**FTS-NASA-VOICE**

**May 31, 2005**

**1:00 p.m. CDT**

Coordinator I'd like to remind all participants today's conference is
being recorded. If anyone has any objections you may disconnect at this
time.

Ms. Ray, you may begin.

T. Ray Thank you. Welcome, everyone, to the CHARM Telecon for May 2005.
We've got a pretty exciting topic here today. For those of you who are
not aware, on the project we have an award. It's not an official award,
but it's an award for the team that has to wait the longest amount of
time to get the smallest amount of data. For the Probe Team it's the
penetrometer. They've waited 20 plus years to get 20 milliseconds of
data and on the orbiter the Radio Science Team has the honor of
receiving this award. They have to wait 25 years to get just roughly 80
hours worth of occultation data in the mission. The first of the
occultations happened just about a month ago and we have Dr. Essam
Marouf with us today from San Jose State, who's going to be talking
about the first radio occultation of both the rings and the atmosphere
of Saturn. We're very happy to have him.

Dr. Marouf, go ahead and take it away.

Dr. Marouf Thank you, Trina.

First I apologize. My colleague Dick French was supposed to join me
today, but he felt ill over the weekend, so I'm doing both of our parts
here. Please feel free to interrupt any minute and ask questions. I've
structured the talk so that the first roughly one-third of it is more or
less an overview for what occultation's are and what do we do and the
basic concepts involved so that when I present some of the results from
our May 2^nd^ occultation then it's covered in context, so to speak. So,
the second part will be essentially results from the ring occultation
experiment and then the third part will be results from the atmospheric
and ionospheric occultation. All of them were conducted the night of May
2^nd^ here at JPL, but May 3^rd^ Greenwich Mean Time. We actually
conducted a second occultation since then on May 21^st^, but none of the
data is in the presentation here yet. Most of the data here is just from
the first occultation.

Since I don't have the slides numbered I will just talk about the slides
by mentioning what's on them. Please feel free to back track me and ask
me which slide exactly I'm talking about.

If you go to the first slide the radio science instrument is one of the
12 on board the spacecraft and one of two that operates in the microwave
region of the electromagnetic spectrum; the RADAR being the other
instrument. The instruments on Cassini complement one another in one way
or the other and in the longest wavelength range here lies radio
science.

If you go to the next slide, which is titled Radio Science Sub-systems,
you will see the configurations with which we do these occultation
experiments. It's really one of two configurations. This one is called
the Downlink Configuration where a radio signal is sent from the
spacecraft and then received at the DSN stations here on the ground.
It's called the Downlink Configuration for that reason.

There is a second configuration, which we uplink a signal from the DSN
to the spacecraft and then the spacecraft returns it back to earth. That
second configuration is usually used to study the gravitational fields
of bodies like Saturn or satellites, which is not the topic for today.
That's why it's not included.

Most of the radio science observations rely on the fact that you have a
very precise sinusoid that you can observe its frequency, amplitude, and
phase as it encounters a target. The target can interrupt the sinusoid,
like in the case of the occultation's here. In the case of gravity
fields the target just remotely perturbs the sinusoidal signal without
the sinusoidal signal really hitting the target itself or passing
through it or encountering it in a physical way.

In the occultation's we transmit from the spacecraft three sinusoidal
signals. All three are derived from the same oscillator. It's a very
special oscillator called ultra stable oscillator where it drifts only
by one cycle in every tenth to the 13^th^ cycles, so it's a very, very
accurate clock, not exactly atomic, but it's about a tenth of the
stability or ten times worse the stability of an atomic clock, but you
can put it on board the spacecraft because the weight is reasonable and
the power it consumes is reasonable, etc.

These three sinusoids then, when they are propagated from the spacecraft
to the DSN you can measure the amplitude and phase and that's very
crucial for this experiment. Not only the amplitude and phase of each
sinusoid individually, but also the relative values, so you can measure
the phase of the X-band, one of them relative to the others. The three
are selected at frequencies that are commensurate with the DSN
capabilities, so they are called X-band, S-band, and Ka-band. X-band is
the link that the spacecraft actually uses to communicate data back and
forth and also to command the spacecraft, so it's the primary
communication link for the spacecraft, but during these occultation's
the X-band modulation is shut off completely and is just a pure
sinusoidal signal that's used.

In addition, the S-band and the Ka-band, the slide shows the wavelength.
The Ka-band is the shortest wavelength, 0.94 cm. The X-band is 3.6 cm.
The S-band is 13 cm. The X and S are sort of traditional kinds of
frequencies for deep space occultation experiments, also for not only
deep space, for Mars, as well as Venus and all other near-by planets,
but the Ka-band is completely new. It was really introduced to Cassini
because during cruise we did a class of radio science experiments in
which we searched for gravitational waves crossing the solar system. The
Ka-band is very helpful in these experiments because it's very
insensitive to plasma, so we can take out the solar plasma by looking at
differential X, Ka using Ka-band as a reference primarily not sensitive
to plasma. But we've realized that this is a very important wavelength
also to use for occultations and it's now part of our suite of
wavelengths we use to do the occultations.

We don't transmit very much power. Cassini transmits only ten to 20
watts, as you can see from the table, but yet, because we use the 70
meter antennas of the DSN that X and S, Ka-band uses only the 34 meter,
so called beam wave guides. The signal to noise ratio we do the
experiment with is quite respectable and allows us a very good dynamic
range to measure both weak and strong signals. The DSN uses an atomic
standard reference to frequencies so the stability of the link really is
determined by the Cassini oscillator. So these are the very simple
sinusoids we transmit, but they are really the key, the stabilities, the
key for doing all of the science to follow.

The next slide, which is called Downlink Configuration is Used to
Conduct one, two, three, etc. When the link then is interrupted by a
target we want to study it's perturbed and from measurements of the
perturbations at the DSN we're able to tell about the medium that
interrupted the signal and its properties, like you will see from the
slides to follow. In the case of the rings, for example, it would be the
particles in the rings, which will introduce both scattering and
diffraction that will be observed at the DSN, while in the case of the
atmosphere it's a gaseous medium, which primarily introduce refractive
effects, but atmospheres also have structure that is at the scale of
diffraction, so there are also some refractive effects in atmospheres,
but primarily refractive effects, not diffractive effects.

We use this one-way configuration also to study the surface. The
atmosphere here is Saturn, but we do the same thing for Titan. The first
Titan occultation is not until March of next year on an orbit called
T-12, but the Saturn occultation happened, as I said, just on May 2^nd^.
We use this one-way also to do some other types of experiments. For
example, the one that you see at the bottom there where the spacecraft
turns to Titan and bounces the waves off the surface of Titan back to
the DSN, choosing the point on the surface of Titan where you get
mirror-like reflection; so this way you study the so-called specular
scattering from the surface of Titan to learn about the material
properties, as well as the roughness of the surface. This complements
the usual back scattering geometry of the RADAR, but the experiments I'm
talking about today are just the first two, the so-called atmospheric
and ring occultation experiments.

The next slide shows because this experiment is tied to the DSN we'll
always look at the Saturn system back from earth throughout the tour
mission. This is a Hubble picture that shows the view of the Saturn
system, or at least Saturn and its rings, as a function of time. At the
top there you see Saturn when the rings are nearly fully open. As time
goes by during the Cassini mission the rings close systematically, just
like when Hubble stayed in orbit for a number of years. It observed
Saturn at these kinds of geometries. This has very important
implications for this experiment because all of the ring occultation's
we're going to propagate the radio signals through the rings and
therefore the geometry of the rings, as observed from earth, is very
important. During the Cassini mission, the lifetime of the Cassini
mission, the four years nominal mission, all of these geometries are
available to observe from the earth.

The next slide shows the geometry of the Cassini orbits as viewed from
the north pole of Saturn and also from the side, actually from the plane
of the rings. You can see that there are very different families of
orbits designs throughout the tour. You've heard from previous speakers,
who talked about signs from different instruments, every instrument
requires certain geometries to do the observations in an optimal way. In
the case of radio science the geometry is at the heart of everything
because you have to get the spacecraft behind the rings or behind the
atmosphere as observed from earth. So in designing this tour, radio
science figured very prominently because we had to get the spacecraft in
this geometry to observe all of the ring geometries I showed you a
second ago.

If you see the next slide, which says, "Typical Ring Occultation Track
Geometry," you see the path of Cassini in the plane of the sky as you
observe it from earth. This is the blue line where there are tick marks
that are separated about 15 minutes. The arrow direction shows where
Cassini comes from, so the ingress side is where the arrow is and the
egress side is the other side. You can see in Rev 7, for example, what
you see is called the occultation track; that's the path of Cassini as
you would see it from the earth. It takes the spacecraft nearly
diametrically across the rings from one end to the other, with a very
slow velocity of about ten kilometers a second, allowing us to map, as
you will see from the slides to follow, the structure of the rings with
exceptionally good quality. This kind of geometry is very, very
difficult to design depending in terms of how you set up the orbit. It
was only possible to do only one time during the mission. Right now Rev.
7 to 14 implement this geometry.

