RAW TRANSCRIPT -- NOT YET REVIEWED FOR CORRECTIONS BY CASSINI PERSONNEL

FTS-NASA-VOICE

Moderator: Trina Ray

May 27, 2008

1:00 pm CT

Coordinator: Welcome and thank you for standing by. This conference is
being recorded. If anyone has any objections they may disconnect at this
time. Ms. Jones you may begin.

Jane Jones: Thank you. Well, welcome everybody to the May 2008 Charm
Teleconference. Today we have Dr. Larry Esposito with us and he'll be
talking about the history of Saturn's rings. I'll give you a little
information about Larry. He's the principal investigator of UVIS, the
Ultraviolet Imaging Spectrograph on Cassini and he was also chair of the
Voyager Rings Working Group.

As a member of the Pioneer Saturn imaging group, he discovered Saturn's
F ring. He's been a participant in numerous US, Russian and European
space missions and used the Hubble Telescope for its first observations
of the planet Venus. I could go on and on about Larry but I think I'll
let him start his talk.

Larry Esposito: Okay. Well thanks Jane.

Jane Jones: You're welcome.

Larry Esposito: Yeah. Very flattering. I'm going to be talking about the
history of Saturn's rings and as part of that I'm going to give an
update on some of the more recent results (to see) and also say a little
bit about that Ultraviolet Imaging Spectrograph that we built here at
the University of Colorado - our experiment on Cassini.

So if you're with me, you're probably looking at the first slide, which
says history of Saturn's rings. Let's go to the next slide, which shows
a picture of the Cassini spacecraft and the spacecraft is about the size
of a small bus. As you probably know, the biggest part of it, the
Magnetometer Boom, which is 11 meters - about 40 feet - long and, of
course, unfurled when we were in space.

But somewhere in the midsection of this, you can see what's called the
remote sensing palette and that includes all the cameras and
spectrometers and the ultraviolet imaging spectrograph is tucked there
on the remote sensing palette, up against the spacecraft and co-aligned
with the camera and the infrared spectrometers. That's the instrument
that we built here at Colorado and it has been operating for now almost
four years in orbit around Saturn and I'm going to show you some of our
results, also some of the implications about the history of Saturn's
rings.

So the next slide is just a retrospective of the launch of Cassini,
which was on 15 October 1997. It was a night launch and you can see the
night sky lit up by the launch of the Titan IV here with the light from
the exhaust flame reflecting off the clouds over Cape Canaveral at the
Kennedy Space Center; beautiful sight and that's how we started our trip
to Saturn more than 10 years ago.

And on the right hand side you can see a top view of the Solar System,
which shows the path of Cassini - not a very direct one - which
eventually, after almost seven years, arrives at Saturn on July 1, 2004
and aboard that Cassini mission was our spectrometer, which is shown in
the next slide. It's a rectangular piece of metal with a picture of a
buffalo on the side - the buffalo is the emblem of the University of
Colorado - we put that there as well as inscribing not our initials but
our names in the aluminum and, which is now in orbit around the planet
Saturn.

And the business end of the telescope is facing out towards right -
those apertures, one circular and four rectangular are where the
telescopes look through aligned with the cameras at the planet Saturn
and any other target that we're interested in; particularly Saturn's
rings. I want to pause for just a second - does anybody have a question
or a comment?

Hearing none, let's go onto the next slide. Some people ask how big is
it really? And the answer is it just meets FAA regulations for under
seat luggage; so it's about the size of a small carryon. Of course, when
we take it place to place before we launched it to Saturn, we didn't
slide it under the seat. It got its own seat on the airplane.

So the next slide is an artist's conception of the Cassini spacecraft
going into orbit around Saturn on July 1 and you can see the main
engines firing here and the spacecraft was above Saturn's rings. If you
click, you'll get the date on that. If you click on your presentation
again, go to the objective of the Cassini mission, which of course is
the entire Saturnian system; not just everything in it but how the
pieces interact.

And on the top part of that slide you can see the various elements of
the system, all to scale, with Saturn - just the edge of the planet -
here visible, showing it is truly the biggest - the giant planet of the
system. And in front of that the various moons of Saturn including the
biggest one, Titan, and some of the very tiny moons on the left hand
side. You can see Pan, which is the moon that is embedded in Saturn's
rings, which are shown in the background here.

And the Cassini mission and the Ultraviolet Imaging Spectrograph, which
we call UVIS, has as a target the entire system. So both investigation -
the big mission - and our particular investigation are looking at the
whole system, which you can see once again down in the bottom here with
a more distant view of Saturn, its rings surrounding it, some of them
very thin rings including the E ring, which covers the moon Enceladus
and, off the screen, some of the more distant moons would be found \--
Titan, Hyperion, Iapetus, Phoebe, etcetera.

Next slide here is showing the scale of Saturn relative to the Earth
moon system. So we've got Saturn with one end of the rings near the
Earth, the other end of the rings would almost reach to the Earth's
moon. This next picture is an artist's conception of Saturn's moon Rhea
and surrounded by a thin ring of chunks of rock and ice. And we're not
quite certain that the artist is correct.

Several of the teams on Cassini have noticed the presence of this ring
but not by photographing it; that's why we have an artist's conception.
But by watching as the spacecraft passed close by Rhea that some of the
particles in Saturn's radiation belts, like the Van Allen belts of the
Earth, were removed selectively when the spacecraft passed near Rhea on
one side and then again on the other side.

So something that would be - that sort of phenomenon would be caused by
several rings of material around the moon and in fact that's how the
rings around the planet Uranus were discovered the same way, showing
symmetric blocking of the light from the star in that case. Well,
unfortunately, the Cassini cameras have taken several close looks at
Rhea - long, long duration exposures and have not seen anything
resembling a ring.

And so at present the only way to accommodate and reconcile both those
observations is that the ring is made up of many large chunks of ice
like shown in this artist's conception. And if that were the case,
unless you were very close to them, you wouldn't be able to see them in
the Cassini pictures but they would still at the same time, absorb and
remove enough charged radiation so that that could explain how Cassini
made those measurements when it flew by.

So in the history of Saturn's rings, we even have the possibility that
not just Saturn but its moons have rings and if that were the case, the
ring around Rhea was probably formed when a small moon in the
neighborhood was shattered by a meteorite; in fact, that's one of the
major theories for the formation of Saturn's rings, which I'm going to
come back to in a second. I'm going to move on to the next slide.

While we look at this beautiful picture of Saturn's rings and Titan and
the moon Rhea here in this view, does anybody have a question or
comment?

Earl Kyle: Does it have any kind of an atmosphere?

Larry Esposito: What, Rhea? As far as we know, we didn't see any
atmosphere. We looked for scattered light and also light emitted by
oxygen atoms and with our investigation, we set a pretty strict upper
limit for Rhea's atmosphere. So as far as we can tell there's no
detectible atmosphere on Rhea.

Earl Kyle: Wow. That is an amazing picture.

Larry Esposito: Yes. Next slide. Here's another one that's pretty good,
too.

Earl Kyle: Wow.

Larry Esposito: This is the moon Dione against the limb of Saturn. At
the bottom left you can see Saturn's rings, which are just about seen
edge on and they would be possible to ignore except for the fact that
the sun shines through them and is casting a shadow, which you can see
in the upper right behind the moon.

