**FTS NASA**

**Moderator: Trina Ray**

**May 30, 2006**

**1:00 pm CT**

Operator: Thank you for standing by. The conference is being recorded.

Woman: Well welcome everybody to the (CHARM) Telecon for May. We have a
very exciting topic for you today and we will just get right to it here
pretty quickly. Just a reminder to everyone, that the presentation is
available on the Web site at www.saturn.jpl.nasa.gov. All the details
are in the email you probably received.

And also press star 6 at any time to mute or un-mute your phone.
Questions are welcome. And today we are joined by Dr. Krishan Khurana.
He is a team member of Magnetometer team. That is the team based in
London.

But he himself is actually a research physicist at the Institute of
Geophysics and Planetary Physics here locally here in Los Angeles at the
University of California Los Angeles. And I will turn it over to you Dr.
Khurana.

Krishan Khurana: Than you. Let's go to my first slide which is a title
slide "Magnetic (Portraitsof the Icy Satellites of Saturn." I have
several co-authors listed there. (Michelle Dougherty) is the principle
investigator of the (Magnetometer) team and responsible for both
fabricating instruments and making sure we all get the data.

The other three authors have helped me in various different ways in both
preparing this talk and also in figuring out some of the properties that
I will talk about today.

So I called this talked "Magnetic Portraits of the Icy Satellites of
Saturn." And I used the term Portraits to indicate that from magnetic
fields you can get information of the type that you actually get from
images which is you can say quite a bit about the surfaces of the moon,
about the interiors and the data set is different.

However, we in many ways, capture regular images that you so often see
from the Cassini Spacecraft. So let's go to the next slide, which is
titled Saturn satellites and ring structures.

Here is a well-known painting in the field and the top one shows
basically all of the known moons. Actually these were at the time the
painting was made; now we know many more. Actually there are 59 known
moons of Saturn and we almost every few months keep adding a few more.

The top figure, the top image shows actually all of the moons in the
exact relative proportions. So Titan is the biggest moon where there is
moons that I am going to be talking about today which are Enceladus,
Tethys, Dione, Rhea and Hyperion are much smaller.

Enceladus has a radius of about 250 kilometers, whereas Tethys and Dione
only are about 5009 kilometers in radius and Rhea is about 700
kilometers. Hyperion is just less than 3200 kilometers in radius. And
the lower figure in the same slide shows where these moons are located
with respect to Saturn.

And you will notice that the moon Enceladus is actually embedded in a
ring. The ring is called E ring and it had been long suspected that the
E ring itself had its origin in Enceladus.

Somehow Enceladus is able to maintain the E ring and today's talk will
have quite a lot to say about that. The E ring actually extends from
about a radial distance of about 4 Rs to roughly 10 Rs which is roughly
the location of Rhea.

And it is an extremely tenuous ring, can only be seen when we are
looking at the rings edge on and with the most powerful telescopes on
thee earth. Luckily Cassini has imaged it now several times.

So let's now go to the next slide, the Zeigler portraits of the IC
satellites. And so there are five moons I am going to be talking about
and these are all pictures that are taken by the Cassini imaging team.

These are visible rings images. The Enceladus at the top left. These are
not to scale. I showed them in a way that you can see most about them.
That is why I kept enlarging them until you could see the features I
wanted you to see. So Enceladus shows, has a remarkably young surface.

If you look at the image on the top left you will see hardly any
evidence of any asteroid impacts and we all know that the more impacts a
moon has the more ancient its surface because in the past in the solar
system almost every object has been pelted by asteroids over the last
4-1/2 billion years.

And if the moon or the planet it able to renew its surface, then you do
not see any evidence or you see less evidence of these asteroid impacts.
For example on the earth we hardly see any asteroid impacts, very few
are know to exist.

It has been hit by these repeatedly but because of erosion and the
remarkable renewal that happens from formations of the platetonics is
able to erase all the evidence of past impacts.

And something similar we suspect is happening at Enceladus. The surface
we say looks remarkably young. We think its age maybe a few million to
tens of millions of years or so in certain places. There are actually
other pictures where it appears to be older.

Then we have Tethys top right and it looks like a regular moon which
hasn't experienced any surface renewal. It is about 500 kilometers in
radius. Then we have Dione which is about the same size as Tethys.

Dione I am showing at the bottom left. It shows something, it is said to
be somewhere in between a moon which is able to renew its surface and a
moon which is completely primordial, because what you see some evidence
of cracks, fractures, ridges which I will show later at a better
resolution.

And also some long trenches. So it appears Dione may be a bit more
similar than Enceladus than say to Tethys or Rhea. That is what the
images are telling us and what I will show you from the magnetic
information, magnetic field is that that also holds up in the magnetic
data.

Dione also seems to be slightly active. Next is Rhea and it in
everywhere looks very much remarkably like Tethys and I am an imagining
expert and I have to struggle each time that I get the images from the
Web to figure out which moon I am looking at.

And then of course you have seen the spectacular images of Hyperion at
the bottom right. These Hyperion certainly is a primordial object. Its
surface has experienced extensive bombardment and most, a very large
event also carved out a very large crater which was then buried under
further crater impacts.

And so Hyperion most probably is a captured object which has been
bombarded by asteroids for all its lifetime and doesn't show any
activity that could renew its surface. S let's go to the next slide
which is physical properties of Saturn's icy satellites.

Just to show you where these moons are located, I will not read off the
numbers and what their orbital eccentricities are. The main points I
want to make are that Enceladus is pretty dense. Its density is 1.6 and
this is a recent number from Cassini.

Previously we though the density was much lower at about 1.2. And what
that means is that in addition to ice there is a large fraction of rock
and maybe even metal embedded inside Enceladus. Most models indicate
that it is mostly likely a fully differentiated body where ice and rock
has separated.

A crust of ice overlying roughly an interior metal whose radius may be
as much as half of the moon itself. And then we have Tethys. Its density
is also somewhere higher than ice by not very high and then Dione
actually at 1.4 and some I have seen recent numbers at 1.5 almost for
the density, 1.5 grams per centimeter cubed.

It is also on the dense side, telling us that Dione and Enceladus share
similarities that they have quite a large proportion of rocky material
inside. Hyperion on the other side is like a fluffy ice ball. Its
density is extremely low as one would expect by looking at the images.

The visual albedos are for the icy moons are pretty high. And the reason
is that the surfaces are being bombarded by also the plasma of Saturn's
Magnetosphere. I will talk more about that which tends to make the
surfaces bright.

With Hyperion is located in the outer Magnetosphere of Saturn where
there is not much plasma. And its albedo is very low. It is a very dark
object.

And the surface temperatures, I am quoting them in degrees Kelvin are
anywhere from in the 70s to 80s degrees Kelvin which is awfully cold and
which means that the ice only shown should not be able to move from the
surface. However, it is bombarded by plasma then it could sputter.

But otherwise it should remain extremely stable and the moons will not
lose any ice even over the long geological history in 4-1/2 billion
years. So let's go to the next figure. Next slide called Field and
Plasma Conditions Near Icy Satellites.

What I am showing you here is a picture of the magnetosphere there of
Saturn. And to understand that, I to tell you that from the sun there is
a constant source of steady wind, extremely hard wind that comes out at
super sonic speed, 400 to 600 kilometers per second.

However it is extremely tenuous. The density for example as measured
near the earth's orbit at about seven particles per centimeter cubed and
by the time they get to Saturn is roughly half a particle per centimeter
cubed.

So this very tenuous sort of wind which is charged plasma impacts the
magnetic field of Saturn. Saturn has a pretty strong magnetic field. The
surface equatorial strength is 0.2 Gauss.

And it is surprisingly axis symmetric, that is the dipole of Saturn is
aligned with the rotation axis to the best measurement we can make.
However its sense is opposite to that of the earth's magnetic field.

By the way the earth's magnetic field the dipole axis is tilted with
respect to the earth's spin axis by about 11 degrees, but no so for
Saturn. The satellites of Saturn are located inside what we call the
magnetosphere of Saturn which is the region where the magnetic field of
Saturn is able to prevail over the solar wind coming from the sun.

And a boundary forms, we call it magnetopause. It is indicated on the
figure. It separates the magnetosphere, magnetospheric field lines from
the solar wind and the magnetic field of the sun which is embedded in
the solar wind.

And the plasma in Saturn's magnetosphere, I should also tell you a
little bit more about what plasma is. Plasma is basically molecules or
atoms of mostly these tend to be hydrogen, oxygen, helium, this type of
elements which has lost one or more electrons. So these are not neutral
particles.

These are charged particles because the plasma is very hot by nature and
as a result the electrons, the nucleus of the elements is not able to
contain these electrons around them. They cannot orbit. They escape and
as a result you get an ion which has lost an electron and electrons
themselves.

So they remain nearby. However, the electrons do not orbit the ion. So
as a result you now have a soup of particles where the electrons and
ions of all kinds and that is what we mean by plasma.

This plasma which has its origin in Saturn itself is embedded in
Saturn's magnetic field and as Saturn rotates this plasma is co-rotating
with Saturn and this plasma now because it is co-rotating now bombards
the moon.

It is overtaking the moons as they go in their own Keplarian orbits. The
co-rotating plasma has much high speed, tens of kilometers per second to
hundreds of kilometers per second. Where the moons they go in their
orbits at speeds of a few kilometers per second. So as a result the
lagging sides of the moons are being bombarded by the plasma.

