Chapter 27 - Planetary Rings

Abstract

Planetary rings are those strikingly flat and circular appendages embracing all the giant planets in the outer Solar System: Jupiter, Saturn, Uranus, and Neptune. Like their cousins, the spiral galaxies, they are formed of many bodies, independently orbiting in a central gravitational field. Rings also share many characteristics with, and offer invaluable insights into, flattened systems of gas and colliding debris that ultimately form solar systems. Ring systems are accessible laboratories capable of providing clues about processes important in these circumstellar disks, structures otherwise removed from us by nearly insurmountable distances in space and time. Like circumstellar disks, rings have evolved to a state of equilibrium where their random motions perpendicular to the plane are very small compared to their orbital motions. In Saturn's main rings (Fig. 1), for example, orbital speeds are tens of km/sec while various lines of evidence indicate random motions as small as a few millimeters per second. The ratio of vertical to horizontal dimensions of the rings is consequently extreme: one part in a million or less, like a huge sheet of paper spread across a football field.