Saturn's rings cast a shadow on the moon Janus in this image made possible only around the time of the planet's August 2009 equinox.
Some of the planet's moons cast shadows onto the rings at equinox, but in this image, it's the rings casting shadows on a moon. The shadow obscures part of Janus (179 kilometers, or 111 miles across) in the image. The planet's thin F ring can be seen in the upper left of the image.
See Reciprocating Rings to see the rings casting a shadow on the moon Tethys.
This image was taken shortly after the planet's equinox. The novel illumination geometry that accompanies equinox lowers the sun's angle to the ringplane, significantly darkens the rings, and causes out-of-plane structures to look anomalously bright and cast shadows across the rings. These scenes are possible only during the few months before and after Saturn's equinox, which occurs only once in about 15 Earth years. Before and after equinox, Cassini's cameras have spotted not only the predictable shadows of some of Saturn's moons (see Across Resplendent Rings), but also the shadows of newly revealed vertical structures in the rings themselves (see A Small Find Near Equinox ).
This view looks toward the northern, sunlit side of the rings from about 17 degrees above the ringplane.
The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Aug. 14, 2009. The view was acquired at a distance of approximately 1.4 million kilometers (870,000 miles) from Janus and at a Sun-Janus-spacecraft, or phase, angle of 90 degrees. Image scale is 8 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