[X] My Assistant

[volcanopele]

The ISS team has officially announced the discovery of a new moon, 7.5 km across, orbiting within the Keeler Gap. They have released two new images and a movie related to this announcement.

Discovery of the Wavemaker
http://ciclops.lpl.arizona.edu/view.php?id=1071

Wavemaker Moon
http://ciclops.lpl.arizona.edu/view.php?id=1074

I have this combined in a post on my blog as well:
http://volcanopele.blogspot.com/2005/05/mo...p-moon-aka.html

[MizarKey]

Why does Daphnis cause ripples ahead and behind but on opposite sides of the rings. In Latest wavemaker image you've got waves on the lower left (inner) ring and on the upper right (outer) ring but not on the upper left inner ring or lower right outer ring. I get turned around in these images...is the orbit counterclockwise in this image, so the trailing wake is the inner disturbance?
Wouldn't Daphnis' gravity tug uniformly?
Does Daphnis have a synchronous rotation about Saturn?

[pat]

Why does Daphnis cause ripples ahead and behind but on opposite sides of the rings. In Latest wavemaker image you've got waves on the lower left (inner) ring and on the upper right (outer) ring but not on the upper left inner ring or lower right outer ring. I get turned around in these images...is the orbit counterclockwise in this image, so the trailing wake is the inner disturbance?
Wouldn't Daphnis' gravity tug uniformly?
Does Daphnis have a synchronous rotation about Saturn?

Its all to do with differential motion.

The particles at the ring gap edges feel the greatest force due to gravitational interaction with Daphnis at the point of closest approach i.e. when they are at the same longitude as the satellite. Yes they are affected by Daphnis' (as well as by everything else in the Universe's) gravity all the time but the effects are neglible. Its only when they are at very near closest approach to Daphnis that there is any discernable effect.

Using the impulse approximation is a very good model for the interaction. The ring particles receive a 'kick' at their closest approach, there is an exchange of angular momentum and the ring particles' eccentricty increases. Its this increased eccentrcity that results in the observed sinusoidal edge waves that we see on the gap edges.

Now remember that the particles at the inner ring edge are orbiting 'faster' than Daphnis i.e. their angular velocity is higher. These particles approach Daphnis from 'behind', recieve a 'kick' while passing the satellite and then move away from Daphnis in the prograde (direct) direction. So on the inner gap edge you see the edge waves 'ahead' of Daphnis while the material 'behind' the satellite hasn't passed the satellite yet, hasn't receive a 'kick' so you don't see edge waves.

The material at the outer gap edge has a lower angular velocity so Daphnis 'catches up' with the ring particles from 'behind', the particles receive their 'kick' and then fall behind the faster satellite. At the outer gap edges you see the edge waves 'behind' the satellite since the particles 'ahead' of Daphnis haven't passed the satellite and haven't interacted with it yet.

Of course all particles at the gap edges 'pass' Daphnis many times over the course of time but the observations show that the edge waves soon damp out due to interaction between the ring particles themselves so effectively they have no memory of their previous encounter when they next interact with Daphnis.

In a nutshell (and not all that well explained :-) ) thats why you only see the waves 'ahead' of Daphnis on the inner gap edge and 'behind' on the outer gap edge. The inner edge wave is the 'leading' one and the outer 'trailing'. Currently the sunlit side of the rings is the south face so in the images we are looking at the rings from 'below' -- so the prograde orbital direction in the image you reference is clockwise. In textbooks diagrams of the orbits of objects in the Solar System are (almost) always shown as viewed from the north hence prograde being anti-clockwise.

I think that synchronous rotation is assumed -- there just aren't enough observations yet to be sure but its a good bet that it is.

pat