**FTS-NASA-VOICE**

**Moderator: Trina Ray**

**November 28, 2006**

**9:00 am CT**

Operator: Excuse me. This conference is being recorded. If you have any
objections, you may disconnect at this time.

Ma'am, you may begin.

Trina Ray: Thank you very much, and welcome everyone to the CHARM
telecon for November 2006. And we\'ve got a really interesting topic
today.

We're joined by Dr. Matt Hedman. He\'s at Cornell right now, and he\'s
been working with the imaging and VIMS teams, in particular on the Rev
28 high-phase observations that we took this last September, which were
really, utterly, utterly unique.

And in fact, the image that came from these has made the cover of
National Geographic this month.

So, it\'s one of the most spectacular images we\'ve taken in the mission
so far. And Dr. Hedman will be joining us to tell us all about it.

Matt, take it away.

Matt Hedman: Okay. Can you all hear me fine?

Trina Ray: Just fine.

Matt Hedman: Okay. Just wanted to check that before the speaker phone
went on.

Okay, like Trina said I\'m going to be talking about these wonderful new
images we have of Saturn's rings. And if there\'s time and interest,
I\'ll also present some new findings we\'ve made on Saturn's faint
rings.

Now, as you all know, Cassini has taken a lot of beautiful pictures of
Saturn, its rings and its moons over the last 2 years. But these
particular images I\'m going to be talking about are pretty unique and
pretty spectacular.

Now, to understand what makes them so unique\...

Man: (Excuse me...)

Matt Hedman: Hello? I thought I heard someone.

To understand what makes them so unique, go on to Slide 2 of the
presentation, which shows a cartoon of the Sun, the Earth, and Saturn.
And just remember, whenever we\'re looking at an object in the solar
system, we\'re always- at least in visible wavelengths, we\'re pretty
much always looking at reflected sunlight \-- at least most of the time.

And so, the illumination geometry of an object is typically described in
terms of this quantity known as the phase angle, which is often written
as - with the letter Alpha.

Basically, what we have in this little cartoon on Page 2 is just a ray
from the sun, hitting Saturn, bouncing off it and heading back to Earth.
And the phase angle is defined as the angle between the ray going from
the Sun to the object \-- Saturn in this case \-- and the ray going back
from the object to the Earth.

So - and we can also define a complimentary angle which here is marked
Theta, which is called the scattering angle, which is, you know, is just
180 degrees minus the phase angle.

So, the thing to just realize is, when Alpha is small, you\'re basically
looking at the lit side of the planet or the object you\'re looking at.
When Alpha is large and the scattering angle is small, you\'re basically
looking at the dark side of Saturn or the dark side of the ring
particles or the dark side of whatever you\'re interested in.

Now, the thing to realize is, when we\'re on Earth, since the Earth and
the Sun are always on the same side of Saturn, we can only really look
at Saturn at low phase angles. Lower than maybe about 10 degrees. I
think is roughly the right number.

And what this means is we\'re always almost looking at the bright fully
illuminated disk of Saturn. The image at the bottom is a typical shot
you get from the Hubble Space Telescope. Actually, this is a little bit
of an exceptional shot taken at extremely low phase angles.

But you see what you typically see in images of Saturn taken from the
Earth. You see very bright back scattering rings and a fully illuminated
planet and little evidence of shadows anywhere.

Now, the great thing about Cassini is since it\'s in orbit around
Saturn, we can view a lot - we can view Saturn from a lot of different
geometries and a lot of different viewing angles.

And if you go on to next page \-- Page 3 \-- you will see one of the
nice images you can get from Saturn from Cassini, which is just
something we can\'t see from Earth.

This is an image of basically a half full Saturn, phase angle of 70
degrees. And here, you can see things you usually can\'t see quite so
nicely as on Earth. You can see things like the shadow of the planet on
the rings extending off to the left. And you can see the shadow of the
rings on the planet at the top of the image. And these regions of the
rings are very difficult to observe from the Earth.

So looking at a bunch of different perspectives is often nice. Now,
there is one sort of geometry that Cassini has trouble reaching. And
this is extremely high phase angles. Phase angles around 180 degrees.
Because if you\'re - if you go back to Page 2, you\'ll see that if you
have Alpha equal 180 degrees, you\'re looking at the dark side of the
planet, but you\'re also looking back towards the Sun.

And looking back towards the Sun, is usually not a good thing to do with
a camera or a telescope. But there\'s one - there\'s a few opportunities
to do - to actually look back in this sort of direction which occur only
when Cassini flies through Saturn's shadow. And that sort of condition -
situation is shown on Page 4.

This is just again a little cartoon showing the basic geometry that
happens when Cassini flies through Saturn's shadows. You have - imagine
the Sun is way off to the left end of the screen, the Saturn rings are
shown in cartoon form, the blue region is supposed to be a
representation of Saturn's shadow and Cassini is, obviously not to scale
in this image, flying through Saturn's shadow.

Now in this geometry, the light from the -- the direct light from the
Sun is blocked by the planet. So basically, Cassini is enjoying its own
private eclipse.

And this means it\'s safe for us to turn our cameras back towards the
Sun and back towards Saturn and take a picture of the - and take
pictures of the Saturn system at extremely high phase angles.

Now, what was special about this Rev 28 opportunity on September 15 was
that Cassini was very far from Saturn at this time. It was about 2
million kilometers from Saturn, which had two nice advantages.

First of all, it moved very - the spacecraft was moving very slowly. So
the spacecraft - so that meant we spent a long time in the shadow. We
actually spent over 12 hours in the shadow. Previous shadow passages
have only taken a couple of hours. So in this time, we had a lot more
time to take a lot more images and other data sets.

The other advantage is because we were so far away, we could actually
take mosaics that pretty much cover the entire Saturn system, allowing
us to get a more global overview of what the Saturn system looks like at
this sort of geometry.

Now, since we\'re basically looking essentially at the dark side of the
planet and the dark side of the rings, you might have first thought that
this would be a particularly boring kind of picture, because we'd just
seeing the dark edges of things.

Well, as you know, that wasn\'t true and maybe, one could think that
wouldn\'t be true because eclipse pictures on Earth are quite pretty.

And of course, what we saw is shown on Page 5. As Trina pointed out,
this is an image that\'s now made the cover of National Geographic. And
I think it is a very striking picture.

You can see basically the dark disk of Saturn surrounded by a bright
illuminated limb. You also see framed around this Saturn - bright
Saturn's rings in various forms.

And of course, the coup de grace was that not only was the Sun which
is - not only were all the rings in the image, but so was our little
planet Earth, which is marked out in the inset at the bottom of this
figure.

And that should have made sense, we\'re looking back towards the Sun,
we\'re also looking back towards Earth. But it's nice to see that, you
know, basically every person in this telecon is in this picture. Just
not very well resolved.

So, this is clearly a nice picture. But I\'ve also heard comments to the
effects that it looks weird or it looks fake. Like one of the comments-
one of the things is, it looks a bit like Saturn - the disk of Saturn is
somehow overprinted on top of the rings in a somewhat non-natural way.

