**FTS-NASA-VOICE**

**January 25, 2005**

**1:00 p.m. CST**

Coordinator I'd like to inform all participants that today's call is
being recorded. If you have any objections, you may disconnect at this
time. Thank you. You may begin at any time.

Amanda Alright, Trina?

Trina Well, Welcome, everyone, to the CHARM telecon. We have some
terrific stuff for you today. Amanda, are you online?

Amanda I'm here.

Trina Okay, why don't you go ahead and do introductions? The recording
has started and we are good to go.

Amanda Okay, and just to confirm that everybody, are we doing question
and answer during the talk or are we waiting until afterwards?

Trina We found that it's pretty good if we do questions and answers
during the talk as questions come up.

Amanda But if people aren't asking questions then they are staying muted
by pushing \*6?

Trina That's right.

Amanda Okay, I think everybody knows that but ... speakers from people
sneezing and stuff.

Let's see. So our two speakers today are John Spencer from Boulder,
Colorado at the Southwest Research Institute, who is on the Cassini CIRS
team, the Composite Infrared Spectrometer. He'll be talking about
results from Phoebe and Iapetus. And Tillman Denk, who is on the imaging
camera team, who will be talking about results from Iapetus, and Tillman
is calling in from Berlin.

> I don't know who wants to go first; we hadn't set that up, so I'll
> leave it up to you two.

John Tillman and I had talked and the plan is for me to go first.

Amanda Okay, so John Spencer is going to go first. Hopefully everybody
has both of their packages that were posted.

John Okay, I guess I will get started here. I don't have a good feel for
the background of the people out there, so if I'm being too technical or
too elementary, please interrupt and I will adjust accordingly.

I am going to talk about some of the early results from the CIRS
instrument, that's the Cassini Composite Infrared Spectrometer, looking
at the different small moons of Saturn, not Titan, but the other moons,
which are pretty interesting in their own right. The first of those
moons that we've got good data from have been Phoebe and Iapetus.

> We go to the first slide beyond the title, entitled "Black Body
> Radiation." I'll just say a little bit about the background of what
> CIRS does. CIRS is sensitive to thermal radiation from warm objects.
> Anything in the universe that has a temperature above absolute 0° is
> emitting heat radiation. If things are really hot, that heat radiation
> is at a wavelength that we can see with our own eyes, and that's when
> we talk about things being red hot or white hot.
>
> But anything that's colder than that will be emitting its own
> radiation, but at a lower temperature, and that means there is a
> longer wavelength because there is a very strong relationship between
> the temperature of an object and the wavelength of the light that it
> radiates because of its temperature. We are looking at pretty cold
> objects in the Saturn system, we're a long way from the Sun so we're
> looking at radiation at quite long infrared wavelengths and that's
> what CIRS is sensitive to.
>
> So this is a nice example of a cooling lava flow, probably in Hawaii,
> shows how lava that is very hot, probably is 1000-degrees is emitting
> red and yellow light, so it looks orange to us. Cooler lava is only
> emitting red light and then as it slows down this channel and cools
> further you can't see any glow from it at all, but if you were to hold
> your hand up to it you would certainly feel a lot of infrared
> radiation coming from that. Looking at objects in the Saturnian
> system, the radiation is at even longer wavelengths, quite long
> wavelengths, infrared.
>
> So going to the next slide, "Black Body Radiation Continued," I show
> pictures of, again, that glowing lava and then a piece of metal in a
> blacksmith shop, which is emitting the same kind of cherry red color
> as some of that lava. The point here is that by simply looking at the
> thermal radiation from an object, it's often hard to tell what the
> object is made of. You can measure the temperature very accurately by
> the color, but the spectrum is often not terribly diagnostic of the
> composition. You look at an object in reflected light and it can look
> very different, depending on its composition, but the thermal
> radiation you can't always tell.
>
> There are some fortunate exceptions to this, the hematite in the
> Meridia Planum on Mars, where the Opportunity Rover landed, was
> identified by small, but distinctive ... we're getting some background
> noise here.

Trina If somebody has an open mike, somebody who has a cough and it
sounds like some sort of a radio on in the background, not with music,
but with talking, if you press \*6 you will be muted, no matter what
your phone is.

John Okay, we're quiet again now. Thanks, Amanda.

So sometimes in thermal radiation there are subtle signatures of
composition and those led to the discovery of hematite on Mars. But
that's often not the case when you're simply measuring temperatures.

So that's what we're doing with the CIRS Composite Infrared Spectrometer
on Cassini. This is a pretty sophisticated instrument; it's a
spectrometer measuring a detailed spectrum over a wide range of
wavelengths between about seven and 300 microns. This is 14 to 600 times
longer than the human eye can see. It is primarily designed for looking
at objects with atmospheres, that's Saturn and Titan in the Saturn
system.

> For atmospheres you do get a great deal of compositional information
> from this kind of instrument; you can see methane and other
> hydrocarbons and all kinds of exotic species in these atmospheres. But
> when you're looking at a solid surface, like the icy moons or Saturn's
> rings, mostly you just measure temperature, though we're certainly
> looking for the kind of subtle clues that you would get from certain
> minerals on the surface. We might see if we're lucky, but we haven't
> really seen anything definitive yet.
>
> So the next slide is, "CIRS and Saturn's Small Satellites" and here is
> a nice montage of Voyager images showing all the largest Saturnian
> satellites to scale with big, fuzzy orange Titan in the middle and
> then the array of smaller satellites. The ones that we've had good
> looks at so far with Cassini CIRS have been Phoebe, which is the
> outermost one and is quite small, it's only a couple of hundred
> kilometers across, and Iapetus, that we looked at more recently.
> Phoebe we observed before we even went into orbit around Saturn. So
> we'll be getting good data on all the other satellites over the course
> of the mission, but not so much yet.
>
> So moving to the next slide we have one of the close-up Cassini camera
> images of Phoebe. This is an irregular satellite of Saturn, meaning
> its shape is irregular, but what "irregular" means in this case is
> that its orbit is irregular; it's not in a nice well-behaved circular
> orbit in the plane of Saturn's equator, like most of the other
> satellites. It's very far out, it has probably been captured early in
> the history of the solar system into orbit around Saturn; it probably
> didn't even form near Saturn at all. But it is very interesting
> because it is our first close look at any of these objects that may
> have been wandering the outer solar system around the time the planets
> were formed and may well have been one of the building blocks of the
> outer planets.
>
> Phoebe is dark, meaning it soaks up 95% of the sunlight that hits it
> and that means it is pretty warm by Saturn's standards. That gives us
> a good, strong signal and allows us to get nice maps of the surface
> temperature.
>
> Is somebody asking a question or is it background noise?

Amanda Once again, I think somebody needs to be sure to push \*6, we
have some background.

John If it is a question, I'm certainly not hearing it.

Okay, we'll move along. A note about, like in most scientific
computations we are measuring temperature in degrees Kelvin, that's the
degree that's measured from absolute zero. So 100 Kelvin, which is a
difficult temperature for these moons is -173° Centigrade, which is
-279° Fahrenheit, so we are talking very, very cold temperatures here.

So as we approached Phoebe back last June, before we went into Saturn
orbit, we obtained a series of maps of the surface temperatures on
approach. The next slide, entitled "Approach to Phoebe" shows an example
of one of these maps. In the upper-left is the scan pattern of the
instrument as we scanned across the surface. The purple rectangle
represents the area that we see at any one time. We have ten detectors
in a row and with the heat and from each of those and then we scan the
... target to build up an image.

Down in the lower right you can see a raw image and then an image where
we've converted the intensity of the heat radiation into temperatures,
so we produce a temperature map of Phoebe here. The image in the
upper-right shows the appearance of Phoebe at around the same time, as
seen by the camera on Cassini.

