Iapetus伊阿佩托斯 is often called Saturn土星's most bizarre奇异的 moon, due to its starkly明显地 contrasting hemispheres半球 – one black as coal, the other white as snow. Images taken by the Cassini-Huygens spacecraft, orbiting Saturn¹ since 2004, offer the most compelling强制性的 evidence to date至今 of why and how the moon got its yin-yang appearance, as well as clues to how other such satellites might have formed in the early universe. Analyzed³ by a research team that includes Cornell scientists, the images are detailed⁴ in the Dec. 10 online edition of the journal Science.

"This is not the most fundamental problem in the world," said research team member Joseph A. Burns, Cornell's Irving Porter Church Professor of Engineering and professor of astronomy. "But it's an enigma谜 that's been puzzling astronomers⁶ for centuries."

Since pictures of Iapetus from the Voyager mission 30 years ago confirmed its intriguing迷人的，有趣的 color scheme, scientists have puzzled over whether Iapetus' dark-light contrast was the result of external debris碎片，残骸 hitting some of the moon, or whether the dark dust was the result of internal activity. Now they know the dust came from elsewhere.

Using pictures taken by Cassini, particularly during a September 2007 close fly-by, the scientists assert断言，主张 that Iapetus' darker half, called Cassini Regio, is the result of the planet's leading side getting bombarded被轰击的 by dusty debris⁸ from another Saturnian moon, Phoebe, which orbits in the opposite direction beyond Iapetus.

It is a longstanding theory, but in a paper published in the journal Nature in October, three Cornell-trained astronomers announced the discovery of an enormous ring of debris ¬– 10,000 times the area of Saturn's famous main ring system – around Saturn, near Phoebe's location, pointing to it as the ring's source. Burns calls this ring the "smoking gun" supporting dust hitting Iapetus and other moons around Saturn.

"The ring of collisional debris that has come off Phoebe and its companion moons is out there, and now we understand the process whereby the stuff is coming in," Burns said. "When you see the coating pattern on Iapetus, you know you've got the right mechanism⁹ for producing it."

Small, white craters¹⁰火山口 that dot Iapetus' darker half indicate a veneer外表，饰面 of dark dust, only meters deep, covering a white, icy surface that matches the rest of the satellite. The imaging data also revealed that all the materials on the leading side are much redder than the shielded and brighter trailing side - another indication that the leading side's dust came from elsewhere.

Other pictures revealed that the transition过渡，转变 from the dark to light hemispheres is not a solid line, but rather a mottled斑驳的，杂色的, patchy不调和的，拼凑成的 array of bright and dark spots.

The pattern, the scientists say, supports a theory described in a companion paper in Science that the darker parts of the moon tend to heat up when struck by sunlight, encouraging the ice to evaporate蒸发，脱水 underneath¹².

This causes any dark spots to get even darker, creating the mottled look.