Pan

Pan does not merely orbit near Saturn's rings; it travels inside them. The small moon circles within the Encke Gap in the A ring, where its gravity helps keep a 322-kilometer-wide lane largely clear and organizes nearby particles into wakes and ringlets. The gap is therefore not empty by accident. It is a visible trace of Pan's repeated passage through the ring system.

Astronomers inferred that a moon should be there before they found it. Wavy edges in the Encke Gap pointed to a gravitational disturbance, and models narrowed down the hidden body's orbit and mass. Mark Showalter then searched archival Voyager 2 images and identified Pan near the predicted position, turning patterns in the rings into evidence for a previously unseen world.

Cassini later revealed why Pan looks so unlike a conventional moon. A broad, polygonal ridge wraps around its equator, giving it the familiar walnut or ravioli profile. The ridge is made from ring material swept up from the Encke Gap and accounts for about a tenth of Pan's volume. Pan is thus both an architect of the rings around it and a body physically reshaped by the same material.