Laboratory experiments were based on a box of unconsolidated material (quartz sand or glass beads) coupled with an external acoustic source to induce fluidization. The first campaign was done at the University of Freiburg. The setup included an air gun accelerating plastic projectiles to velocities as high as ~180 m/s, and a subwoofer as acoustic source. Although any of these shooting experiments lead to the formation of the complex morphology, the better results in terms of particle velocity field was reached at lower frequencies (~100-200 Hz). However, the coupling between the subwoofer and the target box was rather inefficient to transmit the energy of the vibration to the granular material. Investigation of the problem showed that systematic measurements of the material viscosity were a preferable and more genuine criterion to investigate material fluidization. This second campaign was done at MfN. In the first experimental setup, the target was a small cylinder attached to a fixed structure by means of springs to maximize the degree of freedom, and coupled to an electrodynamical exciter. The second setup relied on a Plexiglas box coupled to a vibrating table. The results showed that the highest fluidization was achieved for low frequencies (~100 Hz), independently of the external acoustic fluidization source adopted, and it is inversely proportional to the material grain size.
Numerical modelling was based on a systematic study varying projectile radius (0.1 to 9 km) and BM parameters. The target was assumed Moon-like, with a dunitic mantle overlaid by a 50 km-thick gabbroic anorthosite or basaltic crust. The results showed that (i) decay times produce the largest variations in the final crater morphometry, and whether or not a central uplift forms; (ii) the best fit was obtained for longer lasting decay times (with corresponding depth-to-diameter ratio smaller than ~0.8); (iii) impacts with same kinetic energy occurring in different terrains (Maria or Highlands) can have a difference up to 25% in the d/D ratio. I then derived scaling laws to relate final crater diameter to the transient one, which is fundamental in many questions like the determination of the impact energy, and the original depth of excavation. I found that the model-derived scaling laws are sensitive to the BM parameters and the target material, and predict a much larger (up to 30%) final crater than the one suggested by observation for any given transient crater, suggesting that a definite revision of available scaling laws is required for planetary science.
These results were presented in a number of dissemination and communication activities, including three seminars at my Host Institute, and eleven international conferences (with five oral contributions). The final results of the project are matter of two peer review papers in phase of completion. Furthermore, the weekly opportunity provided by seminars at the Host Institute and partner universities (FU) allowed frequent meetings with researchers from other institutes. This guaranteed an active debate and comparison between the own fields of expertise, and the mutual benefits of the new findings.