And - but, what I want to say is really there\'s nothing too screwy
that's been done with this image. It\'s pasted together out of several
different image frames and the color is an approximation of natural
color based on data from ultraviolet, visual, and infrared filters.

But we haven\'t done anything really violent to this image. We didn\'t
stretch parts of this in different ways. We didn\'t like make part of
the rings brighter and the disk of Saturn darker or anything like that.

It\'s - this is a semi-natural representation of what the system looks
like at this particular geometry. And it\'s just that this particular
geometry is rather unique, and it\'s not something we\'re used to
seeing.

So - and I\'m going to talk about in the next few minutes, just trying
to explain what\'s going on in this image because there\'s a lot going
on and see if I can make it make a little more sense to you. And please
feel free to interrupt me with questions whenever you feel like you need
to.

So first, to go on to Figure 6.

One of the most dramatic features of this image, at least to my mind, is
you can see all of Saturn's rings. In a usual low phase image, you -
like the one shown on figure - on Page 2 is - what you mostly see is the
A-ring and the B-ring. And maybe, if you look closely, you can see the
C-ring in the Cassini division.

What\'s amazing here is in this image, you see not only the A, B, and
C-ring which are marked here, but you can also see the D-ring lying
interior to the C-ring.

The F-ring, lying exterior to the A-ring, is actually the brightest ring
in this image. You can also see the faint narrow G-ring outside the
F-ring. And the E-ring, the bright wide E-ring outside the G-ring.

In normal image - in most other images, you cannot see all of these
rings in a single image without saturating - wildly saturating some and
heavily underexposing others.

I mean usually, when we wanted to display all of the rings in some
fashion for public consumption, you had to go to an illustrator, and
have them draw kind of - fakes - draw a version of the rings where they
made the G-ring and E-ring and the D-ring brighter than they naturally
would be just for the - just so that you could have them all on a single
display.

But in this case, we\'re actually seeing them all at comparable
brightness. And that\'s - that is a product of this really bizarre
geometry where we\'re looking at extremely high phase angles and
extremely low scattering angles.

And the reason this is happening is because basically these different -
the main rings, the rings are usually bright, the A, B and C rings have
different size part - have much different particle sizes on average than
the typically faint rings like the D, G, E and F rings.

Okay. So let's try and understand what\'s going on here.

The first thing to realize is remember, what we\'re seeing in this
image, we\'re not - because the Sun is hidden directly behind the disk
of Saturn, we\'re still seeing scattered light here. We\'re still seeing
light bouncing off of ring particles and various other things.

So there are basically two ways that a ring particle or something can
scatter light from the Sun to our camera. The first is illustrated on
Page 7. This is basically just simple reflection off of the illuminated
surface. And this is what we usually see at low phase angle. We\'re just
looking at the bright illuminated side of many, many ring particles.

However, as we go to higher phase \-- higher and higher phases, we go
from seeing the bright illuminated side of the rings to seeing the dark
un-illuminated side of the ring - of the ring particles.

And as we go to this very high phase limit, all we will see is a very
narrow crescent of illuminated ring particles, which is not going to be
very efficient at scattering radiation to our eye from the Sun.

It actually is even worse than this because of the properties of the
surf - many solar system body's surfaces tend to be more back
scattering. So not only is a smaller fraction of the surface we see lit,
that lit fraction actually gets darker as we go to higher phase in
general.

So, this kind of reflection tends to get - at least this kind of
reflection from a single particle tends to get less efficient as we go
to high phase. Now the other possible way that a particle can scatter
light is shown on the next page, Page 8.

I'm sorry there\'s been a little glitch of the formatting here but
anyway. A small one.

And the other mechanism by which a particle can scatter light is by
diffraction. The simplest example of diffraction of course, is you take
a sheet and punch a hole in it.

If you illuminate that sheet with light, the light coming through the
hole, those that remain collimated, it spreads out over a finite range
of angles. And the range of angles that spreads over is basically given
by the ratio of the wavelength of the light to the size of the hole. If
you have a big hole, the light stays fairly well-collimated. If you have
a tiny hole, the light spreads everywhere.

Turns out that you get basically the same results if you replace the
sheet with a hole in it with just a particle obstructing some of the
light. As light passes by that object, it will be defracted and spread
over a range of angles. If you have a big object, the light doesn\'t get
scattered very far.

If you have a smaller object though, light can be scattered over a
finite range of angles. So in this case, we have some - we have this
fact that the ability of the particle to scatter light by a few degrees
depend strongly on its size.

Was there a question there?

Okay. Now the importance - so now, let\'s see if we can understand --
now, it turns out that this distinction has important implications for
how the different rings -- sorry - it turns out that the typically
bright rings or the typically faint ring have very different particle
sizes.

And so, they tend to exploit these two methods of scattering light in
different ways.

Go on to Slide 9.

Oh, got some backward stuff in there. This is -- we - here we have a
crop of this beautiful image again and below into the left, we have
these little cartoons meant to illustrate how light is getting from the
Sun to our spacecraft in various configurations.

And basically, we have the same picture of the Saturn, its shadow and
the Cassini spacecraft. And we have a little yellow line illustrating
how light is traveling from the Sun way off to the left, to the
spacecraft.

It\'s basically - imagine the yellow line as one ray of sunlight
bouncing off of some particular object. Now realize that again, since
the Sun is blocked, we have to see light scattered by the ring.

So - and as we\'re showing here it - we don\'t mean - it only needs to
be scattered by a few degrees. Basically, everywhere in this image,
we\'re talking about phase angles of roughly between a 179 and 175
degrees for scattering angles between 1 and 5 degrees.

Now, it turns out that particles - to scatter light by diffraction, by a
few degrees, you need particles that are on the order of 1 to 100
microns across. And it turns out that the typically faint rings, the
E-ring, the G-ring, the F-ring and the D-ring, these are composed
primarily of these small particles.

So they\'re fairly efficient at scattering light by a few degrees. So
they appear particularly bright or unusually bright in this particular
geom...

Recording: (mute off).

Matt Hedman: Is somebody going to ask a question?

Okay. If by contrast, if you go to the next page, for the A, B, and
C-rings, these are composed almost entirely of particles between -
bigger than basically about a centimeter.

These particles cannot scatter- diffraction can\'t scatter light
various - these sorts of particles are too big to defract light that
efficiently by the - at this - by a few degrees.

They can defract light very efficiently by a very small amount but not
by a few degrees. So, diffraction really isn\'t coming to play here.

Instead here, what we\'re seeing is more reflection. We\'re seeing
basically the tiny illuminated -- sorry - in part we\'re seeing the tiny
illuminated crescents of light on a bunch of individual ring particles.

Recording: (ringing tone).

That was neat.

So there - and this is relatively inefficient. And this makes these
rings darker than one might expect. And in fact because we\'re not
dealing here with a bunch of isolated bodies but with many, many ring
particles relatively close together, things are actually getting even
more complicated.

