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**Moderator: Trina Ray**

**October 28, 2008**

**1:00 pm CT**

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(Amanda): Thank you. Hi everybody and welcome to the CHARM presentation
for October 2008. We are joined today by Professor Andrew Coates from
the University College London where he is the Deputy Director of the
Mullard Space Science Laboratory.

Professor Coates is also a co-investigator on the Cassini-Huygens CAPS
instrument and on Venus Express. And he's been a co-I on several other
space missions such as Venus Express and Rosetta.

And his research interests include the plasma interaction with
unmagnetized objects such as Titan, Venus, Mars and comets and also
planetary magnetospheres, planetary surface studies, space
instrumentation and space weather. And today he's going to be telling us
about his results studying negative ions at Titan.

So with that please go ahead and start whenever you're ready.

Andrew Coates: Okay, thanks very much, (Amanda) and thanks for asking me
to do this; it's an interesting experience to be doing this from the
middle of Surrey\...

(Amanda): Right.

Andrew Coates: \...in the Surrey Hills in England. So our laboratory is
part of the University College London but we're actually about 35 miles
outside of London and so we're here in the middle of the countryside in
England, which is very nice\...

(Amanda): And you're a good sport to call in in the evening there.

Andrew Coates: Yeah, that's right, it's evening so just a little over a
seven hour time difference to California anyway, usually eight.

Okay, so negative ions at Titan and tholins in Titan's haze. So we think
this is one of the sort of very exciting discoveries of the
Cassini-Huygens mission not only because it included the instrument
which we were involved in but also generally because what we see in
Titan's ionosphere is something which is really unexpected.

And the first slide there just shows a couple of images at the bottom
right to do with Titan's haze and so this goes up to a few hundred
kilometers above the surface of Titan. And then Titan is immersed in
Saturn's magnetosphere so the middle picture there shows you Saturn's
magnetosphere and the sort of donut shaped thing going around inside the
magnetopause and bow shock of Saturn, which are the outer sort of left
hand elliptical type of objects.

The donut shaped thing is actually the region where Titan may be
interacting with the plasma inside of Saturn's magnetosphere. So on that
picture Saturn is really small, right in the center, and the magnetic
field lines associated with that are shown. The solar wind is coming in
from the left hand side. The solar wind is a stream of particles coming
from the sun all the time, a million tons per second of material coming
out of the sun. So these are the charged particles in addition to heat
and light the sun emits these.

So even by the time we get out to Saturn the solar wind plays a
significant role in shaping Saturn's magnetic environment. You can see
that from the shape of the bow shock, which is the outer boundary there
and the magnetopause, which is the next one in.

So, as I say, Titan is usually just inside of the magnetopause of Saturn
so it's inside Saturn's magnetosphere and so you get that donut shaped
thing, which has to do with neutrons coming off Titan. Because of course
Titan is a fascinating object because the - it has an atmosphere and it
has an atmosphere which is much like the early Earth.

And one of the reasons for taking Cassini-Huygens to Saturn was to look
at that primitive atmosphere. It's the only moon in the solar system,
which has got a significant atmosphere. And of course those haze layers,
the tantalizing haze layers was seen by Voyager. The picture on the
right is in fact a Cassini-Huygens image taken with a different
wavelength.

But the Voyager images were shrouded - or the surface was shrouded by
haze and so all that Voyager saw really was the body of Titan with the
haze and then some hazy layers on the top of that.

So, on the left hand side, if you're looking at the PowerPoint version
you'll see one of the beautiful images of Titan, which actually comes up
later in the presentation as well so if you haven't got the PowerPoint
one it doesn't matter because the PDF - it doesn't come out. But anyway
this is one of the really nice images of Titan.

And then if I click again on that - in the PowerPoint version it has a
depiction of the Miller-Urey experiment because Titan's atmosphere
includes mainly nitrogen but there's small amounts of methane and the
possibility of making complex organics. And Carl Sagan and colleagues
and also Miller and Urey thought that that might be a possibility for
atmospheres like Titan.

If you add energy into that type of atmosphere you may get the formation
of interesting substances up at the top of the atmosphere. And so that
is a depiction of the Miller-Urey experiment there, which we'll come
back to later.

Okay, so that's the introduction really. We're interested in Titan, its
interaction in the plasma of Saturn's magnetosphere and in particular
about the ionosphere of Titan and what roles or at least what
significance that might have for this haze of Titan. And we can look at
some of the exciting results from Cassini, which seem to show that there
is a link.

Okay, so just to start off with the next slide then, slide two. You
can't see the number because it's a dark slide. But it says Titan at the
top and there's that picture again of Titan, which is one of the
beautiful early images of Titan; multi wavelength representation using
Cassini's great imaging experiments; a combination between infrared and
ultraviolet. And you can see, of course, right down to the surface.

But you can also see the haze above Titan there as well so I would say
this is a real favorite in terms of images of Titan.

Okay, there are a few facts about Titan here. So on the right hand side
the radius, 2575 kilometers; the atmosphere, as I said before, mostly
nitrogen but also some methane and abundance of that of about 5% near
the surface maybe a little less. It's an unmagnetized object, which has
an ionosphere, so this is unusual for moons. As I say it's the only moon
in the solar system which has got a really significant atmosphere -
high-pressure atmosphere.

In fact the total pressure in the surface, one and a half times the
Earth's atmospheric pressure now. So it's even more significant than the
Earth's atmosphere.

Things like Europa do have stuff near them but much lower density and
lower pressure. But Titan is the real important atmosphere in the solar
system. And as I say it's one of the big reasons for sending
Cassini-Huygens to do all these flybys of Titan.

Okay there's haze in the atmosphere, which is seen on those pictures.
There's also - Cassini-Huygens of course has found dunes on the surface
using the radar instrument, there's evidence for lakes, again using
radar. And lots of evidence by VEMS and other instruments for surface
modification, which has happened recently.

And so, you know, the picture of Titan is a really complex, dynamic,
evolving system with lakes on the surface and presumably precipitation
systems and this sort of thing and the possibility of getting
significant amounts of modification at the surface.

Now what we're really interested in is its plasma environment and this
is the charged particle environment. So as I mentioned the solar wind
blows away from the sun all the time but - because Saturn has a
magnetosphere which has the type of parameters which are shown on that
slide. So the density in the magnetosphere about (0.1) to 1 particles
per cubic centimeter, which is really very low, you know, compared to
the rooms that we're in that's about a ten to the pound, 19 particles
per cubic centimeter so this is really low, you know, 19, 20 orders of
magnitude down on that.

And then the temperature of that plasma in the magnetosphere of Saturn
is something like 100-1000 electron volts. Now if you convert that to
temperature in Kelvin basically one electron volt corresponds to 10 to
the four degrees Kelvin so you've got, you know, a million to 10 million
degrees. So really extremely hot plasma but very rare, as I say, so
that's the magnetospheric environment which Titan, sorry, is bathed in
all the time at that distance away from Saturn.

Now the solar wind, which Titan may spend a few percent of its time out
in the solar wind, the density of that less than (0.1) particles per
cubic centimeter so this is the solar wind outside of the bow shock of
Saturn. And the bow shock, one of the things that it does is compresses
the plasma and makes a larger density inside in the magnetosheath region
and also thermalizes the plasma so there's a higher temperature than
there is outside.

So you have a few electron volt temperature in the solar wind, the
larger temperatures than that in the magnetosheath. But then inside of
the magnetopause, which is the region where Titan spends most of its
time, we have this very high temperature plasma and that's the
temperature in which Titan is immersed.

And so you might expect, you know, thinking back to that Miller-Urey
experiment, this could be a source of energy, which is sort of impacting
on the upper atmosphere of Titan for a lot of the time. Now as it says
on there the solar wind - Titan might spend a few percent of its time
out in the magnetosheath or solar wind region.

And in fact we've only seen one encounter so far, which there was a
paper in the Science a few weeks ago about an encounter on which T32,
which is one of the Titan encounters, where we saw Titan very close to
the magnetopause of Saturn so it was going out into the magnetosheath
and there was evidence for that in that paper. So that was one of the
interesting results.

But of the 44 encounters in the prime mission of Cassini there was only
one when we had that type of situation with basically the spacecraft
going past Titan when the magnetopause was very near. So that's
consistent with the idea that there's only a few percent or very few -
very little of its - of Titan's time is spent in the magnetosheath type
of plasma, it spends the most of its time bathed in this high
temperature magnetosphere.

And that's where the energy perhaps comes from for the types of things
that we're going to be talking about.

(Amanda): Andrew?

Andrew Coates: Yeah.

(Amanda): Does the amount of time that it spends in the magnetosheath or
sort of, you know, on the boundary\...

Andrew Coates: Yeah.

(Amanda): \...depend on how much pressure the solar wind is putting on
the magnetosphere and maybe - does that vary then during the solar
cycles?

Andrew Coates: Yes, that's right. So, yes, there's a huge variation,
absolutely. And this is why the magnetosphere sort of breathes in and
out. So like Earth's magnetosphere where you have the magnetopause
downstream of the bow shock, the solar wind pressure there plays a very
important role in controlling the position of the magnetopause and then
the bow shock.

Because the magnetopause really is the obstacle to the solar wind flow
and then the bow shock sort of forms upstream of that; it's a bit like
the bow and wave around the ship or around Concord when it was flying,
you know, the shockwave around that. And so that whole structure moves
in and out and so it's a really dynamic picture.

I mean, you see the pictures like in the first slide of the
magnetosphere looks pretty static. But as the solar wind changes in
density and temperature the whole thing moves in and out and so, yes,
it's a matter of chance as to whether you see the Titan when we're close
to the magnetopause.

(Amanda): So even though in the 44 flybys there were plenty of times
when Titan was like on the Saturn side, I'm sorry, on the sun side of
Saturn\...

Andrew Coates: Yeah, yeah.

(Amanda): \...there was only one time when it was actually at the
boundary?

Andrew Coates: Exactly.

(Amanda): Is it - oh, okay, interesting.

Andrew Coates: Yeah, so that's consistent with this idea of, you know, a
few percent of its time is spent outside.

(Amanda): Yeah.

Andrew Coates: Yes, we only have direct evidence of where Titan is of
course when we do the flybys because we're making measurements as we do
the flybys and we can see what type of plasma we have upstream of Titan.

(Amanda): Yeah, right.

