FTS-NASA-VOICE

Moderator: Trina Ray

April 25, 2006

1:00 pm CT

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 charm_leads@cdsa.jpl.nasa.gov. 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 charm_leads@cdsa.jpl.nasa.gov.

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.

(Jeff): Okay.

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).

Now the next picture...

Man: I have a question.

Joshua Colwell: Sure.

Man: Do these density waves and the bending waves propagate outwards?

Joshua Colwell: The density waves propagate toward the moon. And in all
the examples that you see here, that is outward, and the bending wave
propagates away from the moon, which in this example is inward. Though
in the slide that's if we go back it's outer A ring and Encke gap slide,
those waves are propagating outward, except for the bending wave. And
you can see in the one that's prominently highlighted that's just
outside the bending wave that the first wavelength is prominent and then
it sort of gets scrunched up. And as the wave propagates, the wavelength
changes and gets shorter as it moves towards the moon, eventually the
wave is stamped out.

Man: Thanks. And duration of the propagation is that in terms of days?
Or weeks? Or...

Joshua Colwell: Well, you will see this wave is going to remain there as
long as the moon is there to excite that wave and the propagation speed
of the wave itself is quite slow, like, I believe, it's about a meter
per second or so. Sort of a walking pace, slow walk. But the pattern is
relatively stable, it's not a standing wave, but the moon is constantly
forcing the wave at the location of these special resonance locations in
the rings. And the wave stamps out at the same location based on
collisions between the particles. So, these waves, in the big picture
view look the same as autumn with Voyager for example, 25 years ago, but
the wave itself is propagating along it.

Man: Good, thanks.

Man: I have a question. Does the wave precede the moon like a bow wave
sometimes?

Joshua Colwell: Well, this -- hopefully in some of the -- in one of the
later slides there'll be a cartoon that might help visualize this. This
wave, if we can zoom out and look at it, would be like a very tightly
wrapped spiral around the planet. So completely going around and the
moon is off the side and the pattern is set up by the repeated
interactions of the moon with ring particles at that location, but all
the way around the planet. So the wave itself is essentially a circular
feature, although it's actually a spiral, very tightly wrapped spiral.

Now there are other features like you would (unintelligible) our bow
wave there are things in the rings called satellite wakes and most
notably near the Encke gap that we see in this picture, the moon Pan is
traveling along in that gap and it's traveling slower than the particles
on the inter edge of the gap and faster than the particles on the outer
edge of the gap. And it produces a sort of an asymmetric bow wave from
that passage which is called a satellite wake. So the particles on the
inner edge are zooming past the moon Pan and so you get the wake
extending upwards on one side of the gap, and downwards on the other
side. So that's very analogist to the bow wake of a ship. The density
waves are a different phenomenon.

And to make things even more confusing, we have a different kind of wake
that I'll be talking about a little bit later.

The outer edge on the next slide, we see the outer edge of the A ring
and you see the Encke gap again and you can see these -- if you look at
the outer portion of the A ring there, you see a number of bright stripy
things, those are -- each one of those is a density wave and they get
closer and closer together as you get closer to the moon because these
special resonance locations get more closely spaced. And there's another
narrow gap near the very outer edge of the A ring called the Keeler gap
and within that blue box there, the -- a new moon discovered by Cassini
inside the Keeler gap called Daphnis and if you can -- you might be able
to make out what we were just talking about that sort of bow wave
pattern on the outer edge of the Keeler gap, if you can see a little bit
of a wrinkled, well that's a different kind of a thing, that's an edge
wave. There's all sorts of edge waves and wakes and wiggles in here that
are all related to the moon. You can see a little edge wave on the outer
edge of the Keeler gap caused by the moon Daphnis.

The next ring in our menagerie here on the next slide the F ring. The F
ring is a narrow ring, it's relatively dusty. We see it here with the
moon Prometheus which orbits closely to the F ring, and in this picture
is you can see sort of pulling material out of the F ring comes closer
to it you can see above Prometheus a gore or gap cutting through the
inner half of the F ring where Prometheus had previously had a close
encounter with the ring and caused those particles to get stirred up and
moved out of that region. And you can see a narrow trace of particles
that appears to be connecting Prometheus with the F ring that just sort
of highlights very close connection between the rings and the moon.

