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

**FTS-NASA-Voice**

**Moderator: Trina Ray**

**August 28, 2007**

**2:39 pm CT**

Coordinator: Thank you for standing by. Today's conference will be
recorded. If you have any objections, you may disconnect at this time.
Ma\'am, you may begin your call.

(Amanda Hendrix): Thank you. Hi everybody and welcome to the Farm
Telecom for August of 2007. I'm (Amanda Hendrix) and I'll be hosting the
telecom today. And today we're going to hear about Hyperion observations
and other observations of Hyperion and are joined by (Dr. James Bauer)
of JPL and Dr. Peter Thomas of Cornell University.

Both of whom worked on observations and data analysis from the -- (Dr.
Bauer) worked primarily on the (unintelligible) spectrometer and Dr.
Thomas worked primarily with data from the subsystem camera.

And so, and (Dr. Bauer) is as JPL and Dr. Thomas is at Cornell.

So we will get started and I think (Dr. Bauer) you going to start.

(James Bauer): Yes. Yes. Thank you.

(Amanda Hendrix): And then we'll segue with Dr. Thomas. Usually we sort
of allow people to interrupt you with questions.

(James Bauer): That's fine.

(Amanda Hendrix): Okay great.

(James Bauer): That's fine. That's fine. And I think I'm going to cover
more of the compositional data and the spectro from the Ven and Peter, I
think is going to cover more of the ISS imaging and the shade model
within and rotation and what not for Hyperion.

But I wanted to start by just kind of thinking of Hyperion in a larger
context and just kind of lead gently into the topics.

But, so Hyperion is one of the over 60 moons of Saturn, sixty known
moons. A large fraction of these moons are a few kilometers in size and
they're either just in their regular satellites, hundreds of Saturn
radii away from the planet or there are a few closer in -- a few Saturn
radii from the planet.

But, there are several things that make Hyperion special. It's the eight
largest satellite and it's one of the -- what are considered the major
satellites of Saturn. It's one of the three darkest satellites and its
albito is about 0.25 and it's the third furthest out of the major
satellites.

Of course, it's a chaotic rotator, so its rotation axis wanders. Its
density is surprisingly low, but again, Peter, I think will go into that
a little more or actually in detail. And it's irregularly shaped. The
surface, of course, looks like sponge, as the lead slide shows.

It's full of pores. At the bottom of these pores are craters. There's a
dark material that tends to exist there. And it's very variable and
patchy with dark and light highly contrasting terrain.

But, if we go to -- that was kind of the purpose behind slide two to
kind of show you the context of Hyperion up against -- compare it to
other satellites just looking over it.

But, if we look -- if we go to slide three, we see that from the ground
it doesn't look extremely unique or impressive, but it also very hard to
observe. This is often due to Saturn's scattered light and the scattered
light from the rings.

The two images on the left are from Saturn ring cling crossing in 1995.
And that's a good time to observe the moons from the ground because of
the profile of the rings, the scattered light from the rings, is
minimized.

If you catch Hyperion where it -- when it's from the planet, you can
take longer exposures and integrate -- collect spectra. On the lower
right is shown an early infrared spectrum from the ground in 1985. And,
the thing about this, of course, is that if you take spectra from the
ground, you can't resolve Hyperion, not even with the Hubble Space
Telescope. It's too small.

So, you get just a fully convolved spectra every part is blended -- it's
a blended spectra of the entire disk and so subtle features are going to
be probably blended away or are more likely to be blended away. You have
to have higher sensitivity in order to see them.

And, of course, you don't know where they fall, whether they correspond
to light or dark terrain and what not. But, you can still tell some very
interesting things.

For example, this spectrum here shows absorption features near 2 and 1.6
or 1.7 microns. It looks like 1.7 in this figure. But, and those
correspond to water ice bands and that was fairly exciting at the time.

When Voyager visited Hyperion, the Voyage Space Craft, Peter probably
recognizes this figure very well, we found, of course that there was
variegated terrain, very high contrast terrain. The satellite was
irregular. That's when we -- and we discovered that it tumbled
chaotically. Initially there were a few subsequent observation from the
ground also helped to really solidify that, that Hyperion was rotating
chaotically, but it was really as a result of the Voyager encounters.

And so...

(Amanda Hendrix): Can I interrupt you with a question?

(James Bauer): Yes.

(Amanda Hendrix): Okay. The views on slide four, the Hyperion -- or the
Voyager view...

(James Bauer): Yes.

(Amanda Hendrix): And I don't if you can see it here or not, but I'm
just wondering now, how does this side of Hyperion that we're looking
at, say at the upper left, do you know if that's the same view that
we're looking at Cassini on the first slide, that same (unintelligible)?

(James Bauer): I think that -- do you see that rim?

(Amanda Hendrix): Yes.

(James Bauer): ...on the lower -- the actually in the middle image on
the right and then again on the lower left I think that rim is the rim
of that kind of ledge that we see towards the top again, if I'm not
mistaken. I think that's right.

Peter Thomas: Right.

(James Bauer): Is that right, Peter?

(Amanda Hendrix): And are we seeing that same rim on slide two?

(James Bauer): On slide two. No. That's the other side of it, more or
less. It's actually at a different angle. It's that rim, Peter you could
probably help -- is that rim the one?

Peter Thomas: Slide two, picture on the left, is way around on the other
side from the usual, you know, the lead in sponge picture and the
Voyager pictures that you had.

And then on slide two that inset of high resolution does go along with
the one on the left of slide two, but you know, it's of the usual sponge
view that you had.

(Amanda Hendrix): Right. Okay.

(James Bauer: So, okay.

(Amanda Hendrix): Actually I have one more question.

(James Bauer): Yes.

(Amanda Hendrix): Sorry. On the spectrum on slide three...

(James Bauer): Yes.

(Amanda Hendrix): Is it -- I usually think of the water as being
(unintelligible). It looks a little different here. Is that real or
meaningful?

(James Bauer): I wondered about that looking at this earlier. I think
this data was actually taken with a CDF Wheel and so it looks like there
may have been some wave length calibration issues with that.

(Amanda Hendrix): They still attributed to water ice?

(James Bauer): I believe they did. Yes. Yeah.

So, let's see. Yeah. Okay.

So if we go to slide five, I see there are three views of -- are there
-- yeah three views of Hyperion with three different instruments with
Cassini and the upper -- the top one is the (VIM's) data cube taken at
about 8800 angstroms roughly.

And the lower left hand view is an Uvis Data, the ultraviolet imaging
spectrometer, and of course, the imaging science system ISS camera in
the lower -- the lower central, but the lower left of the -- lower right
of the images.

If we look at the spectrum that (Amanda) basically extracted and shown
here from (Hendrix)' enhancer, we see that there's this edge at around
1650 angstroms and that is typical of water ice. And we see the high --
the brightest areas have that strongest edge there, so that the brighter
areas are richer in water ice, is what the conclusion is.

If we look in -- if we look at the VIM spectra, we see here -- it's in
slide six, two different -- the spectra of two different terrains. These
are spectra that were averaged over several dark areas, and of course,
over the brighter areas. And we see that the concentration of ice does
seem to -- surface ice does seem to vary.

