http://www.planetary.org/news/2009/0921_Snapshots_from_Saturns_Equinox.html wrote:
Snapshots from Saturn's Equinox
By Emily Lakdawalla September 21, 2009
<<The lighting geometry in this image is extreme. It was taken just hours before equinox, mostly of the C ring, one of the dimmest of Saturn's main rings. The B ring's inner boundary is on the right; the C ring's Maxwell gap (demarcated by two bright arcs, which are bright because they rise above the ring plane) is on the left.
There is a periodic brightness variation in the image. The side lighting geometry means that brightness variations mostly result from changing slopes of the surface of the ring plane due to out-of-plane disturbances in the rings -- in layman's terms,
the rings here are corrugated like a washboard. Previous images had revealed these corrugations in the D ring (located off the view to the left). This image reveals that the corrugation extends beyond their origin in the D ring (which begins not far above Saturn's cloud tops), right through the C ring, to the inner part of the B ring, the densest of Saturn's rings, covering a distance of 17,000 kilometers.
This is a wholly unexpected observation that the science team is now working to understand. Based on the earlier information, scientists had speculated that a comet or asteroid may have collided with the D ring in the early 1980s to cause the vertical disturbance. This explanation fails to work now that scientists understand the true extent of the disturbance.
All they can say is that something happened in the early 1980s to tilt a vast region of the inner rings. Over the intervening years, the tendency for inclined orbits to systematically wobble (or "precess") at different rates, depending upon their distance from Saturn, has created a tightly wound spiral corrugation in the ring plane from the initial tilt to the ring plane.
That close-up view is part of this vast high-resolution panorama across the rings. It must be enlarged to be appreciated. Fifteen narrow-angle camera images comprise this panoramic mosaic. At extreme left, ringlets within the D ring appear bright, indicating that they extend vertically above and below the ring plane.
Next is the C ring. The fact that the edges of the C ring's gaps, and some of the ringlets within it, are bright indicate that they extend vertically above and below the plane of the rings. Some of the C ringlets also contain clumps,
which is a surprise.
Next comes the B ring. We haven't had such a fine view of its intricate corrugations and waves since Cassini entered Saturn orbit more than five years ago. In the outer B ring, bright spokes cross the rings. These consist of tiny particles electrostatically levitated above the ring plane.
Ringlets within the Cassini division are bright just like those in the C ring. Just beyond the Cassini division, in the A ring, is a beautiful corrugation, a bending wave (vertical disturbance of the ring plane) that originates from an orbital resonance with Iapetus. Iapetus is the most distant of Saturn's major satellites but also has a markedly inclined orbit, helping it to create bending waves in the rings.
In the outer A ring, the shadow of Dione shows up several times, as Dione was moving during the time that separated each frame of the mosaic. Apart from Iapetus, the major moons all orbit within the plane of Saturn's rings, so their shadows only cross the rings near equinox.
The ringlets within the Encke gap are very bright, again indicating that they are vertical disturbances. At the very outer edge of the A ring, between the Encke and Keeler gaps, are several spiral density waves. They are, again, bright because they extend above the ring plane, but not because of motion excited by inclined moons. Instead, they indicate places where the ring particles jostle so closely together that, squeezed from the sides, they have nowhere to go but up (and down) -- analagous to the reason that some mountains (like the Himalayas) exist on Earth.>>