Saturn's small moon Pan casts a long shadow across the A ring in this image captured by the Cassini spacecraft a few days after the planet's August 2009 equinox.
Pan (28 kilometers, or 17 miles across) can be seen orbiting in the Encke Gap of the A ring below the center of the image. The thin F ring is at the very bottom of the image. The shadow is interrupted by the crest of a bending wave on the right of the image. See Two Kinds of Wave to learn more about such waves.
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 18 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 obtained at a distance of approximately 1.4 million kilometers (870,000 miles) from Pan. Image scale is 9 kilometers (6 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