RAW TRANSCRIPT – NOT YET REVIEWED FOR CORRECTIONS BY CASSINI PERSONNEL

FTS-NASA-VOICE

Moderator: Trina Ray
July 31, 2007
1:00 pm CT

Coordinator:	As a reminder, all parties will be on open lines so please use your mute button. If you do not have one, you may press Star-6.

	This call is being - also being recorded. If you have any objections, you may disconnect.

	Ma’am, you may proceed.

Woman:	Thank you.

	Okay. Welcome to the July 2007 CHARM Telecon then I’m pleased to introduce Colleen Mitchell and John White. They are both associates from the Cassini-Huygens here at Colorado. They both have received PhDs at the University of Colorado in Boulder. And since then, John is working on primarily simulations of ring and moon-ring interactions, and Colleen works primarily on D ring spokes. And today, they’ll be telling us everything we wanted to know about the rings to Saturn and their current (unintelligible).

	So, why don’t you guys take it away? I think are we starting with Colleen or John?

John White:	We’re going to go John, Colleen, John, actually.

Woman:	Okay.

John White:	To kind of change about constantly.

Woman:	Excellent.

John White:	So if you get bored with hearing my voice, at least you get a break in the middle.

	Okay. So this is John White. Moving on to the second slide which is the title slide - and let’s start by telling you about some of the smallest moons in the Saturnian system which are embedded inside of the ring and then sort of what we can learn from their interactions with the rings about these moons.

	So, go to the next slide.

	There is an image here at the A Ring, so the outer portion of it. And you’ll notice there are two dark bands in the middle of the A Ring. Those are almost completely evacuated gaps.

	So usually, you’ll find gaps like that in the ring. There’s a moon involved somewhere. Either the moon is inside that is our - in these two cases or the moon is elsewhere somewhere with the resonance in there that actually got open the gap.

	In this case, as I said, there are actually moons in these two gaps. Just for references purposes, the left-hand gap here, the big arrow on, this Encke Gap, the smaller one further to the right Keeler gap.

	So, moving on to next slide, here’s a close-up of Encke Gap, and you can see - first of all, it’s not completely empty, there are actually are a couple of ringlets in there which are probably due to interaction with the moon that’s embedded in there, the moon’s name is Pan.

	But you’ll also see on the left-hand edge there, at the inner edge, there is kind of a wavy pattern. That is the result of the particles moving past the moon, and as they come pass the moon, the moon gets in the kick and makes the orbit a little bit more eccentric, so they move inward and outward to the planet and it kind of wave like that.

	The reason they pass the moon, of course, is because of Kepler’s Law. Kepler is - if you’re moving - if closer to the planet, it move faster, if farther to the planet, it move slower so the moon - the particles on the inner edge of the ring pass the moon and the ones at the other edge actually gets fall behind it. In both cases, you actually get waves that are more or less symmetrical but they have it on opposite side of the moons.

	Waves like this were actually used - in fact, these are exactly are used to find Pan back in the Voyager data as a matter of fact. This is rather amazing but effective work. There was a time with a single pic from the Voyager images which is just barely detection, I think, by most standards. But it’s really there, we have images of it now so…

	Just a little close-up of the Encke Gap on the next slide here, you can see there’s a lot of structure here. There are kind of these graded straw features on the very edge of the gap. These are due to the moon pushing the ring, making a compression to expand constantly, it expands, it forms these long (wave) structures.

One of my fellow grad students at CU actually is doing simulations on these and was gratified to see that the simulations actually predicted what they thought. So, every now and then we get something right in the theory side.

Woman:	Can I interrupt with a quick question…

John White:	Go ahead.

Woman:	Is Image A a zoom-out of the one on Slide 4?

John White:	Of Slide 4? No.

Woman:	Okay. Can you tell us how it is relative to each other?

John White:	I’m sure the geometry is accurate. It’s entirely a different image. I think A is an approach image actually…

Woman:	Okay.

John White:	…from approaching the planet.

Woman:	But the dark band in the middle is the Encke Gap?

John White:	Pardon?

Woman:	But the dark band in the middle is…
John White:	Oh, sorry, yes. The dark band is the Encke Gap.

Woman:	And on Slide 4, that whole entire dark band is kind of interrupted…

John White:	Right.

Woman:	…many times is the Encke Gap.

John White:	Exactly.

Woman:	So- okay. Got you.

John White:	The geometries are completely different in these two images.

Woman:	So in Image A, you can’t see the bright bands but we can see in the 4 because it’s zoomed out?

John White:	Right.

Woman:	Yeah. Okay. Got you.

John White:	Okay. So moving on to the next slide, on slide of Keeler gap, when we first actually got into Saturn orbit, right away they noticed a lot of weird features in the edge of the Keeler gap. You can see these little (whiffs) that are labeled A, B, C, D, E, F all through J. Apparently, we didn’t have a very good naming here going.

	That’s sort of indicative of a moon being there. It’s hard to make those (whiffs) to help them continue without just completely smearing out into the background ring.

	So, there’s always been a strong position that there was a moon here. Actually, before Cassini got into orbit, there was, I mean - they always asking question how does that gap remain opened and doesn’t close up when particles just sort of gather out into it? But it took a while to find the moon.

	Two years ago however -- you go to the next slide -- we found it and there’s the moon Daphnis that used to be known 2005 S1. I think that is probably a better name. And you can see the edge waves in both sides of the moon that we use to help find it. It’s not much more of a single (pixel) image but we do have enough edge wave information here to start asking some questions.

	Going on to the next slide - this is where I get kind of text-heavy for a while. The question I’m going to ask here is how can we determine the mass of an embedded moon on this gap moon somewhat affected in the nearby ring? The mass of the moons are actually pretty interesting for us, and I’ll explain a little bit later, the last third of the talk in fact will be largely devoted to some studies of this.

	I wanted to narrow when started into this, if the moon can (do) the time variable structure or are they’re always going to be the same no matter we take the image. This is important for sort of planning observation because it’s going to be time variable. It means we have to come back to the same moon repeatedly and try to take images in different sort of situations but its not going to be changing much, we really don’t care when we get the images, we just took the best images we can get a couple of times and sort of move on. So that is actually significant question sort of from the planning end of thing.

	And kind of going hand in hand with this is how does the eccentricity of the moon change the mass estimate and what about the particles that are eccentric when they come in, what if they didn’t come in in circular orbits either?

	This again is important. If you go when you take a single snapshot and you think you’ve estimated the mass of the moon but the whole edge wave behavior depends on sort of when you take that image, you can be fooled. So again, sort of (unintelligible) need to come back a lot of times to take these images repeatedly?

	Moving on to the next slide is I believe the only math in the entire talk, I hope.

Man:	I didn’t do that.

John White:	Okay. Good.

	Only math in the entire talk, I promise.

	This is the sort of standard analytic formula you would use to determine the mass of the moon, and I’ll walk you through it quickly.

	Moon is the ratio of the mass of the moon to mass of Saturn, sort of just a standard way we (unintelligible) these things.

	AE is the amplitude of edge wave, so, 5 kilometers, 1.5 kilometers, whatever it’s going to be.

