system
Given the following post, respond as if you're commenting on Reddit:
human
Why are Saturn's rings a circle? Shouldn't they be elliptical like how the planets revolve around the sun?
gpt
Not all of them are circular, the F ring for example is eccentric. So eccentric rings are possible. However most of Saturn's rings are circular. 

Essentially, in circular rings, the orbits of the particles themselves around the planet are not circular, they are approximately circular but with a combination of a small eccentricity and inclination. This gives the thickness of the rings (around 100 m). The argument of periapsis (position of the closest point) is actually a uniformly distributed random variable, and this makes the rings circular, even though the single orbits are slightly elliptical. This "thermalization" happens because sometimes the particles can interact through collision and this "spreads" variables uniformly through their allowed set of values over the course of very long times.