Scientists believe a combination of space dust and temperature may explain the mysterious two-toned 'yin-yang' appearance of the strange Saturnian ice moon Iapetus.The moon's leading face in its orbit around Saturn is as dark as soot, while the trailing face is bright as ice. The puzzle is compounded by the fact that the dividing line between the bright and dark faces is amazingly sharp. The two faces and the sharp divide between them are mysteries which have puzzled scientists ever since the moon's discovery by French-Italian astronomer Giovanni Cassini in 1671.
Now a report in the journal Science by Dr John Spencer of the Southwest Research Institute in Boulder, Colorado and Dr Tilmann Denk of the Free University of Berlin, Germany provide an plausible explanation. Using data from the Cassini spacecraft, which is studying the Saturnian system, they've created a model of the moon.
It shows the build-up of dark material on the leading face of Iapetus is dust and debris shed by other Saturnian moons and a recently discovered dust ring which surrounds Saturn.
Dr Simon O'Toole, a planetary scientist with the Anglo Australian Observatory says "the dust ring appears to be generated by ejecta from objects impacting the Saturnian moon Phoebe."
Spencer and Denk's model shows the build-up of material raises the temperature of the dark, leading hemisphere, forcing water-ice to the moon's poles and its colder, trailing hemisphere.
The relatively small size of Iapetus - just 1460 kilometers across - and its low gravity, allow the ice to move easily from one hemisphere to the other.
They say this is unique to Iapetus because its slow rotation produces unusually high daytime temperatures and water sublimation rates, both of which help the process.
But O'Toole says the heat is more likely caused by gravitational tidal forces generated by Iapetus' interaction with Saturn.
In a separate report in Science, a team led by Denk describes how images from the Cassini spacecraft reveals that the leading face of Iapetus isn't just darker but also redder than its trailing face.
They say the same dust and debris which is triggering the heat-driven migration of water is also responsible for this difference in redness.
"It's fascinating that a 340 year old problem discovered by Cassini the scientist, can be explained by some very nice work thanks to another Cassini, this one a spacecraft," says O'Toole.