	The Delta A is how far the moon is from the edge of its own gap. So in the case of the Keeler gap, it’s about 17 kilometers and the case of the Encke Gap in Pan is about 160 kilometers away from its edge. So there’s a big difference. You know what, it’s squared so that comes in pretty strong as a factor.

	And then AQ or the A is the - is some of the major access to the moon’s orbit as about usually is. That’s also a pretty strong factor on all of it.

	Just a quick sanity check , this all makes sense. First of all, if the amplitude goes up, mu goes up, that makes sense, you get a bigger wave for a bigger moon. Delta A goes up and the altitude goes up. The moon gets farther away, you’d expect you need - we have to be more massive to make the same wave. And if you move farthest from the planet, the planet (unintelligible) is smaller, so it takes a smaller kick to make a big wave. So (unintelligible) of all those tracks.

	2.24 is just what you get when the dust settles from a lot of algebra but it doesn’t really signify any - particularly intuitive of the object.

	Now this is a nice analytic expression. We like these things. It’s completely linear on the amplitude, the ae, which means you double ae, you double the mass, so it’s nice and predictable.

	It uses, however, an impulse approximation which means you just assume the moon gives a little tiny kick to the ring particle only at closed approach essentially.

	But this means, first of all, the moon cannot really change the orbital (unintelligible) access the particles to come by, so they always stay on the same location. It just induces the slight eccentricity so they wave in and out quietly.

	This also assumes that the particle to the moon are initially in circular orbit which may not be a very good assumption as it turns out for a lot of these moons. The particles, it might be a little bit better. Ring particles tend to have very small eccentricities, although you never really know.

	So moving to the next slide, here’s what we decided to do. We kind of look at all these assumptions so, you know, this have a little bit to arrive 20 years ago, we got really fast computers actually around these days that are easy to set up to do these kinds of things. So we’re just going to use the computers to integrate the orbit of particles as they pass the moon. Just put all the equation to motion, see what happen.

	What this means is we can see ourselves from some of the assumptions. The particles in moons for instance can come in on the eccentric orbits that they want. Arbitrary space -- space here just means where you are in your orbit, zero degrees space is closest to the planet, 180 is farther to the planet, and just circle around.

	The particles, however, do not interact with each other in our simulations. Assumptions are always being made for these kinds of things. This is for speed, to do a full-up collisional self gravitating ring simulation takes a lot of computer power, and we didn’t quite want to do this, at least for the initial survey that we’re doing here.

	Okay. So moving on to the next slide and we’ll show the results for circular moon and circular particles, initially circular particles, some we encountered in Pan.

	Remember, the Pan is the one in the big Encke Gap there. It’s pretty far from the edge of its ring. And you see two lines, the solid green line is the result of our simulations and red dash line is the result of the impulse approximation, a formula from earlier, and why they are pretty close together and there’s always slight deviation.

	If you look at the dotted line, way down on the lower left-hand corner, starting that 1.6 kilometer amplitude, going over (unintelligible) and down, they give the estimated mass of Pan based upon the two formulas. And you can see they’re pretty close together, in fact, they’re only about 3% difference.

	When I first saw this, my first reaction was “Wow, that really made this entire thing not worth doing. Three percent, you know, is actually, well, less than the measurement error we have going on. So, it didn’t really improve anything for us.

Woman:	What the - what’s the units on your mass there?

John White:	It’s ratio to the mass Saturn actually so…

Woman:	Oh, okay.

John White:	Mass kilogram just multiplied by 5.68 times 10 to 26.

Woman:	Okay.

John White:	I carry that number around so that…

	If we move on to the next slide, Daphnis shows a rather different situation here. Daphnis deviates quite a lot, the lines mean the same thing this time, first of all, the amplitude of that wave is 4.5 kilometers so it’s about three times larger from Pan. Remember that it’s also about ten times closer to its edge.

	The difference between the two approximations or the two mass estimates in this case is about 35% even after physics. That’s a fairly significant amount.

	Now, we’re going to change our density estimate of the moon by 35%, which can be a lot when you’re hovering around 1, so hovering about by 35% (unintelligible) to something that’s got a lot of (unintelligible), for instance.

	So, clearly in this case, the sort of analytic approximations didn’t work very well. The reason why, in that respect, isn’t very surprising is because Daphnis is so close to the edge of the terrain. It’s - the edge is sort of in Daphnis’ chaotic zone. When you get that close, you get these massive kicks from the moon and the sort of linear series completely breaks down.

	Like I said, it’s not shocking in that respect but it was good to actually see the Pan out and show a result that actually meant something, I mean the whole having - worth having done.

Woman:	So the eccentricity that you are now accounting for that was getting assumed out before is important?

John White:	So far, we haven’t put any initial eccentricities of the either body. This is just looking at what happens on circular orbit.

((Crosstalk))

Woman:	Okay. I thought you said particles (unintelligible) eccentric…

John White:	(Unintelligible) the very in the next slide. We’re going to…

Woman:	Okay.

John White:	…sort of relax that assumption. This is just the first half.

Woman:	Okay.

John White:	We have all the analytics theory works in sort of the area where it supposed to work. And the answer is, for Pan, it works quite well, and for Daphnis, it doesn’t work very well at all.

	And as you get the smaller and smaller moon, it probably going to get worse and worse. This - we kind of figure what’s going to happen.

	So, moving on to the next slide, let’s go ahead and relax that assumption. We found out what does eccentricity do to these things. So the following one, we’ll get either the moon or the particle and eccentricity initially. So those even not come in and circle the orbit anymore. Always one of them will be circular. I’m not going to free up both at the same time because I just lead to all kinds of madness.

	The eccentricity I’m getting to both is one and the same in both cases - in these cases. So when you kind of compare apples to apples here to give the ring particles much more eccentricity, you don’t know - really know what you’re comparing anymore.

The variations in the change in position, the (exes), the change in separation from the moon and the ae, the amplitude of the variation, in this case, there’s not going to be amplitude to the edge wave anymore, and I’ll explain why in a minute. But just - can’t trust it completely anymore to be that.

	So moving on to the next slide, here what Daphnis does if it’s eccentric, I’ve given the eccentricity that the current best estimated one from spacecraft navigation estimates. It’s got actually pretty good eccentricity. It moves in and out by plus or minus almost five kilometers. So when you're in 35-kilometer wide gap, that’s actually a pretty substantial variation right there.

	So, the solid line shows what happened with the moon’s eccentric and the dotted line is what happens with the particles coming in eccentricity.

	So, I’m going to actually reverse the order here and start at the bottom plot. See what happens to the particle position relative to the moon? It turns out that these two curves -- the solid and the dash curve - the dotted curve in the bottom -- actually pretty much is the exact same curve just shifted at 180 degrees apart.

	In theory, that’s exactly what you predict. What only matters is sort of the relative position, how close they get to each other when they encounter each other not which one has the eccentricity.

	But you’ll do notice that the particle get pushed back quite far in some cases, almost 15 kilometers. Remember, the analytic theory said that this shouldn’t happen at all. And this is kind of totally as part of why the analytics theory broke down. It didn’t take us to account, and that’s an important factor in this case.

	The top plot is actually a far more confusing and mysterious. I cannot actually fully explain all of this behavior, at least not yet.

