Why are Saturn's rings flat? Why isn't the debris dispersed equally around the whole planet?
That's an excellent question, and the key to the answer is the the entire system is spinning. Each ring particle is orbiting Saturn, like a tiny moon. Since there are a lot of ring particles, from time to time, they run into one another.
Let's ignore the question of exactly how the rings formed (which is a bit controversial) and just assume that at first, there are particles orbiting in every direction. However, the majority ofthe particles will be orbit in one direction or the other (in the case of planetary rings, counter-clockwise as viewed from the north).
Now, particles that are orbiting in the other direction (which we call retrograde) will be moving fast relative to the other particles (think about someone who accidentally ends up going the wrong way on the highway!), so they will be more likely to suffer a collision, and the collision will be more violent. Either the particle will get turned around, or it will be pulverized! Either way, there will be fewer and fewer retrograde particles as time goes on.
Among the particles that are moving counter-clockwise, some will be on orbits that are tilted (or, as astronomers say, inclined), and their inclinations will be more-or-less random. That means that a particle on an inclined path that happens to be moving upward has a good chance of encountering another inclined particle that happens to be moving downward. Thus, inclined particles will clonk into each other and some of the upward motion of one will cancel out the downward motion of the other, leaving both particles on less inclined orbits. This is how the system will tend to settle down into a disk.
As more and more particles settle into the disk, then the remaining inclined particles will have a very dangerous journey! Twice per orbit they have to pass through the disk, where they are likely to suffer a collision.
Particles in the disk, on the other hand, will all be moving the same way at approximately the same speed. Though they might jostle their neighbors from time to time, they do not suffer violent collisions like the inclined particles do.
Over time (a very short time, as it turns out!) the inclined particles will lose their inclination or be destroyed, leaving a nice, flat ring system.
We see this disk-forming process, not only in planetary rings, but in many other astrophysical contexts, such as young solar systems and disks around black holes.
This page was last updated June 28, 2015.