Lord of the Rings

This morning I've been at talks covering the rings and moons of Saturn. As I said before, the rings are very dynamic places and some parts of them are constantly changing. There are so many interesting features, where do I start? Rings, ringlets, moons, moonlets, wisps, streamers, kinks, the list goes on. First, I had better give you an image showing the rings and moons, so that you have an idea where everything is.

Saturns rings
The rings and moons of Saturn CREDIT: NASA/JPL

The rings are made of lots of very small objects, as I said earlier, and are very well defined with sharp edges and gaps. That is why they get names such as A, B, F etc. One talk, this morning, discussed meteor bombardment on the rings. The idea is that a meteor would hit the ring from above and then throw material out over the ring - a bit like a splash. Computer simulations show that this actually sharpens the edges of the rings, although I'm not sure that I followed the reasoning behind that. Still, one consequence is that the icy-redish material of the rings gets polluted by dark material from the meteor impact and this gives a smooth colour change in the ring, even though it has a sharp physical edge.

Spokes in Saturns rings seen by Voyager CREDIT: NASA
Back in the time of Voyager, mysterious spokes were observed in the rings. These have not been observed by Cassini, so it is thought that they must be some kind of seasonal effect. Perhaps when the rings are aligned with the Sun getting less illumination, there is less charging of the ring by UV light. Nobody is really sure. But wait! Stop the presses! During the questions of one talk, it was revealed that the spokes may have just been seen by Cassini. Apparently, this was discussed over dinner the other other night. I can't wait to see those images.

Having said that the edges of the rings are well defined, I'm now going to slightly contradict myself. They are sharp but in some places they are wavy and there are streamers, channels and brightenings too. Quite a few of the talks have shown simulations of these structures which were created by simulating a moon and hundreds of thousands of small objects in the ring. The moon moves in a slightly elliptical orbit around Saturn, so will appear to move slightly closer and further away from the point of view of the ring. This means that there is a changing tugging by the moon on the particles in the ring and this perturbs the orbits of the ring particles. The result is that every time the moon gets near to the ring, there is a sort of gouging of the ring - a channel - and if the moon gets close enough it can create a trail of material out of the ring towards the moon - a streamer. This effect is seen in both the Encke and Keeler gaps (A ring) and also in the F ring.

The Keeler gap is around 40 km wide and is near the outer edge of the A ring. A moon in this gap going by the name S/2005 S1 was found in May and produces wavy edges (towards and away from the moon rather than out of the plane of the rings) about 5-15 km in extent. These ripples are large near the moon and damp down as you go further away. But they can last up to about 12 degrees around the ring in longitude - that is 27,000 km!

Posted in astro blog by Stuart on Friday 09th Sep 2005 (13:00 UTC) | Permalink
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