	First of all, these thoughts are not identical if you ship them 180 degrees. You can kind of tell by looking at the minimum areas. One has got a broad, shallow minimum; one has got a little dip at the bottom. They do have about the same maximum amplitudes, however.

	What you’re seeing is that there’s a huge variation across sort of the encounter phase that the moon or particles as closed approach to Saturn or farthest approach to Saturn or what with ae, the amplitude that they could get, it goes from a little over 20 kilometers but down around 2 or 3, that’s the factor of 10. That’s the factor of 10, the mass estimate, if you just saw that edge wave right there, and then that is very significant.

	Again (unintelligible) my Astrophysics friend, even in their field, a factor of 10 matters, and certainly in planetary science, it does.

	Okay. So moving on to the next slide, this actually is an important point. As I said, we have measured a pretty large eccentricity in Daphnis. This image was taken last September from Cassini. And you can see Daphnis is in the middle and there are edge waves on both sides. The edge wave on the right-hand side has a pretty large amplitude. The one on the left is almost negligible. It’s almost impossible to see there is an edge wave there for sure.

	Clearly, it’s (unintelligible) different edges differently.  If you remember from the Discovery image I showed, at the start of the talk, things were pretty symmetric at that point but clearly there is a time variable change here going on.

	So now I’m going to make things even more confusing for our perspective. If you go to the next slide, why is ae no longer amplitude in this case? Well, before, the particles orbit the exact same kick and so they move in the same (sinuous) little pattern.

	So if you took a snapshot, at different times, the only thing you’d see different would be exactly which particles were where. They all follow the same path.

	They no longer do that. If you take a snapshot, you get all kinds of particles that encounter the moon in different phases. This complicates things, but if you go to the next slide, I’ll show you how.

	You see four different slides showing, this plot, showing situations when the moon is at four different phases, these four different times around the orbit on Saturn, if you will. This is what would happen with stream of particles, as they came from the encountered Daphnis.

	You’ll notice in the bottom right-hand plot, there’s a large amplitude of this wave. It’s about 75 kilometers total or maybe 20 kilometers total, top to bottom with slight above that, the one that is 90 degrees, it’s much smaller, about 5 to 10 degrees or 10 kilometers rather.

	And you notice these things are no longer the smooth (sinuous) (unintelligible) these things didn’t even cross each other which strongly implies this particle should be colliding. So remember the assumption I made the particles don’t interact, that was maybe not a very good assumption for Daphnis, it turns out and the way to admit the shortcomings here.

	It gets even worse. If you go to the next slide and you actually allow for a whole swath of particles to come in, not just a single stream but sort of a ring of them to move in, you can see there’s a lot of intersections between these pads. Clearly an (N body) approaches (unintelligible) but you can also see why ae isn’t really the amplitude, the edge wave. So if these things cross inside of each other, they’re actually cutting through the valleys and the peaks and making them look smaller.

	So if you took those image like in the lower-hand corner, you wouldn’t actually see the full amplitude of the largest wave, you’d actually see something much smaller, therefore, you estimate a smaller mass.

So finally, let’s get to the movie. I always love to show my movies with these things. So hopefully you can all bring this up.

	If you hit Play, this just shows what happens to the edge as a function of time. Particles are coming from the right, they encounter the moon, which you see in that little orange ellipse moving around and then they continue on their way to their little (checks).

	So, it looks kind of like a (slinky) rotating when I run this thing, which I always think so.

	The individual particles, you can watch them carefully, each actually move a nice (sinuous) little wave after they encounter the moon. But the overall effect of this sort of string is to actually look much more bizarre and much harder to predict.

	So, clearly, this thing is pretty complicated, by complicating, it would be interesting and probably worthy of future study and hopefully grant money.

Woman:	So - I’m sorry. Again, the orange ellipse is showing you where in the…

John White:	That’s Daphnis.

((Crosstalk))

Woman:	…Daphnis in its orbit.

John White:	Right. And the red dot that’s on there, that red kind of smear is Daphnis itself where it is on its orbit around on that ellipse.

Woman:	Okay. And then so as it goes, sorry, I’m (unintelligible) here, as it goes around and then the big (slinky)…

John White:	Yup.

Woman:	…is the radial distance from the moon of the particles that it’s passing.

John White:	That’s the position - the overall position of the particles, both downstream distance on the horizontal axis and distance from the moon in the right vertical.

Woman:	Okay.

John White:	If you took a snapshot about one stream of particles at both times, that’s what it looks like, assuming you believe these things don’t collide with each other with those particles.

Woman:	Right.

John White:	It takes a lot of (passes) to fully kind of wrap your brain around this. I’m still working on, to be perfectly honest.

	At some point, I’m going to try to turn this to a screensaver, I think.

Woman:	So this is downstream from Daphnis?

John White:	Right.

Woman:	And that’s because, in the images that we are looking at, still that the wave are always ahead of Daphnis?

John White:	The inner edge is always ahead and the outer edge is always behind.

Woman:	Okay.

John White:	But they’re completely symmetrical. In fact, the equation I used for this, I used some slight simplification. We always have to make approximations, of course. These simplifications resulted in the complete symmetry between the two edges.

Woman:	Uh-huh.

John White:	So, I could have one simulation for the outer edge as well but they should have been exactly identical so it seemed kind of - it cut my work in half.

Woman:	But then we saw in that previous image that they weren’t symmetric.

John White:	I know. So, I didn’t show the outer edge at all here. If I had done the outer edge sort of things, you know, on the upper left (unintelligible) going to the right, they do the exact same thing.

Woman:	Okay.

John White:	At least they had better.

	Okay. So moving back to the presentation and moving - pass to movie slide, so what’s the upshot of all these sort of the conclusion, the takeaway message?

	The analytical formula works only for distance moon edge encounters like Pan. There probably aren’t anymore of these like that. (Unintelligible) it doesn’t work very well even if the moons are in circular orbit. And if it gets eccentric -- the eccentricity causes this time variable changes to the edges. So this means that we have to actually follow the things around like we are currently working on observation programs for the extended mission to try and follow Daphnis around for a while to try and watch these time variable changes to better constrain mass among other things.

	And this is sort of always the mantra these things. We need better observations and we need better simulations. We need to put in collisions and the self gravitation particles because clearly the collisions in the very least make a big difference at least for Daphnis.

	And so who cares about any of these anyway, I mean what’s the importance of all these?

	Well, first of all, this means the small moon here can have really big effect in the edges of the rings, bigger than the theory predicted initially.

	I kind of view this sort of an open hunting license for Cassini people and everyone else for that matter to go looking for these small moons, we’ve haven’t seen a lot of them yet. The only one we found inside the rings (unintelligible) in the orbit was Daphnis.

	But if the moon is small and you wouldn’t have thought initially, that means they could be hiding, so it’s lot more easily. This is a good reason to go back and start paying very close attention for some of the other gaps in the rings where we do some odd behaviors (unintelligible) small moons.

Woman:	So there are gaps where moons haven’t been seen yet?

John White:	Absolutely, all over the place. Actually there are -- C ring especially, but also the Cassini division, there are gap odd behaviors that aren’t associated with residences that we know of. We haven’t found any moon that’s there yet but we’re hoping to.

