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

**FTS-NASA-VOICE**

**Moderator: Marcia Burton**

**February 24, 2009**

**1:00 pm CT**

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Marcia Burton: Great. Thanks very much. Okay, so our speaker today is
Dr. Geraint Jones. And he's from Mullard Space Science Laboratory.

And to the best of my knowledge, it's somewhere just outside of London
in the UK. And I actually pulled up the Web site today to look at where
Mullard was. And I saw what looks to be an English Manor house set in
the countryside.

So I don't know if that's where you work Geraint, but that's quite
impressive. So you'll have to tell us a little bit more about that.

Let's see, I first met Geraint when he was doing a post-doc in our group
several years ago. And he was interested at that time in studying
signatures of comet tails in the solar wind.

And since then he's been involved with a number of the different fields
and particle teams on Cassini including the magnetometer, the MIMI
instrument. And now he's back in the UK working on CAPS.

So he's got a great breadth of experience. And today he's going to talk
about really an amazing discovery that he's made of a debris belt or
rings around Rhea.

The title of his talk is Rings Around the Moon? The Puzzling Case of
Rhea. So I'll hand it over to Geraint. Please\...

Geraint Jones: Okay, thanks very much Marcia. Yes, so I work at the
Mullard Space Science Laboratory which is part of University College,
London.

In the late 1960s, the University was given a grant by the Mullard
Electronics Company which is now part of Phillips to set up a space
science laboratory. And because of lack of space in Central London, they
found this lovely manor house around 30 miles south of Central London.

So it's on top of a hill overlooking beautiful countryside. So it's
quite a pleasant place to work. There are about 150 of us here.

So yes, so I'm going to talk about really puzzling observations made by
Cassini during - to begin with, during a flyby of Rhea in 2005. And then
we followed-up with further observations in 2007.

So if we move from the opening slide to the first, to the next page, I'd
just before I start, like to thank several people and teams that have
contributed to this work.

So on the MIMI team, my colleagues are (Elias Roussos) and (Norbert
Krupp). And colleagues have made a huge contribution to this (Elias)
especially did much of the modeling to get the results that are
presented here.

And the MIMI team is led by Tom Krimigis of the John Hopkins University
Applied Physics Laboratory.

The - I'm also presenting observations from the CAPS instrument. And
that's led by Dave Young of the Southwest Research Institute in San
Antonio.

Also be showing images from the imaging team on Cassini, the PI is
Carolyn Porco. I'd also like to thank (Tilmann Denk) and (Matt
Tiscareno) of that team for discussions regarding various aspects of
this work and also these reporting on observations near Rhea made by
several other instruments including UVIS, RPWS, the magnetometer and the
CDA teams.

Okay so I'll move on to the next slide. So most of the results are
reported on here from one instrument in particular. And that's MIMI, the
Magnetospheric Imaging Instrument. And there are three parts to this.

And the part that's shown on the screen here is called LEMMS which
stands for Low Energy Magnetostatic Measurement System.

So this is mostly designed and built at the Max Planck Institute for
Solar System Research in Katlenburg-Lindau in Germany. And it's designed
to detect very energetic ions and electrons. So the energy ranges are
shown there. So they're really energetic particles.

There are solid state detectors inside this instrument which detect
these particles out in the solar wind and also in Saturn's
magnetosphere.

So if we move onto the next slide, also be showing data from the Cassini
Plasma Spectrometer or CAPS. And that also has different sections. And
the data I'll be showing is from ELS, the Electron Spectrometer\...

Marcia Burton: Excuse me, Geraint?

Geraint Jones: Yes?

Marcia Burton: Can I interrupt just a second?

Geraint Jones: Sure.

Marcia Burton: There seems to be a bit of noise on the line. Is it
distracting to people? Should I try and get that eliminated or are you
guys okay? Do you hear it Geraint?

Geraint Jones: Yes, I have a slight echo but I'm okay to carry on.

Marcia Burton: Any comments? All right, keep going Geraint. Thank you.

Geraint Jones: Okay, thanks. So the electron spectrometer, the lead
co-investigator for that is Andrew Coates here at MSSL.

So this instrument detects electrons. It's a lower energy range than
LEMMS, the part of MIMI I just described.

So it's actually mounted on an actuator so that it sweeps the sky like a
windshield wiper scanning back and forth to get - to measure electrons
and ions coming from different parts of the sky.

And it covers an energy range lower as I mentioned than LEMMS from half
an electron volt up to 26 (kilo) electron volts.

Okay, so we should move on to the next slide. We have an image of
Cassini with the insulation removed so you can see what all the
different instruments and other components are.

And CAPS and MIMI LEMMS are mounted on the fields and particles pallet,
the sensor there at the right. So these instruments can't see particles
coming from every direction at any one time. It depends on the
orientation of the spacecraft and also the CAPS; it's controlled by this
actuation I mentioned earlier.

LEMMS also have been rotated to get better coverage of the sky as well.
But for most of the time it's fixed. So the direction we're looking in
and seeing these particles coming from is controlled by the orientation
of the spacecraft.

So if you move onto the next slide, this shows the trajectory of the
prime and extended or Equinox Missions of Cassini. So we're in the
Equinox Mission now as you know.

So we've completed numerous orbits of Saturn and covered a wide range of
environments in the magnetosphere and passed many of the moons while
doing that.

So the outer blue ellipse there is the orbit of Iapetus. The next one in
is Hyperion. And then just inside that you can see Titan.

And you can see the trajectory of Cassini passes close to Titan's orbit
very often. And that's the reason that Titan because it's by far the
most massive moon, it's very easy to control or relatively easy to
(little) the work a really complex navigation problem.

And by making close flybys of Titan, you can change the orbit of
Cassini. And so you'll see that Cassini passes close to Titan many
times.