The dark line or dark dots in these figures, those are the actual Cube
data's and the red is a spectrofit to the data. The spectrofits are kind
of simple models. They're a combination of water ice and organics that
we're made in the lab to kind of be what we think would be synthesized
in the outer solar system. So these are called tholens, there's a
combination of typen tholen, which is what we would expect to find, you
know, in the environment around...

Yes.

(Matthew Odle): This is (Matthew Odle) with the Saturn Observation
Campaign.

(James Boder): Hi.

(Matthew Odle): Looking at the composition of Hyperion with the ices and
the tholens and the low density, would you agree that if Hyperion was in
the inter solar system, it basically would appear as a comet?

(James Boder): If it were in the inter solar system...

(Matthew Odle): Would there be enough, you know, as far as the solar
flux to basically outcast a lot of these ices, basically form a tail and
basically just appear as a comet?

(James Bauer): Yeah. You would probably get -- yeah you'd get a large
amount of sublimation, certainly with the water -- the fraction of water
ice.

(Matthew Odle): So the composition of Hyperion is basically similar to
what we seen on some comets that we've been examining (unintelligible)?

(James Bauer): In a gross sense. Yes. Yes. It's rich- they're clearly
volatile that -- actually I was going to be covering that in the
conclusion, but yeah. There are large fraction of substances which would
undergo a large amount of evaporation at distances of, you know, a few
AU, so...

(Matthew Odle): Thank you.

(James Bauer): Yeah. Sure.

So, let's see. If we look at some of the features -- the other features
that are present in the spectra, we see a CO2 is present in both the
light and dark terrains. We see evidence of that. And we see an
interesting feature at 2.42 microns, kind of that notch in the spectra.

And that corresponds nicely to -- particularly it's clearer in the lower
albito regions it seems to stand out more in that spectra. And that
corresponds or can be attributed -- it matches well with carbon and
nitrogen bearing species that we see in bearing species.

And that's an interesting feature. It'll become more interesting as we
look at the composition on that.

If we look at the CO2 distribution, we see it is both in the light and
dark terrain. Now this is similar in some respect to distribution of CO2
we see on -- in the Iapetus, but different from what we see in Phoebe.

Some other similarities. If we look at the light and dark terrains of
Hyperion and those of Iapetus, we find that the dark terrains can have a
red color in the optical both in -- in both circumstances.

And that's with -- but if we also look at the -- in the near infrared,
again this is -- now this is slide nine. I guess I'm going through these
faster than I thought I would. But, maybe I'll get a lot of questions,
so, if you look at slide nine, we see that the match is actually better
between for the dark material between that of Phoebe and Iapetus.

So the match is good in the optical for the material being of similar
origin between Hyperion and Iapetus, but better for the material of
similar origin in -- between Phoebe and the Iapetus when we look at the
near infrared spectra.

Now if we move to try and map out particular features that we see in the
spectrum, which is what you can really do with the VIM's, really the
(Franktin VIM's) and other imaging spectrometers (unintelligible).

You see a certain correlations and concentrations of different elements.
The -- in slide 10, the water ice band depth is labeled blue, okay in
the upper image. The red corresponds to -- the reddish regions
correspond to strong CO2 band depth. And the green that the strength of
that 2.42 micron features that we attribute to CN bearing species.

And if we look at the way things are distributed, CO2 actually is
concentrated...

(Amanda Hendrix): Can I ask a question real quick?

(James Bauer): Yes.

(Amanda Hendrix): The green you said was corresponding to the carbon
nitrogen. Is that what you meant? I couldn't hear that part.

(James Bauer): Yes. That's correct.

(Amanda Hendrix): Thank you.

(James Bauer): Yup. Sure. The CO2 is concentrated in certain areas, but
it's not concentrated with respect to brightness. Okay? It's
concentrated over both bright and dark terrains. And we see that the
thing that does correlate very well with the dark terrains, are the CN
bearing or the feature that's indicative of CN bearing species.

So...

(Amanda Hendrix): And is this the same type of thing -- are you showing
us the same kind of thing on page eight?

(James Bauer): On page eight. Yes. That's the same -- that is the same
-- that's a broader. Right.

(Amanda Hendrix): Right. Okay.

(James Bauer): So, it's a larger scale with that.

And if you look again at on page eight or slide eight, you notice, of
course, that the water ice is again correlated with brighter features
and you can see that again with -- in slide ten to some extent too.

To get back to the CO2 distribution, if we look at just the spectra of
the CO2 bands and compare them to what we found on Iapetus, we find that
the bands on Hyperion are shifted somewhat shortward. Okay? At shorter
wavelengths and this would imply that there is possibly -- the species
-- the CO2 is actually up against other different species than with
Iapetus or Phoebe.

And that it could also be bound in part with possibly water ice or other
molecules, but different molecules than on Iapetus.

(Amanda Hendrix): Here CO2 the absorption bands...

(James Bauer): They're shifted again. That's -- it's -- it can't be --
its not pure CO2. So...

(Amanda Hendrix): (Unintelligible) is somewhere else other where we see
Hyperion (unintelligible)?

(James Bauers): Right. It's shifted to a different position. Okay. In
fact it's at 4.27. I think it's even longer.

(Amanda Hendrix): In the Iapetus spectrum you're showing as to the dark
terrain?

(James Bauer): The Iapetus is I believe for the -- I believe it's for
the dark terrain.

I think. It might be -- no it might the bright -- it might be the bright
terrain there actually.

So, okay. Now the fact that CO2 even exists at these distances, is a bit
of surprise and we've seen it on several satellites including Iaepetus
and Phoebe. But, the reason why it's odd to actually see CO2 here, is
that it evaporates fairly quickly -- you know, fairly quickly is a
relative term, but if you look at the plot on page 12 in the upper
right, you see it's a plot of basically evaporation rates in terms of
meters per millions of gears as a function of distance from the sun.

And we look at Saturn's distances, it's, of course, the green line right
there and we find that on the order of tens of thousands of years or
hundreds of thousands of years, we'd expect any, you know, centimeter
size layers to evaporate off of the surface.

And so, of course, we're talking about fairly old bodies and so we would
expect that there would be some sort of source of replenishment for the
CO2. And you can get CO2 from different sources. You can convert it from
CO and you can get it from ultraviolet radiation breaking up water ice
and the oxygen with that, of course, combining with the products broken
up of the left over products of UV breaking up carbonaceous material and
form it that way or you can form it with impacts basically.

But, it's -- as to what the source actually is, again, a bit of a
mystery. Saturn isn't the only place that we see CO2 where we shouldn't.
We see CO2, for example, in -- on Jupiter's satellites, as well. And
again, that source should not be indigenous.

That basically is what I had to talk about with the composition -- just
the composition. Basically, we see that the dark terrain and the bright
terrain both bear CO2 to some extent. The water ice concentration
varies, but the CN bearing feature, this feature that we attribute to CN
bearing species, tends to correlate well with the dark terrain. So,
that's pretty much what I had to say and if there are any more
questions, that's fine. Then otherwise, if Peter wants to talk about...

(Amanda Hendrix): I have a quick question.

(James Bauer): Yes.

