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

**FTS-NASA-VOICE**

**Moderator: Trina Ray**

**April 29, 2008**

**1:00 pm CT**

Coordinator: Welcome and thank you for standing by. I would like to
remind all participants that today's call is being recorded, if you have
any objections you may disconnect at this time. If anyone needs
assistance you must press star 0.

Thank you and Miss (Jones), you may begin.

Miss (Jones): Thank you everybody and welcome to the April CHARM
telecon. Today we're lucky to have Tony Del Genio from the Goddard
Institute for Space Studies. He's going to be talking about Saturn
weather forecasts; hazy, windy with a chance of storms.

And here's a little bit of information about Tony, he received his PhD
in 1978 from UCLA and he's been at NASA/Goddard Institute for Space
Studies ever since. He's also an adjunct professor at Columbia
University Department of Earth and Environmental Sciences.

His research interests include comparative dynamics of planetary
atmospheres and the role of clouds and convection in terrestrial climate
change. He's a member of our Cassini ISS team; that's the imaging team.
He also has worked on the Pioneer, Venus, TRMM/GPM Cloud-Sat, Calypso
and Terra Aqua missions.

He's the Editor of the Journal of Climate an AMS Fellow in 2007 and
received the NASA Exceptional Scientific Achievement Medal in 2008 --
hey, congratulations.

Anthony D. Del Genio: Thank you.

(Jones): And that's -- I think with all that I'll let you take this
presentation and get started.

Anthony D. Del Genio: Okay, thank you everyone for being here today and
I'm going to be talking to you a little bit about the circulation of
Saturn's atmosphere and what are the major questions we have about it
and what we've been learning from Cassini.

And if you're looking at my presentation right there on the cover slide
you can see one of our more recent images of Saturn, that was just taken
back in February with a really beautiful view of the northern
hemisphere, which is only starting to become very well illuminated as we
move towards spring.

Most of the mission so far we've been looking at the southern hemisphere
and I'll show you that the two hemispheres are quite different and the
northern hemisphere is a lot nicer to be doing business with. But that's
mainly just for show for right now just to have a pretty picture to
begin.

If you move to the first real slide, Slide Number 2, I'll try to tell
you what we're going to be talking about today. I'd like to try to
motivate this talk in general from a -- what we call a comparative
planetology standpoint and that is trying to understand where Saturn
fits in among all of the other planets with atmospheres in the solar
system, some direct comparisons between Saturn and Jupiter, ways in
which they're similar, ways in which they're different.

And because we have one particular planet that we know more about than
any of the others, the Earth, what about our knowledge of Earth might be
relevant to our attempts to understand Saturn.

Mostly I'm going to be concentrating on global issues about Saturn's
dynamics; why there's the banded cloud structure that I'm sure you're
all familiar with on the Jovian planets, why are there these alternating
eastward and westward jets that we see on the Jovian planets.

And then at the end though I'll focus on a couple of regional topics of
interest, namely the equatorial jet and how it's been changing over time
and the south polar vortex where we've been seeing some interesting
things going on.

All right if we can move to the third slide then. This is -- don't be
put off by the complexity of that diagram; you don't really need to
understand much in it. Just note that this is basically a classification
of all of the planetary atmospheres -- well I don't have Uranus and
Neptune on this, this is a paper from 1995 and we weren't quite sure
where to put them yet.

But for six of the planets with atmospheres this is an attempt to
classify them according to what are the important things that determine
what kind of circulation they have. And so on the x-axis we have
something called the Richardson number, which essentially tells you
whether the atmosphere in the vertical direction is more stable or less
stable.

The larger values of that the more stable the atmosphere is, the harder
it is to move things vertically; the lower the value the less stable,
the easier it is for air to move vertically.

And on the y-axis we have the rotation rate of the planet or rotation
period of the planet. So small values indicate rapid rotation and large
values indicate slow rotation.

Rotation rate essentially tells you how easy it is for fluid parcels to
move latitudinally on a planet. Thus the more rapid the rotation rate
the harder it is to move in latitude, the weaker the rotation rate the
easier it is to move in latitude.

And those two things become significant because weather circulations in
planetary atmospheres are there primarily for one reason, and that is to
move heat from places where there's too much of it to other places on
the planet where there's too little of it.

And so we can sort of classify the different planetary atmospheres
according to these two parameters. Basically in the upper right hand
corner of the diagram Venus and Titan, our most slowly rotating planets,
those are planets that actually are very stable in the vertical
direction so it's easy to move things horizontally, not very easy to
move things vertically.

And those planets have circulations that primarily are trying to
accomplish the poleward transport of heat from the equator to the higher
latitudes.

The terrestrial planets in the lower right hand side of that diagram,
Earth and Mars, have -- they're sort of an in between regime in which
they try to accomplish both a vertical transport and a poleward
transport of heat. And that's accomplished mainly by what we call
baroclinic storms but what you will know as the low and high pressure
centers that you'll see on a typical weather map of the United States.

And then the Jovian planets are down in the lower left hand corner.
Because they rapidly rotate it's very difficult for those planets to
move things in the latitudinal direction between equator and pole and so
the excursions of the atmosphere and latitude are relatively limited.

But because these planets are internally heated they have lots of heat
to try to get out from their interior and they're very weakly
stratified, we say it's very easy for fluid to move in the vertical
direction. And so we believe that the weather, the circulation on the
Jovian planets is mainly trying to move the heat upward. And that's what
distinguishes them from the other planets in the solar system.

Okay let's move on to the fourth slide. Another slightly different angle
on comparative planetology and that is how easy or difficult it is to
actually learn something about particular planets from observations. And
as someone who does Earth Science part of the time I get spoiled a
little bit because the Earth is actually one of the easiest places to
observe for a variety of reasons.

And Saturn is at the other end of the spectrum; it's one of the most
difficult planets to observe. And here are the reasons why: the Earth is
partly covered by clouds but it's partly clear. And so with remote
sensing instruments we can see the tops of the clouds and the clouds are
at different levels and so sometimes we can see the upper troposphere,
sometimes we can see the lower troposphere, where it's clear we can see
all the way down to the ground.

So it's very easy with remote sensing to observe all of the different
parts of the atmosphere that we would like to observe.

Saturn and the other Jovian planets are just about completely overcast.
And that's a big problem for us because most of our remote sensing is
seeing either near the tops of the clouds or the clearer air that is
above the tops of the clouds. But the meteorology is being driven down
beneath that we can't easily observe with our remote sensing
instruments.

And so we have to be a little bit clever and use what we can see at the
top to try to infer what's going on down at the bottom, and that's a big
limitation.

Another thing about Earth is that it's a small planet. And once you
start to try to observe Saturn, which is an order of magnitude bigger
and develop a remote sensing strategy you learn right away how difficult
it is. With Earth's satellites you can easily observe the entire Earth
with one satellite if you just wait long enough for it to go around
enough times.

But for Saturn you need to stay far away to see any reasonable fraction
of it. And even then you can't really capture the whole planet in your
field of view at distances that are reasonable to resolve the features
that you want. And so you need a more complex strategy of producing
mosaics of images that observe a particular part of the planet at once
and then try over time to build up a lot of observations.

Earth is a planet that does not have rings, Saturn as we all know has
lots of beautiful rings. And of course one person's signal is another
person's noise; a lot of the science that's being done on Cassini is
involved in trying to understand what's going on with the rings. But as
an atmospheric scientist the rings just get in my way, prevent me from
seeing Saturn because they directly obscure the view and they cast
shadows that prevent me from seeing other parts of Saturn.

A more important thing that perhaps many people might not have thought
about is that the Earth is in the inner solar system, a lot closer to
the Sun and so it's strongly heated by the Sun, which means that the
weather processes of interest, the storms, the low and high pressure
centers, thunderstorms and things like that on Earth, that are the
primary agents by which heat is moved around in the atmosphere, are
happening all the time.