	It’s always fun to go hunting for these things although it also causes some eye strain.

	So, the final point here - and this is going to come up in the third part of the talk, the third half if you will. Knowing the mass of the moon can help us understand their origin. I’m going to show you how, like I said later, but first, let’s probably move on to spokes for it. So let’s go from the large to small I guess.

	All right. So can you hear me?

Woman:	Maybe you get a little closer to the mic?

Colleen Mitchell:	Okay. Let me just borrow (John’s). All right.

	Okay. So I’m going to be talking about the spokes in Saturn’s ring. And on the next slide, you’ll see this is actually a Voyager image.

	Now, the reason I’m using a Voyager image here isn’t just because, you know, they were spotted first in Voyager but also because Cassini hasn’t actually been seeing as many spokes as Voyager did. As a matter of fact, we were in orbit for over a year before we saw any spokes, so this is kind of an interesting thing that we’re trying to understand exactly what’s going on with that. It’s not totally nailed down yet.

	So, basically, this talk, I’m going to tell you what we do know but please understand that there’s an awful lot of work to be done, there’s a whole lot of stuff that we don’t know.

	So the next slide gives you a basic introduction. So, the observations that we had at spokes so far they were discovered by the Voyager spacecraft in 1980 and 1981, the Hubble took a large number of images looking for spokes between 1994 and 2003 I think. But - over time actually, they initially started looking around the time of the ring plane crossing in 1995, and as the rings opened up to the front, the spokes basically became - they had - the contrast went down and that became fewer in number.

	And then like I said, Cassini didn’t thought any until 2005 when we finally recovered them. This is an interesting thing we have to explain.

	So what we do know about spokes is that they’re composed of small little dust particles which are probably charged and the features, if you look at the previous image, they start off more or less radially aligned. They can’t have some more complicated stricture to them but they seemed to be radio aligned and they tend to be kind of wedge-shape.

	And the point of the wedge typically is on the side near the core rotation point whereas the - where it’s orbiting at the same rate that Saturn spin. That’s also an indicator that these are probably composed of charged particles.

	They also usually exhibit Kepler shear…

Man:	Yes.

Colleen Mitchell:	Oh, Kepler shear is what John was talking about earlier. Whenever you have something which has a radial extent, the inner material moves faster than the outer material, so things tend to shear out (unintelligible) which is really one of the reasons spokes were such a surprise because people didn’t figure that anything with kind of (unintelligible) structure could exist in the ring then it turned out spokes actually don’t last very long, they have a life term of about five hours or so or less actually.

	And the last thing I want to mention is that there is some - in the Voyager data, we found they preferentially form at particular point in Saturn’s magnetic coordinate system. So if you imagine the system coordinates that are rotating with Saturn, there’s a particular place in these coordinate systems where spokes like to form. That’s another indicator that there’s particle charge.

	Right. Next slide.

Woman:	Can I ask you a quick question?

Colleen Mitchell:	Sure.

Woman:	You - do you think that why they are wedge-shaped and relating that to…

Colleen Mitchell:	Right.

	We have a few dynamic slides.

Woman:	Okay. Great.

Colleen Mitchell:	But short answer is that the particles are affected by the electromagnetic forces that cause the electric field in Saturn or magnetic field of Saturn. And what happens is that the larger particle is the less the electromagnetic forces is affected.

	And so the particles which are - if you’re inside co-rotation, the particles that are on the leading edge of the wedge, those particles are the big articles. And then the particles on the trailing edge, what’s happening with them is they’re small and they’re more highly charged up and so they’re more or less behaving like ions, they’re co-rotating with the planet.

Woman:	Okay.

Colleen Mitchell:	So the electric field will slow these particles down basically and so you get this spread because you have different sizes in the spokes.

Woman:	Okay.

Colleen Mitchell:	That’s what we think anyway.

	Okay. So the next slide, the contrast (unintelligible). This is what tells us that they are made out of small particles. If you look on the left, these are Cassini images by the way. I’m done with the Voyager images.

	On the left, you see a set of spokes at high phase and right against the dark material of the ring. And on the right, they are a little bit - but in the dark region, you can see these spokes that are dark against the dark background part of the ring. Those are at low phase.

	And this tells you that the spokes are made of small particles because small particles tend to (unintelligible) forward, they tend to be forward scatters, while the large particles in the ring that John deals with, those tend to be primarily back scattering.

	Next slide

	All right. So I mentioned that Voyager noticed that there was a periodicity of the spokes. This is just the data from those observations.

	Voyager was able to take images from 90 Saturn rotations, and from this, you got this periodicity diagram which shows that you definitely have your most spokes at this particular period, so 640 minutes or approximately 10 hours and 40 minutes which is the rotation rate of Saturn.

	The next slide shows what you get whenever you combine Voyager 1 and Voyager 2 data. That previous one was just Voyager 2 data. And again, you see the peak at 641 minutes. There’s also a peak at 611 minutes. This actually has something to do with the SED period, which is very interesting, we haven’t been able to actually verify that yet.

	As a matter of fact, since Cassini is only seeing a few of these spokes, so far, none of our long-term movies actually cite out. We had a large number of movies before orbital insertion, staring at the rings from a long distance away. We haven’t been to verify a lot of these. And so during the extended mission, we’re right now trying to come up with times where we can look for long periods of time and see these periodicities again.

	All right. Next slide please.

	Okay. I’ll start with that.

	Okay. So the next slide just shows that the spoke periodicity corresponds with the activity of the Saturn kilometric radiation. Basically, what happens is every 640 minutes or so, Saturn will give off a burst of radiation kilometric range.

	And on the left side, you see that what you look at the activity of these spokes, the images that Voyager took is the greatest amount of activity corresponds very well to the one you see where Saturn is producing this radiation. And if you look at the images where the least spoke activity occurs, again, it’s anticorrelation. So this is pretty good evidence there’s a relationship between the two. And this is what we’re trying to confirm in the extended mission.

	So the preferred geometry, it turns out is, if you look at the next slide, as you can see, there’s this particular magnetic field sector. In this case it’s labeled (Land SOS 115).

	And spokes are most prominent whenever this magnetic coordinate is pointing towards the morning ansae of the ring. And the morning ansae then have a lot more spokes than the evening ansae. And then if you wait a quarter of the Saturn period to have this magnetic longitude pointing towards the sun, that’s when you get the burst of SKR radiation usually. This is just more evidence that there is a connection between these two things.

	All right. So now I’m going to talk a little bit about dynamics.

	So, All right. In the case of large particles, you really don’t have to worry about electromagnetic forces, basically, gravity dominates all of their dynamics.

	So whenever you get down to small particles and subparticles seemed to be about half a micron in size, once you get down to those sizes, you have to worry a lot about the charging of the particles because the electric fields and the magnetic fields can push the particles around a lot more.

	Now, the spokes you saw, they don’t cover the entire ring, I mean there is some sort of the (event) which makes the spoke. And so they’re transient phenomena, there has to be some sort of charging then that causes them.