> You can see this enormous crater, which has been provisionally called
> "Jason" in the upper part of the image and you can see the rim of that
> crater casting a shadow. If you look in the thermal image, you can see
> that shadow nicely in the thermal image as well; there is a dark
> blue-black region in about the same position as the shadow in the
> visible image.
>
> But we also see radiation from the night side, where there is no
> visible light for the camera to pick up at all. ...Has a significant
> temperature and significant thermal radiation. We're seeing
> temperatures there looking at temperatures at around 80 Kelvin and
> then warming up to over 100 Kelvin at the warmest places. We took a
> series of these maps watching Phoebe rotate as we approached.
>
> The next image shows a view as we reached our closest point to Phoebe
> and we scanned across the surface nearer mid-day, so we were seeing
> higher temperatures. The mosaic shows a series of high-resolution
> pictures taken by the camera during that close approach period and the
> image in the lower right with the red rectangle is showing where those
> images are on the surface of Phoebe; it's really quite a small piece
> of the surface we're seeing at closest approach.
>
> And then as the camera was scanning, we were scanning along, riding
> along with the camera and were measuring the temperature at all these
> spots in the surface. Again, I've color coded this so you can see how
> the temperature responds to the craters. We have shaded surfaces in
> craters that are showing blues and purples in this color code and are
> quite cool, and then surfaces facing the sun are much warmer. But all
> these temperatures are around 90 to 110 Kelvin, really very cold, but
> these are the warmest temperatures that we see on Phoebe.
>
> Then as we flew past, we approached on the morning side of Phoebe and
> we left on the afternoon side, and there we actually felt the highest
> temperatures of all. This is a scan across the afternoon side of
> Phoebe, and a picture showing approximately the view that we had to
> there. Again, you're seeing this big crater, Jason, now lit up by the
> afternoon sun. The images at the bottom, color-coded by temperature,
> show what the thermal radiation looked like.
>
> There is a lot of structure here, lighter and dark patches, and you
> can sort of correlate them with the image to see that these are
> controlled by topography, that surfaces that are facing the sun tend
> to be warmer, those that are facing a little bit away from the Sun are
> colder, just as you'd find on the earth, we've got the sunny side of
> the street, if you like, on some of these craters. So the maximum
> temperatures we're seeing here are around 112 Kelvin.
>
> Then as we receded further from Phoebe we could see the radiation from
> the night side, just as the surface was rotating into darkness and we
> are seeing the early evening temperatures. So that's the image shown
> here; the sort of eyebrow shape is, again, the big crater cliffs
> catching the final rays of the setting sun, and that's still quite
> warm, but once the surface rotates into darkness, the temperatures are
> much lower.
>
> So we've got a whole series of maps like this and we can actually make
> a movie of these. The next panel, called "Phoebe Temperature Maps at
> Different Times of Day" shows a series of these maps. These are all
> now in latitude and longitude space, so this is as though you were
> looking at a map of the Earth showing the temperatures in different
> places at different times of day.
>
> The asterisk in each of these panels shows where the sun was directly
> overhead, and as Phoebe rotates, that point moves westward so that
> we're seeing regions that are in the morning -- I should have done
> this the other way out -- but the approach image taken first is at the
> bottom of the screen and then as you move up you're getting to later
> and later times and we're watching Phoebe rotate, we're watching the
> place where the sun is overhead move steadily to the left or west in
> these maps.
>
> So now you can see the big crater, Jason, which I probably should have
> marked in here, is in the second square to the right, or second square
> from the left in each of these panels, just above the equator. You can
> see shadows associated with that, it being very cold in the bottom
> panel before the sun has risen and warming up and then being very warm
> in the afternoon in the panel the next closest to the top one, and
> then cooling off a little bit towards sunset in the final panel at the
> top. The image, again, shows a reminder of the size of this big
> crater.
>
> Then the next series of images, which are kind of like a movie, the
> next series of slides simply walks through these in chronological
> order, so you can see temperatures change through the day. So we can
> just step through those fairly quickly; they're all pretty much the
> same except that the temperature amount has changed in each case, so
> we have movies frame one, two, three -- on three we only looked at the
> night side, but it's in the correct sequence here, but you can't see
> the big crater, it was in the wrong place. Four and five, and - so we
> can construct temperature amounts for individual places on the
> surface.
>
> But moving on to the next slide, this is the Phoebe diurnal
> temperature curve, where now it's simply collecting all the data
> showing the temperature at different times of day. So we see
> temperatures around 80 Kelvin before dawn, warming up to around 110 at
> mid-day and cooling off again in the afternoon.
>
> We're interested in curves like this because it's one of the ways we
> can get a handle on what the surface is like. What we're trying to
> measure is something called the "thermal inertia," which gives a feel
> for how well the surface retains heat at night. If you've got a very
> solid surface, like rock or solid ice, it actually stores heat pretty
> efficiently, and that means heat will soak into the interior of the
> surface and will be released again when the surface cools off, and
> that will tend to keep the temperatures pretty even.
>
> This is like around sunset feeling a wall that has been in the
> sunlight all day and it's still pretty warm, even around sunset,
> because it has soaked up that heat and it's retaining it through into
> the evening, whereas if you had a surface covered in carpet or
> something very fluffy that was very insulating, then it would cool off
> much more quickly and if you'd feel the warm stone wall you would feel
> a much warmer surface than if you felt this rough carpet surface. So
> by just simply measuring how the temperature varies by this time of
> day, we can get some idea of how solid, how dense, how insulating the
> surface is.
>
> So the curves here show theoretical ... of how we would expect the
> temperature to vary for different thermal inertias measured in very
> obscure units that I won't bore you with. But the numbers here, about
> 20,000 to 30,000, are very small numbers for thermal inertia compared
> to solid surfaces. If you had solid rock or ice surface it would be
> 100 times higher thermal inertia than this.
>
> This tells us that the surface is very, very fluffy, like the Apollo
> astronauts saw when they went to the surface of the moon, a surface
> that you could make footprints in very easily because it's very,
> people talk about a fairy castle structure, where you have these very
> complicated shapes of little grains balanced precariously on top of
> each other, making a very insulating surface that cools off very
> quickly at night, so that's what we think we're seeing on Phoebe.
>
> So that's what we've done so far with the Phoebe data. I'll move on
> now to Iapetus.
>
> Iapetus; Tillman will talk shortly in a lot more detail about Iapetus.
> It's one of the most fascinating moons in the solar system because of
> this remarkable light/dark dichotomy; one side of Iapetus is covered
> in very dark material, the other side is covered in very bright
> material and we still don't have a very good explanation for this.
>
> We got our first close look at Iapetus in the new year, essentially on
> New Year's Eve and we receded from Iapetus on New Year's Day, so we
> haven't had this data very long, we've just had a quick look at them,
> but I can at least give you a flavor of them.

Harold John?

John Yes.

Harold Harold Kozak here; I'm the Professor of Astronomy at Wagner
College, also a Solar System Ambassador for JPL. Just one quick thing
about Phoebe that I just wanted to ask you.

John Sure.

Harold Would you consider that Phoebe is actually a captured asteroid of
Saturn?

John It probably is. We wouldn't use the word "asteroid" because we tend
to reserve that for objects in the region between Jupiter and Mars, but
it's probably a captured object of some kind, simply because it's in
this crazy orbit that is not -- it's hard to imagine an object forming
in an orbit like that.

But exactly what kind of orbit it had before it was captured it is hard
to say, it could have come from as far away as the Kuiper Belt and it
could be our first close look at an object from the Kuiper Belt, out
beyond Neptune, because objects from that region are perturbed into the
region of the giant planets and might, early in the solar system, have
got captured.

I actually think a more interesting possibility is that it formed
somewhere in the Jupiter/Saturn/Uranus region, not too far in distance
from the sun from where it is now, and it got captured into orbit around
Saturn and that kind of protected it. There would have been zillions of
objects like this because the giant planets would have been initially
made up or their cores would have been made up of objects like this that
coalesced and then collided with each other and produced the cores of
the giant planets.

But all those objects have gone now from orbiting around the sun because
if they are orbiting around the sun they can collide with one of the
giant planets or pass close enough to get thrown out, but any that
happen to get captured into orbits around giant planets are protected,
so those objects are preserved for us to look at. It's likely that
Phoebe is one of the few objects of these building blocks of the giant
planets that are still there for us to look at.

Harold Do you have any spectral analysis that would indicate any type of
water ice?

John Yes, as I say, the CIRS instrument mostly measures temperatures and
we have only subtle clues to the composition, but at near infrared
wavelengths, just a few times longer than the eye can see, you do get
much more distinct compositional information. There is an instrument
called "VIMS," the Visual and Infrared Mapping Spectrometer, on Cassini
that got very detailed compositional information on both Phoebe and
Iapetus during the fly-bys, and yes, they saw water ice and they saw a
bunch of other interesting things, not all of which they've even
identified yet, but carbon dioxide is one thing that they saw. So it
does have a rich suite of chemicals on its surface, but that's done with
another instrument.

Harold Thank you, John.

John Sure. Okay, so moving on to Iapetus, we can go to the next Iapetus
slide, which is "Iapetus Daytime Temperatures." This shows again, one of
our temperature maps of Iapetus, along with an image taken with the
Cassini camera about the same time. The most striking thing here is it's
quite a lot warmer than Phoebe; we're getting temperatures up to about
130 Kelvin at the sub-solar point, the point closest to the sun.

We also see subtle variations due to craters; you can see in the middle
of the temperature image the subtle appearance of a crater produced by
different slopes facing towards or away from the sun having different
temperatures. But Iapetus is a much more fertile object than Phoebe so
those effects are much less dramatic.

Moving on to the next slide, "Iapetus Temperature Map," we can again
turn this into a map of latitude and longitude and see the temperatures
in different places. Why is Iapetus so much warmer than Phoebe? Both of
them are very dark, they're both absorbing most of the sunlight that
hits them. The difference is that Iapetus is rotating very slowly; it
takes 79 days to rotate once, versus nine hours for Phoebe, that means
the surface during the day has a lot of time to warm up, so that allows
it to get quite a lot warmer than Phoebe, where, by the time it reaches
noon, it is still warming up and then it starts cooling off again
towards the evening. So rotation rate can have a significant effect on
temperatures.

Moving on to the next slide, "Iapetus Nighttime Temperatures," the flip
side of that is that the night side of Iapetus is extremely cold because
it's rotating so slowly that things have a long time to cool down. The
other factor is that we approached looking at the dark side of Iapetus,
these very warm temperatures are on the dark side, but most of the night
side was bright terrain, which is much colder anyway because it doesn't
absorb as much sunlight. In fact, we measured temperatures as low as
about 40 Kelvin on Iapetus, which is extremely cold; it's one of the
coldest places probably in the Saturnian system. So Iapetus has really
the temperature extremes in the Saturnian system; it's one of the
warmest and coldest places.

So again, we can measure the variation in temperature with time of day.
We have a hard time measuring the nighttime temperatures because they're
so cold there's not much radiation. We have measurements with an
instrument that doesn't provide as detailed information so this is our
best map so far of the variation with time of day on Iapetus. Again, we
have an extremely fluffy surface, actually with a very similar thermal
inertia to Phoebe here.