At any given time you can just look at a weather map of the United
States and see low and high pressure centers. And someplace in the
United States there's going to be thunderstorms happening on most days.

And so you don't have to observe the Earth for that long a time period
in order to see the things that you need to see. Saturn is very
different; it's far away from the Sun, weakly heated, it's got an
internal heat source but that actually is fairly weak too. And because
it's very cold it doesn't heat -- and a very deep atmosphere, it doesn't
heat up and cool down very quickly.

So everything happens slowly on the Jovian planets and that means you
have to observe them for a long time in order to see things change. The
other thing about Saturn is that it doesn't have a solid surface; on
Earth storms come and go and they go partially because there's friction
at the bottom boundary to help the storms die out.

But on Saturn there is no solid surface at the bottom to make the storms
die out and so once something occurs it sort of just kind of stays there
for a while. And you don't get to watch it change and by watching things
change that's how we learn what's going on.

And some of the more important things that we care about like
thunderstorms happen only once in a while; on Earth, again,
thunderstorms are happening all the time. On Saturn a little burp every
once in a while helps to clear the stomach of Saturn and you have to
wait a long time before you see the next one.

So that means we've got to spend a lot of time observing Saturn to
really understand what's going on.

And then finally the one obvious thing that makes a difference is that
we do have people on Earth; we launch weather balloons and sample the
atmosphere directly so that we have a good idea how to interpret our
remote sensing observations.

And on Saturn unfortunately we don't have that and so remote sensing is
all we have and we have to always be trying to figure out whether our
interpretations of the remote sensing data are valid or not.

Okay let's go on to the next slide, Slide Number 5. This is just a
comparison of the vertical structure of Jupiter and Saturn. And
qualitatively Jupiter and Saturn are very similar, they both have a
temperature profile that is about as steep as it's allowed to be in the
deep atmosphere where we believe convection is going on driven by the
internal heating inside both planets' atmospheres.

And then the temperature decreases up to a tropopause that is at a
pressure level of about 100 millibars or so and then increases above
that in the stratosphere. And that's qualitatively similar to the
Earth's atmosphere as well.

Saturn and Jupiter also are both believed to have three different cloud
layers; an upper level ammonia cloud layer, a mid-level ammonium
hydrosulfide layer and then a deeper water cloud layer that's the
hardest to observe because the other higher cloud layers get in the way
most of the time.

But every once in a while some of the water leaks its way up to the top
in thunderstorms and things like that and so we can tell directly that
there's a significant amount of water underneath.

The two big differences between Sat and Jupiter are that Saturn, being
farther from the Sun is colder than Jupiter is, and that means that the
water it forms at a deeper level, which makes it harder for us to see
processes going on that have to do with water, with the generation of
thunderstorms and so on, and I'll comment on that a little bit more
later.

The other thing about Saturn is that although both Saturn and Jupiter
have a layer of haze, aerosol particles above the main ammonia ice cloud
decks, Saturn's haze is much thicker than Jupiter's haze is. And that
also makes it harder to see through down to the clouds that we want to
observe where the winds are blowing that we would like to understand.

So if you then go to Slide Number 6, this shows you global mosaics of
images of Jupiter and Saturn and the consequence of that tropospheric
haze that's so much thicker on Saturn than on Jupiter is that on Jupiter
we can see lots more detail in our images whereas Saturn is very washed
out and pale by comparison.

And that means that our job in trying to observe the circulation of the
atmosphere is that much more difficult. The primary way that we observe
the atmosphere on the imaging team is to pick out individual cloud
features, whose morphology, whose shape and color and size we can
recognize, and to try to track their motions from one image to another
image that might have been taken, let's say, one Saturn rotation later
than the...

And you have to be able to identify cloud features in order to track
their motions. And on Jupiter it's very easy to do that, you can see
that there are lots of things that you can track; on Saturn there are
relatively few things that you can track.

And so just to give you an idea of the consequences, Voyager flew past
both Jupiter and Saturn a couple of decades ago and in about the same
time period flying past both planets we got about as -- about 10 times
as much wind data for Jupiter as we did for Saturn. So Saturn is a tough
nut to crack.

And the way we approach observing Saturn and Cassini is that first of
all we try to take most of our images not in -- at visible wavelengths
but at near infrared, longer wavelengths. And the reason that you do
that is that the longer the wavelength you go to the less the small haze
particles overlying the main clouds get in your way; you can kind of see
through them a little bit better as you go to longer and longer
wavelengths.

Essentially once the wavelength of the radiation is comparable to or
greater than the size of the particles you can begin to see through them
a little bit better. So we get greater image contrasts when we look at
those longer wavelengths.

And then the other thing we do, and this is the beauty of having an
orbiter rather than a fly-by mission, is that we try to image Saturn
over and over and over again to build up the statistics that we need to
really determine what's going on with the circulation.

All right, let's move on to Slide Number 7. This is just a
monochromatic; this is one of our near infrared filters that we're
using, that's why it's a black and white image not a color image. And
this is a mosaic actually of images that we have put together at one
particular time showing you parts of the northern hemisphere and
southern hemisphere simultaneously.

This was last year, so the north pole was near the upper boundary there
and the south pole near the lower boundary. And you can see the equator
and the ring plain right in the middle and the shadow of the rings just
above it obscuring part of the atmosphere.

But the reason I'm showing you this is that now that we're starting to
see the northern hemisphere of Saturn in the last year or so you can
notice that it's very different in appearance from the southern
hemisphere. The southern hemisphere is extremely hazy, low contrast, and
by the way it's summer still in the southern hemisphere although we're
approaching spring equinox.

But in the summer apparently the summer hemisphere, the haze thickens up
and it makes it very difficult to see features whereas in the northern
hemisphere we can see much better and the contrasts are much greater.

And so fortunately for us we had some very long observing sequences in
2007 that gave us views of the northern hemisphere and we're just
starting to process those now. And we think that we're going to get
probably 10 times as much information from that data set as we've gotten
all the way up to this point from our southern hemisphere imaging.

Okay let's go on to Slide Number 8. Here's another pretty picture, color
picture of Saturn and showing you the wind profiles, the wind in the --
what we call the zonal direction, the eastward, westward direction as a
function of latitude.

And on the bottom is a profile that we published a couple of years ago
from early Cassini orbits showing how the winds vary with latitude and
then the upper panel is one that we just put together in the last few
months from our first northern hemisphere images showing you something
similar.

And I'm not going to go through the details of what all those different
color symbols are right now but we'll come back to that later. For now,
I just wanted to show you this figure to illustrate, in case you weren't
aware, that Saturn, like Jupiter, has a series of alternating jets, that
is (maxima) in the wind speed and some of them go toward the east and
some of them go toward the west.

And they go back and forth like that with tremendous wind shear, change
in wind, as you move from one latitude to another latitude. And that is
very much like Jupiter. And the wind speeds are quite high; they're
generally of order something like 225 miles per hour at their peak.

And except for the equatorial region -- equator-ward of about let's say,
oh, 15 degrees latitude in each hemisphere, outside that region the jets
seem to be remarkably stable for periods of decades. They are -- the
winds are about the same now as they were at the time of Voyager. And
that's one of the interesting things that we want to understand.

The only exception to that is, as you can see in the equatorial region
there, is this huge equatorial jet that blows toward the east at speeds
of about 800 miles per hour. So we're interested in understanding why
the Jovian planets have these alternating eastward and westward jets and
why are they so stable, why don't they change from decade to decade very
much except in the equatorial region?

All right let's go on then to the next slide. This shows you a little
bit about how those jets on Jupiter and Saturn relate to the cloud
patterns that you will see in our images or if you look through a
telescope at either planet.

