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J. D. O'Keefe (Caltech), S. T. Stewart (Caltech), T. J. Ahrens (Caltech)
We found a new regime of impact physics in modeling the impact of various density and shape projectiles on comets and asteroids. These results provide scaling relationships that not only apply to the impact of solar system objects and solar system probes on comets and asteroids but also to porous low density materials in general. We modeled the comet as a porous rock/ice mixture with separate equations of state for the rock and ice. This gave us the capability to model the cases where the shocked constituents are either in thermal equilibrium or not. In the case of comets, the rock and ice are finely distributed, thus the mixture was assumed to be thermal equilibrium. The model results agree with laboratory measurements of impacts on various porous low density materials. This new regime is defined by impacts of dense impactors into low density porous materials. In this case, the projectile upon impact, flattens and expands normally to the impact velocity vector. The lateral expansion is arrested by Kelvin-Helmholtz instabilities that strip mass from the edges of the expanding projectile. The downward penetration is terminated by Rayleigh-Taylor instabilities, which grow on the front face of the projectile and fragment it. The instabilities in this impact regime are reminiscent of those found in our simulations of the Shoemaker-Levy 9 impact on Jupiter; however, in this case the target medium is a porous solid as opposed to a gas. The resulting crater cavity shape can be bulbous or carrot shaped as opposed to bowl-shaped or flat floored in the case of simple and complex planetary craters. Our results imply that the crater volume scaling changes from momentum scaling at low values of the ratio of the density of the projectile to the density of the target to energy scaling at high values of the ratio and explains the intermediate scaling found for density ratios around one.
This work is supported by NASA grant NAG5-8915.