	And so what I’ve done here is I’m just - produced a little simulation, taken particles of several different sizes, and I've just subjected to a plasma. The idea being that, you know, some of the event happens 400 plasma above the rings, charges up the particles, and then, you know, their dynamics change and they levitate off the rings and that’s when you see a spoke.

And whenever you do this, you’ll notice that the large particles tend to charge faster than the small particles. Now the reason that is that whenever you have a particle in the plasma, the potential that the grain charges too is not a function of the size of the grain, it’s only a function of the plasma parameter but the charging occurrence are a function of the size of the grain. It turns out that the charging occurrence are proportionate to the surface of course. And the actual charge on the grain is proportional to the voltage, assuming they are (unintelligible) particle.

	And so there is a case where the charging time of particles is inversely proportional to its size. And so as you’ll see on the left that red particles, the 1 micron, and the purple particle, the 0.1 micron, there’s a big difference between the different voltages of those.

	And then what I did was, in order to let the particles discharge, I just put the particles in basically the background environment that we believe above the rings. And the little particles were slowly discharged, big particles were discharged much more quickly.

	All right. Next slide.

	With this particle and you put them and orbit about Saturn then what happens is the electric fields were ill also the dynamics of the particles. And what I have here on this slide is I have the angular velocity of the particles around Saturn as a function of time.

	And you can see this vary but what’s most important s that the 0.1 micron particle is affected a lot more than 1 micron particle. This is what I was saying earlier. The large particles in this case will be moving ahead of the smaller particles if you’re inside of co-rotation.

	Next slide please. All right.

	And now, basically, I just laid down a large number of spoke particles at bunch of different points and different sizes. Edge equals zero, you got this wedge-shaped thing.

	This is basically the largest spoke you can imagine, it covers the entire region of the (biering) where spokes have been seen. And the idea is that it at co-rotation, which is the point of 112,000 kilometers on this graph, they basically - they aren’t moving at that point because of the electric field and the magnetic field basically don’t affect the particles at that point. And so the wedge is pointing towards this point regardless of which side of it you’re on. And then, you know, 20 minutes later, you can see on the right that the spoke is shearing out. In the next slide, you can see what happens in an hour, in two hours.

	As you can see, the large particles tend to move on ahead and they shear out normally, but the little particles tend to stay behind. This is why we think they’ve got this wedge shape.

	All right. Next slide please.

	Okay. And so whenever you talk about where these spokes or why these spokes form, you need to explain a few things. First, you need to explain the charging of the particles and levitation of small grains. You need to understand - you need to explain -- excuse me -- the morphology of these spokes. Some of them are, you know, long and narrow, others tend to be broad.

	You also see - it wasn’t clear in that light, so I’ll show you an image later where it is that you often have multiple spokes forming at very nearly at the same location, and of course, you have to explain (unintelligible) set in kilometric gradations, electrostatic discharges.

	And it seems that they might form very rapidly. Based on Voyager images, there’s an image where there is two images taken five minutes apart and one of them, the early one, isn’t the spoke there, and the second one, there’s kind of a little of a hint of the spokes. It seems that they might form very quickly.

	And the two possibilities that we've come up with so far that really I haven’t been discarded yet are impacts and like the (unintelligible). The impact idea is the original one. It was (used) in 1983 by, (I guess in Moorfield).

	The idea is that the plasma above the ring that I was talking earlier about is the result of a meteor impact. The idea is that the plasma charges up the dust, the dust levitate. And then as you saw earlier, the dust doesn’t exactly move at the Kepler rate, it wants to - but it does more or less at the Kepler rate. The dust particles will leave the plasma because the plasma is going to be co-rotated, right?

	And so what happens is you get an electric field set up in the plasma because the dust particles have moved out. And (unintelligible) electric field. And then if you remember from your Plasma 101, whenever you have an electric field and a magnetic field, then you have a plasma involved, the plasma, it get pushed around by the electric field.

	In this case, it gets (unintelligible) perpendicular to the electric field called the (E-Cross) (unintelligible). And in this case it will move the plasma radially. The idea is that this plasma cloud pushes - levitates the bunch of dust, the dust causes electric field, it pushes the plasma radially.

	And so it’s this kind of the roadrunner model where you have this little plasma cloud running along the ring, kicking the whole bunch of dust as it goes along.

	All right.

	The next slide - the next idea, this is much more recent, just last year, Jones, et al. The idea is that whenever you have a thunderstorm in Saturn, then you can have the equivalent of what you see in Earth’s magnetosphere. You get these cases or these features called (unintelligible) we have these large electrostatic - electric discharges along field line.

	Now, on Earth, basically, the electrons goes from the north hemisphere to the south hemisphere and re-impact the atmosphere and basically nothing happens or vice versa, of course.

	In Saturn, the ring is getting away. And so the idea is you have this large discharge of electron moving along these field lines and it comes down, hits the ring, charges up the ring and particles and, boom, you have the spoke.

	All right.

	So, these are the two different idea and one of their features.

	You got o the next slide.

	The impact model, it really explains the long, narrow spokes well. Again, this is the roadrunner model. Clusters of spokes which we have seen, they tend to - you can’t really explain those too well because that would imply multiple impactors which aren’t very likely.

	And finally there’s recently been indications that the radial motion plasma might not quite be as fast as we once thought, so if spokes really do form really fast, maybe that’s not so good for this model.

(Unintelligible) model, on the other hand, it explains why you have groups of spokes really well because as we’re seeing thunderstorm producing all these spokes and spokes tend to show up right around the co-rotation distance which what you would expect to have clusters if you have multiple burst from the single thunderstorm but it can’t explain the long, narrow spokes very well because you’d have to have some sort of weird shape of the lightning discharge in the first place in order to produce long, narrow spoke.

	All right. Moving right along.

Woman:	Colleen, when you say dust here, do you mean ice?

Colleen Mitchell:	Yeah, it’s kind of a jargon that we use.

Woman:	Uh-huh.

Colleen Mitchell:	I do mean ice particles in this case. So little ice cream about half a micron in size.

Woman:	Uh-huh. Okay.

Colleen Mitchell:	All right. So Cassini is also - in the next slide. Well, Cassini didn’t see any spokes and so till September of ’05 and these are they, here’s two little spokes. But, you know, we have been seeing them since then and we sort of to have the chance to put together some statistics that I’ll show you.

	The next slide.

	Remember how I said that the spokes tended to show up predominantly on the morning side of the ansae, we are seeing that as you look at these many more spokes appearing on the morning side than on the evening side.

	And there’s a next image - All right, excuse me. The next slide.

	The magnetic longitude on the other hand, we haven’t really seen enough spokes to get any kind of significant data out of this. Virtually, in the Voyager, they saw 90 rotations of Saturn. Saturn rotates every ten hours. We haven’t had anywhere near that kind of opportunity to stare at the rings yet.

	And so I’m not terribly surprised that if you look at this, you don’t see anything. It really just looks like noise to me for the moment but we’re still working on this. We’ll see if anything stands out.

Woman:	So, again, we sort of - to be consistent with the Voyager stuff, we expecting to see more at 1:15?

Colleen Mitchell:	Yeah.

Woman:	Actually, for…

Colleen Mitchell:	In this new coordinates system that we have, it would be at 100.

