CICLOPS: Cassini Imaging Central Laboratory for OPerationS
Shadow and a Wave

The very faint shadow of the moon Atlas stretches across the top left of this image while a wave undulates across the A ring in the lower right of this image taken around the time of Saturn's August 2009 equinox.

The wave, seen here as narrow light and dark zones, is the Iapetus -1:0 nodal bending wave first seen in Voyager images from 1980 and not reported again until the 2009 equinox (see PIA11670). This bending wave has a long wavelength and a low amplitude that make it difficult to see except under equinox illumination. This nodal wave is the result of a resonance in which the precession rate of the ring particles' orbital planes (the "nodal precession") is in resonance with the orbit of the moon Iapetus. To learn more about bending waves observed in Saturn's rings, see PIA10501.

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 15 degrees above the ringplane.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Aug. 15, 2009. The view was obtained at a distance of approximately 1.8 million kilometers (1.1 million miles) from Saturn. Image scale is 10 kilometers (6 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: October 26, 2009 (PIA 11609)
Image/Caption Information
  Shadow and a Wave
PIA 11609

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