Almost finished with the menagerie. The next slide shows the G ring,
this is one of these very tenuous transparent dusty ring which is beyond
the orbit of the F ring. These dusty rings, since they're so
transparent, we see them usually best when we're looking at them nearly
edge on as you can here. In this picture, for this ring, presumably
there are larger objects in that region that are the source for these
relatively short lived dust particles. We haven't seen any of those
individual objects, but there may just be a relatively small number of
guys in there.

And in the next slide, I didn't put a picture of the E ring itself, I
thought this is just such a great picture from the imaging team of
Enceladus and these plumes from the south pole that are spewing water
ice particles and water ice vapor into the space around Saturn that is
the source of the E ring, which is a very broad diffuse ring of Saturn.

Before we move to the next slide, I just want to kind of give a very
brief recap about some of the big picture things that I promised there.
We saw a couple of examples of things that are changing on rapid sign
scales and Saturn's rings and there are a number of other features of
the rings that appear relatively useful. And relative is always relative
to the age of the solar system. For example, the rings are very bright
water ice and composition and yet they are being bombarded by relatively
dark meteoroids all the time which would tend to make them much darker
than they are on a time short, compared to the age of the solar system.

The interactions between the moons and the ring particles causes the
ring orbits to evolve -- the ring particle orbits to evolve and the ring
shape the change on times that are short compared to the age of the
solar system. So, all of these things point to a pretty dynamic system
and one of the possibilities for the origin of Saturn's rings that has
been proposed in the past is if you break up a moon, for example, if a
comet or a meteoroid hits one of these small moons orbiting Saturn and
breaks it up into a lot of debris, that debris does not re-accrete and
it forms a ring around Saturn relatively recently.

If you remember, though, we said that the overall the total ring system
is about the mass of the moon Mimas and that moon is kind of hard to
break up. It takes a pretty unusual impact for that to break up
something as big as Mimas.

It may be that different parts of Saturn's rings have different ages.
Parts of them have been created or maybe recycled or had their clocks
reset more recently than other parts of the rings. One of the big areas
that people are working on right now is to gather the long time scale
story of the history of Saturn's rings, what would Saturn's rings have
looked like 100 million years ago or a billion years ago and what might
they look like 100 million years from now or a billion years from now.
And the answer is probably certainly that they would look very different
and then I think there are different opinions about whether you can see
them at all a billion years ago or a billion (unintelligible).

One of the interesting clues that Cassini has learned that's related to
this is that some of these new small moons that have been discovered and
measured more precisely now by Cassini are very low density and they're
less dense than ice which is generally what they're mostly made of and
the way for them to be under dense like that is if they are actually
assembled assemblages of particles that are stuck together with gaps and
voids in between those individual particles. So we call that a rubble
pile. And not only does the density suggest that some of these moons or
particles stuck together with gas in between them but the shapes of the
particles are also consistent of these moonlets are also consistent with
what you would expect from assembling something close to Saturn where
it's hard to get things to stick together because of that title force.
The certain orientations are easier for things to stick than other
orientations and you end up producing a sort of a flattened flying
saucer shaped moonlet and in fact, that's the shape that we see for some
of these small moonlets rings.

So, all of this is point towards an interesting genetic relationship
between the rings and the moons that may sort of go both ways. Material
maybe going, for example, from the F ring on to the moon Prometheus and
some time in the future, Prometheus may get smacked hard enough to knock
more material off and add more material into the ring system.

Well, the next couple of slides I have talk a little bit about some of
the things that go into making waves and I don't know how much time I
should spend on that, let me take a quick pause and just see if
anybody's got any questions about the overall architecture, any of those
big picture items I saw -- I discussed.

Too bad I can't take a voice break, but that's okay. We'll move along.

The next slide is a little bit of an introduction to how the waves work
in Saturn's rings and the only thing I want to say here is that Saturn
is a highly flattened planet. If you put a particle orbiting that planet
on an eccentric orbit, a non circular orbit, that is also not in the
equatorial plane as three different periods of motion. Time it takes for
it to go around the planet, the time it takes for it to move closer to
the planet and farther away as it completes it's sort of egg shaped
eccentric orbit and the time it takes for it to go from it's Northern
most extent to it's Southern most extent and back if it's on a tilted
orbit. The fact that those three periods are different is what leads to
a different locations for things like bending waves, density waves that
are caused by the same moon.

The next slide, which has the equations for residences on it, the only
thing I want to point out are the two equations on the lower right part
of the slide. The one that ends with M over M -1, that describes the
approximate relationship between a ring particle orbit and a moon orbit
that gives you the strongest resonance that produces a density wave. And
the one that ends with M +1 over M -1 describes the configuration of the
orbits that gives you the bending wave, or the vertical wave.