On the one hand, the dark - was it - one ring particle - light from one
ring particle can illuminate another making its dark side a little
brighter.

By contrast, in some cases, ring particles can cast shadows on other
ring particles, making them - making some ring particles even darker
than you might expect.

So there\'s a fairly complicated relationship between the amount of
material in here and the - we have in these rings and how much they can
emit light.

And in the ultimate limit, where the ring is extremely dense, so little
light can get - very little light can get through. And this part of the
reason that the outer part of the B-ring in this image appears
particularly dark.

It\'s basically so dense, no light can get through the outer part of the
B-ring to reach our camera \-- or very little.

But it turns out that just having light passing through the rings and be
scattered by rings isn\'t all that\'s going on in this image. There\'s
other more complicated paths light can take between the - between the
Sun and our spacecraft during this time.

And if you go to Page 11, you\'ll see one of the things I'm talking
about.

If you look at the upper part of this ring in this image, I have a line
pointing to this point in the B-ring, where you may notice it looks a
little greenish. Looks like there\'s a little green glow across that
part of the ring system. That\'s not a camera artifact. That\'s a real
effect.

And what you\'re seeing there is Saturn shine on the ring. As the
cartoons on the lower left show, you have light coming-- remember, these
-- this upper part of the rings is actually on the far side of Saturn
from our perspective, okay? Is on the far side of Saturn.

So that part of the rings is actually facing the bright illuminated side
of Saturn.

So we can have light from the Sun hitting the planet as shown in the
little cartoon, bouncing off onto the rings and then bouncing back to
the spacecraft.

Now, this has actually been seen before but this is a fairly nice
example of it. And this shows one of the things that kind of complicates
efforts to understand the scattering properties - the light scattering
properties of the rings because you always have to take into account
this signal from Saturn.

But it turns out we not only have light from Saturn hitting the ring in
this image, we also can see light from the rings illuminating the
planet.

So, first go to Page 12. This is just to point out that what we\'re -
when we see the bright illuminated edge - limb of the planet, what
we\'re seeing there is again, the high levels of the atmosphere which
can - which are sufficiently transparent to allow light.

And light can either be scattered by hazes or defracted by - through the
atmosphere and carried from the Sun to our telescope. But beyond that,
we also notice the dark side of Saturn is not completely dark.

Man: (Ed)?

Matt Hedman: Hello?

Man: Just a minute.

Trina Ray: Matt, I'll see if I can get the operator to isolate that
line. You go ahead and go on.

Matt Hedman: Okay. I just want to make sure nobody - I want to give
everybody a chance to ask questions whenever they feel like it.

Okay. When we look at the - now, why is the dark side of Saturn not
completely dark? Well, just like the far side of -- on - sorry. What
we're actually seeing here is ring shine on the planet.

If you look below, you - we can have a ray come down, hit the rings,
bounce back towards the Sun, low face angle, bounce back towards the Sun
and run into the planet, then bounce back towards the spacecraft.

And on Page 14, we see a similar sort of path can work with -- 14.
Yeah - that works in the southern hemisphere where light bounces off the
illuminated side of the ring, hits the planet, bounces back to the
spacecraft.

And in fact, if you realize in the southern hemisphere, you're going to
have this - it's going to be a lot easier because the light doesn't have
to get through the rings, they just have to bounce off them.

And so, what this means is of course, the dark side of Saturn isn't
truly dark. And for any amateur astronomers out there, you can imagine
how painful it would be to try and do some astronomy from Saturn surface
with all these big obnoxious rings in the way.

But on top of this overall bright background on the planet, there's a
couple of dark bands. And if you go to Page 15...

Man: I'm on another line.

Trina Ray: They're trying to isolate that line. Matt, go ahead.

Matt Hedman: Okay.

There are a couple of dark bands. This upper band, which I mark with the
upper line, actually corresponds to Saturn's equator.

Realize that if you were sitting on Saturn's equator, the rings would
appear as an almost infinitely narrow line across the sky. Remember the
rings are only a few meters across. So, they can't really efficiently
scatter light from the rings, you can't really efficiently scatter light
from the rings onto Saturn's equator.

So, the - Saturn's equator appears rather dark. The other set of dark
bands, the lower set, are actually the rings blocking some of the light
from Saturn. This is - the path of light is again illustrated with a
little cartoon at the bottom. You have light bouncing off the rings,
bouncing on the Saturn.

But then for that light from Saturn to get to the spacecraft, it has to
pass back through the rings. And those rings can block some amount of
the light.

Now, realize that the part of the rings blocking light is in shadow, so
it doesn't emit much light on its own. All it can do is stop the light
from the planet from getting to us.

And I think this is one of the reasons that the image looks so strange
or at least this part of the image can look so strange, because we're
looking at different features of the ring system. When we're looking at
the rings in front of the planet and when we're looking at the rings in
front of the sky.

Now, I try to - now, on Page 16, I've tried to make a diagram to help
make what's going on a little clearer.

Where I've marked our -- marked the A, B, and C-ring and the Cassini
division both on the planet where we're seeing the light blocked by the
ring, and also, the rings on the sky, where we're seeing sunlight
scattered by the rings.

And you can notice some strange discontinuities, where like the C-ring
seems to all of a sudden stop at the edge of the planet disk or - and
the B-ring seems to be wider on the planet than it is on the sky.

But all of these things actually make sense.

First, let's just think about the stuff on the right hand side, where
we're looking at the rings against the dark side - against planet shine.
This is actually simpler.

Here, we're - the ability of a ring to block light basically is directly
related to how much stuff is there. If there's more stuff, it can block
more light.

The A-ring and the B-ring are very good at blocking light because
they're both the densest part of the rings. So, you see a clear band, a
dark band at the location of the A-ring and a clear dark band throughout
the B-ring.

By contrast, the Cassini division between them contains much less
material and so is not as efficient as - at blocking the light.

So, you see quite a bit more light through the Cassini division.

Similarly, the C-ring does not contain nearly as much material as the
B-ring. And so, it blocks very little light.

But if you look very closely at this image, you can see there is a faint
dark band where the C-ring is blocking a little bit of light.

Now, by contrast when we look to the left at the illuminated -- at the
light scattered by -- at the sunlight scattered by the rings, here the
relationship between density and brightness is not so direct.

Something can - in this sort of thing, a region can appear dark either
because it has too little material to scatter light or because it has so
much material it blocks the light.

What this means is that regions like the A-ring and the Cassini
division, even though they have very different amounts of material, can
actually appear to be about the same brightness.

Similarly, the inner part of the B-ring and the C-ring can appear to be
about the same level of brightness, even though they contain very
different amounts of material.

And of course, the B-ring - the outer part of the B ring appears very
dark because it just blocks so much light.

And then, we also have to fold in the fact that differences in particle
size may affect things too.

So - but in any case, there\'s - all of this can actually make sense in
terms of light scattering properties particles and part -- ring's
ability to block light. So there's no need to invoke ideas of us
overprinting images in bad ways.

