Millimeter-Wave Tomographic Interferometry of Ejecta Concentrations in Plume-Surface Interactions

This study explores the ejecta dynamics of Plume-Surface Interactions (PSI). Utilizing radar tomography, a pioneering technique, we achieved the first quantitative mapping of ejecta concentrations in PSI experiments, overcoming the limitations of conventional optical method shindered by the opacity of ejecta clouds. Our experimental setup is a Mach 5 jet impinging on abed of regolith within a vacuum chamber, simulating planetary landing scenarios. We explored the impact of jet expansion on PSI phenomenology across different ambient pressure levels. In moderate under expansion (P_e → Pamb = 2.85 → 2.45) an initial surge in ejecta was followed by a steady-state phase, whereas high under expansion (P_e → Pamb = 338 → 13.6) conditions showed a gradual increase in ejecta concentration near the nozzle. Post-test crater analyses further underscored these differences, suggesting diverse erosion mechanisms. The moderate under expansion condition yielded craters with sunken ridges and gentle slopes (15.6°), likely formed by the collapse of a deep transient crater. In contrast, high under expansion resulted in craters with raised ridges and steep slopes (21.8°), indicative of viscous erosion.