(Amanda Hendrix): And that is let's see, so you see the CO2 in the
bright and the dark terrains.

(James Bauer): Yes.

(Amanda Hendrix): But, I think I was thinking, and maybe I'm getting
confused here, but I think I was thinking that CO2 elsewhere. Again, and
maybe you said that. It mostly occurs in the dark terrains.

(James Bauer): On Phoebe it certainly does. Right.

(Amanda Hendrix): Okay.

(James Bauer): Yeah.

(Amanda Hendrix): And what about like on Calisto, like you show on page
13?

(James Bauer): Calisto it's a leading and trailing side asymmetry. But I
think it's also in the dark terrain and that's a question hit its, right
there...

(Amanda Hendrix): Yeah.

(James Bauer): Right there. That figure.

(Amanda Hendrix): I thought that there was (unintelligible) correlation
with craters. The bright craters had CO2?

(James Bauer): Yeah. I think you're right. I believe that's correct.

(Amanda Hendrix): It's on Phoebe mainly where (unintelligible)?

(James Bauer): Yeah. Yeah.

(Matthew Odle): This is (Matthew) with another question.

(James Bauer): Yes.

(Matthew Odle): Do you have any idea of the bearing strength of the
surface of Hyperion? Say like if we ever landed a space craft there or
sent humans is it a very frangible surface that you would sink into very
slowly or is there enough, do you think there's enough bearings there to
support a space craft or a person basically would set foot on that
satellite?

(James Bauer): It's a good question. And I think Peter will probably
deal with that a little more. It is very low density material. So, Peter
did you want to answer that or...?

Peter Thomas: Well I can get to it eventually. I guess two things it is
very low gravity surface, so when you plop a space craft down, of
course, the space craft doesn't weigh very much, so it's not going to
give high pressure on what it\'s sitting on. And of course, this may be
ice and all these other, you know, ices of many kinds and volatiles and
may have some fine particles.

But it's also, you know, below hundred Calvin's, so a lot of that is
probably feels more like steel then ice to us. So, I doubt that you're
going to have major problems of sinking into it, given that it's a
little bit, you know, compressed and pulverized by getting hit by impact
and that it's, you know, it's pores, but it's not perhaps as pores as
say some comets.

So the with a mass of Hyperion, the space craft would more likely would
be a docking operation, then a landing operation on.

Yeah. Though it's a little -- yeah it's sort of halfway in between. It's
not as bad as some that you'd consider rendezvous and docking rather
than landing.

(Matthew Odle): Okay. Thank you.

(Richard): Fellas. I have a question also on the composition. Yeah my
name is (Richard) I'm with the Space Science for Schools in Reno, Nevada
and I do space science outreach in education in schools in our local
area.

And going to be working with students this year on our moon and also
doing our moon as a basic comparison to other moons in the solar system.
Do we have any evidence of any rocky material or structure to this moon?

(James Bauer): The models that we used, okay, we're -- did not have
silicates as a component. Okay. Whether or not, - I mean a lot of this
-- a lot of the modeling is -- you pick from what you've sampled and you
try it out. And whether it can accommodate or not, this -- an actual
silicate. But the common stuff that we did try and we did try some, did
not -- like olivine and other things, we did not see -- it did not
accommodate.

So I think in terms of pure rock or with that -- or high concentrations
of silicates, they don't seem to fit very well. But, I don't know again,
if Peter wants to add anything. Or...

I think that's true. So...

(Richard): Yeah, so I can pretty much pass onto them with confidence
what were dealing with here is a outer solar system -- I mean out of
planet solar system body, that moon, that's primarily frozen water and
gas at this point?

(James Bauer): And organic.

(Richard): And organic....

(James Bauer): Organic material. Yeah. Just a lot of ice so.

(Richard): Yeah. A lot of ice. Okay. Thank you.

(James Bauer): Sure.

(Amanda Hendrix): Okay.

Peter Thomas: Okay. So should I...

(Amanda Hendrix): Why don't we go ahead with...

Peter Thomas: The second half.

(Amanda Hendrix): We can come back composition questions later on too.

(James Bauer): Okay.

Peter Thomas: Well I will proceed here starting with slide 14. These
slides were somewhat hastily extracted from something else. We may have
to jump around a little bit. But I'm going to be talking about the view
of Hyperion from the standpoint of geologists doing photo interpretation
on the images and also using some of the other data that, of course,
geologists sort of need.

Composition is obviously one of them. And mass is another. And this
slide, number 14, is actually a summary or pretty much all of the good
Voyager views of Hyperion with the Shape Model developed from the
Voyager data superimposed.

Voyager made a fairly leisurely -- or in two made a fairly leisurely fly
by through Hyperion being modestly far away from Saturn. There were
actually a few days when you could, more or less, resolve the object.
And a couple of days when you could see the --some surface features.

And the surface features changed a little bit. They are obviously
changing position and there'd been some notion that maybe it was an
in-synchronous rotation and maybe even chaotic, although that idea was
just coming in at the time.

And the initial during the encounter when everything is a little hectic
and people are, you know, writing press releases and quick science
articles and things, basically we described Hyperion and let go the
question of what the rotation really was.

It didn't really seem to be synchronous with its rotation around Saturn.
And well, I think it was a few months later that (Tom Docksberry) went
and did some stereo controlled models on this and came out with- it was
actually spinning around its long axis, not pointing anywhere near
Saturn, which is very unexpected.

Of course, no one believed his answer, since it was not spinning very
much, the -- it was a very unusual answer. And then sort of sat that way
for about a decade and then some more light curve studies by (Jim
Claviner) and other people and then re-analysis of Voyager data
indicated that, in fact, this thing likely was in chaotic rotation,
which basically means that you can watch it spinning for a while, but
predicting what it will -- its spin state will be a fair amount in time
in the future, is extremely uncertain.

And it means that the orientation of the spin axis changes in space and
it means that the orientation of the spin axis changes within the body,
which creates all sorts of interesting problems saw later with Cassini.

Now let see. Okay, so let's go to slide 15, which is just another
introductory thing, just to remind of us some things. The interesting
has already been mentioned. And Cassini being in its orbit which changes
around Saturn, and is sometimes (unintelligible) to get you near objects
of choice, one of them being Hyperion.

And nearly two years ago now -- it doesn't seem like that, (Scott) went
in about 500 kilometers of Hyperion and the close fly bys, were of
course, designed to do several things. Give you the highest resolution
from the remote sensing instrument and then also, if possible, to track
the space craft and get a mass as you fly close.

Now, all that was done. A good pass. There had been some other passes
before and since, which is sort on the order of half a million
kilometers or less when you can also resolve surface features that
something is good as a really close pass.

When you fly that several kilometers per second, a few hundred
kilometers away, you're actually tracking the object with this humungous
space craft with all the instruments nailed onto it, rather than on scan
platforms, generally are not going to be taking pictures at the closet
approach, and I think, there's about 2,300 kilometers or so is the
minimum range at which we were taking images.

So a successful fly by, lots of views that other times with this thing
in chaotic rotation giving you unpredictable faces every time you flew
by, because you couldn't predict the rotation.

All right. Now let's see. This third slide that I've got number 16 --
actually what I should probably do, is skip to number 17.

