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

**FTS-NASA-VOICE**

**Moderator: Trina Ray**

**February 26, 2008**

**1:00 pm CT**

Coordinator: Excuse me. This is the conference coordinator. I would like
to inform all participants this call is now being recorded. If you have
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Ms. Ray, you may begin.

Trina Ray: Thank you very much and welcome everyone to the CHARM telecon
for February, 2008. Today we have a terrific speaker and a terrific
topic.

Three years ago I think - let me do the math. Yes, 2005 to 2008. Three
years ago the Huygens Probe landed and drifted down to the atmosphere of
Titan and landed on the surface and returned some spectacular data sets.

And one of the most intriguing was the Descent Imager and Spectral
Radiometer, and today we have Dr. Marty Tomasco who is joining us from
Arizona and he's the principal investigator on that instrument.

And thank you very much, Marty, for joining us today.

Martin Tomasco: Well, thank you. It's my pleasure. I wanted to tell you
a little bit about the Descent Imager/Spectral Radiometer experiment,
tell you a little bit about the instrument, tell you something about the
data that we got and kind of summarize where we are in analyzing and
understanding the results that was obtained.

If you all see the first slide, you see the name of this cumbersome
instrument \-- Descent Imager/Spectral Radiometer. We call it the
D-I-S-R or the DISR and I'm Marty Tomasco and we've had a very capable
team, not only in Arizona but in Europe, as well.

We have several German and French and Swiss investigators that also has
(unintelligible) on this experiment.

And now if we can go to the second slide, let me see if I can advance
mine down here. Yes, the second slide. It tells something about the
capabilities of this instrument and what the instrument is.

It's the one optical package on the Huygens Probe that descended through
the atmosphere of Titan, but it's not just a single camera. It's got
many measuring capabilities. It's got a frame transfer CCD. This is the
kind of detector that you can buy in digital cameras now so commonly.

Often they talk about them as 5 megapixel or 7 megapixel or even 10
megapixel cameras. Ours is a 1/8 of a megapixel, but remember we
proposed in 1990 and were selected in 1991 and that's 17 years ago now -
18 years ago almost. And for those days, an 1/8 of a megapixel was about
the state of the art.

But this single CC detector is fed by fiber optics that come from
several different sets of photo optics, and in particular we have a
visible spectrometer with fibers that look up and look down.

We have a four-channel solar aureole camera in two colors; short
wavelength, one we call blue, and a long wavelength one that's in the
near-infrared we call red, and each of those cameras that looks up has
linear polarizers so they can measure the vertically polarized component
and the horizontally vibrating component of the light they see.

And then in addition we have three frame imaging cameras that look down
at different angles from straight down. One looks not quite straight
down, almost straight down. One looks at a intermediate angle and one
looks almost to the side, and so in that way we can cover a range of
angles from six degrees above the horizontal so within 10 degrees of
straight down.

And we also have a near-infrared spectrometer with two linear array
detectors, 132 pixels long and so we can get spectra of the
near-infrared light out to 1.6 microns wavelength, looking up and
looking down.

And so the CCD and the infrared spectrometer cover the range from .48
microns to 1.6 microns and then we also have an ultraviolet photometer
that both looks up and looks down and gets us down to 350 nanometers, so
we go from 350 to 480 with the ultraviolet, 480 to 960 with the visible
and 850 nanometers to 1,600 nanometers for the near-IR.

So those are the channels in which we're sensitive. In addition to that,
we carry a lamp, a small 20-watts lamp with a parabolic reflector which
we can use to illuminate the surface, and this is not so that we have
enough light to take pictures, but it's so that we provide a continuous
spectral range of illumination, so we illuminate all wavelengths in that
range.

And that way we can measure the reflectivity of the surface, not only
through the atmospheric windows where sunlight makes it down to the
ground but also where no sunlight makes it down to the ground because we
carry our own lamp.

Finally we have a sensor to detect the azimuth of the sun. As the probe
is expected to rotate, it has spin vanes on it and the idea is it would
fall down through the atmosphere, the spin vanes would make it rotate
and then we could get data at different azimuths relative to the azimuth
of the sun and the sun sensor is used to trigger those images.

And finally we have a hardware data compressor so that we can compress
all those data and send it back to the - eventually well to the Cassini
orbiter and then eventually back to...

And now if we can look at the next slide, the third slide here, this
shows the placement of the instrument in the Huygens Probe. The Huygens
Probe is about a meter, a little over three feet in diameter and on one
of the shelves we have an electronics box and then a sensor head.

And the sensor head sticks out through the wall of the probe and allows
us a clear view up and down and left and right of the atmosphere through
many different windows.

And then the next slide shows the layout of the sensitive area of the
CCD detector. And remember this is a frame transfer detector \-- that
is, half of the sensitive area of the detectors is covered by an opaque
foil, an opaque film, and it's only the power path that\'s in light,
that is sensitive to light and it's fed by fiber optics.

There's the down-looking imager on the left, side looking imager in the
middle and the other down-looking imager on the right. And in addition
to that, there are strips - vertical strips for the spectrometers
looking up and down and in the above right corner for the solar aureole
camera and its four channels that also looks out.

And so what happens is their sensitive area on top is converting light
into electrical charge and you can transfer the charge rapidly down
underneath the opaque foil and the time between transfers is the
exposure time of whatever data you're collecting.

So there are no moving parts, there are no mechanical shutters. It's all
done with just electrically shifting the charge rapidly down underneath
the foil and then slowly digitizing and reading it out.

That's the way that half of the instrument that\'s fed by the CCD works.

