These two images of Saturn's A and B rings showcase the opposition effect, a brightness surge that is visible on Saturn's rings when the Sun is directly behind the spacecraft.
The opposition effect exists because of two contributing factors. One is due to the fact that the shadows of ring particles directly opposite the Sun from Cassini-the region of opposition-fall completely behind the particles as seen from the spacecraft. These shadows are thus not visible to the spacecraft: all ring particle surfaces visible to Cassini are in sunlight and therefore bright. Away from the region of opposition, the ring particle shadows become more visible to Cassini and the scene become less bright. The surge in brightness falls off in a circular fringe around that point.
Another contributing factor to the opposition surge is an optical phenomenon called "coherent backscatter." Here, the electromagnetic signal from the rays of scattered sunlight, making their way back to the spacecraft, is enhanced near the region of opposition because, instead of canceling, the electric and magnetic fields comprising the scattered radiation fluctuate in unison.
The images were taken in visible light with the Cassini spacecraft wide-angle camera on July 23, 2006 at a distance of approximately 262,000 kilometers (163,000 miles) from Saturn. Image scale in the radial, or outward from Saturn, direction is 13 kilometers (8 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 Cassini-Huygens mission for NASA's Science Mission Directorate, Washington, D.C. The imaging team consists of scientists from the US, England, France, and Germany. The imaging operations center and team lead (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.