Andrew Coates: That's absolutely right. So there were several flybys on
the dayside; there were some on the night side as well of Saturn but the
ones near the upstream region were mostly in the - in Saturn's
magnetosphere.

(Amanda): Okay, thanks.

Andrew Coates: Okay, so that's the upstream conditions then, so the
magnetosphere is the principal upstream condition; it spends most of its
time out there or in the magnetosphere of Saturn and then just a few
percent with different conditions. And so that's the type of plasma that
you've got upstream.

Okay so on to the next slide, the third one. Again, this is a dark slide
so I can't see the number on it and probably nobody else can. But this
is the one with Titan in the middle of it and there's the lower
atmosphere, the thermosphere and ionosphere, the exosphere and topside
ionosphere and then the yellow arrow things, solar radiation.

So this is the sort of classical picture of how Titan interacts with
this magnetospheric plasma. And so we have Titan, which is this
unmagnetized object and this is immersed, as I say, in the magnetosphere
of Saturn. And so there's a flow from left to right, which is the
magnetospheric plasma flow, which is - includes nitrogen and hydrogen,
actually oxygen is a principal component of that as well.

So this slide was done actually before the Cassini mission so we weren't
sure of what the composition was at this region. And then there's the
magnetic field lines, the blue things are kind of draped around this
ionosphere of Titan as we can see there. And so in this case, because
Titan doesn't have a magnetic field, the atmosphere and ionosphere and
really the ionosphere of forming those conducting obstacle to this flow
in the magnetosphere.

And so the magnetic field line drapes around like this as mass is added
to the flow and it's sort of the - because plasma is a very highly
conducting the magnetic field and the plasma are frozen together and so
if you add mass, which Titan is doing, to the magnetic field lines,
which are sort of threading around there, then you get this deflection.

And this is the same sort of deflection, which you get in comets and in
Venus as well. And so there are things one can learn about the
electrodynamic interaction here, which is directly relevant to those
other objects.

And, okay, so that's one thing: the magnetic field structure like that.
Now it was thought, you know, the magnetic field structure even at that
point of - in the magnetosphere would be on average North/South because
this is the - the magnetic field of Saturn has its North Pole in the
Northern hemisphere, that's unlike Earth. And Earth, by the way, at the
moment, has its North magnetic pole in the Southern hemisphere, which is
a bit strange. But it means that the magnetic field orientation is
opposite to that of Saturn.

But so we have this North/South magnetic field by the time we get out to
Titan, which is at 20 Saturn radii in the magnetosphere there. And so we
get the draping as I described like that. Now the - those magnetic field
lines are sort of a conduit for electrons from the magnetosphere, which
can spiral around the magnetic field. Of course we all know that charged
particles spiral around magnetic fields and so there's the electron
doing that on the top of that picture.

And that means that the magnetospheric electronics, which have all their
energy, you know, 100-1000 electron volts, can actually get in by the
field lines and start heating the upper atmosphere of Titan. And so that
is, in addition, another source of heat to the solar radiation, which is
coming in from the left hand side. So the solar radiation, you know,
sunlight, of course is much weaker than at 1 AU where Earth is but
nevertheless it's an important source of heating for Titan on the
dayside of the ionosphere.

And, okay, so - and in addition to that we can see a number of ion
species there; there's a lot of positive ion species, which is doing a
number of different things, it's thought maybe there's some
(ionothermic) (outflow) down the tail and there's other positive ions
there. Those ions pickup, which is coming from the top of the atmosphere
and that's the particle, which is shown below there.

And again, that's a very similar process to what happens with comets so
a neutral particle at the top of the atmosphere of Titan suddenly gets
ionized and then of course interacts with the electric and magnetic
fields of the magnetosphere in this case.

And so that ion pickup, a sort of cycloid-shaped pickup is what happens.
So the particle accelerates along the electric field and then gyrates
around the magnetic field so in the end you end up with this kind of
crazy almost bike wheel type thing.

In fact if you think about the valve on a bicycle wheel as that goes
round, this is something like the drift process, which is happening in
this plasma where we have an electric and a magnetic field. So we have a
sort of convection along as the bike moves along but also a sort of
spiraling around the magnetic field. So the resultant of all that gives
you the cycloid, which is the - depicted there.

But the reason for showing this is that this picture was made before the
Cassini-Huygens mission and there's no mention of any negative ions at
all either or positive. And all the models of Titan's ionosphere had all
positive ions; there was no suggestion of negative ions. Apart from - in
the lower atmosphere, in that region there below 100 kilometers in the
atmosphere there were people who suggested that there may well be
negative ions there.

And normally, I mean, we get negative ions in the Earth's ionosphere at
relatively low altitudes, something like 60 kilometers above our heads
and to 100. And so in that region where the plasma density is high
enough and where the composition is right because we have oxygen of
course at the Earth and that's a very electronegative species and so we
can get negative ions there.

But nobody thought, with the nitrogen and the - with the methane and
with the sort of resultant ions from that that we would have any
negative ions at all. And this picture, you know, is an illustration of
that because there's not a negative ion to be seen. But as we'll see
with the - with what we see with the electron spectrometer which we
think we do see negative ions. And this is really quite significant.

(Amanda): Can I ask a quick question?

Andrew Coates: Yeah, of course.

(Amanda): Let's see, over on the right\...

Andrew Coates: Yeah.

(Amanda): Are you showing that there's sort of bigger ions that get
dragged out into the tail but then the mass loading ions down at the
bottom are simpler? Are those the different processes then?

Andrew Coates: They're illustrating different processes, yeah, different
processes of escape. And they're different - I mean, what species you
get depends on the altitude in the ionosphere\...

(Amanda): Okay.

Andrew Coates: \...and the region that it's coming from and also whether
it's sunlight or not, and, you know, the whole mix of energetics tells
you about the type of ions you might get. So the simpler process at the
bottom has to do with, yes, ion pickup, which is just ionization of the
most abundant neutral species in Titan's atmosphere so you get things
like the H+ and N+ and so on, yeah.

(Amanda): And is that from sort of all over the top of the atmosphere
then?

Andrew Coates: Yes although there is a directional nature to this
because it's at the first acceleration is along the electric field
direction, in fact we'll get to that later. The electric field is from
(-B Cross-B), which is a vector product of the velocity of the
magnetospheric plasma and (B) the magnetic field. And so you can tell,
on this diagram, that would be in a direction outwards basically towards
us, this is a projection that we're seeing.

And so this - we'll get into those directions later. But you get a lot
of the acceleration first is along the electric field and so that's the
direction which you get there. Whereas the ionospheric outflow from the
tail was thought to be some other escape process, which allows material
to be coming out from the tail and so there's already been some
chemistry going on in the atmosphere to - and then to sort of physics of
the outflow happening to take out different species.

(Amanda): Okay.

Andrew Coates: Okay, but basically - so the sort of interesting thing
about the slide for this is the fact there's no negative ions there, not
one to be seen. Okay, so the next slide, slide four, there is the CAPS
instrument so - it's the Cassini plasma spectrometer. And I think there
has been one of these telecons before, which has involved CAPS data so
some of you might have heard this being described so apologies for that.

But there's basically three sensors and the data processing unit and an
actuator and those in the picture on the top left, you can see those
different instruments. And so the slide there includes what we can
measure with CAPS. So Dave Young is the principal investigator of the
CAPS instrument and it includes these three different centers on the
data processing unit and actuator.

So the ion mass spectrometer, which looks at positive ions and so that,
you know, one of the key things for that was to look for those pickup
ions and the other types of ions and escape processes coming away from
Titan. That has an energy range 1-50,000 electron volts and - per
charge, and energy resolution about 17%. I won't go through all the
numbers; you know, but good angular resolution, good angular resolution
in both directions.

And then with the ion beam spectrometer, which was designed to look at
narrow positive ion beams and look at energy and direction with the
similar energy range, a much better energy resolution and we'll see some
data from that in a moment and an angular distribution, which is also
slightly better.

So this was designed, you know, one of the things this was designed to
do was to look for the ions - the positive ions in the ionosphere of
Titan and it does a great job of that. And finally, the electron
spectrometer, the bottom one there, which was produced - we at NFSL led
the team producing the electron spectrometer and I think this was a
collaboration involving Rutherford Appleton Lab and a group in Norway as
well and NDRE.

So we produced for the CAPS instrument and the electron spectrometer and
this has an energy range .6 electron volts to 28,000 electron volts, an
energy resolution of about 17% and good angular range.

Now it mentioned actuator there and so what does that mean? This -
because of an early descope of the Cassini mission we didn't have a
turntable to put these on. What you would like to do is to be able -
with plasma instruments is to see the whole sky and to see the direction
that the particles that you're measuring are coming from.

And what we can - an early descope very early in the mission, meant that
we didn't have the turntable and so we had to basically have an actuator
of our own. And so this is a little motor, which is on the CAPS - the -
just about visible at the bottom there. So this moves the whole
instrument around so the field of view can actually sweep out and get a
larger angular coverage than it would get if you didn't do that because
Cassini-Huygens - because of the images onboard is a three-axis
stabilized spacecraft and so you - for the plasma instruments you need
that extra angle and so there's a motor on it.

And so it sweeps backwards and forwards like a windscreen wiper in order
to get the particles, which we're interested in. So all of these fields
of view are sort of (panel) like and coming out from the sensors. And
maybe I could just sort of briefly say about the electron spectrometer
and how it works because that's relevant later.

The electron spectrometer is a relatively simple instrument; there are
two concentric hemispheres and there's a top cap, so you sort of slice
off the top of one of those hemispheres and that allows particles in.
And you apply a voltage between those two concentric focal plates and
then particles are guided round, and depending on the voltage that you
put on the plate you just bend the particles by a different amount.

And so that actually allows you to select the energy of the particles.
And so that gives you the energy range, which is seen there. So you
sweep that energy and that gives you different energies of detected
particles. And then those are detected on a micro channel plate
detector. And then around the instrument there are different anodes,
which are sort of pointing at different bits of this electrostatic
analyzer where it's in an annulus around the exit there and that gives
you the angle so you're measuring both energy and angle of the electron.

With the ions it's slightly more complicated. The ion - actually the ion
beam spectrometer works in a similar sort of way just with an electric
field. But the ion mass spectrometer has another electric field in it,
which allows you to determine mass because there's something - it does -
it measures the time of flight of the ions so it measures as well as the
energy of the ions it measures the time that it takes to go through the
instrument, which gives you the velocity, which then gives you the mass
per charge of the ions directly.

