**NASA**

**Museum Alliance**

**\
Cassini**

**June 15, 2004**

**2:30 p.m. CDT**

Overview

Dr. Bonnie Buratti of JPL discusses Saturn's icy moons, particularly
Iapetus and Enceladus, as well as preliminary results from the June 11,
2004 flyby of Phoebe. A little about Titan, moonlets in the rings, and
other things.

Coordinator All participants, your call is about to begin. At the
request of NASA, this call is being recorded. If you have any
objections, you may disconnect at this time. Ms. Jones, you may begin.

Jane Thank you. Welcome, everybody. This is starting our third month of
Cassini telecons for the Museum Alliance. Our speaker today is Dr.
Bonnie Buratti. Dr. Buratti became interested in science in the third
grade. She has worked, as she talked about a few minutes ago, at a
planetarium, the Strasenburgh Planetarium in Rochester, New York in the
70's. She worked at the Maria Mitchell Observatory on Nantucket Island,
and then spent some time in graduate school at Cornell University, where
she received her PhD in Astronomy and Space Sciences. After that, she
came to JPL on a Fellowship funded by the National Research Council in
1983 and became a permanent employee here in 1985. She's been a science
team member or principal investigator for various research programs at
NASA, and has published dozens of papers. She has taught Astronomy,
served as a mentor, done all sorts of wonderful things.

What I would like to do now is turn it over to Dr. Bonnie Buratti.

Dr. Buratti Thank you, Jane. Thank you so much. I want to point out that
I actually have some old colleagues on the line. Actually, the director
of Strasenburgh Planetarium, who I have never met, but he has signed on
and one of my old colleagues from graduate school, Dr. Steven Lee, who
is now working at the Denver Science Museum, which had kind of legendary
status when I was at Strasenburgh Planetarium. We thought we were pretty
good, but the one in Denver was the very best in the world. I'm pleased
that Dr. Lee is also signing on and I just want to say, "Hi," to all my
friends. A lot of people from JPL are also signing on.

Jane Everybody is dying to hear what you have to say about what's been
going on with Cassini.

Dr. Buratti Shall I just give a little introduction and then open it to
questions?

Jane Yes, you have up to an hour, actually, even more than that if you
need it; but I thought you might like to talk a little bit about your
research, what interests you in Cassini, and what you're doing with
VIMS. Whatever you want to talk about it, I think our audience would
just be thrilled to hear it.

Dr. Buratti First of all, I'm not sure about the backgrounds of all the
people here, so maybe I'll just go over some general stuff about the
Cassini spacecraft and why this mission is important. Cassini is kind of
the Cadillac of the planetary missions. It's really one of a kind. It's
the most complicated thing that NASA has ever done. There are 12
instruments on the orbiter and then, of course, there is a probe that is
going to go into Titan's atmosphere. It's going to be entering the
atmosphere in early 2005 and land on the surface and hopes to take some
data on the surface. There are also about 10 or 12 instruments on that
probe.

We're going to be exploring the system for a total of four years, at
least. We recently got permission from NASA to go to two more years, so
probably we'll be in orbit for at least six years. We'll be looking at
the satellites of Saturn, the rings, and of course, the planet itself,
as well as the magnetosphere and the particle environment around Saturn
and how it interacts with the magnetic field, the Sun, and the solar
wind.

My particular interest, of course, is satellites. I've been studying
them, I guess, since I was a graduate student at Cornell University. Let
me talk a little bit about some of the things we're trying to do.
There's a whole family of satellites around Saturn and none of them are
really at all alike. I'd say my two favorite, at least until the Phoebe
flyby, were Iapetus and Enceladus.

If you look at Iapetus, in fact, if you're a science fiction reader, you
probably know that Iapetus played a key role in the book, *2001*, and in
the movie, as well. It's a satellite that on one side, it's very bright.
It's like ice, but on the other side, it's very dark. It reflects only
about 2% of radiation that falls on it, that's as dark as carbon. If you
look at a charcoal briquette, it has about the same brightness.
Scientists don't really understand how it got that way. It turns out
that the dark side of Iapetus, the low albedo side, is the half that
leads the motion in its orbit around Saturn. This is called the leading
side, so it looks like it's sweeping up some dust, some dark dust from
perhaps the outer part of the Saturnian System.

One of the big questions that Cassini wants to answer is how did half of
Iapetus get to be so dark. We don't know if it swept up dust. That's
kind of the theory I believe, or if it erupted, there was some kind of a
giant lava eruption onto the surface of Iapetus. That's the other
competing theory.

The other satellite that is my favorite is Enceladus. This is a
satellite that is brighter, actually, than just about anything we know
of. It's even brighter than freshly fallen snow. The only thing we can
think of, as to how Enceladus got that bright, is that it is
geologically active, that there is some kind of geysering process going
on, actively occurring on it. The thing that actually gives us further
evidence for that is that Enceladus is in the middle of a ring, a
planetary ring of Saturn, called the E Ring, and unlike the rings around
Saturn that we all know and love, the bright beautiful rings that are
thin and very visible with the eye if you look through a telescope, this
E Ring is very faint and bloated. We think it came from the E Ring, so
this is evidence that there is some kind of activity on it.

Finally, I should say something about Phoebe because this is the object
that we just got such a close view of on Saturday. The flyby was on
Friday, local California time, but we didn't get the data until
Saturday. This object, actually, there was an interesting model that was
first proposed by Steve Soder, that it was actually material from Phoebe
that was chipped off by micrometeorites, by meteorites coming in and
hitting the surface. These particles would spiral in and hit the leading
side of Iapetus and that was a model for how Iapetus got its dark
hemisphere. It doesn't exactly work though because it turns out that the
dark side of Iapetus is reddish in color, whereas Phoebe is grayish in
color, but at least it's a good model.

The thing that we didn't expect or that we weren't sure about on Phoebe
was the battered surface. It looks like it had a very violent past. We
knew that it had an unusual orbit. One thing that I really didn't
appreciate until this encounter is that it's really far out, outside the
main satellite system of Saturn. In fact, it's about 12 million
kilometers -- that's about 8 million miles -- and it's almost a tenth of
an astronomical unit, which is the distance between the Earth and the
Sun. It's really far outside the system. It's got a very unusual orbit.
It's moving around Saturn in a direction opposite to the sense of
rotation of Saturn, and also the direction that most of the other
satellites, and all of the main satellites, are moving. This is a
so-called retrograde motion.

We knew it was fairly dark. It reflected about 6% to 7% of the radiation
that fell on it. We didn't realize it was so irregular because prior to
the Cassini flyby, the best images we had, which were taken about 2
million miles away, it looked like it was fairly circular. We saw one
bright spot on it, but it looked pretty circular, but we see a fairly
irregular object that is really covered in craters. We know it's got
water ice. We knew that from Earth, and the team I'm working with,
remember, the Visual and Infrared Mapping Spectrometer team, the VIMS
team, we're looking at the detailed composition of Phoebe.