And of course as well as the Titan flybys, we have many close flybys of
other moons as well, the icy moons.

So if you move on to the next slide there's an overview of the moons of
Saturn prepared by (David Seal). And the top panel there you can see the
relative sizes of the moons. You can see the (limit factor) at the
bottom of about - at the panel, just a scale.

And you can see the Titan is by far the largest moon. And you can also
tell that it has a very thick atmosphere.

And then next in size is Rhea in the center there and after that Iapetus
and then going down the sides to (and show) - and MIMI et cetera to the
very small moons which have been enlarged in this figure.

So and at the bottom part of that panel shows where these icy moons
orbit Saturn. And all of the moons in the bottom panel are within
Saturn's magnetosphere all the time.

So the magnetosphere's this magnetic cage surrounding Saturn where the
solar winds can't easily penetrate. So this - so Saturn's magnetic field
holds off the solar wind. And so it's an environment very close to
planet which is quite stable and then further out it gets more variable
as there's different processes like reconnection, et cetera, occur in
the tail.

So and these icy moons, because they sit in the - this region close to
the planet, they're constantly bombarded by particles which are trapped
by Saturn's magnetosphere.

So if we move on to the next page you - of next slide, and this figure
shows Saturn and just as an example, the orbit in Enceladus shown in
blue.

And Enceladus is sitting inside the magnetosphere. And magnetosphere is
largely controlled by Saturn's magnetic field. So as the planet rotates,
it drags its magnetic field around with it.

And particles which are trapped in the radiation belts, the equivalent
of the Earths Van Allen belts, travel north and south, bounce north and
south along the magnetic field lines.

So as the planet rotates, the magnetosphere rotates. And these particles
that are trapped in the magnetosphere in the radiation belts also move
around the planet as well.

So the moon's are trapped on orbits within this constant soup of
energetic ions and electrons that are in the magnetosphere.

So the arrows at the bottom show the general directions. And most of the
plasma, that's the charged ions and electrons move in the direction of
the red arrow.

So there is a slight complication. So all the sort of the two green
arrows show that all the ions, all the protons and the low energy
electrons move around the planets in the same direction as the planet
rotates.

But energetic electrons, that are above around 1 mega electron volts of
the orbit of Enceladus, they actually because of drifts, particle drifts
caused by the shape of the magnetic field et cetera, actually move in
the opposite direction to the rotation of the planets.

So if you move on to the next slide and these particles in the radiation
belt can be lost. So the - they can strike a moon as shown sort of
Particle A comes in, strikes a moon, it's absorbed by the surface. And
it's lost to the magnetosphere.

For Case B, that one is struck a dust grain which is orbiting Saturn for
example in the E-ring. So that's also lost in the magnetosphere.

And in the case of Enceladus, the example C here, the plume of Enceladus
is also an absorber of these energetic particles trapped in Saturn's
magnetosphere.

So if you move on to the next slide, so when energetic particles in the
radiation belt strike a moon or another obstacle such as a dust grain,
material is lost.

So if you - so if LEMMS crosses, if LEMMS, it's the part of the MIMI
instrument on Cassini, when Cassini crosses a region which is passed
close to a moon or another absorbing obstacle, we see a drop in the
fluxes.

So particles will strike a moon. And then when we pass close to that
part of the magnetosphere we'll see a drop in the fluxes of energetic
particles.

And these drops in the fluxes are termed micro signatures.

So in this cartoon, we've got a moon with a magnetic field. We've zoomed
into a region around the moon. And the magnetic field in blue is the
magnetic field of Saturn.

And the red curtain is where you see a decrease in energetic particles.
So they've been lost to the moon. So if Cassini crosses any part of that
curtain, it sees a drop roughly the same size as the moon.

But eventually micro signatures, these dropouts eventually refill.
Sometimes they can wrap around planets, around Saturn more than once.
But usually they refill. So you only see them when you're fairly close
to the moon before the meter static particles have refilled the wake\...

Marcia Burton: Geraint?

Geraint Jones: Yes?

Marcia Burton: I have a question. I didn't know if you want people to
interrupt with questions or if you want to pause every once in a while.

But the rippliness of the red zone is just because of variability in the
magnetosphere, is that correct?

Geraint Jones: Yes, that's right. And the recurrence in the
magnetosphere.

So we find more often than not that these dropouts don't occur exactly
where you expect them to. So we think that these electric currents are
moving the magnetic field around. And so this signature moves around as
well.

So you can think of a, maybe a smoke stack giving out lots of smoke. And
instead of a straight trail as it moves downwind, it encounters a
crosswind so it'll move from side to side.

So we don't always see the dropout exactly where we expect it to.

Marcia Burton: Okay, thanks.

Geraint Jones: So yes, I'm happy for anyone to interrupt with any
questions if anything comes to mind.

Marcia Burton: Thanks.

Geraint Jones: Okay, thanks. So if you move on to the next slide, so
it's another version of the slide a couple of pages back.

So the red region here shows where magnetic field lines of Saturn have
passed close to in this case Enceladus. So this is termed the L-shell.

So this is a way of describing the surface, a set of magnetic fields
that have passed close to a moon. So the L-shell of Enceladus is a
region of the magnetosphere where the magnetic field lines thread the
equator near a moon.

So if Cassini crosses any part of this region that's marked in red, then
it'll see a dropout caused by particles being lost to Enceladus.

So they - so instead of bouncing up and down the magnetic field lines
they've struck the moon. And so we see a decrease in the fluxes.

Okay, so if we move on to the next slide. So this is an example of one
of these micro signatures. So this is for one of the larger moons called
Tethys. And this is data from MIMI LEMMS.

So each one of those lines represents data from a different energy
channel. So we're detecting electrons with LEMMS. And the key at the
right shows you the different energies. So at the bottom we have
electrons from 20 to 32 kilo-electron volts. And that's the top most
trace in this graph.