Earl Kyle: Wow.

Larry Esposito: Let's see what I've got in the next slide here. Another
lovely view of Saturn, its rings and this moon - I can't remember
exactly at this moment - but one of the small moons captured in the
frame. Next slide. This is one of the more famous pictures of Saturn and
its rings and it's taken during a very special opportunity that occurred
only once during the mission when the spacecraft was very distant from
Saturn and caught in Saturn's shadow.

So this is a picture the spacecraft took, a long exposure image looking
back towards Saturn with the sun behind it and in the lower left you can
see a little bright spot right on the edge of the planet, and that's
just the light from the sun refracted through the very upper atmosphere
of Saturn and scattered by high hazes made of little particles.

But for the most part, the sun is hidden, which allows the cameras to
measure very exactly some of the very dim features and you can see here
several rings around Saturn. You can see the main rings in the center,
those are the ones that we showed earlier in the picture. But outside of
them, two very dim rings, the G ring is the closer one and outside is
the ring E.

I can't - I can only say that the rings of Saturn were named in
alphabetical order. So there's not a good way to remember them but this
E ring is the same one I showed in the picture that's associated with
the moon Enceladus and they both start with E; so that's one way to
possibly remember that. And these rings are made up of small particles -
particles about the size of what the particles in the smoke from a
cigarette or small particles about the size of human hair, which tend to
scatter light 10,000th of an inch - tend to scatter light in the four
directions.

So they're particularly visible in this sort of perspective where the
sun is behind the planet. And because the sun is hidden during this
moment, we're able to take a long exposure and see those rings. So we
see the diffuse E ring is the outer most ring there, which I'll say more
about. And if you look at the left hand side of that, between the E and
the G rings, there's a little white dot. Look at the next slide, which
we've blown up here.

And that, as you can see in the upper left hand corner is a dot with
another smaller dot next to it. In fact, some people have been calling
this a pale blue dot because that's actually the Earth and the Earth's
moon that was caught in this particular photograph in the long exposure.
So even all the way from Saturn we can see the Earth and we're lucky,
looking back towards the sun, and the Earth is just barely visible in
this picture.

It gives us the same sort of perspective that Carl Sagan talked about
when Voyager looked back after flying by the giant planets in 1990. And
he wrote a book called The Pale Blue Dot and just to show how tiny and
insignificant the Earth is in the cosmic perspective. But at the same
time, it still has a slight bluish color, which is indicating the
atmosphere and the ocean of the Earth, which makes it possible to live
here.

So we are getting somewhat of this same cosmic perspective from Cassini
although we're in orbit in Saturn and Saturn is the second largest
planet in the Solar System, we're still able to get some idea of the
vastness of space and how small and precious the Earth moon system is in
that. Look at that right there.

Earl Kyle: I have a question.

Larry Esposito: Yeah.

Jane Jones: Could the person who's asking the questions please identify
themselves so that when we do the transcripts there'll be a name
associated with it? Thanks.

Earl Kyle: Earl Kyle. Solar System ambassador, Rochester, Minnesota. On
the previous slide why is it that we're looking at the backside of the
planet? It looks like you can still see superimposed the ring structure
there.

Larry Esposito: Right because the rings are not completely uniform and
they vary in their transparency. And so more or less light leaks through
the rings even when you're looking on the dark side. We first observed
this with Pioneer 11 back in 1979, which was a surprise to us then but
Voyager in 1981, when it also took pictures of the dark side of the
rings, which are not completely dark just as you can see here.

So I'm going to move ahead to the next slide. There's the one with the
insert of the Earth and moon and now here's a close up of the E ring,
which is right in the center of the E ring, you see a tiny black dot
with a little extension, a white extension, toward the bottom. If you're
looking at that, that is the moon Enceladus and that white feature that
we're seeing is light scattered by the volcanic eruptions on Enceladus,
which are coming from its South pole.

And those we can see are just right in the center of the E ring. The E
ring is a little bit cut up and not perfectly smooth near that. So it's
both disturbed and we later learned, created by the materials erupting
off the surface of Enceladus. Next slide. I wanted to show you a picture
we've made with our own experiment here, the Colorado experiment. We
have an imaging spectrograph, so that means it's possible for us to take
pictures.

And the pictures that we make are in the ultraviolet, which as you
probably know is invisible to the human eye. So we make pictures that -
of things that couldn't be seen by humans but are visible in the
ultraviolet. And the way that we make a picture is not by taking a
single snapshot but moving our pointing of our experiment across the
planet. So in our investigation, we only have 64 pixels.

Everybody talks about a camera with megapixels, 1 million or more. You
probably wouldn't buy one today that only had, you know, a half a meg on
it but we only have 64 pixels; that's one of the limitations of space.
And those pixels and this drawing called Saturn's Northern Lights, would
start at the bottom and go up to the top. So Pixel Number 0 would be at
the bottom and Pixel Number 63 would be at the top.

And what we do is we successively expose the planet by moving the slit
from the left to the right and then in the computer we put all those
columns together and we use our 64 pixels to make a picture of Saturn.
And of course, it's not as good as the Cassini cameras, which do have
pixels, but we're able to see in wavelengths that they can't and also
we're able to look at one time at 1028 different colors - different
wavelengths in the ultraviolet.

So what you're seeing here are two successive pictures of Saturn, one on
the left and one on the right, made in that way. We build them up one
line at a time and we have used the colors that humans can see - oranges
and blues - to map out the emissions from the atmosphere of Saturn. And
the blue particularly is an emission from hydrogen atoms. And so what
we're seeing here is the glow of the Saturn atmosphere, the Northern
Lights of Saturn, even though we're looking at the South pole of Saturn;
the Aurora on the Earth also occur in Antarctica.

So essentially what we would call Aurora here on the Earth, we're seeing
exactly the same and we're using both the imaging capability of the
spectrometer and its ability to distinguish constituents in the
atmosphere by the color of light they're emitting to make this picture.
And you can probably see between the left and the right that the Aurora
has changed a little bit, if you look inside that blue circle in the
interior. And so we're actually watching an auroral display here on the
planet Saturn. Next slide.

I want to talk a little bit about one other important aspect of the
system before I come back to the rings and that's about the satellites.
And here the satellites are shown for scale against the United States.
And looking at this you can see that the - on the lower right hand
corner is a dotted circle, which is the size of the Earth's moon. And
then the big yellow circle is the size of Titan, which is significantly
larger than the Earth's moon.

You can see the moon Rhea is about as big as Texas and that Enceladus is
about as big as Iowa and Mimas is about as big as Georgia and the moon
Phoebe would fit across Florida. Some of the smaller moons like Andorra
are not even as big as New Jersey and some would be more comparable to
the smaller Northeastern states.

So let me say what I can say about some of the recent results on
Saturn's moons. And this next picture you're looking at is a grayscale
image of (unintelligible) - of Titan that's made by the side-looking
radar, the radar on Cassini. And you can see that Titan is just
completely covered in this region by long dunes - I'd like to say sand
dunes but they're probably not made of sand on Titan.

They're probably made up of frozen hydrocarbons - grease or gunk - that
collects into tiny little particles, are then blown around by the wind
and form these dunes in the same way that the dunes form in the Sahara
on the Earth.