One of the things we're interested in is whether there are organic
molecules. One of the interesting things about Cassini, one of the
goals, is to look at the kind of conditions that we believe that life
arises in. The object that is probably most appropriate to that is
Titan. This is the giant satellite, the largest satellite of Saturn, and
it has an atmosphere that actually the surface pressure is higher than
the Earth's surface pressure. It has like one and some fraction in
atmosphere.

The atmosphere is made up primarily of methane. Titan is, one scientist
called it "an Earth in deep freeze." It really looks like it could be a
laboratory for the origins of life. Now, we don't expect to see life on
Titan, but we have already discovered some organic molecules, what we
call prebiotic materials. These are molecules that are rich in carbon,
hydrogen, oxygen, and nitrogen, the building bocks of life. We believe
that life on Earth arose from these types of molecules and that they
were transported inward from the Outer Solar System, not just the
Saturnian System, but from outwards, where it's easier for these
complicated molecules to form because the temperatures are lower, and
they were brought in possibly through comets -- we are certain that
comets are rich in biotic materials -- into the Inner Solar System. The
other thing that came into the Inner Solar System was a lot of water.
Scientists believe that some of our oceans and the water in our
atmosphere came from comets.

One more thing I wanted to mention about Phoebe that is very
interesting, it looks like it came from the outer regions of the Solar
System. In fact, the model that seems to be the working one is that it
actually came from the Kuiper Belt. The Kuiper Belt is an asteroid belt
in the Outer Solar System, beyond the orbit of Neptune and Pluto.
Starting in 1992, a series of asteroidal-sized bodies were being
discovered and there are several hundred that are there now, but we
believe there are millions out there. Occasionally, these bodies will be
perturbed by Neptune and will come into the Inner Solar System. We
believe that the gravitational field of Saturn actually captured Phoebe
as it maintained its journey into the inner parts of the Solar System.
That's why it has this unusual orbit and it's actually far out from the
main system.

My main interest is looking at the surface properties and the
composition of the satellites and that's what I will be working on
mainly for the next four or five years. It's really interesting. We've
had a very interesting last couple of days with this Phoebe encounter.
It's almost an embarrassment of riches. We really obtained a lot more
than we expected and one thing that really pleased us is that the
spacecraft pointed at Phoebe so well. As you all are aware, it's
difficult to navigate the spacecraft across the Solar System, come to
this planet that is ten astronomical units away from the Sun, and just
point at this object. It's kind of trying to shoot a hole in one from
across the country. I think that analogy has been made, but I think that
analogy was made for Mars. Think of how much harder it is to get that
hole in one for Saturn. It's so much farther out.

I don't want to talk that much any longer. I'd like to open the floor
for questions that people may have about any aspect of the mission, not
just satellites, which is my specialty, but any questions.

Pete Hello, this is Pete from San Francisco. I was particularly struck
with some of the craters that looked like ant farm traps. What do you
suppose produced that morphology?

Dr. Buratti It looks like -- I have to warn you, I'm not a geologist,
I'm not a photogeologist, so I'm not an expert in this area -- but it
looked to me like this was primarily due to what geologists call mass
slumping. I think that gives us a lot of evidence that there's probably
a fairly tenuous regolith. You'll notice, though, that it only seemed to
occur on the big craters. On some of the smaller craters, they were kind
of bowl shaped, but I think what happened is that after you had a crater
forming, a lot of the regolith material and perhaps some of the material
that was pulverized by the impact, gradually over time, just by the
gravitational force of Phoebe, just fell into the crater to form this,
as you said, it was an ant cone or an ant hill. It's just basically the
force of gravity pulling the loose material from the surface. I think I
used the word regolith. That's just the soil, the loose top of the
satellite, just got gradually pulled in by gravity into the crater.

Pete So, there may be voids in this body?

Dr. Buratti It doesn't look like there are voids. I think that what you
are referring to is by getting a number on the density -- this is
actually a very important number that the radio science team was able to
get -- by getting a number on the bulk density, you can understand not
only the bulk composition, but whether or not it's a so-called rubble
pile. In other words, is it a solid body or is it a conglomerate of
rocks, large boulders. It looks like, and these results are preliminary,
I'm not even sure that JPL has announced them yet, but the rubble pile
hypothesis doesn't look like it holds here. It looks like it is a solid
body.

I think, what I'm talking about is the outer surface, just the
uppermost, maybe few hundreds of meters, the so-called regolith, this
very loose soil of material that has been pulverized by eons of
meteorites coming in and crushing the surface.

M Bonnie, do you have a number we can give people about the strength of
gravity at the surface of Phoebe?

Dr. Buratti I'd have to do a calculation. I don't know it off the top of
my head. I'd actually have to just do a calculation. If you send me an
e-mail, I can do the calculation and e-mail it to you. I just don't know
what it is off the top of my head. It's a function of the mass and the
radius. Now, we do have mass, but see that isn't made public. I might
not be able to do it right away.

M That was my other question. If you had a mass, I could do the.

Dr. Buratti I don't think it's been made public yet by JPL, but if you
know how to plug it into the equation for the "little g," the
gravitational force, you can take the mass, or if it's in the form of a
density and you know the size -- see, that's the other thing. We have to
recalculate the size, also, but I think that JPL does have a mass. It
just hasn't released the numbers, but as soon as you get that, you can
plug it into that equation and get the "little g."

M Good, because we see the pictures of what looks like stuff sliding
down hill, so people want to imagine, "What did it look like when it was
sliding? How fast did it slide?" That kind of thing.

Dr. Buratti Right, right.

M I have a question. I've often heard it stated that when a body, a
satellite, has a retrograde orbit that it's likely a captured object. Is
that purely a 50/50 chance? Are captured objects more likely to orbit in
retrograde?

Dr. Buratti Yes.

M Or is it really more of a 50/50 chance they are going to?

Dr. Buratti Okay, it's easier to capture them. Actually, most objects in
the Solar System, whether they're asteroids or objects in the Kuiper
Belt or satellites, are in the so-called prograde direction. We believe
that was the direction that the solar nebulae, there was a spin
direction in the solar nebulae, and that is the material from which the
planets and the satellites all formed. That's why everything tends to
have the same rotation, but there are objects that are retrograde that
formed in some part of the solar nebulae that wasn't rotating in the
prograde direction, but those objects are easier to capture because it
takes less energy to capture them. If you think about it, they're going
in the opposite direction of the rotation rate of, for example, Saturn
around the Sun. There is just less energy. The relative velocities are
not as high as something that is moving fast, past an object, very
rapidly. It's just going to take more energy to capture than a
retrograde object, which has a smaller relative velocity. Did I answer
your question?

M Yes, so it's likely that a captured object that's in retrograde orbit
had a retrograde orbit to the entire Solar System anyway.

Dr. Buratti Exactly, it was like that from the very beginning.

M Okay, and we actually know the rotation of the Kuiper Belt and that's
prograde. Is that correct?

