Saturn’s moon Iapetus has always had an unusual aspect, one first noted all the way back in the days of Giovanni Cassini (1625-1712), for whom our Saturn orbiter is named. The moon’s discoverer, Cassini correctly noted that Iapetus had a bright hemisphere and a dark one, each visible (because of tidal lock) on only one side of the planet as viewed from Earth. We now call the dark hemisphere Cassini Regio in honor of the Italian-born astronomer.


Image: Cassini-Huygens spacecraft images of Iapetus’ dark, leading side and its bright, trailing side. The high-resolution images shed new light on the long-standing puzzle of how Iapetus got its unusual coloration. Credit: Cassini Imaging Team.

So what makes Cassini Regio so dark? Interior activity on the moon itself is one possibility, but the leading theory is that dusty debris from Saturn’s moon Phoebe is the source. Now images from the Cassini orbiter have been analyzed, with a paper in Science concluding that Cassini Regio is being bombarded by debris from Phoebe. The images can show impact craters down to a resolution on the order of 10 meters to the pixel. Small bright craters are found within the dark hemisphere that indicate the dark surface material is likely only a few meters thick. Moreover, both dark and bright materials on the leading side are much redder in color than on the trailing side.

The leading side’s dust, in other words, seems to have come from somewhere else. Moreover, the transition from dark to light hemisphere is shown to be not a solid line but a mottled array of dark and light spots. Couple this with recent work in Nature that announced the discovery of an enormous ring of debris ten thousand times the area of the main ring system, around Saturn and near Phoebe, a system that supports the dust from Phoebe hypothesis. It is thought that impacts on Phoebe keep the ring supplied with material, and that this material then migrates inward to strike the dark side of Iapetus.

Joseph Burns (Cornell University) thinks the dust question has been resolved:

“The ring of collisional debris that has come off Phoebe is out there, and its companion moons are out there, and now we understand the process whereby the stuff is coming in. When you see the coating pattern on Iapetus, you know you’ve got the right mechanism for producing it.”

Remember that Iapetus is hardly alone when it comes to dark material being found on the surface. Not long ago we looked at studies showing black coatings on Hyperion, Dione and Phoebe as well, suggesting a common mechanism for carrying the material from one moon to another. That work was intriguing because there’s some evidence for geological activity on Dione, but evidently not enough to serve as the source for its dark materials. Here I’m going to repeat a quote I used earlier from Cassini scientist Bonnie Buratti:

“Ecology is about your entire environment — not just one body, but how they all interact. The Saturn system is really interesting, and if you look at the surfaces of the moons, they seem to be altered in ways that aren’t intrinsic to them. There seems to be some transport in this system.”

Indeed, and with Cassini’s help we’re untangling its mysteries. The paper is Denk et al., “Iapetus: Unique Surface Properties and a Global Color Dichotomy from Cassini Imaging,” published online in Science on 10 December, 2009 (abstract). The paper on the ring discovery is Verbiscer et al, “Saturn’s largest ring,” Nature 461, (22 October 2009), pp. 1098-1100 (abstract).