CICLOPS: Cassini Imaging Central Laboratory for OPerationS

Twice-lit Janus
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Janus, Saturn's small moon named after the two-faced god, here displays two illuminated hemispheres.

Janus (179 kilometers, 111 miles across) orbits beyond the thin F ring in the top left of the image. The moon is lit by sunlight on the left and light reflected off Saturn on the right.

This image, taken a little more than a week after Saturn's August 2009 equinox, also shows vertical structures in the F ring casting faint shadows near the middle right of the image. 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 PIA11657), but also the shadows of newly revealed vertical structures in the rings themselves (see PIA11665).

This view looks toward the northern, sunlit side of the rings from about 10 degrees above the ringplane.

The image was taken in visible green light with the Cassini spacecraft narrow-angle camera on Aug. 20, 2009. The view was acquired at a distance of approximately 2.3 million kilometers (1.4 million miles) from Saturn. Image scale is 13 kilometers (8 miles) per pixel.

The Cassini Equinox Mission is a joint United States and European endeavor. 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 was designed, developed and assembled at JPL. 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.

For more information about the Cassini Equinox Mission visit, and

Credit: NASA/JPL/Space Science Institute
Released: November 19, 2009 (PIA 11627)
Image/Caption Information
  Twice-lit Janus
PIA 11627

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