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.

[edstrick]

After the historically known A, B and C rings, rings were named in order of discovery. The D ring was claimed to exist in or the mid 60's based on photography of Saturn with the rings wide open some years earlier. We know know that the detection was almost certainlhy false, and the apparant inner gap between the suspected ring and Saturns' disk was probably a photographic artifact. But the claim stuck the name to that "spot" in the rings. The E ring was claimed, I think by Dollfus, in 1966 edge-on pics of the rings and was a valid claim. Then the F-ring was imaged and detected by charged particle absorption by Pioneer 11. and the G-ring by Voyater, together with the *REAL* D-ring.

Perfect consistency?.. who cares!

[dvandorn]

Besides, while each of these classic ring segments share some gross characteristics, they all show a certain amount of differentiation within the ring (especially in the large rings) and are made up of thousands of "ringlets." After having seen how many thousands of ringlets exist, it seems rather silly to speak of Saturn having "seven rings" (A through G). Maybe we should come up with another nomenclature -- something like "ring domains."

After all, there is precedent for changing nomenclature when we get higher resolution images of planets. Just look at Mars -- while some of the current naming conventions pay homage to the old names based on telescopic observations of the albedo patterns, the modern names are all rather different and now refer to the landforms we've only discovered in the last 30 or so years. If Sinus Meridiani can be transformed into two regions, Meridiani Terra and Meridiani Planum, then why can't Saturn's A ring be redesignated "Ring Domain A" with sub-domains reflecting variations in structure and composition?

And, just remember, for those who wax lovingly over the "olden days," we'll always have the Encke and Cassini Gaps...

-the other Doug

[Rob Pinnegar]

[quote=dvandorn,Jul 19 2005, 04:19 AM]
After having seen how many thousands of ringlets exist, it seems rather silly to speak of Saturn having "seven rings" (A through G). Maybe we should come up with another nomenclature -- something like "ring domains."

There are probably a variety of ways that could be done. For example, I sometimes think of the three inner bands of the C ring (interior to the Titan ringlet) as the "C Minus" ring. There are places where that works a bit (C Ring outside the Maxwell gap as "C Plus", the darker inner part of the B ring as "B Minus", A ring outside the Encke gap as "A Plus" and so on).

I sure don't expect to see anyone else using this though. It's way too informal, and seriously limited. If you designate the outer third of a ring as "plus" and the inner third as "minus" how do you differentiate the middle third from the ring as a whole? Do you call it "C-Zero"? Yeccch. Besides that, it artifically assumes that rings are divided into thirds. It's okay as pet notation, but doesn't make the cut as _real_ notation.

My guess is that ring structures would be least ambiguously defined by (1) basic nature (ringlet vs. gap) and (2) radial distance from Saturn, though ellipiticity could throw a wrench into the second of these. So a ringlet at 73145 km would be R73145, a gap at 85000 would be G85000 and so on, possibly divided into sub-kilometre scale using decimals, i.e. R112730.3. But this would be really confusing for nonscientists, for whom the A-G system is probably best.