So as you move to higher energies, we change color and at the very high
energy. And because as I mentioned earlier, very high energy electrons
go around Saturn in the opposite direction, we start losing the micro
signature and don't see it anymore.

So the scale from left to right, the X axis is in - measured in Tethys
radii. And you'll see that the drop is exactly 1 Tethys diameter wide.
So it's exactly the width of the moon. So it's exactly what we'd expect
to see.

So when we pass through magnetic fields, magnetic fields that are passed
close to Tethys, we're seeing a big drop in the energetic particles
because they've struck the surface and we can't observe them anymore
because they've been lost.

So this covers about 1-1/2 hours of data from a flyby in - of Tethys in
2005. So that's what we expect to see at most of the moons.

So if we go on to the next slide, this just gives a brief overview of
Rhea. And radius is just over 750 kilometers. And it orbits at 8.7
Saturn radii from the planets, from the planet Saturn.

So the close flyby and the closest approach date occurred on day 330 of
2005 towards the end of November. And we approached within 500
kilometers of the surface, so directly downstream of the moon and the
co-rotation flow.

So if you remember the rotation of Saturn, we passed downstream of that
flow of plasma carried by Saturn's magnetic field.

And there was a more distant flyby in August 2007 which I'll return to
later.

So the surface of Rhea as you can see from that image \-- and it's an
extremely old surface - it's practically saturated with craters. There
are some regions with large fractures and a few fairly young, relatively
young craters on this scale thought to be tens of millions of years old.

But there aren't any very young regions like you see at Enceladus with
no craters. It seems to be a very old battered terrain.

So if we move on to the next slide, it shows the geometry of the
encounter that occurred in November 2005.

So we've got Rhea right in the middle of the figure. Saturn is towards
the upper right that - where that blue arrow of the right is pointing.

And Cassini was traveling inbound toward Saturn at the time. So the
track of Cassini is shown there in red. And the - we passed through the
wake. And the region shown in green here which is downstream of this
flow of plasma that continuously flows past the moon.

And so also shown on this figure is the Hill sphere of Rhea. Now this is
a region where the gravitational field of Rhea dominates over that of
Saturn. So in an ideal world if there were no other moons and Saturn was
a perfect sphere, et cetera, if you were to put a particle in orbit
around Rhea inside the hills sphere it would stay in orbit. But outside
it couldn't stay in orbit. It would orbit - it would be in free orbit
around Saturn itself.

But in reality because of other complications like other moons and the
obliqueness of Saturn for example, the stable orbits inside the Hill
sphere probably only extend up to roughly halfway out, maybe a little
more inside the Hill sphere of Saturn.

But it's basically the region where Rhea's gravitational field
dominates.

So the diameter of the Hill sphere is about 15 Rhea diameters, so very
roughly, 11,000 kilometers. So roughly the scale of the Earth, the Hill
sphere.

So the trajectory of Cassini from left to right went through the wake
and also just slightly south of Rhea's equator. So we were about 200
kilometers south of the equator which doesn't really show up very
clearly on this figure.

Okay, so if we move to the next slide, then we can see what LEMMS
actually saw when we carried out this close flyby of Rhea.

And you'll recall the Tethys signature was a really clear dropouts, very
sharp. And then on the other side, the fluxes return to the - to levels
they were before.

And we have these really odd features on either side of Rhea itself. So
right in the middle around 8 minutes or so we see the sharp drop due to
Rhea itself.

But on either side we see these periods where the electron fluxes were
decreasing gradually as we approached Rhea. And then going outbound they
returned again.

Rather than returning back up to a fixed level, they returned gradually.
And this figure shows the data from several energy channels from LEMMS
which is shown in the key at the bottom right there.

And so these wings of the dropout were completely unexpected. And that
was a huge puzzle and some may argue still is.

So if we move on to the next side, we have a cartoon of Cassini going
past Rhea. And we've shown to the same scale, this decrease in electron
fluxes. It's a log scale. So in - if you were to plot it on a linear
scale it would be much sharper than, or much deeper than this..

So I'm going to try and play you some audio of this flyby. So this'll
last about 25 seconds and represents the width of this plot which we
have on this figure.

So we've taken white noise and we changed the volume by the change in
the fluxes detected by the LEMMS instrument.

And so if you listen carefully you'll hear the volume of the flux
decreasing as we get closer to Rhea. And then you'll hear a gap of a few
seconds and then the fluxes will return and will increase again in
volume.

Okay, so I'll start that and it'll last about 25 seconds.

So this sound and one way of describing the curious thing we were seeing
is if you're driving in a car during a heavy rainstorm and you hear the
sound of the rain on the windshield and roof of your car, when you go
under a bridge you expect that sound to stop and then to restart again
on the other side. And that's what we expect to hear or to detect when
we're going past these moons.

But instead when we're approaching this moon or the equivalent of the
bridge, the rain slows down or the flux decreases well before we expect
it to and gradually decreases. And then on the other side instead of
coming back abruptly it gradually increases again.

Okay so\...

Marcia Burton: So Geraint?

Geraint Jones: Yes?

Marcia Burton: Is there an audio file in the PowerPoint or not?

Geraint Jones: There isn't, no.

Marcia Burton: Oh okay. All right.

Geraint Jones: So I was only showing it. But\...

Marcia Burton: That's fine. And then the audio is made from the MIMI
LEMMS data right?

Geraint Jones: That's right, yes.

Marcia Burton: Okay, all right. That was cool. Thanks.

Geraint Jones: Okay so are there any questions at this point?

Okay I'll move on then. So the next slide, so we - the figure at the
upper right shows Rhea looking from the north with Cassini moving from
bottom to top. And then the color of the line represents the fluxes
detected by LEMMS.