And so it is more complicated but it gives you a definite determination
on mass whereas either the electron spectrometer and all the ion beam
spectrometer we're just measuring energy per charge so we don't get a
mass measurement.

And of course with an electron spectrometer we didn't expect to have to
be worrying about mass at all with this instrument; we just expected to
be just detecting electrons in the magnetosphere of Saturn, which of
course we do. And we've had some fantastic results on that and the, you
know, various things, the rotation, the magnetosphere, measurements at
the magnetopause of Saturn, all sorts of things which we've been able to
look at, the plasma (unintelligible) for example as well and so lots of
interesting results with that.

But we got this really unexpected result at Titan, which is what I'm
mainly going to be talking to you about today after this sort of
introductory bit. Okay, so the next slide, slide five, is about negative
ions in ionospheres. So this is just a sort of setup for what do we know
really about negative ions in the solar system because, you know, I
hadn't expected when we started Cassini-Huygens, you know, back in 1989,
1990 - with the instrument involvement - to be talking about negative
ions at all but never - they do exist in other ionospheres.

I mentioned briefly about they do exist in the Earth's ionosphere so the
negative ions are present in the Earth's ionosphere; they're a
electronegative species there so O- and I would say between about 60 and
100 kilometers in the D region of the Earth's ionosphere, it's called -
this is one of the layers in our ionosphere we get negative ions.

Negative ions have been seen in other solar system contacts so they've
been seen in the inner (comer) of comet Haley and there was a nature
paper in 1991 about that. And we were involved in a different experiment
on (Jioto), which proved through the (comer) of comet Haley. But lo and
behold they saw negative ions there as well and they thought that, in
that particular location they thought charge exchange processes.

So this is where a positive charge sort of moves over to a different
atom or molecule and that can happen with negative ions as well and so
that can produce positive and negative ions. It was a really complicated
process, I think it was called the great-granddaughter process so a
couple of different processes had to go on in order to actually get
these things to be born but that was the analysis which was shown there.

So comets seem to have negative ions because they have complicated
species there as well. As you can see these are relatively high mass
ions, which are seen, you know, sort of water group type thing, 7-19 and
then larger masses were seen as well and this was deemed this particular
comet.

They suspected at Europa and some of the deflections in the magnetometer
is - has that - as the Galileo spacecraft went past Europa a few times
was sort of indicative of not only electrons being there but also
possibly negative ions. And again, there are things like oxygen and
chlorine even at Europa perhaps, which are electrostatic -
electronegative species again and that's the suspicion there.

So if in future an outer planets mission goes to Europa this will be one
of the things to look for there. But of course another possible places
it could go and revisit Titan and look at the - follow up on the results
there as well.

Okay, the negative ions, the bottom bullet there is negative ions were
not expected at all high in Titan's ionosphere because we didn't
anticipate any electronegative species. But, as I said before, they were
indeed expected in the (unintelligible) areas (unintelligible), which
have done calculations about that.

And this is atmospheric electricity type of calculations, the sorts of
things, which go on in Earth's D region and lower and so negative ions
were expected lower in the atmosphere but - in the ionosphere but they
weren't expected to be seen directly by Cassini, which was flying by at,
you know, 950 kilometers plus in the different flybys.

Okay, slide six there is, again, drawn before the Cassini-Huygens
mission. And so this was the anticipated interaction between Titan and
its magnetospheric upstream region. And so we have Titan, again, no
magnetic field but the magnetic field is draping around it so it gives a
three-dimensional view of what we were looking at before.

The reason for showing this is that nominally we would have a
North/South magnetic field like that. The direction towards Saturn, on
the left hand side there, in fact it\'s going around a little bit,
Saturn's over to the left. And the orientation of this magnetic field
and velocity is giving us the electric field in the right hand side
direction.

I should just say Titan is going around - is going around - certainly if
you look at Saturn from above it's going around in one particular
direction. And then the plasma associated with Saturn's magnetosphere is
also going around in the same direction, the sort of clockwise direction
but it's going faster than Titan's orbit.

And so you get the - you get basically a wake of material behind Titan.
And the geometry changes as you go around the - as you go around. But -
and the direction towards Saturn on this sort of nominal orientation is
always the same and then the electric field would point in the direction
away from Saturn. So that's a nominal configuration.

So that electric field is important because that is the thing which is
accelerating those pickup ions in the first instance so we get the
pickup ions being produced and accelerated. And the first part of that
gyration is the electric field acceleration and then it gyrates, as I
say, around the magnetic field and you end up with that cycloid path. So
that is the Titan interaction.

As I say, if you - the next slide, slide seven - and I don't think this
comes out very well on the PDF but on the PowerPoint form you can see
what I was talking about with looking at Saturn from above. So on this
picture we have the solar wind coming from the left hand side, solar
radiation also coming from the left hand side. And then Titan, of
course, going around in its orbit in that counter clockwise sense and -
when we're looking from above Saturn.

And, okay, so what about the wakes? Well the solar wake is of course
always in the anti-solar direction so that orange blob coming out of the
various different local times associated with Titan. Titan, by the way,
takes 16 days to go around Saturn so these are different, sort of,
snapshots within that orbit and obviously at different times.

But we have the orange blob going towards the right hand side and that's
the solar wake on the downstream side. So that would be where the
sunlight is absent basically. Whereas it's sunlit on the left hand side,
the upper atmosphere of Titan would be.

Whereas, as I said, the co-rotation of the magnetosphere, the velocity
associated with the rotating - this rapidly rotating magnetosphere like
Jupiter's magnetosphere so this is a sort of giant magneto-disk thing,
which moves around. And so the wake associated with that changes as you
go around in the local time so that's just the angle - we just vectored
Saturn and the sun.

And so we can see, for example, at 6 o'clock local time, which is the
top of the picture there, the wake associated with the co-rotation is
actually opposite the wake associated with the sun. And then that angle
between those two wakes changes as we go around.

As I tried to say on that picture, the electric field in the nominal
case where we have a North/South magnetic field, the electric field
always points away from Saturn. And actually one of the other things
with Titan is that it always has the same physical face towards Saturn
so it's orbit-locked so you always get the same face of Titan, which is
on the Saturn side so that's another interesting point. And that might
be relevant with the negative ions as well.

Okay, so that's the geometry. And so Titan always has the same face to
Saturn and the electric field is always away from Saturn. So slide eight
is the first time Cassini-Huygens actually went very close to Saturn, so
the Titan A encounter, which was back in October 2004, so soon after the
Saturn orbit insertion in July '04. And so this was the first time we
were going to be going really close to Titan.

And so what I'm displaying here is a few days worth of data from the -
in fact it's two days worth of data from the electron spectrometer part
of the CAPS instrument. So the scale on the left hand side is energy
going up that axis, so that's our energy range from less than 1 electron
volt up to almost 32 kilo-electron volts.

Then the time is going along the bottom axis, not very good or legible
scale I'm afraid. But it's basically two days of data. And we're going
from something like 32 Saturn radii on the left hand side of the plot
down to less than 19 Saturn radii on the right hand side of the plot.

And to somebody like me who looks at magnetospheres and that sort of
thing and charged particles, this type of plot says a lot but I'll
probably have to explain it to everybody else. So at the bottom of the
plot - well the color scale, sorry, on the right hand side gives you
number of particles, the number of electrons, which are being detected.

Now the trace at the bottom, that big peak at the bottom there, sort of
less than 10 electron volts so this one in the first couple of divisions
there, that is actually photoelectrons associated with the spacecraft.
So the spacecraft is sunlit and the sun light hits the spacecraft,
there's ultraviolet in the sunlight and that's energetic enough to get
photoelectrons out.

And so you can see those photoelectrons are relatively intense at the
bottom part of the plot there. The traces above that are the real
electrons in Saturn's magnetosphere. And so those are the ones to think
about. And we're going, here, from the solar wind on the left hand side
of the plot, which is that relatively rare, relatively low temperature
environment I was mentioning before, and we're going through some
crossings of the bow shock into the really super heated magnetosheath
type of plasma.

And so you can see, for example, there's a couple of blobs - there's a
blob on the left hand side, which is quite red and then it goes back to
the same sort of thing in the solar wind again, a low temperature and
relatively low intensity type of electron population there.

And then in the magnetosheath again where it says magnetosheath at the
top, there's another crossing of the magnetopause into the magnetosheath
and so we get that significantly different electron population from one
to another. And so that brings us into the magnetosheath so this is kind
of a fly-through - a rapid fly-through of Saturn's magnetosphere.

And then those crossings of the magnetopause, which are over sort of
two-thirds of the way over on the right hand side of the plot. And then
finally it gets into something, which is called Region 1, which is one
of the regions in the magnetosphere, which is a very hot, dense region
but there's lots of fluctuations within it as well.

And so we can see the different type of electrons, which we get in those
different environments. And hidden in all this lot is the first very
interesting Titan encounter. So this is, as I said, the plot is over a
couple of days and we see the Titan encounter on the right hand side;
you may just about be able to see the sort of v-shape and the reduction
in the number of electrons, which we've seen because this color scale
has to do with number of electrons.

So you can see the reduction in that at 20 Saturn radii. And actually
that's the Titan encounter. So that's the sort of large-scale view of
the magnetosphere on that particularly day - or couple of days of data.
And buried within that is the crossing or the Titan encounter. So you
can see that on this particular encounter Titan was, you know, inside
the magnetopause by a - by a significant extent.

And because, you know, it's possible for the magnetopause to flap around
as we were saying before, as the solar wind changes, you know, Titan may
go in and out and conditions may vary. But on this particular day of
encounter we can see that it's definitely solidly in the magnetosphere
region.

But there are still lots of fluctuations, you can see there's
fluctuations of the electrons from large to low energy and this is
something which happens in rotating magnetospheres as well. And - but
some of that is associated with Titan.

So this is giving you the sort of broad plasma interaction view. And
then if we zoom in to the encounter, so the next slide, slide nine, is a
geometry of the encounter - of the Titan encounter. And so if we - the
spacecraft trajectory is plotted in three different - on three different
planes here with Titan as the sort of disc-shaped thing in the middle
there.

So you can see this encounter is taking us above the North Pole of Titan
slightly through the wake and sort of - yeah, slightly to the North and
going Northward because it's starting on the left hand side and moving
over to the right hand side and starting at the diamond on the right
hand plot.