What those numbers mean is, explained on the next slide, so we can skip
over that part if, I don't think we need to go into that in too much
detail. So the slide that says more on resonances strongest resonances
producing density waves are of this first order tight M to M -1 and an
example of that would be, for example, let's take M=20, and the 20 to 19
resonance is the location in Saturn's rings where a ring particle
happens to orbit based by it's distance from Saturn 20 times, or every
time that a particular moon, a little bit further away from Saturn
orbits 19 times.

And at this location there is a density wave that is launched by that
periodic alignment of the ring particles and the moon. And this
resonance is just like if you've got a child on a swing wants to swing
at some particular frequency, and if you push every time the child is at
the back part of that swing, you're pushing in a 1 to 1 resonance. Your
push once for every time the swing wants to swing, and the swing will go
higher and higher. If you pushed, for example, every other time, it
might take a little bit longer, but you would still also build up the
amplitude of that swing and you would be in a two to 1 resonance.

And the same thing is going on here in Saturn's rings. At certain
locations in the rings, there's a resonance between the orbital motion
of the moon and the orbital motion of the ring particles that creates
these waves.

You remember way back when we saw the picture of the C ring with one of
those narrow ringlets, that was called the Titan 1 to 0 ringlets, that
particular resonance is a little bit different because it's 1 to 0
resonance and it has to do with the time it takes for the moon Titan to
go around Saturn once. That's where the 1 comes from and the 0 refers to
the time it takes for a ring particle of that location for its orbit to
rotate -- the actual orbit itself to rotate around Saturn. Though it's
not a commensurability or resonance between the actual particle motions,
but between the orbit of the ring particle and the motion of sight.

So, I don't know if this is totally maybe too much minutia of numerics,
but this is what goes on and how the moons are able to do some of these
features in the ring. If we want to take a look at some of those
features a little closer, this is the same picture we had before on the
next slide and if you see that little blue square on the left, the slide
after that is going to be an image inside that blue square that was
taken when Saturn arrived at Cassini in the summer --when Cassini
arrived at Saturn in summer of 2004, so if you've got the picture up now
that says the bending wave and a density wave with the Mimas 5:3 bending
wave and the Prometheus 12:11 density wave wherever we're zooming in
this image is about 500 kilometers across so you can see the density
wave there is about 50 or 60 miles in extent and the bending wave on the
right is larger.

And the rings are just -- the A ring in particular is just loaded up
with these and as we get closer to the edge of the rings, we're getting
closer to the moon those resonances and those wave that were based.

Okay, what I want to do is move past the next two slides the one that
says streamlines perturbed by moon. And this shows a five armed spiral
density wave without any gravity in the ring itself. And you can see
that if we zoomed in on part of this, we would see a regular wave
pattern going across the ring but looking at the global system we see
that it's a spiral that is repeated M times, in this case five times
around the system. And if we go to the next slide that says perturbed by
moon and ring, we can see the affects the rings gravity is low, if the
moons gravity are taken into account, that the arms of that spiral get
closer together as you move farther away from the planet and closer
towards the base. And that's the pattern that we see in these density
waves and rings is that they get scrunched up as they move away from the
resonance location and the amount that they get scrunched up how quickly
they get scrunched up like that tells us something about the mass of the
ring at that location.

The next slide, size of the particles in the rings, broad distribution
of size in the main rings the common question is how big are the
particles in Saturn's rings and the answer is pretty much every size you
can think of. But the smaller particles are more numerous and the E
ring, the Cassini division, the F ring are significantly more dusty and
not as many big particles, the G, E and D rings that's even more
(through) and for the A and the B rings, a centimeter to many meters at
least, of course, in the A ring, we already have two examples of
moonlets that are kilometers across and larger within the rings and
there are others as I'll show in just a second.

The next slide is a result from radio science occultation data where
they shine a radio beam through the rings at three different wavelengths
of radio waves. And the different amount of the attenuation of that
radio signal at those different wavelengths is due to differences in the
sizes of the particles in the rings. And so the color coding here is
related to sizes of the particles in the rings and the blue is
indicating more centimeter sized particles than the pink and pink and
greenish tones. So, as we get towards the outer edge of the A ring here,
you can see the very outer edge there, there's that little Keeler gap
that's got one moon in it then there's the bigger Encke gap with the
moon in and it the sum of the strong density waves showing up as bright
bands in this picture. And interesting thing to note here is that some
of those strong density waves are showing up particularly blue
indicating more small particles there and that may just simply hit those
waves, particles are getting stirred up more by the moons running into
each other more vigorously and you're getting smaller particles knocked
off as we get out to the very outer edge of the A ring, it's the number
of small particles (unintelligible).

