Damage and rock-volatile mixture effects on impact crater formation

We explored simple geologic strength and material response models to determine which have the capability to simulate impact-induced faulting, complicated ejecta patterns and complex crater shapes. This led us to develop models for material damage, dilatancy, and inhomogeneous materials (mixtures). We found that a strength degradation (damage) model was necessary to produce faulting in homogeneous materials. Both normal and thrust ring faults may occur and extend relatively deeply into the planet during the transient cavity radial expansion. The maximum depth of fault development is about the depth of maximum penetration by the projectile. Dilatancy in geologic materials may reduce the final bulk density compared to the pristine state because of irreversible fracturing. When we include the effects of dilatancy, the radial position of faulting is displaced because of greater upward motions. In addition, the late time crater profile is shallower and the expression of features such as central peaks and rings may be more pronounced. Both damage and rock-ice mixtures effect the distribution of ejecta. The excavation flow field within the heavily damaged region is similar to flow fields in Mohr-Coulomb materials with no zero-pressure strength. In the outer, less damaged zone within the excavation cavity, the material trajectories collapse back into the crater. This effect creates a zone of reduced ejecta emplacement near the edge of the final crater. In the case of rock-ice mixtures, energy is preferentially deposited in the more compressible volatile component and the ejecta pattern is dependent upon the location of shock-induced phase changes in the volatile material.