Planetary science is never boring and often very surprising. Take for instance the new images sent back by the Cassini space probe of Saturn’s moon Iapetus. The moon has always been mysterious since the first Voyager images revealed a light hemisphere and a dark hemisphere. The contrast cannot be overemphasized: the dark side only reflects 4% of the light that hits it, while the light side reflects 60% of the light.

The Voyager images only showed a small portion of the moon. Cassini’s new images were full of surprises, the most shocking being the existence of a ridge running almost exactly on the moon’s equator that may or may not have something to do with the dark material. Some of the mountains on this ridge are three times higher than Mount Everest. At the boundary of light and dark material, the dark material appears to streak out across the light material, as if it were a coating that fell onto the surface. Scientists are scratching their heads, trying to come up with new theories to explain how such structures can be formed. Did the moon erupt dark material from within, perhaps at the mountain belts? Or does the material come from outside, dropping onto the moon’s surface by some mysterious Saturnian process? What is that dark material?

We might have to wait awhile for answers. Cassini won’t return to Iapetus until September 2007, when it will take much more detailed images during a closer flyby.