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The Enigma of Iapetus: "The Moon with a View"

... The ship had long since passed the boundary set by outermost Phoebe, moving backward in a wildly eccentric orbit eight million miles from its primary. Ahead of it now lay Iapetus, Hyperion, Titan, Rhea, Dione, Tethys, Enceladus, Mimas, Janus and the rings themselves. All the satellites showed a maze of surface detail ... Titan alone three thousand miles in diameter, and as large as Mercury would occupy ... months ...

One hemisphere of the satellite, which, like its companions, turned the same face always toward Saturn, was extremely dark, and showed very little surface detail. In complete contrast, the other was dominated by a brilliant white oval, about four hundred miles long and two hundred wide. At the moment, only part of this striking formation was in daylight, but the reason for Iapetus's extraordinary variations in brilliance was now quite obvious...

There was more; already he was certain that Iapetus was his goal. ...

-Arthur C. Clarke "2001: A Space Odyssey"

On September 10, 2007, the Cassini Spacecraft completed a spectacular flyby of Saturn's two-toned moon, Iapetus. The mysterious moon, with a walnut-shaped appearance due to its equatorial mountain ridge and brightness contrast have intrigued scientists for decades, and holds possible keys to the ancient chemical origins of the solar system.

Images taken in 2004 with Cassini's ultraviolet imaging spectrograph on the left indicates water ice abundance across the surface: the bright north polar terrain (shown in red) is the iciest region in this view. Away from the pole, as the color shifts to blue, less water ice is present in the surface. The darkest terrain, which includes very little water ice, is shown in light blue. The dark sky background viewed during the observation is shown as purple in this color scheme.
Images taken in 2004 with Cassini's ultraviolet imaging spectrograph on the left indicates water ice abundance across the surface: the bright north polar terrain (shown in red) is the iciest region in this view. Away from the pole, as the color shifts to blue, less water ice is present in the surface. The darkest terrain, which includes very little water ice, is shown in light blue. The dark sky background viewed during the observation is shown as purple in this color scheme.

Why compared to a walnut?

The ridge at Iapetus's equator extends only over 3000 kilometers (about 1500 miles). This is the feature that often causes the public to compare the moon to a walnut.

Scientists on the Cassini mission are now poring through hundreds of images returned from the flyby, which was nearly 100 times closer to Iapetus than Cassini's 2004 flyby, bringing the spacecraft to about 1640 kilometers (1000 miles) and revealing the moon's yin and yang--a white hemisphere resembling snow, and the other as black as tar.

The exciting new Images show a surface that is heavily cratered, along with the mountain ridge that runs along the moon's equator. Many of the close-up observations focused on studying the strange 20-kilometer high (12 mile) mountain ridge that had originally been captured by Voyager in its past flyby of the satellite.

On September 10, 2007, Cassini zooms in, for the first time, on the patchy, bright and dark mountains originally identified in images from the NASA Voyager spacecraft taken more than 25 years earlier. The image was acquired during Cassini's only close flyby of Iapetus, a two-toned moon of Saturn.
On September 10, 2007, Cassini zooms in, for the first time, on the patchy, bright and dark mountains originally identified in images from the NASA Voyager spacecraft taken more than 25 years earlier. The image was acquired during Cassini's only close flyby of Iapetus, a two-toned moon of Saturn.

"Our flight over the surface of Iapetus was like a non-stop free fall, down the rabbit hole, directly into Wonderland! Very few places in our solar system are more bizarre than the patchwork of pitch dark and snowy bright we've seen on this moon," said Carolyn Porco, Cassini imaging team leader at the Space Science Institute, Boulder, Colo.

"The images are really stunning," said Tilmann Denk, Cassini imaging scientist at the Free University in Berlin, Germany, who was responsible for the imaging observation planning. "Every new picture contained its own charm. I was most pleased about the images showing huge mountains rising over the horizon. I knew about this scenic viewing opportunity for more than seven years, and now the real images suddenly materialized."

"Iapetus provides us a window back in time, to the formation of the planets over four billion years ago. Since then its icy crust has been cold and stiff, preserving this ancient surface for our study," said Torrence Johnson, Cassini imaging team member at JPL.

