The moon Janus casts a shadow on Saturn's A ring but misses the thin F ring in this image captured as the planet approached its August 2009 equinox.
The moon itself is not pictured, and the shadow has missed the thin F ring because the moon has inclined orbit. See Shadow Misses a Ring for a similar image of Pandora's shadow. Janus' shadow here reaches past the Encke Gap of the A ring. The Cassini Division runs from the middle right to the top left of the image. The image has been cropped and zoomed-in by a factor of two.
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, unilluminated side of the rings from about 52 degrees above the ringplane.
The image was taken in visible red light with the Cassini spacecraft wide-angle camera on May 28, 2009. The view was acquired at a distance of approximately 1.3 million kilometers (808,000 miles) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 53 degrees. Image scale is 77 kilometers (48 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