This image is one of a sequence of three, showing the break-up of a small moon due to the gravitational field of its giant parent planet. This is one theory for the origin of the rings of Saturn. If a moon is close enough to a planet, the force of gravity from the planet may be stronger than the gravitational and tensile forces holding the moon together. The distance at which this happens is called the Roche limit. Moons closer than the Roche limit will break up, potentially forming a ring system.