The next slide shows new results from imaging team of evidence for a
small moon in the moonlet in the A ring and these images are taken when
Cassini arrived at Saturn. And those two little faint dashes in the
middle are perturbations in the ring material produced by a small
particle about the size of a football field, small moonlet, a big
particle, in between those two dashes. So we have now evidence for any
moonlets that are not big enough to open up gaps in the rings. So the
answer to the size of the -- the answer to the question of the size of
particles in Saturn's rings is really from dust to mountains, and
everything in between.

The next slide: composition of the rings. Water ice and dirt. And we got
some, I don't know, if you consider any press is good press, but John
Stewart on the Daily Show made fun of us for talking about going to
Saturn to learn that the rings are made of ice and dirt.

Of course, it's not quite as simple as that, they are predominantly
water ice and that's interesting observation because as I said before
rings are gradually being polluted by interplanetary micrometeoroids.
That's the dirt part of this slide and the amount of contamination or
non-icy things that we see in the rings and where that's located and
we're mapping that out is one of the big pieces of evidence that we're
using for studying the history of the ring.

A closer look at this in the next slide. The infrared - the visual
infrared mapping spectrometer shows maps of the A Ring in different
colors of infrared light that are showing variations in the amount of
water/ice and non-water/ice. There's what - dirt for simplicity across
the rings and again this distribution and the amount of non-ice
contaminant and where it's located within the ring tells us something
about how the ring are revolving.

And although Jeff would object on the next slide, and I'll explain why,
this is a picture that has been called a false color/false image image
of the A Ring in the Cassini division. It's false color because it's
from a ultraviolet image so of course we have to convert the ultraviolet
wavelengths to visible light.

And it's a false image because the original picture is just a strip
across the center and to make it easier to see this as rings I smeared
it out as an azimuthally but there's no two dimensional information in
this image. It's a one-dimensional picture that's been smeared out
artificially. And if we had data extending beyond the edge of the ring
that whole black region there would be red as well because in the
ultraviolet, you know, with the color scheme that we used the sky glows
red hydrogen gas throughout the solar system. So where the rings are
transparent we see red and that's why that right red band at the right
there is the Encke Gap. That's ultraviolet light shining through the
Encke Gap.

So the ring summary - the end of the tutorial portion here. That slide -
I think I've said all of this so we can go ahead and skip over it to get
into some of the UVIS stuff here.

The age problem I have also mentioned, different lines of argument
showing lifetimes for evolution of the rings that are much shorter than
the age of the solar system.

And other big picture problems on the next slide - 34, trying to
understand the structure that we see in the B Ring and the C Ring which
is not fully understood yet. The role of limited accretion is the
ability for some particles to get the other in certain orientations very
slowly as seems to have taken place with some of the small moons that we
(unintelligible). And what did the rings look like when dinosaurs roamed
the earth and what will they look like in 100 million years from now?

So Cassini in the next slide just says we're - the answer to these big
questions, we're taking a careful measure of the rings with the various
instruments on Cassini and I want to go ahead and (unintelligible).

Man: Telecon, you know, I'll be off in a minute.

Joshua Colwell: I want to go ahead and move along to the next part,
Stellar Occultation Geometry, which talks about the UVIS observations of
that.

So on that slide which is Number 36 the basic technique that we use for
getting the structure of the rings is to measure the brightness of the
star as the rings pass between our instrument and the star. And we are
able to get that star brightness measured at - samples in the rings that
are only about 10 or 20 meters apart. So we get a very, very high
resolution pan across the rings and the next slide shows one such scan
degraded to a resolution of 50 kilometers. This is (unintelligible)
stellar occultation.

And here we see a different sort of view of the main ring - the B Ring
where most of the material is. And you can see these what looks like
noise in that blue curve, these big strong oscillations correlates with
those different bands - that unexplained banded structure we saw in the
B Ring. And the A Ring on the right there which is relatively bland and
featureless and those peaks that you do see are locations of density
wave.