And that is able to sputter neutral from the moons and that is how one
of the origins of the rings which is the E ring happens. The E ring is
formed from Enceladus as we will see very soon.

So let's go to the next figure. Before I can explain to you how plasma
and magnetic field interacts with the moons and tells us about interior,
I have to tell you a little more, give you a background about what
happens to particles in the dipolar field, the field of Saturn
basically.

One can think of these charged particles as little magnets. And the
reason is, if you take a charge and spin it, actually gyrate it meaning
move it around in a circle. It forms a current loop and a current loop
is a magnet and so the charged particles, electrons and ions are also
magnets.

And we know magnets attract other magnets and so the magnetic field of
Saturn is able to hold electrons and ions tightly coupled with the
magnetic field. Once an ion is born inside the Magnetosphere of Saturn
it remains there for a very long time.

It cannot escape its magnetic field. So the principle motion that these
electrons and ions have is we call it gyration and is shown in the
figure on the left. I am showing you figure in magnetic field lines. I
have shown obviously as green line going up and ions will gyrate around
in one sense and electrons in the opposite sense.

And that is the fate of particles, charged particles. The gyro radius of
the ions is about a few kilometers to tens of kilometers in Saturn's
magnetosphere. So, they are executing this tight, circular motion with a
radius of a few kilometers. Electrons on the other hand have gyro radii
which are only a few tens of meters because they are so light.

In addition to this motion, the particles also bounce around in the
field which I show in the figure in the top right. You will see a red
particle bouncing around in the magnetic field of actually this figure
was taken from the earth. But exactly the same things happen at Saturn.

Why this happens is very easy to understand. You have I suggested that
the gyrating charged particle is a magnet and the magnetic field of a
planet is stronger when you are closer to it than when you are further
from it.

So as the particle approaches the planet it sees a stronger and stronger
magnetic field. And as a result, it gets repelled because it is also a
magnet and magnets of the same polarity repel each other.

And so it moves away from the planet and then gets to the other pole and
then sees a different polarity and again gets repelled. So then sees a
different polarity and again gets repelled, so particles basically
bounce around in the field and gyrate around the field lines, and then I
show you a bottom right figure where I'm showing you drifts of these
particles.

In addition to these two motions, the particles also slowly and this is
very small and almost for, to this discussion can be ignored, particles
drift around in the equatorial plane of the planet.

Because this motion is very small, I will ignore it, but the particle
gyration and bounce will be important in understanding how the plasma of
Saturn interacts with the moon.

So let's go to the next figure, and here I have a table, and it says
field and plasma properties near the icy moons, and I have underlined
some of the properties that I want to discuss.

The magnetic, we know the magnetic field that these moons experience and
where this plasma is. We also know the relative velocity; as I said, the
plasma is co rotating with Saturn. So with respect to the moon, it's
moving for example at, and Saturn is with a velocity of 25.5 kilometers
per second, and it's higher as you move outward.

But that is the only, it's even higher velocities. So the plasma comes
-- the moon is moving in a Keplerian sense. The plasma overtakes from
behind but with a very high velocity, tens of kilometers per second and
bombards the surface. And depending upon what the properties of moon
are, one of the two things would happen.

Either, if the moon is completely inert, meaning it has no conductivity
and it has, and the plasma that comes off from the surface is minuscule,
then the plasma that was bombarding it just gets absorbed.

However, if the moon is conducting and that can happen because it has an
ionosphere for example or it creates its own plasma, so you splatter
neutrals from the surface, and then they get ionized either by electron
impacts or by the EUV from the sun.

These neutrals get charged and so as a result, you create a new plasma
source near the moon. And now that plasma is frozen in the magnetic
field local to the moon and, as a result, it resists all the other
plasma which is co rotating coming from behind the moon, and that plasma
has now to flow around.

And I will go a little more into that before I start showing some really
exciting data. So the main point of this table is that if you look at
the mach numbers, meaning, I alluded that the plasma is moving extremely
rapidly, but what are the sound speeds, for example, in the alignment.

And if you look at the sound speeds of this plasma, and compare it with
the velocity of the plasma, you find that the sonic mock number which is
the last line which has been, I have put red line on MS, that sonic mach
number is at Enceladus 1.14 and at other moons is 1.1 at Tethys, 0.89 at
Dione and 1.3 at Rhea.

This number is almost close to one. So what that means is the plasma --
the alignment is almost supersonic to subsonic; it's in that range. If
the number, mock number is one, then you get into what is called a
transonic regime. If it is greater than one, the plasma is supersonic,
if it is less than one trans sonic.

So the mock number, the plasma is roughly transonic in nature. Meaning
it's moving very rapidly but not fast enough that they'll be strong
shock formed around it -- around the object with which the plasma is
interacting.

So let's go to the next figure and now I'm showing you all the
trajectories of the trajectories of Cassini as it went by the moons. So
imagine those black circles that I have put at the center are the moons.
It can be it can stand for any of the moons, Tethys, Dione, Hyperion, or
Enceladus or Rhea.

And Cassini goes by them, by the moons and what you're seeing are the
trajectories of Cassini. It's in a, now I have to explain to you the
coordinate system.

The X-axis is along the plasma flow direction. And this also happens to
be the direction in which the moon is moving. The Keplerian and velocity
is also in the same direction.

However, plasma comes from the left to right in the top left figure, and
overtakes the moon. And ahead of the moon, a wake is formed. And we call
it a plasma wake, and I show that by two black lines parallel lines.

And then I have color-coded the trajectories. Enceladus trajectories are
red. The Dione ones are cyan. The Tethys is blue. Rhea is black, and
Hyperion is green. And the X direction is along the flow direction; Y
points toward Saturn in these coordinate systems, and Z is pointed along
the rotation axis of Saturn.

So that is the coordinate system in which I will show you all the data
and all of the trajectories. If this is not clear, I would like to hear
from people because this is pretty important in terms of understanding
the data.

Woman: Krishan, would it be accurate to say that the upper left of your
graph, you're looking down from on top of the Saturn...

Krishan Khurana: ...right.

Woman: ...down on top of the moon?

Krishan Khurana: Excellent way of putting it, yes. It's as if you're
looking from top -- above the moon and the plasma is flowing to the
right and Saturn is right at the top.

Woman: And then the lower left, you're actually sitting on Saturn and
you're looking out at the moon as it goes by?

Krishan Khurana: That's right.

Woman: And then the lower right, you're sort of behind the moon as it's
kind of going into...

Krishan Khurana: And the fluid coming out of the plane.

Woman: and you're kind of behind it following?

Krishan Khurana: That's right.

Woman: Okay.

Krishan Khurana: So that's a very good way of putting it. So, and I will
be showing you one or the other projections in the rest of the talk for
magnetic fields. So, as I alluded, they're two ways a moon can interact
with the plasma.

And here I'm showing an interaction which is either -- the moon is
conducting or it's able to produce plasma. We call it, and then which
loads the magnetic field lines and we call that process mass loading.

So the, locally around the moon, there's new plasma generated and these
are just the water molecules which came off the surface of the moon,
were picked up so they became -- they got bombarded by electrons in the
background.

And so as a result, they also ionized, or because the EUV from the sun
ionized the, because there's not a very strong atmosphere around
shielding these moons.

So they, the EUV is able to directly get to the -- any cloud that is
generated around the moon, and is able to then ionize the neutrals, so
anything that comes off the surface has a certain time before it gets
ionized, and the lifetime are typically days to weeks for the neutrals
that come off the surfaces of these moons.

And so if that is the situation, that either the moon itself has some
conductivity because it had what we call an ionosphere, which is a layer
of charged particles around the surface of the moon, and that happens to
be in situations where you have a strong atmosphere.

For example, the earth has a very well developed ionosphere. The upper
layers of atmosphere get ionized just from the EUV coming from the sun.

And we call that ionized region ionosphere and it's highly conducting
and because of that, you cannot actually send signals from the earth's
surface out into space at most lower frequencies. You have to have a
very high frequency of your signal so that it can penetrate that shield
of charged particles around the earth.

And so if the moon either had an atmosphere or if it was somehow able to
mass load the flow, then you will look at an interaction that looks like
the figures I show you.

The left figure, this is Slide Number 9, is showing an interaction in
the X-Z plane. So the flow is coming from the left and Z is along the
axis of rotation of Saturn. All of these moons, they're very close to
each other.

And so what happens is the plasma arrives from the left because it's
easily conducting region around it. It tries to avoid the moon and goes
around it. And if you now look at a field line at a very large distance
above the moon, so in the left figure, if you look at a field line at
very large value of Z, it does not know about the moon.

So the field line keeps connecting with the plasma without any bend.
However, very close to the moon, the field lines and the plasma see the
effect of the moon. And as a result the plasma tries to avoid the moon
and the field lines get bent.

So as a result you form these two rings where the field lines are bent
along that Alfvén Wing and the undisturbed outside of the Alfvén Wing
upstream of the Alfvén Wing and we call it an Alfvénic contraction.

If you look on the figure on the right, which is the figure where the
flow is coming out of the plane, you will see the field lines actually
wrap around the moon. They are trying to avoid, they do not try to go
through the moon, but they wrap around and you see this kind of bend.

And by looking at the sense of this bend of the field line, we can say
what is happening locally to the environment and so this is roughly how
we are able to figure out what the properties of the moon are.

