Coordinator: Good afternoon, thank you all for standing by. At this time I would just like to inform parties that today’s conference is being recorded. If anyone does have any objections, you may disconnect at this time. If you do require any assistance during the conference, please press the star followed by a 0.
At this time I’d like to turn the call over to Ms. Ray. Thank you.
Trina Ray: Okay, thank you. Well welcome everybody. The Charm telecon for April of 2006. We’ve got a really exciting topic for everyone today but before we get started, I wanted to do a little bit of business. We’ve had a request from some of the folks here at JPL to know a bit better what are the demographics of the Charm audience? So what I’d like to do is request that all the participants who are calling in today, and also anyone who downloads the audio file later, to go ahead and send an email to firstname.lastname@example.org. Send an email to that address and just tell me who you are and what your affiliation is so, for example, my name is XYZ and I’m a solar system ambassador, that’s perfectly fine. So we’d like to go ahead and get a sense of who’s calling into the meetings and also who’s downloading the file later. So, for all of you who are downloading this file, this wav file much later, please go ahead – no matter if it’s months after the fact, I’m still interesting in knowing that you downloaded the file and who you are.
And let’s see, that’s the only business we have to do for today. So what I’d like to do is go ahead and introduce our guest speaker today. Dr. Josh Colwell is with the ultraviolet imaging spectrograph team, that’s the UVIS team on Cassini and he is a long time rings expert and he’ll be telling us today about the rings of Saturn. He’s going to do a tutorial, just a basic rings tutorial and then towards the end of the talk, he’ll give some recent UVIS results. So, this is going to be a very interesting talk. For those of you who have been wondering about all that 3D structure in the rings today, all your questions will be answered.
And with that, I’ll turn it over to Dr. Colwell.
Joshua Colwell: Hello. So, yes I’m a co-investigator on UVIS instrument, Larry Esposito is principle investigator on that instrument based here at the University of Colorado. And my role on the instrument has been working with ring observations. And so to start off, I’m going to give a broad overview of Saturn’s rings just some of the basic characteristics and some of the open issues and then I’m going to talk in some detail about some of the recent exciting things that have come about from our observations with the UVIS instrument. Please fire away with any questions as we go along. If I say something that’s confusing, you have an idea for a question that we get into…
Man: Could we ask you to speak into the microphone a little bit more, we’re having trouble hearing you.
Joshua Colwell: Okay, I will try. Is that any better?
Man: Much better, thank you.
Joshua Colwell: Okay, good.
Man: May we also please have the address for the demographics again a little slower.
Man: Yeah, I agree.
Trina Ray: Okay, let me interrupt here quickly, Josh, sorry about that. The address is email@example.com.
Woman: Trina, this is Jane. That email address is in everybody’s invitation. Everyone who received the announcement of today’s meeting, that email is in that – as where you would ask questions. So, if you saved that email, you can always go back to get it there.
Trina Ray: That’s an excellent point. Thank you, Jane.
Okay, back to Josh.
Man: Yeah, could someone repeat the URL for the presentation please?
Trina Ray: That one’s pretty tricky. Who is this and I’ll go ahead and send you an email.
Man: This is (Jeff), Trina.
Trina Ray: Oh hey, (Jeff). I’ll send you an email real quick.
Joshua Colwell: Okay, so let’s see, I’m looking at the first line I’m going to move ahead to the second slide which is just an outline of the talk so, just as I said, talk about the structure of the rings and some of the big questions and then in some detail about UVIS occultation results where we get very high resolution picture of the rings and we’ve discovered that we’re able to put together some of these lines of sight from occultations through the rings at different angles to put together a three dimensional picture of some of the smaller structures in the rings. And there’s some interesting things going on that in the data that we don’t understand yet, so there’s a lot of interesting stuff still to come ahead.