This stunning close-up view shows mountainous terrain that reaches about 10 kilometers (6 miles) high along the unique equatorial ridge of Iapetus. The view was acquired during Cassini's September 10, 2007 flyby of the two-toned Saturn moon.
This stunning close-up view shows mountainous terrain that reaches about 10 kilometers (6 miles) high along the unique equatorial ridge of Iapetus. The view was acquired during Cassini's September 10, 2007 flyby of the two-toned Saturn moon.

Cassini's multiple observations of Iapetus will help to characterize the chemical composition of the surface; look for evidence of a faint atmosphere or erupting gas plumes; and map the nighttime temperature of the surface. These and other results will be analyzed in the weeks to come. Some of the major questions that scientists hope to answer include:

  • Does the "belly band" go all around the satellite?
  • Are the "mountains" seen by Voyager part of the belly band?
  • How deep is the dark material? Is it fresh? Is the deposition ongoing?
  • What is the identity, distribution, and nature of the complex organics on its surface?
  • What do the dark deposits in the bright material look like?
  • What is the morphology of the bright-dark boundary up close?

Scientists on the Cassini mission and elsewhere have long hypothesized about the origins, chemical composition and special features such as the equatorial ridge of Iapetus. Since its arrival at the Saturn system in mid-2004, the Cassini orbiter has successfully performed seven observation campaigns of Iapetus at ranges of 1.4 million kilometers, all the way up to 1640 kilometers during the September 10 2007 flyby. Large parts of the surface have now been observed at 9 kilometers per pixel resolution or better. One hypothesis seems to be that the ridge was caused by despinning in Iapetus' history, and yet, an alternative hypothesis considers an ancient ring system around Iapetus as the cause for the ridge formation. Scientists have also been trying to solve the centuries-old question of the formation of the unique dark/bright albedo dichotomy. Earlier ideas for the albedo dichotomy formation included dust from retrograde outer satellites covering Iapetus' leading side. In this scenario, the dust from outer satellites causes both a significant darkening and reddening of the whole leading side. The higher temperatures on the dark material (think of how much hotter dark asphalt is than concrete on a hot summer day) drive the ice into the brighter cooler areas to further segregate and intensify bright and dark regions. Other scientists speculate that the dark material erupted from deep within Iapetus.

Dark material splatters the walls and floors of craters in the surreal, frozen wastelands of Iapetus. This image, taken during Cassini's close flyby on September 10, 2007 shows terrain in the transition region between the moon's dark leading hemisphere and its bright trailing hemisphere.
Dark material splatters the walls and floors of craters in the surreal, frozen wastelands of Iapetus. This image, taken during Cassini's close flyby on September 10, 2007 shows terrain in the transition region between the moon's dark leading hemisphere and its bright trailing hemisphere.

There are still so many questions to be answered about the enigma of Iapetus. Throughout the Cassini Mission and beyond, scientists hope that this most recent close flyby of the satellite will unveil its dynamic past and future.

References: Denk, T., Neukum, G., Roatsch, Th., Giese, B., Wagner, R., Helfenstein, P., Burns, J.A., Turtle, E.P., Johnson, T.V., Porco, C.C. (2006). "Iapetus: Two Years of Observations by the Cassini ISS Camera." European Planetary Science Congress EPSC2006-A-00571.

References: [1] Porco et al. (2005), Science 307, 1237. [2] Denk et al. (2005), LPSC abstract 2268. [3] Giese et al. (2005), DPS abstract 47.08. [4] Denk et al. (2000), LPSC abstract 1596. [5] Thomas et al. (2006), LPSC abstract 1639. [6] Castillo et al. (2005), DPS abstract 39.04. [7] Ip (2006), GRL, in press. [8] Ostro et al. (2006), Icarus, in press. [9] Soter (1974), IAU Colloq. 28. [10] Buratti et al. (2002), Icarus 155, 375. [11] Spencer et al. (2005), LPSC abstract 2305. [12] Denk et al. (2006), EGU 06-A-08352.

Last Updated: 3 February 2011

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Last Updated: 3 Feb 2011