So that's what we've got so far; we're still analyzing the data and
we'll get more out of it. But we're also looking forward to new stuff
coming up very soon. The Most exciting thing in the near future for
those of us working on the icy satellites, other than Titan, is
Enceladus, this very exotic looking moon with smooth areas and strange
looking craters, regions where the craters seem to have been wiped out
and all these furrows running across the surface. You can even see an
upside-down or backwards "E" in the lower right of this image, as though
it signed its own initial there on the surface.

So on Enceladus with CIRS we will be looking for heat radiation from any
hotspots due to active resurfacing, hot material coming out of the
interior of Enceladus, part of the process that seems to be wiping out
these craters. We think that's a long shot, it's not likely that we'll
see that, but you never know, we might get lucky, so we will be looking
for that. We'll also just be measuring the temperature of the surface,
regardless of whether there are any hotspots or not, because Enceladus
is very cold, it's so bright it's reflecting most of the light that hits
it, so it's a little hard to know what the temperatures will be because
you have to know exactly what small fraction of the light is being
absorbed. By knowing the surface temperature, that will tell us
something about the interior temperature and give us a better handle on
what's going on inside.

> So that's what's coming up next. That was my last slide and I'll be
> happy to take questions.

Harold John?

John Yes.

Harold Harold Kozak here again. Just a very interesting question about
Iapetus, I actually got very interested in it the first time I read
Arthur C. Clarke's novel *2001*, that's really what got me interested.
But I've read some things and I've heard that part of this dichotomy
that you mentioned about has to do with the fact that the dark side
seems to have some kind of organic coating on it. Can you tell me do you
have any information as to the constituents of that coating and the
possible reasons why that coating would be tilted to one side of the
moon rather than the other?

John Well, I'll let Tillman talk in more detail about that, because he
has quite a lot to say about that. But it's certainly true that that
dark material seems to have organic material in it, meaning simply
carbon compounds, of course, rather than biological material. And again,
the near infrared instrument on Cassini, the VIMS instrument, got a lot
of information about the composition of that. All that CIRS can say is
that that dark material is very fluffy and the fluffiness could, in
part, be due to the way in which that dark material has been coating the
surface, but it could partly mostly just be that it has been pulverized
by impact and you would expect it to be that fluffy anyway.

> So we are not the instrument with the answers for those questions. The
> imaging instrument that Tillman will talk about will have more of the
> answers than the VIMS instrument has, again, more of them.
>
> Any other questions?

Tillman Yes, John, maybe I can add two minor points to this. One is that
if on Earth there may be 20 million or so spectra measured that are used
for comparison and none of these really matches what is found on Iapetus
and on the other outer solar system bodies. This is not surprising
because the environments are so different to the laboratory
environments, which are at collectible temperatures and not at these low
temperatures.

Another point is that organics, there are so many different species
there and they look quite similar in the spectra, so it's hard to say
exactly which one is more abundant than another one.

John Right. Any other questions?

Steve Yes, Steve Centers in Rochester, New York. I have a question.

John Okay.

Steve I have my notes here from reading William McKinnon's chapter in
*The New Solar System* book about mid-sized icy satellites and he is
saying, if I've got my notes right, ammonia and antifreeze could explain
ancient planes on Tethys and Dione, but not the planes on Enceladus, the
title heating by Dione is more likely the mechanism there. Do you have
any comment on that?

John I think that's a fair statement. On Dione and Tethys we see
evidence for ancient resurfacing; there are smooth areas but they've
still got quite a lot of craters on them and they may well have been
resurfaced by eruptions form the interior shortly after these moons
formed, when they were still quite warm in the interior. So then all you
need is some antifreeze, like ammonia, that can soften the ices and
allow that bit of leftover heat shortly after these moons were formed to
melt that stuff and erupt it onto the surface.

The thing with Enceladus that is so mysterious is that there are so few
craters on parts of Enceladus and you can see looking at this image,
some areas are almost free of craters, that you have to have had
resurfacing much more recently, when all that residual early heat would
have died away. Enceladus is also smaller than Dione or Tethys, it's
really a tiny body, it's only about 500 kilometers across, so it's very
hard for it to keep any heat inside. So yes, you have to have some
additional heat source and tidal heating due to the influence of one or
the other moons is the most likely.

Steve Alright, thank you.

Pete Hello. This is Pete Goldy from San Francisco. I have a question
about Iapetus. On the equatorial bulge did you find any sign of
non-solar heat on the equatorial bulge?

John No, you can kind of see from the Iapetus daytime temperature slide
that, at the bulge, the kind of level of details that we can see, we
don't see such fine details as the cameras, you really don't see that
equatorial bulge as an obvious temperature anomaly. That's what you
would expect because it's very, very old, it's very beaten up by craters
as a lot of places where that ridge has been is smashed up by craters,
so probably it formed shortly after Iapetus did 4 billion years ago, so
we wouldn't expect to see any residual heat. We'd love to be surprised,
so we've looked, but there's nothing obvious there.

Pete Thank you.

Terry Hello, this is Terry Flower from Minnesota. You said that Phoebe
is more likely a captured an object; Iapetus, was that probably formed
perhaps when Saturn was formed?

John Yes, it probably was; it's orbiting Saturn as the same direction of
the other moons.

Terry Are they in the same plane?

John In almost the same plane; the plane is tilted a bit because it's
far enough out that the Sun affects it. But Iapetus is a bit of an
oddball because apart from the albedo patterns on it, because it is a
long way out from its primary, few other regular satellites are away
from their primary as Iapetus is, so there may be something special that
we haven't figured out yet about how it got to be so far away.

I think we better let Tillman take over now, assuming we only have an
hour for this and we were already over halfway through. So I think I
will shut up now and let Tillman take over.

Amanda We're actually okay.

John We are?

Trina We can go ahead with Tillman, but we'll have more time for
questions later because we do have two hours.

John Oh, we do?

Trina Yes.

John Well, in that case I will give people another chance for if people
do still have questions, I guess we still have time to answer them.

Amanda And then also maybe when Tillman is done we can open it up for
both of you.

John Certainly. Maybe everyone is done.

Jerry Jerry Hillburn, San Diego. Iapetus doesn't show any hint or sign
of an atmosphere of any sort?

John No, it's very small, it's gravity is pretty low, so we wouldn't
expect much of an atmosphere from Iapetus. You could get a very tenuous
atmosphere if there were some exotic ices, maybe ammonia or methane ices
that might evaporate even at these fairly low temperatures, but because
they would escape the surface pretty easily, we wouldn't expect much.

There are, the ultraviolet instrument on Cassini has looked for an
atmosphere and they haven't reported their results yet, but if they
found anything it would be very exciting because you wouldn't expect
anything on an object as small as Iapetus.

Anyone else?

Harold John, are we going to be getting at all into a discussion, any
brief discussion on what's happening on Titan as of now?

John I think neither Tillman nor I are the experts on Titan, and we are
thrilled and fascinated with what's going on and we could certainly
pontificate, but we're not the experts on that.

Harold I'll tell you why, because a number of my students today, we were
discussing this, if there is no evidence at all of any type of microbial
life or unicellular life, is it possible that perhaps the NASA JPL team
might think about possibly seeding Titan in the future with some sort of
anaerobic bacteria?

John I don't know. I think at the moment there is no terrestrial
organism that could survive on Titan because it's so incredibly cold
there; the surface temperature is about 95 degrees above absolute zero
and terrestrial organisms might survive at that temperature and if you
warmed them up then they might be able to still reproduce and live once
you warmed them up again, but there are no terrestrial organisms that
can operate at that temperature because all terrestrial organisms are
water-based and water is absolutely rock solid at that temperature, so
there is very little for a terrestrial organism to do.

Now who knows, genetic engineering maybe could come up with something
that would survive at Titan temperatures, but there are no natural human
organisms. And it's kind of hard to know, given that all terrestrial
life is water-based, how you would make something. You need a pretty
powerful antifreeze to stop things freezing at 95 Kelvin.

Tillman Or you need to create bacteria that uses methane as the solvent.

John Right, so that would be pretty radical genetic engineering.

Tillman Right.

Trina Interestingly, NASA has an entire branch called "Planetary
Protection" whose whole job it is to make sure we don't get bugs from
the Earth out to places like Iapetus and Titan.

Tillman It's probably a big question why we should do something like
that.

John Yes, I think Titan is pretty fascinating the way it is and we
wouldn't want to mess it up. Of course, that hasn't stopped us in other
places, but that's another subject.

Anybody else out there?

Amanda Okay, well maybe we can move on to Tillman then.

Tillman Okay, and in case you still have questions to John, that's no
problem, after my talk.

Amanda We can just open it up at the end again.

Tillman Okay, my talk is about the imaging science of Iapetus and it
will also cover what was found from Voyager data and also we'll go
through the question of the dark side problem that is not solved until
today.

On the second page of my presentation there is the content here, how I
structured my talk. It has this introduction and then the description of
hemispheric brightness dichotomy and then what our planning efforts were
when Cassini arrived at Saturn. This means what we got from a far
distance, what kind of data and how our knowledge was rising over the
last half year. And then we had the New Year's Eve flyby, where we were
quite close to Iapetus. This flyby brought a new mystery instead of
solving the old one. Then this will be the end of the talk.

On the third page, this is the introduction to myself because usually
when I give talks people can see me and we can interact directly, so
it's a little bit strange today, so I have no idea how people are
yawning or doing something else or some might even listen.

> My name is Tillman Denk and I was born in '65. I studied aerospace
> technology at the University of Stuttgart and now I am an employee of
> the Freie University Tech in Berlin at Umlaut. The University Tech,
> you will see that this is in Germany, this is also the reason why I
> might sometimes not find the right words that I want to say; I might
> say it in German, but this might be a problem for you maybe.

