Click on the image for movie of
Looking Saturn in the Eye
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.
This 14-frame movie shows a swirling cloud mass centered on the south
pole, around which winds blow at 550 kilometers (350 miles) per hour. The
frames have been aligned to make the planet appear stationary, while the
sun appears to revolve about the pole in a counterclockwise direction. The
clouds inside the dark, inner circle are lower than the surrounding
clouds, which cast a shadow that follows the sun.
At the beginning of the movie, the sun illuminates directly from the top,
and by the end it illuminates from the left.
The width of the shadow and the height of the sun above the local horizon
yield a crude estimate of the height of the surrounding clouds relative to
the clouds in the center. The shadow-casting clouds tower 30 to 75
kilometers (20 to 45 miles) above those in the center. This is two to five
times greater than the tallest terrestrial thunderstorms and two to five
times the height of clouds surrounding the eye of a terrestrial hurricane.
Such a height difference arises because Saturn's hydrogen-helium
atmosphere is less dense at comparable pressures than Earth's atmosphere,
and is therefore more distended in the vertical dimension.
The south polar storm, which displays two spiral arms of clouds extending
from the central ring and spans the dark area inside a thick, brighter
ring of clouds, is approximately 8,000 kilometers (5,000 miles) across,
which is considerably larger than a terrestrial hurricane.
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 load of precipitation around an interior
circular region of descending air, which is the eye itself.
Though it is uncertain whether moist convection is driving this storm, as
is the case with Earthly hurricanes, the dark 'eye' at the pole, the
eye-wall clouds and the spiral arms together indicate a hurricane-like
system. The distinctive eye-wall clouds especially have not been seen on
any planet beyond Earth. Even Jupiter's Great Red Spot, much larger than
Saturn's polar storm, has no eye, no eye-wall, and is relatively calm at
the center.
This giant Saturnian storm is apparently different from hurricanes on
Earth because it is locked to the pole, does not drift around like
terrestrial hurricanes and because it does not form over liquid water
oceans.
The images were acquired over a period of three hours on Oct. 11, 2006,
when Cassini was approximately 340,000 kilometers (210,000 miles) from
Saturn. Image scale is about 17 kilometers (11 miles) per pixel. The
images were taken with the wide-angle camera using a spectral filter
sensitive to wavelengths of infrared light centered at 752 nanometers. All
frames have been contrast enhanced using digital image processing
techniques. The unprocessed images show an oblique view toward the pole,
and have been reprojected to show the planet from a perspective directly
over the south pole.
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 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 operations center is based at the Space
Science Institute in Boulder, Colo.
For more information about the Cassini-Huygens mission visit
http://saturn.jpl.nasa.gov/. The Cassini imaging team
homepage is at http://ciclops.org.