Why doesn’t Saturn’s ring aggregate to form moons?
This is a question that I was asked to answer for the Naked Astronomy podcast. I thought that, like last time, it would be nice to give a short article on this topic with a few more visual parts. If you want to listen to my answer either listen to the full podcast or the snippet of me giving the answer.
So, lets start of by looking a bit at the rings of Saturn. The rings extend from about 7,000 to 80,000 km above Saturn’s equator. The thickness of the rings changes quite a bit over them from about 10m up to 1km. The rings are mostly made out of pure water ice. The total mass of all of the material in the rings is about that of Saturn’s moon Mimas, that’s the one that looks a bit like the death star. That is 0.000001% of the mass of Saturn, so quite a tiny fraction.
The largest gaps in the rings, such as the Cassini division, can be seen in your back garden with a small telescope – really worth a look. The smaller and more intricate structures to the rings wasn’t found until the Voyager spacecraft orbited around Saturn. The gaps are thought to be due to the gravitational pull of Saturn’s tiny moon which shepherd the material around. A good example of this is Prometheus and Pandora which orbit inside/outside of a ring of material. Though some of the rings are also due to resonances from the motion of the more massive moons.
Before we look into the creation of icy moons it is worth thinking about how the rings were created. It is thought they were either created by:
– a small moon that was shattered by meteor impacts, though the rock content doesn’t really match
– tidal destruction of a comet that came too close to Saturn though this would lead to a mix distribution of ice and rock that is not seen and its more likely that this sort of action would occur around Jupiter
– or a Titan-sized moon with a rocky core and an ice mantle. As the moon spirals into Saturn early in the solar system’s history tidal forces rip off the icy mantle, distributing it into the rings. The rocky core holds together until it hits Saturn’s surface.
Going back to the main point in this article and the question at hand. The large moons of the Solar System are thought to have finished forming at about the same time as their host planets, some 4.5 billion years ago. The smaller moons are different. The smaller moons of Saturn have bright nearly pure ice surfaces that looks undamaged from meteoroid impacts. Their ages are estimated to be only 10 million years old. So new objects are still forming in the Solar System. It is thought hat once the rings are formed the particles of ice start to clump together in the outskirts of the ring. There is a distance from the planet, the Roche limit, at which a moon could form. Inside of this limit the tidal force from the planet is too strong and causes the material to break up and hence no moons can form. Once these moons form they migrate inwards, closer to the planet, and eventually end up into the stable orbits we see them in now.
More info: “The recent formation of Saturn’s moonlets from viscous spreading of the main rings”, Nature 465, 752-754 (10 June 2010), doi:10.1038/nature09096