Asteroid collisional evolution I. Angular momentum splash: Despinning asteroids through catastrophic collisions
When an asteroid suffers a catastrophic impact, with only a fraction of the initial mass reaccumulating into a "rubble pile," a significant amount of angular momentum is carried away by the escaping material. The reaccumulated core will have a lower rotation rate than it would without this effect, and may even have a net spin-down, relative to its preimpact spin rate, due largely to the preferential escape of high-angular-momentum fragments. We call this effect angular momentum splash in analogy with the angular momentum drain mechanism studied by A.R. Dobrovolskis and J.A. Burns (1984, Icarus 57, 464-476) for smaller cratering impacts. It is quantitatively assessed using the model for catastrophic fragmentation events developed by P. Paolicchi, A. Cellino, P. Farinella, and V. Zappalà (1989, Icarus 77, 187-212). The splash mechanism is most effective at intermediate asteroids sizes (∼100 km), where the ejection velocities of fragments are of the same order as the escape velocities of the targets; the corresponding relative spin-down is found to be of the order of the core/target mass ratio. The splash-related spin-down is in competition with the spin-up due to angular momentum transfer from the projectile in noncentral impacts. Although the outcome depends on several collisional parameters, net spin-down prevails in a range of target sizes around 100 km, while larger objects are more often spun up by shattering impacts. This result holds for a wide range of collisional parameters and is in good agreement with the observed distribution of asteroid rotation rates with size.