And the next slide shows a picture of this part of the instrument. You
can see the fiber optic conduit, the metal envelope that covers the
fiber optics, you can see the three lenses on the left side looking in
three different directions. The one that looks most nearly down, the one
that looks intermediate or the one that looks mostly to the side on the
left side. On the right side we have the CCD detector and we have the
fiber optics carrying the light back.

You can also see this little cylindrical tube in the front that it is a
little grating spectrometer that divides the light up into all different
wavelengths bins and we can measure the brightness in different
wavelength bins with that spectrometer.

And then in the front on the sort of the - sort of towards the left and
toward the top a little you can see this device that's our solar aureole
camera that's really four separate lenses, two covered with a red
filter, two covered with a blue filter and each of the red ones and blue
ones is covered with a sheet of polarizing material that's either
vertically or horizontally oriented.

So that's four cameras looking up and three looking to the side or down
there at seven and then there's the spectrometer that looks up and down;
that's nine. So we have essentially nine different instruments in this
little device that's about six inches long.

And the next slide shows how we plan to put the images together, this
would be the footprint of images on the ground so that we can make a
mosaic of the ground and get sort of a skydiver's eye-view of the
ground.

We can look almost straight down and put 12 images around and fill that
circle and then we can look at the intermediate angles and put 12 around
and fill that circle and then finally the 12 around from the one that
looks mostly to the side.

In addition to those images, there is a little picture that almost looks
like a ladder in three different regions and those are the field review
of our down-looking visible spectrometer so we can measure spectra of
different things in the scene and we can try to correlate the spectra
reflectivity of different parts of the terrain with the morphology \--
what the terrain looks like.

So that's how the CCD cameras work. And the next slide shows infrared
spectrometer system. And again there's a six-inch ruler and you can see
the two linear ray detectors are on the left side and then there's a
little grating spectrometer that measures the infrared spectrum of the
light looking up and looking down.

In the bottom of the right-hand end of the spectrometer, there's the one
moving part in the whole DISR experiment package and the that's a small
shutter that can open and close the entrance slit to letting light from
the scene or close it to measure just the dark current of the
instrument, that thermally generated infrared current.

We were careful to have that device spring-loaded so that it would
always stay open if electromagnetic - the electromagnet wasn't energized
or fails for some reason because it was only necessary to use that
shutter in the early part of the descent when the camera was still
fairly warm.

Once we get into Titan's atmosphere, more than 15 or 20 minutes, then we
have a big copper thermal strap that connects the focal plane to the
outside of the probe and cools it down enough so that the thermally
generated dark current is not a problem.

So we put an amount of thought into trying to minimize the number of
moving parts to try to get as high a reliability as we could on the
instrument.

And on the next slide shows the - basically all the optical parts and
built into the configuration which they're going to be used. We had
1,000 machine parts in something about the size of a shoebox and it was
quite a packaging challenge to get this whole instrument in the small
amount of space that we had available on the probe.

So it looks kind of messy, but the next slide shows it when most of it
is in the package in the box and you're just looking at the front end,
the business end of the instrument here and the front cover hasn't been
placed on.

And you see our little parabolic reflector, gold-coated parabolic
reflector with the little 20-watt lamp inside, and just to the right of
that you see the three lenses for the camera that's looking down.

Above that you see the detector for the sun sensor that's going to tell
when we've gone by the sun and various other fibers that run around this
whole system for the visible and the infrared spectrometers that look up
and down in the solar aureole camera.

So it's pretty messy, but - and this way we've got just as much into the
data stream as we possibly could.

And then the next slide shows the whole thing - the whole sensor head
all enclosed in the package. There's a little bit of black sponge rubber
foam material on the right side of the instrument. That's the part that
sticks out into the atmosphere. We didn't want to have a big heat leak
and suck all of the heat out of the probe, so we insulated the front of
the instrument just a little bit.

You can also see the copper strap in about the middle of the box. That's
showing the copper colored piece. That's eventually going to get bolted
to another copper strap that goes out and attaches to the outside of the
probe so that we can keep the focal plane nice and cold.

The center head weighed 3-1/2 kilograms and the mass was 3-1/2 kilograms
out of which 1 kilogram was radiation shield around the detectors. We
didn't want to have cosmic rays or energetic neutrons from the
radioactive generators bombarding our detector and causing lots of noise
in the system. So we used 1 kilogram of radiation shield in the device.

And that turned out to be really very helpful and very necessary \--
even with that 1 kilogram of radiation shield in the device, we could
see the characteristics of the instrument begin to change from the
moment the instrument was placed in the same room with the Cassini
orbiter.

It didn't start to degrade at launch, but it degraded a month earlier
when the two were in the same room and then we could watch the -
basically the dark current in our device gradually rise with time pretty
linearly over the whole seven years of the cruise.

Even though the dark current rose with time, we could get the instrument
so cold when we got to Titan that the dark current was really not a
problem for us.

Okay, so that gives you a little overview of the instrument. So it's
more than just a camera. It's several cameras that look in different
directions. It's two spectrometers that look both up and down. It's an
ultraviolet photometer that looks up and down and it's the sun sensor
and it's the hardware data compressor. So it's a fairly capable package
in a few kilograms and not too many cubic inches of space.

The next set of slides show several images of Titan taken by our
instrument. There's an array of two across by three down images. And we
could project these images in various different ways, but here we've
done sort of a skydiver's eye view and we have them projected as circles
where the center is straight down and as you go out through the circle
you're going more up toward the horizon.

Often you want high magnification in an optical camera that you're going
to use even on a planetary mission. The imaging science system cameras
on the Cassini Orbiter have very powerful telescopes in front of them
but we don't need high magnification. We don't want a small tiny field
of view with high magnification. We're actually there in the atmosphere,
so we're pretty close to what we're looking at. We don't need more
magnification.

