Ejecta distribution and dynamics during high-speed jet impingement on granular surfaces.*

High-speed rocket plume impingement on soil surfaces poses major risks during planetary landing missions. Plume-surface interactions (PSI) can create deep craters and dense clouds of high-speed ejecta, potentially blinding instruments and damaging vehicles and nearby assets. These effects are highly sensitive to flow and soil parameters. We present results from sub-scale PSI experiments conducted at representative Lunar and Martian ambient pressures, varying flow non-dimensional parameters and nozzle elevation. Our focus is on ejecta dynamics and concentration distributions, using glass microspheres as a canonical soil simulant. We employ two measurement techniques: planar Particle Tracking Velocimetry and mm-wave tomography. These measure ejecta trajectories and velocities in a vertical plane containing the jet axis, and ejecta concentration at two surface-parallel planes, respectively. By combining these measurements with particle trajectory models in an optimization framework, we replicate the complete ejecta cloud evolution. This allows us to determine key quantities such as mass transport and kinetic energy from the surface, and estimate far-field ejecta properties.