The third slide is just a beautiful picture of Saturn and its rings taken by Cassini by the Imaging Science Subsystem on approach to Saturn in the spring of 2004. Here we see what I’ll call the main rings which moving from close to the planet out to the outer most ring that we see here in this picture are imaginatively named C, B and A. And the dark, the prominent dark gas between the B and the A rings, which we’ll see a little bit later is called the Cassini division which is actually a very interesting ring in its own right.
And you can see some of the different properties of the rings in this picture just from the sunlight shining through the ring and the shadow that ring casts on the planet, the northern hemisphere of the planet that dark, diagonal band at the top is the shadow cast on the planet by the B ring, which is the most massive and most opaque of the rings, and that little blue, faint band…
Man: (Unintelligible) we still can’t hardly hear you, if you can speak into the microphone a little more.
Joshua Colwell: Boy, I’ve got – okay I don’t - I basically eating the microphone at this point, so.
Woman: It sounds really good.
Joshua Colwell: I’m not sure what else I can do. I will try to speak up.
That blue band above the dark shadow is the sunlight passing through the relatively transparent portion of the rings known as the Cassini division. These are the main rings there are some other rings that are not visible in this image here, which I’ll talk about a little bit.
We move along to the next slide. Here you can see those rings identified, E, B, and A rings in the Cassini division separating the B and the A ring and in this picture it’s clear (unintelligible).
Trina Ray: Tons of feedback.
Man: I think it’s a message from outer space. Call SETI.
Joshua Colwell: So you can see in this picture quite clearly that the Cassini division is not in fact empty and here it looks somewhat like the C ring and, in fact, that similarity holds up under closer scrutiny as well. You can also see in the outer part of the A ring a narrow, dark band there. That’s a mostly empty gap that’s about 300 kilometers across, that’s called the Encke gap. And beyond the outer edge of the A ring is the narrow F ring. And if you can see just interior to the F ring we see one of the moons that is closely associated with the ring system that’s the moon Prometheus. And I don’t know if you’ve got the image of the Earth up on there with this slide or not, but if - I don’t know if you need to click one more time or not, but that’s the Earth to scale with the ring system so it’s very broad system. Tens of thousand kilometers across, but locally only on the order of 10 meters thick or so. So it’s a very flat system but a lot of real estate to look at and we find lots of interesting things going on if we’re imagine this as studying as much real estate as there is on the Earth and there are features that are – that we see down to the scale of the limit of our resolution which is on the order of 100 meters or so or the size of a football stadium.
Next slide gives the overview of the entire ring system and the moons and the schematic. Here you see the main rings and the lower panel the C, B and A ring that we’ve just talked about a little bit. And then there’s some additional dustier more tenuous rings interior to the C ring is the D ring. And beyond the F ring are the G and E rings. The E ring is the largest ring in the system and very optically thin, very transparent. Not a lot of material in it and we’ve got recent dramatic demonstration that that material in the E ring is coming from the moon Enceladus which as you see is about 4 Saturn radii under the planet and towards the inner edge of the E ring there it is spewing material into the system which goes into forming this large, sort of donut shaped cloud of small particles around Saturn that we know is the E ring.
In the upper panel you see many of Saturn’s moons. Of course the large moon, Titan, which dwarfs the others and then a number of intermediate sized icy satellites, the largest of which is Rhea, and then on the left hand side of that upper panel we see some of the moons that are intimately associated with rings. These guys are relatively small, 100 kilometers across or so and smaller and some of them, such as Pan there actually orbits within the rings and we’ve got another discovered example from Cassini of another moon that’s orbiting within the rings which we’ll see a little bit later.
These moons have very interesting affects on the ring system and they may be sort of intimately or genetically linked to the rings as well. So if we take a look at these – well let’s see, on the next slide there’s a very simple minded cartoon now why the rings exist at all. If you, the basic reason is that in close proximity to a planet such as Saturn, there is a strong title force which inhibits the accretion that normally would make particles assemble together when they collide gravitationally and grow larger and larger into a moon.
