HYDROCODE MODELING OF IMPACT EVENTS

Numerical modeling is a fundamental tool for understanding the dynamics of impact cratering, in particular at planetary scales. Hydrocodes (computer programs that implement the continuum dynamics of impact cratering to calculate hydrodynamics and solid state deformations) provide detailed information regarding all variables of interest for the entire simulation, and can thus be tested against observation. Historically, there has been separation between early and late stage modeling of impact cratering. Early stages focus on evaluating shock states in target and projectile, including shock melting and vaporization, and require simulations with high spatial resolution near the impact point. Late stages investigate the process of crater collapse, modeling the formation of the final impact structure, and require long integration time and a large spatial mesh. Continuous improvement in material modeling and advances in computer hardware are approaching the capability of combining the two modeling stages.

Due to computer hardware limitations, most impact modeling work to date has concentrated on the improbable case of a vertical impact allowing a simplification of the model to two dimensions (2D). It is true that moderate (20 to 30 degrees) deviations from vertical impacts have small consequences on modeling outputs, allowing results from 2D simulations to be valid and useful for many applications. However, natural impacts in which the projectile strikes the target nearly vertically are rare or virtually nonexistent. Probability theory shows that the impact angle of maximum frequency is 45°. In non-vertical impacts the axial symmetry typical of vertical impacts is broken and computationally more intense 3D hydrocodes are required to simulate the impact. Thus, the increased use of 3D hydrocodes, facilitated by the continual advances in computer hardware, is providing an important level of realism for simulations of impact events. The capability of modeling oblique impacts has provided the means for understanding problems such as the ejection of matter from planetary surfaces, the giant impact theory for the origin of the Moon, the environmental effects of large planetary impacts, and the spin rate of asteroids