Expert: Philip Stahl - 4/17/2004

Question
I was just reading a book on Planetary Rings and these two questions came to me as I read.

Have the ring systems of the giant planets lasted since the time the planets were formed?

and are rings flat because collisions between particles eliminate crossing orbits?

Thanks


Answer
Hello.

In the case of Saturn, the rings were probably there since its formation in the early Solar System. (At least this is one prevailing theory, the other being that one of its moons was destroyed and the rings are made of particles of that moon - this has since fallen into disfavor).

By contrast, the perturbations of asteroids with the result that tidal gravitational forces destroyed one or more small ones, could certainly explain rings around the other giant planets - especially Jupiter.

The stability and flatness of the Saturn ring system (and probably the others as well) is more likely due to the "shepherding" by nearby satellites.

For example, the 2 satellites - Prometheus and Pandora - which straddle Saturn's F-ring keep its particles "herded" together in a narrow range of orbits. The way it works is that the inner moon accelerates inner ring particles as it passes them, while the outer moon *decelerates* outer ring particles as it does the same.

The balance of all these interactions both constrains the particles' motions and narrowness of this (F-) ring, while also maintaining the sharp edges.

This ring herding process is likely common to all the giant planets.