Okay. And to actually do things like map surface features and find the
volume of the object for determine the mean density and other things, of
course, we do have to deal with this rotation and so it meant that each
one of the passes including the close paths identify surface features,
and you see an example here of a couple of views from the September 2005
fly by with so called control points marked -- you know, on craters in
these two views, you can tell a little bit there's been some rotation of
iterium from the left and the rights and some of the crater on the right
bottom has started to come into...

(Samantha Harvey): Excuse me. May I ask a question?

Peter Thomas: Yup.

(Samantha Harvey): Yes. My name is (Samantha Harvey). I'm a writer here
at JPL -- a science writer. And I'm writing a story about this. I need
to know when you're talking about rotation within the bodies, that's why
your control points change? Is that what you're referring to? I'm a
little bit confused from slides -- the previous slide to this slide.

Peter Thomas: Well, let's do this slide and then I'm going to go back to
the other one.

(Samantha Harvey): Okay.

Peter Thomas: Okay. You're -- you've got these different views here. The
space craft flying by. You don't quite know which way it's spinning when
you go by and so what you do is you look at all different view and the
space crafts moving and the object is spinning. And so you mark...

(Samantha Harvey): Correct. Right. Okay.

Peter Thomas: ...things and the two things and then you have ways of
solving for which way the object itself is spinning and how fast it's
spinning.

(Samantha Harvey): Oh. Okay. Thank you.

Peter Thomas: And what changes -- it actually turns out that the rate at
which it spins, for some reason we've got into the habit on Hyperion of
measuring this in degrees per day, and it's around 72 degrees per day,
which basically means it takes about five more days to spin once.

So it's sort of slow. You can use all these, you know, couple hundred
points on here to determine what we would call an instantaneous spin
factor at this one time of which way it's spinning. And you look at all
the other times, and you find out that, well this spin factor has in
fact changed quite a bit. The rate sort of stays the same, but the
direction in which its, say, North Pole is pointing, changes quite a
bit.

And I will show that in a...

(Samantha Harvey): Thank you. That was helpful. Thank you very much.

Peter Thomas: Okay. The table comes number later. So anyway, this slide
number 17 is just to give you a view that you can tell that the lighting
has changed. There's a little bit especially on the right there just a
few hours later. And you know, there's a whole bunch of things that you
go through and you mark and let the fancy software figure out which way
the thing is actually spinning.

So now, we should back up to the previous one. And of course, then when
you have the spin you know the relative geometry of the space craft and
any point on the surface and that's when you can do things like, make a
shape model and start measuring things on the surface and counting up
craters and doing anything instead that really requires a global map,
for knowledge is sort of a global shape.

And so all these different views you can make partial shape models and
then stick them all together, which was a lot of fun. But anyway, the
numbers on Hyperion here, the mean radius, which is just the radius of a
sphere that has the same volume, since this thing is very irregularly
shaped. You have to have some common definition of the average radius.
One hundred and thirty five kilometers there some uncertainty in there,
largely because of having to stitch these different views together and
you're a little unsure to a couple of degrees of what the orientation of
the spin is.

The range of diameters there is pretty impressive. It's nearly a factor
of two. And then the really important number which comes from the mass
which the radial science and the navigation people actually worked
independently, actually by different techniques, but also involving the
tracking the radio as .5, where in multiple passes it was a little bit
more than half that of water.

And since we've heard that it's likely to be made largely of water ice,
this implies that it's highly porous on the inside, probably more than
40% pore space.

(Amanda Hendrix): Peter.

Peter Thomas: Yup.

(Amanda Hendrix): Did you say grams to meter cubes then?

Peter Thomas: In grams, yes kilograms, not kilometers.

(Amanda Hendrix): Kilograms. Okay because....

Peter Thomas: Sorry. It goes everywhere. But anyway, 540 kilograms per
meter cube and that changes like into half of a gram per cubic
centimeter. And the other thing that translates into is sort of an
average surface gravity of about two centimeters per second squared. So
that's two tenths of a tenth burst.

So that's a very low, low surface acceleration.

All right. So let's skip forward two slides here to more numbers. And
don't worry about this, this table is too much other than on the left
had side it's just for the dates of various observations, different fly
bys and parts of fly bys. You can see 2005 and then Day 160, 161 and so
forth.

Different times, there was, you know, days durations. There's a
orientation of right ascension in declamation of which way it's spinning
and the rate in degrees per day in which its spinning is sort of roughly
the same. Actually the most interesting thing, is this right hand column
called "SS Lat", which means Sub Solar Latitude which means if you're
standing on the equator of the object that is 90 degrees away from the
pole attitude wise, is what's the latitude of the sun? Or what's the
season?

That's basically the definition of season is where the latitude of the
sun is and you can see the sun goes from way south to moderately north
on this and that's mostly motion of the spin axis in the sky. Some of
that is the motion of the spin axis within the -- with any object. But
that's a good indication of just how much the orientation of this spin
changes, you know, over just a few months.

Okay. Going on. Okay, I guess I've already sort of dealt with things
implied in slide 19, about the porosity. Okay. If it's mostly water ice,
then you sort of got low 40% pore space and then if you are mixing
higher density materials to any significant degree, then that -- which
are denser than water ice would be, then the porosity would go even
more. And of course, maybe this may be time for a compositional question
for me in the sense of what are the constraints on say the possible
fractions of the CO2 and organics and stuff like that?

My suspicion is there -- it's hard to tell and they may be low. But is
that true?

(James Bauer): CO2 it's harder to tell. But, water ice is actually
pretty good -- pretty easy to constrain and in the bright material. I
mean, again, we're looking at just he surface. The bright material it's
over 80% and in the dark material it's under 15%. It's actually 14% in
the models. Does that answer your question kind of?

Peter Thomas: Yeah. Yeah. Okay. So in terms of densities, I mean this is
a low density. It's not the lowest density that so far they've been
measured for (unintelligible) satellite in the Saturn system. Some of
the ring related satellites do have lower densities, although they have
some fairly large air bars, but some of them may be down on the order of
.4 or perhaps even less grams per cubic centimeter.

And so, Hyperion may be unique in some of the attributes. The presence
of the low density so of by itself does not necessarily set it apart
from everything else. It may be a combination of other things.

So let's go on to the next slide 20, which is just set of three views.
There's the now a classical sponge view of Hyperion on the left. The
upper right is an attempt to sort of a true visual color image of it and
I guess, we saw a version of this before. This -- the B is, you take the
view on A, and sort of go along ways around to the right about a hundred
and some degrees and that's sort of what you get.

That view -- the view in C is in between the two. You can see the top of
B is also visual, sort of the top right of C some common areas. The bite
taken out on the left side of panel C, is actually what's starting to
take a bite out of the lower right hand side of A. Just to sort of line
those up.

And the key thing about these other views taken on other passes, is to
help you define the shape and have some mean density. They also confirm
that this pattern of dark material in the bottom of craters and other
depressions is a global phenomenon. It's not stuck in just the one area
there that you happen to see at high resolution.

Woman: Peter, I'm sorry I missed it. But where is -- how to B and A
relate then? Where's B compared to A?