So for example, if the moon is very conducting, for example the surface
could have been conducting or if it has an atmosphere within an
ionosphere or if a lot of plasma is being produced, then the field lines
would be bent in the sense that I show you in this figure.

And we'll be able to say quite a bit about the moon itself, and that's
exactly how we were able to see a plume or hypothesize, that there was a
plume on Enceladus way before there was any imaging evidence and I'll
come to that in a minute.

Let's go to the next figure, next slide which is Number 10, entitled
Enceladus. So Enceladus was discovered by William Herschel on August 28,
1789. Like many other moons, the moon radius of Enceladus is 250
kilometers and its density is very high.

And we think it may be fully differentiated with an icy crest and a
rocky core. What is interesting about Enceladus is that it is in a two
to one orbital resonance with Dione. What that means is, every time
Dione goes around Saturn one time, Enceladus goes around Saturn twice,
so they meet each other in a very similar phase of the relationship.

And that tends to pump up the eccentricities of the moons and has many
of the tidal effects on each other. And if we see something interesting
on Enceladus, it may indicate we might see something interesting on
Dione as well, because they are in a resonance with each and they affect
each other's orbits more severely than they do of other moons with which
they're not in resonance.

There's a wide range of surface types. I will be showing you some
figures of Enceladus. You see some very heavily crated old terrain to
very, very young with surface ages of only a few million years.

And Enceladus is tectonically active today and I will show you evidence
of that in a minute and eruptions have been observed. And there are only
two other moons where we know such behavior.

One is Jupiter's Io and the other is Neptune's Triton. Jupiter's moon,
Io, is the most volcanically active moon or planet in the solar system.
At any time, there are hundreds of volcanoes active on Io and we have
both -- we have imaged it and have evidence of that interaction in many
different instuments.

And surprisingly, Neptune's moon, Triton, also shows eruptions, but
those are of basically nitrogen coming out from the interior. And
nitrogen is extremely volatile, so even at very cold temperatures it is
able to remain in a gas form. So that's actually not very surprising.

But Enceladus, which is mainly made of ice, is -- it's very surprising
that there will be a plume there which -- because it suggests that we
are able to heat the interior to temperatures where water is either in a
liquid or vapor form, that it can fall out of the moon.

So we have looked at the outcasting material and it is consistent with a
-- coming from a liquid water source. So it's quite likely that today,
Enceladus has a subsurface interior ocean or lake inside it. We do not
know much about other properties, but the fact that there are plumes
would suggest that there's a liquid water source inside.

So Enceladus has, as I have said in the past, an icy crust on the
outside which has impurities in it and dust particles and all of these
are -- when there's a plume present, all of these also escape along with
the water and populate the E‑ring. E-ring is known to be made of both
dust and very small neutral particles of micron-type of size, and it
appears Enceladus is the main source of that ring today.

Let's go to the next slide, where I'm showing you an image of Enceladus
and it's a spectacular image which shows many ridge like features, many
long trenches and also at the same time, regions like at the top, at the
-- near the North Pole which look pretty old. Because you see asteroid
impacts there, but not any -- much evidence of activity.

To our surprise the most active region on Enceladus was in the -- near
the South Pole and let's go to Figure 12, Enceladus' deep color view of
transitional terrain.

And this is an image of where -- it's actually in an intermediate
region, where we are very close to the South Pole but not exactly the
South Pole, and it shows myriad cracks, fractures. And also at the same
time smooth planes which -- where all evidence of asteroid impacts has
been erased. That is to the top left for example.

And then you see also certain -- a few impacts. And so if I would look
at this image, I would suggest that the oldest terrain is to the
northeast, and the terrain gets younger as we go to the south and it's a
very high resolution image, per pixel is about 150 meters.

So let's go to the next figure and which is entitled "tiger stripes."
And what you see is a spectacular image of Enceladus taken by in the
visible range by the imaging team. And these features, long lineations
parallel to each other have been called as tiger stripes.

And what -- the figure shows that the color of stripes itself is
different, and we believe it's because of the impurities that the water
is bringing out from the surface is what causes the -- some of the color
differences. But the color differences also arise from other effects,
like the crystallinity of ice.

If the ice is extremely cold, then it is brittle and it becomes
amorphous over time. However, if you heat it towards the melting point,
there is a transition and the ice becomes crystallized and the
crystallinities also changes its visual properties. It changes color; it
changes its appearance.

And so what you're seeing on this figure, on this image is a combination
of both impurities that are present in this region, and also the
crystallinity differences that are being caused by temperature
differences.

So just by looking at the image, we are able to say that this is a hot
spot -- the region on Enceladus because the ice is more crystallized.
And later images have shown that there's a plume that comes out of this
region and I will show that to you a bit later.

Let's go to the next slide, now which I'm showing you for the first time
our magnetic field data in the XY coordinate system from three different
fly-bys of the moon.

The moon is at the center; the circle is the moon. And there's a Day 48
trajectory, Day 68 and a Day 195 trajectory. A lot of -- these were all
happened last year in 2005.

And we went by the moon three times, pretty close to the moon, and each
time we saw some deflections in the magnetic field which I show as
vectors plotted along the trajectories, which are the lines.

And you can see that the magnetic field was perturbed by the presence of
the moon over a very large distance, about as much as 8 to 10 Enceladus'
radii. And that is just amazing. That a moon which we knew is active on
the surface, but how could it affect the magnetic field over such a
large distance.

And by -- when we first got the data from Day 48, we were very -- it
showed us that if the moon, which is either conducting or mass loading,
and when we got the observations from Day 68, it confirmed that picture
completely.

And we then suggested to the project to have an imaging campaign for the
third fly-by that was going to happen on 195, where we lowered the
trajectory of Cassini, bring it much closer to Enceladus, and as you can
see, the red trajectory is much closer than the other trajectories.

And also at the same time have imaging campaigns to look for the plume.
And as you'll see, we were very successful, both in persuading the
project to do this campaign and also when the images and other
measurements came out.

Instead of just a magnetometer seeing the evidence of the atmosphere or
a plume, eleven other instruments confirmed our findings, including
imaging where they actually imaged the plume, and also instruments which
are able to directly measure the content that is coming out of these
plumes.

So let's go to the next figure, where I'm showing you how we are able to
model that signature. So we -- I showed you a magnetic field signature,
and by understanding the current flow system in this -- either the moon
or a cloud around it, we are able to build a model and I'm just showing
you a picture of that model in the figure, and then able to model the
signature.

And so let's go to the next figure, where I can show you the results of
modeling from this type of analysis, and the figure name is shifted mass
loading region E3.

So it takes -- it's a very busy figure so I will take a little time
explaining what it is showing you. First of all, let's actually go to
the figure at the bottom right, where I show you once again the
interaction that I've shown you in the past, of a conducting moon.

And the trajectory overlain in the XZ plane and the YZ plane. XZ plane
is as if you're looking from Saturn at the moon and ZY plane is where
the flow is coming towards you out of the plane.

And the trajectory's overlain and you, if you look at the top one, you
expect a bend in the field line which is in the negative sense. As you
can see the field lines are bent away from the positive X direction
where the trajectory is in the left part of the lower right figure.

It's the figure that is showing you the perturbation and if you look at
the left hand side, I have, the trajectory's in the red by two arrows,
by arrows, and the feed lines are bent in the negative X direction.

If you look on the right figure, you will see that as the space cap past
Enceladus it would foresee a positive perturbation and then in negative
perturbation as this dripping pattern is shown on the right hand side.

And now let's go to the left figure where our observations are displayed
as a function of time. I have plotted about half an hour of data there
and you can see that we are, the, what was predicted from this type of,
what we call a draping geometry where the feed line is draped around the
moon and also the predictions from the model.

However, I have to tell you something, to be able to get this kind of
close match I had to shift the region in where the plasma is being
created from Enceladus to below it.

And, as you will see later, it makes perfect sense because the plume is
coming from the south pole of Enceladus so neutrals comes out of the
plume, they travel quite a large distance below the moon before they're
ionized.

So the effective loading region is below the moon, so as a result I had
to shift the mass loading region by about four Enceladus radii below
Enceladus to, and then when I did that, I could get a very good fit for
the data.

Man: Excuse me, I have a question.

Krishan Khurana: Sure.

Man: Yes, in the bottom right, V is where always a velocity...

Krishan Khurana: Velocity's coming out of the plane, yes.

Man: And B is...

Krishan Khurana: The magnetic field and J are the currents that are
flowing in the system.

Man: Oh, okay. What currents are those?

Krishan Khurana: These are the currents that I alluded to in the
previous figure so let's go back to previous figure which is 15, which
is modeling of our Alfvén Wing current system.

Man: Right.

Krishan Khurana: So, think about it this way, there is a flow directed
at this object which it turns out to be is not Enceladus itself but its
plume, and because you're creating plasma there, the feed lines had to,
the feed lines would award this object and the one easiest way to
understand that is that the moon experiences an electric field which is
directed away from Saturn so it's as if you put a conductor with an
electric potential around it.

And as a result, currents flow to the moon. And I'm showing you currents
flowing in a direction which is the currents are flowing away from
Saturn in this cloud region.

You'll see three rings I have drawn on top of this object and these
currents get drawn in along the feed lines on the side of Saturn, on the
side of the moon, facing Saturn, they flow along in the object itself
which is the plasma, in the equitorial plane, and then they exit on the
side away from Saturn, once again along the field line.