On the left hand side we have the wind speed as a function of latitude
for Jupiter and the gray areas that are marked out are what we refer to
as belts, those are the places where the clouds are darker on Jupiter;
and the white regions are the zones and that's where the clouds are
brighter.

And you can see that on Jupiter the colors and brightness of the banded
cloud patterns match up quite well with the winds pattern. Such that,
for example, the westward jets tend to occur let's say in the northern
hemisphere at the northern boundary of a belt.

And then the eastward jets tend to occur at the northward boundary of a
zone. And so there are different signs of the wind shear, the rate of
change of wind with latitude, in the belts and the zones such that in
the belts we have what we call cyclonic vorticity, that is -- vorticity
is basically a measure of how much something placed into a shear flow
would rotate.

So if you imagine a pinwheel put into a belt region on Jupiter then that
pinwheel in the northern hemisphere would rotate in a counterclockwise
direction whereas one put into a zone has what we call anti-cyclonic
shear, it would rotate the other way, in the clockwise direction.

And the winds and wind shears match up very nicely with the cloud
patterns on Jupiter.

On the right we have the same type of information for Saturn where you
can see a wind profile overlaying on an image of Saturn. And it's not
the same on Saturn. And that is something we still do not understand. On
Saturn the westward jets seem to be more in the middle of the bands that
have similar color and characteristics to them.

The eastward jets seem to be in the middle of much narrower bands that
tend to be somewhat darker than the other broader bands are. And so it's
a very different kind of configuration than we see on Jupiter. We're not
sure to what extent that difference is due to the overlying tropospheric
haze that's affecting our view.

or whether that's telling us something more fundamental about the
dynamics that's going on underneath and that's one of the continuing
issues that we'll be trying to deal with as we go forward in time.

Another thing to point out though is that the jet locations on Saturn
seem to be places where the atmosphere is a little bit more active than
it is in other places. You can see, for example, in the big jet location
near about minus 40 degrees latitude, the big westward jet, you can see
a big -- what later I'll try to argue is a convective disturbance there
where the westward wind peaks.

And then at slightly lower latitudes, at about say 27 degrees latitude,
there's an eastward jet and you can see that there are a number of
bright features there as well. And so the jet centers seem to be places
where interesting things are happening on Saturn.

All right, let's go on to the next slide. All right, where do these jets
come from and why do they stay there and not change all the time? There
have been a variety of ideas proposed over many, many decades going back
to just before when I was born in 1952 to try to explain what was going
on there.

And these are sort of the main ideas. And I'll try to at least talk in
some detail about the first three of them as we go into the coming
slides. But early on, just before I was born, (Hess and Pinofsky) who
were familiar with the emerging knowledge of Earth's circulation, tried
to make an analogy between what they knew was going on on the Earth and
what they knew -- or what they thought might be going on on the Jovian
planets.

And they assumed that there were cells of rising and sinking motion just
like we have in the tropics on Earth and I'll show you that in a minute.
And that as air drifted poleward in those cells they would experience,
on the rotating Earth, a Coriolis force, a force that would appear to
deflect the air into the direction of the planet's rotation.

And they proposed that that might be the thing that produced the
eastward jets and going in the opposite direction, the westward jets on
the Jovian planets.

Later on people came up with other ideas that there were instability
processes. In particular something called baroclinic instability, which
I mentioned earlier, is the process that produces our low and high
pressure centers in weather maps on Earth.

That our instabilities that are due to the fact that it's warmer at low
latitudes and cooler at high latitudes on the Earth and that's why we
get those low and high pressure centers and that that might have
something to do with creating the jets.

Recently people have become very interested in the idea of thunderstorms
on the Jovian planets and that those thunderstorms might directly feed
into -- energy into creating the jets.

All of those theories are what we call "Weather layer" hypotheses. The
Jovian planets are different from the terrestrial planets in being deep
atmospheres rather than shallow atmospheres.

And so you can look at the meteorology in two different ways; you can
imagine that the entire depth of the molecular hydrogen atmospheres of
the Jovian planets is intimately involved in the circulation and that
you have circulation cells that are essentially going through the entire
depth of the atmosphere.

And on a rotating planet if you had that type of thing it would take the
form of cylinders that would be rotating around and would somehow pump
momentum into the upper atmosphere.

Or you could take the view that the deep atmosphere is nothing more than
just kind of a bubbling sort of pot of water, although it's not water
it's molecular hydrogen. But like a slowly bubbling pot of water on the
stove that does nothing more than just help move heat up to the upper
levels in the atmosphere.

And then once you got to the upper levels in the atmosphere that's where
all the real action would be taking place to convert that energy into
the weather phenomena that we see and produce the jet streams and the
various vortices and storms and things like that that we observe in the
images.

And so there are these two different, fundamentally different schools of
thought about how the Jovian planets work as to whether the interesting
things are happening up near the top or whether the interesting things
are happening all the way through to the bottom.

All right let's go onto the next slide, this is Slide Number 11. This
shows you the old idea going back to 1951 about the analogy, the
imagined analogy between the Jovian planets and Earth. We don't normally
think of Earth as having banded cloud patterns but here's a satellite
image of the Earth.

And you can see that if you squint a little bit Earth really does have
its own sort of banded cloudiness to it. There is a thin band of clouds
near the equator that meteorologists call the inter-tropical convergence
zone and there are lots of thunderstorms all around the planet there.
The places where those things tend to occur is where we have our
rainforests, the Amazon Basin, central Africa, Southeast Asia, places
like that.

And then if you go to somewhat higher latitudes, what meteorologists
call the subtropics in either hemisphere, then the clouds tend to clear
out, the skies tend to generally be much clearer; you get very little
rain there and those are the parts of our planet that are deserts, okay.

And that is accompanied, if you look at the right hand side of this
figure and look at the schematic diagram here, this shows you sort of a
cross section through the atmosphere, sort of a vertical slice through
the atmosphere at different latitudes. And this is the type of figure
that you'll find in any meteorology textbook for the Earth.

There is, in general, rising motion near the equator on Earth. And then
up near the tropopause the air encounters the stable stratosphere above
and stops rising and begins drifting poleward. And then it begins to
sink in the subtropics and that creates an overturning circulation cell
with, then a return flow down near the ground that produces our trade
winds.

And those are called the Hadley cells on Earth, and those are associated
with the tendency for bright cloudiness near the equator and little
cloudiness and deserts in the subtropics. And that was the idea of (Hess
and Pinofsky) that those things were the analog to the belts -- to the
zones and belts on Jupiter, that the equatorial cloud region would be
like the zones on the Jovian planets and the subtropical deserts would
be like the belts on the Jovian planets.

But Earth's circulation is actually a little more complicated than that.
You see that poleward of the subtropics we have another sort of banded
cloud region, our mid latitude storm tracks where we get these spiral
type of comma shaped cloud forms that you're probably all familiar with
associated with our warm fronts and cold fronts and low pressure centers
and things like that.

But you can see that they're organized all the way around the globe in
latitude bands in the mid latitudes as well in the storm tracks. And
there is another circulation that is associated with those things that
goes the other direction called a feral cell. Okay and remember that
idea; it's a little bit harder to explain; we'll try to explain it but
we think that that actually is the one that has more relevance to the
Jovian planets that the tropical Hadley cell does.

Okay if you go to Slide Number 12. This again is the old (Hess and
Pinofsky) idea of why we have the eastward and westward jets on the
Jovian planets and why we have the zones and belts. And so again their
idea, now this is an imagined cross sectional cut through the Jovian
atmospheres vertically.

And if you look at the blue arrows they imagined that warm air was
rising just as it does near Earth's equator, warm air was rising in the
zones and as that warm air rises it cools off, it condenses water and it
makes clouds and clouds are bright. And that's why we have zones.

