Science Questions

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

Sat, 11th Aug 2012

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Mark Wilson asked:

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.


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 Earthís 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 itís 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 Saturnís 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, donít they?  I mean, if you look at some of the moons around Saturn - Enceladus, for example, squirts out materials into space because itís  being stretched and deformed as it goes around in the gravity field.

Dominic -   Thatís 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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If the moons were within the Roche limit, then moons would be unable to form.  However, according to Wikipedia, the Roche limit for Saturn is 60,268 km.

Most of the Saturn's rings lie between 66,000 to 140,000 km from Saturn.  While closer than many of Jupiter's moons, the rings for the most part lie further out than the Roche limit.

In fact, there are some small moons of Saturn that lie within the outer rings, with the larger ones mostly clearing a path and aggregating material from the rings.

Anyway, it may be that while a captured satellite could potentially exist near the Roche limit, it is impossible for a new satellite to aggregate near the limit.
CliffordK, Fri, 10th Aug 2012

The moons of Saturn exert a strong gravitational effect on the material in the rings. Any particle in the rings whose orbit was a multiple of any of the moons orbital period would experience consistent tugs to pull it into a different orbit.

This clears out gaps in the rings, preventing the rings from coalescing to form a single large body, even though it is outside the Roche limit.

The many particles within the rings will be continually jostling each other as they pass each other in orbit and are disturbed by the moons; this breaks down big particles to form smaller ones. This process would be competing with Van Der Waals force which might allow small particles or gas to stick together to form larger ones.

Given that the composition of the rings is mostly water ice: particles are fairly likely to shatter on impact, and the surface of the particles would be slowly sublimating into a gas. So perhaps the processes breaking down the particles is winning, in this environment?
evan_au, Fri, 10th Aug 2012

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