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.

[Ian R]

Hi Rob,

What's the best way to get a copy of this paper on the D-ring? Is there an internet source where it can be located?

Speaking of the faint Saturnian rings, this is probably one of the best images of the G-ring ever taken:

http://saturn.jpl.nasa.gov/multimedia/imag...7/W00004084.jpg

[Rob Pinnegar]

Hi Ian,

That's a really nice shot of the G Ring. It has a very well defined inner boundary; I assume this must be due to a satellite resonance, but am not sure which one. Also there is a hazy band outside it that looks suspiciously like the E Ring, assuming it's not a glare artifact from Saturn and the main rings.

As for the Icarus paper, I did a quick look round the Internet and didn't find it anywhere, except for the abstract of course. I can give you the reference though: Mark R. Showalter, "Saturn's D Ring in the Voyager Images", Icarus 124, pp. 677-689 (1996). It's a pretty thorough treatment of the subject.

Do you have access to the Web of Science? If you have a university library card, and if your university subscribes to that journal, you ought to be able to download it from there. But Web of Science is sadly not available to the general public.

There are two good images of the D Ring in the Voyager images. The one taken by Voyager 2 is the one you'll normally find in Google searches because it has less smear. The nice thing about the Voyager 1 image though is that it includes the innermost part of the C Ring which gives a better idea of scale. I've attempted to attach that image to this post below (sorry if it doesn't work -- this is the first time I've tried attaching an image). Comparison with the recent Cassini image I've cited above shows the apparent changes in the outer part of the D ring.

 C3494650.BMP ( 626.05K ) Number of downloads: 911


If I didn't know better, I'd be tempted to suggest that D72 has "migrated outward" and merged with D73. However, that just doesn't seem physically possible. We do have to keep in mind here that there are changes in apparent brightness of ring components due to viewing-angle effects. That's probably a contributing factor.

Cheers
Rob

[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.

[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.

[Rob Pinnegar]

The following excerpt, from the last post I put up in mid-August, has turned out to be a load of bollocks. I was right in the first place (see today's new release of D Ring images on the main site) but was too dense to realize it. Buggeration!!!

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.

[Rob Pinnegar]

I've learned something interesting in the past week or so about the way that the inner boundary of the C Ring is defined.

This image shows the innermost thousand kilometres or so of the C Ring:


Intuitively you might expect that the spot marked by the blue X should delineate the inner boundary of the C Ring, which is listed in a number of places (including Murray and Dermott's book) as being 74658 km from Saturn's centre. Ignore the red X for now.

Now, several other well defined C Ring features also have known radii, including the Colombo Gap (the gap that houses the Titan ringlet), which is quite easy to identify and whose outer edge is at about 77880 km. Thus, if you can find an image for which the position of maximum apparent curvature is fairly well defined for these two ring features, you should be able to interpolate between them (neglecting perspective effects) to get rough radii of other ring features. Extrapolation is also possible, which was why I was trying to do this in the first place -- I was trying to use this technique to get the radii of the D72 and D73 ringlets.

I've done this to produce cross-sections through two Cassini images of the C Ring. To make sure that the horizontal axis is correct, we need to see if it correctly predicts the positions of boundaries between major ring features. There's a paper (Nicholson and Perrine) that contains a list of about 40 of these. Three are in the inner C Ring. This gives:


Here the black lines are cross sections through the two images. If the two green lines give the positions of the C Ring inner edge, and the Colombo Gap outer edge, then the red lines *should* be the positions of boundaries, if the interpolation is correct. Notice, though, that they miss the mark in each case. Thus the interpolation is wrong. This is not the correct horizontal axis.

I was completely confused by this until I got a clue from this image of the D Ring which was released on the Cassini web site a while back:


The caption of this image describes the green line as being the inner boundary of the C Ring. Notice, though, that it does not seem to lie at the position of the blue X from the first figure above. In fact, it seems to be at the position of the red X from that figure. So I decided to try running that interpolation scheme again, with the inner edge of the C Ring redefined accordingly. This gives:


Notice that now all of the red lines are landing at real boundaries in the C Ring. Thus it looks like those four little ringlets between the blue and red X's above, which are around 200-250 kilometres wide in total, are not part of the C Ring. They are actually part of the D Ring!

I can't understand why the inner boundary of the C Ring would be defined this way. It seems very nonintutive. Those four ringlets probably have ten times the mass than the rest of the D Ring put together.

For some reason, the D Ring continues to fascinate me. It would probably take an analyst to figure out why.

[Rob Pinnegar]

Just for the sake of completeness, I thought I'd include JPEGs of the two images that were used to produce the cross-sections shown in the last post in this thread:


Incidentally, the interpolation described above also gives an extrapolation that nails the positions of D68 and D73 bang-on, and also gives a radius for D72 that is similar to what has been reported by the imaging team. I'm looking forward to seeing their explanation of the 30-km periodicity in the structure of D73.

[Ian R]

Great work Rob! Due to the general lack of information on the internet with regards to the D-ring, this thread has been a joy to read.