Peter Thomas: Let's see. If you take the A -- the view of A and sort of
go around to the right hand side about 130 degrees or so, then you get
to the B. It's actually sort of easier to go A, C, B in the sense that
the bite on the lower right hand side of A is the bite on the lower left
hand side of C. And you've gone, you know, a hundred and some degrees
around on the bottom.

So C is sort of looking back towards to the B side there and then you go
up on C to the upper right and the upper right of C, you can recognize
in the upper part of B.

(Amanda Hendrix): Yeah. Okay. Got you.

Peter Thomas: It's just a little hard, just because of the way of B is
rotated to actually get B and A hooked up directly, unless you sort of
really know, like one tiny little crater there. But -- and all of that
still doesn't cover the entire object, but that's about slightly more
than two thirds of it, and there were some other images that then
covered the rest.

But it doesn't give an example of really set of a difficulty navigating
around here. When you come by another time, you don't know which way
you're going to look and, you know, then the axis is in a different
place and you've got to used to what it looks like.

But the key thing is, these dark things distributed all over.

Okay, so now actually onto sponges in slide 21 to describe some
different view than this thing we're constantly looking at here. But,
it's still normal is good to look at.

So we had the fly by and everyone said how odd this thing looked, how it
looks different, and, you know, something's going on. Why does it look
like a sponge? You know, maybe there's odd surface processes going on
and eroding things.

Certainly one of the first comments was it looks like sun cups on a
glacier where sublimation and especially a collection of dark material
at the bottom of pits is sort of a positive feedback effect, that it's
going to make deeper holes by sublimating and expansion of tips.

But this is pretty cold out here and there are lots of craters on here.
So one of the first things that we just did, was say, "Okay, let's go
and actually map things that really truly look like impact craters and
segregate them from other types of depressions that might exist."

And the criteria is that, you know, you have a distinct rim that they
cut other features and that are more or less circular. And when you go
do that, go through and map them all out, you're actually not left with
too many things left over, other than the craters that you've map.

So, the media just did a simple mapping discovery was, "Well, what
you're looking at here odd as it may be, is a collection of impact
craters. And the question is, sort of why does this collection of impact
craters look different from many others, other than perhaps the fact
they have dark material at the bottom."

Now, we go to slide the 22 and this is not the best attempt to compare
three different small irregular shaped objects at roughly the same
scale. Hyperion with one of its craters, big craters and lots of other
ones. Phoebe on the upper left and Tilesto on the lower right.

They really do look different. So Tilesto on the lower right really
seems to sort of buried itself. You see sort of some ghost craters that
are pretty well filled in.

Phoebe, you know, very nice looking craters. Some of them may be
slightly conical. Certainly a very lunar look to it.

Hyperion has a different look here. There's lots of craters. It has some
rims on the craters. It has some debris, which you can see moving down
slope into craters. It doesn't have much in the way of ejected blocks,
as you can see. It doesn't, although there are raised rims on craters,
there's certainly nothing that immediately strikes you as being due to
ejected from craters.

And there may be more -- it's turns out there more sort of intermediate
size craters on Hyperion. So we go to the next slide which is one of
these usable horrible, so called, R-plots of crater densities, objects
which R means relative to a particular slope.

The only thing you need to worry about on this plot here is it's goes
from a hundred meters to more than a hundred kilometers in diameter on
the bottom and then the relative crater density on the vertical axis.
The dark symbols are for various counts on Hyperion and the open symbols
are for Phoebe.

And between one and ten kilometers, you just note that there are more
craters on Hyperion then there are on Phoebe, probably by a factor of
two, listing log plots.

So the real difference between what Hyperion looks like and what these
other ones look like is one, it has more of these two to ten kilometer
craters. And two, they are individually sort of well preserved. They
have nice rims. They may not have big ejected blocks or other blankets
or plots around them, but they do have nice slightly bright rims which
you've seen on several of these images.

(Amanda Hendrix): Can I ask a question really quickly?

Peter Thomas: Yes.

(Amanda Hendrix): Does that -- would that imply that these are younger
because of the more distinct rims?

Peter Thomas: Well, individually what it implies is for each, you know,
if you have a collection of well preserved rims, it would be younger in
the sense that yes, in the sense that they have not been eroded.

Now the things that usually erode craters are other craters and so --
but if you have a lot of young craters, then you've had lots of time to
accumulate these, and they're not necessarily young.

So what it says is, that there's some erosional process which typically
operates on Phoebe and apparently also on the moons and Phoebe looks
like a moon that does not eat away at the rims and the bottoms of
craters on Hyperion as well you might suspect.

And if we go to the next slide, number 24, after lots of -- the way some
of this -- you know, we did the crater counts and so we got, "Okay, well
there's a different crater density and so, you know, it's more. Maybe
it's older. Maybe it's something else." But it was just clear that the
impressions -- the visual impressions derived from a lot of these
intermediate size craters that were themselves pretty well preserved and
not as rounded off as say the ones on Phoebe were.

And sort of fussed around with that a while. And of course, being on
Cassini that's flying by other satellites when you're working on
satellites and if you get started on one object and then you get a
little ways on that and the space craft flies by another satellite and
maybe runs off and tries to work on that and forgets what they were
doing.

So it took a long time to get around some other things, one of which
was, just the usual deal optical thing. It's just going back and
contemplating what's really in the images.

And the key element here is the image on the left of slide 24, which is
a real close-up of the bottom of one of these craters that has this dark
material in the bottom part of the crater. The crater itself is pretty
old. Its rim has certainly been beaten down some more than some others
and when you stretch these images rather ferociously to bring out what's
in the dark material, you see low and behold, that the dark bottoms of
these craters are themselves very heavily cratered with small impacts.

And what that really says is, okay the bottom surface is old. It's been
accumulating lots of these smaller craters and which means that the
bottom of that crater has not suffered very much sedimentation of stuff
filling it in. It hasn't suffered very much erosion say from
sublimation.

After the main, the big crater who's rim is outside of this picture was
formed and so you get a picture here of, okay, well the two to ten
kilometer craters are sort of well preserved and sitting there and see
in the bottom of the big one, they're accumulating lots of the small
ones.

So that what's going on is, yes you're still accumulating craters, but
there's really nothing else that's eroding things in any great degree
other than on the very steep slopes where you get some land slides,
which is in the right hand picture there sort of emphasizes what some of
those big slopes.

You know, you have land slides, but this is a picture of stuff that's in
distinction from, say the moon or Phoebe or other things. There's just
doesn't seem to be much extra material that sort of flying around and
doing the usual burying of craters.

One of the usual notions, is okay, well there's low gravity, everything
flies off and there's nothing -- you may be punching holes in it, but
you're not sort of self burying.

Man: Hi. Yeah. If have a question here. Well, we're on slide 24 here and
I'm just looking at the picture on the right and we have what appears to
be like a large cliff type structure right in the middle there. Could
that structure there be an -- similar to like a lunar impact basin and
then that was filled in with smaller craters later on?

Peter Thomas: Yeah. I've got some topography on a later slide on this
whole view. But that face -- the picture on the right, you know, you
have pretty much the whole visual disk. And about two thirds the width
of that disk appears to be a large impact structure which itself has
been heavily cratered since then.

