Oct. 11, 2007 — Poor Iapetus, the bane of the Saturn moons, with a half-black, half-white countenance that speaks of a mysterious past.
The two-toned world has baffled astronomers since its discovery more than 335 years ago, but a recent inspection by the Saturn-orbiting Cassini spacecraft may have cracked the case.
Scientists believe that as the moon orbits around Saturn, it rams into dusty particles left behind by sister moons or other objects, creating a dark mask across its leading hemisphere.
The dark side reflects less sunlight than the bright white surfaces trailing behind. The extra heat melts surface ice and even releases water vapor, which then condenses on the nearest cold spot. That could be on the moon’s poles, or somewhere along the sheltered back side, where it refreezes and recoats the moon’s already bright, highly reflective surfaces.
Losing the lighter-toned water ice leaves Iapetus’ dark side even darker, creating a yin-yang world where there is no grey.
"We think we now understand the essence of why Iapetus looks the way it does," said Carolyn Porco, who heads the Cassini imaging team at the Space Science Institute in Boulder, Colo.
Scientists call the phenomenon “thermal segregation” and believe a similar process has shaped Hyperion, a spongy wedge of a moon that has the distinction of being the largest irregularly shaped moon in the solar system.