Dr. Buratti Most of the objects are prograde. We believe they are. Yes,
they are prograde. We can see the objects orbit around the Sun.

M What about that Oort Cloud? Do we know the direction of that stuff?

Dr. Buratti Good question. I'm trying to think. Comets that come from
the Oort cloud, we have measured the rotation rates of some of them and
most of them are also prograde and the direction around the Solar System
are, for the most part, prograde, but that's a very hard question to
answer because there are billions of comets out there and we've only
really seen a few hundred of them, but for the most part, we believe
that also is, for the most part, prograde.

Carol Bonnie, this is Carol Lutsinger in Brownsville, Texas. I'm so
excited to be on here today. I see that you got interested in science as
a third grader and I'm an elementary teacher. I'm just getting ready to
go back into the classroom after having been a science and math mentor
teacher for four years. I'm going to be working with fifth graders. Was
it your teacher who got you interested, or was it something that you did
at school, or your parents encouraged it? What intrigued you in third
grade?

Dr. Buratti It really came from within. I think that it is possible to
interest children in science in the classroom and that's a good thing to
do. I was brought up in another age. I mean, we're talking about the
50's and early 60's, so girls were not encouraged. In fact, I was
ridiculed about it, so I kept it secret. I was very disappointed when I
would turn on the TV and watch these landings, the lunar landings and
the Mariners, and you would see a roomful of men. It was all these men
in white shirts with crew cuts and they were all sitting in mission
control. There was not a single woman. I looked at that and I got really
discouraged and, "There's no place for me." Somehow, I managed to press
on. I was really interested in it. My parents were mildly encouraging.
They did get me a telescope when I was in third grade or whatever, but I
didn't get any encouragement in school. At that time, you were really
ridiculed. You were different and nobody wants to be different.

I think the times have changed and I think now that girls and boys can
both be encouraged to go into science and I think that it's important to
instill that in the classroom. I think the most important thing is to
show enthusiasm and just to try to instill that in the students.

M I've got a question about the width of Saturn's rings. In the DVD,
*Journey Through the Rings*, it says that the widths of the rings are
only on the order of hundreds of miles, as compared to thousands of
miles wide. Is that correct?

Dr. Buratti Now, are you talking about the width or the height?

M I guess height, edge on height.

Dr. Buratti Yes, the edge on height is small and the reason for that is,
you know how spinning objects like to collapse down? If you've ever had
these, I think a lot of science stores and museums have them, these
little spinner toys that you spin up and then pull a string. It's the
same sort of thing where rotating objects like to relax down to the
equator, let's just put it that way, not to be too technical about it,
and this is the same sort of thing. The rings have angular momentum. I
don't want to use a technical term, but that's a technical term.
Particles like to relax down to the equator. You can intuitively see
that by playing with one of these toys.

I said already that there's this ring called the E Ring that is the
exception, where a lot of the particles aren't in Saturn's equator and
the reason for that is because we believe that these are particles that
have been recently expelled from Enceladus by some process, either
volcanic or geysers or possibly just meteorites coming in and hitting
the satellite. These particles haven't had time to relax, as it were,
down to the equatorial plane of Saturn.

M How does a geyser work when it's that cold? Does it emit dry ice or
something, or how does that work?

Dr. Buratti Yes, it would probably not be water. It would probably be
some type of water, maybe, mixture of ammonia or methane with water, or
even CO~2~. We just don't know. There are several theories. One of them
is that Enceladus is heated by tidal forces from Saturn. Of course,
that's how Io, the large satellite of Jupiter, which has many volcanoes,
large volcanoes, that is tidal energy from Jupiter. There could be a
similar process.

The other thing is there are various models, for example, subsurface
greenhouse effects that could occur on the satellite that cause local
areas of warming and cause geysering. There really isn't a good model. A
lot of it is hand waving.

M I have a question on the grooves that I see in the images. Some of
these grooves look as if a very large boulder was just rolling along,
but seems that a large number of the grooves also begin and end with a
hole. Is that not just my imagination?

Dr. Buratti No, in fact, you're right. It does look like there are
stress fractures from the creation of some of the larger craters. One of
the most interesting things, and we saw this actually on Phobos, which
is one of the satellites of Mars, that when you get a large crater
forming, and there's this large crater on Phobos called Stickney, that
if you look at the direction opposite to that crater, you also see
grooves. Basically what happens is it stresses the whole satellite and
you have focusing of that stress on the opposite side of the crater and
get grooves, as well. You can get grooves coming out of the crater, just
as stress fractures, and you can also get grooves on the other side. I
don't think anyone has looked yet at the pattern of grooves on Phoebe to
try to understand exactly where they're coming from, but yes, there are
some that, as you say, were formed by boulders that are traveling over
this very loose soil that we believe is on the surface, and then some
that appear to come right out of craters.

Emily This is Emily Lakdawalla from the Planetary Society. We've seen
all the fabulous images from the camera for Phoebe, but I was wondering
if you could share anything with us about what VIMS might have seen at
Phoebe, since that's your instrument.

Dr. Buratti I can say things very generally. The problem is we have not
released, and I know you are from the Planetary Society, so this is
especially true, we haven't released our findings to the press yet. I
will just talk in very general terms. We did see ice, which we knew from
ground base stuff and we're looking now for other volatiles, but as soon
as it is released to the press, which is probably going to be tomorrow,
as soon as JPL releases the information, we can be a little bit more
specific about it.

Let me tell you what we were looking for. We were looking for organics,
dark material, minerals, and other volatiles, things other than water
ice. We found some of them, but tomorrow, we'll be able to say
specifically what they are.

Jane I'm sure you all know where to go for press releases, but right on
our Cassini main page, every time there's a new image or a press
release, they're posted to the front page of our Web site,
www.saturn.jpl.nasa.gov, so as soon as something like that is released,
it will be right there on the front page.

Dr. Buratti Yes, I think an answer to the previous question is by
tomorrow, there should be a press release with that information in it.

Kathy Hello, Bonnie, I had a question. Kathy Miles from Virginia. I
recently read about a theory, obviously Phoebe is battered with a lot of
craters, that Phoebe might actually be the source of some of the smaller
moons in the vicinity.

Dr. Buratti Yes.

Kathy Is there any evidence for that and how would you tell, or would
there be a way to tell, whether they actually came from Phoebe or were
just other objects?

Dr. Buratti This is a question right up my alley. In fact, this I can
speak openly about because I've presented it at meetings and stuff.
There are two basic ways you could tell. Just as background for other
listeners, there is a whole family of satellites. There are 13 recently
discovered satellites of the Saturnian System. There are 31 satellites
total, but 13 of these, 12 of which were discovered in the year 2000,
are these small outer satellites, which are exactly at the orbit of
Phoebe and beyond. These are really outer ones.