Okay. Now if you go on to Slide 17, this is not just a pretty picture.
We hope to - we also want to get some science out of this stuff. And we
certainly are beginning to do that.

Of course, it has come down relatively recently, so we don't have any
final results just yet.

But there are things in this image we expected to get. We were expecting
to see interesting variations in brightness and color that would tell us
something about the particle size of these rings.

But we also got some unexpected bonuses. We found some unexpected
structure in the E-ring and we even found some new rings.

So, first the stuff we kind of expected to see. Go on to Page 18.

What you're seeing here is an image and what I want to call your
attention to here is the fact that if you look at part -- look at the
ring and track one part of the ring as you get -- lower - if you look at
the lower part of the ring of any given ring and compare it to the upper
part of the ring, it appears brighter in the lower half.

And the reason for that is that that part of the ring is closer to the
apparent position of the Sun. So, the scattering angle is smaller. The
phase angle is a little higher.

So - and it turns out, since - remember diffraction can only scatter
things over a small - over a range - finite range of angles \-- well
roughly a finite range of angles.

As we get closer and closer to the apparent position of the Sun, the
brightness in general tends to increase, and the rate at which it gets
brighter as you get closer to the Sun, the steepness of that rise,
depends on the particle size.

So, we hope by measuring these - how fast the rings brighten as you get
closer and closer to the apparent position of the Sun, we hope we'll be
able to put interesting constraints on particle sizes in these sorts of
rings.

Now, if you go to Page 19, we not only see difference of the brightness,
we also see some fun variations in color, even within a single ring.

In this --this is just a stretch version of the pretty image, which I've
stretched to just show the E-ring more clearly. And the E-ring is the
big diffuse thing going all around the outside.

If you look to the extreme left or the extreme right of the image, you
notice the E-ring appears rather reddish, whereas if you look at the
ring just above or below Saturn, it appears rather bluish.

So the - and what this seems to be is simply that the ring is getting
bluer as you get closer to the apparent position of the Sun, which
again, is hidden behind Saturn. And that's actually understandable.

Remember the angles over which light can diffract by a particle, is
given by the ratio of the wavelengths of the light over the size of the
particle.

Now, bluer light has a smaller wavelength. So, in general, it doesn't
diffract as much.

So, it's more tightly - so the blue light is more tightly confined
around the apparent position of the Sun than the red light is.

Now, in addition to these variations within a single ring, we also see
interesting variations in color between rings, particularly when we look
at the system in the infrared.

The figure on Page 20 is actually not an image taken with the camera.
It's an image taken with the visual and infrared mapping spectrometer,
which I believe you've heard - all heard about last time.

This instrument basically allows us to take infrared spectra or
spatially resolved infrared spectra of objects like rings and we
observed the rings with this instrument during this opportunity, as
well.

And what we have here is a picture of the rings which basically is false
color where the red, green, and blue are different infrared wavelengths,
but it's still made so that blue is shorter wavelengths and red is
longer wavelengths.

And you could see some - there are very some very interesting color
variations. In particular, look at the right side of the image at the
C-ring and the D-ring.

The C-ring appears as - comes out at a fairly red color. The D-ring by
contrast is very noticeably blue.

This difference is again, we believe due to the fact that the D-ring is
composed mostly of small particles, things like roughly a 100 microns
across. While the C-ring is composed primarily of big particles, bigger
than -- where bigger is bigger than a centimeter. Up to the size of a
few meters.

Similarly, if you - where we can see light from the B-ring, we can see
that it is also red and even the A-ring, although it appears somewhat
muddy, is also has a reddish color. All of these rings are supposed to
contain large particles.

By contrast, the G-ring and E-ring appear greenish or even blue, again,
perhaps indicative of small particles. The F-ring appears as bright
light. That's simply because it's so bright, it basically saturated in
this observation.

One interesting thing to note is the Cassini division. The background
Cassini division looks rather red, but there seems to be a green streak
running through it, right down the middle.

And there - turns out there is a very narrow ringlet in the Cassini
division that happens - that seems to have scatter -- that seems to be
composed of small particles. And we believe that's maybe responsible for
this green streak.

But we're just beginning to try and figure out particle sizes and such
things, from this sort of information, but we also got distracted by
some very interesting surprises.

If you go to Page 21, you get one of the more interesting ones. This is
an image of the E -ring, where with - which just happened to also catch
Enceladus.

Now, as you all know Enceladus has this big geyser jet coming off of the
south pole. And that white - bright white, streak you see in the very
center of the image that's basically saturated or as bright as you can
see, is actually that plume and the little dark dot on top of it, is
Enceladus itself.

Of course, Enceladus itself, we're looking at the dark side of
Enceladus, so it should not -- it itself should not be bright, but its
plume could be bright.

But what was surprising to us was when we look -- when you look at the
E-ring surrounding this, the E-ring normally appears as a rather smooth
fuzzy feature.

But here, we can see tendrils and filaments somehow associated with
Enceladus.

And we're still not entirely sure what's going on here. But one
possibility is that we're seeing particles in the E-ring that are
drifting by Enceladus and Enceladus' gravity is basically sling shotting
them, throwing them in various directions, forming these interesting
structures, these tendrils - this little dark band rising up outside of
-- rising up above Enceladus in this image, etcetera.

And there were - various groups are beginning to model this and trying
to understand what exactly is going on here.

Page 22 of this image - of this sequence \-- sorry \-- shows another
unexpected asymmetry of the ring that we saw on this particular image.

It's again, I've stretched it to show the E-ring better. If you look at
the left part of the image, you see that the core of the E-ring appears
rather red, but it becomes a little more purple as you move out to the
side, which is just because it's becoming a little more blue.

By contrast, if you look at the extreme right of the image, you see a
blue ring interior to a red ring. And we're still very puzzled by this
asymmetry.

Now, assuming color can tell something about particle size, this seems
to imply particles of different sizes are being separated in some way.
And gravity alone can't do that. Gravity treats all particles the same
way.

So, this implies there's some non-gravitational force at work, perhaps
some interaction between the ring particles and the electromagnetics in
Saturn's magnetic field or its plasma environment.

But that's -- that I think is a pretty interesting mystery that we're
going to have to explore.

Trina Ray: Hey, Matt on Slide 22, where is Enceladus? If the ring - if
the E-ring was a clock, is it way over there at 9 pm or is it more like
6 or 7?

Matt Hedman: It is - it would be - this image was composed of a bunch
but of several images where Enceladus moved a bit.

Trina Ray: Yeah.

Matt Hedman: It's at 7.

Trina Ray: Okay, 7 pm. All right.

Matt Hedman: Yeah.

I will say we don't currently think this has anything to do with
Enceladus per se.

Okay. Yes, in fact, you see the little white spot at about 7 o'clock.

Trina Ray: Yeah.

Matt Hedman: That's -- that is Enceladus in one of the frames.

Trina Ray: Okay, thanks.

Matt Hedman: Yeah. Okay.

(Matthew): I have a question.

Matt Hedman: Okay.

