New Clouds Add to Titan's Mystery

http://www.gemini.edu/index.php?opti...sk=view&id=115

New Clouds Add to Titan's Mystery

Wednesday, 15 December 2004

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Using adaptive optics on the Gemini North and Keck 2 telescopes on
Mauna
Kea, Hawai'i, a U.S. team has discovered a new phenomenon in the
atmosphere of Saturn's largest moon Titan.

Unlike previous observations showing storms at the south pole, these
new
images reveal atmospheric disturbances at Titan's temperate mid
latitudes - about halfway between the equator and the poles. Explaining
the unexpected activity has proven difficult, and the team speculates
that the storms could be driven by anything from short-term surface
events to shifts in global wind patterns.

[Images]
Astronomers using Gemini North telescope and the Keck II observatory
observed these mid-latitude features at Saturn's largest moon, and the
solar system's second largest moon, Titan. All images are from
NIRI/Altair on Gemini North except December 2003 and September 2004,
which are from Keck II. Sub-observer longitude and latitude are given.
All images are scaled to show Titan at the same size, although its
angular diameter ranged between 0.73-0.88 arcseconds. In the
stratospheric probing images only the limb-brightened stratospheric
haze
is visible, with a seasonal north-south asymmetry. In the tropospheric
probing images the stratospheric hazes appear limb-brightened, with a
general brightening in the south due to the tropopause cirrus, the
distinct south polar clouds (see especially 9-April, although a cloud
is
near the south pole in every one of these images), and the new ~40
degrees south clouds, which are especially apparent on 8-9 April, 4
May,
and 2 September. The new temperate-latitude clouds are indicated with
white arrows. In the surface-probing images Titan's 22.5 degree-per-day
rotation rate is apparent and the tropospheric clouds are also evident.

"We were fortunate to catch these new mid-latitude clouds when they
first appeared in early 2004," said team leader Henry Roe (California
Institute of Technology). "We are not yet certain how their formation
is
triggered. Continued observations over the next few years will show us
whether these clouds are the result of a seasonal change in weather
patterns or a surface-related phenomenon."

The causes of these storms might include activities that disturb the
atmosphere from the surface. It's possible that geysers of methane
"slush" are brewing from below, or a warm spot on Titan's surface is
heating the atmosphere. Cryovolcanism - volcanic activity that spews an
icy mix of chemicals - has also been suggested as one mechanism that
would
cause disturbances. It's also possible that the storms are driven by
seasonal shifts in the global winds that circulate in the upper
atmosphere. Hints about what is happening on this frigid world could be
obtained as the Huygens probe from the Cassini mission drops through
Titan's atmosphere in mid-January, 2005.

The Gemini-Keck II observations were the result of good timing and
telescope availability. According to Gemini scientist Chad Trujillo,
Titan's weather patterns can be stable for many months, with only
occasional bursts of unusual activity like these recently discovered
atmospheric features. The chances of catching such occurrences depend
largely on the availability of flexible scheduling like that used at
Gemini. "This flexible scheduling is absolutely critical to Titan
meteorology studies," he said. "Imagine how hard it would be to
understand the Earth's diverse meteorological phenomena if you only saw
a weather report a few nights every year."

Like Earth, Titan is surrounded by a thick atmosphere of mostly
nitrogen. Conditions on Earth allow water to exist in liquid, solid, or
vapor states, depending on localized temperatures and pressures. The
phase changes of water between these states are an important factor in
the formation of weather in our atmosphere. Titan's atmosphere is so
cold that any water is frozen solid, but conditions are such that
methane can move between liquid, solid, and gaseous states. This leads
to a methane meteorological cycle on Titan in analogy to the
water-based
weather cycle on Earth.

As it does on Earth, seasonal solar heating can drive atmospheric
activity on Titan, and this could be the mechanism behind the
previously
observed south polar clouds. However, the new temperate-latitude cloud
formations cannot be explained by the same solar heating process If a
seasonal circulation shift is causing the newly discovered features,
the
team theorizes that they will drift northward over the next few years
as
Titan's year progresses through the southern summer and into autumn. If
it is being caused by geological changes, such as methane geysers or a
geologic "warm" spot on the surface, the feature should stay at the
observed 40-degree latitude as the surface activity spurs changes in
atmospheric convection and methane cloud formation. Continued storm
formations will be easily distinguishable in future ground-based
observations using Gemini, Keck and other adaptive-optics enabled
telescopes.

"Using adaptive optics from the Earth allows us to see things that just
a few years ago would have been invisible," said Keck Scientist Antonin
Bouchez. "These observations show that ground-based telescopes are a
perfect complement to space missions like Cassini."

This research is scheduled for publication in the January 1, 2005 issue
of the Astrophysical Journal

http://www.journals.uchicago.edu/ApJ....abstract.html
..