Look at our next slide, this is not a radar image but a camera image
from the descending Huygens spacecraft, which, as you probably know, was
carried to Saturn, built by the Europeans and carried to Saturn as part
of the Cassini mission and separated from Cassini on just about
Christmas Day 2004 and on January 15, 2005, landed on the surface of
Titan. And so that little probe that the Europeans had built landed on
the surface and as it descended, it took a series of photographs where
several have been put together here to show that the surface of Titan
includes what look like rivers and streams - tributary streams - run off
networks.

So presumably it's raining on Titan, at least occasionally, to cut these
streams in the surface; look every bit like rivers in the Appalachians,
for example. And when we can measure the altitude of the surface by
looking at two different angles, we see that all of these streams run
downhill into the flatter area that's in the bottom and lower left. And
we thought that might be something like a sea that all these channels
drained into.

But as it turned out, that's where Huygens did land, in that darker
flatter region, and it actually was solid, not liquid there; although
when the Huygens probe sat there for a while, it was warm enough that it
heated up some of the surface and we saw methane and ethane evaporating.
So perhaps the surface is a little bit slushy or crusty there with
methane underneath. And we think it's methane rain that's cut the
channels that we see in this picture. And I'll say more about methane
because it plays a role on Titan very much like water in the Earth.

On the Earth there's a hydrologic cycle. There's potentially methane
underneath the surface and reservoirs and aquifers, which evaporates
into the atmosphere, forms clouds and rains out and the floods created
by those rains cut these channels and that was told to me once in Europe
that the rain on Titan is predominantly methane. Go to the next slide
here.

So also the methane can collect into larger bodies and here's a
picture - it looks like a lake - almost certainly a lake of methane on
the surface of Titan. You can see that it's flooded the lower lying
areas and that there are some islands that stick out in a crenellated
seashore maybe like Lake Powell that we see in the West here.

And even going beyond Titan, I wanted to show some images of the moon
Iapetus, which has been an object of interest for many years because it
has one very dark side and one very bright side. And at close up, these
are views from just the last year; the cameras are showing that this
dark and black distribution continues even at high resolution. So
there's features here that are just tens of kilometers across and
they're either black or white. There don't seem to be any shades of
gray.

And, in fact, in some of the craters we can see that half of it is
bright and half of it is dark. And the explanation that we have for
this - the next slide will show some close ups here of this same sort of
thing. The explanation is that wherever it was dark on Iapetus - days
are very long on Iapetus and the sun shines for a long time. And where
it's dark, it gets warmer; like wearing a black coat on a sunny day. And
where it gets warmer that evaporates the surface ices, particularly
water.

And the surface therefore evaporates the water evaporates. It's mixed
with some dark material that's left behind sort of a slag and the water
vapor in the atmosphere is the molecules are carried around on whatever
wind might exist and then they snow out where it's colder. So we get a
movement of material ice from the darker regions. The brighter regions,
of course, when the ice leaves the darker regions, that makes them even
darker because they're less icy and where it snows it becomes even
brighter.

So where it snowed then because it is brighter, the temperatures are
cooler and that tends to attract even more ice and where it's dark, it
gets even hotter and that tends to evaporate more ice and we can imagine
after some millions, maybe billions of years that the surface of
Iapetus - if it eventually had some shades of gray, they've all been
driven to the extreme states of either being darker than coal or
brighter than snow.

Next slide. So I'm about ready to go back and do some close ups on
Enceladus and the E ring. Questions or comments?

Kevin Jarigese: This is Kevin Jarigese, ambassador in Albuquerque, New
Mexico. If you look at that last picture, it does look like there are
kind of blowouts. What's the relief between the dark and the light? Is
there a great relief change?

Larry Esposito: So in this part of Iapetus the relief is less than
hundreds of meters and, as you say, it could be that the impacts on the
surface have excavated through that layer of ice that's formed on the
top and exposed some of the darker material underneath. And there's
thought that that might be a depth of 1-10 meters here. But I don't
believe on the stereo there's enough information to actually measure
that better. Does that answer your question?

Kevin Jarigese: It does.

Larry Esposito: Let's go to the next slide, which is a picture of the
moon Enceladus taken from the dark side with the South Pole on the left.
And you can see that there are definitely jets of gas shooting off from
Enceladus. This is a close up of the same phenomenon we saw in that big
picture of the E ring where the moon Enceladus is going through the E
ring and at the same time ejecting material.

And, we go to the next slide; I'd like to explain how we were able to
observe that plume on Enceladus - that eruption. And with an indirect
method, we used with the Ultraviolet Imaging Spectrograph, where instead
of looking directly at Enceladus or at these erupting plumes, like in
the picture, which is the previous picture is from the cameras. Instead,
we watched the star, which is that yellow star, which as seen by the
spacecraft, appeared to set and then rise behind Enceladus.

And over plotted on that is the squiggly line, which is the brightness
of the star. And a very interesting thing happened is that the star,
instead of just blinking out when it got to the edge of Enceladus,
gradually declined in brightness, showing that there's something lying
between us and - that is the Cassini spacecraft - and the star very near
to Enceladus, which is a very thin atmosphere of Enceladus created by
these eruptions.

And now, it's not even a complete atmosphere; it's very patchy. on the
other side of Enceladus we didn't see the same phenomenon. Look at the
next slide. You can see a movie that shows some of these eruptions
occurring and because of observations of the star and because of direct
observations made on Cassini as it flew through the plume, we know the
composition of that plume, which is mostly water vapor.

So it's mostly steam and the water vapor is probably condensing to make
little grains of ice. And based on our estimates of the amount of water
and the amount of ice, the material that's being blown off the surface
or out of the interior of Enceladus is sufficient to actually make the E
ring of Saturn. So we know think that this phenomenon you're seeing in
this movie here of jets of gas blowing grains of ice off of the moon
Enceladus make that ring that we see in the pictures that were taken
from Saturn's shadow.

So the E ring and Enceladus are not just fortuitously both starting with
the letter E but the moon Enceladus is almost certainly the source of
the material in that ring around Saturn. So here we go - talked about
two rings so far. The possible Rhea ring, which is clumps or chunks of
ice an rock from a shattered moon that's orbiting that moon Rhea and
here from the interior of Enceladus, grains of ice that are ejected to
form the ring E - E ring of Saturn.

Let's look at the next slide. So that sort of leads up to questions that
I've been investigating for a number of years. And we've been using the
Cassini data from the cameras and the other experiments and particularly
from our experiment UVIS to say something about the history of the
rings. So this slide we're looking at says Cassini observation shows
Saturn's rings may be ancient.

So what are they rings anyway? You've probably gotten an idea from just
what I'm talking about. They're made up of billions of small pieces of
ice orbiting Saturn. And sometimes when I think about Carl Sagan, I even
say billions and billions of small pieces of ice - and maybe trillions.
And they are really interesting not just because of their own beauty but
because they also resemble the disks from which planets are forming
around the other stars.

And our experiment, UVIS, observes light that's reflected from Saturn's
rings and we also watch stars that pass behind the rings the same way we
watched that star that passed behind Enceladus to measure the
composition and the amount of material in the plume from Enceladus.

Now here's the interesting point. Voyager just a quarter century - and
really the first close up views of Saturn's rings - and a big surprise
from that was how youthful the rings looked; how young they look. I mean
young in a geologic sense so, you know, maybe 100 million years - as old
as the dinosaurs or something but, you know, more than 10 times younger
than Saturn itself.

