These images of Saturn's south pole, taken by two different instruments on
Cassini, show the hurricane-like storm swirling there and features in the
clouds at various depths surrounding the pole. Different wavelengths
reveal the height of the clouds, which span tens of kilometers in
altitude.
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.
At the center of the cauldron of storms spinning around the south pole is
the south pole itself, which literally appears to be the eye of this vast
polar storm system. As in a hurricane on Earth, the south polar "eye" is
relatively clear of clouds and is surrounded by a wall of towering clouds
that cast shadows into the center. However, while morphologically similar,
it is not clear if this vortex operates in the same fashion as a
terrestrial hurricane.
In most of the images, the center of the polar storm is quite dark,
indicating an unusually cloud-free atmosphere in the upper skies, which
are otherwise typically inhabited by bright ammonia clouds. This polar
hole in the ammonia cloud layer represents the eye of the hurricane-like
storm. Unusually dark clouds likely exist at the bottom of this deep hole,
enhancing the murkiness there.
The first image in this montage (at upper left) shows a muted eye, due to
the enhanced scattering of light from the atmosphere itself at this blue
wavelength (460 nanometers), just as in the blue skies of Earth. In the
last image at bottom right, the eye appears relatively bright. This image
is taken at a wavelength of 5,000 nanometers, where the dominant source of
light is the thermal glow of the planet itself. The bright thermal glow
seen in this polar hole again shows that the eye is relatively cloud-free
to unusual depths.
In the imaging science subsystem images, the eye looks dark at wavelengths
where methane gas absorbs the light (728 nanometers and 890 nanometers, at
upper right and lower left) and only the highest clouds are visible,
confirming that the clouds within the eye are deeper than their
surroundings. This effect is also seen in visual and infrared mapping
spectrometer images that show gas absorption.
In the visual and infrared mapping spectrometer image taken at 2,800
nanometers, four times the wavelength of light visible to the human eye,
this cloud clearing appears dark, which is consistent with the idea that
the atmosphere above any distinct clouds is unusually deep there. The eye
is some 1,500 kilometers (930 miles) across, and is surrounded by a
distinct ring of clouds some 300 kilometers (185 miles) across.
The images also indicate the prevalence of smaller but vertically
well-developed storms across the entire south polar region, indicating the
extent to which convection characterizes the area.
Literally hundreds of storm clouds encircle the pole, appearing as dark
spots in the infrared spectrometer thermal image (red image) and as both
bright and dark spots in images taken in sunlight (blue image). Each of
these spots represents a storm. These pictures reveal that Saturn's south
pole is a cauldron of storm activity, unlike anything ever seen on any
planet.
The individual storms surrounding the pole are seen as dark "leopard
spots" in the thermal image (red) taken at a wavelength of 5,000
nanometers, some seven times the wavelength of light visible to the human
eye. Here, these spots are blocking the thermal light, or heat, from the
interior of Saturn. The storm clouds are thus seen in silhouette against
Saturn's thermal glow. The effectiveness of these clouds in blocking
Saturn's interior thermal glow indicates that the storm clouds are
unusually thick, extending deep down into Saturn's atmosphere, and are
comprised of relatively large cloud particles, likely condensates formed
in upwelling air currents.
The large number of dark, circular leopard spots at the south pole seen at
5,000 nanometer wavelength, and their correlation with the features seen
in sunlight at 2,800 nanometer wavelength, indicates that convective
activity extending over dozens of kilometers in altitude is surprisingly
rampant in the south polar region. Why such unusual dynamics exist there
is perhaps linked to Saturn's southern summer, which is the season Saturn
is in now. 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
Saturn.
The images in this montage were acquired on Oct. 11, 2006, when Cassini
was approximately 340,000 kilometers (210,000 miles) from Saturn. The
original imaging science subsystem images have a scale of about 17
kilometers (11 miles) per pixel. The visual and infrared spectrometer
images have a scale of about 174 kilometers (108 miles) per pixel. The
images have been resized to approximately the same scale for presentation
here.
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. The visual and infrared mapping
spectrometer team is based at the University of Arizona where this image
was produced.
For more information about the Cassini-Huygens mission visit
http://saturn.jpl.nasa.gov/. The Cassini imaging team homepage is at
http://ciclops.org. The visual and infrared mapping spectrometer team
homepage is at http://wwwvims.lpl.arizona.edu.