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Cassini stares into the eye of monster storm on Saturn
NASA NEWS RELEASE
Posted: November 9, 2006
NASA's Cassini spacecraft has seen something never before seen on another planet -- a hurricane-like storm at Saturn's South Pole with a well-developed eye, ringed by towering clouds.
The "hurricane" spans a dark area inside a thick, brighter ring of
clouds. It is approximately 5,000 miles across, or two thirds the
diameter of Earth.
Cassini stares deep into the swirling hurricane-like vortex at Saturn's south pole, where the vertical structure of the clouds is highlighted by shadows. Such a storm, with a well-developed eye ringed by towering clouds, is a phenomenon never before seen on another planet. Credit: NASA/JPL/Space Science Institute
Download larger image version here; A movie is available here
"It looks like a hurricane, but it doesn't behave like a hurricane,"
said Andrew Ingersoll, a member of Cassini's imaging team at the
California Institute of Technology, Pasadena. "Whatever it is, we're
going to focus on the eye of this storm and find out why it's there."
A movie taken by Cassini's camera over a three-hour period reveals
winds around Saturn's South Pole blowing clockwise at 350 miles per
hour. The camera also saw the shadow cast by a ring of towering
clouds surrounding the pole, and two spiral arms of clouds extending
from the central ring. These ring clouds, 20 to 45 miles above those
in the center of the storm, are two to five times taller than the
clouds of thunderstorms and hurricanes on Earth.
Eye-wall clouds are a distinguishing feature of hurricanes on Earth.
They form where moist air flows inward across the ocean's surface,
rising vertically and releasing a heavy rain around an interior
circle of descending air that is the eye of the storm itself. Though
it is uncertain whether such moist convection is driving Saturn's
storm, the dark "eye" at the pole, the eye-wall clouds and the spiral
arms together indicate a hurricane-like system.
Distinctive eye-wall clouds have not been seen on any planet other
than Earth. Even Jupiter's Great Red Spot, much larger than Saturn's
polar storm, has no eye or eye-wall, and is relatively calm at the
These images of Saturn's south pole were taken by two different instruments on Cassini. The four monochrome images displayed here were acquired by the imaging science subsystem; the blue and red images in the bottom row were taken by the visual and infrared mapping spectrometer. The images are arranged in order of increasing wavelength in nanometers as follows: (top row) 460 nm, 752 nm, 728 nm; (bottom row) 890 nm, 2,800 nm, 5,000 nm. Credit: NASA/JPL/Space Science Institute/University of Arizona
Download larger image version here
This giant Saturnian storm is apparently different than hurricanes on
Earth because it is locked to the pole and does not drift around like
terrestrial hurricanes. Also, since Saturn is a gaseous planet, the
storm forms without an ocean at its base.
In the Cassini imagery the eye looks dark at light wavelengths where
methane gas absorbs the light and only the highest clouds are
"The clear skies over the eye appear to extend down to a level about
twice as deep as the usual cloud level observed on Saturn," said
Kevin H. Baines, of Cassini's visual and infrared mapping
spectrometer team at NASA's Jet Propulsion Laboratory, Pasadena,
Calif. "This gives us the deepest view yet into Saturn over a wide
range of wavelengths, and reveals a mysterious set of dark clouds at
the bottom of the eye."
Infrared images taken by the Keck I telescope in Mauna Kea, Hawaii,
had previously shown Saturn's South Pole to be warm. Cassini's
composite infrared spectrometer has confirmed this with higher
resolution temperature maps of the area. The spectrometer observed a
temperature increase of about 4 degrees Fahrenheit at the pole. The
instrument measured high temperatures in the upper troposphere and
stratosphere, regions higher in the atmosphere than the clouds seen
by the Cassini imaging instruments.
The Cassini data presented in this view appear to confirm a region of warm atmospheric descent into the eye of a hurricane-like storm locked to Saturn's south pole. The view shows temperature data from the Cassini spacecraft composite infrared spectrometer overlaid onto an image from the imaging science subsystem wide-angle camera. Credit: NASA/JPL/Space Science Institute/GSFC
Download larger image version here
"The winds decrease with height, and the atmosphere is sinking,
compressing and heating over the South Pole," said Richard
Achterberg, a member of Cassini's composite infrared spectrometer
team at NASA's Goddard Spaceflight Center, Greenbelt, Md.
Observations taken over the next few years, as the South Pole season
changes from summer to fall, will help scientists understand the role
seasons play in driving the dramatic meteorology at the south pole of
The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. The Jet
Propulsion Laboratory manages the Cassini-Huygens mission for NASA's
Science Mission Directorate, Washington.