Central Peak/Peak-Ring Dimensions: Observations indicate an abrupt transition in peak morphology with crater size at the onset of peak-ring basins. This is apparent in the trends in diameter of central peaks and peak rings with crater size, which show that peak diameters diverge along separate trends [1-4]. There is currently no systematic anal