Later on in the tour like Rev. 46 or Rev. 46, I forgot to say that the
rings here are mainly fully open, as you see. I'll talk about that in a
second, why is that so important for the ring occultations. In the
figures labeled Rev. 44 and Rev. 46 the occultation is primarily on one
side as far as the rings are concerned. In some cases the atmosphere is
on both sides. Sometimes it's mixed with the rings, so it's only on one
side, but this happens when the rings are about 15 degrees open, both 44
ad 46. Then there is Rev. 63, also an example there, which represents
one member of a third family of occultation's that happen late in the
tour when the rings are less than ten degrees open. So there are three
families of occultations with three different types of orbits designed
to observe the rings: early ones, late ones, and some in between.

This is also evident in the next slide. It's called catching the open
rings. There you will see the angle between the ring plane and the line
of site from the spacecraft to earth called the ring opening angle
plotted versus the time during the four years tour time, so it starts
from 2004 and ends in 2008. As you can see, the rings, through these
systems of oscillations closes down as time goes by, being very open
near the beginning of the tour and being essentially closed near the end
of the tour. The oscillations you see here are because of the orbit and
motion of the earth around the sun. It's roughly a year in terms of a
period.

The red circles indicate the actual times at which occultations will
happen. By design, 7 to 14 were designed very early in the tour for the
reason I'll explain in a second. Then there is a whole set of
occultations, 53 to 67, near the end of the tour when the rings are
nearly six to ten degrees open. Then there are three occultations in the
middle to bridge these two regimes.

Now if you look at the diagram in the middle of the slide where there is
a blue line crossing, that's the line of sight crossing the ring plane.
The attenuation of the signal in the rings relative to its free space
value depends on the normal optical depth of the rings, which I'll
describe in a second, called tau, τ, but magnified along the blue-line
path by one over sin(B) where B is the ring opening angle plotted here.
So depending on B, the magnification of the optical depth can be 2.6 at
the beginning of the tour and near the end of the tour it can be as big
as 9.6 in the exponential. That means if you have large taus, τ, if you
have rings of large optical depth, they may completely block the radio
signal near the end of the tour. Indeed, the Voyager spacecraft, which
is the only other spacecraft that did a ring occultation experiment of
Saturn, was conducted when the ring's opening angle was about six
degrees. Most of ring B was completely blocked in the Voyager
observation, as you will see in a second. So the late orbits look very
much like Voyager, but the earlier orbit is something that can be used
to probe for the first time the optically thick regions of the rings.
They were a big driver on designing the tour to try to get Cassini to do
occultations while the rings are nearly fully open. Right now we're
executing this sequence and the experiment results that I'll report
today belong to Rev 7, which is the largest ring opening of all of these
Revs.

The next slide shows the occultation track for that particular orbit.
Rev seven itself that happened on May 2^nd^, as I said, Pacific Time and
May 3^rd^ Greenwich Mean Time, the spacecraft came in the view you see
here, which is the view from the earth. You see Cassini coming from the
right, crossing behind the rings on the ingress side then if you look at
the times below you'll see that the ring occultation starts with a
baseline that's about half an hour or so. You need to know how the
signal behaves before you start seeing the effect of the rings on the
signal, so we always start an experiment with a baseline and that lasts
for about half an hour. Then for about one hour and a half we're behind
ring A, then B, and then C. Then we switch to an atmospheric occultation
experiment where now the ray goes into the atmosphere of Saturn and we
do about 40 minutes, from about 23:00 at the bottom to about 23:40;
that's from about 11:00 p.m. Pacific Time until about 11:40 we were
doing the atmospheric occultation experiment. Then the spacecraft hid
behind Saturn for about an hour and 20 minutes. During that time the
radio science handed over the spacecraft to the VIMS Team, which did
some other experiments and then took the spacecraft back at about 1:00
a.m. in the morning and did the experiments in reverse; did the
atmospheric occultation and then for an hour and a half the ring
occultation and half an hour baseline at the end.

The event was covered by two complexes; the Goldstone Complex on
ingress, as well as the Canberra Complex on ingress also. We used 70
meters to cover X and S-band and 34-meter antennas to cover Ka-band and
also get the backup X-band. Goldstone covered ingress and Canberra
covered ingress and egress, so we have quite an extensive data set on
ingress and roughly half that on the egress side. You will see some of
the results we obtained in a second.

Before I do that, I want to remind you that we are looking at the
interaction of electromagnetic wave with the rings. The next slide shows
an artist's conception for what the rings look like if you come close
enough to them to observe the individual particles. It's a collection of
discreet random particles that vary in size from millimeters, perhaps,
to big boulders in size, tens of meters in size kinds of particles. We
believe that from previous observations with Voyager, as well as from
many other dynamical evidence, that the rings are not very thick; that
even though they are 60,000 kilometers wide their thickness cannot be
more than a few hundred meters, most likely less than that on the order
of 100 meters or so. So the particles are really packed up in a very
thin layer that is very close to the equatorial plane of Saturn. The
waves go through that and therefore we have to characterize what happens
after the wave interacts with the particles and then understand the
nature of the medium from the perturbations we see in the signal.

The next slide shows how the signal changes, so this is called
Observables and it's a cartoon that shows the rings as a slab of
discreet random distribution of particles with the big, fat, blue arrows
coming from the left, the incident signal. Then the incident signal
leaves the rings perturbed. The perturbations are primarily of two
types. The particles collectively strip some energy away from the
incident signal, reducing its power; so these are the thinner blue
arrows that exit from the rings. In the spectrum that signal remains to
be a sinusoidal signal that you can measure both its amplitude and
phase. We call that the direct signal. It's the signal that just passes
directly through the rings, but gets affected in amplitude and phase in
doing so. In the spectrum that has the characteristic of a sinusoid,
which is a line, in the spectrum sketched below the cartoon this is the
actual measured spectrum from Cassini.

> You see the line called direct signal. We essentially center that
> line, get the amplitude and phase of the signal that directly went
> through the rings. The reduction in the energy of the signal means
> that the particles stripped some energy away and what they do is they
> scatter it everywhere in space. They either absorb it if they have
> some loss in them and in addition, scatter it in every direction in
> space. Part of that energy scattered is captured again by the DSN, so
> in the cartoon sketch this would be the red arrows going in every
> direction in space labeled scattered signal. In the spectrum we see
> that as a broad feature that sits right under the direct signal and it
> spreads in frequency like that because the particles are in their
> Keplerian orbits, so because of the motion of the particles relative
> to the spacecraft and the DSN you get Doppler shift and the Doppler
> shift spreads the energy in a few kilohertz bandwidth, like you see in
> there.
>
> Both of these parts have information about the ring that we use to
> characterize both the structure of the rings and its particle sizes.
> If you look at the left bullets you will see the direct signal gives
> us information about the radial structure of the rings and when we see
> the structure at multiple frequencies, at multiple wavelengths, we
> have three of them, as I said at the beginning, we get the idea about
> the small sizes that make the structure we see. You'll see examples of
> that in a second. There is also information about the thickness of the
> rings from diffraction by the edges from the examples also you'll see
> in a second. The scattered signal itself has a lot of information
> about the particle size distribution, how big are the particles
> relative to the wavelengths and therefore, since we know the
> wavelengths, how big they are really physically, how they are
> distributed in space, whether they are in a thick layer or a thin
> layer, whether they are closely packed, they are clustered or not
> clustered; all of that is very important information we get from the
> scattered signal. So the effects on the three radio signals gives a
> very potent tool for studying the structure and what makes that
> structure of the particles in the rings.
>
> The next slide shows in a heuristic, so to speak, why the structure is
> immediately an outcome of this experiment. The top part is called
> Radio Occultation Profiling of Radio Ring Structure. It shows an image
> of the rings where you see reflected light, essentially. When you have
> a thick ring you have more reflected light and the ring looks brighter
> in the image and vice-versa. In the Cassini division you see dark
> colors because of the fact that there is very little material to
> reflect light. Similarly, in the Cassini division in the ring C and of
> course, in the Encke gap, which is at the very outer part of the ring,
> you see that you see black essentially because there is hardly any
> material to reflect light. Here you have an image in the reflected
> light. In the case of occultation's it's really transmitted light
> because the wave goes through the rings and when it goes through a
> thick material it's attenuated more and when it goes through a thin
> material the attenuation is relatively smaller, so what you see at the
> bottom is the attenuation of the signal as a function of radial
> distance. We see that as a function of time and then use the
> spacecraft trajectory to translate time into distance, so as the
> spacecraft moves behind the rings we make a profile of the attenuation
> of the signal as a function of radial distance.
>
> We translate the attenuation of the signal into an optical depth. If
> you look at the blue arrow next to it there's an equation that relates
> the intensity of the signal to optical depth. Normal optical depth is
> tau, τ, and B is the opening angle that I talked about before. So we
> can translate intensity to optical depth and just to keep the
> attenuation, since we plot optical depth increasing downwards, like
> you see in here, larger optical depth means more attenuation, means
> weaker signal. The plot here shows very clearly ring C with its small
> optical depth on the average and very narrow embedded features in it.
> Then when you go behind ring B you see the signal strongly attenuated,
> almost at the noise level. This is the Voyager profile, so this is the
> profile we measured 25 years ago using the single Voyager occultation
> that happened over the last 25 years. Back in ring A then you go
> through the Cassini division where the signal climbs back in strength
> considerably because the Cassini division is very thin. Then ring A,
> which has a marbled optical depth between ring C and ring B, but you
> can see here that ring B was completely blocked out in the Voyager
> observation.