There's an interesting hypothesis that some of these satellites may have
come from Phoebe. In fact, there were four them. In fact, I kind of
called them the "Phoeboids," -- that word didn't pick up, though -- that
have orbits that are similar to Phoebe. They're also retrograde. They
are about the same distance and eccentricity, that sort of thing. The
question is, especially seeing this large crater, did they come from
Phoebe? My opinion is no, I don't think they came from Phoebe. The
reason for that is that their colors are completely different. If you
look at Phoebe, in the visible, it's very gray. In fact, this is why we
knew it probably wasn't the origin of the dark material on Iapetus, but
if you look at the colors of those satellites, those small satellites,
they tend to be reddish. They just don't match in color. Also, there's
some work by Joe Burns and his colleagues that suggest that they're not
really tied together. Their orbital characteristics are not really
similar enough for them to be associated with each other. If I had to
bet money on it, I would say no. One thing I've learned about science is
you very often just do not know the final answer.

W Will Cassini get a chance to look at any of those "Phoeboids" close
up?

Dr. Buratti No, unfortunately, they're so far out that we don't have
anywhere near enough fuel on the spacecraft, or time because it would
take a long time to crank out there. We're not going to be able to look
at them. The best bet to study these objects is actually from Earth,
from things like large telescopes like Keck and Hubble Space Telescope
and even the Palomar Telescope, the 200-inch. No, we won't get a view of
any of those satellites.

W Is there any evidence that any of the other satellites are also
captured objects?

Dr. Buratti Other than the small satellites that are the outer small
satellites of Saturn, we don't believe that any of the others are
captured. The reason for that is because they are in these kind of very
regular orbits. They're all prograde and they just look like they formed
in place, but some of them have been altered. For example, there's
another fascinating satellite that I forgot to mention called Hyperion
and this is the only satellite we know of that is in what's known as
chaotic rotation.

Most satellites have what we call simple rotations. Phoebe is in simple
rotation, meaning that it rotates on its axis every approximately nine
hours. Other satellites, like the Earth's moon, are in what's known as
synchronous rotation, meaning that they keep the same face toward their
primary.

There is a third rotational state called chaotic rotation. Basically,
the only way I can describe it is the satellite can't figure out what
rotation period it has. It kind of just hops between various periods in
this chaotic fashion. Hyperion, it was believed it suffered some kind of
a collision in the past. In fact, it's another object, it's about the
same size of Phoebe, but it's even more irregular. We got a much better
picture of Hyperion from the Voyager mission. It's irregular and has a
lot of craters on it. It looks like it had a violent past and it's in
this odd state of chaotic rotation, the only such satellite known in the
Solar System that exhibits this state.

One thing I did forget to mention is we expect that Cassini is going to
find more satellites. Right now, we know of 31 in the system, but
Cassini will be looking for more satellites and will probably find some.

M What is the rotation period of Saturn and its relation to the other
gas giants in our Solar System? Does it revolve around a lot faster?

Dr. Buratti It's about ten hours -- I have a little table here in front
of me -- about 10.6 hours and Jupiter is about ten hours, so it's about
the same. Uranus and Neptune are about 17 and 16 hours. The gas giants
tend to have more rapid rotation than the terrestrial planets. Mercury,
of course, is in some kind of odd state. It doesn't keep its same face
toward the Sun, but it makes three rotations for every orbit around the
Sun. It's an odd state. Venus is also in an odd state. The Earth and
Mars have about 24-hour rotation periods.

It seems that the planets, if they haven't evolved by some interaction
with the Sun, have two basic rotation rates. The "terrestrial planets"
have about 24 hours and then the gas giants have about 10 to 20 hour
rotation rates.

W I've been looking at pictures of Saturn's satellites and it's tempting
to look at Dione and say, "That looks like Jupiter's moon, Ganymede,"
and it's tempting to look at Enceladus and say, "That looks like another
one of Jupiter's moons." Is it safe to make comparisons with Jupiter's
moons or are they really different at Saturn?

Dr. Buratti The thing that we're learning about the moons is it seems
that each one is a unique and different world. Phoebe is unlike anything
we've seen. I think one of the earlier callers said, "What about those
craters? They look like ant mounds or ant holes," I forget what the term
was.

M I meant ant lion traps, if you are familiar with those.
\[http://www.wcsscience.com/antlion/page.html\]

Dr. Buratti Actually, I'm not, but they're just like conical shapes. We
haven't seen that on any other satellite. It just seems, so far, they
are all so different. I know that, for example, Ganymede has a so-called
grooved terrain, but the wispy streaks on Dione, we expect the
composition of Dione to be different. There's probably some ammonia, a
lot of ammonia ice, as well as just water ice there. It's colder out
there. There's just a different type of geology happening. I wouldn't be
surprised if it's completely unique. We've been tripped up by analogies
before. That's what I'm saying.

W Bonnie, are there any plans to, in particular, study Mimas? I was
reading a few things about that, in particular, the one really huge
crater that's like 80 miles, like a third of the moon had hit it and it
was compared that it had the highest crater walls in the Solar System,
of all the moons.

Dr. Buratti Actually, it turns out Mimas, unfortunately, is one of the
satellites that we could not get a targeted flyby of. We have seven more
targeted flybys. Targeted means you actually maneuver the spacecraft to
come to swoop in for a close view of the satellite. Mimas, we do not
have any targeted flybys, but we have several flybys that are much
better than the Voyager closest approach.

The Voyager closest approach was -- I'm going to round my numbers off
because I just don't remember exactly -- around 100,000 kilometers,
maybe, 80,000 kilometers, but we have some close-ups, some untargeted
flybys. An untargeted flyby means that the satellite just happens to be
there when you're there, when the Cassini spacecraft is there. We have
four or five that are better and some of them are substantially better,
four or five times closer than the Voyager flyby.

You're right. There is a huge a crater on Mimas, and in fact, that
crater was just called Herschel, named after the British astronomer who
found Uranus. In fact, he's kind of the forefather of modern astronomy.
That crater, the energy from that impact crater was almost large enough
to blow the planet apart. It's just at that borderline where it could
have blown it apart and become a beautiful ring. We believe that the
rings are formed out of collisions of satellites' impacts. The rings are
not very old. They're only tens of millions of years old. Theoretical
calculations show that rings don't last forever, so there has to be a
constant supply of ring material to create new rings and that's one of
the theories, that you have satellites that just get impacted by an
object that's large enough to blow it apart.

W I'm just wondering what would cause such unusually high crater walls?
They're higher than Mount Everest. Isn't that unusually high?

Dr. Buratti There are a couple of things. First of all, the small
gravity. The more gravity it has, the more spherical it becomes. If you
have a crater or a mountain -- this is why, for example, on Mars you see
volcanoes, Olympus Mons, for example, that is much larger than any
volcano on Earth. It's taller than Mount Everest and that's because the
gravity on Mars is less than that on the Earth, so it can't pull down
these mountains or craters. It's the same thing on Mimas. It can sustain
more topography because the gravity is less.

