Iapetus' dichotomy explained

Iapetus is Saturn's icy yin-yang satellite. This Cassini image shows that the side of this satellite that leads in its orbit about Saturn is 10 times darker than its polar regions. This darkening was long thought to be due to contamination from Saturn's even more distant satellite Phoebe. Meteorites crashing into Phoebe will launch small debris that goes into orbit about Saturn, resulting in a vast dust ring that also contaminates Iapetus. This dust ring was only recently discovered by the Spitzer Space Telescope. Some of that dark dust gets deposited at Iapetus' leading face, which is then warmed by sunlight. Recent work by John Spencer and Tilmann Denk show that this can cause surface ice there to evaporate at the equator. That water vapor can migrate towards and then freeze out at the poles, brightening the satellite there, and giving it its yin-yang appearance. Additional images can be found at Cassini's CICLOPS website.