And this was a surprise and we were hard pressed to explain how the
rings around Saturn could have originated so recently in cosmic history;
just 100 million years ago. And there were ideas that had to do with
shattering a moon of Saturn like the explanations for the Rhea rings.
And another explanation that a comet might have come too close to Saturn
and being torn apart by Saturn's gravity and the pieces remained in the
ring.

And the amazing thing - we'll go to the next slide - is that the rings,
which were a 17th Century puzzle - and here I've just got some drawings
that were made in the 17th Century after the discovery of the telescope.
The early telescope makers looked at Saturn and its rings and couldn't
quite figure out what was going on although we with retrospect can see
that all of these pictures here - the rings - are of a planet that has a
ring around it - a flat ring - but also has seasons.

So that sometimes the rings are wide-open and in other seasons, like
near the Equinox, the rings are pointing directly toward the sun and the
Earth and so might even disappear. But this puzzle was eventually solved
by Christian Huygens, who the Huygens spacecraft is named after; but
they remain a puzzle in the 21st Century.

And now here I've got a - if you're looking at this, a blue and red
picture of Saturn's rings. And this is an artificial picture in several
ways. The first is that it was made by the Ultraviolet Imaging
Spectrograph and we didn't actually see the whole expanse of the rings.
We just took a cut through what would be the midline of this from left
to right and to make the picture we assume that Saturn's rings are
symmetric - are cylindrically symmetric or (unintelligible) - symmetric.

So we assumed the rings were the same all the way around the planet. And
the second thing is that this image was taken in reflected light in the
ultraviolet, which as I said, humans can't see. So if we had been on the
spacecraft, we wouldn't have been greeted with this particular view. But
that also is an advantage to us because then we can pick whatever
artificial colors we like.

And so we picked reddish colors to show the emission of hydrogen, which
is filling space - is shining through the rings everywhere. There's a
gap in the rings, we have a red light seen there. And we picked blue to
represent the spectrum of water ice. So wherever the rings are this
bright blue color, that's where the ice is very pure in the rings.

And then we used the computer to turn all that information into a color
photograph based on real information but not really how a human would
see it. In fact, I'm told butterflies can see in the ultraviolet so it's
sort of a butterfly's eye view of Saturn's rings. And you see there's
incredible structure in the rings; that the composition -brightness of
the rings changes from point to point.

And this is something, if the rings were hundreds of millions of years
old, would no longer be true. If there were any original variations in
the rings because the billions of particles keep running into each other
colliding on their orbits - sometimes several times a day - any original
coloring of the rings would be washed out.

So just looking at this picture we know that there are phenomena
occurring that are coloring the rings and are resisting nature's efforts
to smooth everything out. So this was just one of the first indications
that not only are the rings young but they're even younger than we
thought from the Voyager data. Go to the next slide.

This is the one that shows several waves of the rings so it's sort of
corrugated. On the left hand side there's a density wave in the rings; a
wave that's propagating like a sound wave through the rings and in the
right hand side there's a bending wave in the rings, which is
corrugation. Both of these excited by gravity from Saturn's nearby
moons.

And if you were sitting there in Saturn it would be like sitting at the
beach. You would see these waves come and go past you. And just another
indication of the dynamic nature of the rings - even more dynamic than
we expected. So you might think from that that the rings are even
younger than we thought. Maybe, you know, they're not even as old as the
dinosaurs. Maybe they're as old only as the Australopithicine - one of
human ancestors.

This causes kind of a problem in the fact in the data the observations;
we even see differences in the rings between now and 25 years ago. And
so the idea that we had when Voyager flew by that the rings all look so
young in the process of proceeding so rapidly is because they really are
young and they were created just cosmic (idling) just some millions of
years ago doesn't work because we don't think that the rings have been
recreated/created in the last 25 years.

So paradoxically the fact that we see very youthful features in the
rings means that this explanation of the rings' history as being created
quite recently cosmically doesn't work. In fact, we're seeing not only
very youthful ages but ages in between so that we're seeing a whole
range of ages of phenomena in the rings that are inconsistent with a
single creation event. Let's see what I've got here.

So I want to talk a little bit about how moons interact with rings and
come back to this question of how do we get out of this problem that the
rings appear youthful; in fact some places very, very young? But at the
same time, we can't fit that all together with a single event that
created the rings. And there's an interesting relationship between the
moons and the rings that is shown in this next slide here where the
gravity from two moons serves to hold the ring in place.

And essentially to shepherd the rings like a sheep dog would keep a herd
of sheep in place. And in Saturn's rings we definitely find - and in the
rings of other planets we definitely find rings held in place between
the moons. So the moons have definitely a structural relationship to the
rings and, as I said earlier, maybe also a genetic relationship because
if you shatter one of these moons - break it to pieces - that would
leave a ring and that may well be how the rings of Saturn formed.

Now maybe 15 years ago Joshua Colwell and I at the University of
Colorado developed a numerical model of ring formation that was just
based on this idea of shattering a moon to make a ring. Let's look at
the next slide. Here's Saturn's F ring is the one that I discovered and
you can see that it's sitting between two of the shepherding moons just
like in the previous diagram.

And here's some other images of the F ring and this next one shows how
the nearby shepherding moon Prometheus, which is on the inside in
between the F ring and the main rings, which are off to the right, is
creating wiggles and structure in the rings.

Kevin Jarigese: Is there any - this is Kevin again from Albuquerque. Is
there any electromagnetic relationships between the rings and the moons?

Larry Esposito: For particles of this size, you can basically ignore
static electricity. For the moons of Jupiter, there are some pieces of
evidence that show electromagnetic effects are important. But for what
we're seeing in these pictures, the electromagnetic effects are
insignificant.

For some of the smaller particles, though, the ones that are dust, like
in the E ring, it's possible that electromagnetic effects would have
some implications. But here it's mostly gravity that we're talking
about. So I'm going to go to the next slide, which is one called
collisional cascade. This was the idea that Colwell and I had that
there's a sort of collisional cascade.

You start out with big particles, moons of Saturn, and you break them up
and every time you break them up you make a ring. The ring gets ground
up to dust but you smash another moon, make up a new ring.
Unfortunately, as I said, this seems to go too fast. It uses up the ring
material in just some hundreds of millions of years, which, you know,
wasn't a huge problem 15 years ago when we first did this analysis but
now causes us some concern because we're asking ourselves why have we
been so lucky?

Man: What slide are you on now?

Larry Esposito: It's called collisional cascade. If I escape

Man: I don't have titles on my (unintelligible) - this slide (anyway).

Man: Thirty-three.

Larry Esposito: It's Slide 33. Okay? And if I can go back to my slide
show. So you're seeing it. Everybody's seeing it - Slide Number 33. And
that - the problem is, you know, and we humans have been so fortunate,
you know, taking 4 billion years or so to evolve from the very first
forms of life on Earth and then eventually developing intelligence and
space flight, launching a spacecraft into the outer Solar System, flying
by Saturn just at the moment it happens to have rings.

And so this is a challenge, which is similar to the one that at the same
time, we're also finding that there's no unique age in the rings - of
the rings. And so we're having this problem of both when did the rings
form and why are we so lucky to be there at the same time?

