Why is Saturn's moon Iapetus so freaking weird?
Well, everything about Saturn and its moons is weird. But the 1500 km (900 mile) wide moon Iapetus may win the prize for the most bizarro. It has a weird equator-ringing ridge that may have formed when the moon solidified and shrank. It's walnut shaped! It's got several whopping huge impact basins on it.
But the really odd thing is that the two hemispheres of the moon are so wildly different in color:
That Cassini image shows the real mystery. One half of the moon is substantially darker than the other! Why?
Iapetus takes 79 days to orbit Saturn, and it spins at the same rate, so the same hemisphere always faces into its direction of motion (into the wind, as it were) and the opposite half faces away. Astronomers have supposed that there must be reddish dust or some other junk floating around Saturn, and the leading half of Iapetus slams into and accumulates it like bugs on a windshield. The thing is, the entire hemisphere is not coated! Plus, the boundary is complicated and sharply-defined, so clearly there must be something else going on.
Astronomer now think they have the answer. Using visible and infrared data from Cassini, a new model has been proposed where melting and migrating water ice is the key. As the dust piles up on the leading hemisphere, that area gets warmer when sunlight hits it (dark material absorbs heat better). Underneath this material, the ice warms as well. Even though the temperature is -140° C (-220° F), the sunlight can evaporate the water ice, which then migrates around. When it reaches the trailing hemisphere, where there is no dust and is therefore cooler, the water recondenses and freezes out.
Since the bright, shiny ice is leaving the moon's leading half, that half gets darker. That ice, sans dust, reaccumulates on the trailing half, making that part brighter. This is a positive feedback loop, and over billions of years we now have this goofy two-faced moon. The model does a great job predicting what Iapetus should look like; the image here shows the result (click it to see the full graphic of how Iapetus changes with time).
According to the model, Iapetus got to its current condition a couple of billion years ago, and reached more or less a steady state. Note that the poles of the moon are colder than the equator, so they don't have as strong an effect and therefore have relatively less dust.
This model of ice migration is still a hypothesis and will need to be tweaked as more details of the moon come in. But it does explain a lot of the features on Iapetus quite nicely, and may very well turn out to be correct. I'll note that the wierdness of Iapetus has been known for a long time. Heck, Giovanni Domenico Cassini (our spacecraft's namesake) noticed its bipolar* nature shortly after he discovered Iapetus in 1671!
This model of a moon of Saturn was made possible because we did something extraordinary: we went there. Mysteries are easy, solutions difficult. But because we are not afraid of difficult exploration, and we dedicate ourselves to the mystery, the solutions become possible. As I have said so many times before, and will continue to in the foreseeable future, that's what science does. And as much as we scientists like answers, you better believe that it's the questions that keep us going.
* Haha! It's a pun! It's not really polar, since we're talking longitudinal difference here, not latitudinal, but still: haha!