So the - this flow of plasma trapped around Saturn is shown by the red
arrow, the large red arrow moving from left to right. And you can see in
the wake we saw a decrease in that flux as expected. But this wide
decrease in either side was unexpected.

So what causes the extended low energy dropouts on either side of Rhea?
The inbound boundary was at 22 minutes past the hour. And outbound is
at - it was at 52 past the hour plus or minus a few minutes.

But the Hill sphere radius is just over 7.6 Rhea radii. And that was
across the 25 past and 51 past the hour as I showed there.

So these times are fairly close to the times when Cassini entered and
left the Hill sphere, the region where material could possibly be
gravitationally bound by Rhea.

So it may be a coincidence, but it is curious that this dropout has got
the same scale as the sphere.

So if we move on to the next slide and it's a different version of the
cartoon I showed earlier with the curtain.

And so when we pass close to Rhea instead of just seeing a sharp drop
out exactly the same scale as the moon, we also have a wider region
caused by something absorbing electrons and so as represented by that
white fuzzy region surrounding the moon. So we have an extended micro
signature.

The only other place where we have an extended micro signature like this
is Enceladus. And we understand that much better. That is due to the
plume of material coming out of Enceladus. And as I showed earlier, the
magnetostatic particles can strike the screens in the plume and they're
lost in that way.

But around Rhea, we have no obvious explanation for the decrease in
electrons.

So if we move on to the next slide, this shows observations by several
different fields and particles instruments on Cassini during this flyby.

So at the top we have Rhea viewed from the north with the plasma flying
from the top to bottom.

So the pale yellow disc is the scale of the Hill sphere. And the next
panel down we have the LEMMS observations that at two energies.

And you can see the scale of the decrease relatively close to the scale
of the Hill sphere but not exactly.

Then next down is the CAPS-ELS data which shows the electrons. So they
also showed a decrease on either side of Rhea.

And then we have the first color panel. And that's another way of
showing the CAPS data over a larger energy range.

And if you look at to the left of that panel, you can see the energies
and electron volts, the charge for those electrons.

So that red line is where most of the plasma is, that red region running
all the way across the figure except for the gap in the middle.

Then it's the high energy end of the ELS data. You can see this dark
blue region which is the wake. And then on either side you can see the
decrease in fluxes at very high energies in the CAPS data as well.

So the next panel is CAPS IMS. So IMS is part of CAPS which detects ions
rather than electrons. So it detects positive ions.

Those pulses at the start and the end of that are due to actuation. So
as I mentioned earlier, CAPS is on this like a windshield wiper. So that
was operating at that time where you see the dotted line. And then it
was fixed for the Rhea flyby itself.

And you see that ions are missing right from the middle of that plot
where we passed downstream of Rhea's wake and ions were unable to reach
the spacecraft.

Next we show the electron number density from the radio instruments
RPWS. So it's - there are hints that there's something going on there
roughly at the same time as - and the other instruments. But it doesn't
really stand out.

And then the last two panels show data from the MAG instrument, the
magnetometer. So at the bottom you see the magnitude of the magnetic
field and this bump in the wake of Rhea.

And what happens is in the wake, because plasma hits the surface, you
have a vacuum or a stronger vacuum, an absence of particles in the wake
downstream of Rhea.

So the magnetic field increases in strength to make up for the absence
of these particles. So that's why you see this bump in the magnetic
field strength immediately downstream of the moon.

So we looked at all these data sets together. And the magnetometer and
the RPWS and CAPS and MIMI and we're really puzzled by what we were
seeing. There was one possible explanation for this wide feature may be
a wake. So you can - a wider wake.

So you can think of a boat moving through water and its bow wave can be
very wide.

And however, for the speed of the plasma near Rhea, you wouldn't expect
it to expand that quickly so close to the moon.

You can expect features to move at sharp angles to the flow of the
plasma north and south of the moon along the magnetic field lines. But
in this direction we didn't expect to see such features. So it was a
real puzzle.

So if we move on to the next slide, so if we can't explain this by a
plasma wake, so there must be something absorbing electrons there.

So one possibility is neutral gas. So Rhea could have an exosphere. And
it almost certainly does have an exosphere. We expect it to have it
because it's sitting in this plasma and it's continuously bombarded by
energetic particles. And when these strike the surface, they kick up
material such as atoms and molecules. So around the moon we'd expect to
see this very thin exosphere or atmosphere.

And (Saur) and (Strobel) published a work in 2005 which - where they
modeled the expected effect of this process called (sputtering) on the
surface. And they inferred that Rhea could possess a thin exosphere.

So that could be the one explanation for why we were seeing a decrease
in, a depletion in electrons around the moon if this gas which could be
surrounding the moon was absorbing them.

So another possible explanation is a dust cloud. Now we expected to see
a dust cloud around Rhea for in some ways similar reasons to why we'd
expect to see an exosphere.

Because Rhea is continuously bombarded by interplanetary dust grains and
E-ring particles as well. So these are continuously raining down onto
the surface.

And when they do that, they throw a projectile, or small particles. A
few of them may escape, but most of them don't. So they arch out of the
surface and then come back down onto the Rhea surface somewhere else on
the surface of the moon.

And so because they're continuously bombarded by these particles, you
expect to see a cloud of ejecta around the moon.

So this is first detected by the Galileo spacecraft at the Galilean
satellites around Jupiter. And there's no question that we expected to
see these dust particles around Rhea.

And as an aside, by measuring the fluxes of these ejecta particles, It's
a good way of measuring how much material's coming into the Saturn
system. Because we know the mass of Rhea and maybe even try to get the
age of the Saturn ring system.

So models were - of this process have been published already before this
flyby by several workers as listed here. So we expected to see this -
these - this increase in dust around Rhea under the moons.

So if we move on to the next image, so the next slide, this shows an
image of Rhea using the MIMI INCA instrument.