On the next slide the Janus 2 :1 density wave as seen in the occultation
data shows what a cut - slice across one of these density waves, this is
a particularly strong density wave, but extends for hundreds of
kilometers looks like. And you can see that those wave peaks are getting
closer and closer together as we move from the resonance at the
left-hand edge of the plot closer to the moon as we move across to the
right. And that decay of the wavelength tells us something about the
mass density of the rings.

On the next slide, Alpha Virginis A Ring Opacities, this is an
observation where we looked at one star as it went behind the rings
twice at two different locations - two different azimuths around the
ring. And here you're seeing the A Ring and the outer part of the
Cassini division and these curves are high where the rings are opaque
and they're low where the rings are transparent so a gap is about an
optical depth of zero.

You can see an Encke Gap identified there and some of the strong density
waves are showing up as peaks. And one of the interesting things that
you notice is that the blue curve and the red curve do not lie exactly
on top of each other in the middle part of the A Ring and we'll get to
what's causing that in (unintelligible).

We now zoom in to the middle part of the A Ring, sort of the Kansas of
the A Ring, it's relatively broad and flat.

The area outlined by that blue arrow on the next slide shows the - just
that region at higher resolution. And I've highlighted a number of
different resonances so now these numbers associated with the resonances
and the density waves from Prometheus 8:7 means that the wind particle
at that location is orbiting about eight times every seven orbits of the
moon Prometheus and so on.

And if we zoom into the region around that Prometheus 8:7 density wave
highlighted by the red arrow you can see that at higher resolution on
the next slide and the contours and the green slope that are on that
slide show the data analysis technique that enables us to measure the
mass of the rings at the location. And the damping is - enables us to
get the local thickness of the ring which is how we're able to say the
rings are only about 10 meters thick locally from - in one way it's from
analyzing these density waves and how they damp out.

The next slide shows an example of a weak wave in the Cassini division.
Most of the density waves we see are in the A Ring but there are some
now that we see Cassini division. And this shows that it is much less
massive, the behavior of this wave, 1.6 grams per square centimeter,
which means if you could convert the Cassini division in this location
to a solid slab of water/ice it would only be about 1-1/2 centimeters
thick or about a little over half an inch thick. So there's not a lot of
material there and that again highlights this - one of the age problems
for the rings - that bombardment of the rings by dirty micrometeoroids
would grind down something this flimsy mass on a relatively short time
scale.

The next slide shows the - an example of measurements of the mass across
the ring from analyzing a variety of density wave. And you can see
(unintelligible) looking at the A Ring the numbers are around 40 grams
per square centimeter or about 30 or 40 times higher than they are in
(unintelligible).

And the next slide, Density Wave Summary, summarizes those numbers that
has been learned from analyzing not only Cassini but also Voyager data
of density wave in the ring that here you see the equivalent moon radius
of the ring system of 150 kilometers. This is - or what I was saying
earlier about if you put the whole thing together it would be about like
in the (unintelligible).

Now the next slide shows the last thing I want to talk about but also
something I'll spend a little bit of time on which is measurements that
UVIS made.

And if you remember the earlier plot with the red and blue curves that
showed they weren't exactly on top of each other for one occultation
observation on this one we see a very dramatic difference in the opacity
of the A Ring looking at it - looking at the same star shining through
the ring at two different azimuths around the ring system. You can see
that the red and the blue curves are right on top of each other in the
left-hand part where we're in the relatively transparent Cassini
division and then once you hit the inner edge of the A Ring these curves
diverge by a factor of 2. You get a very different amount of light
shining through the ring even though we're looking at the same star with
the same instrument at the same radial distance from Saturn.

And the next slide shows another example of that. This is three
different occultations showing the whole A Ring here. And you can see
that this difference in the amount of light that shines through the
rings varies as we move in position across the A Ring and the amount of
difference is quite dramatic - the amount of difference in light that
shines through the rings.

Well to get a better feel for how we're - how this - what this is
telling us about the rings the next slide is a - this is - this time
it's not a false color/false image. It's a false color/true image of
Saturn's rings with the ultraviolet imaging spectrograph. This is the
slide that says, FUV Observation of 26 Tau 8. And this picture was made
by scanning our slit across the rings at the same rate that the star
appeared to drift across the ring, keep the star in the center of our
slit.

So if you imagine looking through a narrow vertical slit and scanning it
across a scene nothing is changing in that scene, when you put that all
together you'll get a picture. In this case a star is moving across it
and we keep the star in the slit at all times so the star becomes that
bright yellow band across the center and we see the rings in - shown
here in blue that's reflected sunlight in the ultraviolet.

