When a planet is massive enough, it exerts a gravitational pull which is strong enough to rip apart solid bodies as large as moons (or small planets!) which get close enough, even when they’re in orbit. This happens for a number of reasons, but the easiest to understand is the fact that the side of the moon closest to the planet is experiencing a much stronger pull than the side which is further away. This is called “gravitational tidal forces”.
When the difference between the force of gravity on one side of the moon and the force on the other side is larger than the tensile strength of the material which makes up the moon, it will eventually be ripped apart. When this happens, the parts of the moon closer the surface will move faster ahead of the parts that are further away (the former far side of the moon), forming a ring.
This is almost certainly how Saturn got its rings: it shredded a few moons. It’s worth noting that Saturn’s rings are not stable, and within a few millennia they will be gone, sucked into the planet.
Some more fun facts about tidal forces:
– The Moon experiences tidal forces from the Earth! This can cause some minor moonquakes, but more noticeably it creates the moon’s most obvious non-obvious feature: the same side always faces us. The pull of earth’s gravity causes the moon to rotate with the same period as its orbit, kind of like how a tether ball always faces the same side towards the one holding the line.
– Several of Jupiter’s moons and (if I recall?) at least one of Saturn’s are within a radius where the tidal forces of their host body *should* rip them apart if they were made of the same stuff that Earth is made of. From this we can infer that they’re composed of much tougher materials with a higher tensile strength, to resist the forces tearing at them
– Those forces don’t go away though! Tidal effects on moons (particularly Jupiter’s closer moons) causes the moon to bend and stretch and flex. All that energy gets trapped as heat, which is why some of Jupiter’s moons are vulcanic!
– Eventually, the loss of angular momentum to tidal drag (which is the energy that is being indirectly converted into heat and causing vulcanic eruptions) will result in the moons in question drifting closer and closer to Jupiter, resulting in ever steeper tidal gradients which will eventually rip the moon apart, giving Jupiter a very pretty set of rings.
– This won’t happen to our moon. The Earth is too small and the tidal effects are barely relevant. Our moon is in fact spiraling *away* from the Earth, though it’s happening so slowly that we’ll need to worry about the Sun long before we worry about the Moon.