Rings And Moons, A little picture Options

[Jyril]

In this image ringshine makes Saturn look almost shiny. Weird.

http://saturn.jpl.nasa.gov/multimedia/imag...2/W00008589.jpg

[JRehling]

In this image ringshine makes Saturn look almost shiny. Weird.

http://saturn.jpl.nasa.gov/multimedia/imag...2/W00008589.jpg

That's an interesting comment. There's some interesting math behind the explanation (which I'm contemplating hand-waving-style in my head instead of writing down). "Shiny" is the description of a "specular" (or mirror-like) surface, which reflects incoming light out at an angle reflected over the line perpendicular to the tangent plane (eg, billard ball -style bounces only). Lambertian surfaces scatter light completely in all directions (cardboard is pretty Lambertian; cotton more so). Every surface is somewhere on the scale between the two; you can probably count on clouds, like Saturn's, being highly Lambertian. Here, the lighting source is bizarre; the lit topside of the rings are visible over half of Saturn only, and the light incident to Saturn's surface from that ends up producing a spot of light similar to what you'd see on a specular surface that was lit uniformly.

I think it's an interesting area of applied mathematics to see when two different causes lead to highly similar physical effects -- should be its own field of study!

[Bjorn Jonsson]

This is not a specular reflection although it may look like it. What's happening is that there is a brightness minimum centered on the 'midnight longitude' due to Saturn's shadow on the rings. Also the brightness of the rings is strongly dependent on phase angle, thus further complicating matters and making things rather non-intuitive. There are some experimental computer simulations I did of this at http://www.mmedia.is/bjj/3dtest/index.html , including a simple cylindrical map showing the ringshine intensity for a solar elevation angle of 15 degrees above the ring plane.

[ilbasso]

Just saw this headline in the BBC news online: "Saturn rings have own atmosphere" - and it was discovered that the planet's rotation rate has slowed by 7 minutes since the Pioneer and Voyager visits. No news of this in the US media yet!!

Saturns rings have own atmosphere

Of course, there are other stories on the Beeb that don't make it into American press, either, like this one: Hedgehog numbers 'in nose-dive'

[alan]

The radio period is what has changed. It must not be as tightly controlled by the planets rotation as has been assumed. It doesn't seem possible that the rotation period of the entire planet has changed. Where would that much angular momentum go?

[volcanopele]

Just saw this headline in the BBC news online:  "Saturn rings have own atmosphere" - and it was discovered that the planet's rotation rate has slowed by 7 minutes since the Pioneer and Voyager visits.  No news of this in the US media yet!!

Saturns rings have own atmosphere

Of course, there are other stories on the Beeb that don't make it into American press, either, like this one:  Hedgehog numbers 'in nose-dive'

Both the ring "atmosphere" and the changed radio period of Saturn are old news stories, both nearly a year old.

The other one, on the hedgehogs, am I supposed to laugh or cry? I couldn't stop laugh regarding the tidy gardens comments.

[Rob Pinnegar]

Man, those English hedgehogs are in trouble.

Anyways: A while back we had a few posts in this thread regarding visibility of the G Ring in certain Cassini images. Here's another one along the same lines:

In the Raw Images section under "Telesto", the third image (as of July 2nd; the ID number is W00008506) seems to show a really, really faint crescent-shaped ring arc through the middle of the image -- it's so faint that it is only relatively easy to see on the thumbnail. The image was taken June 29th and the "openness" of the ring, in terms of angular distance out of the ring plane, seems to agree with images of the main ring taken at about the same time.

If it is a ring and not just some weirdly shaped "zero point" artifact, then presumably it is either the E ring or the G ring. It would be very interesting if it turned out to be the densest part of the E ring -- in all the E ring images I've seen, the ring only really seems visible when it is edge on. That in itself indicates that it might more likely be the G ring; but it doesn't seem to show the well defined inner rim that can be seen in some of the G ring images. (Of course, at that level of fuzziness, a sharp edge can't be expected to look like a sharp edge).

(Sometimes I really wish there was some optional program available at the Cassini site that could mark perpendicular distance from Saturn's rotational axis on these images -- particularly when dealing with narrow-angle shots of the rings. But I guess that, no matter how much stuff they put on the site, there will always be one more thing I'd like to see.)

[alan]

I got this from Saturn Viewer 2.5
http://pds-rings.seti.org/tools/viewer2_sat.html
time: 2005-JUN-28 03:20:00
viewpoint: Cassini
diagram center: Telesto
field of view: 1 ISS wide angle FOV
http://pds-rings.seti.org/work/viewer_sat_18376.jpg
E-ring is in the right spot, G-ring is at the right edge of the field
Perhaps volcanopele can confirm the time