CICLOPS: Cassini Imaging Central Laboratory for OPerationS
A Complex Transition

The transition region from Iapetus' dark leading hemisphere to its bright trailing hemisphere is a complicated patchwork of craters and highlands, with low elevations filled in by dark material.

An explanation of the pattern visible here might be key to a full understanding of the bright/dark dichotomy on Iapetus (1,471 kilometers, 914 miles across).

The view is centered on the equator and covers an area 711 kilometers wide by 417 kilometers tall (442 by 259 miles).

The giant equatorial ridge visible on the dark leading hemisphere is not present anymore in this region. Instead, large, isolated mountains more than 10 kilometers (6 miles) tall are spread along the equator. These mountains show bright western flanks, while the surrounding lowlands are generally dark.

The bright mountains at center right, surrounded by dark terrain, are also visible in the stereo view PIA08379. The region of Iapetus seen in this mosaic is also visible in the color full-disk mosaic, PIA08384.

The mosaic is an orthographic projection consisting of 21 image footprints across the surface of Iapetus. The view is centered on terrain near 0.1 degree north latitude, 199 degrees west longitude, in the quadrant of Iapetus that faces away from Saturn. Image scale is approximately 83 meters (272 feet) per pixel. An orthographic view is most like the view seen by a distant observer looking through a telescope.

The clear spectral filter images in this mosaic were obtained with the Cassini spacecraft narrow angle camera on Sept. 10, 2007 at distances ranging from 13,857 to 21,846 kilometers (8,610 to 13,574 miles) from Iapetus.

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 Cassini-Huygens mission for NASA's Science Mission Directorate, Washington, D.C. 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-Huygens mission, visit and the Cassini imaging team home page,

Credit: NASA/JPL/Space Science Institute
Released: October 8, 2007 (PIA 08381)
Image/Caption Information
  A Complex Transition
PIA 08381

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Alliance Member Comments
jonsavelle (Oct 9, 2007 at 11:44 PM):
The explanation that this is a "bright trailing hemisphere,,, with low elevations filled in with dark material" appears to contradict the image. I downloaded the picture, opened it in Photoshop, reduced the contrast and brightened it overall, and the result clearly shows a dark cratered surface overlain by white drifts of some bright material. The dark areas appear identical to those on the leading hemisphere. So the question is, what is the white stuff? I don't know, but my guess is ice that strikes the leading hemisphere, vaporizes, then recrystallizes and falls as snow on the trailing hemisphere. The snow then slowly sublimates, leaving the blotchy pattern of light and dark.