I am working with the imaging team through my boss, this is Gerhard
Neukum, maybe one or the other of you have heard about him. He is doing
cratering in particular, and he is a member of the imaging team and I am
working for him. My main task is doing photographing of icy satellites,
which means that I am doing the planning how and when and any details of
the imaging of, in particular Dione, Rhea, Iapetus and Phoebe for the
imaging team.

In the picture to the right, I am the person to the lower-left and I
have selected this one because it shows a little bit of an analogy how I
have control over this computer parts that are finally going to the
spacecraft and doing the imaging.

Number four, the object that I'm speaking about, John gave already a
little bit of an introduction. It's a little bit more detailed here, I
think. It's the third largest of the Saturnian satellites and it was
discovered by a person that was named the same as our spacecraft is
named, in 1671, that's 334 years ago. I give two years here because
Cassini had big problems to determine the orbit and it was more than a
year later that he eventually made it to bound orbit parameters.

The diameter is about 1,440 kilometers; it's hard to say exactly this
number because Iapetus is not close to a sphere, it's not close to a
tri-axial ellipsoid, it's something else that has no name and so 1,440
might be a good value, it will change a little bit in the future with
more Cassini data.

The mean density is very low. This is important, it's close to water ice
and the orbit is far away, we already heard this. The centricity is
modest, not zero, but not too high, about 3%. The inclination is
interesting, it's 7.5° against the local Laplacian plane. This means
that the local Laplacian plane is 14° or 15° against the Saturn equator.
This is because we are not close to Saturn anymore but not still so far
away that the local Laplacian plane is in the plane of Saturn's orbit
around the sun. Saturn is tilted by 26° against its orbit, so the
15° are somewhere in between.

An interesting feature of Iapetus is that it has a precession motion off
its orbit plane and this is about 3,000 years, so we have sometimes a
tilt against Saturn only of 8° and sometimes about 20°.

In the solar system it is a quite large body, without the trans-Neptune
orbits it is number 20 in size, so you have millions of bodies, so it's
something exclusive. The general assessment of Iapetus is it's a very
unusual and very interesting object.

The main reason for this peculiarity of Iapetus is its hemispheric
brightness dichotomy. It was discovered by Cassini immediately when he
discovered Iapetus because he could see it only on one side of Saturn
but not on the other side, and he made a correct interpretation that one
side of Iapetus must be very dark and the other one must be very bright.
The dark leading side, indeed, was found to be as dark as a freshly
tarred street and the trailing side is as bright as fresh snow, or
almost, not exactly, but almost that bright. This difference has been
known since 333 years now and it is unexplained for the same amount of
time.

Every serious explanation attempt since the early 1970s, I will go into
this in detail. But none of this came up to be widely accepted so far;
the only thing that is widely accepted is that none of this is widely
accepted. So it is one of the oldest unresolved riddles of planetology
and maybe it's even the oldest, at least I am not aware of any other
that was not solved in the meantime, like the rings of Saturn or the
great red spot or things like that, they were all solved.

The graphs in this image are by Voyager I and II almost 25 years ago and
the dark hemisphere got the proper name, it was named Cassini Regio, so
when I refer in this talk about Cassini Regio, it is always the dark
side that I mean.

On page six we see a few of the Voyager pictures and there we see the
character of Iapetus a little bit, the dark hemisphere on one side and
the bright hemisphere on the other. On the bright hemisphere we can see
craters and when we look more closely, also some scarps. For example,
the second from top there, you see in the middle running diagonal from
the 10 o'clock to 4 o'clock direction, a scarp that seems also to be a
boundary in the brightness, so maybe large parts of the surface are
tilted a little bit there. This is an indication of the irregular shape.
When you look at a picture of the 21 kilometers, then you see at the 12
o'clock position there is a little bit of a balance, so it's another
indication of the irregular shape.

Number seven shows a sketch how this dichotomy is located on the
surface. To the left are Voyager pictures that are ... projected and to
the right is a sketch drawing. There is a ..., that is the leading side,
this is labeled eastern elongation, has the dark terrain and the post
upright, while on the trailing side, which is labeled western
elongation, it's almost bright completely, but there are a few areas
near the equator where the dark Cassini Regio are reaching into the
trailing hemisphere.

It looks a little bit like an astronaut's helmet and when you see the
original pictures in the inset then you can imagine there the circular
feature looks like the inch of the helmet and some people also named it
"the Moat" because it has this circular structure.

So number eight, now the big question since long ago is why is there
this brightness dichotomy. The first idea that was brought up almost 30
years ago is that Cassini Regio is covered by dust that was originating
by Phoebe. The idea is not bad because Phoebe is orbiting retrograde
around Saturn and Iapetus is prograde and when Phoebe loses material
from micrometeoroid impacts then the material would spiral in the
direction of Saturn in a retrograde orbit and due to dissipative forces
it would come closer and closer to Saturn and the first uptaker would be
Iapetus. It would have collisions head-on, therefore explaining the
orientation of the dark hemisphere. However, a big problem is that
Cassini Regio is red while Phoebe is gray, so the colors are different.

