Hugh Pickens writes "The difference in coloring between Iapetus' leading and trailing hemispheres is striking. NASA's Jet Propulsion Labs has just released a report on a bizarre 'runaway' process that may explain the strange and dramatically two-toned appearance recently revealed in images collected during a close flyby by the Cassini spacecraft. Scientists believe that initially dark material on one side of Iapetus may have come from other moons orbiting Saturn in the opposite direction. Since Iapetus is locked in synchronous rotation about Saturn, as dusty material from the outer moons spiraled in and hit Iapetus head-on, the forward-facing side began to darken. As it absorbed more sunlight, its surface water evaporated, and vapor was transported from the dark side to the white side of Iapetus. Thermal segregation then proceeded in a runaway process as the dark side lost its surface ice and got darker still. Now the leading hemisphere is as dark as a tarred street and the trailing hemisphere resembles freshly fallen snow."
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