The thing you will see also in this chart is that radio occultation
really is an image of the rings, but it's a one dimensional image. It's
the one-dimensional image that you get through a cut along the
occultation track. It's a quantitative image in the sense that you know
exactly what the optical depth is by doing an occultation like that. So
in future slides you will see very similar kinds of plots. All of them
will show optical depth versus radial distance, sometimes versus time
and those will relate to the actual Cassini observation, but an
occultation is a profile along the cut therefore it is called a profile
usually and not called an image, although we will translate that into
artificial images also in this talk.

Before I show you the structure of the rings let me just review very
briefly the fact that the rings have structure at all scales. If you
look at them from the Hubble space telescope, like the next slide shows,
planet Saturn and its remarkable ring system, here we're seeing the
global image of the rings and the structure here, A, B, C, the full
rings from the inner part of ring C to the outer edge of ring A is about
60,000 kilometers, so we're looking at structure here in the tens of
thousands of kilometers.

The next slide shows the structure on much less scale. This is again a
Cassini image from the Cassini imaging team where rings C, B, and A now,
so detailed kind of structure from the tens to hundreds of thousands of
kilometers kind of scale. There is also ring F, which is not very
obvious there, but it's obvious in the insert image, which is a very
unusual kind of ring, but the core is about a kilometer or so wide, but
it's surrounded by a very tenuous material, so it's width really is
variable and can be tens of kilometers around the core part of it. But
what I'm trying to say here is now we're looking at a different scale,
so we're seeing different structure.

If we continue this, again this is Cassini imaging system. You see now
the rings are on the hundreds to thousands kilometer scale. This
structure, ring C, was the inner part and the outer part of it. Then if
we continue the imaging cameras stop roughly at about a resolution of
several kilometers, but the rings continue to have structure on all
scales like we learned from the Voyager experience at the kilometer
level, at the hundreds of meters level, so this slide called Interesting
Ring Structure exists at every spatial scale. It's a slide designed to
illustrate that in a clear way that the rings continue to have structure
at every resolution level you look at. For example, in the image at the
top you see what's known as the Colombo Gap, which is the black region
in the image with a bright feature in the middle of it. That's an
embedded ringlet in the gap. Then if you look at the Voyager profile,
which is the middle panel in that slide, you can see on the right of
that a feature that's at about zero level. That's the Gap and within
this gap you see the embedded feature, which is called the embedded
ringlets, very thin embedded ringlets in the Gap.

There is also a feature, which is labeled in blue here, that if you
expand and look at it in very high resolution, now it's a hundreds of
meters resolution, 200 meters, which is the slide at the bottom, the
panel at the bottom; you see a beautiful wave-like feature. That is now
known to be a bending wave in the rings there. The resonant interaction
with satellites introduces these so-called density and bending waves
where the ring plane gets modulated either in density or modulated in
terms of warping above and below the mean ring planes.

Rings have structure at every detail and these occultations can map them
at the hundreds of meters level, in fact, 50 meters level for Cassini in
these early orbits from the performance I'll show you in a second.
Unfortunately, the picture is not as simple as I just described because
once you talk about hundreds of meters level kind of structure you're
talking about resolving structure below the diffraction limit. The next
slide, called Observations are Diffraction Limited; it shows what
happens as Voyager went behind the inner edge of the Encke Gap where
there is thick material on one side and very little material on the
other side. The intensity of the signal doesn't go from very low value
to the full space value. It goes suddenly, as you can see here
gradually, and in the process it oscillates, which is, for some of us,
the well known diffraction pattern of a so-called knife edge. The
observations are diffraction limited when once you go beyond the Fresnel
Scale of the fraction, which is determined by the square root of the
wavelength times the distance of the spacecraft to the rings then what
you see really is a diffraction pattern of the actual distribution of
ring material, the true optical depth.

In the radio occultation observations to beat the diffraction you have
to do very extensive kinds of processing and the next slide shows that
you can do that if you know the amplitude and the phase of the signals.
The simple picture of just measuring the attenuation through the rings
is not really very truthful in the sense that once diffraction is
important then what you really observe is a diffraction pattern of the
optical depth profile and not the optical depth profile itself. Because
radio science measures the amplitude and phase of the diffracted
sinusoid what you can do then, if you remember the principle of
holography, in a hologram you get an image of an object, but the image
is not really a true image; it's just capturing the diffraction pattern
of the object, so you capture both the intensity and the phase of the
diffraction pattern on an image you call a hologram. Then if you know
the reference signal that was used to construct the hologram you can
play back the hologram and get the true image, a three-dimensional image
of the object that you recorded on the hologram.

The same concept holds here. Because the rings are nearly circularly
symmetric they are really a one-dimensional target and so by observing
the occultation profile you can reconstruct using the same principles of
holography, an image of the rings at a very high resolution. Here I'm
using as an example the F ring, which, is in the image you see on this
slide, is a very tiny structure that's outside the edge of ring A
pointed to by the blue line.