> How can we solve this problem? A while ago it was proposed that other
> outer of Saturnian satellites might be the culprits and not Phoebe
> itself, and indeed in 2000 there were several additional companions of
> Phoebe found which are much smaller and also in retrograde orbits. If
> a satellite is smaller then it is even more easy to knock off some
> dust material. It was also found that at least the largest of them is
> also reddish and might fit the color of Iapetus' dark side.
>
> However, this makes the problem more severe because then we have to
> answer the question why Phoebe is gray and not red if this other
> satellite is responsible for the reddening of Iapetus. Why should
> Phoebe not cover parts of the reddish material? So this is why the
> Phoebe dust idea might not work. There are also other general problems
> like dust particle orbit dynamics and the theorists do calculations
> then. They can find everything they want, but in the case of Phoebe it
> is quite difficult to explain why the poles are bright on Iapetus and
> why only the equatorial and mid-latitudes are covered by the dark
> material.
>
> The second general problem is the complex transition zone on Iapetus,
> like the feature that I called "the Moat" feature. This would require
> some kind of mobility on the surface of the dark material because if
> it's coming head-on on Iapetus it cannot move on the backside behind
> the horizon. If Phoebe dust is coming in, the dark material might be
> no more than a millimeter or so thick, so it's quite hot, not to
> mention how a millimeter thick coating might move over 100 kilometers
> on the surface of Iapetus. There are also dark floor craters on the
> surface of Iapetus on the trailing side, which are a problem of this
> theory also.
>
> Then if you go to the next viewgraph, number nine, another idea that
> came out with Voyager images is that the endogenic origin of the dark
> material might be possible. This idea came up because the transition
> zone was detected to be very complex; the transition zone between the
> dark and the bright material and also because of the dark floor
> craters found on the trailing side. So people thought maybe there is
> some dark material just coming out of the interior of Iapetus.
>
> However, this has severe problems also. We have low mean density of
> Iapetus of about 1.1 grams per centimeters, that's per cubic
> centimeters. If the dark material is something organic then it should
> be rather two grams, so there must be some small layer only in the
> interior of Iapetus and then below this layer there should be the ice
> again and this sounds quite improbable.
>
> Then there's the often difficult question of why is there a precise
> alignment to the apex of motion to the dark material. This is not
> explained at all by any endogenic activity. The energy source for the
> volcanism is also not known and there's also a generally old
> appearance of the surface. So there was a planetologist agreement
> quite soon that the idea for the endogenic origin is a nice one, but
> it's probably a wrong one. But with the Cassini data there was a
> revival of this; I will come to this at the end of my talk.
>
> Okay, next viewgraph; the idea that Phoebe is influencing the surface
> but not that its dust is coming and just landing on the surface, but
> that it's altering the leading side of Phoebe in a chemical or a
> physical way. This idea was proposed by various authors in the 80s and
> the rational is to avoid the color problem, but explain the alignment
> to the apex of motion of the dark Cassini Regio. The obvious mechanic
> problems are the same as for the Phoebe dust theory. No -- I'm sorry,
> what I want to say is the orbital mechanics is one aspect of the
> problem and there were much less Phoebe dust proposed, but also
> interplanetary micrometeoroids that might cause the same head-on; some
> people calculated that it might be possible. Maybe also not Phoebe,
> but some kind of circum Saturn ... source.
>
> On the surfaces there were various flavors of processes proposed. The
> oldest one was just an erosion of the bright material that exposes a
> dark sub-surface layer. But this was later thought to be quite
> improbable, therefore a better idea was from Cruikshank in 1983 that
> the bright material and the dark material were intimately mixed and
> the bright material was happily mated by the dust coming in and this
> enriches the dark material and remaining as the dark surface layer
> that we see today.
>
> Another idea proposed by Squyres and Sagan also in 1983 was that the
> dark material was synthesized by when the bright material was eroded
> that some methane in the surface was synthesized to polymers and had
> then causing the dark surface.
>
> All these ideas have the same problems, like the Phoebe dust series,
> like the complex transition shown and the mobility of the materials
> even harder to explain and also the dark floor craters on the trailing
> side and the dust particle orbit dynamics.
>
> Into that is also another question, why is the dichotomy so pronounced
> on Iapetus? Why is it only on Iapetus? If these processes would work
> they should also apply to other satellites of Jupiter and of Uranus
> for instance.
>
> There are still another kind of idea; this is viewgraph number 11.
> This is the catastrophic collisions and this is also proposed by
> various authors. They have proposed catastrophic collisions with
> Iapetus, with Hyperion, with a former altered satellite and even with
> Titan. So we really have a slew of ideas of what might have been in
> the dark hemisphere.
>
> The idea of the Hyperion was proposed by Matthews in 1992 and the idea
> that speaks for this was that Iapetus has the same unusual color as
> Hyperion, so they have big similarities in color and this might be
> linked to the same event somehow. Then if there is a body crashing
> into Hyperion and then the debris gradually reaches Iapetus, the idea
> is that Hyperion was originally a regular spherical body and now its
> irregular shape would be tracked down to the same event as the one
> that caused the dark Cassini Regio.
>
> However, against this is speaking that the orbit dynamic was very
> weird, I would expect that it is rather somewhere on the Saturn facing
> side or so whatever. So it's very difficult to argue for this from a
> mathematical point of view. Another point is that Hyperion is much
> brighter than Iapetus and if Hyperion is the place where the material
> originated or the comet or whatever, then I would expect that more of
> it will be covered on Hyperion and that Iapetus would be lighter in
> color.
>
> Another idea was that Titan was the cause and this came from Toby Owen
> in 2001. He proposed this because he found some potential
> compositional similarity of Cassini Regio and the ancient Titan
> atmosphere constituents: the aerosols. Of course, he didn't argue how
> the material would reach Iapetus, so the orbit mechanics would be very
> weird for this idea also.
>
> The idea that the catastrophic collision that a former outer, unlucky
> satellite of Saturn was hit by a comet; this was proposed in 2000 by
> myself and my boss, Gerhard Neukum. Why I was for this is because it
> can avoid the problems of the Phoebe dust hypotheses and it might also
> explain the uniqueness of Iapetus, that it's only this body simply
> because it's an improbable event that happened. Maybe the orbit
> procession around the Laplacian plane, the inclination of the orbit of
> Iapetus might help to explain why at the equator it is reaching the
> trailing side, that the dark material is reaching the trailing side
> there. It also explains why Hyperion has a lighter but also a
> reddish-dark color and it might come from the same event.
>
> Against this hypothesis it's probably a very low probability of this
> because it needs quite a strong impact. A satellite might be 50 to 100
> kilometers in size and the impactor also that big or at least 10 to 20
> kilometers and the probability that something like that happened very
> recently appears to be very, very low.
>
> Then there was also no detailed calculation on the orbit mechanics, if
> this is possible at all. And like all the theories, it has also the
> problem that the dark floor craters on the trailing side are not
> explained by this theory.
>
> Then there is still another one from Tabak and Young from 1989 that
> Iapetus itself was hit by a comment and then a lot of material was
> kicked out and water is taken into space or was destroyed by the UV
> radiation of the sun and the dark material was coming back on Iapetus
> and then preferentially falling on the leading side.
>
> This is an interesting idea and the mathematics might be a problem.
> Some people argue that it might work; others argue that it doesn't
> work. Of course, the dark floor craters are also a problem and a
> problem at this time was that there was no potential crater detected,
> but the Voyager pictures might not have been sufficiently resolved or
> the signal-to-noise might not be sufficient to show this.
>
> They, by the way, proposed that it should be on the leading side, but
> they said it might also be a big impact on the trailing side, so it
> must not necessarily be on the dark side because at the time of the
> impact the whole Iapetus would have been bright and just the debris
> falling back would make it dark then.
>
> So the next viewgraph, number 12. This shows a summary of what I just
> said, but I will not go through this now, so you can use this as an
> extra reference if you have some questions later or if you want to
> read it again after my talk.
>
> Thirteen, "May Cassini Help?" We are optimistic, of course, and now my
> talk goes a little bit away from the origin question to the
> description of what we were doing in the last half year in terms of
> Iapetus observations with the camera. So much other stuff was detected
> that the origin question sometimes goes a little bit out of the focus,
> but I will come back to it at the end of the talk.
>
> Next one; we have Cassini observations, the first observations we made
> -- the first scientific observations, not for optical navigation, were
> made in June, July and August. There we had resolutions of up to 15
> kilometers per pixel. The next viewgraph, 15, shows the plan that we
> had, so we are coming in on July 1^st^ and then we had two
> observations on July 2^nd^ and 3^rd^. This is on the top row on the
> right and then in the middle row; observations after the solar
> conjunction period that were just in early July. We were departing
> from Iapetus with resolutions of 15 kilometers per pixel and then
> going away on the face and also increased more and more. We had
> observations every three days and then the final one was in early
> August.
>
> The viewgraph, number 16, shows this very first image just after orbit
> insertion and there was an immediate interesting discovery; we found a
> giant impact in this observation. You see this just above the center
> of the image; it's a very big one with a 550 kilometers diameter and
> just the other observations after the solar conjunction, we found two
> more giant basins. So from Voyager we had zero and now in 1.5 months
> we had three of the size of 400 or more kilometers.
>
> Then these pictures also have shown craters on the southern
> hemisphere. This was not possible by Voyager because Voyager didn't
> see the southern hemisphere, so this was expected, but this was a nice
> confirmation and we extended the known area on Iapetus surface a lot.
>
> And we also detected that there are craters within the dark terrain.
> There's something -- there were only hints for this in Voyager data,
> but now, with the Cassini data, we're clearly confirming this one.
>
> The impact structures are shown in viewgraph number 17. The other two,
> there's a second one within the dark terrain, and it's in the upper
> row of the picture and the lower row of the picture shows very low
> resolution images that show the trailing side, and I did not really
> expect that we will find something interesting in these lower
> resolution pictures, but indeed there's this very big creator that
> slowly rotates into the morning field of view for Cassini.
>
> Viewgraph 18, in October then we had the observation of the leading
> side and the anti-Saturn side at up to 6.7 kilometers per pixel and
> this was the best ever resolution on Iapetus. Voyager, as you
> remember, had 8.5 kilometers, and the Cassini cameras also have better
> quality. And we also made attempts to observe the part of Iapetus that
> is not illuminated by sunlight, but by Saturn shine. (Technical
> interference.)

Thank you. We made this because this was an imaging attempt of an area
that was not very well seen by Voyager and that will also not be very
well seen by Cassini, but during the New Year\'s Eve flyby it might also
have imaged it in the Saturn shine. In Germany, we say gray light of the
planet. This was a test attempt to check the proper exposure times for
the December flyby.

Okay, 19 shows how the pictures of \... is looking. There are lower
solution images and then higher resolution images, and also at very high
phase seen to the right, and Saturn shine observations are shown at the
bottom right. You can see to the left, the moat feature that was
illuminated by Saturn and then the 270 degrees meridian. This is the
boundary where the Saturn light is not reaching the surface any more.
There was a little bit of absolutely dark and then to the very right
then, the part that was illuminated by the Sun itself.

Okay, number 20. This is one of the best images that we got there. There
were also several new discoveries in this one. For example, a peculiar
equatorial wispy streak, that\'s in Cassini Regio. This is the one that
is marked by the red arrow. I think you can see it on your screens, this
very thin line. There are these bright dots to the left, and then it\'s
just a continuation inside the dark area, slightly curved to the limb
and disappearing somewhere near a depression in the hill. This was one
discovery, but we have no explanation for this for this so far in
October.

Then, when you look at the blue arrow, this is another specific feature
that shows which one is the 180 degrees meridian. Then, when you go to
the south, meaning to the lower left, then you can see that all the
areas below this meridian have a somewhat brownish appearance when
you\'re very close to the south pole, but those to the west of it,
meaning up to the upper left of it, these are much more whitish. So this
meridian might form a boundary for the color, which is very suggestive
that some material came from outer space on the surface, hitting Iapetus
just head-on from the direction of its motion. This was something that
was always suspected from Voyager data, but with the Cassini data we
could clearly see it, and this is something very interesting that still
deserves our focus of science research.

Then, the high whitish mountains were confirmed when you go to the next
viewgraph. There is a comparison of Cassini and Voyager pictures of this
area. When you look at the Voyager picture to the right, then you can
see that the yellow and the blue arrows shown. There are very
interesting pictures that we described in the late 90\'s for the first
time. These mountains might be up to 20 kilometers high, and this is
something very strange. We don\'t know something like this on any other
icy satellite or body. The Cassini pictures now have the less favorable
viewing direction, but the one to the right at high phase, you could
also see at least the last one, the one which was labeled with the
yellow arrow in the Voyager picture. This one could be measured to be
about ten kilometers high, so the Cassini pictures also confirmed this
strange surface feature pattern.

Somebody was telling about the Arthur Clark novel, the *2001: Odyssey in
Space.* Arther Clark claims that in the center of the bright terrain
there is a dark erected feature in the surface. So we are not on the
center of the trailing side, not on the center of the bright terrain,
but at least almost of the center of the anti-Saturn side, and we also
have not dark, but bright, erected bodies in not one, but even a few of
them. So there\'s some kind of analogy there, at least if you have some
kind of humor.

The next picture, number 22, shows it even a little bit better, so it\'s
just an enlargement of the Voyager 2 image, with the mountains and of
the Cassini picture with the mountains from a different viewing
perspective. These mountains have a peculiarity that I will come to in a
few minutes. Okay, the next picture, 23, shows the Saturn-shine
imagining attempt. The really surprising thing to me was that it worked.
It even worked with the full-resolution mode of the camera. We had three
minutes of observation. Three minutes the shutter opened, and we don\'t
see any smear on Iapetus. But we could see the stars as streaks, so this
means that the Cassini spacecraft is a great tool for doing long
exposure imagining with very minor jitter and the tracking on the
surface is also almost perfect.