What we want is a wide field of view. We don't want to come down near
something like the Grand Canyon and not happen to be looking over our
shoulder towards where it is and miss the whole object.

So the philosophy here is to build a camera that has the resolution of
the human eye or maybe a little more or maybe a little less, but has
this tremendous field of view so we can see everything that we're
looking at; everything that's in the atmosphere.

So the upper left-hand one is at a high altitude; it's at about a 150
kilometers altitude. And from that altitude, we're completely - the
surface is completely obscured by this photochemical haze in the
atmosphere. That's the haze that causes so much difficulty in seeing the
ground clearly from orbit or even from the earth.

But as we gradually fell closer and closer, we could see the ground
start to become more clear through the haze. The upper right one is at
an altitude of 20 kilometers and starts to show some bright and dark
features and actually there's a little dark haze layer if you look right
out toward the horizon. It's very thin, but you can see that haze layer
there.

We think that's probably some methane condensation at that altitude.

And then the next two down show successively lower ones, the left one is
six kilometers altitude, the right one is two kilometers altitude and
you can see how the scene changes as you get lower and lower and then
finally the bottom two in this display are at even lower altitudes.

Let's go to the next slide which takes one of these and blows it up so
you can actually see a little more clearly what we saw there. This is
the one from six kilometers. And you see this bright terrain on the
upper left-hand part of the scene, but they're covered with these sort
of dendritic drainage channels. And the fact that they're dendritic
means we think they're caused by a distributed - a horizontally
distributed source and we think most likely that's due to methane rain.

We think methane rain cuts these channels with steep sides through the
terrain and then these little creeks and riverbeds run out towards this
dark region in the center of the slide which we think is a dark dry-like
bed that originally was filled with flowing methane but the methane
seeps into the ground or evaporates and we think when we landed it
wasn't there.

The next slide shows a different kind of a projection of the scene but
again, you can see these drainage channels in the center, top third and
sort of center in this bright area, and you can see down into the
lakebed.

In the lower right quadrant of this image, you can see evidence of fluid
flow in the lakebed. You can see these little dark lines and streaks
where the fluid was flowing and oozing along in kind of the muddy lake
bottoms just before it evaporated or sank into the ground.

And we'll have more to say about that and I urge you to look at the
movie, if you have a chance after the talk is over to kind of download
the movie and take a look at that and see what you think of the movie.

Well, because we could see the terrain not only from one perspective,
but the instrument drifted over the surface we could actually see the
ground from different perspectives.

This next slide shows an image of the drainage channels on the right in
a color-coded scheme where what we think the lowest material is is kind
of blue and the higher material gets more red. And then on the left side
we have an elevation terrain map that was constructed from stereographic
images looking in several different directions; several different
perspectives of seeing the ground.

Now we can see that the highest terrain is about 150 to 200 meters high
and the lowest terrain here is about 20 meters below the reference level
that we've chosen for this display. And actually it turns out that the
drainage channels do run downhill and they run toward this dry lakebed
that's at the sort of the zero or minus 10 or 20 meters elevation,
whereas the upland is 150 meters or so higher in altitude.

So the production of these terrain maps was actually a key objective of
the experiment, to not only see what the ground looks like but actually
be able to see how the - what the elevations are of the terrain.

Now, the next slide shows an image taken on the ground. We weren't sure
the probe would survive landing, but it actually did. And I think one of
the reasons is it landed in this soft muddy lake bottom where the mud is
not due to silicates but probably due to ice chips and where these rocks
and boulders that you can see in this image on the left are rounded due
to tumbling during the fluid flows; periods when the fluid is flowing
more rapidly.

On the right side of this image we have an Apollo image of the moon
taken with the same perspective or actually mapped into the same
perspective so you can get an idea of the size of the man on the horizon
and the size of the footprint in the foreground so you can get a feeling
for the size of these cobbles or these rocks - ice rocks here. They're
about six inches across.

And then down in the bottom right-hand corner of the image, you can see
the spot where our landing light was illuminating the ground very
strongly when we got down to this surface.

So I think the images were really very, very interesting, very
spectacular. We have images with resolutions of - got ground image as
some parts are down at the millimeter scale and we certainly have good
resolution at the centimeter scale.

And so it gives us a view of the ground and a view of the surface of
Titan that is just impossible from outside the atmosphere. Right now,
even with the radar image we get down to a kilometer or a couple of
hundred meters, but we don't get a scene like that with the rocks and
the cobbles.

Actually, it looks remarkably earthlike I think. In fact, my backyard in
Arizona before we put any - made any changes to it, looked just about
like that image. So it's remarkably earth-like and the same kind of
processes are happening on Titan as they're happening on the earth, it's
just that the fluid is not liquid water, it's liquid methane. And the
surface is not silicates but is probably water, ice, frozen hard as a
rock.

The next slide shows a spectrum, that is we have reflectivity on the
vertical scale and wavelength on the horizontal scale and the black
curve is the reflection spectrum of the surface of Titan that we
measured using our landing light to completely illuminate the surface.

When I thought, boy, if we could just land there, use our lamp, measure
the reflection spectrum of the surface, we could just identify the
composition of the surface just like that, it would be just great.

Well now we've done it. We've gone there. We've illuminated the surface.
We've measured the reflection spectrum and would you believe now 2-1/2,
3 years after the encounter, we still haven't managed to duplicate that
spectrum exactly in the laboratory, so we don't know what all the
features or actually the smooth shape of the spectrum is due to, but we
have some ideas.

