Iapetus Explained

For over 300 years, astronomers have known there was something odd about Saturn's moon, Iapetus. The leading hemisphere is much darker than the trailing. Recent images taken by the Cassini spacecraft confirm the leading explanation of the contrast.

As Iapetus orbits Saturn, dust-- perhaps from other moons, perhaps from Saturn's rings-- falls onto the leading hemisphere. Dust is much darker than ice, hence that hemisphere has become darker. Reinforcing the process, the layers of dust slightly warms the surface, which allows water vapor to migrate to the trailing hemisphere, where it freezes back into ice. The small size of Iapetus-- about 900 miles in diameter-- and, therefore, its low gravity also aids the migration of the water vapor, further supporting the entire process.