I should also introduce the - what's happening in the plot here: the
plot is done with Titan at the center. So if you can imagine a blob in
the middle of this picture, which is actually Titan, and these are
projections onto different planes of the Titan encounter. And so in the
spacecraft trajectory it's plotted across in black there with the
direction shown.

Now there's also some other directions shown on there, the direction to
Saturn is the red arrow going towards the right and also the arrow at
the top there is towards Saturn. And because of the way the electric
field works the electric field in nominal conditions would be moving
over to the left there on the left hand side. And so we might expect, in
the plasma interaction, an asymmetry associated with that electric field
and we do indeed see that on this encounter.

The other sort of yellow direction there is the direction towards the
sun so you can see we're going from the sunlit side of Titan's
ionosphere into the night side. And so we might expect some affects as
sunlight is attenuated more and more by the dense atmosphere of Titan as
we're flying into that region.

(Amanda): So, Andrew\...

Andrew Coates: Yes.

(Amanda): \...so the dropout, the narrow v dropout that you saw - that
you pointed out on the previous slide\...

Andrew Coates: Yeah, yeah\...

(Amanda): \...is a result of being on the opposite side of the
co-rotating flow or are you saying it's a combination of all of these?

Andrew Coates: Well it's a combination of things. In fact, what it seems
to be is the electron population being basically absorbed in Titan's
atmosphere so we're going low enough in Titan's ionosphere and
atmosphere that we're seeing absorption of those magnetospheric
electrons.

(Amanda): Okay, okay.

Andrew Coates: And so it's somewhere towards the center of the plot,
yeah, where the closest approach is, is where that v-shaped thing is.
Okay, now if we look in great detail at the v-shaped thing, which the
next slide, slide 10, shows and this is a sort of zooming in all the way
now to where we can see some really interesting features. And so the
v-shape now is kind of almost off this plot even but you can see the
electrons coming down on the left hand side reducing in energy as we get
into the ionosphere of Titan.

So the region, which - where it says cold plasma there, this is Titan's
ionosphere, and then we get out into the magnetosphere as well after
about (1540) on the right hand side.

So we can see, on the left hand side, and again on the top there it say
electric field direction over to the left and Saturn direction to the
right. We can see that there is asymmetry in this interaction. So the
left hand side is kind of more, you know, much more gradual a change in
the electrons than it is on the right hand side. And this we think is to
do with the electric field layout of the interaction.

But much more interestingly there's a couple of other things which we
can see so we can see - there's an arrow there, which says ionospheric
photo electrons and there's - the sort of big sort of population, which
is at a few electron volts, between 1 and 10 electron volts. That's the
ionospheric plasma or the sort of tail - high-energy tail of the
ionospheric plasma at Saturn.

But there's another line - where it says ionospheric photoelectrons at
about 20-something electron volts. This is electrons which you get
from - not from the spacecraft now but from the ionosphere of Titan
itself. And so this is an indicator that you're really in the
ionosphere. And you can see that - if you sort of follow that plot from
the left hand side or that line from the left hand side to the right
hand side you can see it dying out after about (1530).

And that is because Titan's atmosphere is attenuating more and more of
the photons coming from the sun and so you get less ionospheric
photoelectrons that all seems to make sense. Okay, the other thing, the
sort of a really interesting thing for this presentation is the vertical
stripe (unintelligible) so you can see the sort of vertical stripes in
the middle of the page leading up to about (1530), which is the closest
approach to Titan on this particular encounter.

There's vertical stripes - but those are actually - have different peaks
in energy. So the energy is going vertically upwards in these plots. So
we have energy increasing as we go up there. And we can see different
blobs at different energies as we go up and the vertical stripes are
shown like that.

Now what is happening at this time is that actuator, which I mentioned,
is actually sweeping the field of view of the electron spectrometer,
which is making these measurements through the RAM direction of the
spacecraft. This is the direction in which the spacecraft is traveling.
So that gives you the sort of relative velocity compared to the
atmosphere and ionosphere of Titan.

And it's precisely, as we go through and (unintelligible) direction,
that is when we see those peaks. And we interpret those peaks as
actually negative ions in Titan's ionosphere. So when we first saw this
we thought well, goodness, what is that? And, you know, we didn't know
what it was. But - and we went through a range of different
possibilities including, you know, something with the instrument or
whatever.

But it seems to be a very persistent feature, which gives us these peaks
in energy. And we can interpret that as negative ions, which we do on
the next few plots. It's actually the next plot, which is slide 11,
shows on the top - it shows the ion beam spectrometer data. This is the
IBS. And, again, we can see, with that, this is measuring positive ions,
by the way, just to remind you.

And, again, we can see a number of vertical stripes but - each of which
has this sort of vertical structure associated with it. And these are
interpreted, in the case of the IBS, as different masses of these ions
in the ionosphere. And taking the potential of the spacecraft into
account because the spacecraft charging, the biggest peak there is seen
at - it's actually seen at mass 28.

And you can use the speed of the spacecraft as we're going through the -
Titan's atmosphere and ionosphere here to - as basically a mass
spectrometer. So I'd just better explain that a little bit better. The
spacecraft is going through at a certain speed and it's about six
kilometers per second in this case.

And so as the spacecraft goes through it - we know what the velocity is.
Now we're measuring the energy so from energy is a half (MB) squared. We
know what the velocity is so we get different masses. And so on the ion
beam spectrometer we can completely understand that; we see these
stripes as we're going through the RAM direction with different peaks
and that, you know is - the spacing is about right to give you sensible
mass separation in the ionosphere.

And this actually corresponds - the IBS data correspond to the envelope
of what is seen if INMS is in the right mode, the ion neutral mass
spectrometer. So if that is measuring ions and sees then much greater
resolution than we can see with IBS, it sees the different peaks at much
higher resolution, you know, even within each of these blobs there's
loads of structure, which Hunter Waite and colleagues, I'm sure, have
spoken about on these types of telecons before.

Okay, so the ions are shown on the top there so that sort of gives the
envelope of the ion distribution. On the bottom panel we can see that at
the same time - or at some of the same times that IBS is measuring those
positive ions we see also the vertical stripes in the ELS with those -
with this peak structure and we think those are negative ions because
they're doing the same sort of thing of a sort of change in energy,
which gives you the different peak energies or change - well, the
velocity is the same, changes the mass, gives you the different energies
and so that's what is being measured by the instrument there.

Man: Quick question?

Andrew Coates: Yeah.

Man: As your rotation rate looks like it's constant at about one RPM?

Andrew Coates: The rotation rate of, yeah, this is - it's about one
degree per second actually of the actuator. You mean the rotation of the
actuator, yeah?

Man: Yes.

Andrew Coates: It's, yes, it's about 1 degree per second. And actually
what the actuator has done in this case is to have a reduced amount of
data which it's taking. And so by looking at the exact orientation of
the spacecraft and exactly where the ram direction is comparing with the
so called SPICE kernels, which are the directional and positional
information of the spacecraft you can tell that this is exactly in the
ram direction.

But, yes, it's about one degree per second is the rotation speed of the
actuator.

Man: Okay.

Andrew Coates: It's sort of moving backwards and forwards as, you know,
through that ram direction.

Man: Oh, it's not rotating completely around, okay.

Andrew Coates: That's correct, yes, it's not rotating completely around
so it's a small - it's a reduced rotation, which is kind of, again, like
a sort of very small windscreen wiper and sort of moving backwards and
forwards to get that ram direction. Because it's the RAM direction in
which we're interested in because that's where, you know, the material
from Titan's ionosphere is being brought into the instrument by the
spacecraft velocity.

Man: So then the changes in the periodicity, is that due to like changes
in the way the spacecraft's pointed?

Andrew Coates: Yeah, so the spacecraft is moving to do different sort of
imaging\...

Man: Okay.

Andrew Coates: \...but if you then, yeah, putting it all together with
the known sort of position of the spacecraft at the exact
characteristics of the way the actuator is moving, etcetera, these are
definitely ram direction, it's definitely the case.

Man: Okay.

Andrew Coates: Okay? So the next slide, and slide 12 shows you the
conversion of energy to negative ion mass and so this is just a simple -
supposed to be simple - but it's just 1/2 mv squared so that's the
kinetic energy. So half times the mass in atomic mass units times the
mass of the proton times the velocity squared equals Q times the energy
in electron volts.

And so from that you can work out and work out what the constants are
and the velocity is always about six kilometers per second so the Titan
encounter is very close. And so the mass in atomic mass units is
equivalent to 5.32 times the energy in electron volts. So you can
interpret an energy spectrum of either these negative ions or the
positive ions - interpret that energy spectrum as a mass spectrum.

But what we're actually measuring with the instrument, because it's an
electrostatic analyzer, is energy per charge and mass per charge. So we
don't get a precise determination of mass; it could be - and so assuming
singly charged ions, which is the first assumption, then this gives you
5.32 times the energy in electron volts.

But if we had multiple charges it might give you larger energies or
larger masses and we'll see that a little bit later. Okay, so the data,
so if we take the data from the virtual stripes, which were on slide 11,
and plot that as a spectrum, so in slide 13, now. So this is a
particular spectrum from the - from TA at this particular time, 15:28:42
and we've got counts being displayed on the axis going up.

And then the energy on the top scale there, that's the energy from the -
effectively the energy from the spectrogram. So it's taking one of those
stripes - the sort of blobby stripes on the color plot and taking a
cross section through it. So if you take a cut through it you get what
is the red line there, which is the total number of counts.

Now what we did, in the analysis of this data, was to take an electron
background away from this so measuring an electron background. Because
the actuator is moving backwards and forwards and in between the times
when we see the negative ions we can see the electron population. So if
we subtract that off that gives you the resultant, which is the counts
due to the negative ions.

So if we take the electron background counts off we can get the signal,
which is just due to negative ions. And so slide 14 is the counts
associated with - the counts associated with the negative ions.

And so because a couple of slides ago we had that conversion between
energy and mass is about 5.3 times the energy. So if you look at the
energy on the top axis there and 10 and 100 electron volts, so if you
multiply that by 5 you get about 50 and, you know, 500 mass units -
atomic mass units on the bottom axis. So that's just using that simple
conversion.

So that then gives you the negative ion spectrum. And seen during one of
those TA blobs, as we call it, the blobs - a very technical term - and
just the blob in the spectrogram there. And so that gives you with a
statistical significance shown at the arrow bars there of what the
negative - the counts associated with the negative ions are.

