Flying over the unlit side of Saturn's rings, the Cassini spacecraft
captures Saturn's glow, represented in brilliant shades of electric blue,
sapphire and mint green, while the planet's shadow casts a wide net on the
rings.
This striking false-color mosaic was created from 25 images taken by
Cassini's visual and infrared mapping spectrometer over a period of 13
hours, and captures Saturn in nighttime and daytime conditions. The visual
and infrared mapping spectrometer acquires data simultaneously at 352
different wavelengths, or spectral channels. Data at wavelengths of 2.3,
3.0 and 5.1 microns were combined in the blue, green and red channels of a
standard color image, respectively, to make this false-color mosaic.
This image was acquired on Feb. 24, 2007, while the spacecraft was 1.58
million kilometers (1 million miles) from the planet and 34.6 degrees
above the ring plane. The solar phase angle was 69.5 degrees. In this
view, Cassini was looking down on the northern, unlit side of the rings,
which are rendered visible by sunlight filtering through from the sunlit,
southern face.
On the night side (right side of image), with no sunlight, Saturn's own
thermal radiation lights things up. This light at 5.1 microns wavelength
(some seven times the longest wavelength visible to the human eye) is
generated deep within Saturn, and works its way upward, eventually
escaping into space. Thick clouds deep in the atmosphere block that light.
An amazing array of dark streaks, spots, and globe-encircling bands is
visible instead. Saturn's strong thermal glow at 5.1 microns even allows
these deep clouds to be seen on portions of the dayside (left side),
especially where overlying hazes are thin and the glint of the sun off of
them is minimal. These deep clouds are likely made of ammonium
hydrosulfide and cannot be seen in reflected light on the dayside, since
the glint of the sun on overlying hazes and ammonia clouds blocks the view
of this level.
A pronounced difference in the brightness between the northern and
southern hemispheres is apparent. The northern hemisphere is about twice
as bright as the southern hemisphere. This is because high-level, fine
particles are about half as prevalent in the northern hemisphere as in the
south. These particles block Saturn's glow more strongly, making Saturn
look brighter in the north.
At 2.3 microns (shown in blue), the icy ring particles are highly
reflecting, while methane gas in Saturn's atmosphere strongly absorbs
sunlight and renders the planet very dark. At 3.0 microns (shown in
green), the situation is reversed: water ice in the rings is strongly
absorbing, while the planet's sunlit hemisphere is bright. Thus the rings
appear blue in this representation, while the sunlit side of Saturn is
greenish-yellow in color. Within the rings, the most opaque parts appear
dark, while the more translucent regions are brighter. In particular, the
opaque, normally-bright B ring appears here as a broad, dark band
separating the brighter A (outer) and C (inner) rings.
At 5.1 microns (shown in red), reflected sunlight is weak and thus light
from the planet is dominated by thermal (i.e., heat) radiation that wells
up from the planet's deep atmosphere. This thermal emission dominates
Saturn's dark side as well as the north polar region (where the hexagon is
again visible) and the shadow cast by the A and B rings. Variable amounts
of clouds in the planet's upper atmosphere block the thermal radiation,
leading to a speckled and banded appearance, which is ever-shifting due to
the planet's strong winds.
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 was designed, developed and assembled at JPL. 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/home/index.cfm. The visual and infrared mapping
spectrometer team homepage is at http://wwwvims.lpl.arizona.edu.