> If you collect data so that if you go up on the blue line and you see
> a horizontal blue line that goes from minus W/2 to W/2 that is
> collecting diffracted data over this extent from minus W/2 to W/2
> around the position of the F ring. If you look then at the curve just
> below the image you will see the amplitude of the signal and the phase
> of the signal, as was measured by Voyager. That is exactly equivalent
> of the hologram. You know full information about the diffracted wave
> front when you get the amplitude and the phase information. The
> amplitude here is in terms of optical depth and the phase is in
> cycles, but then you can play back that hologram on the computer and
> get the true amplitude and phase, which is the panel at the bottom. So
> you see the signal level is almost at zero, zero, zero everywhere and
> then when we get right behind the F ring the signal level dips all of
> the sudden because of the optical depth of the F ring and then goes
> back to free space level. That's the kilometer core of the F ring that
> I mentioned a few minutes ago. The phase also changes right at the
> place of that core and then it goes back and joins the background
> phase. By measuring the amplitude and phase of the signal one can
> reconstruct the optical depth profile down to 100 meters of
> resolution. We can see the rings 100 meters at a time.
>
> The next chart says do you have enough signal to noise ratio to do so?
> Therefore, the chart shows the resolution in meters on the horizontal
> axis versus the optical depth you can probe given the signal to noise
> ratio available to Cassini from the fact that we have finite power on
> the spacecraft, from the fact that the spacecraft is a certain
> distance from the earth, etc. There is a maximum optical depth you can
> probe at a given resolution and that maximum optical depth is what's
> plotted here. It's called a threshold optical depth versus the
> resolution you want to achieve. Here resolutions are depicted from ten
> meters to 100 meters to a kilometer at the end of the chart. Just as a
> typical example, I show what optical depth you can probe at 100-meter
> resolution. The vertical dashed line you see coming out in the middle
> of the chart.
>
> Then there are three curves or three lines you see. One is labeled
> B=24. This is typical of the occultations at the beginning of the tour
> and the intersection of the 100-meter line with a line labeled the
> Cassini-I B=24 degrees is a maximum optical depth of slightly more
> than two and a half, so any ring feature of optical depth 2.5 or less
> can be probably profiled with a 100-meter resolution where the Cassini
> orbits at the beginning of the mission when the rings are nearly fully
> open.
>
> As you can see, that limit goes down. By the time the rings close to
> 15 degrees you can only do that to features that are at about 1.5
> optical depth. Voyager and as well as the last set of occultations on
> the Cassini mission where the ring opening angle can be smaller than
> ten degrees, here it's six degrees, a typical number. Then the optical
> depth that you can profile with a 100-meter resolution is only about
> 0.5 or less. The earlier orbits, the ones that just happened are
> extremely important for that reason, because they allow you to profile
> the rings with high resolution and not only the small optical depth
> regions of it, but also the higher optical depth.
>
> Resolution you can always degrade by going to a coarser resolution and
> digging deeper into the rings. For example, if you go to a kilometer
> resolution the earlier orbits allow you to profile the rings at about
> 3.0 and up to 3.5 optical depth or less. In the profiles I will show
> you in a second the resolution that we process the data so far in is
> 10 kilometers, as well as one kilometer. At ten kilometers the optical
> depth that we can see is a maximum of about 4.5 or 4.3. At one
> kilometer, as you can see from this chart, it's about 3.5. Keep that
> in mind.
>
> On the other hand you can also go to better resolution. You can go to
> 50-meter resolutions, but as I said, there is a trade off between the
> maximum optical depth you can probe and the resolution, but 50 meter
> resolution for features up to an optical depth of 2.2 or so is a
> possibility also.
>
> Finally, why do we do multiple occultations? The rings are not
> circularly symmetric, so the next two slides we'll talk very briefly
> about the fact that the rings have features that are extremely
> important to dynamically understand the behavior of the rings and they
> are not circularly symmetric. In particular, the first one of them
> called Occultations at Multiple Longitudes and Dynamic Ring Features
> shows on the left that two of the very interesting dynamical
> structures in the rings; one is the density waves in which if a
> satellite goes around the rings it acts like an oscillator, which is,
> if you think of it, as doing a certain number of revs per minute or a
> certain number of oscillations per minute around the rings. Then when
> a particle is also an oscillator that goes around the planet, but with
> a different frequency, these two oscillators, their periods are
> commensurate when they relate to each other, like when the satellite
> goes once around the planet the particle goes twice around the planet,
> then the satellite perturbs the rings strongly and the perturbation
> causes the material in the rings to get together in places and move
> apart in others causing the picture you see there where there is a
> density modulation of the plane of the rings and therefore it's called
> density wave because when you cut this by a spacecraft going behind
> the rings you see that profile that's in the picture below where the
> oscillations in the optical depth profile have a very characteristic
> behavior that starts with the wavelength being long and then gets
> short or the wavelength being short and gets long. It has this chirpy
> behavior, which is indicative of perturbation of a satellite to the
> material in the rings.
>
> There are two types of perturbations. One is you perturb the
> densities, which is the one on the left, and therefore the ring
> thickness remains the same, but the material gets together in places
> and apart in others, so this is called density modulation and the wave
> is called density wave for that reason.
>
> If the motion of the satellites is up and down the ring plane and it
> couples into the particle up and down motion around the mean ring
> plane then the ring plane gets rippled, like you see in the cartoon
> also at the top of the figures here. These are called bending waves
> for that reason. The mean ring plane bends up and down in a very
> characteristic way so when you cut it again with an occultation path
> you see the ripples that are in the chart below.
>
> Dynamical features, like density waves and bending waves have very
> important information about how satellites interact with the rings and
> they tell us about the surface mass density of the rings. They tell us
> about how often the particle collides in the rings, the so-called
> viscosity. Then they end up telling us about the vertical thickness of
> the rings because of that, so they have important physical information
> about the rings themselves.
>
> Similarly, on the right-hand side here if you cut a ring so this is
> not Cassini; this is Voyager at Uranus. There was also one ring
> occultation at Uranus where the Voyager spacecraft went behind the
> Uranus rings, both on ingress and egress and Uranus has a very
> different set of rings than Saturn. They are very narrow and they are
> also much denser than the rings of Saturn, but you can see what
> happened, for example, for what's called the Ring Epsilon, the major
> ring of the Uranus rings. You can see that the cut on ingress and the
> cut on egress produce the two profiles you see at the bottom of the
> figure. Those are very different from one another; that's because
> Uranus rings are not circular. Each one of them look like an ellipse
> and the material distribution across the cut from one cut to the
> other. Saturn rings embed many features; in ring C for example some of
> the embedded ringlets in the Cassini Gap, the Cassini division, have
> this characteristic behavior, so by cutting them at different
> longitudes you can characterize the different material distributions.
> This is important because it's a puzzle in dynamics that an eccentric
> ringlet like that, because Saturn is oblate, the laws of dynamics as
> we know them dictate that this ring must disintegrate into a circular
> feature and it should not remain as an eccentric ringlet like we
> actually see them. So the fact that they remain eccentric means that
> there are compensating forces that one has to explain and
> characterize. So far the only explanation that is available to
> characterize why these rings hold together has failed the tests that
> this data implies. That is to say from the Voyager observations here
> when we test the theories against what the prediction of the theory is
> the test fails the theory and therefore we still need more examples to
> understand why such narrow eccentric ringlets maintain their
> eccentricity. In the Saturn data we will have many of these to
> complement the Uranus observations.
>
> Similarly, when there are moons embedded in gaps in the rings the
> moons perturb the edges of the gap and the material next to the gap.
> We know that two of these were found so far in the rings; one in the
> Encke Gap and one in the Keller Gap. Those will perturb the material
> next to the gap, as you can see here from this beautiful image from
> Cassini. If you look very carefully, this is the Encke Gap, at the
> inner edge you'll see it wiggle. If you look even more carefully to
> the material to the left of the edge you will see that it gets
> stratified into a very specific organized pattern.
>
> The line you see here, the blue line, is a cartoon for an occultation.
> If you go behind the structure then this regular pattern translates
> into regular oscillations into the observations. They are called
> satellite wakes in this case and wakes in occultations are indicative
> immediately of a moon embedded in a gap, embedded in the rings. This
> particular satellite here, which is called PAN, was observed in the
> Voyager data, both radio occultation and stellar occultation shows
> this regular pattern, which you see in the diagram to the left that
> were used to infer the presence of the satellite even before it was
> observed in the images. We're hoping to do the same for Cassini also
> where the wakes we will see in other gaps where there are many gaps in
> the rings where satellites have not been seen yet, but some of them
> may contain satellites and the wakes will be one way to tell whether
> there are satellites or not.
>
> Let me now run you through the optical depth profile that Cassini
> observed on the night of May 2^nd^ and 3^rd^ and give you just a
> little feel for the surprises we've seen that night from the pictures
> we already have from Voyager, as well as from other stellar
> occultations that were conducted after Voyager. I'm going to the next
> slide where it says Cassini Rev 7, Optical Depth Profiles of Ring C.
> All of the profiles I'll show the normal optical depth versus ring
> radius except for some will show time instead of ring radius. We're
> still processing the data, so whatever plots are available now are the
> ones I used. In all of them you will see the distribution of ring
> material as a function of radius. Ring C has been observed by Voyager
> very well and the difference here now is we see it with much, much
> better signal to noise ratio, so all of the structure you see here is
> real. There's no noise, almost no noise in this diagram at all. You
> can see that ring C is characterized by a very low optical depth
> background that undulates like a very gentle wave, but on top of that
> there are these very dense embedded ringlets. That is a characteristic
> of ring C. Some of these can be explained by the fact that they are
> coincident with what's called satellite resonances at these locations,
> but not all. Many of these embedded features remain to be a puzzle to
> be explained. Also, why do they have such very sharp edges? How does
> the material remain confined in a feature that has such sharp edges in
> the middle of a background that is of much less optical depth? Why
> doesn't it spread? The structure here is just structure. You don't
> know what particle sizes make it.
>
> The next slide is the three-wavelength observations. The red color is
> the S-band, the longest wavelength. The blue color is the Ka-band, the
> shortest wavelength. The green is the middle, the X-band, which is the
> wavelength in the middle, 3.6 cm. The fact that the three wavelengths
> see three different structures means that the particle sizes that are
> making this structure must have millimeter to centimeter particle
> sizes. This is because an electromagnetic wave interacts only with
> particles that are comparable to the wavelengths or bigger. When the
> particle is much smaller than the wavelength it's essentially
> invisible to that wavelength. So S-band, which is a 13 cm wavelength,
> if the particles are less than 5 cm or so they become completely
> invisible to S-band. The fact that S-band separates from the other two
> means right away that there are few 5 cm or less kinds of particles in
> ring C.
>
> Similarly, when Ka-band and X-band separate it means there are
> particles less than about 1cm, marble size or less, ping-pong size or
> less, for the three wavelengths. Here we see right away a
> differentiation in size because of the fact that the three wavelengths
> sense different structures. This would be the story for the other
> rings. If you look at the very end of ring C you see this tapered
> feature, which we call the ramp, and there you see very peculiar
> behavior where the three wavelengths start from the beginning being
> close to one another and as you move outward towards the end of the
> ramp the red curve becomes very much different from the blue and the
> green, which means there is something there that says that in this
> particular feature there is a larger population of smaller sized
> particles that get generated as you move farther out. What physical
> processes do that is still a puzzle, but the beauty of having three
> wavelengths is that you're able to tell that there is something
> different in terms of sizes for this particular structure.
>
> The surprise of the night was, of course, ring B, which is the next
> slide, Optical Depth Profile of Ring B. That ring until that night was
> completely a question mark; what is its true structure? What's the
> optical depth of that structure? It was not really fully understood or
> fully characterized, neither from the Voyager observations, nor from
> any stellar occultation that was completed between 1980 when Voyager I
> went behind the rings and today. Most stellar occultations see also
> direct and scattered signals, like we do with occultations, but they
> cannot be differentiated from one another, so the scattered signal
> tends to blind the structure, the detailed structure of the rings in
> occultation experiments or at least tends to diffuse it so you don't
> see the detailed structure as well as you see it from radio
> occultations like you see in here.
>
> As you can see here, ring B is much, much thicker than ring C and the
> optical depth, again, plotted downward, increasing downward, so zero
> means zero attenuation. As you go downward there's a lot more
> attenuation as you go to optical depth of 4.3 we hit noise, so the
> dotted line at the bottom here is roughly the noise level.
>
> Close to the noise level you see first if you look at it and you try
> to characterize it as a whole you would say probably there are three
> very distinct kinds of regions. There is the inner regions, which
> extends it about one-quarter of the width of the rings. There is the
> next quarter of it, which shows this high frequency oscillation. If
> you expand and look at the scale of the features you see there they
> are about a few hundred kilometers, 300 kilometers typically. They
> oscillate in optical depth between 1.5 and 3.5. That's a huge change
> in terms of the amount of the material in each of these features on
