CICLOPS: Cassini Imaging Central Laboratory for OPerationS
Tell-tale Clues To A 335-year-old Mystery Spotted In Cassini Images

Preston Dyches (720) 974-5859
CICLOPS/Space Science Institute, Boulder, Colo.

For Immediate Release: Oct. 8, 2007


The appearance of two-toned Iapetus has been deeply mystifying ever since the moon was first discovered by Jean-Dominique Cassini in the late seventeenth century.

Now, high-resolution images of Iapetus recently acquired by the spacecraft named after the Italian/French astronomer during its low pass over the moon last month have uncovered telling details on the moon's surface that may well yield the reason for its strange bright and dark patterns.

The images show that on the moon's bright trailing hemisphere, especially in the equatorial regions, dark material tends to coat the equator-facing slopes of ridges and crater walls and also many crater floors. This finding strongly suggests the warming action of the Sun in removing bright ice from these sunward-facing surfaces and leaving behind the native dark material that is normally mixed with the ice. Subsequent downslope motion is very likely responsible for collecting much of the dark material in the floors of craters and other low lying regions.

"This is somewhat reminiscent of the vineyards in Germany," said Tilmann Denk, imaging team associate at the Free University in Berlin, Germany and an expert on Iapetus. "The grapes get more sunlight when the vine is planted on a south-facing slope. The same mechanism works on Iapetus: the equator-facing slopes get more sunlight, and the bright ice there evaporates, leaving behind the darker stuff."

In this particular characteristic, Iapetus is similar to another moon in the Saturn system with large surface contrasts, Hyperion. "The craters on Hyperion have dark floors, probably for a similar reason, but with a twist," said Paul Helfenstein, an imaging team associate at Cornell University and an icy satellite expert. "Sunlight also warms the surface, but on Hyperion, the terrain is so rugged that it is believed all dark material moves downward to collect on crater floors."

The fact that this process of thermal segregation is so clearly operating on the bright face of Iapetus lends confidence to the two-part suggestion--the first half of which was made by scientists thirty years ago and the second half made more recently by Cassini scientists--that the infall of a thin coating of dark material onto Iapetus' leading side long ago initiated a runaway version of thermal segregation there. With a coating of dark material scooped up by the moon in its orbit around Saturn, all surfaces on the entire leading hemisphere except at high latitudes, regardless of the direction to the sun, became warm enough to evaporate the ice beneath. Once warmed, evaporation proceeded even more quickly until all the surface ice was gone.

The result: a layer of dark material, consisting of both foreign and native material, coating most of leading hemisphere. The differing colors of the leading and trailing hemispheres on Iapetus observed in Cassini images indicate slight differences in composition, as would be expected if the leading side also had mixed in with it material that derived from elsewhere in the Saturn system. The origin of this foreign material remains a mystery, but potential candidates are the small moons at large distances from Saturn or a previously existing outer moon that was broken apart long ago.

Observations of very small bright craters seen for the first time in the recent Cassini images, point to impactors that punched through the dark upper layer to the bright ice beneath and reveal the layer's thickness at no more than a few meters.

In addition to the new revelations about the moon's brightness asymmetry, the recent Cassini images revealed that the tall equatorial ridge bisecting Iapetus' leading side appears to be a competent structure, and most likely tectonic in origin. They also showed, for the first time, giant impact basins on the trailing hemisphere. Enormous basins had previously been observed by Cassini on the leading side, but the new images confirm that the cratering record is similar across the entire surface and that the surface is very old.

Surveying the surface of Iapetus, and determining the origin of the moon's peculiar asymmetry in brightness were two of the key science objectives for this international mission--two that can now be essentially checked off as "done."

"While there are many details yet to be worked out, we think we now understand the essence of why Iapetus looks the way it does," said Carolyn Porco, the leader of the imaging team. "And this discovery too will go down as a major legacy of Cassini's historic exploration of Saturn."

Images from Cassini's close flyby of Iapetus can be found at, and

See Flying by Iapetus."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team consists of scientists from the U.S., England, France, and Germany. The imaging operations center and team leader (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.