So, the field line, the currents are field lined, they come into the
moon on its charge in alignment which is the plasma cloud, travel in the
plasma cloud in a sense of radially away from Saturn, and then exit
along the field lines.

And so, that is the interaction that happens, those are the currents.
Now let's go to slide 16.

Man: Let me ask you, were these plume disturbances made before any
visual evidence was made?

Krishan Khurana: That's right.

Man: Oh, okay.

Krishan Khurana: Yes.

Man: Okay.

Krishan Khurana: So we were looking at this other preservations and we
concluded either that it's an atmosphere or a charged ionosphere near
Enceladus. Both of them suggest that there is either a plume or a
permanent atmosphere on Enceladus.

From our modeling we cannot tell what is, which of the two possibilities
it is but it tells you that there's something interesting occurring near
the moon.

And by modeling these currents, we are also able to say how much plasma
gets picked up near the moon and, on this figure 16 I'm showing, giving
you number, it's about two kilograms per second. It's a very small
number compared to how much of the mutual stuff that comes from, comes
out of Enceladus and the plume.

If we estimate now it's several hundred kilograms per second of
basically water and icy materials that is coming out of the plume every
second, several hundred kilograms per second.

Out of that, about two kilograms per second gets ionized, becomes
charged, and that is what makes the bends in the field, not the rest of
the hundreds of kilograms per second because that's all neutral and the
magnetic field doesn't interact with neutrals.

So, let's go to the next figure where I'm just showing you E4 signature
which was the next flyby and when I do a modeling as a cloud center
Enceladus you can see the red and the black places, black is the data on
the, in the left figure and red is the model (bid on match).

And the reason is, as I've already suggested, that the region which
isdeflecting the magnetic feed lines is actually a plume which is
located below Enceladus.

So, in the next figure, I'm showing what we think is happening, this
figure 18, says Enceladus plume interacts with plasma. So, hard plasma,
which is of Saturn's magnetospheric origin is coming from the left at
the moon, which is Enceladus and there is a jet coming out from the
south pole of Enceladus and it is slowly getting ionized.

And that is where the magnetic field lines are getting deflected. If you
look at where Enceladus is, field lines do not know that there's
Enceladus there.

However, feed lines bend around the plume region where there is, there's
a large quantity of newly formed charged particles and the hard plasma
and the magnetic field tries to avoid it. It, both slows down and that's
what gives the bend in this projection.

And if you were looking in the other projections, the YZ projections,
you would see the feed lines drape around this plume and create
perturbations in the sense that can tell us about where the plume is and
how strong it is.

So, next figure is called fountains of Enceladus and these are the
direct evidence, these are images on the left hand side, the image which
is just a regular image but at a high phase angle and the moon is mostly
dark.

And they have saturated the image to be able to show you direct evidence
of plumes coming out from the South Pole region. That is actually the
South Pole, where you see the plume coming out from.

And on the right hand side, they saturated the color table even more to
show you how big this region is and what the plume look likes. It's the
same image actually but with a different saturation of colors.

And, so here is refutable evidence that there is a plume coming out of
Enceladus and it was this plume which had created the signal in our
magnetic field maximum.

Which by the way, very often you're making at distances of several moon
radii, for example, the first fly by in which we saw the evidence of
this, we were at about six moon radii away, almost 1000 kilometers away
from the moon, and yet we were able to detect the effect of this plume
in our observation. So magnetic fields are extremely sensitive in terms
of the properties that the moons have.

So here in the next figure, which is slide 20, the shifted mass loading
region RE, I tried to improve my model by shifting the plume by four RE
below it and you can see the matching plumes.

It's not perfect because I would have to know quite a lot about the
plume's shape and size, which I do not, to be able to get a good fit.
But, the phasing is in the right sense. Where I expect to see positive,
I see positive.

Where I expect to see negative, I see negative perturbation. And that
tells me roughly where the plume is located.

And E3, once again I'm showing you, is a very good fit but only if you,
if you assume that the mass loading region was located below it, so on
slide 21 which is a shifted mass loaded region E3.

Let's go to the next slide which is a shifted mass loading region E11 is
just another fit showing you that there also I can explain the
observation better if I put the plume below Enceladus rather than have
the mass loading regions right at Enceladus.

So, no more about the modeling, the next figure says weight of mass
loading. As I alluded, the current that is created in the environment is
directly proportional to the weight of mass loading.

And that's all these equations say. What I want to point out is the last
line which M dot equals 1.8 kilobytes per second. Other modeling
suggests that the pickup rate is of the order of 1.82 kilograms per
second of plasma in this region.

So it's this very small amount of pickup which we are able to detect
even at 1000 kilometers away from the moon that gives us the ability to
say something about the properties about this moon.

Now let's move to the next moon, Dione. And I see that I have almost
spent an hour talking about Enceladus, probably because it is the most
interesting object. But, now I will go in a much quicker sense -- speed
to the other moon.

Dione, it was discovered by Cassini in 1684 and it also has a
surprisingly high-density of 1.5 grams per centimeter cubed. It's also
probably differentiated, and as I've already suggested, talked about it,
that it's in orbital resonance with Enceladus. And let's actually get to
the images directly.

So let's go to the 25th one. Here I'm showing you a very large blow-up
of Dione image and what you will see on the top right region, which is
the northeast region of the moon, cracks and fractures and ridges. And
these are nowhere near as dramatic as you saw it in Enceladus but
nevertheless, they are there.

And there's some indication that they have renewed the surface, maybe
today or maybe in the past, we cannot tell. But, Dione has some
similarities with Enceladus, that's what this figure tells us. And you
also see what is called the wispy terrain and for that let's go to the
next figure.

Dione fractures in wispy terrain and on the right inside I'm showing
you, the region is full of wispy regions where there are trenches but
around them is a wispy material.

We do not understand what the origin is but the two may be connected.
And on the left hand side, you see another image of Dione and it again
shows an amazing number of ridges and trenches and cracks which hints at
the renewal of surface either in the past or today.

Let's go to the next slide and there are magnetic field observations
from Dione in the top left figure and what you see in the coordinate
system that I introduced that the main thing is the X-component of the
magnetic field which is positive, a small perturbation that's the top
left figure.

The top trace in that which is marked BXBIIS, which means Dione
Interaction System Coordinate System, DX component magnetic field. And
you see the perturbation is positive.

Let's look at the trajectory now and is shown by the cyan color and I've
also put an ellipse around it to highlight it and so we are located just
below Dione and we are slightly upstream of Dione in the plasma frame.

So at this place if you now look at the bottom right figure we expect
the perturbations the magnetic field would be bent in a positive accent.
You can see draping around and that is exactly what we see in the Dione
data.

So once again this moon seems to be similar to Enceladus which is very
surprising because so far none of the other instruments like in the
past, even for Enceladus have shown us any evidence of any activity from
Dione.

However our magnetic field instrument is picking up a signature, by the
way the perturbation is very small, it's about half a nano- tesla
whereas when we were looking at Enceladus data it was of the order five
to ten nano-tesla, so Dione if it is creating a plume or mass loading,
it's a much smaller source. But nevertheless it is hinting at some kind
of activity inside.

Man: Excuse me. I have a question.

Krishan Khurana: Sure.

Man: You had mentioned before that the best model was for Enceladus was
where the plume was coming from the South Pole.

Krishan Khurana: That's right.

Man: Does this also suggest from the origin is the South Pole?

Krishan Khurana: No. Unfortunately no it is certainly not faultable. It
could be not (unintelligible) for this one or anywhere on Dione. We were
located below the moon so if the plume was below us the perturbation
sense of x component would be different. So we are certain it's not the
South Pole but where it is on Dione from just this one fly by we can not
tell.

Man: Thank you.

Krishan Khurana: So what we are telling the project right now is to give
us more fly bys of that Dione in other locations so we can pinpoint
where the source of plume may be. And by looking at these perturbations
from many different locations around Dione we should be able to point
out roughly where it is, as we were able to do for Enceladus.

So the next figure is Dione data now blown up. On the right hand side
I'm showing you once again the same data set but what I've done is I
have taken out the background magnetic field so I can blow up the data
set a bit more.

And when you do that you can now start seeing even in the Y component a
slight perturbation and near the moon the perturbation where you saw the
X component go positive in the top trays, below that you'll see the B-Y
component and it first goes slightly positive and then slightly
negative. I hope everyone can see that in the B-Y component.

And then I show you trajectory on the model on the left hand side top
figure, and you can see the magnetic feel lines, I expect them to be
bent in the sense to a Y Axis first, that is positive Y and then as I
travel below the moon and away from it in the negative sense of Y. So I
expect to see a positive and then a negative in the magnetic field
perturbation. That's exactly what we are seeing.

So this is further confirmation that the field is draping around the
moon and is not just right going through it, if the moon was completely
inert as I will show you the other moons are, the field lines just go
plow right through the moon.

Plasma gets absorbed and you do not see any perturbation in the Y
component of this sense.

So let's go to the next figure and now I am switching the types of
moons. These are two moons which were mass loading types and now I'm
going to talk about moons that are completely inert. Those are the three
other moons that I'm going to talk about.

So to understand that kind of interaction we have to understand once
again the properties of the plasma which is plowing into these moons
which is the plasma status origin and here I'm showing you what are
called the phase space densities of the plasma.

