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Even further away
Sep. 6th, 2005 08:39 pmJPL just posted a big haul of illustrations of new science results from the various Cassini instrument teams. One of the most interesting is the data on how the visible temperature of the rings, viewed in the infrared, changes not just with the lighting but with the angle at which the rings are viewed.
Unfortunately, the caption doesn't make it very clear how this could be, or how it relates to ring particle rotation. Think of it this way: the rings are made up of many small particles, and if the spacecraft isn't too close, each individual particle, like a little moon, is being seen from approximately the same direction relative to the Sun. If you were to look at the rings from near the Sun (say, from Earth) you'd only see the faces of the particles that were in full sunlight. From behind, you'd mostly see their night sides.
That (along with other things, such as the tendency of the particles to shade one another in the densest parts of the rings) makes the brightness of the rings of Saturn vary with viewing angle. But you can do this not just in visible light, but in thermal infrared, and effectively look at the temperatures of different faces of these tiny moons.
If the temperature of the rings seems to change when you look at it from different angles, that means that the temperature of the visible part of a ring particle is different when you look at it from different angles. And that in turn means that the particles are not spinning so quickly that the heating from the Sun gets spread out evenly over their surfaces, like a pig on a barbecue spit. Apparently this implies that, unlike the shepherd moons like Pan and Prometheus, the ring particles are not tidally locked to Saturn.
Unfortunately, the caption doesn't make it very clear how this could be, or how it relates to ring particle rotation. Think of it this way: the rings are made up of many small particles, and if the spacecraft isn't too close, each individual particle, like a little moon, is being seen from approximately the same direction relative to the Sun. If you were to look at the rings from near the Sun (say, from Earth) you'd only see the faces of the particles that were in full sunlight. From behind, you'd mostly see their night sides.
That (along with other things, such as the tendency of the particles to shade one another in the densest parts of the rings) makes the brightness of the rings of Saturn vary with viewing angle. But you can do this not just in visible light, but in thermal infrared, and effectively look at the temperatures of different faces of these tiny moons.
If the temperature of the rings seems to change when you look at it from different angles, that means that the temperature of the visible part of a ring particle is different when you look at it from different angles. And that in turn means that the particles are not spinning so quickly that the heating from the Sun gets spread out evenly over their surfaces, like a pig on a barbecue spit. Apparently this implies that, unlike the shepherd moons like Pan and Prometheus, the ring particles are not tidally locked to Saturn.