Friday, 24 March 2017

newtonian mechanics - Why are Saturn's rings so thin?


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?



Answer



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.


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