This depression at 1,500 nanometers to 1,600 nanometers can be due to
water ice, but water ice would also produce some bumps in the two places
indicated by the red arrows and we need something to damp out those
bumps. And here in this one we've used methane, ethane and propane ices
and then in the bottom, the red one is with some liquids.

Unfortunately we only have the liquid spectrum at the moment, longward
of 1,400 nanometers and near it looks like it's a pretty good match. But
one of the things people are actively doing the lab now is trying to get
a spectra of liquid, methane, ethane and propane at the slightly shorter
wavelength so that we can make additional comparisons with our
reflection spectrum.

But at the moment we can say that it's a complicated mix of hydrocarbons
\-- probably there's water, water ice. We know that from a variety of
pieces of evidence but the exact composition and the exact mix is still
to be determined from the measurement.

Trina Ray: Hey Marty, I had a quick question. In the slide before, the
pebbles that you see, do we know what those are made out of?

Martin Tomasco: Well, the guess is they're made out of water ice. We
think most of the crust is water ice. They're covering - probably those
pebbles are coated with the aerosol that falls out of the atmosphere and
so they have this kind of orangey color that's typical of the aerosols,
but we think that if you broke one of those open, it would be mostly
water ice with just a covering on the outside of maybe some of the
photochemically produced aerosols.

Trina Ray: Okay. Thanks. I think that's what I had heard.

Martin Tomasco: Okay, so we saw the spectrum comparing the reflection
spectrum of the ground with a couple of candidate materials. One of the
other objectives of our experiment was to be able to measure the
meridianal component of the wind; that is the east/west component of the
wind could be measured by the Doppler wind experiment to high accuracy.

But the north/south component, they didn't have a very good component of
motion in that direction. So they had difficulty determining that but as
long as DISR could see the ground, we could track our motion.

And here we have a ground track with numbers along the ground track
indicating the altitude in kilometers and you can see from 13 kilometers
down to about 8 kilometers. We drifted toward the east and then we
turned around and started to drift toward the west and slightly north.
And then on the last kilometer, it turned around again and started to
drift toward the south and toward the east.

So these ground tracks and the direction of the wind will provide some
input for meteorology people and people who are building global
circulation models about the extent of boundary layers and just how the
winds move in the very low surface.

Actually we could see the ground from about 50 kilometers on down, and
above 50 kilometers, there's information from the long baseline - double
long baseline interferometry using radio telescopes on the earth to
measure the transmission from the Huygens to measure the meridianal
component of the wind and that data looks very interesting.

And it seems to tie into our measurements at lower altitudes pretty
smoothly. I mean you think after we're done we're going to have three
different sources of information; the Doppler wind experiment which
gives high resolution winds towards the east and west, the double long
baseline interferometry from the radio telescopes which gives meridianal
motion, particularly at high altitudes and then the DISR data that gives
meridianal and east/west motions at lower altitudes with high accuracy.

Now all of that has come together pretty well, I think.

Now in addition to learning about the surface, the nature of the surface
and the topography of the terrain, there also have as a major objective
of this instrument to learn about the earth. What are these
photochemical aerosols? How big are they? Where are they produced? Where
do they go? What's their lifecycle? How do they interact with the
radiation field and physics and chemistry of the atmosphere?

And in order to do that, we have the spectrometers the and solar aureole
camera and the polarization measurements. And the next slide shows the
maximum degree of linear polarization as a function of altitude in our
two colors of our solar aureole camera.

And what you see is that at 130 kilometers altitude or so, where we
started taking measurements, light was 60% linearly polarized, and
that's not just singly scattered light, that's including whatever
multiple scattering happened above us.

To produce 60% polarization in light that's been scattered a few times
means you have to be producing basically 100% polarization in every
scattering. And it's different in the red and the blue because the
optical depths are different. The haze thickness is basically different
in the red and the blue but that's consistent with both red and blue
datasets because you have to be able to produce essentially 100%
polarization in a single scattering due to these aerosols.

And the way it\'s turned out is that in order to produce that high
degree of polarization, the aerosols have to have some very tiny
dimensions. They can't be much bigger than about .05 microns or .06
microns in radius, so about a 1/10 of a micron across.

If the aerosols are more than about a 1/10 of a micron across, you can't
produce this high degree of polarization. So we need at least some
dimension in the aerosol that's 1/10 of a micron or less for producing
this high degree of polarization.

And the next slide shows what happens with our solar aureole camera.
This was a camera that was intended and, in fact, measured the
brightness of a vertical strip of the sky, a narrow strip, it was maybe
only 6 or 8 degrees wide, but 50 degrees long. It went from 20 degrees
from the zenith to 70 degrees from the zenith.

And as the probe rotated, we would get some of these images or some of
these maps of the brightness of the sky quite close to the sun. And this
shows a map within 10 or 20 degrees from the sun and the outlines of
constant brightness, these curved circular contours are from a model
with a particular size and shape for the aerosols and the data that we
measured is inside this red elongated roughly rectangular area.

And actually you have to look pretty hard, but if you look at the solid
black contour, you can see a little difference between what's inside the
red rectangle and what the contour looks like outside the red rectangle,
so there's a small difference there. But actually all of the contours
look like they're in very good agreement with the model.

And this thing called the solar aureole is the brightness of light
scattered in a fairly small distance away from the sun. And that high
brightness is a measure of the projected area of the particle.

And what we find is that in order to make good fits, we need something
like 3,000 of these small chunks, each about 1/10 of a micron across at
.05 microns in radius. We need some 3,000 of them stuck together so that
this particle is large enough to provide the scattering through small
angles and yet has a small enough dimension to provide the high
polarization.

