Stormy Weather: Titan's Engimatic Cloud Band is Convective

STORMY WEATHER: TITAN'S ENGIMATIC CLOUD BAND IS CONVECTIVE
>>From Lori Stiles, UA, University Communications, 520-621-1877
October 20, 2005

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Contact information is listed at the end of the news release
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University of Arizona scientists say that the peculiar clouds at middle
latitudes in Titan's southern hemisphere may form in the same way as
distinct bands of clouds form at Earth's equator.

"Titan's weather is very different from Earth's," said UA associate
professor Caitlin Griffith. "If you walked past Titan's
minus-40-degree-latitude line, you might be showered with liquid
natural
gas. If you decided to visit Titan's south pole, you might encounter a
storm
the size of a hurricane which also consists of methane, more commonly
known
as natural gas," Griffith said. "Otherwise, don't expect clouds on
Titan."

Titan's weather forecast has remained the same for years, and that
baffles
scientists. They don't understand why clouds a thousand miles long
stretch
over the temperate latitude.

"Imagine how curious it would be if beyond Earth's poles, clouds
existed
only at the latitude that crosses New Zealand, Argentina and Chile,"
Griffith said. "Furthermore, Henry Roe (of the California Institute of
Technology) and his colleagues find that most of these peculiar clouds
bunch
up at zero degrees and 90 degrees longitude, analogous to Earth
longitudes
southwest and southeast of the Cape of Good Hope," she added.

The highly localized nature of the clouds suggests that they have
something
to do with Titan's surface, Griffith said. Scientists think ice
volcanoes
must be venting methane -- the gas that condenses as clouds -- into
Titan's
hazy, mostly nitrogen atmosphere. Otherwise, the moon's atmospheric
methane
would have vanished billions of years ago because methane is destroyed
by
ultraviolet sunlight.

Griffith, Paulo Penteado and Robert Kursinski of UA's Lunar and
Planetary
Lab studied the origin of the clouds by analyzing cloud height and
thickness
using images from Cassini's visual and infrared mapping spectrometer
(VIMS).
This instrument is among a suite of instruments on the Cassini
spacecraft
orbiting Saturn. It measures light at 256 different wavelenghts.
Griffith is
a member of the UA-based VIMS team, headed by Robert Brown of UA's
Lunar and
Planetary Lab. Griffith and her colleagues analyzed images that gave
them a
3-D view of the cloud and a six-frame movie that shows how it evolved
over
three hours.

"The structure of the clouds turns out to be complicated," Griffith
said.
"We detected not one region, but many regions of cloud formation. Each
long
cloud consists of a number of vigorous storms where clouds rise to 40
kilometers altitude (25 miles) in a couple of hours and dissipate in
the
next half hour. The rate of cloud ascent and dissipation suggests that
we
are witnessing the formation of convective clouds, likely similar to
thunderstorms, that disappear through rainfall.

"Over the next several hours we see the clouds form long tails,
indicating
that strong westerly winds stretch out the clouds and carry the
particles
downwind a thousand kilometers (more than 600 miles). This detailed
look
into the structure of these clouds reveals that the clouds evolve from
a
number of small active cloud formation centers lined up like an uneven
string of beads long 40 degrees south latitude. These localized storms
cause
a healthy rain, and very long clouds, once the wind has stretched them
out."

Griffith argues that it's improbable that many ice volcanoes, all
aligned
at 40 degrees south latitude, are forming these clouds. In addition,
the
scientists estimate that the cloud activity at zero degrees longitude,
if
volcanic, does not appear to spew out enough methane to create the
mid-latitude cloud band. Smaller clouds actually lie upwind of the main
cloud at zero degrees longitude, they note. The team also conclude that
the
clouds aren't obviously caused by Saturn's tidal pull on Titan's
atmosphere.
They also don't find evidence that mountains and lakes might cause
mountain
clouds or marine clouds, Griffith said.

"We believe that it's no coincidence that Titan's south polar cap of
smog
extends from the pole to 40 degrees south latitude -- exactly where the
methane cloud band appears," Griffith said. The researchers suggest
that
global circulation may cause the air to rise at this latitutude on
Titan,
much as clouds form in a band around the Earth's equator and rain on
the
Caribbean islands. "Such rising air would cut off air from the south
polar
region from mixing with the rest of the moon's atmosphere, causing smog
to
build up and form a cap over the pole," Griffith added.

Theoretical modeling supports the UA team's conclusion, Griffith said.
Pascal Ranou and his group in Paris studied Titan's circulation with an
elaborate and complicated general circulation model. His model predicts
that
solar heating naturally creates rising air on Titan at 40 degrees south
latitude.

The next mystery is why Titan's southern mid-latitude clouds are
bunched at
zero degrees longitude. There's no evidence yet that volcanoes,
mountain
ranges or Saturn's tides are involved, Griffith said. "What's causing
the
bunching is unclear, and likely involves unknown features on Titan's
still
largely unexplored surface," Griffith said.

Griffith, Kursinki and Penteado are publishing an article on their
research
in the Oct. 21 issue of Science.

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, Calif.,
manages the 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 Visual and Infrared Mapping Spectrometer team is
based
at The University of Arizona in Tucson.
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Contact Information
Caitlin Griffith 520-626-3806 griffith@xxxxxxxxxxxxxxx
Paulo Penteado 520-621-4682 penteado@xxxxxxxxxxxxxxx
Robert Kursinksi 520-621-2139 kursinsk@xxxxxxxxxxxxxxxx

Related Web sites
NASA Cassini -http://www.nasa.gov/mission_pages/cassini/main/index.html
JPL Cassini - http://saturn.jpl.nasa.gov/home/index.cfm
VIMS - http://wwwvims.lpl.arizona.edu/
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