Catastrophic Disruptions Revisited

We use a smooth particle hydrodynamics method to simulate colliding rocky and icy bodies from centimeter scale to hundreds of kilometers in diameter in an effort to define self-consistently the threshold for catastrophic disruption. Unlike previous efforts, this analysis incorporates the combined effects of material strength (using a brittle fragmentation model) and self-gravitation, thereby providing results in the "strength regime" and the "gravity regime," and in between. In each case, the structural properties of the largest remnant are examined. Our main result is that gravity plays a dominant role in determining the outcome of collisions even involving relatively small targets. In the size range considered here, the enhanced role of gravity is not due to fracture prevention by gravitational compression, but rather to the difficulty of the fragments to escape their mutual gravitational attraction. Owing to the low efficiency of momentum transfer in collisions, the velocity of larger fragments tends to be small, and more energetic collisions are needed to disperse them. We find that the weakest bodies in the Solar System, as far as impact disruption is concerned, are about 300 m in diameter. Beyond this size, objects become more difficult to disperse even though they are still easily shattered. Thus, larger remnants of collisions involving targets larger than about 1 km in radius should essentially be self-gravitating aggregates of smaller fragments.