(Matthew): This is Matthew with the S.O.C.

I see a linear feature at about - actually two linear features at about
10 o'clock, are those just camera effects?

Matt Hedman: Those are just camera effects.

(Matthew): Okay. Thank you.

Matt Hedman: Yeah. Yeah.

Okay. And moving on to Slide 23, of course, is one of the big fun things
we saw on this was, we got to see a couple of new rings.

The first ring we found is shown actually on the right hand image. It's
a ring between the orbits of - the orbit of the G-ring and between the
main rings. It actually lies in the orbit of the two small moons, Janus
and Epimetheus.

Another ring that was discovered is shown in the left image, it's found
at the - you can see it just below the limb -- the south pole of Saturn,
it's a narrow little streak and it turns out that that ring is - lives
at the same orbit as the small moon called Pallene.

So both of these rings seem to be associated with small moons and
because of that, what we tend to think is going on here is that
basically these moons are supplying the ring material.

The moon is constantly being bombarded by small meteorites, you know,
things that can be even small - smaller than sand grains. And each time
one of these particles bangs into the surface, it releases a cloud of
dust. And because these particles -- these moons don't contain - don't
have that much gravity, that dust can in principal escape and fill out
the rest of the ring.

Now, one question with this sort of idea is there's another small moon
that lives between the orbits of the G-ring and Pallene I believe. This
is the moon called Methone.

And if you - we've looked at that image and it does not seem to
produce - that moon does not seem to have produced a dust ring as bright
as Pallene has. And it's not clear why that would be the case.

And that is yet another mystery that we have to work out with these
wonderful new images. So that was all I was going to say about this
particular set of images.

I'd like to turn now and talk something about the various - some of the
new things we learned about some of these rings, but maybe I should stop
and ask if there are any questions about these pretty images.

Trina Ray: I guess I had one more question.

Matt Hedman: Sure.

Trina Ray: The green that is the -- reflects the -- the green that you
see on the rings that sort - in the upper part of the image...

Matt Hedman: Yes.

Trina Ray: ...reflected from Saturn, I mean, is that green a real green
or is that just - I mean, is it because that the rings are taking out
the red light, and so, you're kind of getting mostly a reflection of
blue light, but then when it hits the rings again, it's more - where is
the green coming from? I mean, why is it green?

Matt Hedman: Why is it green? Okay. I will say, like I'm not sure
exactly what, you know, why it appears green to our eyes. I will say,
you are seeing a complicated convolution of basically the spectrum of
ice or the ring spectra and Saturn spectra.

Trina Ray: Okay.

Matt Hedman: So, you have -- you are basically - the rings and Saturn
are both chunking out pieces of the spectrum and I must admit I
haven't - like I've looked at reflected spectra in infrared, but not
enough at the visual to know exactly why it appears green in this image.

Trina Ray: Yeah, it's also tricky because this is basically the light
that's shining back on the rings here, it's sort of from that blue
hemisphere of Saturn, because the red gets scattered away by - as it
goes through the rings the first time. So, you kind of get this blue
Saturn - it probably complicates things on top of that also.

Matt Hedman: I think that - I'm not sure if that complicates it too
much, just because the back lit of - if you think about the back side of
Saturn, the dark side of Saturn...

Trina Ray: Uh-huh.

Matt Hedman: ...we're also seeing basically light has gone - hit the
rings, hit Saturn and come to us. And it also appears green.

Trina Ray: Okay.

Matt Hedman: So, I think it's just basically - right, it's basically,
ring spectra times Saturn spectra gives you this color which I will
admit I'm not quite - I couldn't tell you why in visible is exactly
showing up as this greenish glow.

Trina Ray: Okay. Well, sounds good. Thanks.

Matt Hedman: Okay. But it is a complicated mixture of the two spectra.
Okay.

Man: Another question, please.

Matt Hedman: Sure.

Man: Yeah. With the new images, do you have a new count on the number of
rings and number of moons in Saturn?

Matt Hedman: These images aren't very good for looking for new moons
because we're looking at the dark side -- we're looking at basically the
dark side of the moon. So, the moons are in general fairly dark.

For the new count of rings, we have, you know, these two discrete ring
features, the one associated with Janus and the one associated with
Pallene, these are fairly, you know, you can feel pretty good about
calling them rings.

However, you can also see in some of these stretched images that there's
quite a diffuse amount of material - there's quite a bit of diffuse
material kind of throughout the system and so it gets a little hard.

What do you call something that's - what do you call a ring what's not
-- what do you call part of the ring? From this set of images the number
of rings has definitely upped by two, I would say. But what's the total
number of rings? I think that's a little bit hard to define question. Or
hard to define the answer to.

Man: I have a question about the Enceladus close up, showing the
disruption of the E-ring. It appears that there's a moon off in the left
side of the image. Wondering what moon that is.

Matt Hedman: Tethys

Man: Tethys. Thank you.

Matt Hedman: Yeah. And actually, you notice it looks kind of half
full...

Man: Yeah.

Matt Hedman: ...that's not sunlight making it half full. It's - realize
we're looking at the ansa of the ring. So, Saturn is 90 degrees off -
we're looking at half illuminated Saturn...

Man: Okay.

Matt Hedman: So, that's actually Saturn shine...

Man: Saturn shine.

Matt Hedman: ...illuminating half that moon.

Man: Okay. Well, thank you.

Matt Hedman: Yeah. That's Tethys, I believe. Okay.

(Dave Doody): Another question.

Matt Hedman: Sure.

(Dave Doody): This is Dave Doody. Looking at Slide 23, at about 8
o'clock and also at about 4 o'clock, there's a dark arc just above the
haze of Saturn's high atmosphere, could you talk about that?

Matt Hedman: Could you tell me what - the dark arc around -- sorry,
could you give me the - what time of year -- what time of day? Sorry.

(Dave Doody): If Saturn were a clock...

Matt Hedman: Yeah.

(Dave Doody): ...it would be at 8 o'clock and also at 4 o'clock.

Matt Hedman: Excellent question. That - what you're seeing there is the
edge of Saturn shadow on the ring. Remember in this particular geometry,
we're almost looking straight down the shadow of Saturn.

So the disk of Saturn and the edge of Saturn shadow are almost on
perfectly on top of each other, but not quite. And those little dark
bands that you're seeing - this little -- these little dark bands on
either side of the high haze that's on the rings, right?

(Dave Doody): Yeah, great. Thank you.

Matt Hedman: Yeah, that's actually the shadow, which extends a little
bit further out because the rings are a little closer to us.

Any more? Okay.

For the next little bit, if I can indulge you guys, I'd like to talk
about the new discoveries we've made in Saturn's faint rings, the G and
the D-rings.

And this kind of goes on with what we're talking about here, just
because these are rings composed mostly of small particles and appear
particularly bright in these beautiful images.

And on Page 25, I've got a text line that just says, why we should be
interested in these rings.

For the first fact, these rings have much less mass than the main rings.
So self-gravity is an important - also particles don't collide all that
often. So, in some sense, the physics is simpler.