Woman:	Oh, okay.

Colleen Mitchell:	So yeah. Yeah, we would expect to see a fairly large peak at 100. All right.

	And finally, just to show you the complicated morphology of the spokes, this is probably one of the most complicated spokes that we’ve seen so far. You can see there are several different spokes here. They are long very long and narrow. They seem to curl a little bit, if you look at the top part, they seemed to be a little bit more radial than they are the bottom where they’re kind of curved in Kepler shear.

	And, you know, we just have to explain how those come about and we really just aren’t there yet, so there’s a lot of work to be done here.

Woman:	Some of your thoughts suggest that the long, narrow (unintelligible) haven’t yet evolved into wedges?

Colleen Mitchell:	Yeah, that’s possible.

	Another thing is that if the charging on the spoke isn’t significant, then the little particle would not behave in the co-rotating sense anyway and so they will move in Kepler orbit too.

	And so, you really have to - it’s a very complicated problem. If a spoke is not highly charged, then the grain will all move in the Kepler fashion anyway until, you know - I think that basically, the different morphologies of the spokes tells us, you know, the specifics of their formations. But again, we haven’t been able to work that out yet in any detail anyway.

	All right. So, the conclusion on the next slide, the spokes have returned, alleluia. They do appeared to be more on abundance on the morning ansae than on the evening ansae which we what we expect from Voyager result. We haven’t yet seen any significant dependence on the magnetic field sector that - don’t worry about that because we haven’t seen enough spokes to say anything, and of course, you know, we haven’t yet seen a spoke formed which is we’d be very excited if we’re able to do that, but, alas, we haven’t been able to.

	And of course, like I’ve been saying along, there’s a whole lot more work to be done, and the necessary questions that I think I’ll past it over to John.

((Crosstalk))

John White:	Okay, okay.

	All right. Moving back to the big particles, the ones for whom electric filed don’t matter, I want to talk about the growth of ring moons.

	And we’ll tell the story kind of in chronologic order on how we came across it.

	So first, if you move on past the Growth of Moon slide, the next slide after that, you see a picture of Pan. Remember I said when it was first discovered, it was a single pixel detection. We’ve got more than one pixel on it now, happily. It looks like a lemon, it is not a lemon, I’m just waiting for conspiracy theorists to get a hold on this, they will.

	Just keep moving along here. I just want to show you pictures, kind of get a feel of the shapes of these, guys. Here’s Atlas just beyond the edge of the A ring. It looks like a flying saucer, no, it is not a flying saucer, please do not call it that because people already trying to.

	And then Prometheus on the next slide here, kind of potatoey shape, also has a fair distinguished shape of previous too. And Pandora, these are two moons, Prometheus and Pandora, they’re the ones that shepherd the F ring, just beyond the A ring there.

	And then Telesto is far away from the ring it has a somewhat similar shape though it’s a far more kind of round and stubby.

	So these moons have, first of all, (unintelligible) shapes, not spherical at all, you’ll notice. They’re not quite are irregular asteroids though.

	So one of those goals of the Cassini imaging team was to actually study the shapes of these moons and try to deduce pretty much whatever we could about the moons from just the shapes and morphology.

	Now, that was one project that was going on kind of one end of the team. Meanwhile, another discovery would turn to have some bearing on this shortly.

	Moving to the next slide, we have kind of some serendipity. On the left we have a context image, you see the B and A rings, the A ring is the outer part. And then blowing up, getting closer, we go to these two on the far right, two Cassini orbital insertion images. This flew actually right over the A ring (unintelligible) looked down upon on the dark unlit side.

	And you can see little tiny streaks circled in here. Now you attempted to call those things cosmic ray hits or some kind of noise in the space or the ECD detector. If you go to the next slide, we show that in fact these things are aligned all of them exactly what the orbital motion at those locations.

	You would best expect maybe one of them that could be exactly (not) aligned. They have four precisely aligned like that, tells you something else is going on. So this little propeller-shape features must be some kind of interesting feature or some properties of the ring, there is something going on here.

	So going to the next slide, what was that? Well, it turns out that this will be predicted. A few years ago, somebody was doing some simulations of the ring, they discovered a moonlet in the ring. In this case, about 50 meters in size. It can create those kinds of pattern by just the way it influences the ring around it. Remember we plotted those waves on the edges. Well, if the moon doesn’t clear a gap completely, those waves just turn into (color) features.

	But we’re - in the Cassini imaging team, we’re wondering once we saw these things what were we actually seeing with those (colors)? Are those the gaps that are opened up, little tiny evacuations, or is it a place where those particles get (chubbed)?

	It turns out that the geometry in - with these observations were done, on the unlit side, 90 degrees above the ring, looking straight down, it’s very hard to understand these kinds of things. A brighter feature could be either less material or more material in some cases. And so we weren’t sure what we’re looking at.

	This is important because it’s about the factor two in the mass estimate for this moonlets and we wanted to know how big they actually where.

	There is really one thing to do which is you got o the next slide, simulation. It took longer than just clicking the buttons but we did them.

	You see in the middle with the red little sphere, that’s the moonlet. And then you could see the propeller of the - form, the top has particles that don’t have gravity with each other and the bottom of the particles do. In both cases, you see the propeller.

	But we also noticed something, completely independent of the propeller that was also very intriguing to us which kind of our brains are working which is the moons, the little cores have kind of thrown a whole bunch of particles on top of them. They’re kind of (glommed) on.

	Little light bulbs went on over our heads and said, aha, we should have thought it was going to happen. It just didn’t occur just, you know, go this direction.

	But if we go to the next slide, I can get - show you some blowups of this exact thing same property.

	So this sort of simulation, you can see the big red core, and the area around it where it’s gravity dominates over the gravity of Saturn is also shown, so all the green particles are just the ring particles that are pretty much just happened to be inside of that region of influence.

	If you go to the n ext slide after that, it’s the same shot but from the side view. So you’re actually in the ring side looking at the core.

	And, you know, again, there’s a few particles in there but it’s not what’s the (unintelligible). But if you come back several orbits later, I guess 15 orbits later, the next slide, you can see what happened. It’s kind of like (B beards) I always think that people, you know, sometimes trying to get in the Guinness Book of World Records. All these particles are (glommed) on to this core and you could barely even see the core anymore. A tiny bit of red in there.

	And if you go to the next slide, you’ve got two side views, one from down the orbital direction and one from (end-on) looking from the planet outward.

	And you could see we formed kind of a moonlet here. And again, it’s very not spherical. In fact, the shape is quite a lot like those moons I showed you earlier -- Pan, Prometheus, Atlas, Pandora.

	So what’s going on here? Well, what we think that’s happening is of course is the moon’s gravity to track in the particles is hanging on to them. But at some point, it has to stop, we find. It doesn’t keep on growing, keeps on (unintelligible) reading material.

	The reason why we think is the material (unintelligible) does not have the same density as the moon. The particles probably have a lower density to begin with plus there are a lot of spaces between the particles, and that’s (unintelligible).

	And vacuum of course is a very low density, lower the overall. It’s like pouring marshmallow fluff on a bowling ball. If you look at the overall density of the flying objects, it’s much less than the bowling ball.