And then when the air got to the top in the zones it would start to
drift, in this diagram, let's say northward from the zone toward the
belt that would be toward the left. And as it did it -- the rotation of
the planet would give it a Coriolas force that would deflect it to the
right and that would create, as you can see in the red arrow, an
eastward jet in between the zone and the belt region.

And then in the belt region, just as in the Earth's subtropical deserts
the air would begin to sink and warm up, the clouds would clear out,
you'd have lower cloud tops and you would get darker colors in the belts
than you got in the bright zones where the air was rising and forming
clouds. And again you would have one of these Hadley cell type
circulation systems.

And so that's a proposition that we then begin to test if we have a way
of determining whether air really rises in the zone in the anti-cyclonic
shear regions on the Jovian planets and whether it's actually sinking in
the belts.

The hard thing is that we can't measure those vertical velocities
exactly because they're much weaker than the east/west velocities. We
can measure the eastward jets and the westward jets quite easily but we
can't directly measure the vertical velocity. And so we need to be a
little clever about that.

And the way we wind up doing that is we wind up looking for
thunderstorms because thunderstorms happen in places where the overall
vertical motion of the air is upward and they're suppressed in places
where the air is downward. So that gives us a motivation to try to look
for thunderstorms on the Jovian planets.

Okay if you go to Slide Number 13. Here is a completely different idea
about what might be maintaining the jets on the Jovian planets. And the
idea is that if you had -- if you have eddies, any kind of disturbance
in the mean flow, and for this purpose imagine that you have a
disturbance in the mean flow that causes air initially to let's say move
around in a circle, in a vortex like motion, just like, again, the low
and high pressure centers on Earth do that routinely.

And so if you have air moving around in something like a circle and then
that eddy, that little disturbance in the flow, then moves into a region
where there is a wind shear, a systematic change in the wind with
latitudes. And in the figure in the picture I have taken an example from
a latitude where in the center of that image there is an eastward jet,
so a maximum in the wind speed moving toward the east.

And there are eddies, little disturbances on either side. And if you
take this little circular eddy and you put it into this region of wind
shear then the wind shear is going to deform the eddy and stretch it out
into a more elliptical form; it's going to tilt it.

And that tilt is going to create a bias. And what the net results of
that bias is going to be is that momentum is going to be transported by
the eddy into the jet. That's a hard thing I think to imagine at first.
People usually don't have a hard time imagining that air motions can
transport heat.

So for example we know in the United States that before a low pressure
center comes by usually the wind is blowing from the south and on a day
like that the temperature will be warmer than normal because the wind is
carrying with it warm air from the south and bringing it up to the
north. And then after the cold front goes by the wind direction shifts
and cold air comes down from Canada into whatever location we're at and
the temperature goes down.

So I think we have an easy time imaging that air motions can move heat
around. We also have an easy time imagining they can move humidity
around; we know it gets more humid and less humid associated with these
motions in front of and behind the fronts on Earth.

People are less used to the idea that such motions can carry momentum
with them as well, but they can. A good way to imagine that is imagine
you're watching a baseball game and a ground ball is hit slowly to the
right hand side. And the second baseman has to charge in to pick up the
ground ball and then throw back across his body to first, in the other
direction from the direction that he's moving.

The second baseman in order to actually get the ball to first base has
to correct for the fact that he's moving toward home plate. And has to
direct the ball a little bit to the left of where first base actually
is. Because once he lets go of the ball the ball is going to maintain
that component of the momentum toward the plate due to his own motion.

And if he doesn't correct for that when he lets go of the ball he'll
find that the ball drifts away from first base to the right and he won't
have made an accurate throw.

That's just an example of how that ball having been thrown away carries
with it whatever momentum it had been carrying in the direction that it
had been moving.

The same thing is true with air parcels moving in planetary atmospheres.
Once these air parcels become tilted as I have indicated in the diagram
here, they've taken on these elliptical shapes.

Then if you follow the arrows every time there is an arrow that is
moving, let's say, toward the center of the jet if you look at, for
example, the three uppermost ellipses, then you'll see that on the half
circle of their transit in which they're moving toward the center of the
jet, they are also moving in the positive, the eastward direction and
therefore carrying faster than average momentum with them.

And on their other half of their circuit, where the arrow is pointing
the other way and they're moving in the opposite direction, away from
the jet, then they're carrying momentum that is slower than the average
amount of momentum. And so they bring high momentum air in and they take
low momentum air out.

And in that way they systematically transfer momentum into the center of
the jet and help accelerate it to keep it moving. So that's another idea
and there are several different versions of that idea as to how it might
happen.

One version of the idea suggests that the eddies are formed by
thunderstorms. And another version of the idea suggests that they're
formed by this baroclinic instability, this low and high pressure center
mechanism that exists in the latitudes on the Earth.

We can get a clue that something like that is happening just by looking
at the image because you can see that the cloud features, if you look at
the bright linear cloud features you can see that they're all tilted in
the direction -- they produce this sort of chevron, V-shaped kind of
structure that sort of is like the head of an arrow that's pointing
toward the center of the jet.

And that's usually a giveaway that this type of process is happening.
But just the shape of the clouds itself doesn't actually tell us that
that's really happening. And so we have to really go in and measure the
wind speeds to see whether those types of relationships are really
occurring.

So let's go to the next slide, Number 14. And we'll talk a little bit
about how we view that. Well first of all we can't image Saturn all the
time; there are lots of instruments on the Cassini orbiter, each with
their own science goals and each with a preferred location and distance
from the planet to do their best observing.

And so the figure on the left shows you the petals of the various
Cassini orbits as they go around Saturn. And in time the white ones at
the bottom, so it's pointing toward the bottom that I have the arrow
saying, "2004-2005", those represent the early mission orbits. And then
as time went on the orbits drifted more so that the apoapses, the
farthest distance from Saturn would take place on the night side of the
planet; those are the green ones.

And now we've moved back again over to the day side so that in 2007
where the yellow ones are at the top of the page there, you can see that
during that time period the apoapses were far from Saturn on the day
side rather than the night side and the periapses very close to the
planet.

Again, Saturn is so big that what we want to do to images, we want to
stand far away from it; that's different from many of the other
instruments on Cassini that want to get right up close and see things in
as much detail as they can.

We want to stand far back from Saturn and see as much of the planet as
we can. And so those two sets of orbits, early in the mission, 2004-2005
when our apoapses were on the day side of the planet, and by the way we
need to be on the day side of the planet because the images are
reflected sunlight so we have to be on the day side or else we can't do
business.

And so early in the mission, 2004-2005, and just last year, 2007, we had
those day side apoapses and that's when we've been doing most of our
business. If you look on the right hand side this gives you an example
of what we do near apoapsis, we put together a mosaic of individual
images and you can see them numbered 0 through 8 and so it's like a 3x3
array of individual images that allows us to build up a regional picture
of what's going on on Saturn at any given time.

And we try to take images at several different wavelengths. I mentioned
that we use the near infrared wavelengths a lot because they can see
through the haze better than other filters do and allow us to get more
detail in the cloud images.

But at the same time sometimes we want to know what's going on higher up
and so for those we use images in filters that are right in the middle
of a methane absorption band. And when you are observing Saturn in the
middle of a methane absorption band you can't see very deep because the
sunlight as it's coming into the planet's atmosphere is being absorbed
by the methane and if you get too deep it's all absorbed and none of it
gets reflected back out.

So in a methane absorption band filter you only see sunlight that's
reflected from high altitudes. And so by using those two different
filters we can see things going on at two different altitudes.

All right let's go on to Slide 15. And this gives you an idea of exactly
what we do. This shows you a little latitude/longitude strip from two
different images, top and bottom, of the same region on the planet
separated by one Saturn rotation. And the little rectangle in the middle
identifies a little cloud feature, a little kind of innocuous, little
bright thing that you see toward the right hand side of that little
rectangle.

