Sorry it has taken me so long to get to these images. I guess it was too much for me to swallow all at once! On September 10, Cassini flew by Iapetus. Two weeks ago, on the first day of the Division of Planetary Sciences meeeting, several of the science teams released beautiful products that give an early look at some of the science results from the encounter.
The images are spectacular, of course. I've already posted an amateur grayscale version of this one; here, now, is an enhanced-color view of the trailing side of Iapetus in all its detailed glory.
OK. Next, here's an image of Iapetus from the Visual and Infrared Mapping Spectrometer, or VIMS. I think it's quite notable that the stuff on Iapetus appears almost as segregated to VIMS as it does to the cameras. The news at DPS from this instrument team is the discovery of "Rayleigh scattering" in Iapetus' bright ice. This indicates, the VIMS team says, that there are extremely tiny, widely separated particles buried in Iapetus' ice on the trailing side. The Rayleigh effect is strongest where there is the least amount of pollution of the ice, near Iapetus' poles, again pointing to cleaner ice near the poles and dirtier ice near the equator.
OK, next, here's an image from another spectrometer, CIRS, that looks at part of the electromagnetic spectrum even longer than the cameras or VIMS. Once again, I think it's quite unusual how well you can match features visible in the CIRS image to the camera (ISS) image. You can see temperature differences between crater walls and floors. Everwhere you see dark material, you see warm temperatures in CIRS, and everywhere you see bright material, you see cool temperatures.
So the Cassini team seems to be converging on agreement on an explanation for how Iapetus came to look the way it does. It's a combination of two factors: external pollution by a small amount of fine, dark particulate material being preferentially deposited on the leading hemisphere, and Iapetus' unusually slow rotation rate. Slow rotation means that its dayside gets much hotter than the daysides of others of Saturn's icy moons, which can help get the ball rolling with John Spencer's runaway thermal segregation model. And once that gets started, it's just a positive feedback cycle; ice is sublimed from the equatorial regions of the leading side, which darkens the leading side, which makes it warm even more during the long day.
If the Cassini team is reaching consensus, though, the rest of the outer planets community is not. I talked with two different outer planets people not involved in the Cassini mission who told me they think the model is no good. And one of them mentioned something that's been a topic of broader conversation about Iapetus: is Iapetus' surface white-on-black or black-on-white? I mentioned this question to Tilmann Denk, who laughed and proposed the following experiment. Take a look at the images below. They represent the same area on Iapetus. One shows the image as it was taken by Cassini's camera; the other has been inverted -- it's a photo negative. Which one is which? Can you tell?
So now I've written too much; but there'll be much more to come, I'm sure!
For more images from this encounter, here's the page on which I collected all the raw Iapetus images...