Why don't Saturn's rings aggregate to form moons?

12 August 2012

Question

Naked Astronomy,

We know that planets form around stars when all the left-over gas and dust clumps together to form them. I would like to know why all the material that makes up the rings of Saturn doesn't do the same thing. It seems logical that it would come together to form one or more moons but that is not what we see. In fact all this material appears to be uniformly smooth without any clumping whatsoever. Please explain.
Mark Wilson
San Diego, California, U.S.A.

Answer

Dominic - The problem is that the material is so close to Saturn that it feels a very strong tidal force. The moon, we know, induces tides in the Earth's oceans, but similarly, the Earth also induces tides in the rock of the Moon, and that would also happen for a body orbiting Saturn. And the closer that body is to Saturn, the stronger those tides are, and those rings are really quite close in to a very large gas giant planet and so, they feel incredibly strong tides. And that means anything that tries to clump together where those rings are, would immediately be pulled apart by those tidal forces. Chris - So basically, there are some little moonlets, but it's really hard for them to exist because they're just going to get ripped to pieces. But what about the moons that do exist? Because, say Jupiter has got some very large moons around it, so has Saturn. They're not being pulled apart. So, how did they get stable in the first place then? Dominic - Now this, I should say, is not well understood and people are researching this at the moment. I think the current thinking is that those moons must have formed further out in Saturn's system and then they must've migrated in as a result of exchanging energy with other moons. Chris - Because they stretch and bend quite a bit, don't they? I mean, if you look at some of the moons around Saturn - Enceladus, for example, squirts out materials into space because it's being stretched and deformed as it goes around in the gravity field.

Dominic - That's right. You can have moons where a tidal force is so strong that they effectively have anti-gravity. So, the tidal interactions are stronger than the gravity which is pulling the moon together - like Phobos and Deimos orbiting Mars.

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