Here, the brightness pattern on the surface is very complex. The mountain in the center of this view is part of the range informally named "the Voyager mountains" that were first detected on the limb of the moon in NASA Voyager spacecraft images. Interestingly, its eastern (right) flank is dark, while the other flanks are bright. This suggests that external material arriving on Iapetus from its orbital motion might play a role in the darkening process. One plausible source, the outer moons of Saturn, might provide a very thin but steady stream of very dark particles from the eastern direction as seen from this mountain.

The mosaic consists of six image footprints across the surface of Iapetus. The view is centered on terrain near 0.1° and 199° west. Image scale is approximately 46 metres per pixel.

The clear spectral filter images in this mosaic were obtained with the Cassini spacecraft narrow-angle camera on 10 September 2007. Distances for the blue portion of the image range from 7744 to 9135 kilometres from Iapetus; distances for the red portion of the image range from 20 267 to 21 595 kilometres from the moon.

A separate, non-stereo version of the scene is included for comparison.

Iapetus is 1468 kilometres across.

Credits: NASA/JPL/Space Science Institute

An explanation of the pattern visible here might be key to a full understanding of the bright/dark dichotomy on Iapetus, 1468 kilometres across.

The view is centered on the equator and covers an area 711 kilometres wide by 417 kilometres tall.

The giant equatorial ridge visible on the dark leading hemisphere is not present anymore in this region. Instead, large, isolated mountains more than 10 kilometres 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. The region of Iapetus seen in this mosaic is also visible in the color full-disk mosaic.

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° north and 199° west, in the quadrant of Iapetus that faces away from Saturn. Image scale is approximately 83 metres 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 10 September 2007, at distances ranging from 13 857 to 21 846 kilometers from Iapetus.

Credits: NASA/JPL/Space Science Institute

The dark material is preferentially found at the bottoms of craters. Bright water ice forms the 'bed rock' on Iapetus, while the dark, presumably loose material apparently lies on top of the ice.

The mosaic consists of two image footprints across the surface of Iapetus. The view is centered on terrain near 42° south and 209.3° west, on the anti-Saturn facing hemisphere. Image scale is approximately 32 metres per pixel.

The clear spectral filter images in this mosaic were obtained with the Cassini spacecraft narrow- angle camera on 10 September 2007, at distances ranging from 5363 to 5884 kilometres from Iapetus.

Credits: NASA/JPL/Space Science Institute

At left, below center, the eastern rim of a great and ancient impact basin can be seen. With a diameter of almost 500 kilometres, it is one of the largest impact structures on Iapetus, 1468 kilometres across, and in the entire Saturn system.

The mosaic consists of three narrow-angle camera footprints across the surface of Iapetus. This view is centered on terrain near 35.1° south and 218.5° west. Image scale is approximately 231 meters per pixel.

The clear spectral filter images in this mosaic were obtained with the Cassini spacecraft on 10 September 2007, at a distance of approximately 40 000 kilometres from Iapetus and at a sun-Iapetus-spacecraft, or phase, angle of 31°.

Credits: NASA/JPL/Space Science Institute

These wavelengths represent reflected solar light and indicate where the surface is brightest and highest in water ice abundance. (Red indicates the brightest regions, purple the darkest.) The bright 'Voyager Mountains', part of the equatorial ridge, are seen as bright spots against a dark background. The dark material that covers one hemisphere of Iapetus is indicated in purple and is seen on the right side of this image.

The middle image is a color composite: blue-green (longer ultraviolet wavelengths) indicates where the surface is bright and probably richest in water ice. Red (short ultraviolet wavelengths) indicates where the surface is low in water ice and relatively high in dirty material. The sky background is also bright at these wavelengths, making the limb, or edge, of Iapetus where the surface is dark indistinguishable from the sky background.

The image on the right, taken by the imaging science subsystem, is for reference, with the regions observed by Cassini’s ultraviolet imaging spectrometer outlined in red.

Credits: NASA/JPL/Space Science Institute

The visual and infrared mapping spectrometer is like a digital camera, but instead of using three colors, it makes images in 352 colors, or wavelengths, from the ultraviolet to the near-infrared. The many wavelengths produce a continuous spectrum in each pixel, and these spectra measure how light is absorbed by different materials. By analyzing the absorptions expressed in each pixel, a map of the composition at each location on the moon can be constructed.

The left image in the figure shows the amount of reflected light at a wavelength of 1.75 microns in the infrared (green light seen by our eyes is 0.53 microns). The color image on the right shows the results of mapping for three components of Iapetus' surface: carbon dioxide that is trapped or adsorbed in the surface (red), water in the form of ice (green), and a newly-discovered effect due to trace amount of dark particles in the ice creating what scientists call Rayleigh scattering (blue). The Rayleigh scattering effect is the main reason why the Earth's sky appears blue.

The Rayleigh scattering effect on Iapetus provides evidence that tiny grains, less than the wavelength of visible light (less than 0.5 microns) have been embedded in the surface of Iapetus. The tiny grains must be well-separated for the Rayleigh effect to become prominent, so the abundance of particles must be less than about 2%. The Rayleigh scattering effect shows in all areas, although weakly in dark regions (the red carbon dioxide dominates the colour image), and it appears stronger away from the equator. Investigating the trend from dark to bright areas, the Rayleigh effect changes with the amount of dark material in the ice, and becomes weaker as more dark material is added. This points to cleaner ice as one moves north or south from the equator and away from the dark leading side of the moon (toward the right in the image).

This provides additional evidence for an external source for the dark material coating Iapetus, and for ice transport away from the warm dark regions and equator to the cooler poles. The ice transport away from the equator increases the concentration of dark material there and reduces the Rayleigh effect. With the volatile transport from the dark warm regions, the strong carbon dioxide signature is a surprise because frozen carbon dioxide is more volatile than water ice. Therefore, the carbon dioxide must be trapped, making its presence stable in the warm equatorial region. The trapping mechanism is currently under study.

Credits: NASA/JPL/University of Arizona/USGS

The equatorial region includes the equatorial bulge which shows no differences in these compositions compared to surrounding regions.

The color image on the right shows the results of mapping for three components of Iapetus' surface: carbon dioxide that is trapped or adsorbed in the surface (red), water in the form of ice (green), and a newly-discovered effect due to trace amount of dark particles in the ice creating what scientists call Rayleigh scattering (blue). The Rayleigh scattering effect is the main reason why the Earth's sky appears blue.

There is a complex transition zone from the dark region, on the right, which is high in carbon dioxide, to the more ice-rich region on the left. Some crater floors are filled with carbon dioxide-rich dark material. As the ice becomes cleaner to the left, the small dark particles become more scattered and increase the Rayleigh scattering effect, again indicative of less than 2 percent dark sub-0.5-micron particles.

The visual and infrared mapping spectrometer is like a digital camera, but instead of using three colors, it makes images in 352 colors, or wavelengths, from the ultraviolet to the near-infrared. The many wavelengths produce a continuous spectrum in each pixel, and these spectra measure how light is absorbed by different materials. By analyzing the absorptions expressed in each pixel, a map of the composition at each location on the moon can be constructed.

Credits: NASA/JPL/University of Arizona/USGS