Dynamic fragmentation in the solar system: Applications of fracture mechanics and hydrodynamics to questions of planetary evolution.

Abstract

Impact phenomena shaped our solar system. From the accretion of the planetesimals 4.6 billion years ago to the comparatively recent spallations of meteorites from their parent bodies, which take them to Earth, this ceaseless process has left no bit of solid matter untouched. As usual for most solar system processes, the scales are far different than we can address directly in the laboratory. Impact velocities are often much higher than we can achieve, sizes are often vastly larger, and most impacts take place in an environment where the only gravitational force is self-gravity. Laboratory studies, by contrast, are limited to disruptive impacts with typical velocities ∼3 km/s, involving targets smaller than a kilogram in an imposed terrestrial gravitational environment. We must extrapolate from these data by twenty orders of magnitude before we reach the mass range of asteroids, comets and planetesimals. The complexity of fragmentation phenomena, and the role both strength and gravity play in most interesting catastrophic impacts, make numerical models of catastrophic disruption the most viable research tools. But numerical models must be subject to careful scrutiny regarding numerical accuracy and the proper representation of physics. For this reason two very different code models of fragmentation and catastrophic disruption are presented here. They not only are both good predictors of laboratory outcomes, but they also largely agree about predictions involving large-scale extrapolation. A simple analytical model for fragment size distributions in the strength regime is also presented. Each of these models is suited to a particular class of problem, depending on the complexity and the sophistication required. It is hoped that the ideas and models developed in these pages will contribute to a better understanding of fracture and fragmentation events with regard to the evolution of solar systems and planets.