Iapetus is often called Saturn's most bizarre moon, due to its starkly contrasting hemispheres -- one black as coal, the other white as snow.

Images taken by the Cassini-Huygens spacecraft, orbiting Saturn since 2004, offer the most compelling evidence to date 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. Analyzed by a research team that includes Cornell scientists, the images are detailed 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 that's been puzzling astronomers for centuries."

Since pictures of Iapetus from the Voyager mission 30 years ago confirmed its intriguing color scheme, scientists have puzzled over whether Iapetus' dark-light contrast was the result of external debris hitting some of the moon, or whether the dark dust was the result of interior 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 that Iapetus' darker half, called Cassini Regio, is the result of the planet's leading side getting bombarded by dusty debris 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 and near Phoebe, 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 is out there, and its companion moons are 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 mechanism for producing it."

Small, white craters that dot Iapetus' darker half indicate a veneer 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 showed that the transition from the dark to light hemisphere is not a solid line, but rather a mottled, patchy array of bright and dark spots.

The pattern, the scientists say, supports a previous theory described in a companion paper in Science that the darker parts of the moon tend to heat up when struck by sunlight, causing the ice to evaporate underneath. This causes any dark spots to get even darker, creating the mottled look.

The research team includes Paul Helfenstein and Peter C. Thomas, both senior research associates at Cornell. The paper's first author is Tilmann Denk, Cassini imaging scientist at the Free University in Berlin, Germany. The Cassini program is an international cooperative effort involving NASA, the European Space Agency and the Italian space agency.