http://ciclops.lpl.arizona.edu/view.php?id=912

CASSINI IMAGES DISCOVER A WINDY, WAVY TITAN ATMOSPHERE

MEDIA RELATIONS OFFICE
CASSINI IMAGING CENTRAL LABORATORY FOR OPERATIONS (CICLOPS)
SPACE SCIENCE INSTITUTE, BOULDER, COLORADO
http://ciclops.org


Preston Dyches (720) 974-5823
CICLOPS/Space Science Institute, Boulder, Colo.

For Immediate Release: March 9, 2005

CASSINI IMAGES DISCOVER A WINDY, WAVY TITAN ATMOSPHERE

The dynamic atmosphere of Saturn's haze-enshrouded moon Titan is
revealed in the first Cassini Imaging Team report on Titan, to appear
in
the March 10 issue of Nature.

Imaging scientists, analyzing images of Titan designed to allow views
of
the surface and lower atmosphere, have discovered that the winds on
Titan blow a lot faster than the moon rotates. In contrast, the jet
stream of Earth blows a lot slower than the surface of our planet
moves.

Titan is a particularly slow rotator, taking 16 Earth days to make one
full rotation. Yet, despite its slow period, model simulations made a
decade ago predicted that winds in its atmosphere should blow faster
than its surface rotates, making it, like its slowly rotating cousin
Venus, one of the solar system's "super-rotators".

"It has long been known that winds in Venus' atmosphere blow many times
faster than the solid planet itself rotates," said imaging team member
Dr. Tony DelGenio of NASA's Goddard Institute for Space Studies, or
GISS, in New York, who made the first computer simulation predicting
Titan super-rotation a decade ago. "Models of Titan's atmosphere have
indicated that it too should super-rotate just like Venus, but until
now
there have been no direct wind measurements to test the prediction," he
said.

Titan's winds are measured by watching its clouds move. Clouds are a
rare occurrence on Titan, and those whose motions can be tracked are
often small (about 100 kilometers or 60 miles across) and faint; in
other words, the clouds are too inconspicuous to be seen from Earth.
The
discovery of moving clouds required careful manipulation of Cassini
images in which cloud features are hard to distinguish through the
moon's ubiquitous haze and against the backdrop of Titan's complex
bright and dark surface. DelGenio and his associate John Barbara, also
of GISS, used Cassini images that had been taken through special
filters
designed to see through the haze to detect surface features as well as
clouds. "To discriminate clouds from surface features, I took images of
the same region at different times and subtracted them from each
other,"
said Barbara. "When I did this, time-variable clouds stood out as
regions of changing brightness."

Ten such clouds have been tracked, giving wind speeds as high as 34
meters per second (about 75 miles per hour) to the east - hurricane
strength - at an altitude somewhere in Titan's middle and lower
troposphere. "This result is consistent with the predictions of Titan
weather models, and it suggests that we now understand the basic
features of how meteorology works on slowly rotating planets," said Del
Genio.

Cassini images also reveal much larger cloud streaks - 1,000 kilometers
(620 miles) long - elongated generally east-west. These clouds occur at
preferred locations and move at only a few meters per second.
Apparently
these streak clouds originate closer to Titan's surface, perhaps from
places where methane is released to the atmosphere from below Titan's
surface, or places where wind blows over topography.

In Titan's hazy stratosphere, it looks as though modelers may have to
go
back to the drawing board. Voyager images of Titan detected a faint
detached haze layer above Titan's main stratospheric haze, at altitudes
of 300-350 kilometers (190 to 220 miles). Cassini ultraviolet images,
which are sensitive to scattering of sunlight by small particles,
detect
a similar detached haze layer, but at an altitude of 500 kilometers
(310
miles) instead.

"The change we see in the detached haze over the 25 years since Voyager
suggests that either the photochemical process that produce the
hydrocarbon haze particles, or the atmospheric circulation that
distributes them around the planet, may change with the seasons," said
imaging team member Dr. Bob West of the Jet Propulsion Laboratory, who
designed all the Titan atmosphere imaging sequences for the Cassini
mission. "It will be a challenge for models to be able to predict how
and where these detached hazes occur," he said.

Images of Titan's night side, in which high haze layers are backlit by
the Sun, surprised scientists by showing evidence of an entire series
of
haze layers. These may be evidence of gravity waves, the atmospheric
equivalent of ripples on a pond, propagating up to Titan's upper
stratosphere by disturbances that originate at lower levels. If so,
then
analysis of the properties of these waves may yield insights into the
temperature and wind profiles of Titan's stratosphere and how they
change over the course of the mission.

Images associated with this release, and information about the
Cassini-Huygens mission, are available at http://ciclops.org,
http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini.

The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. The Jet Propulsion
Laboratory, a division of the California Institute of Technology in
Pasadena, manages the Cassini-Huygens mission for NASA's Science
Mission
Directorate, Washington, D.C. The Cassini orbiter and its two onboard
cameras were designed, developed and assembled at JPL. The imaging team
is based at the Space Science Institute, Boulder, Colo.