This was the best ever observation of the moat feature so far. There are
other examples from Voyager and the Cassini observations to the left in
sunlight, where we see the moat feature also. You can see that you see a
lot more detail there in the Saturn shine. Also, the illumination by
Saturn is a little bit more difficult than sunlight because Saturn is
not a point source on the sky of Iapetus, so we don't have the very
sharp shadows. In this case, it\'s also relatively close to the zenith
of the sun, and so we have high sun conditions and casting off from the
full moon. This is something where quite a lot of our morphology is very
difficult to see.

Okay, the next viewgraph, number 24. Then we came close to Christmas,
and the Huygens probe was released, and we had three days of optical
navigation imaging to look for the probe. There was also an imagine of
Iapetus done. The first two were done for optical navigation image
purposes, and the final one there was no star in the field of view
anymore, but the person who is doing the optical navigation images at
JPL is a very friendly person, and so he just smuggled in another
Iapetus picture that was a very nice one. Number 25 is the plan that we
had. It\'s not exactly what we got because the trajectory of Cassini was
changed very late in the process in October. So they\'re a little bit
closer to the equator and were doing a flyby then just over the north
polar area instead of the southern hemisphere as was originally planned.

Number 26 shows some discoveries from December 2004. A major one was
from images that were acquired in mid-December. These are shown at the
bottom left and center, and they show for the first time, very clearly
the moat feature on the terminator, near the evening terminator. It is
very obvious here that this is a big crater. It\'s the size of about 250
kilometers. This is a little bit smaller than the albedo, than the dark
albedo pattern, but the suspicion that there is a big crater is now
clearly confirmed and we know this from these images.

Then, another discovery was a fourth large basin, about 400 kilometers
in diameter. That is shown on the upper right. There were hints in
Voyager I data for this one, but these were never published because
everyone would have laughed if we would have shown it. But it was indeed
there, and so we have now three large basins in the dark terrain, and
one large basin in the bright terrain, just found within about half a
year.

The biggest surprise is shown in the next viewgraph. This is the
stunning rich -- somebody called it the bellyband because it really
looks like if Iapetus would be a Cuban cigar or something like that.
It\'s some kind of a noble satellite. The bellyband is about 2,000
kilometers long. It starts somewhere near 50 degrees western longitude;
this is just left to this very big basin. Then it runs across the dark
terrain and it\'s ending at least near these bright Voyager mountains.
This is somewhere near 250 degrees west. So it goes over almost half of
the planet, and maybe it goes even further; this is something we do not
know so far because we don\'t have sufficient imaging information.

Really stunning is that it is exactly aligned along the equator, so if
you need a natural landmark on the satellite spin for Iapetus, it will
help you to know exactly where the equator is. It is up to 20 kilometers
high. This is also intriguing; you don\'t have that high of mountains on
the satellites. You might have it within craters, with impact craters,
the walls might be that high, but not at any ridges or mountains like
this. It depends on the reference radius that you select. If you select
the 718 kilometers that are currently adopted by the International
Astronomical Union, then this mountain would even rise 28 kilometers
above the mean radius of Iapetus. When you look exactly at the right
arrow, there you see this little peak on the horizon. But then there is
also a little bit in elevation in to the north and to the south of it
before it reaches the circular structure of Iapetus. So there might be a
larger bulge where the narrow peak is residing on it, but this is
something that still needs more research to do.

In the next viewgraph, there is the Iapetus picture again. When you look
at the blue arrows, you might have proposed this from the Voyager
pictures already, the one with the 12 kilometers per pixel, there is
this very tiny, bright line that is a continuation of these bright
mountains. These are seen in the pictures, but nobody got the idea that
these might be just the continuation of a big ridge. In the nine
kilometer picture, the mountain show an elevation, but from this
viewpoint, it looks likes there\'s just a big bulge somewhere in the
dark terrain, but this is from a quite narrow ridge. Nobody would have
believed it if you had told it.

Okay, 29. From Cassini, we had extensive observation on New Year\'s Eve,
2004-2005. It is the second closest approach to Iapetus of the whole
nominal mission. We were at an altitude of 123,000 kilometers, and this
resulted in pixel resolutions of 740 meters. This event was a gift from
the Tour Gods if you believe in them, and the reason for this is because
it was not in the original tour that was planned, but then there was the
detection of the Huygens receiver problem in early 2000, and this forced
the responsible people to re-plan the first few orbits of Cassini so
that a distant flyby of Cassini could be done for the Huygens mission.
This re-design of the tour led to this quite close flyby of Iapetus.
Until October, it was even closer -- as close as 55,000 kilometers, but
then another problem with the Huygens made people cautious, and the
decision was made to shift it out a little bit because it was not quite
clear if Iapetus could disturb the path of the probe too much so that
proper entry into the atmosphere would be in danger.

The imaging system was doing eight mosaics, and these were from a
moderate 50-degree phase angle to 140 degrees phase angle, and the
resolution was quite similar over the whole one and a half days of
observations. It started at one kilometer and went down to 740 meters,
and then back to 1.1 kilometer per pixel.

Then you go to number 30, you see an example of one of the eight
mosaics. This is just to show you some of the details that we were
deciding to use for this observation. The same is also the next one on
31. This is the mosaic with the highest resolution. This is a 3x2
mosaic, so the green squares show the field of view of the camera, which
has about 1,000 pixels in each direction. The somewhat thicker frame to
the left, this shows an observation of the moat feature in Saturn-shine.
The blue dots indicate area that is not illuminated by the Sun, so this
was one of the pictures that we tried to do in getting mapological and
surface information from an area that was not lit by the Sun at all.
More attempts were also made in the seventh mosaic. We made five
different footprints for this area and this planned at a time where it
was not clear how accurate the pointing of the spacecraft would be, but
then we found out that it was very accurate at the end.

Number 33 shows the orbit of Cassini. The blue arrows show activities of
the Huygens probe, and the red arrow shows the flyby of Iapetus, and at
this scale you can really see that it was very fortunate that we got so
close to Iapetus when Cassini was there on New Year\'s Eve. Iapetus just
passed by, it was a little bit faster than the spacecraft and Cassini
used this opportunity to observe.

The other fortunate thing was that activities for getting rid of Huygens
were all done before Christmas and right after Christmas, and then the
probe mission itself was 14 days after New Year\'s Eve, so the Iapetus
flyby was really at a time -- maybe it were the only two, three, or four
days of the whole mission within the Huygens\' activities where
observation of something else than something is related to the Titan -
Huygens mission was possible. So we were really lucky with that.

Okay, in the pictures, viewgraph 34 shows the very first of these
observations. Again, we can very nicely see the bellyband and the big
crater that was detected in the December OPNAV (Optical Navigation)
observations. To the right, we see this big 550-kilometer basin. Then on
top, there\'s a transition zone. Unfortunately, we are not able to see
the pole itself because the pole was not in sunlight at this time; it
was the South Pole that was illuminated by the Sun. Then to the left, we
can see the shape of the ridge and that it is composed of different
mountains of different height.

When you go to the next picture, this is a close up of the bellyband
from two different views from the first request of our eight mosaics,
and from the second mosaic. I think you can see very nicely how steep
the mountains are, so it\'s up to 20% or even 30% in steepness in some
cases. You can also see that these are cratered very much. This is
indicative that the ridges are indeed a very old structure on the
surface of Iapetus. I wonder sometimes why it was not destroyed
completely by any bigger impacts.

Into the lower left, you might also see some double or even triple
ridges. It\'s not very clear if these are really triple ridges, but it
looks like that. You can also see that the ridges are a little bit
brighter than the flanks. This is probably an indication that the dark
material is less thick in these parts than at the flanks. Maybe because
there is some steep peak where the material cannot stick or whatever,
but it\'s still unclear at the moment.

Then 36 shows the central part of Cassini Regio. It is centered at 90
degrees western longitude on the equator. The ridge goes somewhere
through the picture. It\'s very hard to see here because we are looking
straight on it. From this picture, it\'s very clear that the dark area
on Iapetus, the Cassini Regio, is also heavily cratered. This means that
if the dark layer is a quite young phenomenon, then it must be
relatively thin because you can see the crater through it. If it\'s an
old phenomenon, then it must be so thick that the craters did not punch
through, and you cannot see any crater that has a bright floor within
the dark terrain. I think it\'s rather relatively young because the
bright rims are indicative that the dark material layer is not that
thick.

The next slide, 37, shows another interesting feature. This is what we
call the landslide crater. It is at the edge of the big 550-kilometer
spacing, and it shows a big landslide that is partially inside the
crater, partially outside. What is particularity interesting in this
crater, you can see this in the image to the lower right very nicely,
that there is no small crater inside. It appears to have a very flat
ground, and almost no crater at all. Even the landslide has very few
craters; maybe they are two or three inside, so this is indicative that
the landslide is even a little bit older than the flattening of this
crater. This is something mysterious because Iapetus has so many
craters, and it is believed to be very cold. The question is why is
there a place on the surface where maybe something warm from below was
melting the surface or weakening it in a way that all the topography was
lost there? That\'s another interesting thing we found in the New
Year\'s Eve data.

Then the next one shows the color picture; one of a few color pictures.
What is really interesting in this picture is that you can actually
discriminate between the dark material in the craters and the shadows.
When you look at the upper part of the picture, in the upper right part,
then you can see the shadows that are very dark as expected, and
standard dark material, which is brownish. There were suspicions in
Voyager\'s data and confirmation in Cassini data, low resolution data,
but here, you can really clearly see that the walls of the crater that
are facing to the southern direction are covered by the brownish
material, while walls that are facing towards the pole, especially
within the dark terrain are not covered by this dark material, but are
bright instead.

