Saturn has two main types of moons: the first are regular moons, like Enceladus, that are similar to moons around other giant planets and orbit in Saturn's equatorial plane. The others are tiny, icy moonlets that reside on the outer edges of Saturn's rings. They weren't discovered until about six years ago when the Cassini spacecraft began imaging the Saturn system, and they were an unexpected find.
The regular moons are as old as the planet; they formed about 4.5 billion years ago from the same stuff that makes up Saturn. But the smaller "ring moons" are less than 10 million years old, they are far less dense and they look different, like flying saucers with patchy surfaces. How did they form?
In a new study published last week in Nature, scientists from France and England created a computer model to simulate the Saturn system. That model confirms the prevailing theory of the moonlets' origin, that they formed from the accretion of material in the planet's rings. When ring material moves beyond a certain distance from the planet—called the Roche limit—it becomes gravitationally unstable and clumps up to form the tiny moons. Gravitational forces, angular momentum and tidal forces then all act to push the moonlets clear of the rings and to force the contraction of the rings.
More moonlets may be in Saturn's future, the scientists say. The ring moon Janus currently keeps the rings in check, but as it moves farther from the planet, the rings could spread and provide more material for new moonlets.