This was a major mystery some years ago before Cassini was flown, and it
was one of the drivers of our design of our instrument that we could
actually understand how this all works.

The next slide shows kind of a picture of what an aerosol particle would
look like if it had 2,000 of these little monomers stuck together in a
wreath arrangement where you could still see a small dimension to
provide the high polarizations.

So this is the picture of one of those haze aerosols, if you like, with
2,000 monomers and we actually - we think they're actually slightly
larger. We think they have more like 3,000 monomers.

They think what happens is this small individual clumps of solid
material of 1/10 micron across are produced in photochemical reactions
at about 400 kilometers altitude, but then as they fall, they stick
together.

And first, a pair of them stick together to make two and then a clump of
two would collide with another clump of two and they would stick
together to make four and they would continue to grow by this binary
aggregation into these loose fluffy structures that are - that include
several thousand components.

It only takes about 10 collisions to get up to 1,000 components, 1,000
monomers and we have maybe 10 or 12 or 15 collisions that link these
particles with several thousand little bitty pieces stuck together.

So that's what we think the particles look like. And the reason we care
about what the particles look like is we'd like to understand how
radiation scatters and is transferred through the atmosphere. And if you
have - if you know something about the size and shape of the particles,
that's connected with their probability of scattering light in a single
scattering through various angles.

And the next slide shows a thing we call the phase function, the single
scattering phase function as a function of angle from 0 to 180 degrees.
And so this is the relative probability of scattering a particle in a
single scattering through 3 degrees or 5 degrees or 30 degrees or 180
degrees.

And what you can see is they have very different forward peaks and if
you had a particle with 500 pieces in it at least .05 micron pieces, it
would have a lower forward peak. If you had 3,000, it has a higher
forward peak. If it has 10,000 it has a still higher forward peak, and
the height of that forward peak is important for understanding how light
scatters in the atmosphere.

So we think that the right one is the green one there with 3,000
monomers and that tells us how the light scatters from a single
scattering event.

The next slide shows the data that we got with our spectrometer, our
visible spectrometer in particular. And on the vertical scale is the
brightness of a particular measurement in two colors; one at 529
nanometers toward the short end of our visible spectrometer, one in the
red at 830 nanometers in the near-infrared, actually, and the horizontal
scale is altitude. So as a function of altitude, this kind of double
scattering diagram shows you what the data looks like.

And when we first got the data back on the night of the descent, we had
all kinds of plans for rapidly analyzing this data but when we took a
look at it, we just were pretty puzzled. This is not what we expected to
get. This is not what we thought we would get.

But the reason for it is that our sun sensor which was supposed to make
the instrument collect data at various specific angles from the sun
didn't work exactly as it was planned. It - the probe was not
particularly stable. It had high frequency, rapid, small oscillations
which in a field would be operation of our sun sensors from time to
time. And so we got more nearly a random sampling of data at different
angles from the sun.

And this scattering, this data, is not noise. The data has 100 to 1
signal to noise ratio, so it's really high-quality data, but as the
probe rotates, as the probe rotates, we're looking toward the sun and we
have the sun in our field of view and it's very bright. And as we rotate
180 degrees away, we don't have the sun in our field of view and it gets
darker and that's really what's going on.

So before we could really understand how to understand this data or
model this data, we had to figure out how to probe the data and in the
next diagram you can see how this data looks if you plot it as a
function as azimuth angle from the sun.

And as you look at 30 degrees from the sun and at 330 degrees from the
sun, we have a field of view that includes the sun and then if you look
at 180 degrees from the sun, we don't include the sun any more. And so
all the data really goes together on this nice curve. We know where it
is.

This is an activity that took us six months to a year to figure out the
attitude of the probe. But once we knew the attitude of the probe, then
the rest of our analysis fell into place and we could model the data and
understand the vertical distribution and the size and the shape of the
aerosols pretty well.

So let's go to the next slide now. This shows the upward looking visible
spectrometer, that's ULVS, upward looking visible spectrometer. The
intensity this measured, divided by the flux from the sun as a function
of altitude in many different wavelengths. And the points of the
measurements and the jaggedly lines are a model.

And the lines are a little jagged because the probe was tipping and
tilting. And as it tipped and tilted, the sun landed on a different part
of our diffuser where it had different transmission and so the jags go
up and down.

But the data has those jags and actually the models have the same jags
because we compute it, the models for the same orientation as we had,
and so this is actually a pretty good fit, fits are good to better than
10% and all the wavelengths in the continuum, that is where there's no
methane absorption and at all altitudes. We were quite happy with this
fit.

The next slide shows the fit looking away from the sun so these are
azimuth of 180 degrees plus or minus 30 or 40 degrees, and you can see,
again, the data represented by points and the models are represented by
these solid lines that reproduce most of the jagged pieces which are due
to tips and tilts of the platform.

So not only do you have know roughly the azimuth to which you're
pointing, but you also have to know the tip towards the sun and the tip
perpendicular to the sun in order to model the data and analyze the
data.

And that's all made the data analysis rather more difficult, but finally
we're at the stage where we think we have a very good model.

In addition to looking up toward the sun and away from the sun, we could
look down and the next slide shows the downward looking visible
spectrometer; the intensity divided by the flux from the sun, plot it
against altitude for all the continuum wavelengths.

And if you could look both up and down, then you can measure the amount
of energy absorbing into these layers and you can get the amount of
energy absorbed in that slab. And so we thought that was important to
do, as well, and we did that on this experiment.

And then finally the next slide shows the spectrum on a downward looking
visible spectra at many different altitudes according to that key on the
top of the slide, and you can see that agreement between the points and
the curves is really pretty good.

