Daphnis (Greek: Δάφνις) was a small, fast-moving moon of Saturn. It was discovered in 1980 by Dan Pascu and Carolyn Shoemaker from photographs taken by the Voyager 1 spacecraft, and was provisionally designated S/1980 S 6. It was named after Daphnis, a nymph in Greek mythology.
Daphnis has an irregular shape, measuring approximately 5 kilometers (3 miles) across. Its surface is very dark, with albedo values as low as 0.05. This makes it one of the darkest satellites of Saturn. There is a bright ridge or “waviness” running along the equator of Daphnis that is about 2 kilometers (1 mile) wide. This feature is likely an optical illusion caused by shadows cast by the waves on the satellite’s surface. The waves themselves are only about 20 centimeters (8 inches) high.
Daphnis orbits within the Keeler Gap, a 3:1 mean motion resonance with Pan, another inner satellite of Saturn. As a result of this orbital configuration, Daphnis creates spiral density waves in Saturn’s A ring; these can be seen in images taken by the Cassini spacecraft. The resonant interaction also forces Daphnis to undergo libration around its orbit; that is, it appears to wobble slightly as it goes around Saturn. This effect is most pronounced when viewed from above Saturn’s north pole; from this perspective, Daphnis can be seen to trace out a lissajous figure over time.
The discovery of Daphnis has helped to solve a longstanding mystery regarding the structure of Saturn’s rings. The gap in which it orbits was first detected in images taken by the Pioneer 11 spacecraft in 1979; at that time, however, its existence could not be explained satisfactorily without invoking an unseen shepherd satellite orbiting just outside the gap (a hypothesis later shown to be incorrect). The presence of Daphnis within the gap provides enough mass to explain its existence without resorting to such ad hoc explanations