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]

A quick, probably simple question... I wondered if this is a more or less correct guess: Are the E and G rings are kind of reflective or glossy like snowflakes? I see the way they are illuminated in the famous Saturn ring portraits and the bright points line up with the sun in the same way a glossy reflection would. The rings are of course made of many particles so the effect is diffuse and smooth. I circled what I am describing in the picture to ensure there is no confusion about what I am asking.

As a way to teach myself how to use Blender effectively I have been trying to copy various physical effects. I found that a good way to simulate the outer rings was with a hacked glossy material (I can't make a glossy volume...). My fake Saturn is here if you are curious. (If Björn Jónsson happens to read this, you are awesome and I thank you for providing all those image maps.)

[john_s]

"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

[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.