As ring particles emerge from the darkness of Saturn's shadow, they pass through a region of twilight. The Sun's light, refracted by the planet's atmosphere, peeks around the limb, followed shortly by the Sun itself (see PIA08329 for an example of this effect).
The "penumbra" is the narrow fringe region of the planet's shadow where part (but not all) of the Sun is visible around the side of the planet, creating only a partial shadow there and making the shadow edge look fuzzy.
The A and F rings are captured here. This view looks toward the unlit side of the rings from about 20 degrees above the ringplane. Two faint ringlets can be seen within the Encke gap, which stretches out of the blackness at center and toward right.
The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Sept. 12, 2006 at a distance of approximately 1.5 million kilometers (900,000 miles) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 163 degrees. Image scale is about 9 kilometers (5 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 operations center is based at the Space Science Institute in Boulder, Colo.
Credit: NASA/JPL/Space Science Institute