Okay, so - all right, so this is an interpretation; this is an
assumption that these are negative ions because they're doing the same
sort of thing as the positive ions, we get the different peaks. And so -
but if we just think about this for a minute because we didn't expect
negative ions and so slide 15 is us thinking, well, you know, are these
negative ions - are they really negative ions? Could they be something
else?

So the evidence that they are negative ions is they're firmly in the ram
direction, as I say, the detailed analysis shows that. There are
extremely narrow distributions in both energy and angle. They can't be
electrons because then you would have to have a very strange population
of electrons to give this blobby structure like that, it's most unlikely
to be happen; we have to be non-gyrotropic, that means it's not the same
distribution all the way around the magnetic field and you have to see
that several times. So it's really unlikely that they're electrons.

And we also ruled out if that's - in the instrument because we are
flying low in Saturn's ionosphere - atmosphere here; we're flying down
to, in this case the closest approach I think was 11 and 70 or - just
over 1000 kilometers, something like that.

And so it's relatively low in the atmosphere. But it's - we thought
about, well, could there be some sort of discharge in the instrument?
Could Titan's atmosphere be getting in and causing, you know, some sort
of discharge between the high voltages, which there are in the
instrument. Could there be scattering in the instrument? But we've ruled
all of those out and so we convinced ourselves that that was not the
case and that we're indeed seeing ions.

And so, you know, this was a real surprise. So it was seen in this first
close flyby of Cassini and they were seen basically in mass groups - if
you look at those vertical peaks and where they fall - mass groups of
about 10-30 - it's not very good resolution because of course it wasn't
designed to look for negative ions, this electron sensor. And so if one
went back, you know, you'd want to do it with better resolution of
course and look at the real detailed chemistry, which is going on.

But we see with our broad resolution, we see mass groups of 10-30,
30-50, 50-80, etcetera, etcetera and even over 500. So you can see that
in that first spectrum on Slide 14. But Slide 15 is summarizing the
(unintelligible).

Okay so Slide 16, then, we originally saw this - this data on TA in 2004
and on the left hand side there's the spectrogram, which we've just seen
and had a look at the spectra within it. We can see those peaks and
there's also the photoelectrons but we first saw that in 2004.

And then we thought, well, you know, we basically - while we thought the
data was interesting we were busy with other aspects of Titan\'s
magnetosphere and we didn't go back to it. But then all of a sudden we
started getting a huge range of encounters when this was happening. So
the first time this was really seen in anger again was T16.

And this turns out to be the most interesting encounter where we're
seeing these because, again, these are all on the same sort of time
scale these spectrograms on page 16; they're the same type of things. So
again it's energy up the left axis, time along the X axis, it's the
upper plot. And so we have just over half an hour of data on each of
these plots.

And, again, it's as the actuator is going through the ram direction on
the closest approach. So at T16, 17, 18 and 19, you know, this is just a
comparison, we see the same sort of signal not always quite at the same
energies because the spacecraft - the potential of the spacecraft can
change. And so we have to take that into account when we're analyzing
these.

And this is because the density in Titan's ionosphere can change as
well. And so that means that the flux of particles from Titan's
ionosphere to the spacecraft can change so that changes the whole
charging scenario. But we're left with signals, which are there clearly
on all those encounters. And so those were confirmed in those more
recent than TA low altitude encounters.

So we went on and continued to analyze this. And so the next slide,
slide 17, which for some reason the number doesn't appear on that, and
that shows the data from T16. So that, in the previous slide, and in
slide 16, that's the one on the top left of those - of that string of
four spectrograms there.

And we can see that the energies on that plot are going up to maybe 2000
electron volts. And so that corresponds - and that's right in the center
there. So with this energy conversion - that corresponds to 10,000 amu,
so really, really heavy species of negative ions actually being seen in
Titan's ionosphere.

So we can analyze this in the same way that we did with TA so that the
top of the plot here, the top panel of the plot shows the number of
counts per bin on the left hand axis and energy per charge and the
conversion to mass per charge shown on the top and bottom axis
respectively.

And, again, just showing the complete signal, there's the background
signal there, which is the lower plots. If we subtract the background
signal off we're left with basically the bottom plot.

Now to get the bottom plot we run this through the instrument and
efficiencies and conversion factors make assumptions about what the
electron - or what the efficiency of the micro channel plate detector is
and the micro channel plate detector is the heart of this instrument,
which is the detector behind the - it's an annular shaped detector just
behind those concentric metal plates. And so this is the thing which is
giving us - telling us how many electrons or negatively charges species
are there.

And so we didn't expect this to be measuring ions so we've made some
assumptions about what the efficiency is for that detector with ions
because we've determined that, in measurements in the laboratory and
both here at Mullard Space Science Lab and also in the US at Southwest,
we got the, you know, we understand the efficiency for electrons but we
don't know it so well for negative ions.

But if we make some assumptions - reasonable assumptions based on how
electrons - how much - channel plate behaved with ions we can then
determine the density. So if we do that we get density, which is plotted
on that bottom panel there as number of particles per centimeter cubed
and per bin. And so we get these peaks again, the peak type of structure
on the low mass end, on the left hand side below 100 masses per charge
give you those peaks.

But it's a much broader structure on the right hand side with some
structure within it. But this apparently is a whole conglomeration of
negative ions going up to 10,000 amu/Q.

So we can see, you know this - these are really huge. I mean, this is
compared to - if we go back for a moment to slide 16, where we we're
getting energies of maybe 60 electron volts or something on TA on the
left hand side, which would correspond to about a mass of, you know, a
few hundred, 300 or something atomic mass units but here we've got
10,000 so it's well beyond the chemistry of the models which exist so
far of the Titan's atmosphere, you know, and not only that but these
things are negative ions.

Okay so the - click on that again if you've got the PowerPoint just
shows you, again, the total at the top and then the negative ion
signature and then the electron background at the bottom.

Okay, so then we thought, well we're assuming at the moment that - we're
measuring energy per charge, which corresponds to mass per charge of
these things. But what might be the (Q), what might be the charge - be,
so charge - slide 18 is going into that a little bit.

So this is - the electron spectrometer measures energy per charge so
what is the charge? And if we make a reasonable assumption about the -
about the size of a 10,000 atomic mass unit ion, okay, and if we make a
reasonable guess about the density of that, that shows that these things
must be the size of aerosol, so these are really quite big, you know,
10-30 nanometers depending on what density that you actually suggest or
assume for that.

Okay, so we get a size. And then if we look at the local plasma
conditions, so we're in the ionosphere so the electron temperature,
something like (0.1) electron volt, which corresponds to 1000 Kelvin;
this is reasonable from, for example, from the Langmuir probe, from the
RPWS experiment. And also the electron density, which is about 1000 per
cubic centimeter and that corresponds reasonably with, again, with the
Langmuir probe but also with other determinations of the density deep in
Titan's ionosphere there.

And that gives you, in the plasma sense, the Debye length, which some of
you might have heard of, some of you not, but this has to do with the
shielding of a particle in plasma and so the Debye lengths are something
like seven centimeters.

And so you can use that, you can use the plasma conditions and these
things floating around that - tiny little almost spacecraft. I mentioned
we get spacecraft charging on Cassini and all other spacecraft, which
are in a plasma environment. But these little aerosol-sized things are
also behaving like tiny spacecraft in themselves. And so they're bathed
in the plasma and so that means that they can actually assume a
potential.

And if we use some theory to predict what that potential might be with
this size of grain and with that temperature of plasma you get something
like a quarter of a volt, which doesn't sound very impressive but. And
then if you work out the charge on that just from basic plasma-(cific)
type of theory this corresponds to about five electrons.

And so if the charge on each of these things - so this is a bit of a
long calculation - but if you work out five - of the charge on these
ions is something like five electrons then that - we've determined our
(Q) then. So we've got five electrons and so that means that the 10,000
that we measure could actually be 50,000 atomic mass units, so a huge
mass.

And so, you know, we're getting really, really very complex species,
which are being formed here. But of course we can't determine that
charge so this is a number of assumptions and it's theoretical and
stuff. So 10,000 is what we measure for sure but if we went back there
with another instrument we'd like to measure the charge space so we
could actually determine this and see what the actual mass of these
really high mass negative ions is.

Okay, but if we take it with the - with the singly charged and then look
at the density of the function of height we can see the next slide,
which is - there's not a number on it but it's after 18, so it must be
slide 19. And if we calculate that density in the way that I just
mentioned and I should mention - I should have mentioned the
collaborators at the beginning - they were on the first slide.

But Ed Sittler, one of our collaborators pointed out we should be using
the ram assumption to calculate the density. And so in the paper that we
wrote up in Geophysical Research Letters we used that assumption. So if
we split that out with mass these two plots are just different -
altitude plots between 900 and 1150 kilometers for that T16 encounter.
And the top panel of that is to the light ions and the bottom panel for
the heavy ions.

And so the largest amount of density there is in the very high mass ions
at the bottom panel there. But each of them is showing a reasonable
thought of structure, you know, with an ionospheric peak, which is
something like we get with the other instruments, which can measure
total density. And so it's another species in the ionosphere by the look
of it from the profiles of those densities. And so assuming that singly
charged those are the masses which we would get from that.

Okay, the next slide, slide 20, shows us the possible negative ions,
which the different mass species or mass groups would correspond to. So
as I say they're not very well determined because of the finer energy of
the width of the instrument; I mentioned that's about 17%. And they're
also, presumably, this - if it's complex chemistry which is building
these up, there's presumably different peaks, which are actually
corresponding to - or it's actually contributing to these larger peaks
and so you get a sort of convolution between the instrument response
function and what is really in the ionosphere there.

And so the determination is not accurate. But the type of species, which
we've suggested in - we had a paper last November in Geophysical
Research Letters and Hunter Waite's also had a paper earlier in the
year, in May, which these observations were mentioned in as well. And so
between us we came up with the identifications of these ions.

And so we have the possible negative ions on the right hand side there.
So the low mass ones, we think that's oxygen - the one on the right hand
side O- is unlikely. That is the candidate in Earth's ionosphere where
this kind of thing happens as well as I've mentioned. But things like
CN- or NH2-. If you look at the electron's affinity, which is the sort
of likeliness of producing negative ions, if you've got enough electrons
around to do that, then these are all reasonable electron affinities and
something where the electrons in the ionosphere might actually cause
that.