So it's as you may have heard before by - in presentations by Don
Mitchell and others from the MIMI team, INCA detects energetic neutral
atoms. So it's a camera. But instead of detecting photons, it detects
atoms.

So MIMI INCA was fortunately on during this Rhea flyby. And this shows
one of those images with the disc of Rhea in the middle. And it's one
way of measuring the column density, the thickness of any atmosphere
that's surrounding Rhea.

So when we fly by Titan, INCA shows a bright halo around Titan because
the atmosphere is there when energetic particles in the magnetosphere
hit the atmosphere they release energetic neutral atoms. So we see this
glow around Titan which helps determine the thickness of the atmosphere
there.

And we can use the same thing - same technique at Rhea. But we didn't
see any significant enhancements in the energetic neutral atom flux
around Rhea at all. So it gives an upper limit to how much gas is there.

And also UVIS, the Ultraviolet Imaging Spector can detect the presence
of oxygen and molecular oxygen. And from observations by UVIS there was
no positive detection of - (O2) around Rhea.

So these combined suggest that there's - if there is gas there, there's
not much there, so the exosphere is quite tenuous.

And to see the decrease in the electron fluxes that LEMMS sees, we need
much higher column densities, much, much higher than the upper limits
returned by INCA and UVIS.

So I think we can quite safely conclude that neutral gas is not the
reason why electrons are depleted around Rhea.

So if we move on to the next slide, so we move away from Rhea for a
moment. And just to briefly mention, the solid grains and bodies can
absorb electrons.

Earlier on in the talk I mentioned - I showed the example of a dust
grain near Enceladus absorbing a particle.

And by using this technique, we can learn a lot about the regions around
Saturn and can even detect bodies before they're observed by cameras.

So this figure in the middle shows the fluxes measured by LEMMS during
one (periapse) pass in 2006.

So this shows the data from LEMMS for three energy channels, the
electrons labeled E5, E4 and E6.

So the (dipole) L-shelf, the scale at the bottom is - you can think of
it as the equivalent to the distance from Saturn. We were quite close to
the equator.

So the arrows show the lines when we were moving inwards towards Saturn.
And then we moved out again. And you can see how the fluxes changed
moving away from the planets.

And now there are two tiny moons of Saturn discovered by the imaging
team and called Methone and Anthe. They're only two or three kilometers
across each. And the - their positions of their orbits are shown by the
vertical red lines there.

And you can see that in the LEMMS data we saw wide decreases both
inbound and outbound near the orbits of these two moons and suggesting
that there's something at the orbits of these moons absorbing electrons.

And dropout there is around 1000 kilometers across. Now those two moons
as I mentioned are only a couple of kilometers across. So these dropouts
are far too wide to be explained by the moons themselves.

So several people from the MIMI team lead by Roussos and proposed that
these dropouts are actually evidence for possible ring arcs or rings at
the orbits of these small moons - Methone and Anthe.

And then if we move on to the next slide\...

Marcia Burton: Geraint?

Geraint Jones: Yes.

Marcia Burton: Before you go on, why is the dropout associated with
Anthe displaced to the larger L-shell?

Geraint Jones: Well as - yes, as I mentioned earlier, there are currents
moving dropouts around. So the magnetic field carrying a signature can
be moved inwards or outwards away or towards Saturn. So the same reason
as we put that wiggle in the - that sort of curtain cartoon showing an
example of a dropout near a moon.

But also we should mention that because of the persistence of some of
these dropouts, because they might not persist all the way around the
orbits, it's possible that both of those dropouts have come from one
moon or arc. And one of the signatures inbound was moved away in one
direction and the other one moved outbound.

So it basically shows that there's definitely something at approximately
that distance absorbing particles.

Marcia Burton: Okay.

Geraint Jones: Okay.

Marcia Burton: Thanks.

Geraint Jones: So if we move on to the next slide, you can see that the
imaging team targeted these moons and took long exposures or several
exposures and collided them.

And sure enough, Anthe and Methone have got arcs. And so one or both of
these arches are responsible probably for these dropouts that we saw in
the LEMMS data in the proceeding slide.

So before these images were taken showing the existence of these arcs
and there - the existence of at least one of them if not two was already
detected by the LEMMS instrument.

And so it's when these dropouts that are observed by LEMMS, it's a bit
difficult to infer how far away the source is. And so they can be really
close or if they're far away they might be responsible for a deeper
absorption. But because they're quite persistent, they can spiral around
Saturn, be detected quite far away from the source.

So one example of such a detection is Pioneer 11 which flew by Saturn
just before the Voyagers did in the early 1980s and detected a similar
dropouts at the orbits of the G-ring before it was observed by any
spacecraft.

So if we move on the next slide, we return to Rhea. And this shows data
from three instruments during the 2005 flyby.

So Panel A shows data from CDA, the Cosmic Dust Analyzer, the dust
instrument. And that shows the number density of dust particles detected
by that instrument and the size range it covered.

So you can see the Hill sphere is shown or the boundary of the Hill
sphere is shown by the dotted lines, the vertical dotted lines. And
close to the moon we see this increase in fluxes probably caused by or
almost certainly caused by the bombardments of the surface by
interplanetary dust particles. So what was expected based on earlier
models.

And the RPWS instruments and - can also detect dust particles. So when
dust strikes Cassini it creates a small plasma cloud. And then this can
be detected by the radio instrument. So the middle panel shows where
these dust detections occurred during the flyby. And again you can see a
sharp peak near closest approach to Rhea.

And then the bottom panel shows data from the MIMI INCA instrument. So
this is a little harder to interpret. And it's not entirely sure what
size range we're detecting. But with the INCA instruments there are
occasional discharges between the plates of the instruments which detect
the energetic neutral atoms.

And you can see that there was a slight increase in the - in this
current due to these discharges, our closest approach to Rhea again
suggesting that a small dust particle striking the instrument.