And you can see that the starlight is attenuated in the B Ring, that
there's not as much shining through the B Ring, which is this broad
central blue region of this - of the thing here. That big gap there in
between the two main predominant rings is the Cassini division which is
not showing up in its ultraviolet color picture. We just see the red sky
shining through and then the A Ring you can maybe make out the Encke Gap
towards the outer edge. And I've got a couple more examples of this and
one of them will show this asymmetry that is so interesting.

The next slide shows the same sort of picture for the star Delta
Aquarius and in this one you can actually see at the - running
diagonally from the bottom center to the upper right a very narrow
purplish line. That's the F Ring. And again the right line running
horizontally across the middle is starlight which is blocked out by ring
material where the ring is dense and you can see it disappearing behind
the central part of the B Ring where it's so opaque and partially
blocked out by the A Ring on the right hand side. So it's by - this is
just visualizing how we convert these occultation traces into these
detailed radio profiles.

And the final one of these particular images here, the FUV Observation
of Alpha Leo, Slide 49, you can see this has azimuthal difference. This
is the same observation that had this big difference between the blue
and the red curves. The geometry of this picture that's sort of dark
blue here is that the Cassini division is the dark band that is near the
right hand part of the picture, A Ring is in the upper right hand
corner, and we've got - moving to the left the Cassini division and then
the B Ring. Then there's a very abrupt dark boundary in the middle of
the plot cutting across the blue ring. That's the shadow of the planet
on the ring.

Now the bright band running horizontally across again is the starlight
and if you look at where the star cuts it cuts the A Ring twice, once
right in the middle and you can see that the starlight is attenuated.
It's going from bright white to sort of the faint-ish pink and blue.
Part of it is blocked out by the A Ring over there and on the fair right
hand edge of the picture the star is behind the A Ring again and it's a
very bright signal of the star through the ring. So we're seeing much
more starlight through the A Ring on one part over here on the right
than we are in the middle.

And the next slide is a cartoon. We call it the Granola Bar Model of
Saturn's rings that explains how we can have this sort of asymmetry. If
that blue spot can be imagined to be the star at one particular instant
and we're zooming in at very high resolution at ring particles here.

If the ring particles are clumped, instead of being distributed
randomly, if they are clumped in some sort of aligned fashion as is
shown here the amount of starlight you see through the same patch of
ring material depends on how your line of sight is oriented with respect
to those aligned clumps of particles.

If you're looking parallel to those aligned clumps, you'd see more light
through, would infer that the rings are less opaque. Looking across them
as is seen in the bottom one of these three cartoons then you see very
little starlight through and you would assume that the ring is more
opaque.

The next slide is a summary. It's a complicated slide. I'm not going to
go through all of the information here. I just want to get one point
across. These various colors traces across the A Ring are color coded
with brightness and shown in their correct azimuth as we cut across the
rings and where the curves are green we infer that the rings are more
opaque and where they're darker blue they're less opaque. And you can
see that that varies with azimuth across the ring system consistent with
what you would expect from aligned structures in the rings.

And by constructing a very simple idealized model of the rings in this
way which is shown in the next slide with a few parameters that
identified orientation of these granola bars and their size and their
shaping and their spacing then we can try to see if that explains these
differences in the amount of light that we see through the rings from
the occultations.

Bear in mind that these individual structures here are maybe only 10 or
10s of meters across - smaller than we're actually directly sampling
with the occultation. But by virtue of their organization being parallel
and in line like this we're able to say something about them by having
multiple views through the rings.

The next slide - it says, Example Model Fit to Data, shows just one
particular location in between a couple of density waves. All of our
measurements of the transparency ring shown in the blue asterisks and
the model fit to those using this aligned self-gravity wake model and
shown in the red diamonds and we get a good fit using this.

And the next slide then shows the results of running - applying this
model to all our data so far where we're able to measure the amount of
particles that are in between aligned clumps in the rings, that we're
able to measure the shape of these clumps.

Those red asterisks go with the right hand axis and they show that the
clumps are very flat, about 1/4 as vertically thick as they are wide,
and that there is not - the green curve shows that there's not much
material in between these clumps - that the rings are relatively empty
in between clumps.

And the next slide shows the spacing of these clumps and how that varies
across the ring with red pluses that the gap width or spacing between
the clumps is about one or two times the width of the clumps themselves.

