  We demonstrate that the chaotic orbits of Prometheus and Pandora are due to
interactions associated with the 121:118 mean motion resonance. Differential
precession splits this resonance into a quartet of components equally spaced in
frequency. Libration widths of the individual components exceed the splitting
resulting in resonance overlap which causes the chaos. A single degree of
freedom model captures the essential features of the chaotic dynamics. Mean
motions of Prometheus and Pandora wander chaotically in zones of width 1.8
deg/yr and 3.1 deg/yr, respectively.
