[thedoc (OP)]

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.
Asked by Mark Wilson


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[CliffordK]

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.

[evan_au]

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?

http://en.wikipedia.org/wiki/Rings_of_Saturn#Formation