A RECENT COMETARY ORIGIN FOR SATURNS RINGS

Gravitational torques and erosion by micrometeoroids may cause rapid evolution of Saturn's rings, possibly suggesting formation of the rings within the last 108 years. Optically thin and/or narrow rings at Jupiter, Uranus, and Neptune may form repeatedly by disruption of inner satellites. However, present-day creation of Saturn's more massive rings in this way is unlikely. An alternative scenario for ring origin is tidal disruption of a large comet, such as Chiron, passing within Saturn's Roche radius. Subsequent collisions among the fragments lead to a flat, equatorial ring, while collisions with inner satellites produce a Population II-like set of craters on the moons. Assuming a nominal flux of Saturn-crossing bodies, 10-100 close passages occur in 4.5 Gyr. Most close passages leave no bound debris, so that approximately one ring capture event occurs in the age of the solar system. This rate is comparable to the expected frequency of disruption of ring parent satellites by cometary bombardment. These results imply that the rings result from a recent unlikely tidal or collisional event, or that the arguments for rapid evolution of Saturn's rings are erroneous and the rings are primordial. However, the flux of Saturn-crossers is highly uncertain; if the flux is much higher, the rings are probably the recent result of tidal disruption. Ongoing searches for outer Solar System planetesimals and future data from Cassini will help determine the rings' age and origin. © 1991.