	If you go to the next slide, why would all this matter? First of all, you have to understand the concept of where (Roche) zone is. The (Roche) zone is just a region on the moon with the moon’s gravity dominates over that of Saturn.

	So if you assume the particular shape for this (Roche) zone, and there are analytic expressions for this, you can actually work out at what point the growth should stop. (Unintelligible) point has to do with the density of the final moonlet, not with the actual linear size or the mass. The density turns up to be about half a gram per cubic centimeter in Saturn’s ring.

	So what’s going to happen here is you start with the density core, you start pouring on much less dense material, it’s below this critical density. You venture the average density, it’s going to drop below the critical density or reach the critical density, at which point, any new particle trying to get on is actually outside the (Roche) zone and so Saturn’s gravity tears it away again. You can’t actually grow anything.

	Now, as I said earlier, the density is independent of the size of the moon, which is a little bit surprising in some ways, but it means that it doesn’t matter if the moon is 15 meters in size, 1,000 meters in size, a kilometer or, you know, 50 kilometers as the moons I showed you really are.

	So, this should have a bearing on what we’re seeing with moons like Pan and Prometheus and Pandora.

	How long does this take to actually happen? Well, it’s actually pretty quick, in the simulations takes just a few orbits to quickly gather up as many particles it’s ever going to get.

	Some of the particles though stick around it. There’s constantly collisions going on that knock particles loose, but as soon as that happens, new particles move in and fill in those gaps.

	So this is, you know, a steady state process, a lot of the particles stick around for a very long time.

	Our thought is that our simulations get include the idea that the particles can weld each other - into each other, the ice gets heated and cooled. You get little bit of melting, things can actually bond together. But the real Saturnian system, this probably can happen.

	So, eventually, a lot of cycles of the heating and cooling, if you go into the Saturn shadow and back out again, we think it actually may form a fairly solid moon.

	Does growth always stop? It doesn’t always have to stop. The answer - short answer is no. And if that’s what you want to know, then you can stop listening to the rest of slide I suppose.

	The reason is if the critical density is below the dense of the material that’s coming in, the growth can continue forever and you never going to reach that cutoff point.

	Just as a sanity check, I want to actually check, it turns out that if you use this theory in the planet, the planets can grow as large as they want to them to be, which is a nice check because otherwise the planets also should have something like the critical density, they should have - orbiting the sun and none of them do. It’s always good to check these things and make sure that they don’t come up the wrong way.

	So moving into the penultimate slide here, this is just the plot showing the density of ring moons I showed you earlier. If you look at the point here with the (aero bars), they’re, Pan, Daphnis, Prometheus, Pandora or Atlas, Prometheus, Pandora, and the two on the far right are Janus and Epimetheus, the core orbital moons.

	And you could see that once that are close to the ring, ignoring Janus and Epimetheus for a second, all have a very low density, around half a gram per cubic centimeter which is close - very close to the critical density.

	It’s hard to explain that, it’s sort of a shard of a moon that broke up because you expect the density to be somewhere around that of water which is around one.

	But if you believe this thing is sort of accreted, which we do, then this actually does make complete and total sense.

Janus-Epimetheus (unintelligible) are much higher in density but still below that of water by quite a bit. Because probably it’s telling us that this *5630 some material but didn’t decrease all the way to completion so there wasn’t enough stuff up there for them to keep going forever, the ring basically went away around them.

	So that’s where we think the moons are probably coming from. We’re still working on revising the theory coming out - dotting some of the Is and crossing some of these Ts here. But it turns out the density of these guys in working out their master’s has strong implication for where they came from.

	This also means these things could be recycled. If you break a moon up right now and you leave sort of core in place, it will gather up more particles. It will be a completely different looking moon probably and will have different carters and things like that. But these things can be recycled. So the rings could be older than we think if the cycle is going on constantly.

	So the final slide is just a pretty picture. The Encke gap with some nice density waves on the side. And a shameless plug telling you to go and visit our - the CycleOps Web site to see pretty pictures.

	And then with that, we will take questions.

Woman:	Thank you very much.

	I have a few questions but I’m sure other people do too, so.

John White:	Fire away.

Man:	Yeah.

Woman:	Okay. I’ll start. Let’s see. Okay.

	So, at SOI, you saw these propellers…

Man:	Right.

Woman:	…but they aren’t in gap…

Man:	Yeah.

Woman:	…that’s telling us that there are moons - moonlets that are not in gap.

Man:	Right.

Woman:	Like - okay. So you start telling us about moons that are in gap but these aren’t in gaps and so is that mean that they are not massive enough to have cleared out gaps?

Man:	Basically.

	What happens is you can start to clear a little bit of gap but the particles you just pushed have to go all the way around the Saturn relative then you come back again. And that’s actually a long period, typically something like a `100 orbits or something like that or more.

	And by the time you come back, a lot of the particles are just diffused through interactions with each other back to where they were to start with. So if you can’t push things far enough away or if they diffuse back to you back in time, you don’t open the gap, you just (unintelligible) a propeller.

There is probably there going to be a continuum between gap and propeller, you know, as you (unintelligible) slowly. Eventually, one will turn with the other.

Woman:	So the propeller is a lack of material.

Man:	Right.

	It is one of the things that we’re looking at right now. But you have to go somewhere, of course, and a bunch of stuff in sort of a bank (unintelligible). It’s a little harder to see in these pictures and the gaps but they’re there.

Woman:	And then initially, you were talking about using the scallop edges and need some stuff to get us the mass…

Man:	Right.

Woman:	…I think of those moonlets. And then you show us the density, the estimated densities of these guys, I mean next to last slide. Are those somehow you’re your previous work or are this in different types of…

Man:	The two in the left, for Pan and Daphnis, are.

Woman:	Okay.

Man:	You know, Daphnis has those massive, massive aerobars, that’s because we don’t completely have a handle on how to get mass there.

Woman:	Uh-huh.

Man:	We also don’t know what shape very well which is another problem, as opposed to like the density, of course.

	For Atlas, Prometheus and Pandora, we’re also using interactions with the ring but we’re using different interactions. For Atlas, you can use density wave inside the rings and see - use measurements of those, these waves they keep kick up like resonances to get the mass.

	In Prometheus and Pandora, I believe we get those mass estimates from the interactions with the F ring with each other.

	Finally, Janus-Epimetheus, the easiest way to get their mass, since they actually sort of sling off each other, every time they get close, they change places one is inside, one is outside, they swap. You could actually get their masses from that.

	And so actually their masses are pretty accurate but their exact shapes are causing most of those aerobars.

Woman:	Oh.

	And then so I think you were saying that the low densities imply at a particular amount of velocity?

Man:	Right.

Woman:	And which would then making with the formation?

Man:	Exactly.

Woman:	Okay.

	And so maybe (unintelligible) I missed it, what kind of (unintelligible) velocity numbers are you talking about?

Man:	In the mantel? The core is sort of the solid chunk, we assume is probably close to that of water.

Woman:	Okay.

Man:	Inside the mantel, a fluffy layer of particles that gets accreted (unintelligible) probably around 50%, 60%.

Woman:	Oh, okay.

Man:	It’s pretty high but things don’t pack very densely.