At the same time, these dust -- this dust can't survive in the Saturn
system very long in astronomical time scales.

The thing is, these dusty particles are living in Saturn's magetosphere,
which contains plasmas and such - which contain plasma that can - and
ions that can run into these particles and change their orbits or even
erode them.

And it turns out the time scale for dust particles to erode inside an
atmosphere, is on the order of thousand years or so.

So, you can't just have this dust ring just sitting there forever, you
need to actively supply it. That means that these things can at least
\-- sorry - since these rings are particularly young, or should be
rather young, we can use them as some sort of probe of ongoing processes
in the ring system.

And in fact, we've observed in several of these dusty rings that they
can change on time scales as short as decades.

The other important thing to realize is, there are dusty rings around
all the outer planets, that's around Jupiter, Uranus, and Neptune.

So, we also have a broader base for comparative studies when we study
the same rings, which is also a nice feature.

Okay. So, first of all, I want to talk about the G-ring, which I am
showing a picture of on Slide 26. This is the narrow ring between the
F-ring and the E-ring.

It typically looks like the picture at the top, which is a close to edge
on view of the ring. You can see though, it has a narrow \-- sorry - a
sharp inner edge and a more diffused outer boundary. And this is
typically what it looks like.

Now, this is probably one of the - prior to Cassini, this was one of the
more obscure rings and was least known about it, because it was so
faint - basically, because it was so faint.

Now with Cassini, we've had a lot more chance to look at it and we're
beginning to get some clues as to where this ring came from. Remember
this is - we're seeing small particles here.

Something must be supplying the thing with dust. But what was supplying
with dust was not really clear and it's still not 100% clear, but at
least we have a clue.

If you go to Page 27, you will see one of those clues is. This is an
image - the bottom panel image there is one of several we've obtained
which shows an unusual feature on the inner edge of the G-ring.

You notice that, in addition to the regular G-ring, there's a sharp
narrow feature right near the inner -- right on the inner edge of the
G-ring. We only see this - we don't see this all the time. It appears to
be a longitudinally confined bright arc of material living on the inner
edge of the G ring.

And since it's, you know, a fairly faint and fuzzy feature we had a-
we've only been able to catch this thing maybe 12 times in the last 2
years. But we have caught it multiple times.

On Page 28, I'm showing three images, each taken about 45 minutes apart.
The top are the actual raw images, the bottom are high-pass filtered
images for clarity and the arrows are pointing at this bright arc.

Now you might have some trouble convincing yourself it's there, but I
think at least in the center image, you can convince yourself the very
tip of the ring is brighter than it is on the other two images.

And if you want a really convincing picture of the arc, I advise you to
download an animation that is available from the Saturn Web site that I
just wasn't going to force you guys to try and download now.

But that also shows very clearly an arc of material moving through.

So, we've been trying to figure out what this arc was doing and
fortunately we were eventually able to actually figure out how fast it
goes around the planet.

And it turns out by geitting these 12 observations over the course of 2
years, we were able to measure its period pretty accurately.

We give it on Page 29. Turns out the period of this arc is 0.80813 days.
So it takes for about 20 hours for it to go around Saturn once.

Now, that was exciting because it turns out Mimas' period, the period of
the famous Death Star star moon, is 0.94242 day.

So, if you take the ratio of the Mimas' period to the arc's period, you
get 7:6 to very good level of precision. So, every time Mimas goes
around Saturn six times, this arc goes around seven times.

And as you probably heard a few months ago, ring dynamacists love it
when there are these whole number ratios in orbital periods because it
means resonance can now operate.

And that the gravitational pertubations of these moons can have an
important influence on sculpting the ring.

And in this case, we think that Mimas is actually being able to confine
this ring in longitude and to keep it from spreading and becoming a
complete ring all the way around Saturn.

To see how this works, go to Page 30. This shows basically kind of the
geometry of the system if we're looking straight down. The blue thing is
supposed to be the arc material and the white ellipse is meant to
represent the orbit of Mimas.

Mimas has a finite eccentricity, so it gets closer and further away from
Saturn. Now, the eccentricity has been greatly exaggerated here, but
still basically Mimas is going to move around an ellipse in this ring.

Now, what if we weren't just, you know, this is the view if we were
hovering above Saturn, but let's now go - switch to the perspective of
somebody sitting on that arc moving with it.

In that frame, Mimas is still going to move closer and further away from
the planet, but because we're in this rotating frame, where we're moving
around with the arc, the orbit of Mimas isn't just going to be this
eccentric feature, it's actually going to be something more complicated.

And if you look on Page 31, you'll see what you actually get. It's
because of this 7 to 6 whole number ratio, it turns out Mimas executes
this funky six lobed pattern which effectively create six little pockets
\-- six gravitational pockets, where material can just stably be
confined. And this arc occupies one of those six regions. Now, you may
wonder why aren't the other five possible spots occupied?

And we're not entirely sure, but one possible explanation is, remember
what we're seeing here is the dust.

What we imagine might be supplying the dust are bigger particles, a few
particles that are meters or bigger. These big particles, which we can't
see very easily, may exist in all six pockets.

But recently, two of those particles crashed into each other in one of
the pockets and released a bright cloud of dust lighting up that
particular one of the pockets and producing the arc that we see today.

Now, what we're also hoping and this might provide us a clue with what's
supplying the rest of the G-rings. It could be that material - that once
you liberate this dust, that dust begins to interact with Saturn's
magnetosphere and Saturn's plasma environment.

That maybe able to cause it to leak out of the arc slowly and populate -
form the rest of the G-ring that we've been seeing -- that we see all
the way around the planet. We're still working on that. But at least,
there seems to be a possibility that that'll work out.

And \-- sorry \-- on Page 32, I just also want to say this model that
the arc is confined by gravitational perturbations from a moon is not
unique to Saturn. It's also - it's been used before to explain a series
of arcs seen at the orbit of Neptune, which is shown on Page 32.

This - and in this case, the idea is there\'s a resonance with a small
satellite and this is a much higher resonance for - I think it's 43 to
42, which is confining these arcs. So this is not a unique process just
happening with - between the G-ring and Mimas, it also happens in other
situations.

But in this case, we're in the particularly fortunate case where we have
the Cassini spacecraft there, we can watch this region over a protracted
period of time and study in great detail, and then really understand
what's going on there.

So that's what I wanted to say about the G-ring. And are there questions
at this point? Then I'll talk quickly about the D-ring.

Man: Could you perhaps talk about the little equation with the symbols
on the top of 31 and 32?

Matt Hedman: Okay. That equation - I apologize on the second page it
looks like something funky happened. But that equation is the equation
that basically describes the - that describes this resonance.

If - basically, what we have here is six times the longitude of the arc
minus seven times the longitude of Mimas plus the pericenter of - the
longitude, pericenter of Mimas must equal pi or 180 degrees.

This is basically - you take the fact that - and this equation appears
to be true for the arc and is the condition you need for the arc to
occupy one of these stable pockets.