And these features are due to methane absorption in the atmosphere that
are gaseous absorption features whereas the wavelengths that I've shown
before have just been at the peaks of the spectra, that is where there
is no gaseous absorption where it's just due to the aerosol themselves.

So that's the downward looking spectra. And the next slide shows the
upward looking spectra, and this is on a linear scale and the top one
shows the more or less continuous spectrum without much methane
absorption working up toward the sun when you're at high altitude.

And then as you gradually fall deeper and deeper in the atmosphere, the
methane features get stronger and stronger and we can actually reproduce
them with our model and then understand the distribution of methane and
the absorption through the methane, as well as the continuum properties
of the aerosols.

The next slide shows the downward and upward looking spectra in the
infrared at continuum locations, and why don't you just move ahead to
the next slide after that which shows the upward and downward looking
spectra in the infrared. Actually this is the upward looking spectra in
two halves, the measurements of the points and you can see the error
bars on the logarithmic scale get big when you get to very dark level.

And the next slide after that shows the downward looking spectra, again,
in two panels at the short half of this spectrum and then the long half
of the spectrum.

And then we have a model that fits in the continuum looking up and
looking down, in the methane bands looking up and looking down, and
really allows us to understand what's going on with the aerosols.

Okay, so the next slide after that shows the optical depth of the
aerosols as a function of wavelength where the aerosols are broken into
three vertical regions; above 80 kilometers, between 30 and 80
kilometers and under 30 kilometers.

And the optical depth is kind of - if the optical depth is one, then you
have about a 30% chance of a photon getting through that layer without
hitting an aerosol particle. If the optical depth is two, then you have
about a 10% chance of a photon getting through. If the optical depth is
3, then you have maybe just a few percent - a 3% chance of a photon
getting through without hitting an aerosol particle.

Well it's interesting how these curves vary with wavelength. The red
curve at the highest altitude varies most deeply with wavelength and the
steep variation with wavelength is an indication of a small aerosol
particle. And then on the next layer, from 30 to 80, the curve is less
steep indicating slightly larger particles, and in the lowest region,
under 30 kilometers, the curve is the least deep, and that's an
indication of the largest particles yet.

Now let's move to the next curve. Okay, and this curve shows how the
brightness of the aerosol particles are actually what's called a single
scattering albedo, which is the fraction of the energy that's reflected
from a single bounce off an aerosol particle as a function of
wavelength.

And what we see is that at high altitudes, we have the red curve where
the particles are a little bit darker. At lower altitudes we have a
higher curve, where the particles are a little bit brighter. And we
think the particles are brighter at the lower altitudes because there's
a little bit of condensate material that's been coating the particles
and so the light can't get into the particle and get absorbed.

The absorption is strong in the blue end and the red end, well, there's
a peak in the curve and then drops off longward of about 800 nanometers
or so. And that's actually the same shape as we see in the surface
spectrum, so I think the drop-off in the surface and the drop-off in the
aerosols is related, and I think the aerosols coat the surface and
probably cause that drop-off in the surface.

So it may be that the surface spectrum is not telling us so much about
the ices that are there, the clean ices that are there, but is telling
us about the photochemical material in the aerosols and I think that's
something we're going to have to include in the modeling work in the
future.

Okay, and then the next slide shows the optical depth of the atmosphere
as a function of altitude for all these different wavelengths. And what
you can see is that the blue end at 400 nanometers or 350 nanometers,
the atmosphere is very thick there, optically thick. There is no chance
for a photon to get all the all the way through, but as you go to longer
and longer infrared wavelengths, the optical depth gets less and less
and you have a better chance to get through.

It's interesting to look at the curve for 934 nanometers, the green long
dot-dash curve and that shows that the total thickness of the atmosphere
at 934 nanometers which is the wavelength of the imaging camera on
Cassini. The total optical depth of the atmosphere is about 3. So the
chance of the photon getting through that atmosphere of 3 with 3 optical
depths, there's only about 3%.

So about 3% of the light that the camera is looking at comes right from
the point on the ground that's in a straight line with the camera and
the rest of the light. Ninety-seven percent of the light has undergone
at least one scattering and has come from some distance away and then
acts to blur the view.

So that's the reason that it's so difficult to see the ground clearly
from - even from the Cassini optical camera in orbit, which is fairly
close. We'd have really very high spatial resolution, but three optical
depths of haze makes it difficult to see the ground very clearly from
orbit.

The next slide shows a numbered entity of aerosols as a function of
altitude. And above 80 kilometers, we see a red and a blue curve that go
together and it goes to a peak of about 5 particles per cubic
centimeter.

And that calculation was done for the shade particles that I showed when
I showed the picture of the shape of the particles and that all works
just fine but below 80 kilometers, we think that the particles are
somewhat coated with condensate material. That makes them shinier and
also makes them scatter a little differently than the exact shape of
that particle that I showed you, so the red and the blue curve diverge.
If you use light from the red end of the spectrometer from the blue end
of the spectrometer, you conclude a slightly different number density.

Well there's only one real number density, but the particles are a
complicated shape with a little coating of liquid on them so they're
very difficult to make in fact calculations for.

In the lower 30 kilometers of altitude, we think the particles are still
larger and that they look like different sizes they would get different
number densities. If they were sort of three microns, spherical clumps
of stuff then you'd have several tenths of a particle per cubic
centimeter. If they were 10 micron clumps, then you'd get several
hundredth of a particle per cubic centimeter.

So there's a family of solutions there between size and the number
density. But we know the opacity and we know how they obscure the light.

