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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - IMPACT (Physics of Impact Cratering Collapse)

Teaser

Summary

Complex crater collapse is a mechanism still poorly understood, because standard materials models fail to explain the development of central peaks or rings, flat floors and terraced walls from a much deeper and narrower transient cavity. One current model invoked to explain such a collapse is the Acoustic Fluidization (AF) model, which relies in the temporary softening of heavily fractured target rocks by means of an acoustic field in the wake of an expanding shock wave originated upon impact. The Block Model (BM) is one AF simplification, differing from AF for being described by a time-varying Bingham rheology, which depends on viscosity and decay time. Both these two variables were suggested to linearly depend on the projectile radius. However, BM is still lacking of the coupling between the model equations and target properties.
A clear understanding of how complex craters form is essential in numerous aspects of planetary geology, including investigation of stratigraphy and composition of the near-surface structure of planetary bodies, derivation of reliable crater-based chronologies, and unravelling the bombardment history on the Earth-Moon system, and the origin and evolution of life on our planet.
The overall objective of the project was the improving of the current understanding of the mechanics acting during the final stages of the impact cratering process, and it was fulfilled through a multidisciplinary approach including laboratory experiments and numerical modelling. Both the methodologies were required to comprehensively address the challenge of crater collapse. Laboratory experiments take advantages of direct measure of material response to fluidization, while numerical modelling can simulate assesses the individual effect of any variable on crater formation.

Work performed

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.

Final results

\"The state of art about impact crater collapse indicated that a temporary and weakening mechanism is required to explain the collapse of large craters. One current model invoked to explain such a collapse is the AF model. While a number of specific applications of the AF model are available, only few studies focused to assess the accuracy of the model itself. The systematic modelling activity pointed out that the suggested linear relationship between the model parameters of viscosity and decay time, and the projectile radius might be an oversimplification. If on one hand, such a relation was derived assuming Mohr-Coulomb model instead of a pressure and damage-dependent model for describing the target material, on the other hand at the larger crater diameter the fluidized motion of the block rocks can be affected by the larger production of melt. I showed that the study of complex crater collapse can be feasibly achieved also by means of small-scale analogue experiments, where the target was conveniently chosen as unconsolidated material and couple by an external source of fluidization. The behaviour of the target material under fluidization can be then connected to the oscillation waves delivered by the external source, and testing any influence of material properties.
The innovative research program contributed to the Host Instituteâ€™s reputation as regards the high quality research environment and the excellent infrastructures. This ensured the development of new synergies with universities and other institutes (the project engaged the attention for a future collaboration regarding laboratory experiments under vacuum), to attract Bachelor and Master students (MfN had a visiting student of Dr. Melosh, the worldwide standing expert on impact cratering), to promote new fund raising for PhD students and Postdocs, and to foster scientific matters among people (educational programs at MfN). Museum like the \"\"Rieskrater Museum\"\" (NÃ¶rdlingen, Germany) can benefit from the up-to-date findings to actualise the collection, enabling better exchange with relevant scientific institutions (Ries crater was training location for the Apollo astronauts), and increasing interest for the museum visitors. Furthermore, the nature of impact craters favoured collaborations in the space exploration programme (ESA HERA space mission), which will provide benefits as regards new research cooperation, educational program, and new technologies for commercial applications.\"