Everyone loves a good mystery story, and in all of astronomy few can come close to a long-running saga called "The Case of the Two-Faced Moon."
In Act I, Gian-Domenico Cassini discovers, in 1671, a strange satellite orbiting Saturn or at least a moon he sees half of the time. He can spot Iapetus, as it came to be called, when it's on one side of Saturn but not the other. In Cassini's words, the moon goes through "a period of apparent Augmentation and Diminution." His puzzlement is understandable, as astronomers eventually realize that half of Iapetus is as white as snow and the other half dark as charcoal.
In Act II, planetary astronomer Steve Soter posits in 1974 that Iapetus is an otherwise icy satellite with one hemisphere that's become coated with dark dust. The proposed source of this face paint is Phoebe, the next large satellite outward, which is circling Saturn "backward," in a retrograde orbit. Soter argues that dust blasted off Phoebe by impacts eventually drifts inward and splatters onto the side of Iapetus that continuously faced forward in its orbit. But others argue instead that something has oozed from the moon's interior and blanketed its dark face.
Voyagers 2 confirms the moon's bizarre nature when it visits the Saturnian system in 1981. But it comes no closer than 600,000 miles, too far away to reveal what's really going on.
Act III begins with a close flyby of Iapetus just a month ago, on September 10th, by the Cassini orbiter. Swooping past at a distance of just 1,000 miles, the spacecraft scrutinizes the strange moon's surface with telescopic cameras and probes its chemistry with sensitive spectrometers. Then the scene shifts to Orlando, Florida, where planetary specialists from around the world have gathered to learn just what Cassini saw.Iapetus might not be colorful, but it's still a stunning study in black and white.
Soter and other "externalists" apparently got it right. The dark half of Iapetus is covered with fine, ice-free material that's knee deep in places. It's "a remarkably fluffy surface," reports Bonnie Buratti (Jet Propulsion Laboratory, exactly what would result from a gradual build-up of fine particles raining down from above. The chemical make-up remains unclear, though carbon compounds are present, and researchers can offer only educated guesses as to its source. Phoebe is still in the running, though its surface isn't a perfect spectral match.
But the scientists meeting in Florida at least can agree on what happened after the dark stuff arrived. Midday temperatures on the dark face, named Cassini Regio, can climb to 220° a long way from Orlando's swelter but warm enough to make water ice turn to vapor.
"The reason Iapetus gets so warm is that it rotates so slowly," explains John Spencer (Southwest Research Institute). Its leisurely 79-day-long spin, in synch with its orbital period around Saturn, lets the dark side bake in sunlight for weeks at a time. Once vaporized, the water migrates to colder climes on the back side and at the poles. And it's a runaway process that keeps driving the water out. According to Spencer's calculations, it'd take less than 100 million years to create the stark dichotomy seen today.
We don't yet know what the dark stuff is or where it came from. Those answers will come in time. But I'm sure that Cassini would have been pleased to see, 336 years after first spotting this two-faced moon, that modern-day astronomers are just as fascinated by it as he was.