And I'll get to a little bit more of that later. But yes, that's a...

Man: So, that would be similar to a larger lunar impact basin then?

Peter Thomas: Yes. And it has perhaps a central teeth which scales back
the lunar things of higher gravity. I do have a couple of slides later
on dealing with that specifically.

Man: Yeah. I appreciate you bringing that out. I'm just seeing a -- that
rounded domelike area right in the middle there. I've never noticed that
before.

Peter Thomas: Yeah. And on Voyager sort of the top half of that picture,
as you remember from the pictures shown earlier, that the Voyager you
sort saw the top half of this thing and the bottom part you could see,
but it was much more apparent in the Voyager images that scarp on the
top. And it wasn't necessarily connected in our Voyager views as sort of
those round thing possibly being a whole big crater but a possible
central teeth.

The views have improved. All right. So, back to the some sort of
physical reason as to why the craters on Hyperion might be better
preserved than average. In -- it was 1997 when the Mir Spacecraft flew
by the asteroid Matilda, excuse me, my throats going here. I've got to
get a drink.

...flew by Matilda which turned out to have a low density and also
turned out they have examples of impact craters which may be a little
deeper than average in which didn't seem to have a lot of ejected. That
started inquiries as to what happens when you have impacts into things
that are much more porous than say the rocks on the surface of the moon
or other things that we're used to.

And some laboratory experiments and some scaling by (Hasmit Hossaple)
and other people suggest that when you get above porosities of 30% that
you greatly reduce the amount of ejective that's thrown of a crater when
it's made. The craters is made much more by compression, rather than by
excavation.

It's also that when you have everything being equal and if you have a
larger crater in a higher gravity, that you will throw less -- whatever
you may eject is going to get thrown less distance unless not out of the
crater. And so, those two things higher porosity and actually higher
gravity both can conspire in some cases to perhaps greatly reduce the
amount of material that gets thrown out when you make an impact crater.

And so we took the things of mean density and mean sizes for a whole
bunch of objects for which we sort of gleaned surfaces reasonably well,
and of course, some of these mean densities are well known and some or
somewhat guessed at and the porosities are even more a guess, since you
have to guess what's -- what the material is into which you're putting
the pores.

But you can make some reasonable estimates of the porosity. And so, this
plot here on the bottom is porosity of percent and the vertical scale is
the local gravity times a sort of scaled size of crater, a typical
crater that you're going to see on these. A small object, you know, a
three kilometer diameter satellite you're not going to have a ten
kilometer crater.

So, we just took an average crater size of a third the size of the
object. And so there's this perimeter gravity times crater diameter,
which (Hasmit Hosapple) and others regards as sort of a scaling of how
effectively you can throw things out of craters if you make it and the
porosity being also the greater porosity of less material that might
throw out.

And so, you plot the known objects. Hyperion with the age and the Phoebe
in the upper left with the P, Janus, Epimetheus. Matilda, the thing that
started all of this is this really porous and lots of other ones are way
down because they are small and they have low gravity.

And the interesting thing is of the objects we've seen so far, Hyperion
on this plot sort sits alone in terms of likelihood. You can think of
everything basically to the left of about 35%, porosity is not going to
show porosity effects of cratering and then, among the other objects,
the higher it is in the plot, the more likely it is to also hide what
little eject that there is.

And so, on this plot, only Hyperion, and perhaps, Janus, which we've
haven't seen very well and still argue about its porosity, only Hyperion
is really a good candidate satisfying the model of (Hasmit Hosapple) in
terms of how you make craters and porous objects.

So it's at least consistent story. It doesn't necessarily mean that's
really going to wind up being the final answer, but taking their model
of how you make craters and porous objects, at least simple mindedly
Hyperion sort of fits.

Now, when we begin to look at other things, of course, there may be
other attributes which become more important, but at least, at this
stage of the game that sort of was a nice little correlation to come
across and perhaps, explain some of the things going on.

And so let's go to the next to last slide here, which we do get back to
--most of what I was talking about was these two to ten kilometer
craters, the things that make it look like a sponge.

Well, when you look on the scale of the whole object, the remembering
this thing as like three hundred and some kilometers top to bottom in
this picture here. And so on this plot, the upper right is the usual
sponge picture. Next to it is a color coded shake models and the color
code is basically the height of the surface above an equipotential
surface.

So this is really sort of the gravitational height or whatever you want
to call it, but it's the kind of topography we think about on the earth
when you're -- most of the topography is the -- is intuitively -- the
topography is intuitively very close to the sense of above and
equipotential, say sea level.

And so you see there's actually forty some kilometers worth of relief on
this object. You know, if you're an astronaut driving around that's sort
of the vertical distance you have to drive through. On the other hand,
you're driving against gravity of two centimeters per second squared.

Now on the left is just a topographic profile through this crater in
sort of crisscrossing direction of -- I've got four of these profiles
across the crater with sort of southeast, northwest, and things like
directions of those profiles sort of crisscrossing this large 240
kilometer diameter crater or at least whole.

The very top inset there shows just the actual cross section of the
whole satellite. And 32 is the central peak, as it were, and one and
three are the sort of southern and northern rims. And so you can see on
the whole object, this big crater may not be too dramatic when you
actually folded the gravity in, and of course, exaggerate the critical
release, then you can see that it is a good hole in the ground with a
distinct central peak.

Now, this may be like lunar craters of a modest size up to, you know, 60
kilometers, give or take a little bit. Those that are slightly larger
than simple bowl shaped craters, those are the ones that start having
central peak. The lunar gravity is vastly more than on this object, and
so it turns out that you would expect on a very low gravity like this to
barely be able to get a central peak crater form. All of them should be
bowl shaped, which is pretty much what they are.

But the largest ones, in fact, which scale by gravity back down to a
lunar crater that has a central peak. So, there's some -- I think a
reasonably good expectation of this big thing on the sort of typical
sort sponge face is a big crater, fairly old, since it has lots of other
craters on top of it and large enough to have been a central peak.

And, you know, with some subsequent land slides at top and bottom, but
those haven't really changed the size of the crater very much.

And, then let's see. There's just one more slide, which really just
rehashes everything I'd said. So we might as well get into more
questions here.

(James Bauer): I've got a question about that large crater.

(Alexandria): (Alexandria).

Peter Thomas: Yes.

James Bauer: (Alexandria) go ahead.

(Alexandria): Why don't with the question about the large crater.

Man: Okay. I just mapped you again. Just from the first time I've seen
that same image of Hyperion with that large crater with the landslide,
it almost looks like the landslide was like a slumping action of the
rather low velocity of -- that formed these cliffs. Almost like the
object that hit was at such a low velocity that just caused that
material basically slump in. Is that a good assumption?

And then afterwards you have these smaller craters that impacted the
central area afterwards.

Peter Thomas: Yeah. If you go back to -- let's see here, which one.
Yeah. Go back to slide 21. Okay. That's a good set of overview of the --
and you see -- actually what I should have had is a plot not just of
topography but of slopes.

Relative to gravity those -- so the top and bottom of this view you have
these sort of long slopes that are a little bit brighter and, you know,
look like they have the landslides coming down them. And those -- the
slopes relative to gravity on those are in the sort of high twenties of
degrees. Nearly 30 in some cases.

