This could be true, but simpler explanations exist — specifically, that Saturn’s rings could have been formed more recently by smaller objects like comets, asteroids, or small moons. Simulations suggest that objects like these could have collided near the planet, their debris spreading into the rings. Alternatively, small bodies could have been sucked in and then torn apart by the planet’s powerful gravity. These explanations would also explain the near purity of the rings’ icy composition better than the primordial ring theory: If the rings were 4.5 billion years old, they would have gotten “dirty” by now, by accumulating dark material from meteorites and interplanetary dust. It is still possible, however, that the rings could be ancient but continuously “cleaned” by new icy material, keeping them bright despite their age.
One thing we do know for certain is that some of Saturn’s rings are fed by its moons. Cassini confirmed that a lot of the material for the E-ring — a diffuse ring outside the bright, main rings — comes from icy particles venting from the moon Enceladus. Cassini also found that many of Saturn’s inner rings are made of particles from the moons that orbit within them, likely blasted off by micrometeoroid impacts.
This is similar to how Jupiter’s rings are formed. Scientists think that the thin Jovian rings are in a constant state of decay and replenishment. While material is constantly falling into Jupiter’s atmosphere, new material is steadily added to the rings from meteoroid impacts on the small inner moons Metis, Adrastea, Thebe, and Amalthea.
Uranus and Neptune’s rings are less well understood, but the leading idea is that they formed from the debris of small moons that were shattered by collisions or gravitational disruption.
The Roche limit
A major factor in how planets form rings is called the Roche limit.
When an object orbits a planet, there are two gravitational forces at work: the pull of the planet, and the satellite object’s own gravity that holds its matter together. When a satellite orbits close enough, the gravitational pull of the planet can compromise that self-gravity. This can lead to the smaller object being ripped apart as the side of it nearer to the planet gets pulled on more than the far side.
The conditions in which an orbiting object will be ripped apart is called the Roche limit, and it mainly depends on distance from a planet. Within that limit, loose material also doesn’t coalesce into a solid object like a moon. The Roche limit of a planet (or dwarf planet, or smaller body) depends primarily on its size and density. However, the size, density, and material composition of an orbiting object are also all factors in whether it will be torn up within the Roche limit. Saturn, for example, has “ring moons” — tiny moons that orbit within its rings. These likely have enough material strength to withstand Saturn’s gravitational effects, at least temporarily.