> such short scales. The material essentially doubles in density over
> hundreds of kilometers kind of width, something we haven't seen really
> very clearly in any of the other rings. Neither ring C nor ring A
> shows this kind of a deep fluctuation in the amount of material
> encountered over short distances. Then there is a similar kind of
> behavior for the outer one-quarter of the rings. This third quarter
> shows a very thick feature, which we call now the core of ring B. That
> core is very dense. It's an optical depth that exceeds 4.0 everywhere,
> this is 10-kilometer resolution, and may have caused the signal to
> saturate the noise at this kind of a resolution. Until we look at even
> coarser resolution we don't really know for sure what's the true
> optical depth of this core features. It could be slightly more than
> 4.3, like this diagram shows. But even this core feature is not solid.
> As you can see, it's divided into regions and between the middle part
> of it is about 1,500 kilometers wide. The scale at the bottom here is
> 5,000 kilometers per tick mark, so the feature extends roughly from
> about 105,000 kilometers to 110,000 kilometers. You can see it's
> divided into regions. The densest part is the center part and it's
> about 1,500 kilometers. This is the first time really we see ring B.
> The structure we see here cannot be explained by the same reasons we
> invoke to explain some of the structure we see in ring C and ring A,
> mainly satellite resonances, interaction with satellites. We think
> we're seeing totally new physics here that will take some time to
> assimilate and to understand. It's not completely obvious what
> physical mechanism could drive structure like that to exist.
>
> The thing you should keep in mind is none of this structure really is
> noise. I said at the beginning within ingress and egress and I'm
> showing you here the egress profile. If you plot the ingress profile
> and put it on top of this one the alignment is amazing. Most of the
> structure repeats almost to the smallest detail between ingress and
> egress. Some of it is not exactly circular, but most of it aligns
> almost perfectly, so this is not noise. This is, as I said, true
> structure, the like of which we haven't observed in ring C or ring A
> before.
>
> The next slide shows the three wavelengths' behavior. In this case you
> will see that in the inner one-quarter that there are regions where
> X-band, Ka-band separate from S-band, the red curve, and therefore,
> there are some limited regions here where S-band is separate and
> therefore, there are centimeter-sized ping-pong or smaller sized
> particles. But almost everywhere else there is all three colors,
> almost lock one another. You can't see any differential between the
> three, except in the core. The core is noise limited; I just said so.
> So S-band has saturated in noise at the top dotted line you see and
> then X-band saturates a noise at the green bottom-line and Ka-band is
> in between. These three wavelengths have three different capabilities
> and therefore the X-band has the highest signal to noise. It can probe
> deepest. The S-band has the lowest signal to noise. It can probe the
> least deep. The Ka-band is in between. In the core we're hitting
> noise, so we can't tell exactly at this resolution what's the true
> differential between the three, but to the right of that, to the left
> of that we see all three wavelengths seeing exactly the same
> structure, which means right away that ring B is nearly devoid of
> centimeter-sized particles. The only exceptions are these flat regions
> in the inner part of ring B where the S-band shows separation from the
> other two and therefore we see small, 5 cm or less, kind of population
> of particles.
>
> The next slide shows ring A and the next slide shows that ring A is
> different from ring C, different from ring B. We're seeing three very
> different kinds of structures for the three main rings of Saturn. The
> Cassini division is a part of the left of that slide. Ring A itself
> follows a nearly flat kind of a region with these dense kinds of
> fluctuations in it. These are waves that I'll show at higher
> resolution in a second. Because ring A is very close to the moons,
> just outside the edge of the rings the moons couple very effectively
> into the motion of the particles close to the rings and therefore
> introduces a host of density waves and bending waves in the structure
> of ring A. Maybe I'll do that on the next slide.
>
> The edge of ring A is at the very right of that slide and interior to
> it is the Keller Gap, which is a very narrow feature that goes all of
> the way to the zero level, which is free space. Before that is the
> Encke division where, again, the attenuation is minimal in the Encke
> division and it's about a 320 kilometer wide kind of a feature and the
> signal level goes back almost to free space. The region I'll show you
> is between the Encke division and the outer edge of the A ring, so all
> of these noise-like kinds of features, which are not noise at all, are
> really waves in the rings. This chart here is a 10-kilometer
> resolution. The next one is a 1.5-kilometer resolution. On the very
> left is the edge of ring A and because this is time and we're going
> into the egress occultation the scale here is time, not distance.
> That's why the edge of ring A is on the left line, while in the
> previous slide it was on the right. Then the Keller Gap and then there
> are three panels. The fourth panel ends with the Encke Gap. All of
> these regular oscillations are the density waves that are clearly
> visible in ring A and we see about 40 of them. Here in this part I
> show there are only about 20 of them. They have never been seen before
> with this clarity and never in this massive number. This is the first
> really detailed mapping of the structure of ring A and the wave
> phenomenon that populate that ring and that populate that ring simply
> because of the fact that it's very close to the satellites immediately
> outside the edge of ring A and therefore the coupling is very
> effective, very efficient. The gravitational coupling between the
> motion of the particles and these satellites is very strong.
>
> If you continue with charts like that throughout ring A you see a
> similar number of waves, so we see over 40 waves all together in ring
> A, most of them will be mapped with 100-meter resolution when we
> complete the processing and will give us very important information
> about the physical properties of the ring themselves, but waves are
> also very important for determining the lifetime of the rings because
> there is always energy or momentum exchanged between the satellites
> and the rings. The satellites get propelled away from the rings as
> they supply momentum to the ring particles; therefore this density
> wave is also a clue for the lifetime of the rings. In fact, they tend
> to imply that the lifetime of the rings is very short, millions of
> years, which is much less than the age of the solar system. But again,
> this is based on theory for the interaction that reflects our
> understanding of the interaction between the rings and the satellites.
> The more detail we have the more we can test these theories and
> improve them, but the bottom-line is they are important for
> determining whether the rings are a new phenomenon or a more recent
> phenomenon or an ancient phenomenon that existed when Saturn was
> initially created. They carry very, very important information about
> the physical state of the rings, as well as their origin.
>
> The three wavelengths...very differently, as you can see from the next
> slide. You can see here the three curves in the inner part of ring A
> are almost on top of one another, but as we move outside they
> separate, expand, separate, direct curve, X-band separates very
> clearly and systematically. The X and K, the blue and the green, stay
> together for a while, but as you move also near the edge of the ring
> they tend to separate also. What that means is that even though the
> particles inside ring A may be big, as you move outward the population
> of centimeter-sized particles increases systematically and then close
> to the very end of the ring even the population of millimeter-sized
> particles also is present. I think this is probably because the
> density waves that populate ring A causes more frequent conditions
> between the big particles and therefore it grinds them down into
> smaller and smaller ones, increasing their population as you move
> outward. But that's just one conjecture. I mean the data says here
> that the big particles at the very beginning of the ring and smaller
> and then a larger fraction of smaller and smaller particles as you
> move outward.
>
> Notice also the Cassini division. There is a ramp outside the Cassini
> division like there was a ramp outside ring C. They are almost
> identical in width, which is extremely puzzling. They look identical
> almost in shape and width and in behavior. You see the S-band again
> and the three wavelengths separating in almost the same way they did
> separate in ring C. Having three wavelengths really is extremely
> useful in terms of not only learning about structure, but what
> particle sizes also make that structure.
>
> The Radio Science team, just for fun and also for outreach purposes
> translated the previous curves into images and used the size
> information you see in the chart, so if you look at ring A here, how
> do you translate the picture of that? You start from big particles and
> you go to smaller and smaller particles as you go towards the edge. So
> we've used these three wavelengths' information to create the
> following images. The first image, which is the next slide, shows ring
> A, the part we just looked at, where the color here depicts particle
> sizes. When all three wavelengths suffer the same extinction, so they
> are essentially showing the same structure, we color the image purple.
> As X-band separates we increase the proportion of green, so when you
> see green shades you know that the X-band separated and you see more
> smaller sized particles, less than 5 cm sized particles. When the
> Ka-band also separates, when we see millimeter sized particles we add
> the color blue or we emphasize the color blue, which only happens near
> the tip of ring A in this case. These color images, first the
> structure you see in here is all real, but it's only one-dimensional
> structure. As you know, occultations yield one cut, so we sweep this
> structure azimuthally and get a two-dimensional image, but it's really
> a repetitive radial structure. Then the color is added by just
> artificially coloring the different effects on the three different
> wavelengths. This "image" in here is exactly the data that was in the
> previous chart; it's just in a graphical way that depicts the
> information in a perhaps more visually, easy to perceive kind of a
> way, at least for the general public. It shows that there is something
> different about ring A as you move outward; big particles at the
> beginning and then increased population of smaller particles at the
> end.
>
> I said also the same for ring B where I said that the three
> wavelengths are almost identical everywhere except in these flat
> regions at the beginning of the ring. Then I said also in ring C you
> see differential everywhere between the three wavelengths. The next
> image shows the complete size information, the small size information
> coded in the form of a colored image. It's called Radio Occultation:
> Unraveling Saturn's Rings. Again, here the purple is when all three
> wavelengths are affected almost equally. The green is when X-band
> separates. The blue is when Ka-band also separates. So in ring C you
> see a mixture of greens and blues because of the fact that all three
> wavelengths were affected differently. In the B ring only the inner
> part of it shows a bit of green in two bands because these are the
> regions where S-band separated, but it shows also that there is purple
> everywhere essentially because the three wavelengths are nearly
> identical. In the very dense core we color that white because of the
> fact that we don't really have much information about the differential
> effect. In the inner ring A we see the purple and then it tapers to
> green near the outer edge with a hint of blue at the very outer edge.
> These colors are just coding for the size populations.
>
> The final image that I show here is just a comparison between the
> structures we see in our radio observations with the structure you see
> in the real image. First we hid the visible light and we hid the
> radial signal and asked people which one do you think is the real
> image and which one is the radio science image. We got about a 50/50
> split, although after a while, if you think about it, you probably
> would be able to tell which is which, but here we're just trying to
> say that multiple instruments complement one another in terms of what
> they see. Occultations don't see two-dimensional structure, so that's
> something that will be always the privilege of the true images of the
> rings. You see the rings as they really are in terms of radial, as
> well as azimuthal structure, but, as I said, with resolutions that are
> kilometers and above while the radio signals will give you multiple
> cuts, but each with very high resolution. These images are all on the
> Cassini Web site. I put links to them here in addition to others that
> are not included in the talk here.
>
> Finally, regarding the rings I want to mention that the radio science
> team did analysis only of the direct signals so far. We have not
> started on the analysis of the scattered signal yet, but we see clear
> evidence for the scattered signal in the data and that evidence has to
> do with the spectra we observe as a function of time, which we call
> spectrograms and we can see that there is a population of meter-sized
> particles everywhere in the rings, in addition, of course, to the
> special reasons where we see centimeter-sized particles most of the
> population of particles in the rings is broad and it extends all of
> the way to the meter size. We see that very clearly in the scattered
> signal signature that we see.
>
> I'm not going to go through this in detail, but I just wanted you to
> know that we have data regarding the meter-sized particles that will
> also be useful in the future to get the size distribution all of the
> way from the millimeter size to the tens of meters kind of size. As I
> said here, the energy strip from the rings from the direct signal is
> rescattered everywhere in space. That shows in the broad feature in
> the spectrum in the slide called Observables.
>
> In the next slide the actual footprint of the antenna as Cassini went
> behind the rings is depicted in the picture on the left as these
> ovals, the blue ovals. Every blue oval is just looking at the ring at
> one instant of time and as Cassini went in where the arrow at the top
> is until it went out it was moving away from the rings. Therefore, the
> size of the footprint got larger as we moved out. If you take any one
> of these footprints, and here I took one close to ring B, and plot it
> at the top; there are three pictures on this diagram and I'm talking
> about the top right one now where the oval is just one of the many
> ovals you see on the left diagram or left picture; you see that
> crossing an artificial sort of a feature drawn as two red lines here,
> which is about 300-kilometers' width. As the beam crosses a feature
> like that the echo from that feature, the signature of the scattered
> signal appears in the data at different frequencies and that's because
> the particles are moving and therefore every particle within the
> footprint of the antenna scatters a signal at a different Doppler
> shift.
>
> The colored contours that divide the oval that you see are contours of
> constant Doppler shift, so the color bar on the right shows that red
> is five kilohertz Doppler shift. Blue is minus five kilohertz Doppler
> shift. The colors in between are different Doppler shifts. As a beam
> crosses a ring feature the features start showing a large positive
> Doppler shift and as a beam continues to cross it the echo moves to
> different Doppler shifts and then by the time the beam leaves the
> feature the feature would be at a negative Doppler shift.