You can also see that near the boundary, there are some dark shadows
visible. Just immediately northwest of the shadows, there is also the
material -- much brighter -- then it gets a little bit brownisher and
darker again. This might be indicative that there\'s a big slope there
or something, like a wall, a mountain, that is protecting the area; just
part of it by being hit by that material. Then there\'s also this
impression of streaks. This might also be caused by local topography if
the material was coming from outer space ballistically somehow, but
there were some hills or hilly terrain that were protecting parts while
others were hit in an easier way because they were close to depressions.

The next one, 39, shows close up the bright crater walls. It\'s very
interesting that these are all facing to the north in direction, so away
from the center of the dark terrain. We are not able to see if there any
layering between the surface and the bright walls. Just to the upper
right is this crater that seems to have some dark shadows on top of the
bright rim of the crater, but this is probably shadow and not dark
terrain.

From these pictures, which have a resolution of one kilometer, we
believe that the thickness of the dark layer at this part of Iapetus is
probably no more than about 500 meters. It might be even less, but we
cannot say with these pictures because the resolution is not good
enough. This is about 30 degrees northern latitude, so it might be
thicker of course near the equator, but this is the area where we have a
little bit of insight into the bright terrain that is located within
dark terrain. The further you go to the north, you can also see this in
the picture, the more bright terrain becomes obvious, so there\'s a
gradual reduction of amount of dark terrain the further you go to the
north.

Next frame, number 40, shows one image that was done in Saturn-shine.
The bright area from the top, this is some stray light that was coming
in because the sunlit area is not very far away from the Saturn lit
area. Here we see the moat in even more detail. I put in the
low-resolution image to the right so you can see a little bit the
location on the surface, how it is related. What is obvious is that the
dark albedo pattern is a very complex one. There is material, certainly
not in a ballistical way, been placed on this part of the surface.

When you are going just above the moat, you can also see a lot of very
small craters that seem to have dark floors, but these are all very
small. When you go to the right, the one that is indicated with the blue
arrow, this seems to be the largest certain dark floor crater on this
hemisphere. This was a little bit of a surprise to me because we had
dark floor craters in Voyager pictures obviously on Saturn-facing
hemisphere. These are more or less absent except of this single one, but
when you go further to the right where the terminator is, if you can use
this word for Saturn-shine, there are more craters that seem to have to
dark floors, but there it\'s not clear if this is shadow or if this is
Saturn -- if this is material. So unfortunately, we did not make color
images. Maybe we could see it in there, but I doubt that the color
imaging would have worked under these illumination conditions.

Okay, next one. This is repetition of the content and where we are now.
What I talked about in the last few minutes and now, let\'s look again
at the problem of the dark material; how it was placed on the surface
and what was the reason for this. See if we learn something for this
from the Cassini pictures that we have now.

First, this was the title of my whole presentation. What we hoped to do
to is to solve the brightness dichotomy problem. This may be the data
now or at this data that we will look at later. But, I have to admit the
brilliant idea for this is still missing. This is next viewgraph, the
42. We did not come out here with the solution of our mystery, but we
came out with a second mystery. So instead of having zero mysteries, now
we have two, and the second one is the bellyband. It is a very unique
and exciting, and unexplained feature. Even as exciting as the dichotomy
because it\'s so unique and we don\'t have it anywhere in the solar
system, at least not observed so far. We have a strange peak on the
Uranian satellite Oberon, but this is probably a strange central peak of
a crater and not a bellyband. So it\'s absolutely unclear to us at the
moment how this one formed. Maybe one of you has an idea of what was
happening there and might help us a little bit?

The brightness dichotomy, how do the theories hold under the new Cassini
data? I talked about the Phoebe dust theories. That Phoebe is covering
the surface or also the theory that Phoebe dust is causing an alteration
of the surface and removing some of the bright material. What is good
for the Phoebe dust theory is that we found that Cassini Regio is
probably a thin, dark layer, so less than a half a kilometer. Okay,
that\'s not a millimeter, but at least there\'s no proof that it\'s a
thick one. There is the possibility that it is indeed thin. Then, we
have the streaks in the transition zone. This is very indicative for a
ballistic placement of the dark material, so this is also good for the
Phoebe theory.

Then we have the lack of the punch through craters. I mean the craters
with a bright floor within the dark terrain. These are absolutely
absent; we have not seen any. This means that the Cassini Regio must be
younger than most of the craters that are located on it, that it\'s just
covering them. This is just a strength of the Phoebe theory, that
Cassini Regio should be a young thing, even ongoing today the process.
So if a new impact is creating a hole in the blanket, then the process
of the Phoebe dust would cover it again, but this is a big question, how
quickly could it cover a new impact? There would be an impact big enough
to be visible for Cassini and its pictures, say every ten million years
or 50 million years, we don\'t know. Then would the process be able to
bury the bright floor of that crater and also the bright ejecta?

This is a big problem of course, and the unsolved question. The
assessment for the Phoebe theory is that it remains valid, but of course
the other problem that I mentioned earlier, especially if there are dark
floor craters on the trailing side and this complex transition boundary
remains also. So the Phoebe theory therefore, is not really a stronger
one now after the Cassini observations.

Then, another point was the endogenic origin. I mentioned that the
Cassini came up with this idea again, when we first saw the pictures
then the idea came up that the big ridge might be the original source
for the dark material. This sounds very weird, and it\'s probably weird,
but the elongated shape of the Cassini Regio is arguing for this, as are
the streaks in the transition zone. So this ballistic emplacement is not
necessarily from outer space, but might also be something that was
coming from the center of the region itself. The big strength of this
would be that we might combine two unique features to one origin.

However, there are several severe arguments against this. The ridge is
probably very old; it is highly crated. Then the ridge is not exactly
symmetric. The Cassini Regio ends somewhere near 50 degrees west, so the
question is why there more of the dark terrain east of this? Then
Iapetus and the rich interior are probably bright, so the icy material
is probably inside, and the dark is only a layer. The question is why
should material erupt and the bright component should disappear very
quickly, while the dark component only would return back to the surface?

Then, we also have the bright mountains on the anti-Saturn side, which
are a problem because we have the dark mountains at Cassini Regio, but
then there are some mountains that are absolutely bright. This is also
not consistent with something of a ridge that is spewing out dark
material. The main obstacle for this of course, is the energy source,
because Iapetus is believed to be very cold and very dead in its
interior. It\'s very unclear why material should come out of the
surface.

Then we have catastrophic collisions with Iapetus itself. The idea that
a big impact on Iapetus excavated material and only the dark component
came back to the surface. I was starting to believe in this idea when we
has the low resolution images because this idea requires a big basin
somewhere on the leading or on the trailing side of Iapetus, with a
diameter of up to 300 kilometers was calculated by the office at this
time. Now, what Cassini can offer \-- not one basin with 300 kilometers,
but four basins with more than 400 kilometers, so there is plenty of
impact material available to support this theory.

However, now with the New Year\'s Eve data, I\'m not leaning to this
idea anymore because in this case, Cassini Regio should be an old
structure, and we don\'t see any bright floor craters again. And the
appearance of Cassini Regio is not that thick that it can be considered
as relatively old compared to the craters. So it appears unlikely that
the craters formed after the dark blanket was in place. But, the big
basins are certainly older than many of the craters and therefore, I
think that this theory has now a bigger problem than it had before with
the new data.

Then, the question is the theory that came from me and my boss, with the
collision of a former outer Saturnian satellite with a comet. I think
the BC data that -- I have this data from the New Year\'s Eve flyby --
supporting in the way that the streaks and the transition zone are
supportive and that there are these dark crater walls in the transition
zone that are facing to the center of the leading side of the Iapetus.
All the arguments are quite similar to the Phoebe dust arguments.

However, the things that are speaking against this theory were not
weakened by the new data. The big question of the low probability of
such an event. It must really be a recent event because there is no
bright floor filling mechanism any more. The event is once and the break
out would cover Iapetus after maybe 100,000 years or whatever, and then
it\'s there. If a new crater is coming, then there must be a hole in the
dark blanket. This is not observed, so it should be a quite recent
event, and the probability for this is very low. Then of course, the
dark floor crater on the trailing side are not explained as well by this
theory.

So what can we do now is the big question? The next thing that we will
do is just homework. This means that we have to look into the data very
carefully. Meaning that we have to map the streaks in the transition
zone, the orientations for them, the mapping of the orientation of the
bright crater walls. How they effect the lead side and windfall side for
incoming material or if you can see there is some particular angle where
the material if it is coming in will not cover the surface anymore,
things like that. Then you have to map the color in detail. If there are
latitude effects and the longitude effects that I already mentioned with
the boundary between the leading and the trailing side.

Then how the streak colors are fitting. If they are the same anywhere,
and if there is any color heterogeneity within the Cassini Regio. From
just looking at the pictures, I think there are small hints that we have
indeed color differences, and this will be interesting to map them out
and see if they are correlated somewhat with the leading and the
trailing side, for instance. Then a very important thing is doing crater
counts. The crater counts are important to determine the relative ages
of the surface. So we look for relative ages of the big basins for
instance, compared to the overall dark terrain, and then see if the
bright terrain has a similar crater age at the dark terrain or if it\'s
very different. Then looking maybe you can count craters on the ridge
only, and see if there the ridge itself is a younger or older
phenomenon. Then looking for the landslide crater and things like that,
so it has a lot of work to do.