Title force is basically a differential force of gravity based on the different distances of particles from the planet. Though, when you’re a little bit closer to an object, that object exerts a gravitational pull on you and the difference in that pull is large when you’re close to the planet and it’s not as big a difference when you’re farther away. So a pair of particles that’s far from Saturn, for example, is able to gravitationally stick together, whereas a pair of particles that’s close to Saturn is not able to because of this differential gravitational attraction that Saturn exerts on it.
Another way of thinking about this, which should become clear in the later part of the talk where we look at some of the results from UVIS, is to imagine a countless ring particles as runners on a racetrack running around Saturn. And because of the way particles orbit, particles that are closer orbit faster than particles that are farther away. So if we imagine two particles on neighboring lanes at a racetrack running around a planet, the gravity between those two particles is like those guys holding hands trying to stick together to grow into a moon. The fact that the one that’s on the inner lane is going around faster means that it eventually gets pulled away from the one on the outer lane and they don’t end up sticking together.
Though the existence of the rings at Saturn as well as at the other Jovian planets just have to do with this aspect of how gravity works in close proximity to a planet.
The next slide we’re going to go into a series here of where we look at some of the rings in some detail just to get a closer look. Here is a nice comparison of the D ring seen from Cassini and Voyager and one of the most exciting things about this picture is that the D ring has changed in the short period of time between Voyager and Cassini. Of course the imaging capabilities has also changed a lot so the Cassini picture is much higher quality image, but you can see that the features, while some of the features are lining up you look at the right hand edge there near the words Cassini and Voyager we see a narrow bright feature in both that is lining up at the same position. If you look over on the far left you see another narrow bright feature that is lining up between Voyager and Cassini.
But then a little bit towards the middle, the brightest band in the Voyager image seems to have moved inward in the Cassini timeframe, that bright band in the Cassini timeframe, a little bit closer to Saturn and in the period of just a little over 20 years there’s been a large change in the structure of the D ring. Now this may seem like not such a huge shift in the structure, they’re generally looking the same, but on the time scale of the solar system or the history of Saturn, it is quite a huge change. So we’re seeing things changing on rapid time scales in Saturn’s rings.
The inset picture in blue up there shows an interesting periodic structure in the D ring that’s zooming in on one of those gaps there showing structure that we didn’t have the resolution to see before. And to my knowledge there were still searching for an explanation for the origin of that periodic structure. We’re going to see examples later on of other wave like features in Saturn’s rings and this is not consistent with those sorts of (unintelligible) other cause or explanation at periodic structure in the D ring.
D ring is mostly dusty, relatively transparent ring. Mostly small particles close to the planet. The lifetime of the individual particles may be quite short because dust particles get relatively easily pushed around, they get pushed around by Saturn’s extended upper atmosphere. They can get pushed around by radiation pressure, force of light from the sun acting on them, they can get pushed around if they get charged by Saturn’s magnetic field and all of those things that can end up putting those particles on orbits that might take them into the upper atmosphere of the planet.
So, these dusty rings are interesting tracers because the individual particles may have short lifetimes and that would tell us then by looking at those something about where those particles are coming from. Presumably some larger, longer lived particles.
Moving out then to the next slide, we see the C ring and here’s the inner half roughly of the C ring, the D ring would be off to the left inside of this picture. And there’s a lot of interesting structure here, most of which is not really understood. I’ve just pointed out one feature in the C ring and that is the narrow ringlet that’s in a gap. That dark (unintelligible) surrounding that bright light, narrow ringlet that’s heightened 1 to 0 ringlet is associated with a particular resonance, meaning particles, orbits at that location in the orbit of Titan. And I’ll explain how that particular resonance works later on.
