When Cassini gazes down at Saturn's rings with the Sun directly behind the
spacecraft, an unusual phenomenon called the "opposition effect" can be
seen. The effect is visible here as a bright region, near right, toward
the inner edge of the A ring.
The precise nature of the effect at Saturn is still under scrutiny by
imaging scientists. However the effect in Saturn's rings can be witnessed
from Earth, when the viewing conditions are right. It can also be seen in
photographs of the lunar surface taken by the Apollo astronauts.
To understand the effect, imagine an observer standing on a dry, sandy
beach. When the Sun is directly behind the observer, the shadows cast by
the grains in the field of view in front of the observer will fall
directly behind the grains and will not be visible. When the Sun is at
any other angle relative to the observer, the shadows cast by the grains
will be visible to the observer. These shadows in the field of view make
the scene a bit darker. This effect would cause a centrally bright spot
to appear on the sandy surface in the first case, but not in the second.
For Cassini, the opposition effect is seen when the angle between the Sun,
the rings and the spacecraft is extremely close to zero. For the sequence
of images during which this view was obtained, Cassini maintained this
viewing angle and the bright spot appeared to move across the rings with
the spacecraft's motion.
The moon Janus (181 kilometers, or 113 miles, across) is seen here at
lower left.
The image was taken in visible light with the Cassini spacecraft
wide-angle camera on June 7, 2005, at a distance of approximately
738,000 kilometers (458,000 miles) from Saturn. The image scale is 40
kilometers (25 miles) per pixel.
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 team is based at the Space Science
Institute, Boulder, Colo.
For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov.
For additional images visit the Cassini imaging team homepage http://ciclops.org.