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]

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.