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.

[Ian R]

I have collected together the best images of the D-ring that I’ve been able to find, and labelled (where possible) the positions of the ringlets that Rob mentioned in his initial post to this thread:

- D68 ringlet (Red circle)
- D72 (Green circle)
- D73 (Blue circle)

I have omitted the D72 (Green circle) in some of the Cassini images, as according to Rob it seems to have either disappeared, or merged with another ringlet.

Two of the other Cassini images are absolutely puzzling and I cannot fathom them out. If anyone can help me identify what exactly is shown in the pictures labelled 'Cassini Puzzle', then I would be most appreciative.

Thanks,

Ian.

Attached thumbnail(s)

 

[Rob Pinnegar]

Hi Ian,

Your identification of D68 and D73 in the Cassini images agrees with what I've come up with so far. For what it's worth, here's my interpretation of the two "puzzle" images (sorry this is so long, but I thought I ought to explain my reasoning):

Puzzle #1 is actually "mislabeled" as an image of the D Ring because almost all of the bright ring material shown is actually the innermost 1,500 kilometres or so of the C Ring. The part of the C Ring that lies interior to the Titan ringlet is about 3,000 kilometres wide in total, and can be roughly divided into three ringlets that have about the same width. The outer two of these are the brighter ones, and they are separated from the third innermost one, which dominates the Puzzle #1 image, by a gap which is visible in the upper left corner. If you look at other images of the inner C Ring you should be able to identify this interior ringlet by its appearance, which is quite distinctive.

The gap in the upper-left corner of Puzzle #1 is the same one that shows up in the lower-right corner of Puzzle #2. Notice that the four sub-ringlets at the inner edge of the C Ring match up in both images; the other C Ring details are obscured in the second image because they are overexposed. This means that the fairly prominent D Ring component (the one with the two starlines through it) in the second image is almost certainly D73. Some of the faint components of the outer D Ring that are very clear in other images don't show up all that well in this one. I suspect that this may be due to differences in the illumination angle, which seems to favour dust at backlit angles and larger particles at forward lit angles. (I don't have Mark Showalter's paper handy at the moment, but he did mention this there.)

Some of the other images in the D Ring section of the Cassini website (N34711-N34714) show a bunch of narrow bright ringlets clustered together. Unless I'm very wrong these are close-ups of the D73 region. D73 also appears at the top of image N35232. There is a diffuse ringlet at the bottom of that image which also seems to show up in image N35235. Presumably this is the same diffuse ringlet that can be seen interior to D73 in other Cassini images? I'm still not convinced that it is D72 though. It seems to have the wrong width, and it just doesn't seem to lie at the same distance from Saturn, when compared with the Voyager images. Close, but... no banana.

Cheers and happy hunting,
Rob

[Rob Pinnegar]

Can't resist resurrecting this thread to point out the nice fully-processed shot of the D Ring which is currently up as the Cassini website's shot of the day. There are a number of features visible in this image that cannot be seen in any of the other wide-angle Cassini views (although they do show up in some of the narrow angle shots). The dust features are harder to see than in the Voyager images, but the resolution of narrow features is much, much better.

In reference to some of my previous posts here: I'm now pretty sure I was wrong about the D72 feature having completely disappeared. The broad feature at the inner edge of the "dark zone", which was visible in earlier Cassini images in addition to the new one, does in fact appear to be at the same distance from Saturn as the D72 feature from the Voyager images. What threw me off is that the core of D73 (the very bright ringlet in the lower-left part of the new image) is about 250-300 kilometres further from Saturn than I thought it was. Ooops.

In any case, things have changed in this part of the ring system. In 1980, D72 was way brighter than D73. Now it's the other way around.