And now this is not a detailed chemical model; there are people making
detailed chemical models to explain these results at the moment because
as I've said before, we had not expected negative ions at all in Titan's
atmosphere but here they turn up.

Okay, so the mass 30-50 there we have obviously most complex species and
then getting even more complex between 50 and 80 there. And by the time
we get up to these 80 plus, so - and up to 10,000 we have really complex
species, which could be doing this.

And again we looked at the electron affinities for all of these. So
there's (polyimes), there's high order nitriles, there's PAHs, which are
polycyclic aromatic hydrocarbons and, you know, that is an interesting
issue in itself because those things have been seen in the interstellar
medium and in other contexts.

And there's also cyano-aromatics, those complex sort of ring-type
structures. And so by the time you're getting up to those high masses
goodness knows what these species are. And so if the interpretation is
right then they're negative ions though some of those could be really
interesting in terms of what might be happening lower down and it
relates back to the Miller-Urey experiment as we'll talk about in a
moment.

Does somebody want to ask a question or - you okay? All right, so
carrying on to slide 21, so this is from our paper where we - where we
published this last year. And so basically this is a summary of the
encounters up to that time when this paper was accepted that we'd
actually seen these negative ion species.

So going all that time from TA, which was - is that bottom - the top
line there, sorry - all the way down to T36. And so basically looking at
these - the different columns there, of course there's the different
dates, different altitudes, different local times and so on, slightly
different local - relative velocities and different latitudes - they're
starting to build up a picture of where these things are happening.

And the column in the right hand side is Cassini in Titan's shadow -
it's at night. And so there's a few of these encounters where we see
these complex ions where indeed it is at night but most of them are not
at night, most of them are during the day.

And so the types of things that could be contributing to forming these
ions might be things like sunlight or electron attachment from the
ionospheric electrons depending on what species it is and so on and so.
By building up a picture of the different conditions in which we get
these - these ions we can build up a picture of the processes which
might be forming them.

And so this is sort of collecting together the observations. And in this
paper we were not looking at trends particularly just sort of
documenting. So this is quite a few of the encounters up to T36, which
we actually saw them but there was some where we didn't. So basically
it's wherever the altitude of the Titan encounter is low enough that we
see the - we do see these ions. So we need to be going low in Titan's
ionosphere in order to be seeing them.

And some of the other features have to be right as well but we'll get to
that a little bit later. Okay, so Slide 22, again, is a rather boring
list of more encounters of where we see them. And so after that GRL
paper we're of course still looking at negative ions and we've got a
research student here and will be looking at these in detail.

And some of the more recent encounters where we've looked at - where
we've looked at these ions are T39 to 43 and those are tabulated here as
well. So again we can build up from this a list really or a sort of data
set of the conditions in which we're seeing the negative ions.

(Amanda): So, Andrew, so the Titan flybys that aren't listed here\...

Andrew Coates: Yeah.

(Amanda): \...are ones that the altitude was too high and\...

Andrew Coates: Yeah.

(Amanda): \...therefore you didn't see negative ions?

Andrew Coates: That's correct, yes. So, yes, if the altitude is too high
it seems that we don't see them so we're not going deep enough into
Titan's ionosphere to do that. But, yeah, what we're doing sort of with
the next paper, which we're working on at the moment is to actually look
at - and we presented this at the Titan book symposium back in July - is
to look at the conditions in which we see these ions and to try and sort
of build up a picture.

And certainly one of the definite pictures that we get is that if the
spacecraft flies past Titan too high we just don't see them because
we're not going into the ionosphere. It's when we're going deep enough
into the ionosphere that we see them.

(Amanda): Okay.

Andrew Coates: Okay, the next picture - number 23 is a summary of a few
more - well a few of those encounters, which we just talked about. So
again here's those telltale vertical stripes, which are the negative ion
things. So here they keep cropping up at these different encounters. And
again there's the energy structure, the sort of energy peak structure,
which we see, which we interpret as masses and that's seen in all those
encounters.

And one thing you can see from this slide is the energy, which of the
maximum - the closest approach is basically in the middle of these most
usually, in fact, you know, it's not necessarily exactly in the middle
of these signatures because that depends on whether the spacecraft was
(skewed) around so there was things slightly away from the ram direction
or not.

But the highest mass that we see on these - if we look at that that
actually changes quite significantly. So if you look at - in slide 23,
T42, which is at the bottom right hand side, we have the maximum energy
there of about 100 electron volts, which corresponds to about 500 amu,
whereas the top left one is getting up to almost a kilovolt, maybe 800
eV or something, which would correspond to about 4000 amu/Q.

So you can see that the maximum energy of these negative ions changes
depending on which encounter that you're on. And so you can start
studying that with the different parameters in those tables we can start
studying that and that's in a couple of slides time.

The next slide, slide 24, again, is that summary slide maybe comes out
better on the PDF of where these wakes are, you know, the solar wake
anti-sunward and the co-rotation wake to do with the rotation of the
magnetosphere of Saturn, that green one in this case. So you can see the
geometry changes as you go around there.

And the next click on the PowerPoint, I don't think it probably comes up
any way on the PDF, is the encounters where the negative ions were seen.
So we had a group of - or well, we first saw them on TA, which is
towards the sunlit side of Saturn's magnetosphere so the - you know,
near to the magnetopause there.

And then we had another group, T16 to 23, which was more in the tail
there. And another group T25 to 32 at a particular set of local times
and 36 to 42. Now the mission so far and the locations where the Titan
closest approach is does not include all local times around Saturn yet
so this is a subset. But this is just showing the types of local times
where we do see negative ions.

We didn't see them on T5, that's because the T5 encounter, which is at
the top there, actually happened to be on sort of through the darkened
side and so we didn't - and it also didn't go quite as close - and so we
didn't see the negative ions on that one. But we do see them on, you
know, at a wide range of local times around Saturn.

But as the mission goes on and during the Solstice mission and hopefully
all the Equinox mission and hopefully the Solstice missions we will
see - we will fill this in and see whether we see them at all local
times or not.

Okay so then we go into using these numbers and trying to understand
what they're telling us in terms of what the - where the highest masses
are actually seen. And so if we take that maximum energy on which we see
the negative ions and look at those with a few different parameters.

So, for example, the closest approach altitude is one, and so slide 25 -
it's the first of these types of slides. So if you look at this closest
approach altitude of these encounters the ones which are getting lower
and down to 950 there - the ones which are getting lower seem to be
where we see the maximum ion mass being highest.

And so that is telling us that there may be some process which is
causing these things to coalesce and get bigger and bigger as we're
going down into the atmosphere or - or into the ionosphere - deeper into
the ionosphere of Titan so as we're getting down to 950 kilometers. That
highest one that we're seeing, T16, it's shown on the plot there, that
outline plot on the - point on the right hand side there, so that trend,
although the statistics aren't great - there is a trend, which seems to
be that at lower altitudes we're seeing higher mass.

And so that, you know, perhaps is telling us something about the
processes of formation. So these things are perhaps coalescing and maybe
descending through the atmosphere and there's more chemistry going on to
create bigger and bigger ions. That's a possibility.

Okay, and the next one of this type, plot 26, is on the vertical axis
now we have latitude, so this is latitude in Titan's terms; and so
Titan's equator would be at 0 degrees there and up to the pole at 90
degrees. So with the fantastic range of flybys, which Cassini has given
us, we've got a good range of latitudes and so quite a few in the
Northern hemisphere but some of them in the Southern hemisphere as well.

And there seems to be a trend that at higher latitudes, again, we're
seeing heavier and heavier ions because that heavy - that heaviest one
we saw, T16, was where we had a very high Northerly latitude of the
encounter. And similarly in the South, again, there's a trend, there's a
few points which don't agree with it but there's a trend which seems to
be that as we go lower or to more negative latitudes on the Southern
side, and again, we're getting heavy ions.

And so if that's right, if that trend is right, it may be telling us
that the ions are getting larger if the local part of the ionosphere is
less sunlit. So if there's less energy going in at that point from the
sun then this is something which may be playing an important role but
we'll summarize all that in a minute.

And then slide 27 is something called solar zenith angle. And so this is
the angle - if you sort of draw a line between the center of Titan and
the position where the spacecraft is and call that line the local
vertical and where is the sun compared to that. So it's the sort of
sunshine angle if you like or something like that.

So 90 - well 0 degrees would be the dayside, so noon is completely above
your head. If you're on the terminator of Titan, so on the side of it
then the angle between the sort of local normal - well that line between
the middle of Titan and the spacecraft is at 90 degrees, the sunlight
line, so the solar zenith angle is 90 degrees there. And on the complete
night side you'd be 180 degrees.

And what this plot seems to tell us is the highest masses, which we're
seeing, are actually near the terminator, near - somewhere near 90
degrees perhaps. Again, there's not enough data really to draw very good
conclusions from it but it seems to be, you know, not unreasonable or at
least the couple of points where we've seen really high masses seem to
be in the sort of 90-120 degree type of region.

And so, again, that is consistent with the idea that these ions perhaps
are larger if the local ionosphere is less sunlit. So there's less
sunlight, which means perhaps that there's less sunlight to break up
these large negative ions. So the next slide, slide 28, is sort of
putting together the results, really, from Hunter's paper in '07 and
ours later in '07.

And Hunter had this beautiful sort of summary slide in the middle in his
paper or summary of what is happening. And so sunlight and energetic
particles are interacting with Titan's, you know, unique and very
interesting atmosphere; all sorts of different processes are going on,
which are causing benzine in the INMS, which he may have talked about
before and other complex organics, which are seen in the positive ion
sensors.

But the INMS is limited in what it can measure on the spacecraft, it's
up to 100 Daltons or atomic mass units. There are - they are actually
with the IBF, seen the positive ions up to about 350 Daltons so far. And
that's - the plot on the bottom left shows that. The plot on the top
left shows the neutrals with the exquisite resolution, which the INMS
has.

And so the neutrals and the ions are showing that - there are complex
species there. And then we have these negative ions, 20-1000 atomic mass
units, so 1000 from our paper and then the example, which we had in
Hunter's paper was up to 8000 atomic mass units.

So the idea here is that we have sunlight and energetic particles acting
on the atmosphere having all these processes. And we have these
negative - large negative organic ions forming. And then these are
perhaps floating down through the atmosphere and become the tholins in
Titan's haze. And then - and this is speculative because we don't know
that this happening for sure and then, you know, maybe even interacting
with the surface.

