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."

ESA Report
NASA scientists are on the trail of Iapetus' mysterious dark side, which seems to be home to a bizarre "runaway" process that is transporting vaporized water ice from the dark areas to the white areas of the Saturnian moon.

This "thermal segregation" model may explain many details of the moon's strange and dramatically two-toned appearance, which have been revealed exquisitely in images collected during a recent close flyby of Iapetus by NASA's Cassini spacecraft. Infrared observations from the flyby confirm that the dark material is warm enough (approximately minus 230 degrees Fahrenheit or 127 Kelvin) for very slow release of water vapor from water ice, and this process is probably a major factor in determining the distinct brightness boundaries.

"The side of Iapetus that faces forward in its orbit around Saturn is being darkened by some mysterious process," said John Spencer, Cassini scientist with the composite infrared spectrometer team from the Southwest Research Institute, Boulder, Colo.

Using multiple instruments on Cassini, scientists are piecing together a complex story to explain the bright and dark faces of Iapetus. But yet to be fully understood is where the dark material is coming from. Is it native or from outside the moon? It has long been hypothesized that this material did not originate from within Iapetus, but instead was derived from other moons orbiting at a much greater distance from Saturn in a direction opposite to Iapetus.

Scientists are now converging on the notion that the darkening process in fact began in this manner, and that thermal effects subsequently enhanced the contrast to what we see today. "It's interesting to ponder that a more than 30-year-old idea might still help explain the brightness difference on Iapetus," said Tilmann Denk, Cassini imaging scientist at the Free University in Berlin, Germany. "Dusty material spiraling in from outer moons hits Iapetus head-on, and causes the forward-facing side of Iapetus to look different than the rest of the moon."

Once the leading side is even slightly dark, thermal segregation can proceed rapidly. A dark surface will absorb more sunlight and warm up, explains Spencer, so the water ice on the surface evaporates. The water vapor then condenses on the nearest cold spot, which could be Iapetus's poles, and possibly bright, icy areas at lower latitudes on the side of the moon facing in the opposite direction of its orbit. So the dark stuff loses its surface ice and gets darker, and the bright stuff accumulates ice and gets brighter, in a runaway process.

Scientists say the result is that there are virtually no shades of gray on Iapetus. There is only white and very dark.

Ultraviolet data also show a non-ice component in the bright, white regions of Iapetus. Spectroscopic analysis will reveal whether the composition of the material on the dark hemisphere is the same as the dark material that is present within the bright terrain.

"The ultraviolet data tell us a lot about where the water ice is and where the non-water ice stuff is. At first glance, the two populations do not appear to be present in the pattern we expected, which is very interesting," said Amanda Hendrix, Cassini scientist on the ultraviolet imaging spectrograph team at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

Because of the presence of very small craters that excavate the bright ice beneath, scientists also believe that the dark material is thin, a result consistent with previous Cassini radar results. But some local areas may be thicker. The dark material seems to lie on top of the bright region, consistent with the idea that it is a residual left behind by the sublimated water ice.

Some other mysteries are coming together. There are more data on the signature mountain ridge that gives Iapetus its "walnut" appearance. In some places it appears subdued.

One big question that remains is why it does not go all the way around. Was it partially destroyed after it formed, or did it never extend all the way around the moon? Scientists have ruled out that it is a youthful feature because it is pitted with craters, indicating it is old. And the ridge looks too solid and competent to be the result of an equatorial ring around the moon collapsing onto its surface. The ring theory cannot explain features that look like tectonic structures in the new high resolution images.

Over the next few months, scientists hope to learn more about Iapetus' mysteries.

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