A propeller-shaped structure created by an unseen moon is brightly illuminated on the sunlit side of Saturn's rings in this image obtained by NASA's Cassini spacecraft.
The moon, which is too small to be seen, is at the center of the propeller structure visible in the upper left of the image, near the Encke Gap of the A ring (marked with a red arrow in the annotated version). The A ring is the outermost of Saturn's main rings.
The moon is likely about a kilometer (half a mile) across. Disturbed ring material to the upper left and lower right of the moon reflects sunlight brightly and appears like a white airplane propeller. Several density waves are also visible in the ring. A spiral density wave is a spiral-shaped accumulation of particles that tightly winds many times around the planet. It is the result of gravitational tugs by individual moons whose orbits are in resonance with the particles' orbits at a specific distance from Saturn.
Last Update: 23 Jan 2013 (AMB)