And we saw an increase near Rhea. So dust is definitely there but it was
expected.

So if we move to the next slide, so this shows how we estimate how much
material is around Rhea. So this cartoon for the 2005 flyby is looking
towards Saturn. And Rhea's the disk in the middle and the - surrounded
by the Hill sphere with the plasma moving from left to right.

Now the blue vertical lines of Saturn's magnetic field. And they're
constantly moving from left to right as Saturn rotates in the
background.

The yellow arrow shows an example electron moving along the magnetic
field. And just to the right of Rhea, you can see where Cassini passed
closest to the moon in the wake quite close to the surface.

So if you move on to the next slide, you can see that if we take this
example electron, as it bounces north and south along these blue
magnetic field lines that travel south, now when it gets close to Saturn
its direction reverses and then it travels north again along the
magnetic field line until it gets close to Saturn again and then reverse
direction.

And so these trapped particles are constantly moving north and south as
the magnetic field is dragged from left to right by the rotation of
Saturn.

So we know the scale of the Hill sphere. And the different energies of
electrons we can estimate how quickly they drift through the Hill
sphere.

So the red lines show the integrated, the path lengths through the Hill
sphere before it was eventually detected by Cassini.

So if we move to the next slide, the observed dust flux by the CDA
instrument was not out of the ordinary. It's fairly close to the E-ring
peak number density. So there wasn't the huge increase in dust necessary
to explain the decrease seen in the electrons.

So if we were to take a spherical dust cloud caused by bombardments of
the surface, it's - the shape of that dust cloud would fall off too
rapidly with distance to explain the shape of the electron decrease we
saw as well.

So another thing that helps us understand or gives us a clue at least to
what is going on is the CAPS was looking at electrons that were coming
straight through the Hill sphere rather than bouncing north and south
whereas LEMMS at the bottom right was detecting the electrons that
bouncing north and south several times through the Hill sphere before
being detected.

So we have the fluxes from the two instruments. And we can combine those
to try and understand the possible distribution of material in the Hill
sphere to explain what we saw.

So if we move on to the next slide, the only solution that we were able
to come up with was that rather than a spherical cloud around Rhea, we
needed a debris disk, a disk of material fairly close to the equatorial
plane. And so the particles or the electrons detected by LEMMS crossed
this surface several times north and south before being detected by
Cassini.

And so another advantage of having material in an equatorial orbit is
that it's more stable. This isn't my field of research. It's all that if
you go to high inclination orbits around Rhea, they're not stable for
long time scales at all.

So if you have particles in an orbit near the equator, there's in much
more stable orbits and can stay there for much longer time periods.

So an equatorial debris and an equatorial orbits makes a much more sense
than a spherical distribution of trapped particles.

The disk will be mainly observable by its effect on the charged
particles, so those that travel through the disk before reaching
Cassini.

And it can be of low optical depth. So that means it wouldn't
necessarily be a very obviously easy to observe debris disk. Because the
particles that we were detecting went through the debris disk if it's
there several times rather than just once. It doesn't need to be as
thick or as dense as optically dense as needed to be observable with a
camera for example.

So by combining the CAPS and the LEMMS data, we found a model, a fit to
the observations that with a thin equatorial debris disk that's agreed
with both the CAPS and the LEMMS data.

So Elias Roussos did lots of this work. He came up with a relationship
between the typical or largest grain size in the debris disk, the
optical depth of the disk if we were to observe it from one side.

And I'm not showing them here, but the images obtained by the imaging
team between the report of this explanation for the electron decrease
and now haven't been able to detect any sign of a debris disk.

So if it's there, it's extremely difficult to observe. And it pushes the
sizes of the particles or pebbles or boulders in this debris disk to a
large scale. So we're talking about centimeters, decimeters or even
meters in size for the particles in the debris disk.

So if that's true, then it is a real puzzle. Because when you have large
particles, you also expect small ones to be there as well. So when they
hit each other you get a cascade of sizes. We have big ones, slightly
smaller ones and then going through smaller and smaller sizes.

And the base should be observable by the imaging instruments on Cassini.
And they haven't been detected so far.

So if the debris disk is there and the typical sizes of these bodies or
particles in the disk are large, we'd expect to see small particles as
well. But that hasn't been detected so far. So it's a real puzzle to
explain everything that Cassini is telling us about the region around
Rhea.

Okay, so if we move on to the next slide, there are some aspects of the
electron fluxes which are consistent with there being absorbing
obstacles orbiting Rhea.

So this figure shows the inbound data from that flyby in 2005 from
several different energies. So each panel shows electrons of different
energies.

And as you go to higher energies, the width of some of these features
increases. And so that's tends to agree with what you'd expect to see.

So as you go to higher energies, the electrons have slightly larger
(gyroradii. So that means if you have an electron trapped by a magnetic
field, it spirals around the magnetic field. But if it has a higher
energy, then the size of that spiral gets bigger.

So if you have energetic electrons, they follow a larger helix or
spiral. And they will form a wider dropout if they hit a particle.

So as you go to higher energies, these signatures do widen which sorts
of agrees with what we'd expect to see. But the rates at which they're
increasing is a bit larger than expected.

It's another - another aspect of this is that electrons going through
the Hill sphere around Rhea go through at different rates.

So if you go to higher energies, they spend longer going north and south
through the Hill sphere before being detected by Cassini.

And combining all these different factors to explain the observations we
have is quite complicated. So all we can do is give ballpark figures.
And the debris disk explanation is the only one that we've been able to
come up that explains what we see.

So if we move on to the next slide, one of the more curious aspects of
the data returned by LEMMS, these really short dropouts.

So I showed you the really wide decrease earlier on which you can see in
the top panel. For example in the red line at the top you see it covers,
it decreases quite far away from Rhea and then increases again gradually
on the other side.

