Photos sent back to Earth from the Cassini space probe have thrown more light on why Saturn’s moon Iapetus is dark on one side and light on the other.

“This is not the most fundamental problem in the world,” says Joseph Burns, of Cornell University. “But it’s an enigma that’s been puzzling astronomers for centuries.”

The leading suspect in the case of the light/dark sides is dust collecting on the leading face of the moon. In this week’s Science photos from Cassini show metre-deep dust on the dark side covers around 40% of the leading face with bright, pole-facing slopes close to the edges and small bright craters.

However, as John Spencer and Tilmann Denk note in another article in this week’s Science, mere accumulation of space junk wouldn’t completely explain the shape of the dark patch. Instead, they propose that the deposition of dark materials raises the temperature of the leading face, as the darker colour means less sunlight is reflected away. This causes water molecules to sublime.

As Iapetus has hardly any atmosphere these molecules follow ballistic trajectories for hundreds of kilometres before re-impacting the surface, having migrated most of the way round to the trailing side, which gets lighter from all the ice build-up.

“The resulting increase in temperatures and sublimation rates produces a runaway process that proceeds until sublimation is cut off by formation of a dark ice-free lag deposit on the leading hemisphere,” they write. “…This mechanism is unique to Iapetus among the Saturnian satellites because its slow rotation produces unusually high daytime temperatures and H2O sublimation rates for a given albedo.”

Image: Cassini imaging team (click for larger version).