So you would expect stuff to be slumping down. It's --most of that
slumping -- it's not clear to me how much that slumping -- you know the
relative timing of a lot of that slumping relative to the formation of
the original crater. You've certainly accumulated an awful lot of
craters -- smaller craters since the big one formed.

Then if you go to the next slide, it's -- the Hyperion part of the slide
-- has Pheobe, Hyperion and Tilesto on it. Actually at the very bottom
of this sort of sliced off extra little appendage of the Hyperion
picture just to the left of Tilesto there is one of these slopes --
steep slopes coming on -- from the southern side of that big crater.

And then the bigger is just above that has some fill in it, so yes,
there's -- stuff has come down there as you said at low velocity. This
is, you know, creep or you'll see occasional slide. Some of that within
the smaller craters, say if it's the upper half of this Hyperion picture
clearly greatly post date the formation of the big crater, because it's
sliding into a younger, you know, craters that have been superimposed on
the other one.

Certainly one of the interesting things about this object is, you know,
there is this down slope movement in some places. There of the very
interesting correlation of the different compositions with different
morphologies and I was sitting here thinking earlier as we were
listening to the compositional presentation of just how much sort of the
materials of the surface are probably what has survived or been
processed through a bunch of different activities.

You know, cratering, a little bit of sublimation. There, you know,
charge particle interactions or something like that. We're only at the
beginning instead of figuring out sort of what that compositional
residue may mean for sort of surface processes and then tying whatever
that might mean into what the pictures of the geology show.

You know some of them show big down slope movement. Most of them show
that nothing has been covering it up by way of craters. So, we're in the
infancy of actually trying to put all of this together in terms of, you
know, how -- what's really working on the surface.

That was a long battling answer.

Man: Okay. Thank you.

(Amanda Hendrix): Did someone then have a question?

Okay. Let's see. I'm going to chime in with a question that someone
emailed me. Someone who I think normally calls into the Charm Telecom,
but couldn't make it today.

And she has a question. And I think it might mainly be for Peter. But,
it's from someone called (Diedra Teleham) who's the president of the
Irish Astronomical Society.

And she sent along some pictures of limestone getting eroded because
organic material buying with frozen water makes an acidic solution. And
so it makes these sort of holes or craters in the rocks that look
Hyperion like.

And I think she was wondering is there any possibility that something
like that could be contributing to the appearance of Hyperion?

Peter Thomas: I wonder...

(Amanda Hendrix): And one day, maybe all the holes are Hyperion aren't
due to impact.

Peter Thomas: Yeah. Well we -- I mean -- I guess -- some of us have been
working on that probably would assign most of these to impact for a
variety of reasons. There's also the question of, I mean, one of the
erosional things that was, you know, was this sort of sun cup, and
obviously the limestone sort of solution is another case where you can
have things collecting in holes and sort of providing feedback for the
erosion down into a surface.

You know, you've got a little pit. You collect, you know, organic
matter. The organic matter will hold onto the water. It will raise the
acidity of the water, leak the limestone out and grown more stuff and on
and on.

And so, as with the sun cups, that's a very good natural thing to
question on this as to whether there is some sort of feedback like that
going on. I guess our approach was that we could find lots of things
which really did look like craters as them having the distinct raised
rims and cutting things.

And so once you go with those, then you're not left with very much. So
there might be some other things, but again, at the low temperatures
here, less than a hundred Kelvin, or at least for most of it, I guess
noon time and the darkest materials there.

I'm not quite sure what the temperature gets up to, but it may be --
might be more than a hundred Kelvin, but that's still ferociously cold
and low rates of, you know, sublimation at least are waterized and
anything likely to be a major constituent and certainly solution
involving liquids in terms of the limestone thing requiring an actual
liquid could be present to engage in dissolving things and then being
able to carry the material away dissolves species away, that would seem
to be ruled out by the temperatures.

(Amanda Hendrix): Right. Okay.

Peter Thomas: But yes. I mean the lime --it's a reasonable thing from
the standpoint of, you know, limestone as with the other things, that
look at this and say, "Okay, are there these sort of feedback mechanisms
which can deepen holes?" And it just gets very hard to apply them. They
look like there are a lot of craters. And then we do see that thing on
slide 24 where it looks like the bottom of these big craters are them --
are old and, you know, nothings eroded them and nothings buried them.

(Amanda Hendrix): Right. Right. Yeah, I think that might be the smarter
(unintelligible). Well, they may combine with the temperature evidence.

(Richard): This is (Richard) again from Space Science for Schools. I
just had a very general question here for both the gentlemen. Generally
what's the point of view that most of you are having on the origin of
this body? Are we dealing with a captured body or is this something that
was actually, you know, part of the Saturnian System since its
inception?

Peter Thomas: Who goes first? This sounds like a compositional question.

(Richard): Yeah. Really because I know that this question is going to
come up with my students, as you know, as we -- as they first become
students of our, you know, proficient basics of our moon and then we
begin to go outward into the solar system.

But I know this question will come up, so please give me your best shot
at it.

(James Bauer): Well. I don't know. Peter.

Peter Thomas: Okay, well I'll babble at little bit then you should,
anyway...

(James Bauer): Babble.

Peter Thomas: Some completely different view. In terms -- okay, it does
seem to be icy so at least it's not, you know, a captured asteroid
probably. Certainly it might well have formed there. Mind you, I'm a
surfaces guy, not a dynamicist or an origin guy.

It may well have been battered from something larger, but that still
doesn't answer the question of where it came from. It's a small guy in
between two big guys which, I guess, unless the dynamicists tell you,
you know, it'll be sort of like an asteroid belt, as nothing big can
form there, but something small might form there.

I don't really know. It's -- I guess my prejudice might be that it is
some slightly odd ball version of something that formed with the Saturn
System.

(Richard): Right. It doesn't have a highly inclined orbit or highly
elliptical orbit around Saturn, right? Is it more of a circular orbit
around Saturn or?

Peter Thomas: It's sort of modestly eccentric. And it's certainly not
totally circular and part of the reason for its chaotic motion is in
fact, it does have some eccentricity. But it's not, you know, it's not
like Phoebe or something and, you know, a completely retrograde orbit or
anything like that.

So it's slightly odd, but not in my amateur dynamicist viewpoint,
sufficiently eye catching to require that it be captured.

(Richard): Right. Okay.

Peter Thomas: The key words there, amateur dynamicist.

(Richard): Of course, very much agree with what Peter said. And,
actually I have a question for Peter and that is in terms of the
chaotic, the source of the chaotic motion of Hyperion. I'm not a
dynamicist and Peter, you know...

Peter Thomas: I, at least, inhabit a building that has dynamicists in
it.

(Richard): There you go. There was the theory that its chaotic motion
might partly be attributable to its interaction with Titan or a
residence. It is in the three to four residence or four to three
residence with Titan, if I'm not mistaken. Is that -- do you know any
more recent theories in terms of or modifications to that theory?

Peter Thomas: I think that's sort of what I recall in the sense that the
interactions with Titan will prevent you if it's -- well I guess there's
several requirements. One, that it be somewhat slow rotator and two,
have these varying torts from Titan. And that's where the eccentricity
comes in, I think.