You can see some other interesting things in the C ring that’s got sort of a mixture of very high resolution sharp structure. There’s another narrow ringlet a little bit further out beyond the Titan 1 to 0 ringlet. And then there are these gradual, gentle fluctuations and undulations in brightness that we see retching out across the upper right corner of the image. There’s a sort of – the E ring is characterized by this sort of mix of smoothly varying over large length scale fluctuations in material and very sharp abrupt brighter more dense ringlets within that smoothly varying background.
We go to the next slide showing the outer C ring you see another more examples of that. Here we see that in the lower left hand part is this sort of roughly periodic smooth undulations in the C ring and then all of a sudden these very abrupt even bright bands are called plateaus that punctuate the outer half of the C ring and, you know, they look like they’ve drawn in with a magic marker almost. So, perfectly uniform and begin and end so abruptly and we see that same abrupt behavior when we look at it in very high resolution…
Man: Josh, could I ask one real quick question?
Joshua Colwell: Yeah, fire away.
Man: What kind of shudder speed is this? Because, you know, I, you know, if you saw something like this on Earth you would suspect a slow shudder speed.
Joshua Colwell: I cannot answer that question because I didn’t take these pictures and if there’s somebody from imaging on or who knows, maybe (Jeff) or (Brad).
Man: Less than a second probably.
Joshua Colwell: Say that again. Can you repeat that.
Man: Probably less than a second.
Joshua Colwell: Fraction of a second.
Man: Thank you.
Joshua Colwell: You see the, then there’s just I pointed out here another individual ringlet in a gap in the outer part of the C ring there. So there’s some interesting structure here. Some of these narrow ringlets are associated with resonances with moonlets. And some of them are not and we’re not sure exactly what’s producing either the slowly varying structure that constitutes the background of the C ring or these particular plateaus that we see in the image.
Moving beyond the C ring now to the next slide. And I guess I couldn’t find a nice picture of the inner B ring or I neglected to include that so we’re jumping straight into the central part of the B ring. Which is a really interesting location. It’s the one part of the ring that we have not been able to see through yet by any of the stellar radioccultation techniques where we look at light that’s transmitted through the rings. We haven’t yet been able to see through – light through the central part of the B ring. But in imaging, we can see that there’s a lot of structure there from the occultations we have that go through part of this region you see that the amount of material in the ring goes through very abrupt transitions from a lot to a whole heck of a lot on very short time scales. Sort of like you see in the C ring you get these abrupt transitions from a very transparent ring to a moderately opaque ring. It is abrupt transitions on a number of different length scales in the central B ring as well. And we’re trying to understand what’s causing this structure and their various ideas having to do with instabilities, gravitational instabilities, and for some of the structure also patterns developed by effective meteoroids impacting the rings. But most of the structure that we see here in the B ring is not very well understood.
The B ring is where most of the mass of Saturn’s rings is contained. If we took all of the rings and assembled it together into a single object, that object would be about the size and mass of the moon, Mimas, which is a moon with the famous death star crater orbiting not too far out beyond the rings.
Man: Say that again, please.
Joshua Colwell: I said the mass of all of the rings, if you put the rings all together into a single object, that it would form an object about the size and mass of the moon Mimas.
Man: Moon Mimas.
Joshua Colwell: Mimas. That’s the moon with the famous death star crater.
Man: Is that a moon of this planet?
Joshua Colwell: Orbiting Saturn.
Man: Yeah, thank you.
Joshua Colwell: The next slide shows the outer edge of the B ring and the Cassini division. And I’ve just, because the Cassini division is not really an empty place, I highlighted for you where the edge of the B ring itself is.
The edge of the B ring is actually related to the moon Mimas, that we just mentioned. The particles at the edge of the B ring are orbiting Saturn about twice for every orbit of the moon Mimas, and this is one of many resonances between ring particle motions and satellite motions in the ring system that help produce some of the structure that we see. And those sort of satellite related structures in the rings are the ones that we understand the best or we can at least identify the best. But, they do not makeup the majority of the ring structures that we do see. Just beyond the outer edge of the B ring, there’s that black space there is empty space. That’s called the Huygens gap and within it is the Huygens ringlet which is identified there it’s a narrow and very opaque ringlet. And then we see some low optical depth bands, that means low opacity relatively transparent bands that make up the Cassini division on the right half of this picture.