Now with this model we're not actually saying how big those particular
clumps are but we are able to measure that and the next slide with the
black background shows one way that we're able to get an actual size
measurement for these particular small structures in Saturn's rings.

If you imagine measuring starlight shining through a cloud of small
particles like we see in the left-hand panel every measurement you would
get the same amount of light through. Every one of those little ellipses
there needs the same amount of attenuation. On the right hand cartoon
you see if there are big particles sometimes your star would be totally
obstructed by one of those big particles and sometimes it would be
totally in between them even though the total amount of material between
the left panel and the right panel is the same and so you get a noisier
signal if the particles are big.

And in fact if we look at the next slide - in fact let's go ahead to the
one that - Slide 58, Expected and Measured Variance. The green curve
shows how much noisier the data is than it would be if all the particles
were very small which is what is shown in the blue curve. And in the A
Ring over on the right - this is just for one particular occultation you
can see that there's this huge difference between the green curve and
the blue curve which is telling us that we're seeing the signal of big
particles or clumps of particles in the A Ring.

And next slide just illustrates that in slightly different way where we
took the ratio of those two curves. So anywhere that curve is bigger
than one on its - just the green curve, Slide 59, anywhere that curve is
bigger than one tells you that we're seeing quote/unquote big particles
in the ring and big in this case is meters are larger.

And if we take a closer look on the next slide at the A Ring we can see
that this measurements of particle size also depends on the angle that
we're looking. And that's because if you're not looking at spherical
object you get a different size depending on how you look at it. If you
look at a granola bar face down on the table it looks big and if you
look at it end on it looks small. So by virtue of getting stellar
occultations through the rings and the number of different viewing
angles we're able to measure the size of these oblong structures in all
of their three dimensions.

And the next slide just shows a computer simulation that illustrates
this. This is done by John Weiss and Glen Stewart. John Weiss now on the
Imaging team and Glen Stewart's here at Colorado.

And that computer simulation the red and blue particles on the right are
showing structures of particles that temporarily collect together and
then are torn apart getting back to this idea of particles running
around Saturn on a racetrack with the particles closer to Saturn going
faster than the ones just a little bit further away. And that's why you
have these slanted clumps of particles here. The ones that are on the
right edge are moving slower than the ones that are one left edge and so
the ones on the left edge pull ahead of the ones on the right edge and
eventually tear these clumps apart.

And the scale here is showing that these clumps are on the order of 10
meters or so across where the spacing is maybe a few tens of meters. And
by shining starlight - looking at starlight at a number of different
angles through the ring system we're able to put together something
about - actually measure this structure of these microstructures in
Saturn's rings.

And the next slide just summarizes what we find for these things which
are - we're calling self-gravity wakes. They've been known about for
some time because we see this asymmetry in images of the A Ring as well
and now with the detailed occultation measurements we're able to measure
them with much greater precision.

And of course this ties back if you think about that computer simulation
on the previous slide for the details of how the particles behave when
they run into each other. Are they very sticky? Are they very bouncy?
All of those fundamental aspects of how the ring particles behave have
these is now observable consequences structures in the ring.

And finally the last two slides just show - remember I asked you to take
a look at that little tiny ringlet beyond the Huygens Ringlets and one
of the images of the B Ring (unintelligible) give you a reference for
which slide that was. That was on Slide 11 - way back on Slide 11 I
pointed out this very narrow feature beyond the B Ring. Well here we see
that feature in occultation data and what I was talking about is how
it's not the same around Saturn.

The Huygens Ringlet is this big feature on the left showing up in the
red and the blue slightly offset because it's not a circular ring. And
then at the outer edge of the gap in one occultation we see that ringlet
shown by that prominent red spike and then - but it's not there in the
blue. In fact that blue spike is within one of the bands of the Cassini
division.

And the final slide shows another example from two different
occultations - yet two more occultations showing that ringlet acting
like some sort of discontinuous ring. Just one example of the things
that we're interested in looking at.

And with that hopefully people have some questions that I can answer.
I'd be delighted to.

Man: Yes can you tell me how wide the Keeler Gap is?

Joshua Colwell: The Keeler Gap is about 30 to 50 kilometers across. I
think its width is not constant because the moon inside there produces
some pretty strong perturbations on the edge of that gap but it's about
that order - 35 or 40 kilometers across.

Man: Thank you.

(Matt): I have a question.

Joshua Colwell: Go ahead.

