Temperature changes mapped with Cassini's composite and infrared
spectrometer throughout Saturn's main rings show the ring temperatures
decreasing with the increase of the Sun-spacecraft-ring angle (called
phase angle) on both the lit and unlit sides of the rings. These
temperature changes indicate that the ring particles spin slowly compared
to their orbital periods of 6 to 14 hours. They may spin several times
per orbit to less than one time per orbit.
Four scans are shown for the lit and unlit rings, at relatively low (less
than 60 degrees) and high (more than 130 degrees) phase angles. Warmer
temperatures about minus 262 degrees Fahrenheit (110 Kelvin) are shown in
red and cooler temperatures about minus 343 degrees (65 K) are shown in
blue. Other colors indicate temperatures between minus 343 degrees and
minus 262 degrees (65 K and 110 K). The top two scans are for the lit
rings and the bottom two scans are for the unlit rings. The change in
ring temperature between each scan can be seen clearly.
The thermal characteristics of each main ring vary noticeably with phase
angle. Radial scans of the A, B and C rings show a decrease in temperature
with increasing phase angle for both the lit and unlit sides of the rings.
The C ring and Cassini Division exhibit the largest change in temperature.
The temperature of the lit C ring decreases by about 22 degrees (12
Kelvin) between low and high phase angles. A similar contrast is present
for the unlit side of the C ring. The C ring and Cassini Division are
darker than the A and B rings so they can absorb more heat from the Sun.
The lit B ring shows a temperature contrast of approximately 18 degrees
(10 K) while the unlit B ring shows very little thermal contrast. Very
little sunlight may make it through the thick B ring to its unlit side.
The lit A ring is particularly interesting because the magnitude of the
thermal contrast decreases with increasing radial distance from Saturn.
The outer A ring shows only a small temperature change with phase angle,
possibly because it contains smaller, or more rapidly rotating ring
particles, which would have more uniform temperatures with phase angle.
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
composite infrared spectrometer team is based at NASA's Goddard Space
Flight Center, Greenbelt, Md.
For more information about the Cassini-Huygens mission visit,
http://saturn.jpl.nasa.gov and the instrument team's home page,
http://cirs.gsfc.nasa.gov/.