And that's the cloud feature that we've identified in the first image.
And then we go to the second image and we try to find the same cloud
feature in the second image.

If we can find the same feature in the second image then we note how far
it's moved in the intervening approximately 10.5 hours, a little bit
more than 10.5 hours. And if we know how far it moved and how long a
time interval that has occurred over then we can derive a velocity from
that distance over time.

And if we're lucky then that gives us an indication of what the wind
speed was at that location and that time.

In the early days people used to do this manually; they would actually
just look at images the way that we're doing, identify features visually
and then just try to find the feature in the second image using the
eye/brain combination.

These days we do it in automated fashion on the computer by just having
the computer cross correlate regions in the first image with regions in
the second image and look for the places that are most highly correlated
to each other. And those then are our cloud tracked features that give
us the wind speeds that we need to understand what's going on. And
that's how we map the circulation on the Jovian planet.

Now what we're looking to do in particular is we're looking for places
where the east/west wind and the north/south wind, which we call u-prime
and v-prime, are moving faster than average in a particular north/south
direction or slower than average in the opposite north/south direction.

So we're looking to see whether those east/west and north/south winds
are correlated or uncorrelated with each other. If they're uncorrelated
with each other than this eddy tilting mechanism that I was talking
about a minute ago can't work. But if they are correlated with each
other than that's telling us that momentum is being transported in some
direction.

And depending on the sign of the correlation it's either being
transported into the jet and speeded it up, accelerating it. Or it's
being transported out of the jet and slowing it down, decelerating it.

And on the right hand side we have a graph that shows how the
correlation between the east/west and northwest, north/south, excuse me,
wind speeds, how that varies systematically according to, on the y-axis,
the shear of the -- the latitudinal shear of the zonal wind.

And the sense of that -- you can see that there is a correlation -- and
the sense of that correlation is that in the upper right hand quadrant
there is a flux, a transport of momentum toward the north in places
where the wind itself is strengthening toward the north, in other words,
moving into an eastward jet.

And in the lower left hand quadrant we can see that the correlation is
negative in places where the wind is decreasing with increasing
latitude, which also then gives you a net transport of momentum into the
jet.

And so this idea of tilted eddies, eddies that have been distorted by
the shear and are now systematically transporting momentum into the jet,
seems to be something that is working on Saturn.

Previously during the Jupiter fly-by my ISS colleagues demonstrated that
this is taking place on Jupiter as well. And so we now have evidence
that on both Jupiter and Saturn this process seems to be happening.

And the net result of that is that these eddies are essentially giving
up their kinetic energy and it's being converted into the kinetic energy
of the jet, of the mean flow everywhere. And so that is the process that
seems to be accelerating these jets at the cloud level.

Okay let's go on to Number 16.

(Jeff): Got a question.

Anthony D. Del Genio: Yes.

(Jeff): This is at more than one latitude?

Anthony D. Del Genio: Yes, this is at all the latitudes at which we have
data. Now this particular figure is from a paper that we published last
year. And at that time we had only been able to observe the southern
hemisphere when we did that analysis. And so these results are southern
hemisphere only.

The other thing about Saturn is that I mentioned that it is less
(contrasty) than Jupiter is. And in particular in the eastward jet
regions there are very few contrasts. And I also mentioned that the
southern hemisphere on Saturn right now is very hazy.

And so we had a very difficult time making these measurements near the
eastward jet. So the vast majority of what you see in that figure are
from the shear regions and the westward jet regions and not very much
from the eastward jet regions.

However, since then, with the 2007 images of the northern hemisphere
where the haze appears to be a lot thinner and we could see a lot more
detail, we've been able to do a much better job with that and we've made
analogous plots for the northern hemisphere where now we can make these
measurements both in eastward and westward jets and we're getting the
same answer. So it does seem to be a pretty universal thing.

(Jeff): Okay.

Anthony D. Del Genio: Okay, Slide Number 16. This shows you a close up
of the most dramatic convective storm that we've detected to date. How
do we know this is convection? Well we don't. On Earth there are things
that we use to determine whether something is a thunderstorm or not.

Obviously we know that thunderstorms produce torrential rain most of the
time near the ground; although that's not an absolute requirement, it
doesn't always happen. But most of the time thunderstorms produce
torrential rain. We have no way of measuring rain on the Jovian planets
with the instruments that we have. So we don't, for a fact know that
these things are making rain.

Thunderstorms on Earth produce lightening. And on Jupiter we were
actually able to observe lightening associated with cloud patterns that
we thought were convective storms. We've attempted to do the same thing
on Saturn but we have not been able to observe lightening directly.

And we believe that that is associated with the fact that, as I
mentioned earlier, the water cloud on Saturn is buried at deeper levels
than it is on Jupiter because Saturn is a colder planet and the water
condenses out at deeper levels. And so even if it's producing lightening
the lightening flashes just can't get out through the overlying
thickness of cloud.

Furthermore you have the thicker haze tropospheric hazes on Saturn than
on Jupiter. And all those things we believe are making it difficult for
lightening to get out to places where we can detect it with our cameras.

However we have other ways of concluding that the things that we're
looking at in this image are indeed convective storms of the kind that
we have on Earth.

First of all one of the defining features of thunderstorms is that they
are very tall; they're taller than other type -- taller and thicker than
other types of cloud patterns that we see on Earth. And the same thing
is true on the Jovian planets.

So we look first of all for things that are very bright compared to
other cloud features, that is, that things that are reflecting a lot of
sunlight. And we look for things whose tops are very high. And this is
why we image Saturn both in a near infrared filter that sees through the
haze down to the deepest level that we're able to see in.

And then we simultaneously take images of Saturn in a methane band
absorption feature that is sensitive to clouds at very high altitude.
And by looking at those two things together we can figure out the places
where we have clouds at one altitude, clouds at another altitude, or
thick clouds that are spanning the range of altitudes.

And so the image that you're looking at here is a false color image that
essentially marks how much sunlight is being reflected in the two
different wavelengths simultaneously.

And the false color -- the way you interpret the false color is that in
places where the image is blue those are places where the image is
bright in the methane band and dark in the near infrared continuum band.
And that means that you have a high cloud that is not very thick and
deep.

There are other places where just the opposite is true where you have a
low level cloud that does not reflect much sunlight at high altitudes
but it does at low altitudes. And those places show up as pinkish or
reddish.

And then in the midst of that little cluster of things there in the
middle of the image you see a little region in the middle where it is a
brighter, whiter, yellowish color. And those are places where sunlight
is being reflected both in the methane absorption filter, which tells us
if a cloud top is high, and in the continuum filter, which tells us that
the cloud is very thick and deep.

And so that's how we identify a convective cloud. And that's not all
that much different from the way that people do that on Earth as well.

There are other pieces of information that suggest to us that this thing
is a thunderstorm type cloud. First of all it is a small, rapidly
evolving thing. And that doesn't happen too often on the Jovian planets.
Most cloud features are bigger and once they form they last for a long
time.

But the convective clouds kind of come and go, they brighten and then
they go away. And so you can tell that they're different.

And then finally we got some help from another instrument on Cassini,
the radio and plasma wave spectrometer experiment, which measures
electrostatic discharges coming from Saturn.

And what was noticed was that when this particular feature that we saw
on our images, which came to be known as the Dragon storm. When this
appeared early in the mission every time it would come into view as
Saturn was rotating and it would come over the horizon and come into
view, the RPWS instrument would notice electrostatic discharges
occurring.

And then the thing would disappear over the other side and really the
electrostatic charges would disappear.

So the electrostatic discharges are presumably the signature of
lightening. And the fact that they're occurrence is correlated with the
appearance of this Dragon storm cloud feature in our images suggests to
us that there really is lightening associated with this particular storm
even though we can't in our images actually see the lightening itself.

