Ring occultation

I'm sat in the first planetary rings session which is discussing the results from the Cassini mission. The only trouble is that the slides have been zooming past at such a rate that it makes it difficult to keep up when note taking.

Marouf presented data on radio occultations - measuring the radio signal from a distant object such as a star, as it passes behind the rings. As you might expect, the strength of the radio signal is reduced when it goes behind the rings as the particles reflect, absorb and scatter the radio waves. By measuring the change in signal, you can get an idea about how much ring material is in the way. If you watch with time, you get to measure different parts of the ring as the background object moves.

Cassini has been observing at three different radio frequencies (about 1 cm, 3.6 cm and 13 cm wavelengths) and each tells you about different sized particles. Basically, if the ring material has a similar size to the wavelength of the radio waves, there is more scattering and you detect less signal. So, comparing the three frequencies tells you about the distribution in sizes of the grains in the rings without the need to go there with a tape measure.

Let me try to summarize what I've just heard about the A, B and C rings. The A ring is full of dynamic structure with lots of millimetre sized particles near the edge. Cassini seems to have measured a different optical depth than Voyager did but I missed the reason why. The B ring is very dense. The C ring has more centimetre sized particles over the ring than A or B. There seems to be different physical structure at each of the three wavelengths in the C ring. That means that the different sized particles are distributed in different places.

Posted in astro blog by Stuart on Friday 09th Sep 2005 (08:58 UTC) | Permalink
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