The inclination is I just looked up is very low. It has an eccentricity
of only .1, so it has some, which is just enough, I think to help deal
with the chaotic rotation.

But, I don't remember my dynamics well enough on when I was working with
(Greg Black) on actually measuring this of all the input to it. I mean,
the chaos -- we see this thing sort of spinning around nearly its long
axis. And I guess (Greg Black) and others in modeling this have
determined that that kind of activity can go on for a long time, like
thousand of years.

You can't predict exactly where it's going to be pointing or its rate,
or where the spin axis is in the body. But given the various torques
from Saturn and Titan and things, I think he was even worried, you know,
worried about Saturn's, you know, obliqueness.

You know, you will get a chaotic, since you can't predict it. But the
same kind of spin will go on -- you know, can go on and on and on, and
suddenly it will shift to something else. And so it doesn't boil to the
influence on something that's rotating slowly. All these, you know, very
minor torques that I guess can build up and cancel out, should leave you
the same kind of activity for a long while, and then flip you.

And I'm an amateur.

(Samantha Harvey): And I have a question as well. This is (Samantha
Harvey) again. I have a very general question I believe, it's
compositional. So, it's about porosity. It's effect on craters. Would
you say that it would be mostly water ice and organics that was in these
pores between the craters, and in that affect, would it be spreading
them? I'm just really confused about that concept and if you could
clarify that it would be awesome.

(James Bauer): Well okay. So the dark material...

(Samantha Harvey): Yes.

(James Bauer): ...is primarily within the craters. And that is -- that
has a large organic component. So -- and not so -- not as much water ice
is in the dark material. Like I said, it was something like 15%.

(Samantha Harvey): Right. Right. Right. Right. Okay.

(James Bauer): The models -- if we look at the models, and we take this
kind of toy dark material that we try to fit to it, we -- that ends up
comprising, you know, 7 -- on the order of 70% of the material in the
dark.

(Samantha Harvey): Okay. That makes a lot of sense. I just wanted to
make some clarification on that, because -- but would it cause the
craters to spread at all, or would they remain in similar states for
many, many years?

(James Bauer): So -- well -- okay that would more or less kind of have
to do with the sun cup theory. And Peter did you want to...

Peter Thomas: Okay. Yeah. Let me just back up a little bit here and....

(Samantha Harvey): Thank you.

Peter Thomas: Something that comes up frequently in terms of when we're
speaking about dealing with a porous object. Okay. Pore space. You know,
there -- the stuff in the pore space is vacuumed and we're also
calculating the pore space relative to the whole body, that's the mean
density, you know, of this thing that's 300 kilometers long.

When we're talking about the dark materials and all the other things
which, you know, which we're seeing, are very much surface phenomena.

(Samantha Harvey): Okay.

Peter Harvey: And so one, in terms of making craters and things like
that, the pore -- the porosity we're talking about is, you know, the
open spaces which may be on microscopic scale, but throughout the body
and that gives its low density.

We -- what the actual density of say, the layer of the dark material at
the bottom of craters is, you know, it might have very low porosity and
be pretty much the density of whatever those organics or the other
components with the dark material is. But that's a trivial part of the
volume of the whole object.

So that wouldn't affect the determination of Ross. Overall, you have
this thing which is probably mostly water ice, with a few other things
mixed in the body and a lot of porous space and if that mechanics of
dealing with the pore space that probably give you the shape of the
craters and not much stuff flying around.

But down on the surface the things that, you know, we're seeing in terms
of the (abitos), the different compositions and things, that's a
reflection of surface processes which may be in only a few meters deep.
And, you know, it may be concentrating stuff that so everything we see
morphologically here sort of says that once you make an impact crater on
this object, it doesn't erode very much. You know, it doesn't expand. It
doesn't get buried very much.

(Samantha Harvey): Okay.

Peter Thomas: It's clear from, you know, the dark material and, you
know, the compositional variation that we've heard about that there are
these processes going on that in some areas, you know, will allow you to
see different components of what's mixed in with the material
undoubtedly of depth and then at the surface both by things moving on
the surface or solar radiation or particle ration and other things, you
know, allow you to see these sort of sorted out other components.

But how that relates to things of real depth, we can only guess.

(Samantha Harvey): Thank you so much. I really appreciate it.

(Matthew Odle): This is (Matthew) with the final question.

(Amanda Hendrix): Go ahead.

(Matthew Odle): Being that Hyperion tumbles chaotically, eventually
there's going being to be a map made of this satellite and how do you
determine what's -- where the poles are going to be and where the
equator is?

Peter Thomas: Great question, because we ask this occasionally and
cartographers have their standard and things. For a satellite that's
asynchronous rotation, the usual thing is that, you know, it has a
stable spin pole and that tells you where north is and tells you where
the equator is and the prime meridian is stuck on the part that faces
the primary.

For guys for this, the solution is probably going to be that will pick a
-- since the spin pole while at least while it's been seen, has sort of
wondered around a limited area of the satellite for the long end, I
suspect when we get around to this, what we're going to do is we're
going to pick one particular time, and it's probably the time of closet
approach of Cassini in that fly by, when we have the best data. Okay.

Pick the spin pole at that. That gives you, you know, a north. It gives
you an equator. And then you just pick one particular feature on the
crater and call that a zero longitude and this is what was done on many
other things that were on asteroids.

Or you calculate the moments of inertia of this object assuming it's
homogeneous and then you put poles along the principal moment actually.
It's not clear which way we're going to try at. The day they've been out
there, I guess on the imaging team, are probably the ones that are in
the best position to do this.

I should have done this a long time ago, but there's too many other
things. But, it's a great question because it does come up periodically
and when are we going to get to it?

That's -- actually when are we going to get to it, is probably the best
question...

(Matthew Odle): And then just an observation just from an amateur
astronomer's viewpoint. If you set up a telescope on Hyperion, it seems
to me that you have a devil of a time getting into some kind of a polar
liner for tracking, you would have to use basically a Dophsonian or
something because the sky would be basically viewed in chaotic
motion\...

Peter Thomas: Yup.

(James Bauer): Yes.

Peter Thomas: Yeah. Your latitude changes. You stay in the same place
and your latitude effectively changes because the pole moves within the
object.

(Matthew Odle): It's quite a crazy place to try to set up for an
outreach, that's for sure.

Peter Thomas: Yeah. To say nothing of the low temperature.

(Matthew Odle): Right. Thank you.

(Amanda Hendrix): (Unintelligible). Okay. Well let's wrap it up then.
And I'd thank our speakers, Dr. Thomas and Dr. Bauer again very much for
a really interesting talk.

Peter Thomas: Thank you.

(Amanda Hendrix): And I'd also like to invite everyone to join us for
the next term telecom which is going to happen on September 25th and
that is when we will hear about results from the RSS or the Radio
Science Subsystem on Saturn.

So we will talk to you all then. Thanks again everybody.

(James Bauer): Okay.

Peter Thomas: Okay. Thanks.

(Amanda Hendrix): Bye.

Peter Thomas: Thanks.

(James Bauer): Thanks

END