Also, mapping of dark floor craters on the trailing side will be an
interesting task to see if there is some pattern in the distribution of
these craters that are a main obstacle to all the endogenic theories for
the origin of the dark terrain. Then also an important part of the work
will be stereo work, meaning that we are creating three-dimensional
models of the surface. This will allow a better discrimination of the
ridge morphology, and then we want to see if the dark and bright
material pattern is correlated to the topography -- to hilly terrain as
I suspect it when describing these pictures.

That is the question, if there is large area topography, meaning that
hundreds of kilometers are elevated compared to other areas. The general
shape of Iapetus. Then a big question, that is the end result is also if
there some kind of the ridge in the bright terrain also? It is not
obvious in the Saturn shine images, but when we are doing stereo, I hope
that we can determine areas where it should be at a crater, and that we
can determine if there is indeed an elevation also. And so on; there are
a lot of things, and we have done this homework, then I hope we can go
back to a question and think about how all fits to the origin theories
and maybe gets the brilliant idea then.

Okay, I will end my presentation with viewgraphs 43 to 45. We have
another great opportunity to see Iapetus; this will be in September 2007
on September 10^th^. There we will have a flyby at about 1,000 to 1,500
kilometers above the surface. This will lead to about 12 meters per
pixel image resolution. This is very, very much bigger than we have at
the moment, so it\'s about 50 or so times better. Here, we will fly over
the trailing side and to some of our best images. You can see this in
viewgraph number 44. We\'ll be over these bright mountains that we
observed first in Voyager\'s time, as indicated by the blue arrow.

Green frames again, are showing the size of the narrow angle camera
field of view. So, you see that if it just really gets plain running. If
it will work, then we\'ll not only get great data and a lot of detailed
information about this very heterogeneous part of the surface of
Iapetus, but it will certainly also be a spectacular image if it\'s put
together to a big poster. Maybe it will be one of the biggest posters
that we will ever get in the Cassini mission.

Okay, I will conclude with a proposal that you can look for more images
at the Cassini imaging team homepage. It\'s
[www.ciclops.org](http://www.ciclops.org/). This is where most of the
pictures are put in a regular daily pattern. Then the things that I
said, a little bit more condensed, are also written down in to LPSC
abstracts, and these will be available in early February at the URL
given at the bottom of this viewgraph. If you want to re-read a little
bit of what I have just said today, then you might find this one as a
source. Okay, I will quit here and thank you for your attention

Trina Tillman, I guess I had a question. I can\'t think of any other
body on the solar system that has so many large, flat basins, other than
maybe the moon or something. Can you think of one or is it just me, that
I can\'t think of one?

Tillman Yes, that\'s right, at least in the Saturn system. There is
Tethys with the big Odysseus basin, which is 400 to 450 kilometers.
Then, we have on Rhea the Tirawa basin, and maybe there\'s another one.
The floor on Iapetus, yes, that\'s really a good thing.

Trina It\'s very unusual.

Tillman Yes, and you\'re right on the moon. There are lots of craters,
that\'s right, but I have to think about this, I cannot say now at the
moment. It\'s very unusual for the Saturnian system, and this is also
indicative for me that it\'s really an old thing. If you assume that big
basins are very old structures, but many craters inside them are
indicative of this.

Pete Tillman, this is Pete Goldie from San Francisco. I have several
questions. The bright edge of the crater walls relative to the distance
from the albedo change, is that orientated from the equator? Did you
have enough images to see whether there was any relation between the
bright walls and the equator or just the bright walls and the edge of
the albedo change?

Tillman These are facing away from the equator, that is right. If you
are referring to viewgraph number 38, I think, this is the first one
with color image, then the equator is somewhere to the left of the
image. Just outside the lower left edge of the image and it\'s running
approximately parallel to the boundary between the dark and the bright
terrain. So this means that the bright crater walls are indeed facing
towards the northern hemisphere -- the North Pole -- and away from the
equator. Is this what you wanted to know?

Pete Yes. Is that true south of the equator? Were you able to tell that?

Tillman The New Year's Eve flyby did not observe the area south of the
equator, but from low resolution images that are a little bit further to
the south where you can really see the craters that are in the dark
terrain, also it\'s possible near the edge. There it\'s also very
obvious in the same way. Let\'s say the bright faces, yes, these are
obvious somewhat. What\'s even more obvious is that the dark equator was
facing within the dark terrain; this was also very obvious.

Pete Yet the bellyband appears too extremely old, and the albedo seems
to be very young.

Tillman Right, that\'s our current picture.

Pete My last question is about when tidal lock occurred on that planet?
Can you tell by any of the crater counts or anything, when the planet
became synchronous orbit for rotation?

Tillman That is difficult because one fashion is from theoretical
consideration and I heard many years ago from a colleague, Douglas
Hamilton is his name, he made calculations where he said it needed about
one billion years for Iapetus to come into a synchronous rotation. But
I'm not sure if this is really the reality. From the crater counts that
our group made on all the outer planet satellites so far, we cannot see
any difference between the leading and the trailing hemisphere. This
means that either the satellites were locked into a synchronous rotation
very early, before most of the craters were created, or what appears
more probably to ask that the objects that were creating the craters
were captured into a Saturnian orbit before they were eventually falling
onto the surfaces of the satellite.

Pete None of the very largest impacts would have affected when tidal
lock would have occurred?

Tillman That\'s a difficult question. It\'s possible that Iapetus was
forced into some kind of a vibration when these impacts happened.
That\'s possible, and then the gravity forces from Saturn would slowly
damp this down so that it went into synchronous lock again, but I don\'t
know how long this would take. Maybe only a few years, maybe a few
hundred or a few thousand years -- this vibration period.

Pete During the extended mission, do you think you\'ll get a lot of
attention?

Tillman It depends on the flyby in 2007. However, an extended mission
will be planned before the Iapetus flyby will go on. This is maybe also
of worth to the leaders of the satellite group of Cassini that are also
listening in part. That we should strongly argue for another visit at
Iapetus somewhere maybe in 2009 or 2010. I think this would be a very
interesting thing, especially if you could go over the Saturn-facing
hemisphere because this moat feature. In the pictures you have seen just
now are among the best pictures that we will ever get of this feature.
These are the gray light images. If you could really go over this area
somewhere near, say in the morning or evening, the illumination
conditions then we would learn a lot more about the details of this
area. I will propose that we will argue for doing another Iapetus flyby.

But I have to admit that I have doubts a little bit we will indeed get
it because we are ending in 2008 at the very high inclination orbit
where the apoapse part; so the part where the spacecraft is for most of
the time is over the illuminated side of Saturn, while the periapse is
over the dark side of Saturn. To go down from the high inclination of 76
degrees down to zero degrees, this needs at least half a year if you
start with doing this immediately. But there are many groups -- the
atmospheric people and the ring people, and the magnetospheric people
that like to be at high inclination, so it might last even up to a year
until we are down in the equatorial area.

Then we have the next ring plane crossing, I think it is in summer 2009.
This ring plane crossing will certainly force the shape of the tour.
That\'s my guess for what people will do. Then it\'s questionable if you
will find time to go out to Iapetus, which is not exactly in the ring
plane and which is also very far out and very difficult to reach because
we always have to stay into an orbit that is leading us back to Titan
somewhere. So it must be in resonance with Titan. This is somewhat
tricky and problematic for the Iapetus research. But I hope that we will
get another one.

Pete Thank you very much.

John I had a couple of follow-up answers to that. One is you were
pointing out how you have these bright frost deposits facing away from
the equator, which is also the direction to the bright material. But it
could be that one thing we\'re seeing is simply that the frost is
evaporation from equatorial regions and it\'s collecting in regions that
are colder, which would include the shaded sides of north-facing in the
northern hemisphere, south-facing in the southern hemisphere, outside to
craters which are colder. There could be, that whatever has caused the
leading trailing dichotomy has been modified by just movement of ice due
to the contrasting temperatures on the surface.

Tillman Yes, I think mapping of these features, very carefully, might
show us this.

John Yes. The other thing about the bellyband is that it may be
significant that Iapetus is the object that took the longest to come
into synchronous rotation probably, of any satellite in the solar system
because it\'s so far away and the tidal forces are so weak. And it\'s
the only one that has this equatorial band, and maybe it\'s in some
mysterious way, associated with it coming into synchronous rotation? We
really don\'t know that yet. Maybe it\'s one of the unusual things of
Iapetus that might explain why it has this unusual feature.

Tillman Yes, we might of course think in the other direction. Maybe
Iapetus is the oldest place in the Saturn system and that all the
satellites had this kind of belly band for whatever miraculous reason,
and on Iapetus it was preserved because it was freezing fast enough. But
this is also wild speculation only. Another question?

Trina Okay, well it sounds like there are no more questions. Amanda, did
you want to close out?

Amanda I just wanted to say thanks so much to John and Tillman for
volunteering to do these nice talks and taking the time to prepare them
and give them. We really appreciate it, and it was really very
interesting.

M It was very interesting and informative, and we certainly appreciate
the information.

John Well, thank you.

Tillman Thank you.

Trina Okay, everybody, next month\'s telecon is going to be on February
22^nd^. Of course, we\'re trying to get some Titan talks for that
considering our recent Huygens mission, but everybody is really busy and
we\'re having trouble getting their attention. We\'re still going to try
to do what we can, but if not, surely there will be other interesting
Cassini science results that we can pull into the CHARM discussion for
next month. Have a terrific month.

M Thank you very much.

Tillman Thank you.

W Thank you both.

W Thank you so much.

All Bye.