(Matt): Yes it's (Matt) (unintelligible) with the Saturn Observation
Campaign. I've had a lot of people that I work with in outreach that
have some theoretical ideas. One of being at end of mission to basically
dive the Cassini into the ring plane and attempt to image ring
particles. Is this a feasibility? Is this feasible to do or...?

Joshua Colwell: Well one of the problems that we've got with that - with
the end of having a very exciting end of mission for Cassini compared to
Galileo is that Galileo could be in radio contact with the Earth at the
same time it was taking pictures of whatever we wanted it to take
pictures of. Unfortunately for Cassini the cameras are bolted onto the
spacecraft...

(Matt): Oh that's right.

Joshua Colwell: The antenna is bolted on so.

(Matt): No scan platform.

Joshua Colwell: You don't have - right and that makes it a little bit
tricky to do stuff, to get data from the very last.

However, you know, if the mission, you know, we're hoping for multiple
extended missions and at some point when it really is looking towards
the end one possibility would be to put the spacecraft in a more risky
orbit that took it in closer to the rings where you'd have a closer look
at some of the rings. But imaging - I think we're better off saying that
we've actually got images of ring particles and their names are Pan and
Daphnis than imagining that we would get a picture of a 10 meter size
ring particle.

(Matt): Thank you.

Man: I joined the talk a little late so I don't know if you already
talked about the - but can you tell us a little bit about the E Ring?

Joshua Colwell: The E Ring? Yes. Well the E Ring is a - extends from
about 3 Saturn radii out to at least 8 and possibly much more. The CDA,
the Cassini - the Cosmic Dust Analyzer on Cassini is - measures the
impact of dust particles on that instrument and they've measured impacts
out quite far, out to the orbit of Titan. So the E Ring may have a very
broad extended part out there. Those particles may somehow be not really
genetically related to the E Ring. They may be coming from another
source but the main E Ring is produced by the activity that has now been
directly observed on Enceladus.

The South Pole of Enceladus is this geologically active region with
water/ice particles as well as water vapor spewing out of these
fissures - the so-called tiger stripes at the south pole of Enceladus.
And that material then goes into orbit around Saturn at the orbit of
Enceladus and those small particles get strongly perturbed by radiation
pressure from the sun and that puts them on eccentric orbits so that
those particles end up filling a large volume in space around Saturn
that makes up the E Ring. Very tenuous, not very opaque ring, which is
why it doesn't show up in most of the pictures that you see of the ring
system. Usually we - that were standing well back from Saturn at a large
distance and looking at the system edge on, very long exposure.

Man: Thank you.

Trina Ray: Okay are there any other questions?

Well Josh I'd like to thank you today. We really appreciate the time
that you've taken to put together such a fantastic talk on the rings of
Saturn.

Joshua Colwell: I'd be happy to answer any questions by email if they
occur to people later.

Trina Ray: Oh absolutely. That's a good idea.

Then to let people know that we will be having our talk in May - on the
last Tuesday of May is May 30. It will be Krishan Khurana and he will be
talking about Magnetic Portraits of the Icy Satellites. He'll be talking
about both Jupiter's moon and Saturn's moons also.

So until then everybody have a great month and don't...

(Jane): And but - Trina before you sign off.

Trina Ray: Sure.

(Jane): This is (Jane) again. For those who joined late at the beginning
of the telecon Trina asked everybody to send a note saying if you were
here on the telecon or if you download the material later. So I just
thought I'd remind everybody because some people did join late.

Trina Ray: That's a great idea - thanks (Jane).

So again that email which - like (Jane) said we're trying to get a sense
of the demographics of the Charm audience so if you could drop an email
to Charm, C-h-a-r-m, underscore Leads, l-e-a-d-s, at
c-d-s-a.jpl.nasa.gov and just tell me who you are and what organization
you're with we'd really appreciate it. And also if you look at your
original invitation you can see the Charm Leads email on that.

Man: What is Josh's email?

Joshua Colwell: Sorry. It's joshua.colwell@colorado.edu.

(Jane): Or just send it to Charm Leads and we'll be sure to get it on to
Josh. Okay so...

Trina Ray: For anybody who didn't get that.

(Jane): Yes if, I mean, if you have any questions and you can't get a
hold of Josh just send an email to the Charm Leads and we'll forward
that on to him.

And again, Josh, thanks very much. We really appreciate it and everybody
have a great month and we'll see you in May.

Joshua Colwell: Bye.

Trina Ray: Good night everybody.

Man: Bye.

Woman: (Unintelligible).

END