 | | Shepherd Moon - This unprocessed image shows the moon Prometheus (near bottom center) at the edge of the thin F Ring of Saturn. The image was made on July 1, 2004 as Cassini passed just outside the rings just after going into orbit around saturn. A ring particle near Prometheus must orbit Prometheus with a certain period, and orbit Saturn with another certain period. This turns out to only be possible if the ring remains a certain distance from Prometheus all the time. It appears that the ring is not in orbit around Prometheus, but imagine standing on the little moon and watching one grain of dust in the Ring. As you orbit Saturn, the dust orbits at a slightly slower rate so it appears to be going around you in the opposite direction that you are going around Saturn. After a great while, you "lap" the ring particles, but that means they have gone all the way around you once. This requirement of being "co-orbital" contains the ring to within certaindistances from the moon. The moon is called a Shepherd because otherwise the ring particles could drift out into a wide band. |
 | | A-Ring Resonant Wave Mosaic - LARGE FILE, c 700K - This 10-image mosaic shows the outer part of the ring apse in a band the center to the left edge of the Prometheus image above. This image is just plain full of stuff. It is the most crudely made mosaic of all of mine. The spacecraft was moving quickly and dropping below the ring plane as it took these images. (I'm using up = north, not up = sunlit side. It's southern summer on Saturn and the rings are illuminated from the south.) It is very difficult to make the images match up against my template, which is the lower resolution image behind the mosaic frames. Novertheless, the sequence of waves can be seen in some detail. Waves are not ringlets, but appear to be. They are tightly would spirals caused by resonance with satellite orbits. When the chunks and dust that make up the ring are orbiting with a period that is nearly a simple fraction of the orbital period of one of the moons (such as 1/2, 3/5, or 10/11) then they always catch up to the satellite at nearly the same place in their orbit and therefore are subject to a persistent extra gravitation in a particular direction, which perturbs their orbits. The net effect of this on the collection of objects making up the rings is spiral waves. Since the orbital period is controlled by distance from Saturn (Kepler's Law) distances at which orbits resonate are marked by bold waves. The big players that make bright waves here in the A Ring are the moons Mimas, Prometheus and Pandora. The Encke Division, which is the large gap to left of center, is cleared by a tiny satellite, Pan, that orbits within it. Pan is also responsible for the "wake", an obvious spiral pattern and wavy boundary on the inner edge of the Encke Division. You can see the wake and bright "ringlets" just interior to it "overlap", which is a demonstration that they are waves in a sheet, not true ringlets. |
 | | Titan - This image from July 3, 2004 displays 727 nm near infrared light as red, 619 nm orange light as green, and 343 nm near ultraviolet light as blue. The brightnesses in these colors approximate those in real red, green, and blue, so this is a near true color image. There is a NASA image using these colors, but I have altered the color balance to more reflect the whole of Titan color data and also so that the mean color approximates that I have seen on Titan in telescopes. The orange color is caused by hydrocarbons that absorb blue light. The violet upper haze is the diffuse outer limits of this atmosphere, where the only effect on color is the scattering of blue light similar to what happens in Earth's atmosphere. |
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