But on either side of the deep decrease where - which are caused by the
moon itself, you also have these very brief dropouts lasting only a few
seconds.

So the bottom panel shows some of these features in the data from the
high time resolution channel from LEMMS. And you can see three clear
dropouts on one side of Rhea, another three on the other side and then
another possible one towards the end of this period.

So if we move to the next slide, and the simplest explanation for this
is that there could be extended arcs or rings of material orbiting Rhea
in its equatorial plane.

So these dropouts, these set of six dropouts are almost symmetrical. And
you can fit the positions of three discreet rings to the locations of
these dropouts.

And so maybe they're caused by another process such as features on
either side of the wake downstream of Rhea. But the symmetry is very
intriguing. So we'll have to look for those features again during future
flybys.

So just to show you at the bottom there, we have - we return to the data
from Tethys. And you can see how that compared to what we see at Rhea.
So at Tethys we see really sharp dropouts. And Rhea we have these
features on either side.

And also the dropout in the wake at Rhea is a bit more ragged because of
differences in the plasma temperature that the rates at which these -
this wake fills in downstream of the moon.

So the - so these brief dropouts are really curious aspect of the data.
And the symmetry could be a coincidence. But if it is, it's an amazing
one.

Okay let's move on to the next slide. So just to briefly show you that
when Voyager-1made its closest approach to Rhea in the early 1980s, a
wide decrease in electron fluxes was observed then as well.

So at the bottom panel you have a plot covering 12 hours. And then the
next panel zooms in to 10 minutes. And then the top panel moves into a
period covering only 3 minute around the closest approach to Rhea.

So you can see in the middle panel that there's a broad decrease around
Rhea and then suggestions of small bits as well.

And so these were observed further away from Rhea than the ones during
the close flyby in 2005.

And interpreting the data from Voyager is complicated because the pitch
angle, the direction in which the instrument was looking was constantly
changing. And there are lots of data gaps as you can see in the plots.

Marcia Burton: How far away was this flyby?

Geraint Jones: To be honest I can't remember. But I - it's well outside
the Hill sphere, so much further away than the flyby by Cassini in 2005.

Okay, so if we move on to the next slide, we have further data from a
flyby 5700 kilometers downstream in August 2007. So this is just outside
the Hill sphere.

So we've shown on the scale of L-shell. So this is distance from Saturn
and the data from LEMMS during those two flybys. So the 2005 data is at
the bottom and then 2007 data at the top.

And so we were further downstream. But again you can see a hint of the -
a broad feature around Rhea. It's not as obvious or as symmetrical as it
was in 2005 in the LEMMS data at least.

But there is an indication that the broad depletion is there again. It's
been shifted regularly again by these currents that move or we believe
by these currents that move these signatures in and out away from
Saturn.

And also we see these short period dips again surrounding the dropouts.
We suggest that they - once they're formed near Rhea they're carried
downstream so you can detect them further away.

We've looked at the positions of these dropouts. And because the whole
signature's being moved inwards towards Saturn, it's a bit difficult to
be completely sure that we're seeing them at the same distance from the
axis of the wake as when we observed them in 2005 during the close
flyby.

Okay so LEMMS showed a hint of a broad decrease being there again at the
same scale as the Hill sphere.

And if we move on to the next slide, we show the data from CAPS CLS
during the 2007 flyby. And this time ELS shows the broad decrease more
clearly than LEMMS. So on either side we see this decrease in electron
fluxes inbound, then outbound again on the other side.

So the change in the spikiness of the data is just a change in the time
resolution of the instrument. So for the flyby itself at around zero 50
hours, it changed into a high resolution mode. So that's why the data
suddenly gets more spiky.

But you can see on either side of Rhea we again saw a broad decrease in
electrons and again, roughly the scale of the Hill sphere. So the
feature is still there.

Okay, so if you move on to the next slide I can briefly mention future
plans. And so for the Equinox mission just over a year from now we've
got a targeted flyby of Rhea planned to within 100 kilometers of the
surface. So the flyby speed will be 8.6 kilometers a second and then
will pass close to Rhea's North Pole.

So in 2005 Rhea was on the day side of Saturn, this time will be on the
night side. But again, Cassini will be moving inbound towards the
planet.

And so LEMMS and (ELS) will be placed in orientations designed to check
for the recurrence of these features I've been describing.

So we've seen the wide wake already in 2007. So we expect to see it
again. It will be small scaled (dips), be present again during this
flyby and at similar distances.

And the instruments will also be fixed so that we get good pitch angle
coverage. So we're seeing electrons that are bouncing north, south along
the magnetic field lines. But also those that stay fairly close to the
equator of Saturn. And so we hope to get good coverage of the electrons
from several directions.

Okay so that's occurring a year from now. So if we move to the next
slide, is there any other evidence for the existence of a debris disk
around Rhea? Let's move to the next slide.

We have an image of Rhea captured by the imaging instruments on Cassini
ISS.

This is taken - its infrared wavelengths. And it shows a feature that
was first discussed on an online forum unmannedspacefilght.coms where
amateur and several professional planetary scientists stitched together
images as they're released on to the planetary data system as they're
released to the public. And some members there pointed out this curious
feature.

If you look at the image the equator of Saturn is more or less in the
center moving - it's horizontal in this view. And you can see a row of
dark features running across the image and - from left to right. And so
it's a row of dark patches.

And the position of these patches is actually very close to the equator.
So if we move to the next slide, several images taken at different
wavelengths with ISS have been combined.

And if we zoom in on to one of these patches, you can see that these
features which are shown in blue here are actually tend to occur on the
Eastward facing slopes of craters. So the color's been stretched. It
isn't natural at all.

So these features tend to occur on these Eastward facing sides of
craters and also at what appear to be high elevation regions.

So if we move to the next slide, this isn't - this is not inconsistent
with material in orbit around Rhea's equator. So circular orbits but
orbits that are gradually decaying.