>
> If you look at the last diagram at the bottom right it depicts in
> frequency and in time now the intensity of the echo as a function of
> time and frequency. You can see that as the beam goes into the feature
> you start from the shortest time, which is about 50 seconds in here,
> the feature starts showing at the large, positive Doppler shift. As
> time goes forward you get stronger and stronger scattering and then
> the feature moves to a smaller and smaller Doppler and by the time the
> footprint or the antenna leaves the feature it has a
> minus-five-kilohertz kind of a Doppler shift. The color is the
> strength of the scattering in this way in this curve. So the
> scattering gets stronger and stronger as you move towards the center,
> as the center of the beam moves right on top of the feature. The color
> here is a translation of the strength in decibels; two, four, six,
> eight, etc. is the strength in decibels. This is a feature that has
> particles simulated to be like a power law distribution of a maximum
> size cut off over five meters.
>
> If you go to the next slide you'll see the same simulation for
> particle sizes of different sizes, a maximum size of three meters,
> five meters, ten meters. The three features you see simulated on the
> bottom right diagram, you will see that we see the same shearing in
> frequency as time goes by. The frequency changes almost on a linear
> kind of behavior, which we call linear chirp. But the strength of the
> echo, the left most one is for a particle size of three meter maximum,
> five meter the next one, ten meter is the next one. You can see that
> the colors indicate that the signal gets stronger and stronger as the
> particle size gets bigger and bigger. This is also depicted in a
> three-dimensional diagram on the left side that you see to the left of
> that. The three features there, you see three features that have very
> systematically increasing heights. Those are the amplitude of the
> scattered signal. The signal gets stronger as the particle size gets
> stronger, but it also, in terms of the bandwidth of the signal, which
> is the last one on the right, but at the top of the diagram, you will
> see that all here normalize to the same peak value and then the green
> curve, which is the largest particle, has narrower width. Both the
> width of the feature you see, as well as its strength, is a direct
> translation to the particle size and therefore these are simulations
> here, but when we see features like the ones depicted here in these
> simulations we are able to tell what size they are, what maximum size,
> if we assume a power law distribution or do we actually get the
> distribution if we don't assume a power law distribution. This is a
> task still to be completed.
>
> The last slide regarding the rings show the data, the actual data
> observed in ring B and it shows this shearing kind of behavior,
> exactly like the simulation where the scattered signal here, again,
> the bottom there are two halves in here. The top part is a structure,
> as we've seen it from the direct signal. To the left you can see at
> the zero level that's a gap. This is the gap in the Cassini division
> called the Huygens Gap and now we're entering ring B. So the structure
> we see at the top is the outermost part in ring B. At the bottom is
> the time sequencing diagram now as actually measured, not from the
> simulations we had on the previous slides, but as actually measured.
> You can see that individual features in ring B are scattering exactly
> like the model said with this shearing kind of behavior that can be
> used to determine their particle sizes, in particular, the meter
> particle sizes. We see this typical signature in ring B, ring C, and
> in ring A, so we have clear evidence in the data that the size
> distribution extends to the meter-sized range, but exactly how they
> differ between the rings is still work to be done.
>
> Now I'm going to move to the atmosphere. I'm sorry this is taking a
> lot longer than I expected and I haven't had any questions yet. I'm
> going to try to move a little faster and finish the atmosphere. This
> is not an area of my specialty, but I will still try to do the best I
> can here to explain what they did.
>
> We did the ingress ring occultation then followed by an ingress
> atmospheric occultation. Then we did an egress atmospheric occultation
> followed by an egress ring occultation, so the data now is from the
> ingress atmospheric occultation. Now the medium is gaseous, not
> particles anymore, so diffraction is really the primary effect on the
> signal and when the signal goes through the atmosphere the atmosphere
> is not homogeneous and the ray then starts refracting much like it
> does in a lens.
>
> I'm going to skip to the next slide. The next slide is really Titan
> and again, Titan is a future target for us, but it's not one we're
> discussing today. However, it's a very interesting target because it
> has a very extensive atmosphere, as you know, and the radio
> occultation is one of the few observations that can map the structure
> of this atmosphere with the exception of the high resolution all of
> the way to the surface, but today's Saturn, so let me just concentrate
> on Saturn for now.
>
> The next slide shows what's called Atmospheric Limb Track Maneuver.
> That's what I was trying to describe from the previous slide, which is
> if you take Saturn out of the way then you would point the antenna,
> high-gain antenna back directly to earth along a straight line that
> connects to the two, but once Saturn is on the way and the ray bends
> in the atmosphere the ray that connects actual earth now to the
> spacecraft is not any more a straight line connecting the spacecraft
> to earth. It's a curved path that goes through the atmosphere, bends
> in the atmosphere and comes back and then exits the atmosphere in the
> direction of the earth. To do an atmospheric occultation it's a much
> more difficult job than doing a ring occultation where you keep the
> antenna pointed to earth. Here you have to constantly move the antenna
> as a function of time to point it to the point on the Saturn limb here
> so that the ray that goes into the atmosphere and bends comes back out
> and goes to the earth.
>
> We do that by using a model of Saturn atmosphere to find what angle to
> point at and then upload that to the spacecraft based on the best
> knowledge of the trajectory we can get, just ahead of the experiment,
> usually a few days before the experiment and then hope that we are
> pointing in the right direction. So far in the two occultations we did
> the pointing was almost perfect. We got very good atmospheric
> occultation data, but it's a much harder experiment to do, as I said,
> because of this maneuvering of the spacecraft so that we point at the
> direction where we expect the signal to go to earth.
>
> The fact that the ray also is bent means that there will be a Doppler
> shift due to the fact that the ray bends in the atmosphere. This is
> one of the observables we use to get to the structure of the
> atmosphere. The bending angle relates to how the atmospheric structure
> is and the bending angle translates into a Doppler shift that we can
> measure, so the Doppler shift, as a function of time, gives us
> information about the bending angle as a function of time, which gives
> us information about the refractivity of the atmosphere, the density
> of the gas, as a function of height. The fact that the ray bends in
> the atmosphere also means that two rays that are going into the
> atmosphere at slightly different bending angles will continue to
> diverge from one another so the intensity of the signal as a function
> of time goes down.
>
> Two effects we measure: The intensity of the signal as you go behind
> the atmosphere; the intensity goes down because of the differential
> refraction of the rays in the atmosphere and also the Doppler shift
> because of the fact that the rays bend relative to the straight path
> to the earth. From these two we can determine the density of the gas
> as a function of height and from the physics that relates density to
> pressure and temperature we can also get to the pressure and
> temperature. The data that was reported here so far has only the
> intensity as a function of time and therefore I'll go through them one
> at a time to just give you an example, but there is a whole suite of
> products that come out of this experiment, including the temperature
> pressure profile and also including the small scale structure of the
> atmosphere that has not been completed yet and is work in progress.
>
> The next slide shows the intensity as a function of time. Again, we're
> coming from free space. We finished the ring occultation experiment
> and this is Ka-band. The signal level is almost zero dB the full
> strength of the signal used here as a reference. Then when we go
> behind the atmosphere the signal intensity dips, as you can see here,
> and then fluctuates as it oscillates, as the signal goes through the
> atmosphere it fluctuates and this is again not noise, this is the
> structure of the atmosphere, the small structure of the atmosphere
> itself being measured here by the Ka-band signal at station 26, which
> is one of the 34 meters at the Goldstone Complex here in California.
>
> Just to try to convince you that this is not really noise the next
> slide shows the station 25 measuring the same signal and station 34 in
> Canberra also measuring the same signal. Here there are three colored
> curves that lie almost on top of one another in terms of the large and
> small scale structure not exactly because the signal in Canberra goes
> through a different earth atmosphere than the signals at the Mohabi
> Desert here in California, but the correlation between the
> fluctuations you see here is really very impressive.
>
> The X-band does the same thing. This is the attenuation of the X-band
> as a function of time. Again, from station 14, which is the 70-meter
> here at the Mohabi Desert and then five X-band stations observing
> almost identical structures. These are five curves on top of one
> another that essentially observed the same structure. As I said, the
> large-scale structure gives you the temperature and pressure profile.
> The small scale structure gives information about layers in the
> atmosphere, gravity waves in the atmosphere and all of that's work in
> progress that has not been completed yet. The signal is lost here
> because of the differential refraction at the beginning, but as the
> signal goes deeper and deeper in the atmosphere the final turn in the
> curve is not because of refraction; it's because the signal gets
> absorbed by different microwave absorbers within the atmosphere. I'll
> show you in a second that the three signals are absorbed in very
> different ways. The S-band shows very similar behavior, but the S-band
> is also very similar to plasma, very sensitive to plasma, so it's used
> to also sense the ionosphere before we get into the neutral
> atmosphere.
>
> The first dip in the curve here just before, so we're coming from free
> space with the curve flat, but then there is a dip at the beginning,
> that's the beginning or part of the ionosphere. Then the signal
> recovers and goes back through the neutral atmosphere, much like the
> three signals before did.
>
> The next curve shows the X, S, and Ka-bands and how they were
> differentially affected. Again, here you see that during the
> refraction because of the ray bending they more or less suffer similar
> effects, they attenuate in a similar way, but not near the end of the
> observation where different microwave absorbers can affect these
> wavelengths very differently. S-band gets cut off at the end here by
> ammonia, so is S-band, so is Ka-band, but there can be also a loss of
> the signal because of absorption by other gases, like phosphine, so
> the differential absorption you see in here will also be very
> important information regarding the microwave absorbers in Saturn's
> atmosphere.
>
> In the middle region where you see all of the three wavelengths
> behaving very much similarly there will be information about the
> temperature and pressure profile, but also information, the experiment
> is not exactly sensitive to temperature, but temperature over mean
> molecular weight. So we rely on an instrument like CIRS where they
> observe the infrared to say something also about the composition of
> the atmosphere in that particular region where we did the occultation.
> The combination of the two gives information about the absolute
> temperature/pressure profile and also gives information about
> composition, like hydrogen to helium kind of ratio, which is a very
> important characterization of an atmosphere, whether it's solar light
> kind of a ratio or it's depleted in Saturn like Voyager has suggested
> before is something that comes out of the combination of these two
> instruments, Radio Science and CIRS.
>
> Also of the products that come out of the temperature/pressure profile
> is the winds in the Saturn atmosphere. Voyager has shown that Saturn
> has equatorial winds that are enormously large, 500 meters per second
> or so. Recent observations from the Hubble space telescope showed that
> the winds are not that large, probably 300 meters, slightly 300 meters
> per second or so and there is a big puzzle. Did the winds really slow
> down? What happened at Saturn between the time where the Voyager
> observations were conducted and where the Hubble space telescope
> observations were conducted? The imaging system, by tracking features
> in Saturn's upper atmosphere, suggests a velocity or a speed of winds
> that's close to some number between the Hubble and the Voyager
> observation, but in analyzing the radio science data the winds play a
> very important role and in the future by analyzing the data we will be
> able to also constrain the winds of Saturn.
>
> Finally, the neutral atmosphere affects the data by affecting the
> amplitude and phase. The ionosphere primarily affects the phase or the
> frequency of the data, so the S-band, the longest wavelength, senses
> the atmosphere and ionosphere. X-band senses also the atmosphere and
> ionosphere. The differential between using these two combined you can
> remove the neutral atmosphere and keep only the signature of the
> ionosphere. What you see on the slide labeled RSS Saturn Ionosphere;
> Entry Ionosphere Data is a differential frequency observed between
> these two when we use the S and X-band as the wave went through the
> ionosphere of Saturn. Again, that's used to determine how the number
> density as a function of heightened ionosphere looks like and that's
> the next slide. That shows the number density as a function of height
> being a direct curve on entry and the data here is from the S and X,
> but you can also use the X and Ka-band. The Ka-band is very
> insensitive to plasma, so it can be used to remove the neutral
> atmosphere from X-band observations. That's the blue curve you see and
> it shows very similar results. The entry and the exit happened when
> the sun was about to rise on one side and about to set on the other
> side, so these are different kinds of conditions, so when we repeat
> these for egress we see very different results.
>
> The next slide shows the egress Doppler residuals we see due to the
> ionosphere.
>
> The final slide shows the electro density profile looking very
> different from the ingress side. Here the egress, as I said, was
> happening at dusk, while the ingress happened nearly after a long day
> in sunlight, at dawn. So the latitudes are very similar in terms of
> being about ten degrees south or actually seven and nine degrees
> south, but the solar zenith angle is comparable, but one is at sunset
> and one at sunrise, so the electro number density looks very
> different, as you can see in here.
>
> The final slides: That's just the scientific objectives in words. I
> don't need to repeat them in here. The work is really done by a whole
> team and team members are listed there. Most of them are busy in
> conferences, giving presentations, or they are just working with the
> data right now. I got stuck doing the presentation as a whole.
> Somebody should have been here to talk more in detail about the
> atmosphere and ionosphere, but that's what you get.
>
> Questions?