Another reason is, we think, that the ice on Mimas and other satellites
is rigid, not only because it's very low temperature, but also because
it has impurities. One of the things that Voyager did not expect to see
was craters. The theoretical indications were that if you made a crater
in ice, it would just slump away, so there was some surprise when the
Voyager spacecraft encountered Jupiter and saw these satellites like
Callisto that have all these craters on them that didn't slump away.
They were still there. The current models are that because you have all
these impurities, because it's low temperature, it's more viscous, and
it's very rigid, these craters just stay. They can be maintained for
many hundreds of millions of years, if not the age of the Solar System.

W I have another question. I read somewhere online that people think
that there might be little bitty moons constantly forming and falling
apart in the rings. The stuff is clumping together. Is that true?

Dr. Buratti It's possible. Whether or not it's true, we can't say
because it's a model that we don't have any evidence for, but it is
plausible. One of the interesting things that I think has come out of
planetary research on small bodies in the last decade is that some small
objects, comets, we think for sure, the satellite Amalthea, which is the
satellite of Jupiter, because it has a density less than one -- that's
the density of water ice, which is about the lightest thing you can
think of out there -- because it is less than one, the only way we can
explain that is that you have void spaces within the satellite. In other
words, you have a rubble pile. You have boulders that are held together
by the self-gravity of the object, but there are void spaces between the
boulders.

We know that objects like this exist, so in the rings, we know that the
rings clump. We've seen the outer rings, the narrow braided ring and the
G Ring of Saturn, do have clumps in them that Voyager has observed and
now Cassini has actually observed it. Those clumps could form a
quasi-satellite, a rubble pile, or almost a sand bank of materials that
you could call a satellite. Sometimes it's hard to tell, what is a ring
particle and what is a satellite? It's just a matter of size. It's not
unreasonable, let me say that. We don't have evidence for it yet, but I
think it's a model that just might work in some cases.

M Regarding Saturn's rings, when Cassini is in orbit, what sort of
resolution will we see of the ring material?

Dr. Buratti Let me do a couple calculations. I don't have my calculator
with me. It will be several tens of meters, let's just put it that way.
Some of the best resolution is going to occur actually right after
Saturn orbit insertion. During the actual insertion of the spacecraft
into orbit around Saturn, we're not going to be observing because we're
just going to focus on getting into orbit. We're firing the engines and
that's all we're going to do, but right after Saturn orbit insertion,
we're going to turn the spacecraft. We're close to the rings, so we're
going to look at the rings. That will actually be some of the best
resolution, much better than Voyager resolution. There is a danger. We
can't get too close because as you get close to the rings, you threaten
the spacecraft with a stray particle that might come and hit it, so it's
a tradeoff between science and safety. It will be tens of meters, I
believe, of spatial resolution.

M Shall we expect to see individual objects?

Dr. Buratti It depends. It depends on what you call an object. As you
know, the satellite Pan was actually found in Voyager images, in the
rings of Saturn, about ten years after the images were taken by Voyager.
We expect to see small satellites within the ring. What you call a ring
particle and what you call a satellite, I guess, is just a matter of
terminology. Yes, we expect to find small satellites right in the rings.

There are indications from Earth-based radar observations that the
particles in the rings are meter size and less, but the occasional large
chunk in the rings may be a satellite or may be a large ring particle.

M Will any orbits be close enough to point the radar at the rings?

Dr. Buratti I don't believe---

Dr. Lopes If you'd like, I can answer that.

Dr. Buratti I don't think there are any.

Dr. Lopes Actually, not scatterometry, but we are doing radiometry of
rings. That is essentially looking at the measure of the brightness,
temperature, and it might give you some indications of ring particle
sizes, assuming a bunch of things. That's not exactly up my alley, but I
know that we have some of those observations.

Dr. Buratti That was actually Dr. Rosaly Lopez from JPL, who is on the
radar team, who is listening in and was able to answer that question.
Thank you, Rosaly.

The answer is yes. There will be some scatterometry.

Dr. Lopes Only radiometry.

Dr. Buratti I'm sorry, radiometry.

Dr. Lopes The scatterometry is the active radar, that is when we
actually transmit and receive an echo back. Radiometry is passive, so
that's all we're going to do is passive radiometry.

M There will be no attempt to, say, shoot the microwaves through the
rings toward the Earth? I guess they wouldn't reach either.

Dr. Lopes No. I actually have heard that they talked of that. That was
long before I joined the team. I'm still fairly new, but it wouldn't
work. We wouldn't be able to get enough signal. I really have to say I
don't know the details. It was before my time.

Dr. Buratti There are going to be some experiments with radio science,
some occultations.

Dr. Lopes Yes, radio science.

Dr. Buratti Radio science is going to shoot, from the same antennae,
through the rings to Earth. By looking at how the radio waves go through
the rings, the team will be able to understand the thickness and the
particle sizes of the rings by looking at how the radio waves are
received at the Earth.

Also, of course, there are going to be occultations of stars. As the
stars go behind the rings, we should be able to tell, also, how thick
the rings are and the sizes of ring particles.

W I have another question about Mimas, if you don't mind. As I
understand it, the diameter of Mimas is around 218 miles and again, as I
understand it, Mimas is responsible pretty much for the Cassini Gap. The
question is, the gap is like 2600 miles. How can Mimas, being that
small, be responsible for a gap that large? The theory that I had heard
was the gravitational effect of the rings amplified Mimas' gravitational
effect, in pushing things out of the way in the gap. I was wondering if
you had any more insight on that.

Dr. Buratti If you look at other gaps in the Solar System, like in the
asteroid belt, they are pretty wide, too. A lot of this is heavily
theoretical and we're not really sure. The answers are based on computer
models, where you take n particles, where n particles is usually much
less than the actual numbers in the ring, so a lot of this is
theoretical. If you look at analogy, and you're right in that the reason
that you have the gap is due to the fact that a satellite like Mimas,
its gravitational pull is preferentially on particles that are in what
are known as resident orbits. In other words, orbits that the particles
happen to line up close to Mimas, much more often than other particles.
The particle might make two orbits for every orbit of Mimas and that's
the basic mechanism.

In terms of the width of it, you pointed out that there is some
gravitational force from the rings themselves, but also, if you look at
gaps, for example, in the asteroid belt, they also tend to be pretty
wide. We do know that these clearing resonances do tend to be pretty
wide for reasons, which aren't really perfectly understood at the
moment. That's all I can say.

M How fast do the rings orbit, and do they all orbit about the same
speed?

Dr. Buratti They're in what's known as Keplerian motion, which means
they have their own individual orbits that follow Kepler's laws. That
means that the ones inside are going faster than the ones on the
outside. Just like in the Solar System, Mercury is orbiting faster. Yes,
they are going at all different speeds. They're basically objects in
their own orbit, around Saturn.

Jane Bonnie, this is Jane. What are you going to be doing at Titan? What
is the VIMS instrument going to be concentrating on during the many
flybys of Titan, the ones you get?

