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.

[geckzilla]

"Glossy" is the wrong word- that implies a specular (mirror-like) reflection. What's going on in this image instead is that there are small particles in the G and E rings which become particularly bright when the sun is almost directly behind them (in the same way that smoke is most conspicuous when backlit by the sun). The parts of those rings that are closest to being exactly backlit are the brightest, and those parts are close to Saturn, because the Sun is behind Saturn.

John

That's what I thought at first, too. However, upon further reflection (har har) one must conclude that if that was the only effect at work, the shape of the highlights would take on a circular appearance which fades with distance from the light source rather than a vertical one which extends in brightness from the light source like a column. The sun is near the top edge of Saturn's limb (near the pole) in this picture. Its position is important to understand that the bottom portion of the rings are much brighter than one might anticipate for a circular halo extending evenly around the light source, even considering the variations in density. I do not understand why the bottom highlight is off center from the upper highlight but I suspect it has something to do with the probe's movement as it was making observations.

[geckzilla]

Considering it again, perhaps it is more easily explained by refraction of light through hexagonal ice crystals rather than reflection. The glossy material in Blender may mimic this effect but the physics aren't the same. Either way, I had not considered the optical properties of the ring materials before exploring this matter through the simulation. We know the rings are composed of ice so it makes sense that hexagonal crystals would form, right?

[geckzilla]

I spent a good number of hours today trying to figure out if my idea is unreasonable or ignorant. So far two people have told me that the E ring needs to operate something like a simple fog with the sunlight emanating evenly and roundly outward from the sun. I've searched and searched for some reference images other than the large mosaic and I've investigated the mosaic itself to understand better how much Cassini moved during the imaging process. The lower ring reflection does indeed coincide with the sun passing diagonally from the upper right to the lower left behind Saturn. That helps partially explain why the background portion of the ring is slightly brighter than the foreground portion and it also explains the movement. Moreover, I'm impressed with whoever put the mosaic together because it looks like a difficult job with lots of blown out white areas and yet the result is virtually seamless and hardly blown out at all as far as I can tell.

So my question is why do you think this, John? Is it just your intuition or is there some imagery I'm missing? Is there a previous discussion on this topic somewhere?

I'm feeling a very strong sense of deja vu as I write this post. I think I might just be nuts.

[hendric]

Geck, on Earth ice crystals falling through the air can get aligned to form sundogs etc. I imagine there could be forces aligning the crystals at Saturn, but I don't think the environment is that benign, so to speak - look at how quickly spokes dissipate for example.

If you take your arc and make a circle with the brightest parts, I can believe that it is just a partial halo around the sun. I suspect there may be distance effects also, which cause the nearer rings to have a different sun->ring->camera angle than the further side.

[Gerald]

Interstellar dust is thought to be polarizing (at radio wavelengths) due to precession along galactic magnetic field lines (http://www.rssd.esa.int/SA/PLANCK/docs/esl...0_Boulanger.pdf).
Such partial alignment might occur near Saturn, too, due to its magnetic field.
Depending on the size and shape of grains around Saturn there may occur several types of scattering, most promising may be Tyndall scattering, provided the grains are smaller than about 1 micron.
You may check, whether Tyndall scattering of partially aligned, asymmetric or needle-shaped grains can cause asymmetric scattering patterns.

[geckzilla]

Even randomly aligned ice crystals can create halo effects. I wasn't thinking they would be aligned but that would be neat if they did have any sort of alignment. I better look into more possibilities. Thanks for the suggestions.