T. Frueh, D.A. Kring
Ballistic sedimentation describes the emplacement of crater ejecta and its interaction with local material through secondary cratering, mixing, and subsequent flow. Scaling laws that predict the final dimensions of these processes typically assume a flat pre-emplacement surface. However, most solid planetary bodies exhibit significant topographic variability, which may act as barriers to ejecta and modify ballistic sedimentation processes, producing ballistic shadows. Here, we conceptually study terrain slope effects on ejecta emplacement using the lunar south polar region as a case study. We evaluate these effects, by analyzing ejecta trajectories from eight prominent south polar craters and their interception by South Pole–Aitken (SPA) basin massifs. Intercepted ejecta experiences reduced time of flight and emplacement velocity, affecting ejecta thickness across the massif or potentially shortening ejecta blankets. Ejecta is shifted toward the source crater, and uphill spreading efficiency is reduced, leading to thickening on the near slopes and thinning on the far slopes and summits. Thinning increases the likelihood that smaller, younger craters have exposed deeper stratigraphic layers. Reduced emplacement velocities may also limit mixing, decreasing the proportion of local material in the ejecta blanket. We suggest these processes be considered in landing site planning and in assessing the provenance of returned and in-situ analyzed sample. This work provides a first-order approximation of ballistic shadowing and slope effects on impact ejecta emplacement on solid bodies and highlights the importance of quantitatively and empirically investigating these processes when planning geologic activities at specific landing sites.