Okay, let's go on to Number 17. Just to illustrate that that's not the
only thunderstorm that we think we see on Jupiter. There are other
places on -- I'm sorry, on Saturn, there are other places where we also
see these small bright cloud features that evolve quickly and we believe
that we're seeing these storms happening in a variety of places.

And so what we are trying to do is to map them to try to understand
where they occur and what that tells us.

Then if we go to Slide Number 18. You can see that on the left hand side
is a plot of the wind field that I showed you before, the zonal wind,
the east/west wind is a function of latitude. And in the middle there is
a bar graph showing you the number of convective storms that we've
measured at each latitude.

And don't worry about the difference between the black and the red, it's
irrelevant for what we're going to talk about here. The one point I
wanted to make is that if you look at where in latitude the
thunderstorms are occurring there are a lot of them that occur in the
cyclonic wind shear regions, those are the things that on Jupiter we
would call belts.

And there are relatively few of them occurring in the anti-cyclonic
regions, which are the things on Jupiter we would call zones. On the
right hand side is a similar type of diagram for Jupiter itself and you
can that the association with the belts and zones is even more dramatic
on Jupiter than it is on Saturn. Partly that might just be because it's
easier to observe these things on Jupiter; partly it might be a real
difference between the two planets.

But the main message is that the convective features are mostly in the
belts; and they are almost never in the zones. And if you go back to
this old picture of why we have belts and zones on the Jovian planets,
the old idea was that the zones are bright because they're the regions
of rising motion and the belts are dark because they're the regions of
sinking motion.

These observations are telling us that that's not true; it's just the
opposite. All of the thunderstorms are happening in the belts and very
rarely do they seem to be happening in the zones. So apparently because
thunderstorms are associated with rising motion, the air on the Jovian
planets is actually rising in the belts and is sinking in the zones
instead.

And so that old picture of how the circulation of Jupiter and Saturn and
the maintenance of their jets works seems to be wrong.

(Jeff): Question?

Anthony D. Del Genio: Yes.

(Jeff): Could that be a selection effect?

Anthony D. Del Genio: We have tried our best to try to deal with that.
And the way we do that is that you can't just tell by looking at the
images but if you start plotting the data you can plot, as I say, we
identify the convective features by simultaneously looking for things
that are bright in this continuum filter that sees relatively deep and
in this methane band filter that sees cloud tops that are high.

And so if you just make a scatter plot of how bright a particular region
is in one wavelength versus how bright it is at another wavelength, if
you do that in one of the regions that has a convective cloud the
convective cloud pixels stand out as an unusually bright area both in
the methane band and in the continuum band.

And so you can do that, you can see that very easily for the zones --
I'm sorry, for the belts where everything else surrounding it is
relatively dark.

Then what we can do is we can go to the zones where in general
everything is brighter than it is in the belt. And we can ask is there
anything in the zones that is as bright as these brightest areas in the
belts that we think are thunderstorms.

And the answer is we see nothing in the zones even though overall the
zones are brighter than the belts are, the bright areas in the belts
that we think are thunderstorms are brighter than anything we see
anywhere in the zones.

And so we're pretty convinced that the thunderstorms are almost
exclusively happening in the belts. And we've pretty much convinced
ourselves that it's not a selection effect.

(Jeff): So they couldn't be just obscured by these supposedly high
clouds that form in the zones?

Anthony D. Del Genio: Not without having a signature -- because those
higher clouds that are forming in the zones would just add to the
brightness of them. And so if they were being preferentially obscured by
those then you would see things -- then you would have basically
something bright on top that was hiding something that was also bright
underneath it.

So that the total brightness of the column would be even higher than
what we saw in the belts where you didn't have the advantage of the
extra stuff on top to reflect even more sunlight. So if that were the
case, if it were being hidden by higher hazes then we'd expect to see
areas that were brighter than anything we would see in the belts.

And instead the brightest things we see in the belts are brighter than
anything we see anywhere in the zones.

And it's not foolproof. The other thing we can say for Jupiter at least,
where we can observe lightening, is that lightening is observed only in
the belts not in the zones.

But that is something that could be a selection effect because if you
had thicker clouds that made it difficult for lightening to get out then
you might just be obscuring the lightening in the zones and it would be
easier for it to get out in the belts and then you'd be seeing a
latitudinal dependence that really was a selection effect.

So we feel that although lightening is a more direct indicator that
something really is a thunderstorm than just looking at the brightness
and the height of cloud features we feel that the fact that the
brightest cloud features in the belts are brighter than anything we see
anywhere in the zones is probably a more reliable indicator of something
that is a real difference and not a selection effect.

Okay so going to Slide Number 20. This is just going back to the
different ideas that I mentioned earlier about why we have the
alternating jets on the Jovian planets. We've basically ruled out the
old idea that you have rising motion in the zones and sinking motion in
the belts like we have on the tropics of the Earth.

And instead we seem to be coming toward a picture that is more like the
mid-latitudes on the Earth where eddies -- eddies that may be like the
low and high pressure centers that we see on our weather maps in
mid-latitude on Earth, yet sheared out and tilted and they transport
momentum into the jet from both sides.

And associated with that is a circulation that is just the opposite of
the one that was imagined 50 years ago, 60 years ago, mainly that there
is rising motion in the belts and sinking motion in the zones.

There are these other ideas for how the eastward and westward jets might
be driven. So people have suggested in recent years that the
thunderstorms themselves are actually feeding their energy into the
jets.

We've looked for that but we haven't been able to find good evidence of
that. And I realize I skipped -- I skipped Slide Number 19 and that was
an attempt to actually to test this particular mechanism. Slide Number
19 that says No Sign of Preferred Drift Direction or Concentration of
Large Momentum Fluxes.

Basically what we did was we went into the images and looked at what
these momentum fluxes were near where the convective storms were and we
looked to see whether the convective eddies systematically drifted
toward the jets, which would be a requirement for them to be dumping
their energy into the jets.

And basically we saw no evidence that that was happening. And so we
can't rule that out but we've yet to see any evidence that suggests that
that is a viable hypothesis for actually making jets. There have been
numerical simulations to try to produce that effect. And different
people get conflicting results.

There was a study done by one of the members on our team that was able
to make that process happen in a numerical model. And then there was
another study by someone else, (Adam Showman) and his colleague at the
University of Arizona that was not able to make that process happen.

And so it can't be ruled out yet but it doesn't look like it's as viable
an idea as the one that is more analogous to the normal process that
goes on in mid-latitudes on the Earth.

Then there's this other theory of the deep convective cylinders, a
circulation that goes all the way down to the base of the molecular
hydrogen layer. There are recent numerical simulations that are making
people think that that might be a relevant process especially at low
latitudes where the strong equatorial jet is driven on Saturn.

It's not obvious yet from those simulations what you would observe at
the cloud top to help you decide whether that theory was correct or not
correct but it still must be considered a viable theory.

All right let's go on to Slide Number 21. And this now is kind of
summarizing the shift in our view of what we're seeing when we're
looking at the Jovian planets. This is a repetition of what I showed
before, the old view in which the rising was in the zones, the sinking
was in the belts and the analogy was to Earth's tropics and the Hadley
cell.

And then if you click to Slide 22, now in the red you see this
mid-latitude circulation that goes the other way that's associated with
the mid-latitude low and high pressure centers in which the upwelling is
actually occurring in the belts and the sinking is in the zones. And we
think that's actually the better analogy.

So we think that the Jovian planets may indeed be like Earth but like a
different part of the Earth than what has been imaged for most of our
history of thinking about the Jovian planets.

(Jeff): So question. How does that relate to the colors then, this
traditional idea the white stuff is condensate? So that no longer works
I guess?

