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.

[Rob Pinnegar]

There are quite a few new images of the D Ring now up on the Cassini website. Since we're so close to the ring plane at the moment, the rings have a very "foreshortened" appearance. However, D68 and D73 are quite visible, as are the fuzzed-out remains of D72.

One of the interesting things about these new images is that, if you look at some of them closely (particularly the ones for which parts of the rings are hidden by Saturn's shadow) you can see vertical "bands" outside the main rings that look very much as if they might be the E and G rings. These could of course be artifacts, and it wouldn't surprise me at all if they turned out to be just that... but if they really are rings E and G, this would probably be the first time we've seen all seven main rings in the same image. I hope this isn't wishful thinking on my part.

Since Cassini's first images of the D Ring came back from Saturn more than a year ago, these new images may give some idea of whether there have been further changes in the D Ring during the Cassini mission. (Hopefully, at least.)