And the reason the pericenter comes in is because you need the orbit of
Mimas in the arc's frame to form a closed loop pattern. So you have to
take into account the fact that the pericenter of Mimas can actually
process or change over time.

But, also - right. And the lower fact of the equation is actually to
write the equation of the ellipse you're seeing, of the funky 6-lobed
pattern showing that the longitude of arc minus Mimas - sorry, I think
I - yeah, sorry. Is that sufficient or do you have more specific
questions?

Man: That takes care of it. Thank you.

Matt Hedman: Okay. Okay. Last thing I want to talk about is the D-ring.
Page 34 reminds you where the D-ring is. It's probably my - it's one of
my favorite rings, it's -- but most people don't know where it is.

It lies between the C-ring and the planet, which makes it extremely
difficult to study, because it's sandwiched between two, in general,
much brighter objects.

In fact, the only really good shot we got of the D-ring prior to Cassini
came from Voyager. And on Page 35, we get one of the pretty - one of the
nicer pictures from the Voyager spacecraft.

In this image, we have Saturn is basically off the lower left, the inner
edge of the C-ring - the overexposed thing in the upper right is the
inner rays of the C-ring.

And in the D-ring, we see three discrete ringlets named - which are
named D68, D72 and D73. And there's a diffuse sheet of material between
all of them. And what I want to point out here is D72 is by far the
brightest feature in the D-ring at this time.

Now, with the Cassini, we were able to look at this region again. And on
Page 36, we have a comparison image shown from Cassini - taken by
Cassini at a similar but not quite the same geometry, but close enough.

The little, green dash line marks the inner edge of the C-ring. Again,
Saturn is off to the left. And you can, again see three discrete
ringlets \-- D73, D72 and D68. But the most striking thing is that D72,
which used to be the brightest thing in the ring, is now actually
fainter than D73.

And not only that, its actually shifted interior, inwards by 200
kilometers relative to the other features in the D-ring. This was one
the largest secular changes in the ring system that we've observed to
date.

And we're still not entirely sure what happened here, but this goes to
show just how dynamic these faint rings can be.

Now, the part of the ring I want to focus on is the outermost part of
the B-ring -- D-ring because we think we have some evidence that this
region of the rings was recently disrupted by perhaps an impact.

This part of the ring is slightly odd because it appears brighter at low
phase angles than at high phase angles.

If you look on Page 37, I compared two images of the D-ring taken by
Cassini. The upper image is taken at low phase angles- backscattering.
The bottom image is taken at high phase angle- forward scattering, like
the images we were talking about earlier.

And what you may notice is if you look between D73 and the C-ring at
high phase angles, you see maybe a few ringlets. But at low phase
angles, you see a continuous sheet of material.

The trick here - now, if we look at that sheet of material at high
resolution, which is shown on Figure 38, Page 38, that sheet of material
breaks down into a series of bright and dark bands, and the series of
bright and dark bands are very regularly spaced with a characteristic
wavelength of about 30 kilometers.

And every time we've looked at this region, we see this characteristic
pattern, with this characteristic wavelength of somewhere between 34 and
30 kilometers.

Page 39, I don't know why I threw this in, shows a whole bunch of
brightness profiles showing brightness as a function of radius. And in
basically - I just threw all these together to show this very clear
periodic variation in brightness shows up in all of these profiles.

And it always exists between radii of about 73,000 and 74,000
kilometers. These are radii from Saturn center.

Now, we were very confused by this structure for a long time. We
couldn't think of any plausible mechanism for producing it. But earlier
this year, we got a couple of pieces - critical pieces of information
that allowed us to make sense of this.

The first piece of information comes from - came from actually a Hubble
observation done 10 years ago, which is shown on the next page. It's on
Page 40.

It's -- turned out in 1995, the Hubble Space Telescope observed a
stellar occultation. You watch the star go behind the ring. And by doing
that, you can get a measure of how much material is in the rings along
the line of sight.

And this occultation turned out to be very good for measuring the amount
of material in the D-ring. And the profile shown here shows a rough
measure of the amount of material, the inner edge of the C-ring is found
at the far right of the plot.

At the left edge of the plot, you're basically seeing just noise. But if
you look between 73 and about 74.2, you see a series of spikes. These
are very regularly spaced. And we believe these are representative of
the same periodic structure we've seen in our images.

But what's neat is, in this case, the characteristic wavelength isn't 30
kilometers, it's 60 kilometers. So the wavelength of the structure
apparently has decreased by a factor of two in the last 10 years.

The other cool piece - so that's an interesting clue. The other clue
came from the - came from images taken in January, which are shown in
Figure 41 - on Page 41 where - which were taken with the rings almost
edge on. We were only a few degrees out of the ring plane. But we were -
actually, we were less than a degree out of the ring plane.

And what - the top is the actual image, just cropped, the bottom is the
image stretched vertically by a factor of five to show the rings a
little more clearly.

And what you see is some broad - some sort of broad brightness
variations with three broad peaks working outward. But on top of that
there's some fine scale brightness variations.

And those fine scale brightness variations have the proper
characteristic wavelength of about 30 kilometers. But if you start
looking at these brightness variations, you start to see some very
strange things.

Various bright bands seem to coalesce and merge as you go across the
rings. You see structures that look like inverted Vs or maybe even
regular Vs. And it doesn't quite look like just a series of simple
bright and dark bands.

If you look at Page 42, I drew some lines to guide the eye. The
horizontal line is a line going across the center of the ansa, and
just - that basically bisects the ring.

And if you look at this image, you notice that wherever there's a
brighter than average spot above the ring, there's a darker than average
spot below - sorry. Wherever you see a brighter than average spot above
that line, you tend to see a darker than average spot below that line
and vice-versa.

This is something called a contrast reversal. And it turns out it's a
characteristic signature of a vertically corrugated ring.

To understand how that works, look at Page 43.

Here, we have a little cut through cartoon of a vertically corrugated
ring. And the lines are meant to indicate lines of sight passing through
the ring. If you have a vertically corrugated ring, your lines of sight
can pass through different amounts of material depending on where you
look through the ring.

At some places you pass through the ring perpendicular and you don\'t
see much material. So the ring appears dark.

At other places, you're looking through a lot -- a relatively lot of
material and it appears bright. So a simple periodic corrugation can
produce periodic variations of brightness.

But, realize that unlike just, say density variations on the ring, in
this case, the reason that if you're bright or dark depend on your
viewing geometry. And in particular, they can produce these strange
sorts of patterns we see.

An example is shown on Page 44.

The cartoon at the bottom shows a wire-mesh diagram of a vertically
corrugated ring. It's just -- and what you can see is they're apart -
they're - if you look above the ring, above the center of the ring on
the far side of the ring, you will see a series of bright and dark arcs
as the mesh gets closer and get squished together and pulled apart from
our perspective.

You see a similar pattern of bright and dark arcs, if you look at the
other half of the ring. But notice that the two sets of bright arcs
don't meet up with each other. They're actually 180 degrees - sorry.
They're actually 180 degrees out of phase.