Anthony D. Del Genio: Right. So that no longer works and I can give you
half an answer to that question and not the other half an answer, the
other half we're still trying to figure out.

So the way the first half works, the easier part to understand is the
belts, okay. If you look at the Earth and you look at where air is
moving upward on Earth, where we have thunderstorms, for example, near
the equator, actually the equatorial band of clouds, the Intertropical
Convergence Zone on the Earth, is a place that is really a study in
contrasts.

You have a small number of places where you have these torrentially
raining storms throwing off cirrus clouds and (guilds) at the top that
reflect a lot of sunlight. And then in between those clouds everyplace
else the air is sinking and it clears out and you have clear skies.

So on the Jovian planets where you don't need nearly as many
thunderstorms in order to get the heat out from the center of the planet
as you do on the Earth, you have the occasional thunderstorm that comes
by every once in a while that makes the belt very bright in one
particular location.

Like if I were to take you back to Slide Number 17 where I just had some
sample images of convective storms on Saturn, you can see there are a
few individual places where it'll get very bright and then every place
else it will be darker because the air is sinking every place else.

And so we believe then that the belts are dark because the convective
storms that are occurring in the belts are actually occurring over a
very small area. They're carrying all the mass and all the heat that
needs to get out from the interior of the planet but all of that is
happening concentrated in very small areas.

And every place else there is a gentle sinking motion that actually
causes clouds to dissipate and cloud tops to lower. And that's why the
belts are darker. That by itself then does not explain why the zones are
bright.

I have an idea about that and I have no idea whether it is a viable idea
or not. There are places in the subsiding regions of the Earth's
subtropics that are not cloud-free. In particular, since this telecon is
being run out of JPL, if you head west from Pasadena out to the coast,
especially on a summer morning, most of the time you will find -- or a
lot of the time at least you'll find that it's cloudy in the morning
near the coast.

And as you go out over the Pacific Ocean and there is what we call a
marine stratocumulus cloud deck there that is fairly persistent. It
tends to go away in the afternoon a lot of the time because the sun
heats it but not all the time. And the farther north you go in
California the more persistent it is.

So perhaps different kinds of clouds like that are a better analogy for
what's going on in the zones on the Jovian planets. But at this point we
just do not know. And that's something that remains to be understood.

(Jeff): Oh, okay. Thank you.

Anthony D. Del Genio: Okay if we go to Slide Number 23, this is just
sort of a summary, sort of a cross section of what you might imagine
that we're seeing on the Jovian planets. And this figure, Z is the
vertical direction and Y is the latitudinal direction, let's say
pointing toward the north pole, for example. And this figure is actually
not something I took from a paper on Saturn or Jupiter but it's a paper
I took from a review article about Earth's meteorology in mid-latitudes.

And it's showing what goes on there. And really we think that what's
going on, on Saturn and Jupiter is actually quite analogous. We think
that there are these baroclinic storms, these storms that are trying to
transport heat poleward and upward like we have on Earth, that are down
beneath the level mostly that we can see in our images and they are
generating a circulation in which air is rising at higher latitudes and
sinking in lower latitudes.

And up near the level that we can see in our images around one to two
(bar) of pressure, then there are these eddy momentum fluxes, u-prime,
v-prime that are pumping momentum into the jet. And the Coriolis force
on this mean circulation that is going around in the opposite direction
is actually opposing that and the ballast between those two forces is
what's keeping the jets stable over a long period of time.

Okay just in the last couple of minutes I know you all want to get
going, I just thought I would talk about a couple of regional features
that people might be interested in.

First at the equator, these are in Slide Number 24, these are three
different images of Saturn taken at three widely spaced times. On the
left we have a Voyager image back from 1981 when there was a relatively
quiescent character to the equatorial region on Saturn.

Then in the upper right hand corner we know that in the 1990s all hell
broke loose in the equatorial region and we started to get eruptions of
these apparently very vigorous thunderstorms that began in 1990 with the
so-called Great White Spot.

This is a Hubble Space Telescope image taken from a few years later,
1994, when there was another outbreak of these clouds. And you can see
right in the middle of that image just above the rings you can see an
outbreak of what looks to be a very vigorous convective storm near
Saturn's equator.

Now at the bottom right you see what Saturn looks like during the
Cassini era. And it looks a lot more visually like it did at the time of
Voyager than during most of the time that Hubble was imaging Saturn in
the 1990s. With one big difference and that is when we go in and look
more closely at that equatorial region the cloud tops are a lot higher
now than they were back at the time of Voyager.

If you go to Slide Number 25, on the left is a repeat of that
latitudinal profile of the east/west winds that I've shown you several
times before. But now I want to talk about what all those different
colors mean.

Those colors tell us how the wind profile has varied with time from
different missions that people have used to make measurements. So the
solid dark line is the Voyager wind profile from 1980, 1981. And the
orange circles represent wind measurements that were done by
Sanchez-Lavega's group in Spain from Hubble Space Telescope images.

And you can see that at most latitudes the orange circles line up very
nicely with the Voyager wind profile that's given by the dark line. But
when you get closer to the equator then about 15 degrees latitude in
either hemisphere all of a sudden the Hubble data starts to disagree a
lot with the Voyager data. The wind speeds are dramatically lower near
the equator during the Hubble era than during the Voyager era.

The green and the red symbols are measurements -- and the blue symbols,
I'm sorry, are measurements that we have made from Cassini in recent
years. The green and red symbols are measurements made in our continuum
filter.

And the only difference between the green and the red is that the
measurements were made manually by two different people working
independently just as a kind of a sanity check to make sure one person
wasn't biased in getting answers that were systematically different just
because of the way they were looking at the images and choosing things
to track.

So we had two different people on opposite sides of the country take
images and try to measure wind speeds. And they got the same results,
which is a nice thing. And those results are stronger than the wind
speeds near the equator that Hubble got but weaker than the wind speeds
that Voyager got.

And so the question is have the wind speeds really changed and if they
have really changed by that much it's a very dramatic change; it's a
change of over 100 meters per second, a change of several hundred miles
per hour. And to imagine that wind speeds could actually change that
much on these huge planets is a very difficult thing to swallow.

And so we've thought about other explanations for why that might be
happening. And it goes back to the change in the visual appearance of
these planets. Namely that once the storms broke out in the 1990s, as
seen by Hubble, the cloud tops in the equatorial region started to come
up.

And now during the Cassini era even though the equatorial region doesn't
seem to be quite as active as it was during the time of Hubble, the
cloud tops still appear to be very high.

And the thing about remote sensing imaging of atmospheres is that you
can only see down as high as the clouds themselves are. And so if the
cloud tops have shifted upward the region that you're -- the altitude at
which you're measuring the winds is going to be different from the
altitude at which you were measuring the winds when the cloud tops were
lower.

And so the question is do the winds vary vertically and could that be
giving us an impression of weaker winds when instead we're just
measuring the winds at a different altitude.

And so on the right hand side another experiment, the infrared
experiment, (Sers) on Cassini, measures temperatures above the clouds
and from the temperature patterns you can deduce what the wind field
looks like using something called the thermal wind equation of
meteorology that basically says if the temperature is getting colder
toward the pole the wind speed should increase with height. And if the
temperature is getting warmer toward the pole the wind speed should
decrease with height.

Well above the clouds the temperatures generally decrease with latitude
in places where the wind is westward and increase with latitude in
places where the wind is eastward. And that gives you a systematic decay
of the wind speeds with increasing height above the clouds.

And so one possible idea that would explain this apparent change in the
equatorial region is that the winds haven't changed at all but we're now
looking at a higher altitude where the winds are just weaker.

In support of that we made other measurements with our methane band
images, which we know see to higher altitudes than the continuum band
images do. And those are represented by the blue squares on the plot on
the left.