But there are things which are seen, which by using occultation
measurements the ultraviolet instrument UVIS, has actually seen a
signal, which it associates with tholins, which has a non-zero value at
about 950 kilometers as well.

And so it could be, you know, we're measuring in situ what UVIS is
seeing from the occultation measurements. And this is indeed a
population which is then populating the haze, which is lower down in
Titan's atmosphere and then, you know, even interacting with the
surface. But, you know, this - a little bit of speculation in there.

Okay, slide 29, so there's recent relevant work and I mentioned the UVIS
stellar occultation measurements and so these are supportive of these
heavy organics up to the 1000 kilometer level, which is where the lowest
altitude we can measure them with Cassini with the electron
spectrometer, the CAPS instrument.

And - but there is also additional supporting evidence because there are
discrepancies between some of the methods of measuring density from the
Langmuir probe and from the INMS. And it could be that these negative
ions are actually what, you know, is the missing link, which actually
allows you to cross-calibrate properly between the RPWS and the INMS. So
it's consistent with that idea so that could be - that our negative ion
density is, you know, it's maybe 1% or something of the main density of
the plasma there. But it could be enough to explain the discrepancies,
which are seen.

There's also an interesting theoretical approach which is going on,
which is perhaps that heavy positive ions from the lower altitudes might
be levitated by electric field structures, by ambi-polar electric fields
in this region and (Tunescon Gombozia) has suggested that.

And then the heavy species could then perhaps acquire a negative charge
and then - are accelerated downwards. And so this is trying to explain,
you know, how does all this fit in with the overall structure. So if
that happened that would encourage these large negative ions to be going
downwards into the atmosphere.

Okay, so how does all this link? I mentioned in the title of this thing,
tholins and so tholins - this was a term which was coined by Sagan and
(Karen) in '79 - this is Carl Sagan. And, again, so this shows the
Miller-Urey experiment on the right hand side here. So tholins are
basically products from energetic processing of mixtures of gasses
including methane and nitrogen, which we get in Titan's atmosphere and
water.

And so at this time these guys were trying to simulate in the lab what
may happen in the atmospheres if you add basically some sort of energy,
which in their case was a spark as you can on the picture there. In our
case, you know, perhaps it's the hot magnetosphere of Saturn, which is -
and possibly sunlight also, which is doing that. And so there's
energetic particles coming in at the top of the atmosphere and the
sunlight.

And so tholin, the word, as they coined it, is the Greek for muddy and
so these are brownish and sticky residues, which perhaps formed by the
extensions of the Miller-Urey experiment, which was done originally in
'53 to simulate the Earth's early atmosphere. And so there's the link
with Titan.

And, you know, something which could be happening here is we are
actually seeing this process in action with these huge negative ions
that we see and the large positive ions as well. And so this gives us
something really exciting, actually, for future missions to be able to
look at.

And so the source of energy may be electrical discharges or may be
ultraviolet radiation. Then the last bullet there on slide 30 is Sagan
and others tried to simulate the atmosphere of the planets and moons so
different mixes of these in order to look at what was happening. So
maybe we're seeing, you know, this process in action in a planetary
ionosphere/atmosphere system at Titan for the first time.

Okay now slide 31 was one which we presented at the Saturn Book
Symposium in London. And so this is actually from an Enceladus encounter
so it's switching moon and switching gear. In fact the flyby speed here
is something like 14 - over 14 kilometers per second so it's much
higher. And so that means that the - if there are negative ions here
then there's a different conversion between the energy and the mass.

And so we think we basically see them at Enceladus as well. And so I
won't go into the detail of this but, as I say, we did present it in the
London meeting of the Book Symposium so - actually we've got a
publication out on this as well. But this shows the energy on the left
hand axis and we see vertical sort of peak structure at the type of time
when we're passing Enceladus. So this is between 10 electron volts and
100 electron volts. There's also sort of other interesting structure as
well, which other people are looking at and I won't go into.

But the stuff at high energies could be ice grains and so that's that
picture there on the right hand side, these could be ice grains. But the
stuff in the middle there could be negative ions. And these are - look
like 16-500 amu. Again we have separate peaks, some of this is due to
water but there may be other stuff in there as well and so keep your eye
out for future publications on that because we think it could be pretty
exciting and something to compare, again, with the positive ion
instruments and what they're seeing.

Okay, so the last slide, slide 32 there is the conclusions. And so we've
seen, you know, from the 16 encounters, which we had with the earlier
paper plus six more recent encounters, 22 of the Titan encounters in the
Cassini's prime mission. So these are times when the altitude is low
enough - and I forgot to mention when somebody asked earlier about the
ram direction about the altitude. It's not just that, actually CAP has
to be able to see in the ram direction; the pointing has to be right so
that we actually have to see in the ram direction.

Whenever we have those - both those conditions fulfilled it seems that
we actually see the signal, which is consistent with negative ions. The
lower mass groups and the, you know, below 100 amu are similar at all
encounters with the except - you know, once one has done the correction
for spacecraft potential, which may be different on the different
encounters as I said.

And - but, you know, some of them stand out. There's some extremely high
masses, a few thousand amu, up to 10,000 on T16 and we talked about
that. And there was another encounter at other - at a Southerly latitude
where we saw that significantly high mass as well.

We feel these are very significant for the upper atmosphere chemistry of
Titan. And we find, in this sort of systematic study that we did of this
with only 22 encounters, we find that the higher mass negative ions are
preferentially seen at low altitudes and so we see the highest mass ions
at 950 kilometers and this is Cassini's lowest altitude so far. You
know, maybe if we go even lower we might see higher masses.

As I said also, depending on the charge assumption, which one makes, we
might actually be seeing higher masses already. But the affect on
altitude is apparently to increase the maximum mass that we see. Also at
high Titan latitudes and in the region of the terminator, so both of
those are telling us that in the absence of sunlight, so where
photo-dissociation is less, then these things can build up and this may
be what's happening.

And so from the admittedly not huge number, but still 22 encounters,
from the trends which seem to emerge from that, that give us some idea
then of what the - both the sort of formation process and the
destruction process is. Because the formation - it seems that at lower
altitudes where the density is higher that's what we need in order to be
making these things.

And then to be - and of course the number that we see is a balance
between production and loss. So the production is associated with the
low altitude. And then the loss is associated with - perhaps with, you
know, the most likely thing is the destruction by sunlight. And so the
destruction by sunlight is consistent with those last two sub-bullets
there.

So what we could be seeing - and why we're really excited about it, is
this could be the early stage of tholin formation. And this, as I said,
is consistent with those UVIS measurements of the stellar occultations,
which showed us the density of tholins as a function of height and forms
a link back to Sagan and colleagues and the Miller-Urey experiment and
so on.

So it really sort of links the plasma interaction directly with
processes going on in the atmosphere like that. But also (parts) with
the surface because this stuff once it's coagulated into haze and maybe
is dropping down through the atmosphere, forming the haze layers,
eventually may interact with the surface and this is something which the
dunes perhaps on Titan perhaps they have their origin in something like
this process going overhead.

So the two papers published about this so far, Hunter Waite back in May
'07 had a paper in Science, which showed some of this data and also that
beautiful schematic. And then we had a study in GRL in November last
year about this process (at time). So we're seeing this with the CAPS
electron spectrometer. And the colleagues that worked with me on this
are on the beginning of - beginning of the presentation, as I say, I
should have added (unintelligible).

And the bottom bullet there, maybe we're also seeing negative ions and
tholins as well and so this, again, is quite an exciting result, which
we presented at the meetings just earlier this year and again we're -
these are the sort of things which will be published eventually.

So with that I'll stop having been going on about one hour forty minutes
or something and see if there's any further questions?

(Amanda): Thank you so much, so it's a really interesting talk. Does
anybody out there have a question?

(Amanda): I was thinking of a couple. Let's see, let's go to Enceladus
and Europa.

Andrew Coates: Yeah.

(Amanda): And what were - we don't really think that they're tholins -
it isn't really dark. Well I guess at Europa there's dark on the surface
but what do you think the implications are then for potentially\...

Andrew Coates: Well, yeah.

(Amanda): \...negative ions?

Andrew Coates: Yeah, I mean, when I say it's similar it's not similar to
the extent of having these really heavy things, okay? So we don't - so
Titan is where we're having the really heavy stuff going on.

(Amanda): Right.

Andrew Coates: But then they're nowhere near as heavy at - with the
evidence that we got so far from Enceladus so these are - something up
to - so maybe 500 amu. But they're not - and if they're water clusters,
I mean, the analysis which we presented at the London meeting showed
that it seems that they could be water clusters but there may be some
other stuff in there as well.

But whether this has to do with, you know, it's unlikely that this is
really and particularly the water stuff is nothing really to do with
dark material and that it's nowhere near as heavy as the material which
we're seeing at Titan.

So while it, you know, may be producing different substances, etcetera,
it's interesting but, you know, they're probably water clusters really
mainly but we need to do some more analysis on that I think to be sure.

At Europa, of course there hasn't been really in situ measurements,
which have shown the existence of negative ions apart from inferring
that there may be there from waves seen in the magnetometer. And so
those - there was a paper a while ago by a magnetometer team for work
(unintelligible) I think and that showed that there could be chlorine,
negative chlorine ions. And so this is to do perhaps, you know, even
associated with a subsurface ocean of Europa, the stuff, which has sort
of sputtered off the surface and then, you know, interacts with the
local plasma environment presumably quite high density.

And the electron affinities are good enough and the density of the
material is good enough that you can get chlorine there. So these - so
there isn't an observation of very heavy negative ions there but there's
a possibility of maybe light ions, which are sort of elemental species,
which may be there anyway formed by the processes and maybe stuff from
the subsurface ocean coming out. And so this could be - it's not proof
of a salty subsurface ocean but it's a possibility.

But what we need is direct measurements to see if we've got negative
ions there as well\...

(Amanda): Thanks.

Andrew Coates: Yeah, and so going back to Europa, you know, this would
be something which would be exciting. And in addition, going back to
Titan where, you know, we just had this charge thing, which is just
completely undetermined and we're making assumptions, we'd really like
to measure the charge of those ions and that would tell us actually how
heavy they are so it'd give us more information for the chemistry and
the sort of chemists, which are working on this.

(Amanda): So you would carry a different instrument or what would be
the\...