Dr. Buratti We want to find out what the surface composition is like.
One of the things about the VIMS instrument is it's an infrared
instrument so we can actually see through the atmosphere. We'll be able
to see the surface and we want to look at not only the structure of the
surface, but also what ices might be there. Just like, I think, the
caller from the Planetary Society was asking, what materials has VIMS
seen at Phoebe. Because we can look through the thick atmosphere of
Titan, which we can't see through from Earth, we'll be able to see
whether there's water ice, CO~2~, carbon dioxide ice, methane ice,
possibly ammonia.

It looks like, from radar data from the Earth, that it actually has an
ocean, or at least some lakes on the surface. The radar instrument, not
on the spacecraft, but from Earth, Arecibo on Puerto Rico, has observed
glints from what looked like lakes or oceans, so that's one thing we'll
be looking at. We're going to look for hydrocarbons, for this prebiotic
material, the building blocks of life. We're also going to be looking at
hazes in the atmosphere.

We're going to be looking at weather on Titan. Titan has seasons. It
takes about 30 years for Saturn to orbit around the Sun, so it has
seasons, just like the Earth does. Actually, Saturn isn't tilted very
much to the equator. It's only a couple of degrees -- it's actually
three degrees. I'm sorry; it is actually about 26 degrees. I have it
confused with Jupiter. That can't be right. This table I'm looking at, I
think has a mistake in it. It says that it's tilted 26 degrees, but I
don't think that's right. I think it has very little tilt.

M No, that's correct.

W It is 25.33.

Dr. Buratti Is that right? I'm just confused with Jupiter. That has to
be right because we know about the rings, how they tilt back and forth.
Yes, 26-degree or 27-degree tilt, so that's why, from the Earth, we see
the rings edge on. Sometimes we see them open up.

Saturn has seasons and also, Titan has seasons on account of that. We're
actually going to be trying to observe that from the spacecraft
throughout these four years. We're going to be comparing data to what we
see on the Earth and what we've seen from Voyager, to see if it is
getting warmer. There are some indications that it is getting warmer.
The atmosphere is changing. We'll be looking at clouds forming. Maybe
there are weather systems and storm systems. We'll be trying to measure
wind strengths, wind velocities, looking at the composition of the smog
on Titan. Titan, like the Earth, has smog in the upper atmosphere. We'll
be looking at that, and a whole slew of things that we just have not
understood at all before.

W Didn't Hubble take an image of Titan and show what looked like what
would have been a continent or something, like a land mass?

Dr. Buratti Yes, in fact, you can, if you look at the proper spectral
wavelengths, you can actually faintly see the surface from the Earth. In
fact, methane, which is the primary gas in the atmosphere of Titan and
it's also why we can't see through to the surface in the visual, there
is a slight window, an absorption band at 0.89 microns. That's 8900
Angstroms. It's just in the near infrared, just long-ward of the
wavelength that we can see with our eyes.

There were some images that mapped out, as you said, this kind of
continent-shaped object on Titan. We're not sure if it's a continent, or
it's an ocean, or if it's mountain system. We just don't know.

W Can you explain what the methane window is?

Dr. Buratti Yes. What happens is there are areas that the atmosphere is
very absorbing and there are also areas, if you look at the spectrum of
methane, it has characteristic bands, spectral bands, that are a
function of its electronic structure where in the bands that absorb, we
just cannot see through the atmosphere because it is absorbing the
sunlight. If you look outside of those bands, the atmosphere is clear
and then we can see to the surface. It just basically means at certain
wavelengths, the atmosphere is opaque. It's just thick and you can't see
through it, but at other wavelengths, it's clear, like the Earth's
atmosphere and the visible.

M Do you understand the energy budget of Titan? In other words, how do
the hydrocarbons stay liquid?

Dr. Buratti First of all, there is some greenhouse warning on Titan.
There's methane there and it is a weak greenhouse gas, so the surface is
warmer than just an object there without an atmosphere would be. Also, a
lot of these hydrocarbons, like ethane, for example, are believed to be
liquid. They have low freezing temperatures, that we expect to be liquid
at the temperatures that Titan's surface is.

In fact, it's very interesting in that methane is near the so-called
triple point, which is the temperature at which the substance, in this
case it is methane, can exist as a liquid, a solid, and a gas. For
example, on the Earth, water is actually near the triple point, so we
have liquid water. We have snow or ice, and we also have H~2~O vapor in
the atmosphere, water vapor in the atmosphere. Water is near the triple
point of the Earth's temperature, whereas on Titan, methane is near the
triple point. Methane may have the same role of being the core of
weather on Titan, that water -- ice, vapor, and liquid -- has on Earth.
Something like ethane freezes at about the surface temperature of Titan,
so we don't really know until we get there.

M Do you expect tidal heating to be happening on Titan?

Dr. Buratti That's a good question. We're not sure, and the reason we
don't know is because we really don't know what the shape of Titan is.
In fact, there is a very interesting experiment that the radio science
people are doing in which when Titan is closest to and farthest from
Saturn, they are going to measure the bulge. That's hard to do because
it's got an atmosphere, so you have to use radio waves that can go
through the atmosphere. As you know, a tidal bulge is a function of how
far you are away from the primary object, so by measuring how much
larger the bulge of Titan is, when it's farthest away -- I think I used
some lingo. I think I said perihelion, ... chrone and perichrone -- it's
the minimum distance and the maximum distance that Titan is away from
Saturn. By how much that bulge changes, we'll be able to measure what
the tidal effects are.

M How will you be able to tell the difference between liquid methane and
just a very flat surface? How might you be able to tell the difference
between that, or how will you really detect if it's liquid or not?

Dr. Buratti There is a radar experiment, which is going to do altimetry.
If you have a liquid surface, it's going to be completely flat. It's
going to be, I should say, a sphere, but every planetary surface that we
measure, even ones like Europa, the Galilean satellite, which looks like
it has ice flows on it and ice plains and very recent active geology and
melting, the average tilt on it is a few percent, but it should be
fairly easy to show from the radar altimeter whether or not there is an
ocean there.

The other issue is we'll be looking for glints from an ocean, actual
visible glints. We may be lucky enough to see one of those. Of course,
if the probe lands on an ocean, we'll know that. The probe is actually
designed to operate for about a half an hour after it lands, whether it
lands on a solid surface or an ocean.

M Everybody asks that, whenever I do a presentation, if it's okay if the
probe lands in liquid. Okay, thank you.

Dr. Buratti We don't know. I could say it's not going to be as rough as
a terrestrial ocean is, but when you think about the winds on Titan,
some parts of it could be rough. It will be much smoother than the
surface. The radar altimeter will be able to pinpoint that.

M Will the surface be mapped out with the altimeter before the probe is
released, or will that happen after?

Dr. Buratti Parts of it. There are actually going to be, not including
this untargeted flyby, there are going to be two close flybys. They are
called Ta and Tb. There are going to be two close flybys of Titan, prior
to the probe release and we're only going to get a chance to look at a
fraction of the surface close up. We'll see it at some resolution, on
part of the surface, but the altimetry is going to cover only a small
fraction. Hopefully, it will be a representative fraction of the
surface.