To understand this you do a cut in the velocity phase space. Basically,
you look at all the particles and see what their velocities are, and
then you, according to the direction of the field, you look at the
velocity for the parallel to the field or perpendicular to the field.

And then you ask the question, "How many particles have a particular
parallel and a particular perpendicular velocity. And that number of
particles we call the phase space density. So it's just a graph of how
many particles have what kind of speed.

So it's a similar way to how we look at things on a map in terms of
location, and the location in velocity-space is given by these types of
plots. So, on the left-hand side is a plot that contours of phase space
density of the plasma in the v-parallel, z-perpendicular,
two-dimensional plane, but on the right-hand side I show you cuts of
that.

So in the top right is the z-perpendicular plane, what the velocities
are in the perpendicular direction, and on the y-axis plot is the number
density. So, particles with a certain perpendicular velocity have that
much number density. That's how you look at these plots. So because the
plasma, for example, the earth moon is sitting in the solar wind and
that is interaction of an inert moon we understand very well.

There the phase space densities look just like these actually I took
from the Earth's moon. So this is these are phase space densities of
plasma, which is supersonic. And, let's go to the next figure.

And now you can see why I was interested in showing you the previous
figure. Here you will see the moon as the circle on the left-hand side,
and once again z-x plane and z-y plane, exactly the same coordinate
system.

Flowing, coming from the left, and because I said that the plasma is
flowing mostly in the direction perpendicular to field line, there is
very little parallel velocity in the plasma. It's very cold.

So, it is not moving up and down the field much, but it is being just
carried by the flow to the moon. So as a result, when the moon, when the
plasma plows into the moon, at that time it has no time to escape, move
along the field line.

So as a result a particle which is located on the same field line which
is in contact with the moon but is above the moon or below the moon,
those particles do not have the time to fall into the moon by the time
the field line convects past the moon.

So as a result you get a plasma flow, a wake behind, I call it plasma
flow shadow, which is roughly the diameter of the moon itself. So, it's
a cylinder with diameter the same as that of the moon.

And the reason is the particles have no time to escape. If, for example,
the particles were also moving up and down rapidly, then this plasma
flow shadow would be much larger in volume and in cross-section.

And, because the plasma is now depleted in the shadow, when you look at
it cut behind the moon. So this figure on the right-hand side is showing
you a cut at roughly two moon radii behind the moon, and it's showing
you what happens to the field lines.

You have evacuated the plasma there, and as a result the plasma pressure
there has fallen. And something has to make up for it because the total
pressure in the environment has to remain the same. And as a result the
field lines get drawn into the wake.

And I'm showing you those in the right-hand side figures, but, when you
look, but the size of that disturbance is roughly the size of the moon
itself, and that is the supersonic interaction.

Let's go to Page 31, which is the phase space densities in a sub-sonic
plasma -- and this is like I previously showed you, but now the plasma
is hotter. So it has velocities which are pretty large now, both along
and perpendicular to the field line.

So as a result when you look at the space of these densities instead of
a very narrow beam, you see this lots of contours, meaning that a lot of
particles have high enough velocities both along and perpendicular to
the field and that's what this figure is showing. Let's now take this
kind of distribution of plasma and have it interact with the moon.

Let's go to next figure, which is mass absorbing sub-magnetosonic
interaction, and what you see is actually what you would expect to
happen. I'm showing the flow coming from the left, the left hand side
figure.

This is the XZ plane figure. And now think about particles which all
have very large velocity along the field line. And let's start with a
particle which was located below the moon but was connected with the
moon.

So as it is, and let's assume it was actually gyrating. It was actually
bouncing along the field line towards the moon. So let's look at this
cylinder that I have drawn at the bottom left, an incline cylinder.

Any particles which are located in that cylinder, which would be
particles of a certain parallel velocity which have been carried by the
flow, none of them can escape the moon because as the flow gets closer
to the moon.

The particle is getting, edging upwards closer to the moon and will plow
into the moon, and so all the particles with that particular pattern of
velocity and in that cylinder get absorbed, forming a shadow downstream
of the moon, shown in yellow, so that would be the shadow.

The cylinder shown by yellow, yellow cylinder at the top of the figure,
top of the figure which says particle shadow ring, is created by those
particles which are absorbed which are coming from below and now are not
there anymore.

Similarly, particles which are traveling towards the moon from above the
moon, they will be absorbed and their shadow ring will be below the
moon. And so all the particles that are coming, depending upon their
energy, create these shadow rings.

And let's now look at a cut in the YZ plane, the figure on the right.
And this is again drawn at about roughly two moon radii behind the moon,
so it's a cut at X equals two moon radii, for example. And what you
would see would be all these particle shadow wing.

Right at the center the gray one is the cold plasma wing. This is akin
to the supersonic plasma wake that we have observed and studied at the
earth's moon itself interacting with the solar wind.

However, particles with higher parallel velocities, their shadows would
be at a larger distance from the central wake. And these I'm showing you
by blue, fine, green, yellow and red colors, and you'll notice that
because the particle shadow rings are inclined, their cross sections
would be elliptical and more and more elliptical as you are a larger and
larger distance from the moon.

So one would see behind the moon shadows of these particles, meaning the
way you'd expect to see certain particles with a certain velocity, and
you don't see them. And but the main effect is that you have reduced
particle density, at not only right in the cold plasma wake but all
along the field line behind the moon, on the downswing of the moon.

And that means now the field line would be drawn in, not only in the
cold plasma wake, but also at very large Z values. So I'm showing you
with the dotted lines, field lines, what they would look like.

And if you compared that with the previous figure I had shown you for
subsonic -- supersonic interaction, this interaction is extremely broad
because there were very large values of Z. And that is exactly what we
saw for, towards the moon, and I will show you that in the coming
figures.

But I also want to point out what happens to, I also have shown you what
I call energetic electron shadow. The electron even for the same energy
as an ion, for example, moves much more rapidly because the mass is so
small.

So for the same energy they have much higher velocities, and as a
result, they move up and down the field line extremely rapidly.
Actually, if the electrons have energy of say a few hundredths of
kilowatts and higher, the whole field line gets evacuated of those
energetic electrons, and that's what I'm showing you.

There's a shadow which is, which extends all across the field line, and
so you have to imagine the field line as ultimately terminating into the
ionosphere, both north and south, ionosphere of Saturn, so these are
dipolar field lines. I'm just showing you in the equatorial plane
uniform feed lines, but one end goes into Saturn in the northern
ionosphere of Saturn and the other end returns to Saturn in the southern
region of Saturn.

And so the whole dipolar field line gets evacuated of electrons, and
that is what causes a very large signature. But notice the sense of this
signature is opposite to that of the mass loading case.

I'm showing you here field lines which have been drawn in rather than
draping around an obstacle, and so by looking at the sense of these
perturbations, we are able to say if the moon is inert or if it is
active today.

And let's go with that introduction look at the signature of Tethys.
First a few points about Tethys: it was ...

Man: Excuse me. I have one question.

Krishan Khurana: Sure.

Man: Does, so would you characterize Enceladus then as non-absorbing?

Krishan Khurana: No, that's right. Most of the plasma is able to avoid
Enceladus. It drapes around Enceladus. Some of this does fall into it,
but most of it is able to avoid the moon.

Man: Okay, because I noticed on the slide you said it's, you know,
consistent with an absorbing moon model. I was wondering if it's an
opposite in Enceladus's case, but it's not. It's just a low, much lower
number then?

Krishan Khurana: No, this moon, now I'm going to be talking about
Tethys.

Man: Okay.

Krishan Khurana: And Tethys is a mass absorbing. There's hardly any
plasma generated near Tethys, so all of the upswing plasma that was
coming at the moon gets, falls into the moon and gets absorbed.

And actually as I've shown you in these figures, it's not just the
plasma which is directly aimed at the moon, but even plasma which was at
large distances but in the same field lines which got connected with the
moon.

All of that plasma along the field lines will get absorbed, a good
fraction of it will get absorbed.

Man: Absorbed onto the surface, correct?

Krishan Khurana: Onto the surface of the moon, yes.

Man: Okay, I just have one more question. I 'm still a little confused
with that term mass loading because I associate mass loading with, you
know, material being deposited on the surface, which is not correct,
right?

Krishan Khurana: No, mass loading of the field lines. That's the term,
so you have field lines, and they have picked up new plasma near the
moon or fenced the moon in new plasma near the moon, so because new
plasma has been deposited in the field lines.

They slow down because the total momentum has to be conserved. And you
are now taking momentum from the existing particles in the field lines
and giving them to the new particles that have been just born.

Man: Thank you.

Krishan Khurana: That's what we mean by mass loading. And thank you for
correcting me, for making it clear for others. So it's the mass loading
of the field lines.

So let's go to Tethys discovered by Cassini in 1684. It's similar size
to Dione, and we do not know much about it in terms of sector, but we
suspect it's mostly pure ice because its density is pretty low.

So it has some rocky material, but some of the higher density
enhancement from one is just from the compaction of the ice that gave it
depth.

It\'s in a 4 to 2 orbital resonance with Mimus, which is in a moon,
which is a very small moon, so it belongs to almost nothing and it's
maybe just fortuitous that they are in this resonance today.

They're not affecting each other's orbit in any major way that we can
see. The terrain is mostly heavily cratered. The surface age is several
billion years. That's what we are seeing from the images.

