Width of a planetary ring system and the C-ring of Saturn

PLANETARY ring systems such as those of Saturn and Uranus seem to have an inner limit analogous to the Roche outer limit. The analysis of the C-ring of Saturn described here gives an insight into its structure and dynamics. The quasi-steady distribution of particle sizes in a ring requires that besides collisional comminution and sputtering by protons, electrons and meteorites 1 there are processes which permit the particles to grow. On Saturn, one of these processes is condensation 2 on to the inner rings of water molecules sputtered from the outer edge 3 of ring A. Another is the adhesion of smaller particles to the larger ones after low velocity collisions produced by gravitational attraction and by the differences in Keplerian velocities 4 . This process can include ejecta formed during cratering. The question arises under what condition these small particles will stay long enough on the surface of the larger ones to develop a permanent bond.