So you can imagine the particle in orbit and slowing its altitude is
decreasing over time and then eventually it will strike the surface.

And if it's in a retrograde orbit that's moving from right to left in
this view - this has got north towards the top of the image, then they
tend to strike the eastward facing slopes of craters and any high
elevation regions.

So if this scenario's correct, then the impact velocity for material in
orbit such as this is around 460 meters per second. And that's fast, but
it's not sort of hyper velocity speed. So you - so the impactors would
probably (fragment) without forming large craters.

So it's a curious idea. We're looking more closely at this. And it's
extremely preliminary. It's just a very interesting set of features that
were found by amateur observers. And we're paying a lot more attention
to these now. And we'll study these features in more details over the
coming months.

So as I said, very preliminary, but it is extremely curious that these
features are occurring close to the equator and may have an explanation
similar to the explanation that we've put forward for the debris disk
orbiting Rhea.

Okay so move on to the summary. And observations by MIMI LEMMS and CAPS
ELS clearly show that there's a deficit of electrons in the region
around Rhea.

Now the only explanation that we've been able to come up with is that
Rhea's surrounded by an electron absorbing population of solid material.

Whatever's going on there, it's an apparently unique moon magnetosphere
interaction.

So from the model we produced to fit to the electron observations, we
have three components to this material. A spherical ejecta cloud which
was expected and flat equatorial disk of material and maybe discreet
rings or arcs at low altitudes.

But these rings or arcs that are there are also very difficult to
explain because you need something to constrain the orbits of particles
into a narrow range of altitudes.

So the next point is to the elephants in the room. There's no
observational evidence so far. So the remote observations of Rhea by ISS
hasn't detected any evidence for this orbiting material to date.

If big boulders or pebbles are there, then we expect to also see smaller
particles. But these haven't been detected to date.

So a possible explanation is that the smaller particles are somehow
removed. Maybe they're not stable or they can be ground down by some -
by magnetospheric particles et cetera. But no easy explanation. So it is
a big puzzle.

And then finally the equatorial surface features and other absorption
signatures from other flybys are currently undergoing analysis.

So I'll end with the final slide which is just an artist's impression of
what the region around Rhea may look like, heavily exaggerated. But
there could be a disk of boulders or pebbles orbiting Rhea in the
equatorial plane.

So maybe in a year's time, in the March 2010 flyby we'll get a lot more
data to help us understand exactly what's going on.

But whatever's going on at Rhea, it's unique and we look forward to
finding a few more pieces of the puzzle then. Okay, thanks a lot.

Marcia Burton: Thank you very much Geraint. That was a great talk and
your graphics are amazing. I expect that the artist representation on
the last view graph is you right? Because I know you're really good with
Photoshop. Did you\...

Geraint Jones: No it wasn't.

Marcia Burton: No? Oh that's lovely. That's lovely. So certainly there
must be some questions for Geraint. People want to chime in with their
questions?

No questions? I've got a couple. But so for the XXM we're - the extended
extended mission, this is a seven years that we're proposing to fly, I
think we're trying to get some close Rhea flybys. Do you know what is in
the plans for the XXM?

Geraint Jones: Yes. We've got several really close flybys on the scale
of I think the closest is a couple of hundred kilometers. The next one
out is less than 5000 kilometers. And then quite a few other ones which
are a bit further out.

So if XXM is approved, then we're bound to get much more useful data. I
mean there are other techniques as well which we hope to use such as
UVIS staring at a background star as it passes close to Rhea or Rhea
moves across the path between Cassini and the star.

So there's a chance that if there are boulders orbiting Rhea that we
might see a dip in the starlight as these pass in front.

So that's another technique that will need to be tried out if possible.

Marcia Burton: Is that being planned or\...

Geraint Jones: Yes there are plans, yes.

Marcia Burton: Yes, that's cool.

Geraint Jones: So with each flyby, I mean we've got several more distant
flybys that have also taken place which I haven't shown here. But yes
the data from MIMI and CAPS, there's curious for several of those as
well. So we've got a lot of work already but we look forward to getting
more close flybys.

Marcia Burton: Anybody, any other questions?

Man: You have a question?

Woman: Question. When you first looked at your data, was your immediate
reaction that you had a problem with your instrument or was it more that
you had something really interesting going on?

Geraint Jones: I think it was the latter. This flyby in 2005 occurred
when we had the run of several amazing flybys during the past several of
these moons. So within a period of two or three months we had really
close flybys of Tethys, Dione and Rhea.

And so we suddenly got all this fantastic data.

And when we compared the new data side by side from the different
flybys, it was clear that the Rhea data was unusual. It - there was no
obvious explanation for it.

So but we were constantly looking at the data from the other flybys that
there was no obvious problem with the instrument. And we were seeing the
feature in CAPS as well as MIMI. So that helped us be confident that it
wasn't an instrumental effect.

Marcia Burton: Anyone else, questions?

Okay, well I guess we'll bring it to a close then. That was a really
great talk Geraint. It was fascinating. I really appreciate it.

So just to let the participants know, in a month's time we're going to
have (Ralph Lorenz) talking about Titan. The title of his talk is A New
Window on Titan's Winds and Internal Ocean. So that'll be at next
month's CHARM telecon.

And if anybody has any suggestions for topics they'd like to hear
whether it be additional science topics or even topics that pertain to
engineering, running the spacecraft, just send an email to CHARM leads
and we'll try and get those in the schedule.

I'm scheduling talks for the whole year, so now is a good time to let us
know about that.

And with that I will say goodbye to everyone. And again, thanks very
much to Geraint.

Geraint Jones: Thanks very much Marcia and thanks for everyone else for
listening. Okay, thank you.

Marcia Burton: Yes, bye-bye.

Geraint Jones: Bye.

END