The Huygens "Gap"

Caption NASA:"The sharp outer boundary of Saturn's B-Ring, which is the bright ring region seen to the right in this image, is maintained by a strong resonance with the moon Mimas. For every two orbits made by particles at this distance from Saturn, Mimas makes one orbit.
The moon's repeated gravitational tugs force ring particles away from this region.
The dark region is called the "Huygens Gap" and it includes the bright, eccentric Huygens Ringlet, also visible here near center".