A Black And White World

Iapetus, one of Saturn's moons has long baffled scientists. It is distinctly black and white with a clearly defined dark leading edge facing the direction of its orbit and a very bright white trailing side. NASA/JPL scientists now believe they may have a handle on why this oddity is happening. They are describing a runaway process that transports water vapor from the warmer dark side to the frigid cold side.

NASA scientists are on the trail of Iapetus' mysterious dark side, which seems to be home to a bizarre "runaway" process that is transporting vaporized water ice from the dark areas to the white areas of the Saturnian moon.

This "thermal segregation" model may explain many details of the moon's strange and dramatically two-toned appearance, which have been revealed exquisitely in images collected during a recent close flyby of Iapetus by NASA's Cassini spacecraft.