Now, just as a teaser or a preview for something we’ll see towards the end of the talk, if you look just beyond the edge of the Huygens ringlet, before you get to the first band there, there’s another really narrow and not as bright ringlet extending along at the outer edge of that first gap. Just beyond the edge of the Huygens ringlet.
That feature there is pretty peculiar, it doesn’t seem to be there all the time or all the way around the planet. In our occultation data, we see that sometimes, but not all the time, and I’ll show some examples of that. That’s just one of countless examples of how any time you look anyplace in the rings and you’re interested in something, this is happens to me all the time, I start to focus in on one piece of data or look for one particular phenomenon, something over on the side of the plot catches my eye and it’s something that I haven’t seen before and I don’t know what’s producing it. There is just a tremendous amount of new and exciting things we’re seeing in here.
Moving on then beyond the Cassini division, the next slide shows the outer portion of the A ring. Here in the A ring is a different looking beast than either the B ring or the C ring. And the B ring had these big oscillations in brightness that we do not understand very well. And the A ring is relatively bland except for a large number of waves that are launched by resonant gravitational interactions between ring particles and moons.
And pretty much every feature you see in this picture is a wave that can be related to a perturbation by moon orbiting just beyond the edge of the rings. So in the middle we see the Encke gap and within the Encke gap there is a ringlet that is highly variable as we travel along that ringlet there are clumps and bends and kinks in that ringlet. That material is interacting strongly with the moon Pan which is orbiting inside that gap. Now this gap, remember is about 300 350 kilometers across, the moon Pan is about 10 or 20 kilometers across and orbits near the center of this gap and this material is approaches Pan and interacts with it and a complicated structure along this ring as it sort of plays catch up and tag with the moons Pan and that gap.
Now, I’ve only highlighted a few of these waves but almost every feature, except for the blurry looking bending wave on the left, is in fact a density wave. Even these things that look just like a single little ringlet are in fact the peak, the initial peak of a density wave. And these waves are caused by stirring, particularly vigorous stirring of the ring particles by a moons gravity just at a particular resonant location, I’ll talk about how that works in just a second.
Man: A question.
Joshua Colwell: Sure.
Man: Yeah, are these ring waves that we seeing a visual effect of depth or is it just a different color variation?
Joshua Colwell: This is a change in the packing of the particles. So the particles where, in this picture here where the particles are closer together and more densely packed you see a brighter signal and then right next to it where there more rarified and not as closely packed you’d see a darker signal so that there’s actually – it’s actually a change in the packing in the ring particles. So if you can imagine a slinky and putting a pulse wave along a slinky, it’s in some ways similar to that sort of wave where the rings are closer together in some points and farther – the rings of the slinky are closer together and farther apart, and closer together and farther apart. Now we’re actually seeing changes in the packing density of particles in the (unintelligible).
Man: Okay, thanks.
Joshua Colwell: On the other hand, the waves that looks different than the others that’s labeled bending wave is a different phenomenon. That is a wave where the ring has been distorted sort of like corrugated siding: vertically. Out of the plane of the ring it’s been pulled. The ring particles have been pulled out of the plane. In this case the moon Mimas, which is on an inclined orbit, it’s not on exactly the same plane that the ring particles are in, and as that moon goes up and down as it orbits Saturn, it gravitationally tugs on particles up and down and at certain locations those tugs can build up over time and that produces a what’s called a bending wave. Where you sort of get a kind of a Mexican hat pattern in the ring, and in this case then that appearance there is due to just a geometry effect of how the ring is locally bent and warped and the - how light from the sun gets through that to the camera taking the image. That’s a slightly different (unintelligible). We have some (unintelligible).