And something very interesting about this moon is that two other minor
moons, Telesto and Calypso, and they inhabit the same orbit as that of
Tethys, but they're located within Tethys's Lagrange points, L4 and L5,
which is sixty degrees ahead and sixty degrees behind Tethys in its
orbit.

And it's a point where the gravity of Tethys and Saturn cancel each
other and anything which is located there can have a very stable
relationship with Tethys and Saturn. So these must be some captured
objects.

When this happened we do not know and how stable these moons are also is
not known.

So let's go and look at the picture of Tethys in slide 34, which is just
Tethys, and you can see there is a very large gash on the surface, which
appears to have some impact origin. No, we do not see any evidence of
internal activity. It's all craters basically. The surface hasn't been
moved today or in the past.

And now let's go and look at the observation from Tethys in the next
figure, which is actually Tethys and has three figures in it. On the
left is the figure showing the magnetic field perturbation.

On the right top is the trajectory plot, and the model that I showed you
earlier is as below but with the trajectory now of Cassini superimposed
on it.

So actually let's begin with the bottom right figure. The blue traces,
the blue arrow in the right hand figure YZ projection is showing us that
the Spacecrafttravels from positive Y to negative Y, meaning from
Saturn.

It was moving away from Saturn at that time, and it was located below
the moon Tethys at that time when it went by Tethys. And so this figure
would suggest that you would, if the moon is inert, you would expect to
see first a bend in the field line which is toward Saturn, BY positive,
as you can see in the dashed lines that I have shown in that figure, and
then perturbation which is away from the moon, BY negative.

And now let's go and look at the observations in the left hand panel,
top left, and you can see that there is a clear BY positive first and
then a negative perturbation.

You do not expect to see much signature in the X component for moons
which are inert. And actually X component is the strongest component for
moons which are active, which have either atmospheres or mass load
plasma, the background plasma.

So the fact that we do not see major perturbations BX and we see a
perturbation and Y component consistent with mass absorbing tells us
that this an inert moon.

And in the next figure I just dwell on that a bit more. I have taken out
the background from the magnetic field so the data on the left hand side
is the same one as I showed you before, but now from the top three
traces I subtracted out the background so that I could expand the
figure.

And you can see that the perturbations I was alluding to in the BY
component is really very strong and clear and that the other thing is
magnetic field strength becomes more southward, meaning stronger in its
strength, and that is also expected as you can see.

The field lines are drawn in toward the wake, so they get closer to each
other, and this is in response to the fact that we have evacuated
plasma, reduced the plasma pressure, and something has to take its turn.

And it's the magnetic field which gets sucked in, drawn in, and enhances
its strength and becomes, more southward and also bipolar signatures and
Y component.

I have been avoiding talking about all the hashes here on these figures.
These I'm going to call ion cyclotron based, and these will be with a
very pure tone, and they are from the gyration of ions on the magnetic
field lines.

And they are seen throughout the E ring, and they in general have
nothing to do with the moon, but they have everything to do with the E
ring. The E ring is mostly very fine particles, mostly perhaps ice
particles which are slowly being ionized by the UV from the sun and from
electron impact.

That creates new ions, and when new ions are created, they create waves,
and that's what you see. So these are waves created by the E ring
itself, and if there was a local source of plasma at the moon, then
these waves get enhanced.

I forgot to mention that for Enceladus we do see that enhancement near
the moon, so that's kind of indirect evidence that plasma is being
picked up by the magnetic field near Enceladus but nothing like that
happens at Tethys.

Let's go to Rhea, which is slide 37, discovered by Cassini on December
23, 1672. It's a bigger moon than all the others. It has a radius of 760
kilometers, and its density is on the high side, but for such a large
moon, it's not too high because, as you know, self gravity tends to
increase the density of even icy objects.

And the internal structure is not known, but we think it may be mostly
ice with some rock mixed in. It doesn't seem to have differentiated.
It's in a resonance, five to three resonances with Dione. It's actually
not exact.

It's a near resonance and normally those don't count because over a long
period, they cancel out. You have to be in the perfect resonance with
another moon to affect its orbit. And the terrain is once again heavily
cratered and doesn't show any activity.

So let's go to the next figure which shows a picture of Rhea, and you
won't be able to tell from that of Tethys. Once again, it looks like the
earth's moon or an inert object which has been pelted by all kinds of
impacts, and it doesn't look like an active moon.

And when it, however, in the next figure 39, it does show some wispy
features. We do not understand the origin of those, but you can see
those wispy features are not associated with any surface geology.

They're just some markings on the surface. It's as if something got
deposited on the moon, and in the past people have suggested that
materials from the E ring or from other moons actually somehow gets
deposited on Rhea and that this is not of internal origin.

Now let's go to next figure, which shows the magnetic field data. It
shows Rhea on top, and it shows three figures. The one on the left is
the magnetic field data. You're familiar with it by now, and you're
seeing now the biggest perturbation is in the Z component.

It gets stronger from minus 24 to minus 28. The field is southward in
Saturn's magnetosphere, and it becomes more southward. At this time the
spacecraft is located I show that in the - on both on the top right
panel and on the bottom right panel by a little dash. That is the
trajectory when this data set was taken on the left hand side.

And so, as you can see, Cassini was in the cold plasma wake when it flew
by Rhea, and it was slightly below the central location of Rhea from the
equator of Rhea.

So it was just below Rhea actually when these observations were taken.
And one expects a certain type of perturbation. Let's go to the next
figure, and I sort of illustrate that in the bottom right figure, what
you expect the field line to be.

Again, because it's an inert magnetic moon, the field lines are getting
drawn in. And if you're located below the moon, then the sense would be
that as you move from left to right, you would first see field lines
which are pushed away from Saturn direction, having negative Y
perturbations, and then towards Saturn.

And that's exactly if you look at the BY component you see and also
because the field lines have been drawn in. I draw them much closer to
each other in the direct wake where the colored circles are, and that
indicates the BZ component will get strengthened. And that's exactly
what we see.

So once again by just looking at the magnetic field data remotely but in
the alignment of Rhea, we are able to say quite a lot about its surface
properties, that it's an inert moon, that there's no plumes on it, and
that all the plasma that interacts with the moon gets absorbed.

And finally Hyperion and then I have to actually disappoint you. We have
observations, but we, hello, are you still there?

Woman: Yes, we're still here.

Krishan Khurana: Okay, and we cannot make much sense of it. Let's
quickly go through it, so Hyperion was discovered by Bond, by two
brothers Bond and Bond, and William Lassell, all independently, all in
the same month in 1848.

It's a very irregular object. Its internal sector is unknown, but from
the very low density, we think it's purely ice, and it's in a four to
three orbital resonance with Titan, which could do some orbit
modifications.

But what it actually does most is that because Titan is such a large
object and Hyperion is such a small object, that Hyperion is not even
able to have a well-defined rotation axis. So it just keeps tumbling
chaotically because of its resonance with Titan.

However, it doesn't appear that there's any much tidal heaping that is
generated by this resonance. And it has a very, very low albedo, which
tells us that the surface is mostly covered by a dark material, and
we've seen that before.

For example, comets in general are extremely dark. Their surface is very
dark, and the reason is most of the times, the impurities that are
present in the water that are left behind coat the comet, which is able
to evaporate away when the comet gets close to the sun.

And we think something similar is happening at Hyperion that some of the
water is able to escape. Water molecules are able to escape, but the
impurities are left behind and they coat the surface and that's what
gives the low appearance.

And the next one is an amazing picture of Hyperion and all kind of names
have been given to it: Honeycomb and all different people describe it in
different ways, but it's one of the most stunning images I think that
have come from the Cassini imaging team.

And it shows a surface completely inert pelted by every imaginable rock
and with a very large gash around its equatorial region and doesn't show
any intervening surface renewal.

Let's go to next figure, which is almost the last figure, the last
slide, and then I will conclude. And here I show on the left hand side
the location of closest approach to the moon, and we could not find any
evidence of any perturbation in the magnetic field.

There are perturbations occurring away from the region, but not right at
the closest approach. And the trajectory was such that if the, if we had
actually flown in the place that we think we did, we should have seen
some indication of plasma absorption.

And so this is puzzling to us, but it is actually explainable in the
sense where Hyperion is located, it's located far away from Saturn in
the outer magnetosphere of Saturn. The plasma there is not fully
co-rotating.

It also moves rapidly in other directions in response to changes that
occur in the magnetosphere of Saturn. So we do not know the flow
direction of plasma. What I've shown you are figures, which assume a
nominal flow direction, but there's no wake in the nominal direction.

It's quite possible that the flow was deviated from the nominal
direction and as a result the plume, the wake is located not where we
think it is, but it is at some other location. It either did not go
through the wake at all or we went through the wake at a different time
and we are looking at this data more closely to figure out where the
wake is which should be present.

So finally I would conclude, the last slide so there is no internal
magnetic field in any of the satellites I should have mentioned that
right away. One of the properties of some moons may be that there is an
internal magnetic field.

And as you may know, observations from the Galileo Spacecraft which were
actually made by our team in UCLA showed that Ganymede, one of the moons
of Jupiter has a strong internal magnetic field just like the earth one
and the same sense.

And so we always try to look for internal magnetic fields in any of the
satellites that we visit but we haven't seen any evidence of that.
Enceladus interacts with the plasma over a very extended region as I
showed you in perturbation plots.