T. Ray There are no questions? Maybe everybody is still taking it in.

Essam, I was wondering if you could just spend a few minutes talking a
little bit about your background and how long you've been working on the
Cassini project and working towards the radio science ring occultations,
because I think that's a compelling personal story that is, I think,
almost unequalled by anyone else on the science team.

Dr. Marouf Well, I was a graduate student in the early 1970s working on
a thesis topic. It was suggested to me that radio occultation is done on
atmospheres and there was a new mission called Voyager coming up and
could we do a similar thing for rings of Saturn. My original work was on
developing the basics for a radio occultation experiment that can say
something about the rings of Saturn. I wrote the thesis in the mid-'70s
about the rings of Saturn that was just the theoretical predictions of
what would have happened if a spacecraft went behind the rings of
Saturn.

Then, of course, Voyager came along in the early '70s and I was able to
sort of peer my thesis work to Voyager and then Voyager did the
experiment in 1980. I was a post-doc at the time. It worked very well,
but Voyager had it limitations, like I described, which is there is a
geometry there that the rings were, at the time, nearly closed and we
could not get through ring B.

Cassini was a dream come true so to speak, because it gave us the
opportunity to do multiple occultations being an orbiter and also gave
us the opportunity to look at the rings over a broad range of geometry.
So I started really working on Cassini in the Phase A study where that
was about the mid-'80s, so 20 years ago, which is hard to believe. But
even during that time these diametric occultations that were envisioned
when the rings are nearly fully open was the driver. We said that we had
to get ring occultations early in the Cassini mission and for the
specific reasons, one, two, three, which is the fact that we probe
denser ring regions. Ring B is the most massive of the rings and it has
probably the most interesting dynamics in terms of the fact that it has
the most material and therefore must be a big driver on the system, but
the planning was a very long process. We started actually planning these
occultations physically in 1995 and we considered so many tools at the
time, one after another, implemented some aspects of these occultations,
but I remember that all of them did the early sequence, which is the
sequence that we are implementing right now.

The work today is really a product of many, many years of not only by
myself, but the whole ring working group, which many of its members were
Voyager members also and therefore they have seen the limitations of the
Voyager observations and have tried very hard to gear the Cassini
observations to capitalize both on what we learned before and what we
hope to learn new from Cassini. Cassini, because of its long time has
given us the opportunity to think things over very, very carefully, but
the combination of the long effort was that occultation on May 2^nd^,
which almost worked to perfection. It's a long wait, but it's a great
reward when things work the way you envisioned them many years ago.

This summer we have eight total occultations from May 2^nd^ to September
5^th^. We have six more to go and then we have 12 more near the end of
the mission, the second half of the mission where we look at the rings
at different geometries. The story is almost true for most of us, which
is we are products of the Voyager generation as young people who saw the
limitations and saw an opportunity to do better.

> Cassini is a marvelous spacecraft in terms of capabilities. Some of
> the capabilities are Cassini, like pointing, like the fact that the
> ultra stable oscillator is 20 times better than Voyager, but the radio
> system is not really very different. It's essentially a Voyager kind
> of technology. What's really different here is that the orbits are
> very, very different and you get the slower occultation. It's a much
> better opening angle and you have much better results not only for the
> rings, but also for the atmosphere because these diametric
> occultations give exceptionally high quality, nearly equatorial
> atmospheric occultations, which will be relevant in the winds, which
> is also a very important issue right now. But the other occultations
> will complement this set in terms of covering different latitudes,
> telling us about the ammonia abundance at different latitudes and
> combined the occultations will give us a lot more detailed picture
> about Saturn's atmosphere, as well as Saturn's rings. I think when you
> add to that other observations that we'll observe the rings also we
> will know a lot about the rings at the very end of this mission, a lot
> more than anybody had imagined.