W Is one of those close flybys the one on July 2^nd^, right after SOI?

Dr. Buratti No. Actually, that is not all that close. That is an
untargeted flyby of Titan. The Cassini team has dubbed it T0. We have
Ta, which is the first close flyby and Td and then Tc is the probe
release. We're not going to be observing then, but this serendipitous
view of Titan is going to be from about a little bit more 300,000
kilometers closest approach. Just to give you an idea of how close I
mean by close, the targeted flybys are going to get down to less than
1,000 kilometers. This view on T0, which will be July1st -- I think the
data should be available the 2^nd^ or the 3^rd^, down linked on the
2^nd^, available sometime later on in the day -- that will have better
than HST, Hubble Space Telescope, resolution. It will be nothing like
the October flyby on Ta, the close flyby of Titan.

W I see the next thing on the calendar, after the Saturn orbit
insertion, is that Ta encounter in October. Are there going to be any
other views of anything happening in between SOI and Ta, or is it going
to be quiet?

Dr. Buratti I'm not in my office, so I don't have my list in front of
me, but there is going to be a Voyager-class flyby of Dione, I believe,
before that. Rosaly, do you have any of your stuff out now?

Dr. Lopes Yes, I can get that here in a minute.

Dr. Buratti Yes, while we talk, because I think there is a Dione at some
point. It might be toward the end of the year. There's a better than
Voyager-class Dione. I think the resolution is about four times. It's an
untargeted flyby. Yes, the first targeted flyby is Ta and that's in
October.

Dr. Lopes Dione is December of '04, so the 15^th^ of December.

Dr. Buratti Okay, so it's after the Ta.

Dr. Lopes And Mimas. Actually, I don't think those are targeted, are
they?

Dr. Buratti No, no. The first targeted is going to be Ta.

Dr. Lopes Yes, that's right. There's not actually much happening between
SOI and Ta.

Dr. Buratti That will give us time to reduce the data. Some of you
people may be wondering why we don't know off the top of our heads when
these flybys are, but that's because there are so many of them. Other
than the 52 targeted flybys of the satellites and Titan, there are also
about 40 Voyager-class and better flybys. This is so much work to do all
this, we just focus on what we're doing at the moment and things that
happen six months down the road, we're not worried about that.

We have planned them. Let me emphasize that all the activities have been
planned. Everything that we're going to do, we've put into action, but a
lot of the detailed observations haven't been worked out and we are
going to try to observe them, just in time. We're going to do all our
work just in time, so that's why we're not even thinking about Dione in
December right now. Right now we're going to be spending a lot of time
just reducing all that data we got at Phoebe.

Jane Will some of the results from VIMS be available later on the VIMS
Web site?

Dr. Buratti Yes, we are going to post some of them. There's going to be
a press release sometime tomorrow or the next day. It talks about some
of the items, some of the things on the surface that we've been able to
say are there for sure.

Jane I know some things will be on the Cassini Web site or in press
briefings, but I thought maybe there would be even more, from time to
time, on each instrument's web site.

Dr. Buratti Yes, there will be. In fact, I believe you can link to that
-- I don't know it off the top of my head.

Jane It's right on the Cassini Web site under "Spacecraft." Each
instrument's web site is listed there. For those people who are maybe
hungering for or following every single instrument or certain
instruments, you will be able to go to those web sites and that's
already all set up.

Dr. Lopes Yes, and some of them are more updated than others.

Jane Yes, I noticed, Rosaly. Some of them need web masters, I think.

Dr. Buratti Yes, it's all a matter of budget. Some of our budgets are
... Yes, if you're interested in specific things, if you start on the
JPL web page, which is www.jpl.nasa.gov, everybody probably knows that,
and then go to the individual instruments. There are also different
objects and there are a lot of images that you can link to. That's
probably the best place to start. All the press releases, of course, are
there, too. The images are released on the JPL Web page and also, the
Ciclops. That's the ISS team. It is, I think, www.ciclops.org. It's
Ciclops, but there's an "I" instead of a "Y".

Judy Right, because it's Cassini Imaging.

W Can you tell me something about how you try to explain to people, if
you show them a VIMS image? The VIMS instrument is just so cool because
it sees in so many different colors, but it's hard to show that in one
picture.

Dr. Buratti Right, that's difficult. In fact, we just prepared a press
release and we went through the argument as to how we can do that. We
don't actually have a good explanation of that. I think the easiest way
is to say, most people know that color TV is made up of, I guess, it's
red, green, and blue, three images at once. We have 356 images at once,
356 colors. Instead of just three colors, we have 356. This is what is
so sensitive to composition. We can actually detect volatiles, rocks,
minerals, and organics, on the surface because we can look at the
characteristic colors or spectra, if the audience is educated in
science, the spectra of each of these individual components. It's
because we have so many colors.

Jane There's a great description of the instrument on the VIMS web page.
What exactly is a Visual and Infrared Mapping Spectrometer? That helped
me understand it.

Dr. Buratti It's basically like taking images, but you are doing it in
356 colors, all at once.

Judy We have a little more time. Does anybody have more questions?

W Off the ... question, it Titan has seasons, what season is it on Titan
right now?

Dr. Buratti I should know that. Of course, it depends on where you are.
It's like on the Earth, it's winter in some cases and summer in the
other. I think -- don't quote me on this -- I think it's southern
summer. I'm actually not sure. I wasn't going to worry about that until
early October, right before the Ta flyby. It's summer someplace because
of the tilt.

M This is a question about the VIMS instrument. Does it acquire all of
those channels instantaneously?

Dr. Buratti Yes. The way that the VIMS instrument works is it actually
is an imaging spectrometer, but we put together our images with a mirror
system. It's all done with mirrors. Although we obtain the full spectrum
at the same time, all 356 channels, we have this mirror that moves back
and forth, to put an image together just by moving those mirrors. We can
put the image together, and it depends on our integration time, anywhere
from a minute to an hour, depending on how long we want to integrate. In
other words, our exposure time is longer, so we take longer to put the
image together. Yes, the spectral information is gathered all at once.

I might add that the technology is about 25 years old. Now we have much
more advanced imaging spectrometers, but this thing was launched in 1997
and it was actually designed more than 10 years prior to that.

Judy Any further questions?

Dr. Buratti I want to encourage everybody to check out the web site
periodically. I see today we got bumped off by this spectacular image
from the Spirit, from the Mars Exploration Rover in the Gusev Crater. It
was a really spectacular image, but I think that more and more images
from Phoebe and from Saturn, as we get closer, are going to be updated
on the web page. I think looking at that regularly would be a smart
thing to do, a fun thing to do, I should say.

M Could I go back to something that was spoken about very early in the
call? It had to do with the different albedo on the planets that have
one face continually in the direction of orbit. The limit of what I
understood about the difference in the albedo was that it was dark in
the front end and it's usually been attributed to organics. Is there a
situation where you do not attribute that dark color to organics?

Dr. Buratti In other words, you're asking what can you get that's dark
that isn't organic?

