When a snowball becomes a moon: When enough stuff accretes onto a body within Saturn's rings, they can become a Shepherd Moon. These moons rule over their own ring gaps. They also shepherd the rings, keeping them in order. If they have enough gravity, they can create disturbances in the edges of their gaps. Daphnis orbits within the Keeler Gap and as the material passes it the moon gives it little tugs creating rhythmic waves.

NASA/JPL-Caltech/SSI/CICLOPS/Kevin M. Gill

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