The excitement of scientists upon Cassini-Huygens entering the Saturnian System was reserved mostly for Titan, Saturn itself, and its rings. That the other moons might be something more than cratered and dead ice bodies was hardly expected.

Enter Iapetus. This strange moon between Phoebe and Titan helped write the exciting new chapter of Saturn moon exploration. Upon the side of Iapetus facing toward its motion around Saturn is a coating of debris “as dark as asphalt,” according to the official NASA Cassini-Huygens page for the moon. The other half of the moon is “bright as snow.”

Perhaps even more surprising was the discovery of a mountain range that neatly straddles the equator of the moon, a ring around Iapetus’ middle that makes little sense at all. Some scientists believe that the formation of this mountain range may provide an explanation for the dark debris, although others believe the debris falls onto the surface from an outside sources.

More Information

• Cassini-Huygens Website
  • Saturn’s Moons section
  • Iapetus Information

Table of Contents

• Introduction
• Pan
• Atlas
• Prometheus
• Pandora
• Janus
• Epimetheus
• Mimas
• Pallene and Methone
• Enceladus
• Telesto
• Tethys
• Calypso
• Helene
• Dione
• Polydeuces
• Rhea
• Titan
• Hyperion
• Iapetus
• Phoebe

The dark material on Iapetus would look dark brown to the human eye, according to this near-true color image of the Saturnian moon. Why the moon is stained this color closer to a mysterious equatorial ridge and less so at higher latitudes remains a mystery.

Okay, get those 3D glasses out. Iapetus, the strangely-shaped and strangely-colored moon with strange landforms that defy current scientific explanation, gets the stereo treatment, with never before seen basins and craters.

Planetary science is never boring and often very surprising. Take for instance the new images sent back by the Cassini space probe of Saturn’s moon Iapetus. The moon has always been mysterious since the first Voyager images revealed a light hemisphere and a dark hemisphere. The contrast cannot be overemphasized: the dark side only reflects 4% of the light that hits it, while the light side reflects 60% of the light.

The Voyager images only showed a small portion of the moon. Cassini’s new images were full of surprises, the most shocking being the existence of a ridge running almost exactly on the moon’s equator that may or may not have something to do with the dark material. Some of the mountains on this ridge are three times higher than Mount Everest. At the boundary of light and dark material, the dark material appears to streak out across the light material, as if it were a coating that fell onto the surface. Scientists are scratching their heads, trying to come up with new theories to explain how such structures can be formed. Did the moon erupt dark material from within, perhaps at the mountain belts? Or does the material come from outside, dropping onto the moon’s surface by some mysterious Saturnian process? What is that dark material?

We might have to wait awhile for answers. Cassini won’t return to Iapetus until September 2007, when it will take much more detailed images during a closer flyby.

Images taken by various space probes are made available to the public soon after they are uploaded to Earth, but because of the sheer number received, most images remain in raw form until they can be calibrated and corrected. These raw images are full of artifacts from data loss, cosmic rays, and dust on the camera lens (see the background image of Iapetus). The images may also be overexposed or variable contrast.

The availability of these images is a boon for space enthusiasts and amateur scientists. During the Voyager missions of the 1970s and 1980s, the public had to wait for their newspaper to arrive, the nightly news to come on, or a new book to be published on the subject to see the latest images of the outer planets and their moons. With the advent of the Internet, awe is just a click away.

The public database of images taken by Cassini is available at JPL’s Cassini-Huygens website.

The Cassini spacecraft flew by the Saturnian moon Iapetus on December 31, 2004 and snapped some images. The moon is recognizable because of it has one bright side and one dark side, perhaps due to sweeping up debris on its leeward side as it orbits Saturn. Cassini will pass much closer to the moon around September 10, 2007.