M Yes. What would be opposing theories?

Dr. Buratti Phoebe is a good example of something that is dark that
doesn't look like it has a lot of organics. It may have none. We're just
not sure yet. It's what is known as carbonaceous. Basically, it's
composed of carbon and that's very dark, and some comets are also
carbonaceous. The thing that you can distinguish organic materials and
carbonaceous materials are that things with organics tend to be reddish.
If you look at the spectrum of the dark side of Iapetus, it's actually
more red than carbon, so that's the thing that really clinches it for
the organics. Also, there are some compounds in the infrared around
three microns that show characteristic absorption bands of organic
materials.

M Are you looking for, say, double bonds and such things?

Dr. Buratti Yes, exactly. The OH stretch band, that sort of thing.

M For the ones that are pure carbon, as opposed to organic carbon, what
would be the primordial origin of that, super nova outside?

Dr. Buratti No, I don't think you have to have super nova because if you
look at the origin of carbon, it's primarily in the interiors of any
giant star. For a star to be carbon burning, it doesn't have to be big
enough to form a super nova, but you are right in that it is
regenerated. It is at least second generation. In the case of the Sun,
we believe it's a third generation star. That material was formed in the
interiors of other giants. Some of them were super novas and that kind
of spews it out into the universe, into the galaxy, I should say, and
into the cloud of gas and dust from which our Solar System formed.

In terms of just a local Solar System, carbon seems to come primarily
from the Outer Solar System and these polymers seem to like to form in
low temperature environments, these kind of organic polymers. That's why
we think that the organics formed in the Outer Solar System, and then
were transported into the Inner Solar System through comets or other
mechanisms.

M Could you restate, again, what the difference between pure carbon is,
and I guess, organic compounds?

Dr. Buratti A pure carbon is just the element carbon, whereas organic
compounds have other things, specifically, hydrogen, oxygen, and
nitrogen. These are the biological materials that are mixed in. They are
much more complex and they have a redder spectrum in the visible.

M But it is safe to say that if it's dark, it's carbon, either organic
or non-organic.

Dr. Buratti Let's see, is there anything dark that's not carbon? It
depends on what you mean by dark. For example, if you look at the moon,
the moon in the visible has about 9% albedo and it's primarily rock,
some rocks. Some areas of it are like 7% albedo. It all depends on what
you mean by dark.

M I guess, when you have such a contrasting albedo on one satellite.
Those situations, it's always coming back to carbon, organic or carbon
elemental. Is that correct?

Dr. Buratti There are certain areas on the moon that have dark, that are
7% albedo, and other parts of it that are twice that and that's all
rock. You really have to look at specific things and actual numbers.
There's not carbon, that we know of, a lot of carbon, on the moon.

M But in these satellites that are orbiting with one side facing
forward, it's usually a carbon.

Dr. Buratti The only satellite that has that kind of very stark change
in albedo is Iapetus and we're pretty sure it's got to be some type of
carbon, whether it's a complex carbon compound or just pure carbon. Yes,
I think that would be true. There are minerals that are fairly low
albedo, low in reflectivity. That's why I'm hedging here. Scientists are
loath to say never, ever. There are always some exceptions to the rule.
It's almost like language and linguistics. There are all these irregular
verbs. It's the same thing in science. There are all these irregular
situations.

W I understand that the VIMS instrument can actually detect different
isotopes of some elements and I'm wondering if you could say what that
does for you. What are you looking for? What questions can that answer?

Dr. Buratti One of the things that you can tell from isotopic ratios is
whether the atmosphere has escaped. I must say that is kind of on the
edge of what we can do. I don't think we would advertise that until
we're actually able to do it and validate it because to look at isotopic
ratios, you really need to look at very small changes in the wavelength
in which that band occurs, the position of the band. Those changes are
very small so it's kind of right at the edge of what we can look at, but
heavier isotopes of atoms, or molecules in the case of something like
nitrogen or CO~2~, something like that, are less likely to escape from
an atmosphere. You can tell how much of an atmosphere has escaped
through time. If it has more of the heavy element than you would expect,
that means a lot of the atmosphere has escaped. If you have an
atmosphere that is very new, for example, if its being recycled through
volcanoes, you would have more of the light isotope. Basically, you can
tell the history of an atmosphere, the evolution of an atmosphere, by
looking at the isotopic ratios.

W Would you be able to tell anything about the E Ring and whether it
came from Enceladus or not by looking at them?

Dr. Buratti No, it's not anywhere near sensitive. I don't even think
we'll be able to see the E Ring. Even the imaging camera will be able to
barely, barely look at it. No, we won't have anywhere near a signal to
look at an isotopic ratio on the E Ring. It's very, very faint.

M One last question about the rings. When Voyager went by, there were a
handful of unexplainable phenomena with the ring movement, such as the
spokes and the braiding. Are there now hypotheses that most ... are
comfortable with to explain those?

Dr. Buratti Yes. I think the spokes, I think there was a fairly good
model in that they were small dust particles that were, in fact, small
enough that they could get easily electrically charged by the high
energy particles that are in the Saturn environment. In fact, they were
corotating, that is rotating with the planet's magnet field, which is
not the same as the rotational rate of the rings. Remember, we said
earlier that each ring particle was in a Keplerian orbit just following
the laws of motion, Kepler's laws of motion, around Saturn, whereas the
magnetic field is rotating at 10.6 hours with Saturn, so that it
actually looks like it's raised up slightly from the rings and actually
rotating faster. I think that was well defined.

The other thing about the braiding and the kinks in the narrow rings,
that we never understood as well. I think the canonical model on that is
that it is due to gravitational pull by some of the satellites that are
close to Saturn, gravitational resonances and that sort of thing. It may
also be due to the clumpiness in the rings that we talked about earlier.

Jane We've got just a few more minutes and I want to just announce the
next speaker, if everybody is finished with questions. I think we have
another five minutes on the line for any last questions.

Wow. This has been fantastic, just wonderful.

Dr. Buratti Thank you all. This was really a great experience. Thank you
all for listening in and for your interest.

Jane Our next Cassini telecon will be on June 29^th^, which will be a
very exciting time around here with SOI just around the corner. Our
speaker for that talk will be Dr. Linda Spilker. She is the Cassini
Deputy Project Scientist and she is a ring specialist, so she'll have
some information about the excitement of SOI and also, she knows lots
about the rings. She will be our next speaker and I'm sure there are
other telecons that Anita has planned for MER and also, I saw that
there's some interest in a Stardust one, also. Anita will be
communicating with all of you.

I just really want to thank Dr. Buratti for this wonderful Q&A.

Dr. Buratti Thank you for asking me. It was a pleasure.

Jane It was a lot of fun.

Dr. Buratti It was also good to talk to some old friends.

Jane Right. Yes, I have a few old friends on the line, too.

M We can clap by turning our mutes on and off.

Jane Right. Let's all thank Dr. Buratti and then we can all go back to
work. Thank you so much. Good-bye.