The transportation of ice to the bright, trailing hemisphere of Saturn’s moon Iapetus is being sped up by red ‘gunk’ being deposited on the moon’s leading hemisphere, say scientists from Germany and the United States.

Ever since it was discovered in 1671, Iapetus has been known as the moon of two halves. Its leading hemisphere is dark, whilst its trailing hemisphere is very bright, leading to a difference in brightness at opposite sides of its orbit of 1.7 magnitudes as seen from Earth. In recent years, NASA’s Cassini spacecraft has been able to spend time observing Iapetus up close, and findings have shown that the trailing hemisphere is so bright because 80 percent of its surface is covered in ice that has accumulated there. The leading hemisphere, by comparison, is covered in a dark red material – it looks like Iapetus has passed through a great cloud of dust.

Where this material originated was a mystery until the discovery earlier this year of a very faint and thin, but very large, outermost ring of Saturn (see http://www.astronomynow.com/news/n0910/07satring/) that matched the orbit of the outermost moon, Phoebe. The idea is that micrometeorite impacts smash dust off the surface of Phoebe, creating a ring of dust that spreads out, only for Iapetus to pass through it and get a coating of gunk.

According to research by John Spencer of the Southwest Research Institute in Boulder, Colorado, USA, the dark red dust – mostly nitrogen bearing organic compounds – absorbs sunlight and heats the leading hemisphere, encouraging the transportation of water-ice from the dark hemisphere to the bright one. Similar to the phenomenon seen on Earth’s own Moon by the Chandrayaan 1 lunar orbiter (http://www.astronomynow.com/news/n0909/24moon/ ) whereby the sunlight warms molecules of water in the lunar regolith, causing them to begin hopping to the cold traps at the lunar poles, water-ice is migrating from Iapetus’ warmer leading hemisphere to the colder trailing hemisphere. The loss of ice on the leading hemisphere leaves more dark surface exposed, which absorbs more heat, exacerbating the situation in a runaway process.

“Most of the ice migrating from the leading side is settling on top of existing ice, so it is not increasing the total ice cover,” Spencer told Astronomy Now. “In fact, we might expect some slow decrease in the ice cover even on the trailing side in the future, because the ice is likely to be slowly lost from the equatorial regions and head for the poles, but this might take a few more billion years.” Indeed, the poles are already where the ice is concentrated the most.

Why are other ice moons of Saturn such as Phoebe and Hyperion not behaving in this lopsided fashion? According to Tilmann Denk of the Freie Universitat in Berlin, Iapetus is in a unique situation. “It’s slow rotation (compared to Phoebe) and synchronous rotation (that’s the difference to Hyperion) make Iapetus unique in the Saturnian system,” he says. “Indeed, I believe the reddish colour of Hyperion represents how Iapetus’ leading side would if the runaway migration of water-ice were not active there.”

Individual papers featuring Denk and Spencer as lead authors that describe the research are published in the 10 December edition of the online journal Science.