Andrew Coates: A modified, yeah, a modified instrument, which would
allow you in this case to look for charged - well, at Titan we'd like to
be able to determine the charge on these particles in addition to just
measuring the energy so you need to do something similar but you also
need to measure perhaps total energy as well. And so that would give you
the charge also.

At Europa, you know, you'd just like to have, you know, this type of
instrument but we suspect that negative ions are there so it would be
worth again, you know, trying to actually measure those in detail and
trying to look at negative ions as well. So in these different solar
system environments we've, you know, the models may say - may be
positive ions but it seems like negative ions play a really important
role as well and our minds shouldn't, you know, should certainly be open
to that for future missions and include that type of instrumentation on
there.

(Amanda): Right. So - is INMS on Cassini\...

Andrew Coates: Yeah.

(Amanda): \...does not - is not able to measure these negative ions?

Andrew Coates: INMS can measure positive ions\...

(Amanda): Okay.

Andrew Coates: Yeah, so that - and, yes, it cannot measure the negative
ions. And so again, you know, if one's going back to either of those
targets you'd like to be able to measure with good match resolution, so
with an INMS type technique, but you'd like to be able to measure
negative ions as well. And so this is something for the possible outer
planets future missions that, again, is being suggested.

So there's a few things about this, which, you know, really open up the
possibilities and, you know, at Titan it could be particularly
interesting because of course you have these things being apparently
manufactured in what turns out to be almost, you know, (in press terms)
sort of chemical factory of organics and nitriles in the upper
atmosphere of Titan. And then these are falling down through the
atmosphere and may be interacting with the surface. You'd like to
measure those all the way down, you know, from the ionosphere down
through the atmosphere and then the interaction with the surface to see
if the sort of dark dune-like material is actually to do with this
because that would be extremely exciting.

So following that with a combination of techniques, you know, maybe with
balloons and landers and this sort of things could be something, which
one would like to do in the future.

(Amanda): So did you say that the, you know, the sort of is manufactured
in the atmosphere and it falls down but I thought you said that it's
actually accelerated down to the surface?

Andrew Coates: That's a possibility, yeah\...

(Amanda): Okay.

Andrew Coates: \...I mean, you know, some of the electrodynamics say
that yes they may be accelerated down. Now if it is accelerated down
there's a possibility that, you know, because of the way the electric
fields are setup and so on it may be that you get a different sort of
flux of this type of material on one side of Titan compared to the other
so you may get a different, you know, different types of material coming
down through the atmosphere on one side than the other because of the
predominantly electric field in the, you know, in the region away from
Titan.

Now whether those fields penetrate far enough in order to make that
asymmetry real, you know, that's something, which, again, would need to
be looked at\...

((Crosstalk))

(Amanda): That would be an asymmetry with respect to Saturn?

Andrew Coates: Yeah because\...

(Amanda): Okay.

Andrew Coates: \...that electric field points outwards from Saturn,
that's the electric field to do with - to do with the motion of Titan
through the magnetosphere of Saturn, the relative motion there so the
(-V+B) electric field points outwards on average, I mean, it wiggles
around because the, you know, the velocity of the magnetic fields are
not always in that direction, in fact, sometimes they're quite different
as we discovered on some of the encounters and, you know, the mag team
has done a lot of work on that.

But on average, you know, it could be that it means that one phase of
time rather than the other has a preponderance of this stuff coming down
from the high atmosphere\...

(Amanda): Oh, interesting.

Andrew Coates: \...something for future missions or perhaps even for
Cassini to look for.

(Amanda): And what do you think would happen to these negative ions as
they fall down or are accelerated down to the surface in order to make
the gunk, the tholins, on the surface? So to go from the charged
particles to the gunk?

Andrew Coates: Yes, well there must be - there must be - well, it must
lose its charge, I guess, and then sort of - the whole relationship
between these 10,000 amu ions, at least, and then the haze - I don't
think we really know because we haven't been able to experimentally, you
know, work out how they - the chemists are only just working out how we
can make light - light negative ions; they haven't worked out yet for
sure although, you know, there's evidence from the Miller-Urey
experiment and other things that you can do it with the right sort of
energy and so on - making the negative ions.

But the relationship between those negative ions and the haze low in the
atmosphere and the surface, I think that's a real unknown and something
which is definitely for future missions to look at in terms of following
the process all the way through.

(Amanda): Right.

Andrew Coates: So that could be really exciting. So I wouldn't, you
know, I haven't really got the background to be able to guess what the
chemistry might be but I guess you must be losing the charge from the
negative ion\...

(Amanda): Right.

Andrew Coates: \...and, you know, in a glomeration of further particles
because I guess also, you know, the mass increases apparently as we go
down in the ionosphere and so maybe it continues to do that; maybe it,
you know, plateaus out at a certain level. But it's producing the gunky
stuff and maybe - the stuff in the haze and then maybe it goes to the
surface. But I think we have to do more measurements to really find out
the details of how that actually happens.

(Amanda): All right, that's really interesting though. Any other
questions out there? Okay, well maybe we'll wrap up now then\...

Woman: Well I\...

(Amanda): Okay.

Andrew Coates: Yes.

((Crosstalk))

(Amanda): Okay, sorry. I'm sorry.

Woman: As they go down they're losing their valences, could they be
combining with the positive ions and creating a more complex molecule?

Andrew Coates: Well they're certainly - yeah, there's a whole range of
probably phsysical and chemical processes, which can go on as the
density increases because of course collisions become more likely, yes.
So things like conglomerations or reactions, which are happening
together, the chemistry probably gets more and more important as you go
down in altitutde.

But, you know, funnily enough, I mean, this region - not a lot is known
about it and, you know, some people call it the, I mean, even in the
Earth's environment because it's difficult to get spacecraft into that
type of region.

But certainly Saturn and, indeed at Titan, there's been no spacecraft
which has made measurements because Huygens started making its
measurements sort of 450 kilometers above the surface. So there's a real
gap between 950 and 450. So some people call that these (norosphere) and
some people call it the (agnostisphere), you know, you can believe what
you like in terms of what's going on there because we can't constrain
the physical processes.

So, I think, you know, the future missions in fact - there's the Titan
and Saturn system mission, which is being proposed as one of the outer
planets candidates and Europa/Jupiter is the other outer planets
candidate; they're both really exciting.

But TSSM would be trying to understand that (agnostisphere) that just -
we don't know what's happening and it would be great to know because
we're seeing, you know, it's really tantalizing seeing these huge
negative ions at the top of the atmosphere knowing that you've got the
black stuff on the surface and, you know, what happened in between is
kind of, you know, we just don't know and we'd like to get back and find
out.

Woman: Could this be in any way maybe how the Earth was?

Andrew Coates: Yeah, in the Earth's early atmosphere, you know, people
talk about the Earth's early atmosphere being similar to - being similar
to Titan's atmosphere. And certainly the existence of hydrocarbons there
is important.

Of course the surface temperature is way different compared to Earth's
surface temperature because of course Titan is so much further away from
the sun. But the fact that there is an atmosphere - and a significant
atmosphere with things like nitrogen and methane in it, this is why, you
know, Cassini-Huygens for a long time has been, you know, from proposal
stage through to actually going there and measuring, this has been one
of the exciting things that actually, yes, this is a mimic of the
Earth's early atmosphere and it could be telling us something about what
our Earth did early on.

And so, you know, of course the words life and astrobiology and things
like that spring to mind. But Titan, you know, the surface is certainly
too cold for, you know, one would think for life to be there. I know
some people have theorized about that. But so we could be seeing, yes,
one of the building blocks of much more - of complex molecules; we're
already seeing that - building blocks up to 10,000 amu so this is quite
complex stuff.

So I say, the (polyines) and PAHs - polycyclic aromatic hydrocarbons,
you know, this is just - as I say, it's something which is seen quite a
lot around the universe. And so maybe those are playing a role in
building up, you know, I'm not an expert in that particular field but
maybe these are - the types of things, which are the building blocks for
more complex things later on. But Titan's much to cold at the moment
probably.

Woman: Right, but, you know, who knows because they also said the bottom
of the ocean - there was no possibility for life either. So\...

Andrew Coates: Well that's right, you know, so I mean in principal
anywhere where you've got water or enough - and enough energy and, you
know, the right conditions basically, yes, who knows? And, yes,
extremophiles in the bottom of the ocean can survive in very harsh
conditions, you know, so astrobiology wise, I mean, the search for life
anywhere beyond Earth is something which is very important. So Mars is
obviously one sort of nearby possibility and so missions like, you know,
the current Phoenix and the rovers there looking for water and other
substances.

And then (exo-miles) further in the future and looking at the science of
life on Mars. And then Europa is another potential target where you
might have the right mix of things going on. And indeed, Titan, and,
yes, although the temperature is what we think for life as we know it is
too low then who knows, you know, it's a possibility.

People have suggested even Enceladus because we see these huge plumes of
water vapor and the geysers coming out of Enceladus with, again, complex
substances in there and so that, again, building blocks. We don't see -
the negative ions that we do see at Enceladus are probably stuff which
is coming, you know, certainly stuff which is coming out from those
cracks and plumes rather than stuff which is precipitating to the
surface through the atmosphere because of course we don't have an
atmosphere.

But the stuff we see coming out is principally water but there's other
stuff in there as well. So, you know, it opens up really these outer
solar system targets and - as well as Mars - as possibilities for life,
you know, because, I mean, we used to think of the inner solar system
and the Earth as being in the right habitable zone and, you know, Venus
just being too hot and Mars being tool cold and so on.

But I think, you know, a lot of thinking is going on now to suggest that
where you have the right conditions and the right mixture of conditions
then maybe. But of course as scientists what we're after is proof and
there is no proof yet for this, so I mean, so all we can work on is the
data that we've got. But I think these 10,000 amu particles are really
sort of maybe the point of what we should be looking at the future at
Titan and who knows they might even be related to building blocks of
life.

Woman: Thank you.

Amanda: Any other questions? Okay, well thank you, again, Professor
Coates, for a great talk. This is really interesting and really nicely
put together and easy to understand for a pretty complicated topic.

Thanks everybody for calling in and please join us next month for the
last CHARM telecon of the year, which will happen on November 25. And at
that point we'll hear from a couple of speakers maybe two or three
speakers about the latest results from the recent Enceladus flybys. So
thanks again everybody. Talk to you later.

Woman: Bye.

Man: Bye.

Woman: Bye.

END