Okay, so that really concludes what we've learned about the aerosols. We
know their vertical distribution, we know their size, we know their
shape, we know their number density, we know something about their
optical properties and the scattering properties.

Another part of the experiment was to learn about the gases in the
atmosphere, particularly methane. The methane in the atmosphere is at
very cold temperatures and it's for very long paths as an awful lot of
methane molecules along the path.

And that's a situation that's very difficult to simulate in a laboratory
on the earth. You can make a methane cell several hundred meters long.
You can have many bounces so you have a very long path, you know, half a
kilometer or a kilometer long, but it's difficult to cool such a cell
isothermally enough so that all the bounces survive.

And so what you really have in the lab is a small cell that's cool to
cold temperature or you have a large cell that's at one temperature and
it's very hard to measure the properties, the methane under the right
path when it's not the right temperature.

And we thought that one thing we could do with our data is since we know
how methane bands form in Titan's atmosphere, that's exactly the case
we're interested in, that's the long paths and the cold temperatures. So
we should be able to figure out an improved methane absorption
coefficient for these cases which would be useful then for studying
other planets in the outer solar system.

So this slide shows the upward looking spectrum as points on the left
panel and the model for how the methane would absorb has solid curves,
and this is from laboratory measurements that were extrapolated to Titan
conditions. And what we see is they're not a particularly good fit.
They're not a good fit in the near continuum weak regions and are not a
good fit in the strong regions, either.

But on the right panel you can see the methane absorption coefficient
that we've derived from the measurements in Titan's atmosphere and they
can make a very good fit.

And so we've published this and we hope that it will be useful for
modeling the methane in the atmosphere of the other outer planets.

The next slide shows that in the visible, as well, and there's one band
in the visible in the near-infrared and is visible where again, the dash
curves are the old standard numbers and the solid curve and the points
are their revised numbers that we've got from the DISR data. So there's
been significant improvement in getting the methane absorption
coefficient.

The next slide shows the spectrum of the ground when the ground was
illuminated with our lamp from an altitude of 25 meters and it's
compared with different methane mixing ratios near the surface. Now at
25 meters, now just in talking about how we don't have good laboratory
measurements in methane, well we don't have good laboratory measurements
in methane where the pressures are appropriate to the stratosphere where
the pressures are some millibars.

It's difficult to get with low pressures a lot of methane in the cell
but you can increase the pressure to a bar or five bars or something
like that and measure the methane absorption characteristics.

And if you're only 25 meters altitude on Titan, the pressure there is
1-1/2 bars and there the data are in good agreement with the laboratory
measurements. And so now all you have to do is vary the methane mixing
ratio and see how much methane you have. And the points are the
different measurements, the curves are for different methane mixing
ratios is 4.7, 5.1 and 5.5% methane.

And what we derived in showing the bottom of that slide, we get 5.1 plus
or minus or the less than 1% methane mixing ratio near the surface, and
that's in very good agreement with the direct measurements made by the
gas chromatograph and the mass spectrometer on the Huygens Probe.

So we think these optical measurements are in good agreement with the
actual measurements at low levels and we then trust the direct
measurements throughout the atmosphere so we know the amount of methane
in the atmosphere and then we get the methane absorption coefficient
from the DISR; from the DISR data.

And finally, in addition to methane as one we're saying we can do with
these kinds of data and that is if we now know if we now have a model of
the aerosols on Titan, we know how they're vertically distributed, we
can calculate how they absorb energy, and we can get the solar heating
and we can get the heat balance of the planet. And we can compare the
solar heating with a thermal infrared cooling and get the net radiative
forcing which is what drives the wind from a dynamics in Titan's
atmosphere.

And so this first slide shows the energy depth position rate and watts
per cubic meter for micron and wavelength at different altitudes. And
you can see that at high altitudes, the red curve, it's mostly due to
the aerosols, but when you get down to the lower altitudes, 15
kilometers, it's mostly due to the methane band.

And we know the mix of how much is due to methane, how much is due to
aerosols and how much occurs in all these different wavelengths so we
can calculate the total of the heating rate.

And the next slide shows the heating rate averaged over the disk
compared with the cooling rate measured by the infrared spectrometer on
the Cassini orbiter. And averaged over the disk, the heating and the
cooling rates are in pretty good agreement as we can see on this next
slide.

And the slide after that we show the heating rate and the cooling rates
at the latitude of Huygens' entry and there the solar heating rate is in
degrees Kelvin per 24 - well, for 24 hour day and the thermal cooling
rate is in degrees Kelvin for 24 hour day.

And you can see that the heating exceeds the cooling and so the
atmosphere would be warming up and gas would be expanding and then it's
driven to flow into another region and that's the forcing for the wind
field.

They've done that at the landing site and the next curve actually shows
the net heating minus cooling and you can see that's of the order of 1
degree Kelvin per Titan day. Of the order of a degree Kelvin per Titan
day and we have the profile as a function of altitude.

And then if you have that, then you can calculate the net rated of
heating and cooling at all altitudes and at different parts of the disk
for different latitudes and so I have two slides that show that, but
let's just skip through those sort of quickly.

So it was actually both slides need to be recalculated including the
right aerosol distribution at the different parts of a planet which
we're now getting from the Cassini orbiter, so those have to be redone.

What the last slide shows a summary of results from our DISR experiment.
The first point is that the images of the surface show evidence of rain
and fluid flow. Rain had been speculated before, but to actually see the
river channels and the drainage basins is really very reassuring and
really a good feeling.

We made measurements of the heights in the terrain because we have
stereographic baselines we can look at the same terrain from different
points of view. We have measurements of the reflection spectrum of the
surface which eventually will tell us about the composition of the
surface, although that's a work still in progress.