And you can see that our measurements and the methane band images match
up nicely with the Hubble Space Telescope measurements lower speeds. And
so we do think that a large part of what we've seen going from the
Voyager to the Hubble to the Cassini era might just be an artifact of
the fact that the clouds have come up higher during this particularly
stormy epic on Saturn and that that now is causing us to just measure
winds at higher altitudes where the winds are weaker.

However, there are numerical models, one that was published by (Syanogi)
and (Showman) just last year that showed that if you have convective
storms you really can expect the winds to get weaker.

And so there is a decent chance that what's going on on Saturn is some
real combination of the fact that we're looking at a higher altitude and
a fact that the storminess that we've seen in the equatorial region has
actually caused the winds to decrease over time.

And then the last thing I want to talk about, if we move to Slide 26, is
what's going on near the south pole on Saturn, which has gotten a lot of
attention. This is a polar projected mosaic of images on the left hand
side looking down at the south pole.

And this is another one of these false color composites of several
different filters. And so where you're seeing the reds and pinks, those
are where the skies are relatively clear and you're seeing down to
relatively deep into the atmosphere, clouds with low tops. And the
places where you're seeing the bluer colors are places where you're
seeing higher tops. And the places that are the brightest are places
where we think we have high, thick, convective type clouds as I was
talking about before.

And there is this very unusual looking feature surrounding the south
pole of Saturn, an apparent vortex that has an inner ring of very bright
high clouds and then an outer ring of more high clouds.

And if you look at the lower three black and white images there labeled
B, C and D, one of the ways that we know for sure that that ring of
clouds surrounding the central core there are high cloud tops is that at
these latitudes you get the sun casting a distinct shadow.

And so if you have very high cloud tops then you can notice that they're
high by noticing the shadow cast by the sun.

And so the little arrows, the one on the top in B and the one extending
from the C inward in the middle one and the one on the left -- the arrow
on the left hand side in image D, you can see as a function of time, how
the sun's shadow is moving around; you can see a little dark area inside
that ring of bright clouds. And it's moving around with the sun and
that's telling us that that's a real shadow and that those are high
clouds surrounding this inner core.

And on the right hand side you see an image of Hurricane Katrina on
Earth and that's exactly the type of structure that we see near the
center of a hurricane. There is a central eye in which clouds are either
low or absent. And then that's surrounded by an eye wall of very high
thick heavily precipitating clouds and very strong winds and that's
where most of the damage from a hurricane comes from.

We know that hurricanes have these spiral bands extending out from the
center. If you look at the Cassini images you can even see a couple of
very weak spiral bands extending out from the center here. So that made
people think, well maybe this is something like a terrestrial hurricane.

And if we move to Slide 27, here's the evidence pro and con. On the
right upper hand is the thermal image associated with a terrestrial
hurricane. And the purples and reds represent the cold temperatures and
the greens represent warm temperatures.

And you can see in the eye, the hurricane is warm, it's a so-called warm
core system.

On the left you can see the Cassini temperatures as derived from (Sers)
data. And you can see it's also relatively warm in the center. And so
that's a feature that it has in common with hurricanes.

Below that you can see wind profiles going from the outside to the
center of the system. Unfortunately the Cassini one on the left, outside
to the center, goes from left to right and the one for a terrestrial
hurricane on the right, outside goes from right to left so you have to
flip them in your mind's eye.

But you could see in both cases, as you go toward the center of the
hurricane the winds increase and peak near the eye wall and then get
very weak in the interior.

So all of those things make us think that it had something to do with
terrestrial hurricanes. There's just one big problem. We know now where
terrestrial hurricanes come from. And we know that the one ingredient
that you really need to make a hurricane on Earth is that you need warm
ocean waters underneath. And those things provide the energy source.

As you get strong winds whipping up near the ocean's surface that causes
water to evaporate from the ocean's surface. That water that evaporates
from the ocean's surface is what goes up inside the clouds in the eye
wall and releases latent heat. And that's what drives the hurricane,
gives it its energy source.

Well on Saturn there is no ocean underneath us. And so if this thing is
to be related to a hurricane there is this huge question of where does
it get its energy from. There are other things that make it unlike
hurricanes, which move around -- propagate; the Saturn vortex is fixed
to the south pole. It looks like it might be long lived rather than a
very transient feature.

And so all of those things make us think that although morphologically
it shares something in common with hurricanes on Earth, really when it
comes down to why it exists and what it is, probably it's something
else. What that something else is and why it's there, why we don't have
one at the north pole, why we don't have one at the south pole on
Jupiter and so on, we don't know the answers to any of those questions
yet.

So finally I've gotten to the last slide here, which is just a summary
of what we've been talking about. That our current understanding of both
Saturn and Jupiter is that their circulation may, in fact, be analogous
to Earth but not to Earth's tropics, instead Earth's mid-latitudes.

For Saturn in particular the relationship between the winds and the
(albedos), the brightness of the cloud bands is something we have yet to
understand. There have been dramatic changes near the equator. Maybe
some dramatic changes in the winds but we need to look at that more
strongly.

A big question is, whatever is changing at the equator, why is it
changing? And one of the big differences between Saturn and Jupiter is
that Saturn axis is tilted to a much greater extent, 26.7 degrees
compared to Jupiter, which is just a little bit over 3 degrees. And so
Saturn has a much stronger seasonal cycle.

And so perhaps something having to do with the progression of the
seasons on Saturn may be involving shading by the rings is creating a
tendency for storms to occur near the equator preferentially during
certain seasons. But that's something that's yet to be understood.

And finally the south polar vortex that we see on Saturn does share some
characteristics in common with hurricanes on Earth. But we think it's
probably not really the same kind of animal.

Okay, thank you for your patience and listening to me today.

Miss (Jones): Thank you very much, Tony. Does anybody have any
questions?

Woman: I was wondering if the -- Saturn gets warmer as you go down so
could the liquid hydrogen area act like an ocean underneath?

Anthony D. Del Genio: Probably not because that is really, really deep.
Of course we don't know how deep any of these features actually go. But
the other thing about is that the hydrogen, what you need is for a phase
change to occur that is tied into the winds themselves.

So on Earth the relevant constituent is water, which exists in liquid
form. And then when strong winds come along and bring dry air into
contact with the wet ocean surface that causes water to evaporate.

On the Jovian planets although you eventually get down beneath a
molecular hydrogen region to a zone in which you have this metallic kind
of more liquidy hydrogen, that's a different kind of phase change and
it's not obvious that, first of all, these things would extend down that
far, certainly the water clouds associated with what we see in the
images aren't extending down anywhere near that far.

And what kind of interaction there might be at great depths between any
residual wind motions and that phase change at greater depths is harder
to understand. And why it would happen just in that location as opposed
to the things we see on Earth, which originate primarily in the tropics
as isolated vortices and then go propagating off somewhere, that's not
what we're seeing.

What we're seeing seems to be sort of a more fixed feature of the
general circulation. So I'd say probably not.

Miss (Jones): Any other questions?

Well Tony, we'd really like to thank you for this presentation. I
thought it was just awesome. And, you know, the Earth/Saturn analogies
are things that a lot of our volunteers in museums will probably like to
incorporate in some of their programs because that's always kind of a
popular and friendly topic to compare our planets to Earth.

So thank you so much.

Anthony D. Del Genio: Okay my pleasure.

Miss (Jones): And with that I'll conclude the telecon. And goodbye
everybody.

Woman: Thank you.

Man: Goodbye.

Man: Goodbye, thank you.

Man: Tony, you still on?

Coordinator: (Jane)'s still on. I don't know if Tony's still on.

Man: Oh no, this is (Jeff). I just had another question for Tony. I
didn't want to take up the time. I'll just send him an email. Thanks.

END