So - and I've shown another blowup of that structure - of the structure
in the D-ring up above, and that shows a similar pattern of bright and
dark arcs that don't seem quite matched up at the middle as you might -
would think a normal sort of flat ring would do.

So we really think what we've got here is vertically corrugated ring.
And it turns out that this is also not - is not an unheard of thing.
There's actually evidence that the main ring of Jupiter also has
vertical corrugation.

So - and I will fully well admit the resolution in our data is not quite
enough to make to it really, really convincing, but I think it's the
easiest explanation for this very strange data.

And what's really nice is if you put the time variability together with
this evidence for vertical corrugation, you can actually make a
consistent story, and you can explain both of these things at the same
time by just taking the entire ring and tilting it out of Saturn's
equator plane by a small amount at one particular time in the path.

Go to Slide 45 to show - to see what I'm talking about. At the top
image, we have the picture of the ring that's just tilted out of
Saturn's equator plane.

Now, the thing to realize - so, the ring particles are on inclined
orbits. The trick - the thing to realize is because Saturn is not a
point mass, you just can't have inclined orbits remain stationary over
time. They have to evolve.

And in particular, the nodes have to regress. In other words, the point
where the ring particle orbit crosses the - at the Saturn's equatorial
plane has to clock around as a function of time.

So this entire tilted sheet of material have to - this entire tilted
sheet of material clocks around Saturn, we have a time scale I believe
in the order of a week.

But, the other thing to realize is the rate at which this -- at which
your nodes regress depends on the orbital semi-major axis. So if you're
closer to Saturn, you'd go around faster than if you're further out.

This means that not only does the structure clock around, it actually
starts to twist up. And that's what the other two images are showing.
The structure will naturally tend to twist up and form a vertical spiral
that becomes - when it's sufficiently tightly wound, looks like a
vertical corrugation.

And the thing to realize is as the spiral gets wrapped tighter and
tighter, the wavelength of that corrugation will get smaller and
smaller.

So, this could explain why we see time variability because we're seeing
the spiral wind up and its wavelength is getting smaller. But it can
also - and it can also explain why it's a vertically corrugated
structure.

Now, this is just a pretty model, but it actually turns out that works
quantitatively. It turns out that this sort of winding spiral structure
predicts that basically the wavelength goes inversely with time or
equivalently the wave number, which is two pi over the wavelength,
should increase linearly with time.

And if you look on Page 46, we actually have a plot of wave number
versus time. Wave number measured in inverse kilometers.

Now, the data point at 1996 was the data from the Hubble Space
Telescope. The data around 2004 to 2006 is the Cassini data. And so you
have two classical points you can - yes, you can just draw a line
through them.

But, if you look at the inset on Page 46, which shows the Cassini data
blown up you can notice that there has been a secular trend over time.
The wavelength does seem to have been - the wave number is been
increasing, which means the wavelength is decreasing.

Now, more to the point, in late 2006, we got two more observations of
this structure, and we were again able to measure its wavelength. Those
points are shown on Page 47.

And it shows quite - and amazingly, it turns out that those points fall
very close to the predicted trend line, showing that the structure has
continued to wind up at the rate we predicted - we would have expected.

Now, the other thing is, knowing Saturn's gravitational field, we not
only can know the slope of this line we can also predict \-- sorry - we
not only expect this data to fall along the line, we actually can
predict what the slope of that line should be, from Saturn's
gravitational field.

Now, the slope of this line is basic - the slope of this line is 2.5
times 10^-5^ inverse kilometers per day. The rate you would predict,
given Saturn's gravity, is 2.4 times 10^-5^ inverse kilometers per day.

So they match within 10%, which gives us some confidence, even if all
the data was a little low signal-to-noise that we're really on the right
track. But this really does - could be the explanation for what we're
seeing.

We're seeing basically - we're basically seeing a vertically corrugated
ring that was - that is winding up due to the evolution of the orbits
due to Saturn's gravity.

And so we can explain what we're seeing here. All we need to do is take
Saturn - take part of the D-ring and tilt it a little bit out of
Saturn's equator plane and at some specific time in the past.

And in fact, we can figure out what time in the past -- at what time
this happened by extrapolating this trend back in time and figuring out
when we would just have a simple sheet of inclined material.

And it turns out that that condition must have occurred in - some time
in spring of 1984.

So, some time in the spring of 1984, the outer part of the D-ring had to
have been knocked \-- something must have happened to have taken part of
the D-ring and tilted it out of Saturn's equator plane.

Now, there's a variety of possible, you know, we're not entirely sure
what happened then. But one possibility is that basically a meteorite or
a comet collided with something in the D-ring and knocked basically the
entire ring off its axis.

It turns out that an icy body only a few meters across would carry
enough momentum into the ring to do this.

Now, we're still not sure how that momentum would be transferred into
the ring and distributed amongst many ring particles spread over a
relatively large range of semi-major axis. But, we're looking forward to
trying to model that.

But if nothing else I think this data shows \-- this is -- this sort of
shows just how dynamic some of Saturn's rings can be. That they're not
kind of eternal objects that are just there, they're actually dynamic
systems that are evolving and changing in significant ways over time.

And that's it. Sorry if I ran over.

Trina Ray: No, that's fine.

Do we have any questions for Matt?

Matt Hedman: Oh dear. Freaked them out.

Man: Yeah. I have a question about a lot of these images.

Matt Hedman: Uh-huh.

Man: I know they're taken with relative little light levels and I see a
lot of spots in the background. Are those mostly noise on the (CCD) or
are they actually stars?

Matt Hedman: Let's see. I'm trying to think of a good...

Trina Ray: Aren't most of them cosmic ray impacts on the CCD?

Man: Yeah. They could be -- yeah.

Matt Hedman: Yeah. A lot of them are stars and cosmic ray impacts.

Man: But you can't really differentiate between the - between them
unless you looked at a - like a star chart, something, right?

Matt Hedman: Yeah. And we actually do do that periodically.

Man: Okay. Thank you.

Matt Hedman: Yeah. Yes. You can actually see stars through these rings
which...

Trina Ray: Any other questions?

Well, thank you very much, Matt. We really appreciate you taking the
time today. That high phase image is so amazing.

You know, the more I look at it, the better it gets. And I'm not even -
I'm, you know, I'm a ring fan, but I'm not a huge ring fan, you know.
But I mean, that image is unbelievable.

And I'm - it's going to be one of the top images this mission ever
takes. So I really appreciate you taking the time today and explaining
it to us and all that details because it was - it's really spectacular.

Matt Hedman: Thank you, thank you. Thank you for giving me the
opportunity to talk.

Trina Ray: And just a reminder to everybody that the CHARM telecon in
December is over the holidays, so it will be canceled. And so our next
CHARM telecon will be the last Tuesday in January. And that date is \--
hold on a sec \-- here it is, January 30 in 2007.

And other than that, everybody have a great holiday, and we'll see you
next year.

Matt Hedman: Okay.

END