Here’s a conundrum for you: why do Saturn’s rings have such sharp edges?
It’s question that has puzzled planetary scientists for many years. Various ideas have been put forward but none adequately explain the structure we see today.
To understand just how sharp the edges are consider this: pictures from Cassini show that the density of particles at the edge of the outer B ring, for example, drops by an order of magnitude over a distance of only 10 metres or so.
That’s extraordinary given that the ring is 25580 kilometres wide.
A few back of the envelope calculations show how strange this state of affairs is. The ring contains particles ranging in size from 10 metres to less than 1 cm, with distances between them of only a few metres.
You’d expect these particles to be colliding at a fair old rate and just a hundred collisions per orbit should create enough debris to blur any sharp edges between rings within a few orbits.
And yet Saturn’s rings are some 10^12 orbits old.
Now Dima Shepelyansky at the Université de Toulouse in France et amis think they have worked out what’s going on. They say that the moon Mimas exerts a gravitational force that “kicks” the rings every time it orbits causing the motions of the particles to become synchronised with Mimas and each other.
The result is a dramatic reduction in the number of collisions between particles, far less debris than expected and hence a much lower tendency for the rings to become blurred.
They’ve even simulated the sharp edge effect with a computer model of 1000 particles.
As for physical evidence in favour of this idea, the team points out that the outer B ring has an epicyclic frequency that appears to be in resonance with the frequency of Mimas’ orbit.
That’s exactly what you’d expect but they’ll need more data from other rings to make certain of their claim.
That should be forthcoming from Cassini data. So we should know one way or another soon whether Shepelyansky’s ideas hold water.
Ref: arxiv.org/abs/0812.4372: Synchronization Mechanism of Sharp Edges in Rings of Saturn