We've made measurement of the zonal and meridianal winds and that
compliments the Doppler wind experiment which measured the zonal wind
and the double baseline interferometry which make good measurement at
high altitude.

We have measurements of the size, optical properties in vertical
distribution in the haze aerosol, understanding their production and
lifecycles. We have measurements of methane absorption coefficients from
(long paths) at low temperatures which is really useful for analyzing
the atmospheres of the other planets.

We have measurements of the methane mixing ratio near the surface which
confirmed the measurements made by the gas chromatograph, and finally
we've computed the solar heating rate and the thermal cooling rates and
compared them at the landing site for studies of a heat balance of the
Titan atmosphere and the drives for the wind field.

So that's a summary of the DISR results and was quite an ambitious
experiment with cameras and spectrometers and sun sensors and data
compressors and so on, but I think it was actually highly successful.

It did pretty much everything we had in mind for the experiment to do.
And in addition, returned the nifty movies that you have a chance to
download and look at in your leisure.

Well that concludes what I was planning to say and if there are any
questions that anybody cares to ask at this point, I'm happy to try to
answer them or otherwise you can go download the movie and take a look
at the movie.

Trina Ray: So do we have any questions?

Martin Tomasco: Well it was pretty technical, I understand it was a
pretty technical talk but I was told it's a really technically savvy
audience, but...

Trina Ray: Well, it was challenging but very, very interesting Marty. I
guess I have sort of a question. What do you think - I mean, this really
was a very ambitious experiment and as you say, just remarkably
successful.

What do you think is sort of the big question that you have about either
Titan surface or atmosphere, you know, coming sort of out of the Huygens
Probe mission? What do you think sort of is the big question?

Martin Tomasco: Well I think there are a couple of questions that remain
that are really quite important and quite interesting to understand. One
is we see now that it rains. One question is how often does it rain and
where does it rain. We know there are lakes at high latitudes but we
don't see actual rain failing on the DISR site, so where does it rain
and how often does it rain is an important question.

The other thing is how does the methane actually get out of the surface
and get back up into the atmosphere? It can't just get from those small
lakes at high latitudes. I think there has to be another mechanism for
getting the methane out of the interior.

And finally, what's the composition actually of the surface? And what's
the composition of the aerosols exactly? People have made laboratory
experiments and tried to simulate the aerosols but we'd like to really
fit that reflection spectrum of the surface in greater detail to
understand the composition a little better.

So those are three kinds of questions that are kind of important.

And finally, we know what the terrain looks like around the probe
landing site, but you know, what we've seen from the radar images and
from the images of camera on Cassini, the terrain varies an awful lot on
Titan.

You have places where you have sand dunes. You have places where you
have river channels, you have lakes, you have drainage basins. How does
all of this sort of active - evidence of active geology, how does that
all work together and how is the surface formed and what are the surface
processes that are still ongoing and still forming the surface. So
that's another set of questions that deserves a little more study, I
think.

Trina Ray: Those are big questions.

Martin Tomasco: Yeah, yeah.

Trina Ray: We definitely need another mission to Titan.

Martin Tomasco: That would be really great. It would be really great.
You can imagine sending one probe to the earth and having it land either
in the Pacific Ocean or in a dessert or in a rain forest and you might
draw very different conclusions about what the earth was like if you
only had a single probe.

So maybe we can use a few more probes or something on a balloon that
would be blown around by the wind and get close up looks at other places
or maybe higher resolution radar. There are a lot of things you could
imagine for another mission to Titan that would be really important and
interesting.

Man: I have a question. The fact that one of the data channels was
nonfunctional during the descent, did that impact your experiments at
all?

Martin Tomasco: It did impact us. It was recommended that we put the
same data on both channels. And actually all of our spectral data was
put on both channels and we got all of that spectral data.

But as far as the images go, we could have put 350 images on the first
channel and the same 350 images on the second channel, but we really
thought that was not maybe the best things to do. We thought, well, if
one channel fails, we'll still have 350 images. If they both work, we'll
have the same 350 twice. You know, what good is that?

So we put different images on the two channels. We put 350 on the first
and a different 350 on the second. And because we lost the second, we
still got the same number of images, but we didn't have as much
overlapped and we couldn't do the three-dimensional terrain height
analysis of as many places in the scene as we would have been able to if
would have had both channels working.

So that was the biggest impact for us. We got pretty good spatial
coverage, but we didn't get the double coverage that we would have
needed if we had really wanted to do or tried to do a stereographic
analysis from different pieces of terrain.

So we got a few places where we could the 3-D stereographic analysis but
not as much coverage of the 3-D stuff as we would have wanted.

That was the biggest impact.

Man: Thank you.

Trina Ray: Do we have any other questions?

Okay, well with that, we will thank Marty for joining us today. We
really appreciate it. It was really a terrific talk, Marty and we
appreciate you taking your time.

Martin Tomasco: You might mention where that Web site is or maybe they
have it in their information about where the movie is.

Trina Ray: In fact, I will mention that again. There is a video that
goes with this presentation and it's on the password protected site. And
everyone should take a look at the original email that they received.
The information for logging into the password protected site is on
there.

And if you have any trouble, you can contact the
charm_leads_01@cdsa.jpl.nasa.gov.

And we will have a very interesting topic next month. I don't know what
it is but I'm sure that Dr. Amanda Hendrix has planned something
spectacular.

And with that, we'll let everybody go and we will see you next month.

Martin Tomasco: Thanks very much Trina.

Trina Ray: Thank you Marty.

Man: Thank you.

Trina Ray: Fantastic.

END