Scientists unravel riddle of Saturn's yin-yang moon Scientists in Germany and the US have solved the 300-year-old mystery of how Saturn's moon Iapetus came by its bizarre, asymmetrical appearance - the moon is black on one side and white on the other. In two papers in the journal Science, the researchers explain how dusty debri... Scientists in Germany and the US have solved the 300-year-old mystery of how Saturn's moon Iapetus came by its bizarre, asymmetrical appearance - the moon is black on one side and white on the other. In two papers in the journal Science, the researchers explain how dusty debris from other Saturnian moons builds up on one side of the moon; this in turn alters the temperature of the moon and drives the migration of water ice to the other side of the moon. Iapetus was discovered in 1671 by the French-Italian astronomer Giovanni Domenico Cassini. He quickly noticed that the moon had an unusual appearance, being much darker on its leading side (that faces the direction of orbit) than on the trailing side. In fact, the trailing side is some 10 times brighter than the leading side. 'This is not the most fundamental problem in the world, but it's an enigma that has been puzzling astronomers for centuries,' commented Joseph Burns of Cornell University in the US. Over the years, a number of theories have been put forward to explain Iapetus' appearance. One theory suggests that the dust on Iapetus comes from a debris cloud from a meteor impact within the Saturn system. Some have suggested that interplanetary dust could have gradually accumulated on one side of the moon, while others believed that geological processes inside Iapetus could be releasing dark materials onto the surface. A fourth theory, which combines the deposition of dust from external sources with thermal processes on the lunar surface, was proposed in 1974. However, this theory has been largely overlooked in the intervening years, even though its predictions of how the moon's appearance would change over time have proven correct. In this study, researchers on both sides of the Atlantic studied images taken by the Cassini-Huygens spacecraft since 2004, with a particular focus on pictures taken during a close flyby on 10 September 2007. Using the data, the team was able to model what was happening on Iapetus. Their research reveals that dark dust breaks away from Saturn's other moons, especially Phoebe, and falls on Iapetus' leading hemisphere. Studies of craters on Iapetus suggest that the layer of dust is metres deep. However, the story doesn't end there. 'It is impossible that the very complicated and sharp boundary between the dark and the bright regions is formed by simple infall of material,' explained Tilmann Denk of the Freie Universität Berlin in Germany. 'Thus, we had to find another mechanism.' The dark surface absorbs large amounts of heat from the sun, and because Iapetus rotates so slowly (once every 79 days), the dark surface is exposed to the sun for a long time. Temperatures at the equator become high enough to trigger the evaporation of the ice under the dust. The evaporating ice recondenses at the moon's poles and on the trailing side, giving it its distinctive bright white appearance that contrasts so sharply with the dark, dusty leading side. According to the researchers, Iapetus' small size (it is just 1,500 km across) and low gravity mean that it is relatively easy for ice to move from one side of the moon to the other in this way. John Spencer of the Southwest Research Institute in Colorado, US, concluded: 'Iapetus is the victim of a runaway feedback loop, operating on a global scale.' Countries Germany, United States