If the region is almost 6 to 10 radii, Enceladus radii, so it is a big
wall of space which gets affected and that is the reason why when we go
by the moon we are able to see in surface activity. The plasma pick up
region is below Enceladus. We know that.

And the pickup rate of plasma is very small. It is only two kilograms
per second. Compare that to several hundred kilograms per second of
neutrals that are coming out in the plume. So the plume mask that is
coming out is several hundred kilograms per second.

Dione also mass loads correlative plasma and is the biggest surprise for
our team. We were I could say that we were almost expecting Enceladus to
give us some surprise. But Dione we were not expecting and so we were
wondering if there was a hidden plume and we are now, we have a program
that we are gong to look for it by flying there several times.

And also other teams will be doing imaging and other measurements to
look for evidence of a plume at Dione. So far none have been found in
the first fly but that we had by the moon. And the pick up rate must be
at least a factor of ten lower because our magnetic field signature is
weaker by about that much compared to what we saw at Enceladus.

So the pick up rate may be just a few hundred grams per second at Dione.
Tethys and Rhea clearly show that they absorb incoming plasma and form a
plasma depleted wake which we can sense by seeing enhancement in the
field strength.

One curious thing has been and which is something that has bothered us
is that all the trajectories which were upstream of the moon which were
Enceladus and that of Dione have shown evidence of plasma pick up. And
every time we went to the down stream region we saw the moon which was
Dione, which were Tethys andRhea, they showed us evidence of plasma
interruption.

So, could it be that upstream of a moon you see evidence of pick up and
down stream you see evidence of plasma absorption. I personally don't
think so but it could be just a coincidence that is how our trajectories
work, that for the moon which absorbs plasma we happen to be downstream
and the moon which do not we were up stream.

But we will need more fly bys. We would like to go at least once through
the wake of Enceladus. We have so far been only up stream and once
through the up stream region of Tethys or Rhea to be able to say
conclusively that these two Tethys and Rhea are absorbing type and
Enceladus and Dione are mass loading type. I will stop there and take
questions. Thank you.

Woman: Krishan, I guess I had a question about some of the other moons
in the solar system. It sounds like; you know Enceladus and Dione are
very interesting. How would you compare them to just sort of you know to
some of the moons of Jupiter system?

Krishan Khurana: I would put Enceladus in the same class as Europa.
Europa was one of the biggest surprises for us when we got observations
from Jupiter and from Galileo. Europa shows the presence of an ocean
inside. It shows up in many observations including the magnetic field.

And the reason we could see a magnetic signature of an ocean is that
there was an electromagnetic induction effect that came from the ocean
in the magnetic field of Jupiter.

Jupiter is a fast rotator, so the magnetic field basically creates a
wave of ten-hour wave period in the frame of the moon Europa, Ganymede
and Callisto. And if there is any conductive reason, current flow on the
surfaces of those regions which would be an ocean for example, and
produce a magnetic field of its own that current system.

And we were able to measure those, detect those. So we directly detected
an ocean in Europa and we also believe in Ganymede and Callisto.
However, the Europa Ocean seems to be much closer to the surface.

That is what we are concluding from our observations, maybe ten to 20
kilometers depth, whereas, Ganymede and Callisto's oceans are very deep
maybe about at a depth of 200 kilometers or so.

But we were still able to, you know detect clear signatures from those.
So Enceladus is in the same class as Europa which is one of the most
interesting objects. Then Dione is somewhere in between.

However what is interesting is that we, some of the physics that I
described about Tethys and Rhea is brand new. We have flown by the earth
moon and so we partly understood inert interactions. But it turns out we
had not.

We have really a new class of physics where the plasma gets absorbed not
right near the moon but on the whole field line which gets in contact
with the moon and so they are also teaching us a lot about plasma
interactions and plasma physics which will be extremely useful in
understanding other bodies in the solar system.

After all we have now millions of comet like bodies in the outer solar
system and it is quite likely one other spacecraft will go by one of
those.

And to be able to understand those interactions, these moons are really
providing us a great lesson in how to look for interaction of objects
either inert or active with respect to both magnetic fields and plasma.
Any other questions?

Man: Do any of Jupiter's moons have magnetic fields?

Krishan Khurana: Yes, Ganymede has a very strong internal magnetic field
in addition to the induction magnetic field that gets generated from its
oceans. But yes, it has a strong magnetic field. The surface strength is
about 1500 nanotesla which is 1/20 that of the earth's magnetic field,
but that is still very strong.

Actually this field is stronger than what Mercury is able to generate
which is a planet and much, much bigger planet. And so actually it is
bigger but not that much. I am talking about Ganymede which happens to
be the largest moon in the solar system.

And so, what is surprising is that Ganymede which is an icy moon, has a
surface made of pure ice and most likely has a liquid ocean, inside has
a core which we think is metallic, which we think this magnetic field,
which must be metallic and fluid, so fluid ion motion which are causing
that magnetic field.

That is the best way to explain the data from Ganymede.

Man: Do you think that the reason that Mars supposedly became desolated
was because it lost its magnetic field?

Krishan Khurana: I would say that the same reason why it lost the
magnetic field may also be why it is so inert and also may explain why
its surface looks the way it does today. So what I mean by that is, most
probably does it, we know for sure there is a dynamo inside Mars which
switched off maybe about four billion years ago.

So right in the first few hundred million years after Mars was formed,
they were a dynamo active up to that time. But then it shut off. And
normally what that tells you is that the internal source of heat that
was present either dried up or whatever was sustaining it changed its
type.

And you know for example if it was something like platectonics that
happened like the Earth, and it stopped on Mars, that would take away a
source of energy in the system. And once that happens, then because
there\'s no heat being put out from the interior to exterior, the
convection inside the core of Mars would shut off. And if that happens,
then you automatically expect the surface techtonic activity would also
die down because the techtonic motions is in response to the motions in
the mantle of the planets. For example the Earth\'s mantle is convecting
and it\'s moving the plates in the ridged crust and that is causing
plate tectonics at the surface of the Earth. But the source is always
interior. There\'s a heat source both in the mantle and in the core, and
that\'s what drives; somehow at Mars either it was not enough because it
was small or it finished for example. There may have been a lot of
radioactive materials present at the beginning which rapidly decayed,
like Aluminum 26, which depletes very quickly and so within a few
hundred million years the source of heating was gone and Mars became
inactive. And that\'s what shows up in the surface.

In terms of water, we really do not know if there is water present today
at Mars or not. It maybe there locked up beneath the surface even today.
Or if it was present at the surface, and because now there\'s no
magnetic field, it will get blown away by the solar wind. It will get
picked up in a major way and be lost.

Man: How dangerous is the Earth of having that happen? How much in
danger is the Earth of having it loose its magnetic field?

Krishan Khurana: All the studies I have seen show that the core of the
Earth will remain active over several million years. I recall reading a
beautiful article; I cannot recall it either in Scientific American or
somewhere, where they looked at a scenario how the Earth will evolve
over time, and that scenario says over the next several billion years,
maybe as many as another three to four billion years, you won\'t notice
much difference in the interior activity.

> Beyond that, then some of the heating that is coming from
> radioactivity, and the primordial heat that is locked inside, and the
> core would have grown to its maximum size, the inner core. What
> happens at the Earth is there is a core which is molten, but from it
> interior keeps getting differentiated and settles into the interior
> core which is solid. And it is the difference between the densities of
> the two materials that the outer core and the inner core that drives
> convection. So it acts as a source of heat. When you have
> differentiated the core completely, and there\'s no more
> differentiation to be done, then your source of heat goes away. But
> all of that will happen over many many billion years, so in the short
> term, or even the geological medium term, the Earth will look like
> what it does today.

Man: Therse icy moons that orbit some distance from Saturn in the
magnetic field, my question is very simple, is the magnetic field
strength lethal to human life at those distances?

Krishan Khurana: No, magnetic field is actually never lethal to
anything. It\'s extremely benign. What is lethal actually is the charged
particle alignment. I alluded to all the ions and electrons which have
stuck to the field lines. And so these are the plasma particles I was
talking about. They certainly are nasty things. And so they create
radiation. They also, because they move at very fast velocities, they
interact the same way everyone interacts at the plume on the surface of
Enceladus with the human body. So one has to have a very effective
shielding in order to be able to travel in space.

And that actually is a problem not just near the icy moons, but
everywhere in space. There is no region of space which is free of
plasma. So as soon as you leave the earth's atmosphere get into
ionosphere and beyond, you're in a space alignment where there is plasma
present.

Some of it is coming from the solar wind, other getting generated from
the atmosphere of the planets, and all of it, it moves very rapidly.
It's pretty hot. It's temperature in millions of degrees Kelvin, and as
a result any living organisms coming in contact with this plasma receive
a lethal dose very quickly.

The biggest doses in the solar system other than near the sun, occurs in
the Jovian magnetosphere. In Jupiter\'s magnetosphere for example we
have Europa. You get lethal doses within a few minutes.

At Saturn is better, it will take several days for you to get lethal
doses. in Saturn\'s magenetosphere.

Man: (Unintelligible), similar situation at Saturn, that Jupiter, as far
as, having ability (unintelligible).

Krishan Khurana: Could you ask your questions again?

Man: The amount of radiation present in the area around Saturn is just
as lethal as around Jupiter to human beings when talking as far the
future.

