Take a look at this picture (from APOD https://apod.nasa.gov/apod/ap110308.html): Saturn's rings at APOD

I presume that rocks within rings smash each other. Below the picture there is a note which says that Saturn's rings are about 1 km thick.
Is it an explained phenomenon?


3 Answers 3


There seems to be a known explanation. I quote from Composition, Structure, Dynamics, and Evolution of Saturn’s Rings, Larry W. Esposito (Annu. Rev. Earth Planet. Sci. 2010.38:383-410):

[The] rapid collision rate explains why each ring is a nearly flat disk. Starting with a set of particle orbits on eccentric and mutually inclined orbits (e.g., the fragments of a small, shattered moon), collisions between particles dissipate energy but also must conserve the overall angular momentum of the ensemble. Thus, the relative velocity is damped out, and the disk flattens after only a few collisions to a set of nearly coplanar, circular orbits.

I think the key is that particles in a thick ring would not move in parallel planes but would have slanted trajectories, colliding all the time and losing their energy very fast.

  • $\begingroup$ So essential point, in my opinion is, that it is a matter of initial conditions. Picture You are pointing out begins with "fragments of small shattered moon" and it is the same for comment of anna v above. I would like to wait if a few answers appear here, but I am ready to accept Your answer here. $\endgroup$
    – kakaz
    Mar 9, 2011 at 14:34
  • 3
    $\begingroup$ In inelastic collisions between the ring particles angular momentum is preserved but kinetic energy is lost. So the final rotation axis of the rings is given by their initial angular momentum but it is the dynamical evolution of the rings that makes them flat and thin. $\endgroup$ Mar 9, 2011 at 14:54
  • $\begingroup$ I would add that the tides should also be taken into account: the gradual increase of angular momentum of the rings due to the tides the rings induce on Saturn. It is not only collisions, because the orbits will expand differentially, depending on the mass of the debris, contributing to thinning. $\endgroup$
    – anna v
    Mar 10, 2011 at 5:56

If one could assume that the ring is a continuous distribution of mass, we could try minimizing the total energy of the system (self energy + energy of interaction with Saturn). These two conditions along with the condition that total mass of the disc is a constant, would (I think) leave us with a unique geometry (inner and outer radius, thickness).

EDIT: Some Googling gave this paper: http://dx.doi.org/10.1016/0019-1035(79)90084-8

  • $\begingroup$ It is clever idea but too simple and vague. I would like to know why internal structure of the rings, namely scattering do not destroy its thickness, and continuous model is probably wrong here. Note also here that it is not clear if energy is the only possible bonds here: there is angular momentum, possible entropy, maybe perturbations from other Saturn moons etc. For example consider Earth and its artificial satellites - they do not form "rings" in natural way, so probably idea that mineralisation of energy is enough is too simple - You have at least prober initial conditions. $\endgroup$
    – kakaz
    Mar 9, 2011 at 14:39
  • $\begingroup$ @kakaz I agree, but how would you know which parameter is to be given more importance? $\endgroup$
    – Manu
    Mar 9, 2011 at 14:56
  • $\begingroup$ @mnnttl - by numerical experiments for example. You cannot explain something by just assuming other things. There should be an evidence for certain assumption - energy is usually very general notion, and so it works, but also is usually not enough alone to provide correct model - see thermodynamics for example. $\endgroup$
    – kakaz
    Mar 9, 2011 at 20:32

The rings have formed where they are because there is greater gravity there. The reason is the shape of the gas giant. It is far from circular. It is wider at its equator. Centripetal force causes a thicker equator. Therefore because the thickness is far wider at the equator, the gravitation at the equator is more than that at the poles. There is more mass pulling you down at that point. The particles of the rings fall into a gravity well.

This explanation is self evident because as can be seen from the above photo, the rings are so incredibly thin, and are PERFECTLY aligned with the equator.

Compare the reason why the moon only shows us one face. It must have more mass on one side. It is like a ship floating on the ocean. The ship has more mass in its hull that gravity pulls into the water.

On the gas giants, given time every asteroid has worked their way to the equator. I would imagine being an asteroid, it would take a very long time of maybe thousands of years and collisions to achieve such a thin presence. It is very beautiful though!

  • $\begingroup$ Some numbers to support this argument would be useful. $\endgroup$ Apr 9, 2013 at 17:50
  • $\begingroup$ it is somehow correct intuition-rungs cannot form around say 2/3 of the planet, it has to be on the equator. So thank You for pointing this. But it does not mean they should be so thick in reality. It may look innocent but explanation requires statistical physics approach in my opinion $\endgroup$
    – kakaz
    Mar 31, 2021 at 8:51

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