Old rings?
Published on Jan 28, 2008 at 5:16 pm.
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Saturn’s rings are always fascinating to look at. Whenever I do a public star party when Saturn is visible, people are always so amazed by looking at Saturn through a telescope. The planet is great, but it’s the rings that they are looking at. They are simply fascinating.
But, the rings have also been a mystery. Where do they come from? What are they made of? For over three centuries, we’ve known that the rings can not be solid, because the tidal forces on them would tear them apart. So, the rings are composed of a swarm of smaller bodies. The rings are thin, scarcely hundreds of meters thick. And, they are composed of icy particles, which is why they are so bright. But, where do they come from?
The rings are inside Saturn’s Roche Limit. The Roche Limit is the distance at which tidal forces of the planet overcome self gravitation of a body. In other words, if something is held together by gravity rather than by other forces, it will be torn apart inside the Roche Limit. So, when I was in graduate school, we were taught that something icy got inside Saturn’s Roche Limit and was torn apart. The spreading material gave rings. What was this mystery body? No one knew, but speculation ranged from a comet to one of Saturn’s own moons that somehow had an orbit that became unstable and drifted too close to Saturn. Or, perhaps, a collision with a large comet shattered a small icy moon just outside the Roche Limit. The only problem, though, is that these models also predicted that the rings are temporary, lasting no more than a few hundred million years or so. And, that seemed fine, as long as Saturn was the only world know to have rings. But, then we discovered rings around Jupiter, then Uranus, and finally Neptune. Rings seem commonplace in the outer Solar System. Granted, the other planets rings seemed far less prominent than Saturn’s, but it was still hard to explain how all four could have such a singular event as had been suggested for Saturn.
A problem for planetary scientists was that even if the ring material could survive for much longer, it should be darkened by the planetary dust and micrometeorites coating the ring particles. In order to be as bright as the rings appear to be, they would have to be pretty young (at least as far as the Solar System goes!).
But, it appears that the rings may be old after all. Work by Dr. Larry Esposito suggests that perhaps collisions between ring particles could be constantly recycling the ring material, so that it always has a fresh surface. If that is true, then the rings may in fact be as old as Saturn itself, and they may last well into the distant future.
-Astroprof
Image courtesy of NASA, JPL