Krishan Khurana: That's right. Yes it is lethal. It is much less intense
by, you know factors of 10 to 100 but still it is very lethal. You could
not, an astronaut just in a space suit would not be able to survive that
radiation for any length of time, you know, over days for example.

You could protect yourself inside some spacecraft, which have very large
mass amount of shielding, but even then you would not want to spend
months or years, certainly in Saturn\'s magnetosphere, and in Jupiter\'s
magnetosphere even days.

Man: All right, thank you.

Man: Can you talk a little bit about reasons of the albedos being so
high for (unintelligible)?

Krishan Khurana: There was a big static and I completely lost you.

Man: Yes, sorry about that. Can you talk a little bit about reason for
the high visual albedos, of Enceladus and Tethys.

Krishan Khurana: Yes, I can. Actually the albedos of all the icy moons,
both in Saturn's system and Jupiter's system is pretty high, and part of
the reason is this plasma which bombards these surfaces.

So, as I said at Enceladus most of the plasma gets deflected, but not
all, and some of it does fall onto the surface and it sputters the
surface. So if particles coming at tens of kilometers per second hits
the surface and it creates its own cloud with molecules from the surface
are ejected.

However the velocities of these molecules that have just come out are
not large enough to escape the moon's gravity, so they once again fall
down on the surface and are redeposited.

So this way the surface of the moon over time gets renewed. It's not a
major renewal, the only top few inches, actually is a few milliliters of
the ice surface gets renewed. So it does not erase any craters or any
geological features, but it does make the surface have a very fresh
appearance.

And, it creates these very pristine, crisping clean ice layer, frost
layer, on top of the moon, which is highly reflecting, and that is why
the high albedos.

And, this is different than if for example, if the material was able to
escape. Then what happens is because the material that would be escaping
would be mostly water, and it would leave impurities behind and those
impurities actually will coat the surface.

And that what happens at comets for example. At comets the water is able
to escape into long comet wakes, and it leaves impurities behind, and
then so the surface slowly gets dark, tarnished because the carbon
particles that are there are left behind.

But, at these moons, not especially the non-active ones, not much water
is able to escape, so as a result, there's always a clean frost that
gets re-deposited on the surfaces of these moons.

Man: That's really interesting.

Krishan Khurana: Any other questions?

Woman: Well, actually, I guess Krishan I had one more question. You
didn't talk much about Titan and Titan is clearly one of the interesting
moons. If you had a minute, where would you, how would you summarize the
magnetic signature of Titan?

Krishan Khurana: Okay, very good. Titan is a mass-loading object and the
reason I did not go to Titan is; you saw it took me almost the first 45
minutes to describe Enceladus and it would do about the same, I would
need about the same amount of time to describe the interaction of Titan.

It's extremely complex. And it's not surprising. It has the biggest
atmosphere of any moons we know. Actually, the nitro, its atmosphere is
very similar to that of the earth. It is a nitrogen atmosphere.

And the surface pressures are many times higher than that of the earth.
And as a result it interacts with the plasma in a very strong fashion.
If the plasma, if the interaction is plasma-loading type, meaning the
(field) lines are not able to go through Titan's atmosphere, but they
drape around Titan.

And a good fraction and some of the plasma also escapes from the
atmosphere of Titan. So, it provides additional mass loading to the
magnetosphere of Saturn.

The interesting thing, though, is that when you look at the
magnetosphere of Saturn and look at its plasma, the origin of most of it
is Enceladus. And the way it works is Enceladus is, makes first the
E-ring.

The neutrals come off Enceladus from the plume and they create an
E-ring, and then these E-rings interact with the background
magnetosphere of Saturn and get eroded, the E-ring gets eroded.

And that eroded stuff becomes ionized and fills the magnetosphere of
Saturn. So, E-ring is a source region for the magnetosphere of Saturn.
So Enceladus indirectly supplies all, almost all of the plasma to
Saturn's magnetosphere, but Titan does as well.

And the ratio is roughly 80 to 20. 80% of it is coming from Enceladus'
E-ring and other inner icy moons, and about 20% from Titan. Titan's
interaction is also complex in other ways because it has a very
interesting chemistry.

Titan, in addition to the nitrogen, has all kind of hydrocarbons present
inside, and as they get picked up they create an amazingly interesting
chemistry which many different instruments are able to measure and we
are able to say from the measurements of even plasma instruments quite a
lot about the composition of the atmosphere of Titan.

And also something about what's happening deep in the atmosphere of
Titan, even though the measurements that we make are always quite a
large distance from Titan.

Woman: Well, I think that sounds like a good opportunity for me to go
off and see if I can get a Titan specific talk rolling here at the Charm
Telethon.

Krishan Khurana: Oh I agree completely. If you and I can, you can even
discuss with me and I can provide you some excellent names of people who
would be perfect for this type of talk.

Man: Is there a relationship between that would be fascinating,
relationship between volcanism and magnetism in terms of these planets?

Krishan Khurana: Only indirectly. The earth has volcanism and it has
magnetism. Io has volcanism, but so far we haven't been able to detect
magnetism. In general, both have the same origin, you require internal
heat.

So for a moon or a planet to be active you require a source of internal
heat and that for the earth is part radioactivity and part primordial
energy, the energy the earth was born with and is slowly leaking out
from the interior, and also differentiation, as I alluded to, of the
inner and outer cores.

And so, it's that source of energy which as it convects out through the
mantle and the crust creates these very large plumes of liquid melt
which cause volcanism on the earth. Or through plate tectonics, motion
of the plates. But the force is an internal heat. And is the same
internal heat which drives convection in the core of the Earth, making
the magnetic field.

So they are related. Now let's go to Io, which is the most volcanic
object in the solar system, more than the Earth. It's like Hawaii all
over, everywhere on the surface.

And yet, when we flew by, we saw strong magnetic vibrations, but they do
not appear to be a strong internal magnetic field. So there is a source
of heat, but sometimes that source of heat is not able to create an
internal magnetic field.

And we are also almost certain that the interior of Io is molten, and
that we can say by looking at gravity data. So there is a fluid core
there, and yet it is not able to make a magnetic field.

And people that come out with explanations, if you want to know I can
give you. But there's not a 1-to-1 relation. I would say if you see
volcanism, there is a high probability of an interior magnetic field,
but not 100%.

Man: It sounds like you're saying that the chemistry of the magma or
whatever, the internal...

Krishan Khurana: No, actually, it's not the chemistry. It may have to do
with the source of heat. The heat source for the Earth is interior. The
heat is mostly coming from the core, and it's propagating outwards. At
Io, the heat source is tidal heating. It's in the exterior.

So the moon is getting heated from the outside, and the heat is
traveling inward. At Earth it is traveling outward and that's the main
difference people think, which in one case makes a magnetic field, and
in another case is not, able to do that.

Because you will not be able to drive convection in the interior if the
outside is hot. For convection to be driven, you need the exterior to be
cooler than the interior, as you know from a boiling pot of water.

Because the exterior's cooler, and as a result, you have convection,
which is able to give heat to the exterior, and the water self-convects,
and so basically, you may not have any convection in the core of Io.

Man: Do we know why it's hot on the outside?

Krishan Khurana: It's hot on the outside, yes, because of the tidal
heating from its proximity to Jupiter, and its residence with other icy
moons.

Man: Interesting. Thanks.

Krishan Khurana: And that is, by the way, also the reason for it's
amazing volcanic activity. The surface is younger than even that that of
Europa, actually. There are so few craters known in Io of impact origin
that is the pristine surface in the solar system, in terms of asteroid
impact.

It may be even younger in some ways than the Earth. The Earth has more
asteroid impacts than Io does.

Man: Thank you, that's really interesting.

Woman: Are there any other questions today? Okay, well, I'd like to
thank Dr. Khurana for joining us. We had a very interesting Charm
telecon, as always. We really appreciate the taking the time to join us
today.

Krishan Khurana: Great, and I thank for everybody for very interesting
questions, and for actually making me interested in going back to the
data, and bring more with it.

Woman: If anybody has any questions, feel free to send an email to the
Charm lead, and we'll make sure that we forward that on and get that
question answered for you. Also I'd like to advertise the next Charm
telecon that will be the last Tuesday in June, which is June 27.

This one will be a particularly interesting one, as it is the second
year anniversary talk that we'll be giving. So we will have five
distinguished panelists.

Currenly we have Dr. Andy Ingersal from Cal Tech, who will be talking
about Enceladus and the atmospheres of Titan and Saturn. We have Dr.
Elizabeth Turtle from let's see Arizona, University, yes University of
Arizona, and she'll be talking about the Icy satellites, and Titan's
surface.

We have Dr. Jeff Cuzzi, who is a world specialist in the rings of
Saturn. The meeting will be hosted by our very own Dave Seal, who is the
mission planning lead for Cassini.

This will be the week after the private science group meeting, so many
of the scientists will be in Europe supporting that, so Dave has agreed
to host the telecon.

And we're still looking for our final panelists, which will be the
magentospheric reps, but we'll have that wrapped up in the next week or
so. So everybody should really bookmark June 27, the last Tuesday in
June, at 11 o'clock Pacific Time, for a really, really terrific Charm
telecon. And everybody have a great month!

Krishan Khurana: Thank you.

Man: Thank you.

Woman: Bye-bye.

Man: Hello?

END