Woman:	Right.

Man:	So that’s actually, from what we found, not that shocking compared to, say, asteroids -- from actual measurements of asteroids.

Woman:	But, obviously, I’m thinking about Hyperion which is super underdense and its appearance is a result of that family (unintelligible).

Man:	We actually haven’t applied to this to Hyperion. Something to think about actually, now that you know mentioned it.

Woman:	Might be interesting - I don’t know if there’s any comparison.

Man:	Right. Hyperion is far enough, critical density, the point that which stops growing is fairly small number like a * per cubic centimeter which you - it will never reach.

	So I think it’s one of the reasons we haven’t even thought about it. But yeah, we should look at that moon actually, you’re right.

Woman:	Do other people have questions?

(Ken):	I got a question. This is (Ken) at Princeton.

	Have you thought about applying to Iapetus at all actually with that big bulge?

Man:	No. Iapetus is actually sufficiently dense that it’s not close enough to sort of (Roche) zone, but that (bulge) would be caused by the same effect. Moons that are that dense actually should be pretty close to spherical.

Woman:	So you’re saying that that equatorial ridge must…

Man:	It can’t have the exact same cause here as what we here. is what I’m saying.

Man:	Okay. Thanks.

Man:	Which is kind of comforting because all the big moons, you know, are roughly spherical, so obviously an explanation for why this doesn’t apply.

Woman:	Other questions?

	I have one about the spokes.

Man:	Oh, okay.

((Crosstalk))

Woman:	And that is…

Man:	Yes?

Woman:	And again, you were saying this and I didn’t quite get it. But at least it seems like there used to be a lot of ideas about how the spokes were seasonally dependent.

Man:	Uh-huh.

Woman:	Is this still the case and its linked to the magnetic environment?

((Crosstalk))

Man:	Oh, yes.

	So what we’re thinking right now, the best ideas we’ve had do far is that the seasonal dependence is because the plasma immediately above the ring is caused mainly by photo electrons. And so if you open the rings up to the sun, what happens is the UV (unintelligible) from the sun causes more and more photoelectrons to create the sheet above the ring.

	And what we’re thinking happens is whenever - no matter what event - what type of event forms the spokes, whenever it stops, or the spokes leave the - excuse me - the particles lave the plasma, what happens is the (unintelligible) this background environment where there’s a lot of photo electrons flying going on. And so the grains would charge out negatively and quickly if there’s a lot of photo electrons.

	It turns out that - of course, we have, you know, photoelectrons above the ring. The ring itself has to be positively changed, right? And so this large electrical field that could stuck particles back down on to the ring. That’s the current - my current idea of why we haven’t seen spokes in the region between like 1998 and 2005.

Woman:	Okay. Because we expect them to be in summer when the rings are open to the sun.

Man:	We expect to be near the equinox whenever the rings are not open to the sun and you don’t have this stick flare of photo electrons above the ring.

Woman:	Oh, okay, okay.

Man:	So then these particles can get away from the big background ring.

Woman:	Okay.

Man:	Probably we’re looking forward to a busy next two years.

Woman:	Oh, yeah.

Man:	Very much so.

Woman:	One more spoke.

(Robert):	I actually have a question, just a clarifying one. (Robert) from Salt Lake City.

	On Slide 56, I just wanted to clarify the direction we’re looking at on the side views of the…

Man:	Right.

(Robert):	…accreted objects.

Man:	Okay.

(Robert):	The view - if we’re looking from the planet, looking out of it, would that be the one from along the y axis?

Man:	The right-hand one.

(Robert):	The right-hand one.

Man:	I operate in a funny coordinate system where x is outward, y is in the direction of the orbit and z is the perpendicular to the orbital plane.

(Robert):	Okay.

	So, if we were looking at the one on the left-hand side then, that’s - as if we were in along with the orbit of the object.

Man:	Just ahead of it.

(Robert):	Just ahead of it. Okay. Thank you.

Man:	Good question, sir.

Woman:	I have one more question.

Man:	Okay.

Woman:	And this kind of goes like really a question. On Slide 4, this image of the Encke Gap, I assume not clear on where is Pan - where is Pan’s orbit?

Man:	Hold on one second. I got to get to back to slide real quick.

Woman:	Okay.

Man:	If you wouldn’t mind if I’m looking at.

	Okay. Pan, let’s see. It depends on which side of the ring, obviously. This is a (unintelligible) image so we’re actually on the south side.

Woman:	And what is Pan has to do with anything with the smaller rings that are in the Encke Gap?

Man:	The central ring is actually right on Pan’s orbit more or less.

Woman:	Oh, okay.

Man:	Very close to.

	Particles actually right there to never actually get the Pan. They get close and then they get the orbits tweak slightly so they’ve actually move away.

	The other two ringlets, I don’t think we really understand very well. That is actually another target for the extended mission we’re planning to look at.

Woman:	Do you know that the Pan…

Man:	The Cornell with Joe Burns and (Peter Thomas) are doing this.

Woman:	Oh yeah.

	So, you know…

Man:	I'm still skidding my brain around the exact geometry. I’m not used to thinking of the south side of the ring which means post-equinox is really much happier for me.

	Pan should be to the top of the slide.

Woman:	Yeah, heading around to the other side…

Man:	Right.

	The material what you’re seeing on the right-hand or the left-hand edge, the inner edge there, has passed Pan is moving down to the left.

Woman:	Okay.

Man:	I have a question. So, I guess if Pan is in the middle…

Man:	Uh-huh.

Man:	…there are kinks to that like in the F ring?

Man:	They are close like in the F ring. I don’t think that ever saw any real kinks. The ring is do appear and disappears them. On some images, you’ll see, you know, one or two of them, some you’ll see all three and their brightness has s just completely changed. There’s a lot of time variability.

	So I don’t notice on any kinks.

Man:	I have a question.

Man:	Uh-huh.

Man:	Also on Slide 4 that ring that’s in the middle of the gap that you said lines with the orbit…

Man:	Right.

Man:	…in the middle of the slide, there seems to be a flash mark across the ring. It looks like almost the scratch. I thought it was on my screen at first, when I move your slide around it, it looked that “What is that?”

Man:	That I guess it’s probably like a cosmic (grade) or something that’s what we completely claims.

Man:	Okay. So that’s on the CCD?

Man:	Probably.

Man:	Okay.

Man:	We constantly have this problem with - exposure just cosmic (grades), get the things and they always hit the worse possible spot every time.

Man:	Thanks.

Man:	I've spent a long time trying to track moons, they’ve turned out to be just, you know, high-energy protons on the projector. Wasted many hours.

Woman:	Other questions?

	Okay. Well, if nobody has any sort of questions then I will thank John White and Colleen Mitchell one more time very much for a very interesting talk. And I’d also like to invite everyone to call in for our next CHRAM telecom which will be August 28 and the topic will be Hyperion.

	So, thanks again very much everybody.

	Bye-bye.

Man:	Bye.

Man:	Bye now.

Woman:	Bye.

Man:	Thank you.

Man:	Very nice, too organized with this…

Woman:	Bye everyone.

Man:	See you next time.

Woman:	Thank you.

Woman:	Bye.

	Hello?

END