So I'm going to go on to the next slide. Colwell and I did some analysis
of the evolution of these ring moon systems showing the collisional
cascade and what you see in this chart is just the size of the largest
ring fragment as a function of time and what happens is the original
material is very rapidly ground up into dust. The size of the largest
fragment goes to zero very quickly. And the various symbols here and
lines are just the different mean, medium and mode of the distribution
of particle sizes.

So very quickly in our simulation in the time - about a tenth of the age
of the Solar System - all those original moons will be broken to bits.
In the meantime, there would be great rings around Saturn such as, you
know, like the ones we've photographed with Cassini and Voyager but that
would all come to an end very quickly in less than 10% of the age of the
Solar System.

So I wanted to show you some - I'm going to come back to how do we
answer that question but our answer is based on some detailed
investigation by Cassini of Saturn's F ring. And here, the slide you're
looking at now has four different views of Saturn's F ring showing
various features within that look to me like clumps or maybe moons that
are inside the F ring. And some of these are very short lived. We
photographed them and then they are gone the next time we look.

Man: Wow.

Larry Esposito: So that there seems to be at least in Saturn's F ring
some sort of process that's creating aggregates, maybe even little
moonlets in the rings that are short lived. And we with UVIS have been
looking at that and this is sort of how we're coming to our answer of
the question of what's the history of Saturn's rings. And we're trying
to answer the question basically are Saturn's rings young or are
Saturn's rings old?

So I'm going to show you a movie here. If you're watching this, you'll
see the star, which is a star that we've been watching, gradually start
moving. It's a little clunky in my (grain). If you go back you can start
that again. Yeah, that's a little better. The star is occulted, blocked
by Saturn's F ring. And so just like before as I talked about how we
measured the Enceladus plume by indirectly watching a star go behind it,
we've been measuring the nature of Saturn's F ring by watching now more
than 60 stars pass behind them.

And here's an artist's simulation of a star passing behind the F ring.
And you can see the artist has drawn in a little moonlet - a little tiny
moon. And when the star passes behind that, it blinks out. And naturally
that's what we observed also in Saturn's rings. So now I'm to the (word)
slide, which says UVIS finds clumps and moonlets.

So that's exactly what we did. Using the star applications we found
clumps and moonlets in Saturn's F ring. Thirteen different events,
probably most of them are temporary aggregations and one is a possible
moonlet. And this potentially is a solution to the problem of the age of
the rings. Now I don't know where everybody's from. I know in
Albuquerque they have a big recycling program. Certainly here in
Boulder, Colorado recycling is very big.

But this is a way potentially to make the rings last longer is to
recycle and that is when you destroy a moon and make a ring, if the
particles recycle by collecting again to make a new moon, that could be
used again. It could be shattered again to make a ring later. And so for
this to happen we need to have an alternate process that Josh Colwell
and I didn't consider 15 years ago in our collisional cascade.

That is, we considered breaking up moons but we didn't consider the
pieces of moons coming back together to make new moons that could be
then the source of future rings. So the story I'm heading toward here in
the history of the rings involves significant recycling like modern
America. So this next slide shows some of these features that we're
seeing in the rings and we're back to squiggly diagrams here.

Keep in your mind the artist's conception - that movie that I showed
before. And we have data here; we have a smooth line that's the visual
and infrared mapping spectrometer - one of the spectrometers on
Cassini - that's watching the same star that UVIS is watching. And,
amazingly, at one time the light from the star pretty much blinks out.
And that shows that it's not just a fluke or an error in our
observations but there's something that got between us and the star.

And I was giving names to these temporary aggregations; this one I'm
calling Pyewacket. And I'm using cat names because we're imagining that
these clumps come and go again and again. So they have something like
nine lives. And you probably remember Pyewacket from the movie Bell Book
and Candle. It's a familiar name. It's a name of Kim Novak's familiar;
she plays a witch in that movie with a little black cat.

Excuse me. So there's another star that we watched, Alpha 34, the star
(Spika). And here we see two other features, which we call (Butterball)
and (Fluffy). And we have, you know - I'm still looking for nominations.
We have features that still have numbers and don't have cat names yet.
But basically what we're seeing when we watch stars pass behind Saturn's
F ring is one, the ring is different all the time; that's the dynamic
nature of the ring.

But occasionally we have seen a number of events, which are not
repeatable that show a lot of starlight blocked. Sometimes with
(chucked) edges like you can see in Features 8 and 7 here and we're
interpreting these as temporary aggregations or as evidence for the fact
that the processes go both ways. That is, competing with meteorites
shattering the rings there's also a process where the materials, the
pieces, come back together.

So in addition to that in the F ring we're also seeing features in the
main ring, which have been called self-gravity wakes. And this is
something we could never have done with Voyager but because on Cassini
we're in orbit around Saturn, we get to watch one star after another go
behind the rings. And each one of them gives a cut through the ring,
which is just like a CT Scan.

We have scans through the rings in a variety of different angles and
just like with a CT Scan, you can turn that into a 3-dimensional image.
And in fact we see in our data features that are too small to be seen by
the cameras. And these are elongated, transient clumps that are called
self-gravity wakes for historical reasons and I'm going to show you some
pictures of those in the next slide.

So here's a numeric...

Man: What slide are you on now?

Larry Esposito: So let me escape here. I don't see the numbers.

Trina Ray: I'll get it. Let's see. Forty-two.

Man: Forty-two.

Larry Esposito: Forty-two.

Man: Yeah. Also, you keep referring to some video clips and they weren't
on the site that we downloaded; at least on my thing. So I'm not - are
you actually showing movies on...?

Larry Esposito: I'm showing movies. I sent them to the site. We had...

Man: They're not up there. I just downloaded it as you started.

Trina Ray: So it's on the password protected site. So check your
original email and you'll see a link to the password protected site. And
it'll have - and it's very case sensitive - and you'll be able to
download two movies from there as well as the original PowerPoint for
repackaging.

Man: Okay.

Trina Ray: And the movie that we've watched so far I think is called
Mittens.

Man: All right. I'll get it later. Thanks.

Jane Jones: Trina, this is Jane. Trina, this is Jane. I wasn't able to
get the Mittens either. So maybe it's not up - maybe it's on the one
you've gone to but I don't - I didn't find it either.

Trina Ray: Yeah. It's on the password protected site. I just downloaded
both the simulation and Mittens and they work just fine.

Jane Jones: Oh Okay.

Larry Esposito: So I apologize but I'm glad that it is possible to see
them. So I've got one more movie to show, which is coming up soon, which
is also going to be related to these self-gravity wakes. So the picture
I'm showing now is Slide 43 and on the left hand side is one of these
artificial color images of the rings and on the right hand side of that
is a numerical simulation of particles of two different sizes - red and
blue.

And what you can see is when we make a computer model of the rings,
because the individual particles have gravity, they attract each other
and the rings become very heterogeneous. You see all these structures in
the rings and what that means is when we take a tomographic view of the
rings, we take this sort of CT Scan from our multiple occultations, we
can build up the shapes and structures of that, which is below the
capability - it's invisible to the cameras.

So we're proud of that capability. I'm going to move on to the next
slide, Slide 44. So here's another artist's conception of what it would
look like in Saturn's A ring if you looked at some of these temporary
structures and you see the particles tend to clump up. We always had a
model of the rings that was simpler; that was more heterogeneous, where
everything was uniform.

