Shepherd moons Prometheus and Daphnis race around Saturn’s rings within their respective gaps, affecting the ring material in their own distinct ways.
Prometheus, on the left side, pulls the bright, ropy F ring into streamers while tiny Daphnis, embedded in the 25-mile-wide Keeler gap at lower center, sends up a mound of darker A ring material in its wake.
Shepherd moons are so called because they keep the ring material in check with their gravity as they orbit Saturn within the same plane as the rings. They keep gaps in the rings clear, and in some cases pull at the fine, icy particles as they pass. They are some of the most interesting and dynamic members of the Saturnian family of moons.
Raw image: NASA/JPL/Space Science Institute