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]

In the news today... some new theories about a possible recent collision between the D Ring and a Saturn-crossing interloper:

http://ciclops.org/view.php?id=2281&flash=1

There is one thing here that surprises me: namely, that the authors haven't tried to connect their postulated ~1984 impact with the atrophying of the D72 ring feature. To summarize some of the earlier posts in this thread, D72 was very bright when the Voyagers passed Saturn, but it has drastically faded since then, and what is left of it also seems to have moved a few hundred kilometres closer to Saturn. So there's been some radial movement of material since 1980/81.

Well, since the proposed impact date of 1984 is just an estimate, it stands to reason that it could be off by a few years. If that's true, it's very tempting to think of the D72 feature seen by Voyager as being the initial result of that collision. D72 could have been brand-new when the Voyagers passed by in 1980/81, maybe only a year or two old. A recent origin would help explain how it disappeared so fast... it was a transient feature that we just happened to catch at its brightest.

The changes in the spacing of the ringlets in the outer D Ring also seems to be connected to D72. The current spacing is 30 km, but apparently was 60 km in 1995 when Hubble took some observations of this region. In 1981, it was 130 km... and D72 clearly marked the demarcation between the outer D Ring and a longer-wavelength region in the inner D Ring. Showalter's 1996 paper in Icarus shows this. (It's odd that these three values of spacing are nearly in the ratio 1:2:4. This could be coincidence, but it makes me wonder whether the 1981 and 1996 estimates were limited by the resolution of Voyager and Hubble; perhaps these were picking up subharmonics of the main pattern?)

There's no way the authors aren't aware of these points. The Showalter paper is the only major D Ring paper out there, so it's *gotta* be one of the key references of their work. That means there has to be some good reason why they aren't making a connection with D72 in the press release. Presumably we'll find out what that reason is when the paper gets published.