But Saturn's rings are more complicated than we thought and we can tell
this because at some angles we can see through the rings and at other
angles we can't. And that can only be the case if the rings are
heterogeneous. And those tomographic pieces of information have put
together this 3-dimensional - put together here in this 3-dimensional
artist's view of the rings on Slide 44.

Kevin Jarigese: Is there any shepherding along with these large
clumpings - kind of like beading or...?

Larry Esposito: I'm sorry. Ask me again.

Kevin Jarigese: This is Kevin from Albuquerque. In looking at your
3-dimensional view there, it appears that there's kind of like beading
or gravitational - maybe some clumping. And you're talking about the
clumping but from clump to clump is there like gravitational beading, I
guess is the only word I can use? Or maybe some trailering going on?

Larry Esposito: So each of these structures is trailing. They have long
tails going out and those are the same sort of trailing structures you
can see in Slide 43. Let's see if I go back to that. That's the
simulation on the right hand side.

Kevin Jarigese: Forty-three. Yeah - I've got it.

Larry Esposito: Yeah, so that - so because of the differential rotation
because the particles closer to Saturn are orbiting faster than the
particles further away, all these structure are sort of stretched out in
that direction and so there's...

Kevin Jarigese: Is there a compositional change between the mass and
the...?

Larry Esposito: Not that we know of. And so it could be that if these
things are more permanent that the outer parts are getting more
meteoritic bombardment; there could be some compositional differences.
But what we think is that they're very temporary. That they come
together and come apart and I'm going to show you that just in a movie
in a just a few seconds here.

Kevin Jarigese: Got it.

Larry Esposito: So I'm on Slide 45, which is a frame from a movie that -
where we follow in a computer the dynamics of a distribution of
particles where it has various sizes running from oh a few centimeters
up to about 25 meters. And the sizes are drawn in this picture - so you
can see dots of different sizes. And in the center, we've put a big
particle, which could be one of these moonlets or - and it could be
something that had grown from the sort of processes you're going to see
in the movie here.

So if we look at that - we go to the next slide. Now this is another
movie here - and hopefully you'll see it start. What I'm seeing on my
screen is that original scene gradually growing clumps of material, some
of which sticks and some of which comes off. Anybody else see that?

Trina Ray: Yeah.

Larry Esposito: So this is what we think is happening in the rings. It
is these temporary aggregations are caused by the gravity of the
particles and the particles stick together to make temporary structures
and then the temporary structures disrupt. And both the photographs of
the F ring and our own occultations when we watch stars pass behind the
F ring are showing us some of these structures that show that the
accretion process can't be ignored in the rings; that these particles
stick together as well as breaking apart.

So that's the process, which I'm claiming can recycle the material in
the rings; use it again and again and make the original material in the
rings be able to last a lot longer. So the next picture I've got is...

Jo Pitesky: I - this is Jo Pitesky at JPL.

Larry Esposito: Yeah.

Jo Pitesky: On the simulation what is the time interval over which that
runs?

Larry Esposito: I think this is about 100 orbits, which would be a
couple months.

Man: Wow.

Larry Esposito: So if the rings are billions of years old, this has had
the chance to happen many, many times. And so even if many of these
clumps come apart, maybe some of them stick together and those are the
ones that could potentially grow to be big enough to be moons that could
be shattered later to form future rings.

Jo Pitesky: It also looks like that (seed) is about - the radius is
maybe about a tenth of a kilometer.

Larry Esposito: (I don't know) - yeah you can see that on the side.
It's - I think it's 50 meters - 25 meters in radius is what I think.

Jo Pitesky: Do you have any sense yet about if there's a minimum size
for what an effective seed needs to be or...?

Larry Esposito: For this (work) - it has to be bigger than the next
biggest particle by a factor of several. So in this distribution the
next biggest particles that you can see are a few meters across - I
think it's 12 meters - 6 meters. So the seed has to be significantly
bigger than that otherwise you don't get this phenomenon occurring.

Jo Pitesky: And it looks like this is mostly stickiness as opposed to
self-gravitation or...?

Larry Esposito: No. It's all gravity.

Jo Pitesky: Wow.

Larry Esposito: There's no adhesion in this.

Jo Pitesky: Wow.

Larry Esposito: I was in Finland last week and (Hikki Sallow) showed me
some similar simulations where he's put in stickiness and you get even
more clumping as you might guess.

Jo Pitesky: One last question for now. Since you've got a - what you
think might be a minimum size in some ways about what would be an
effective seed and you have some idea about how long it takes for the
particle size to go to zero, does this give you some sense of some kind
of time scale or - what am I asking?

Larry Esposito: So the time scales of accretion are very rapid. They're
on days to months. So they're infinitesimal compared to the age of the
rings; even if they're only 100 million years old - what we would call
young.

Jo Pitesky: I guess I'm...

Larry Esposito: So there's plenty of time for this to happen. If you're
asking what sort of rate do we have to make bodies that are bigger?

Jo Pitesky: Yes.

Larry Esposito: So then it's sufficient if this simulation, if we ran it
100,000 times and in one case we got a moon out of it. That's fast
enough to recycle the material in the rings.

Jo Pitesky: Okay.

Larry Esposito: Let me go on to the next slide, which is the 2-D
autocorrelation. This just shows from our occultation data that we can
actually see - maybe see is too strong; we can detect these waves. Go on
to the next slide. If we take that data in the previous slide and turn
it into a picture of the rings - this is our picture. This is now not
theory or a computer simulation - this is what our data show.

And you can see that in the rings we get these clumps of material with
trailing sort of leftward-leaning structures. So our data, our
observations from Cassini are a direct confirmation of predictions of
these models like the one I showed you the movie of. And here, from a
different model, this by Stuart Robin and Glenn Stewart of what happens
to the rings as you put more and more material in it.

And you can see from the left there's six of these simulations for
different values of the amount of material, which is given by this
little Greek letter Tao, which would be the measure of how much the
particles geometrically cover the whole area of the area of the rings.
So where Tao is .25, the ring particles cover 25% of the whole area of
the rings.

And what happens is very quickly they clump up as you can see. And
especially as the optical depth Tao, as the amount of material gets
bigger. And what's second, below each of these figures there are two
numbers. The first one is what we started with and the second one is
what the coverage is after we run the model to the equilibrium.

And you can see that even though - let's take the bottom right one -
even though the amount of material would cover the whole rings to a
factor of two; that is, on the average there would be two particles,
you'd have to go through, if you shot a bullet through the rings, when
we get done with the numerical simulation, that the actual coverage is
only .46.

So that would mean more than half the bullets that we shot through the
rings would not hit anything. And so this is - and I'm talking about
bullets, but you can think of the same thing with photons from a star,
the light from a star. So what this means is even though there's a lot
of material in the rings that a lot of light gets through because it's
all clumped up. And I admit to this myself. When I estimated the mass of
the rings back in 1983 from the Voyager data, I assumed the rings were
completely homogeneous and therefore the amount of light that got
through the rings told me how much mass there was in the rings.

And now I can see from our data from Cassini that that correlation, that
sort of extrapolation is totally incorrect and would underestimate the
amount of material in the rings by a factor of maybe five. So the rings
then could be much more massive than we had thought from the Voyager
data and that's not a bad thing because if the rings are much more
massive, there's much more material that can be recycled and used to
make future rings and they'll last a lot longer.

