Gravitational wakes in Saturn's rings

Abstract

THE outer parts of Saturn's rings display a variety of local non-uniformities in their particle distributions. Azimuthal brightness variations are seen in the A-ring^1,2, and may be attributable to the gravitational aggregation of particles into linear wakes that trail the rotation of the ring^3–5; Voyager's stellar occultation experiments revealed large differences, on length scales as small as 150 m, in the surface density of particles⁶. Theoretical arguments^7,8 suggest that local instabilities may occur in both the A-and B-rings, the instability criterion depending on the velocity dispersion of ring particles and the orbital velocity and mass density in the ring. These arguments, however, are derived from the purely gravitational dynamics of an idealized, infinitesimally thin ring, made of identical particles. Here I use numerical simulations, including both gravitational interactions and dissipative impacts between particles, to study realistic models of Saturn's rings. For the C-ring there is no instability, but for the B- and A-rings gravitational wakes form. In the A-ring these wakes are so strong that particles trapped in them form metre-sized aggregate particles, which themselves lead to further instability. These different behaviours are consistent with the observational evidence.