Faint Ring Thread, Saturn's D, E and G rings Options

[Rob Pinnegar]

There are two new "Raw Images" up that give a good view of Saturn's D Ring. As of today (July 17th) they are on the first page of the Raw Images section. The better of the two is image number W00009347.

The very narrow inner ringlet is called D68 and it is the innermost well defined ringlet of the entire ring system -- it's only about 7250 kilometres above the cloud tops, about half-way from the planet to the inner edge of the C Ring. If you search the "Saturn-D Ring" section of Raw Images, there is a nice narrow angle view (N00035241) which I am pretty sure is a close-up of D68. D68 is an oddball, it really is sort of "in the middle of nowhere".

The brighter ringlet in the upper right is called D73. About a thousand kilometres inward from D73, there is a noticeable "dark zone". In the Voyager images, there was a third bright narrow ringlet inside this zone, D72, which seems to be gone now, strangely enough. The relevant Voyager images are Voyager 1 image 34946.50, and Voyager 2 image 44007.53. If the diffuse ringlet at the inner edge of the "dark zone" is what is left of D72, it looks to have migrated a bit closer to Saturn in addition to spreading out a lot. (By the way, I'm not making up these ringlet designations on the fly -- they are given in a paper by Mark Showalter that was published in Icarus in 1996, which is pretty much the only major paper on the D Ring.)

To give some idea of scale, the three bands of material in the far upper right corner are part of the innermost ringlet of the C Ring (this can also be seen on some images of the rings taken on May 3rd of this year).

Since it is so faint and doesn't appear in many images, the D Ring rarely attracts much attention. But it's kind of neat to look at if you haven't seen it before, particularly because of D68, which is sort of the "anti-F ring" in a way.

[volcanopele]

Thanks for the treatise on the D-ring. Are there any known causes for the D68 ringlet, what controls it? For the F ring you have Pandora and Prometheus, but I wonder what keeps D68 confined.

[Rob Pinnegar]

There is some discussion of confinement mechanisms in the 1996 Icarus paper. Apparently D68 doesn't really need to be confined because its has so little mass. It seems that the rate of viscous spreading varies roughly linearly with opacity, and D68's opacity is very low. It can stay the way it is for eons without help from an outside source.

There are a couple of satellite resonances in the vicinity of D68 (one is the 3:1 Atlas resonance) but they are so weak that they probably aren't actually having any confining effect. It's also possible that the internal oscillation modes of Saturn itself could be involved in the D Ring structure -- in theory, these can play a "shepherding" role very similar to that of satellite resonances. (I don't claim to understand this, and am just trying to summarize part of the paper.)

I should probably add that my identification of the features in the new Cassini images as being the D68, D72 and D73 ringlets from the Voyager images is based entirely upon my own amateur analysis. I did my best to be careful, but the fact that my interpretation requires D72 to have virtually "vanished" over the last 25 years does make me wonder whether it is entirely valid.