So going on to the next slide, we can say something then about what's
the model for the ring history and this is my particular model based on
my experience with the Voyager data modified very seriously by what we
found out from Cassini­ these last few years. So we believe that Saturn's
rings were created when a moon was shattered by a meteorite impact.

And of course, the pieces formed a ring around the planet. But there's
recycling; the pieces can recollect to form new moons, which are then
shattered later to form new rings, which then form new moons and so on
to the next slide. And therefore recycling - just like it allows us to
extend our natural resources on the Earth - allows the rings to be as
old as the Solar System although continually changing.

And because the rings have more mass than previously thought, they can
last that whole time and still be bright and icy after 4 billion years.
And so this is sort of the Cassini answer or my answer based on Cassini
results of the answer to the question of are the rings young or old? And
the answer is yes. The rings are young and the rings are old. The
material has been around for a long time and it is in the form now of
rings and moons.

And the moons are recollecting to form - I'm sorry - the rings are
recollecting to form moons and the moons are being shattered in
competing processes that give at every moment a different realization of
the rings. But rings will be forever in this example. You know, usually
wherever I'm telling this story I take an example of a metropolitan
area. When I told this explanation at the (COSPA) meeting in Beijing I
explained that the, you know, Beijing is just full of the most modern
construction.

There's cranes everywhere, they're building all sorts of new roads and
apartments and buildings but at the same time, the material that's being
used in some cases, the marble came from ancient dynasties and had been
used in buildings before. And even if you walk down the street, you'll
never see a person in Peking that's much older than 100 years old but
Beijing has been around for thousands of years.

And so in the same way, because of this continual renewal in the rings,
the features we see in the rings are quite useful and short lived but
the rings themselves are continually changing and continually renewing
and will likely be there a billion years from now if another
civilization sends out a spacecraft to the moons of the planet Saturn.

So that's my story here. That's an implication of how the Cassini data
are extending and perhaps paradoxically extending what we knew from
Voyager to get exactly the opposite answer. So from Voyager we concluded
the rings were young only as old as the dinosaurs. Now we can conclude
that the rings are as old as the planets - billions of years old - but
continually renewed. So thanks for your attention. I'll take any
questions anybody has.

Jane Jones: Yes. Thank you Larry. This is Jane. I think Trina's on also
still and if anybody has any questions we have some more time. Also, for
those of you who didn't, like I didn't, download the two movies, they
are definitely on the password-protected site. So you can go back and
look at them again afterwards.

(Pete Oldy): I have a question.

Jane Jones: Sure. And could you please state your name please?

(Pete Oldy): Yeah, this is (Pete Oldy) and my question is...

Jane Jones: Hi (Pete).

(Pete Oldy): Hi Jane. Do you think as part of the extended mission
you'll actually be able to image some of the clumping in a way to
support your theory?

Larry Esposito: So the - for to get pictures of those we need to get
close to the rings again. And that's not planned; we'll never get as
close again in the extended mission as we did in some of those first
photographs I showed you when we went into orbit.

But in the extended, extended mission, which is a proposal we're going
to make to NASA to extend the Cassini mission to the time of midsummer's
day, the summer solstice on Saturn - in that mission, near the end, the
spacecraft gets very close to the rings and flies just over them
multiple times. So it's possible in an extended, extended mission that
we would get close enough to the rings to take even higher detail
pictures to look at - look for some of these temporary moons and clumps.

In the meantime, we still watch the stars going behind the rings and
every week we get a new set of data. So in the extended mission we're
going to look at maybe 50 more stars, which is going to allow us to
improve the detail of our tomography but probably that's not quite like
seeing it yourself in a photograph.

(Pete Oldy): On the extended, extended mission, you're talking about the
possibility of doing orbits. Is that inside the D ring?

Larry Esposito: Yes.

(Pete Oldy): And what distance do you think you'd be if you were able to
image those?

Larry Esposito: So what would be the range to the rings? Could be as low
as 10-15,000 kilometers.

(Pete Oldy): Wow.

Larry Esposito: And that...

(Pete Oldy): And the velocity would be quite high, would it not, of the
orbiter?

Larry Esposito: The velocity would be higher. So there's a question of
smear, which hasn't been worked out yet but you'd have to ask the camera
team lead about that.

(Pete Oldy): Thank you.

Trina Ray: Larry, one of the questions that I'm getting a lot and I
thought maybe I would just ask you is what about spokes? We haven't
really seen such dramatic spokes as we saw in the Voyager days and what
are you thinking about that?

Larry Esposito: Well it's not a complete embarrassment because, you
know, eventually, after the start of the mission, we did see some
spokes. But they're in no way as spectacular as what was seen by
Voyager. And I think that's also a seasonal effect. So I'm predicting as
the season advances toward the equinox in the next year or two that we
will see much more spectacular spoke images because that was the season
when Voyager saw them.

Trina Ray: Okay. Thanks.

Man: Related to the spoke question, is there movement of those
spoke-like particles along the radius of the change in distance - think
that those particles change distance or orbits around Saturn?

Larry Esposito: Yeah. From the Voyager data we saw spokes form over
radial distances of 10,000 kilometers from Saturn in a matter of
minutes. So that would either imply that the particles are moving that
rapidly or there's some sort of electromagnetic phenomenon. And I would
say it's more likely the latter and what the individual spoke particles
do is they have very limited range. They move up and down mostly but
very little inward or outward.

Man: So they would be lifted, if you would, off of the rings and then
descend back down to the same orbit?

Larry Esposito: Right. So it's sort of like you're at the stadium and
the wave goes by­. You know, people raise their arms and put them down?
The wave goes around the stadium in about 10 or 15 seconds but nobody's
running - nobody's moving that rapidly. So that would be how I would
explain those apparent velocities as a wave-like phenomenon -
electromagnetic phenomenon where, like in the stadium, the little
particles stand up and then come back down.

Man: Would the radio science experiment be able to detect whatever
energy is producing that?

Larry Esposito: So radio science is not the best but the radio plasma
wave spectrometer has detected radio signals from the rings that are
consistent with some of these electromagnetic phenomena. One possibility
is that a lightening strike on Saturn could start one of these waves and
so that's a proposal to cause the spokes as a result of electrical
activity in Saturn's atmosphere.

And definitely the RPWS experiment has seen electrical activity,
lightening storms, in Saturn's atmosphere that could power the spoke
activity. But the visibility of the spokes is still, as Trina said, not
as good as it was in Voyager. So we may, if they do come back as I
predict, it will be easier to do those cross comparisons in about a
year.

Man: One other question. On approach to Saturn there was some - I think
I remember hearing of a detecting of a burst of oxygen being seen
perhaps related to a meteor impact. I haven't heard much about that
since.

Larry Esposito: Yeah. Those were our results, which we reported in
Science Magazine from the Ultraviolet Imaging Spectrograph. We were able
to map out the system and in the same way that we looked at hydrogen in
those auroral images we can focus on the color of oxygen and map the
oxygen in the Saturn System.