M I have a question, Professor Marouf. It has to do with the artist's
conception that was a slide you threw in. Based on what you've learned
now what is your picture of how the rings might appear close enough to
see the individual particles?

Dr. Marouf Well, the picture is never put together until you have all of
the pieces of the puzzle solved, I mean worked out in terms of data
analysis and put in front of you. In the case of the picture I showed it
was really driven by some of the dynamical studies that followed the
Voyager observations. What we started with as a picture of the rings,
which is a cloud-like kind of a distribution of particles, very thick
where the particles are far apart from one another and mostly small
particles. That's the picture I started with a long time ago. It has
changed completely. Now we think the rings are a compact layer of large
particles mostly in the same plane with the smaller particles being
spread around them in a thin layer, but not a very thick layer. Most of
the material really is very highly confined to the equatorial plane. Is
this material distributed uniformly? Well, there is evidence in the
data, even the data we haven't analyzed yet, that no, the material may
be distributed in a very peculiar way, that the particles are really
clustered together and they are not clustered uniformly, so they have
this long elongations, which theoretical dynamical studies predicted
some time ago. So the picture is continuously evolving and we have to
bring all of the pieces of the data to veer on how the rings really look
like. The picture I showed at the beginning, the slide you referred to,
still shows the particles uniformly distributed everywhere. That may not
be the case. In parts of ring A and ring B the particles may be
clustered in formations rather than being just randomly distributed
everywhere.

M So it's not so much a random herd of boulders?

Dr. Marouf Right. It's a herd of boulders, but they are in a chain-like
formation, not separated from one another like the picture shows.

M Another question has to do with spokes. I know they have not been
observed optically during this mission so far. Do you think that the
radio science would pick those up should they appear during more closed
rings?

Dr. Marouf Yes. That's a good question. The spokes are a combination of
dusty particles and plasma. The dusty particles would be much smaller in
size than any of the wavelength would sense, but if there is plasma
associated with them then we can sense that in terms of the effect on
the phase of the S-band and the X-band signal, the differential phase of
the X and S or the X and Ka-band, much like an ionosphere affects the
signal. But in terms of the amplitude of the signal, I doubt we'd see
spokes because of their very small size compared to the wavelengths of
all three signals we use.

M I have a question. I'm an ambassador from Picket, Arkansas. What size
would you say the smallest ringlets are or does it go all of the way
down to **... (time 01:44:30)** resolution? How many ringlets do you
expect to find in the whole system?

Dr. Marouf The smallest size we can see is about a millimeter or so and
we see very little of these in most of the rings. Mostly in ring C and
the outer part of ring A like I discussed, but in most of the rings the
smallest size appears to be in the centimeter size range and in places
like ring B it could be even much larger than that. There can be smaller
sizes, but what I'm trying to say is that there is a mean population
that we think exists and when we talk about size we talk about sizes
that belong to that population, but there can be smaller numbers of
particles that are very small and we can't see them just because they
are not really part of the mean size population.

M Right. Do you see any evolutionary changes in the rings from Voyager
to Cassini? You said that the rings would be possibly temporary or
relatively new.

Dr. Marouf Not in the evidence we saw so far. We see, of course, the
main ring features more or less identical to what Voyager showed, but of
course, Voyager did not see a ring B, so ring B for us is new, but we
see a ring B on ingress and egress being about the same and also between
our first occultation we did and the second it's about the same, so it
depends on what time scale we're talking about. By the end of the
mission maybe we'll be able to say something on a longer time scale, on
a four-year or three-year time scale . There are always changes in terms
of when you do an occultation at one longitude and the other, the
changes are primarily because many of the dynamical ring features we see
are tied to satellites, so as the satellites go around in orbit they
drag all of these features with them, so when we cut them at one
longitude we see them differently from another longitude and that has a
lot of information about the dynamics of these features, but the main
features of the rings, like the edges and the boundaries between major
rings and the embedded features that we see from one occultation to
another, those all appear to be not changing on a small time scale.

C. Marble This is Chuck Marble, the Solar System Ambassador out of
California. This has been a very good presentation. Thank you for taking
the time to spend with us. As one of the other questions alluded to, one
of the big mysteries is were these rings a permanent fixture of Saturn
or are they very temporary? I know that's one of the things we wanted to
see with Cassini. I know it's hard to believe, but it's almost been a
year now since Cassini arrived, so we're almost a quarter of the way
through our primary mission. Do you believe we will be able to determine
how long these rings have been around and how long they'll last? If so,
what role will radio astronomy play in this?

Dr. Marouf Yes. The ultimate question anybody who works on rings wants
to address is where did these rings come from? Are they a recent
phenomenon? Is this a moon that shattered and created these rings or is
this something that has been there forever, since the formation of
Saturn, not a relic of the original disk that formed the satellites
around Saturn?

We're constantly collecting evidence. It's an evolutionary process and
we're constantly collecting evidence to try to veer on this question.
The most relevant part of the measurements in our radio science data
that relates to this question is the waves that we see because these
have, as I said, a clue regarding the interchange of energy or momentum
between satellites and rings. Satellites confine rings. They keep it in
place. Otherwise rings spread out and essentially vanish at the end. You
need the satellites to confine ring material and essentially not to keep
just expanding outside and then disappearing, but at the rate that
satellites are being repelled by rings from the data we see right now,
the lifetime of the rings is very short. It is only a few million years
compared to the billions of years for the life of the solar system. The
evidence right now is leaning more towards more recent rings, rings that
shattered essentially, formed by shattering a big satellite that
wandered inside the Roche limit and therefore we may be seeing a recent
structure. But you can't really say for sure until you have more data.
You just have to collect pieces of evidence that support this kind of a
claim. As I said, whatever evidence we have right now is more supportive
of this hypothesis.

M Dr. Marouf, is it possible that given the different characteristics of
the different rings that these rings have formed at different times in
the planet's history?

Dr. Marouf Well, yes. That's a second order, kind of an inference that
is not also very clear. There is definitely very different attributes to
these three different main ring features, but they share more than they
differ. They appear to be all made of the same material except that the
Ring C and Cassini Division show some different colors in the other
types of data that perhaps indicate a slightly different origin, but
mostly they share more than they differ. They all appear to be a product
of the same kind of event, although as I said, I can't really say
conclusively whether three different bodies could have created these
three different rings, but that's a possibility.

M Very good. Thank you for being with us. We wish you a great mission
and, hopefully, a long extended mission.

Dr. Marouf Thank you very much.

M Kind of along the same line, this may be outside of radio science, but
do you have like a percentage of composition of the rings as what are
ice and what are the contaminants?

Dr. Marouf The radio science is not really sensitive to the composition.
VIMS is one of the instruments that will give the visual infrared
mapping spectrometer is one of the instruments of the 12 instruments
that I listed at the beginning. It will give us very good clues about
what impurities is a contaminant, water ice, but like you said, they are
primarily water ice.

M Right. Thank you.

M If there are no more questions I'm just thinking out loud here. Is it
possible that these different rings did form at different times and
therefore, is it at all possible that it's an ongoing replenishment
process and as one of the rings fades away, so to speak, another body is
captured and creates another ring? I mean could this be a self-feeding
process that could continue for millions of years?

Dr. Marouf Yes. I mean there are conjectures right now saying exactly
that. For example Pan, which is a satellite in the Encke Gap will
eventually shatter and create new ring material that will make the rings
look a little different from what we see today and that there is a
continuous process in which new rings are being created from shattering
bigger bodies to create the stuff we see, but on the time scales that we
have right now we can only conjecture these kinds of possibilities. As I
said, we look into data for phenomena that allow us to estimate the
lifetime of these features. Of course, if the lifetime is too short then
probably the rings are there because it's a recent phenomenon. If it's a
recent phenomenon then it will recur and more bodies will get shattered
and we'll see the rings change, but from long time observations we have
so far from ground based, as well as from the recent Voyager and Cassini
the rings haven't changed very much, but when we talk about changes
we're talking about changes over a much, much longer kind of time span,
so yes. I agree with you that it is possible that the rings are being
rejuvenated by the break up of new satellites and that therefore it's a
dynamical phenomenon that changes as a function of time.

M For future generations I hope so only because it's so hard to imagine
an earth night sky without a ring planet in it. Thanks again.

Dr. Marouf You're welcome.

T. Ray If that's it for the questions, I'd like to thank everybody for
attending the CHARM telecon today and thank Dr. Marouf for taking time
out of his busy schedule to speak with us today. We really appreciate
it. We wish you the very best of luck on your upcoming occultations for
the rest of the summer.

Dr. Marouf Thank you very much, Trina. Thank you, everybody.

T. Ray We don't have a particular topic for the next telecon. We're sort
of asking around to see who's available and who's got some good
material, so I don't have anything for you all today about what the next
telecon will be in terms of topics. I do know it will be on Tuesday,
June 28^th^ at our same time. Everybody have a great month.

Participants Thank you, Trina.