the middle. This ringlet exhibits irregular azimuthal variations of geometrical width and optical depth, which may be caused by the nearby 2:1 resonance with Mimas and the influence of the eccentric outer edge of the B-ring. There is an additional narrow ringlet just outside the Huygens Ringlet. A Ring The A Ring is the outermost of the large, bright rings. Its inner boundary is the Cassini Division and its sharp outer boundary is close to the orbit of the small moon Atlas. The A Ring is interrupted at a location 22% of the ring width from its outer edge by the Encke Gap. A narrower gap 2% of the ring width from the outer edge is called the Keeler Gap. The thickness of the A Ring is estimated to be 10 to 30 m, its surface density from 35 to 40 g/cm2 and its total mass as 4 to kg (just under the mass of Hyperion). Its optical depth varies from 0.4 to 0.9. Similarly to the B Ring, the A Ring's outer edge is maintained by orbital resonances, albeit in this case a more complicated set. It is primarily acted on by the 7:6 resonance with Janus and Epimetheus, with other contributions from the 5:3 resonance with Mimas and various resonances with Prometheus and Pandora. Other orbital resonances also excite many spiral density waves in the A Ring (and, to a lesser extent, other rings as well), which account for most of its structure. These waves are described by the same physics that describes the spiral arms of galaxies. Spiral bending waves, also present in the A Ring and also described by the same theory, are vertical corrugations in the ring rather than compression waves. In April 2014, NASA scientists reported observing the possible formative stage of a new moon near the outer edge of the A Ring. Encke Gap The Encke Gap is a 325-km (200 mile) wide gap within the A ring, centered at a distance of 133,590 km (83,000 miles) from Saturn's center. It is caused by the presence of the small moon Pan, which orbits within it. Images from the Cassini probe have shown that there are at least three thin, knotted ringlets within the gap. Spiral density waves visible on both sides of it are induced by resonances with nearby moons exterior to the rings, while Pan induces an additional set of spiraling wakes