And we think that that oxygen is coming from those same plumes on
Enceladus. So it's not coming from a meteoritic impact but from water
molecules that are erupted off of the moon Enceladus that are then
impacted by electrons and broken apart by sunlight to make oxygen and
hydrogen atoms. And those oxygen atoms eventually escape the system and,
in the meantime, we're able to measure their distribution and their
lifetimes, which are consistent with eruption processes and processes in
the magnetosphere of Saturn.

Man: Thank you very much. That answers a lot of old questions.

Kevin Jarigese: One question here. This is Kevin from Albuquerque again.
As you look at some of those photos, you really see that it appears that
the coagulation of the various moons are very different. And I guess my
question is, in thinking about what you just presented - because I'm
going to have to rewrite some of my lectures here - how do you determine
the mass of a moon when it's embedded in a ring like that? And if it is
embedded in that ring, would the edges along the ring - would that tell
us about mass?

Larry Esposito: So of course, since we're looking at gravitational
phenomena that directly gives the masses here. And one way we can tell
the masses of objects in the rings is because they are able to shepherd
the ring edges or exclude some ring particles. And the cameras have
shown features that they call propellers, which are sort of nascent ring
edges where the moon is just big enough to sort of push particles away
from it by its gravity but not big enough to make a gap - an angular gap
in the rings.

And so from those gravitational interactions we can estimate the sizes
of some particles in the A ring being 50 to a few hundred meters across.
And in the same way, when we look at our star occultations we can
measure the width of the features that block the light. We find widths
between 27 meters and 9 kilometers. Pictures of moons in the rings - the
moons that have been found - are typically a few kilometers - 3 to 6
kilometers across.

Kevin Jarigese: Wow. Would the - as the moon coagulates, would that
cause any - maybe not pertubulations but maybe eccentricities of the
path of the moon in the ring itself?

Larry Esposito: No. There's an interplay between the moon and the rings.
And so the moon has a directly observable effect on the rings, which has
been observed in the camera images in these propellers. Over time, the
interaction between the gravity of the particles and the moon can't be
ignored and the growing moonlet will be pushed around. But we can't say
exactly how far or how much yet.

Kevin Jarigese: Wow. Thank you very much.

Larry Esposito: Yes, you're welcome.

Robert Bigelow: This is Robert Bigelow, Solar System ambassador from
Salt Lake City. I'm still a little bit confused about the end result of
the collisional cascade. If that were the only thing happening. When you
say the ring materials would be used up, does that mean it just, you
know, really small sized or does it actually get pulled out of the...?

Larry Esposito: Yes. It's ground to dust - that's the first part. So
both your answers are correct. The ring particles are ground to dust and
then the dust is small enough that it can be pushed around by the
electromagnetic forces.

Robert Bigelow: Oh okay. So it will be pushed either or pulled into
Saturn or ejected from the system?

Larry Esposito: Right. Exactly. And the ones that are pulled into
Saturn, those little grains will become meteorites; flashes in the sky
for the Saturnian residents. So yes, the - I wasn't clear on that. The
particles are eventually ground down to dust and the dust is dragged
away out of its orbit either into the planet itself or to far space.

Robert Bigelow: Okay. Thank you.

Larry Esposito: Yes, you're welcome.

Trina Ray: So Larry, what do you think is the big open question about
the rings right now?

Larry Esposito: So I'd like to get data that would confirm this model or
not. To show directly that the particles in the rings are either ancient
or recent. And so the questions that are important to that - what are
the key questions? What are the ring particles made of? What's their
composition? And what are the sizes of the ring particles? What's the
distribution of sizes and then how do those two things evolve? How does
the size change and the composition change with time?

And so the key questions about this are tomography - the questions could
be answered with Cassini data - of tomography from more occultation,
some close up images and some spectroscopy of the rings that measures
their composition. So those - Trina, you're probably thinking those are
the same questions we had when we went into orbit. They're still the
same questions. What's the structure of the rings? What's the
composition of the rings and what's their distribution of sizes?

And now we know better but we can't - we haven't yet got a definitive
answer from that about what's the history of the rings.

Trina Ray: Okay. Thanks.

Bob Kaplow: Larry, Bob Kaplow in Dundee, Illinois.

Larry Esposito: Yes.

Bob Kaplow: I've got another question for you. The thickness of the
rings - it seems like every time I hear new information, latest estimate
on the thickness of the rings keeps getting thinner and thinner and
thinner.

Larry Esposito: Right. This has been going on for 30 years.

Bob Kaplow: Where is that now and are we going to stop before we end up
with paper-thin rings?

Larry Esposito: So rings are already thinner in a relative sense than a
sheet of paper. I mean they're definitely flatter than a pancake. They
stretch from here to the moon and in some places where we measure the
thickness it's like 20 feet. So they're really some of the flattest
systems we know and there are a couple of points to be remembered.

I've been talking about that ring particles are not all the same size.
So the rings may be thinner than the biggest particles in them. So the
largest ring particles may be tens of meters across or hundreds of
meters. And the rings themselves could be just meters thick. And
furthermore, the little particles tend to have more vertical excursions
than the bigger particles. So we can't just talk about the thickness of
the rings.

The most massive particles are - biggest particles are most likely in a
very, very flat system. The smaller particles like the dust that causes
the spokes may have excursions of tens or hundreds of meters away from
the ring plane. I don't know if that's helping you visualize this at all
but they are absolutely one of the thinnest, flattest phenomenon that
we've ever observed.

Bob Kaplow: Okay. So it's like sheets of sandpaper with bumps on it.

Larry Esposito: Yeah. And in fact, embedded stones - sandstone in the
sandpaper.

Jane Jones: Well unless anybody has any other questions, I'd like to
thank Larry for this awesome talk. It was really great and I thought a
lot of people had really wonderful questions. Our next Charm Telecon
will be on June 24 and it'll be our annual anniversary episode. So what
we'll be doing is gathering kind of the summary of what Cassini's been
up to for the last year - the fourth year of the prime mission.

So that one will be good to put on your calendar. I know

Trina Ray: And Jane, could I just mention that since we have so much
material in the last anniversary talk, the plan is to split the
anniversary talk across both June and July. So we'll have the first half
in June and the second half in July.

Jane Jones: Fantastic.

Larry Esposito: So I want to - I've got just a moment left. Thanks for
those kind words, Jane, but I just want to give a slightly simpler
answer to Trina's question about what can Cassini tell us about the
history of the rings.

Jane Jones: Okay. Great.

Larry Esposito: And so we can't directly measure the history or the
origin of the rings. There's no single experiment that does that. But we
can measure the composition of the rings from the spectrometers, we can
measure the structure of the rings from the cameras and the
occultations, and that tells us the dynamics of the rings - how they're
moving, how they're changing.

And from that, we can play the movie backwards to get the history and
the origin of the rings. So that's - that was one of the first tasks
that Cassini set for the mission and we're still working on that task.

Trina Ray: Okay. Thanks Larry.

Jane Jones: Thank you Larry.

Larry Esposito: You're welcome.

Jane Jones: And thank you everyone for calling in. I know this is a busy
day here at NASA with a press conference on Mars also at 11:00. And a
lot of our museum alliance people were called into a special Mars after
party and so what I'm going to do with these people is as soon as the
wave file - the audio is done I'm going to remind all the museums that
this whole package is there for them for those who missed it. With that
we'll say good-bye.

Larry Esposito: Good-bye.

((Crosstalk))

Jane Jones: Thanks everyone. Bye-bye.

END