Captain's Log December 30, 2004 Iapetus Up Ahead With another circuit around Saturn completed, we are once again far from the planet, out near the orbit of Iapetus, and surveying all around us. Our last orbit of the planet brought us another low flyby over Titan's Xanadu region and even closer inspection of its strange variegated geography, a rare detailed look at a long streak of a cloud hovering at southern mid-latitudes in Titan's troposphere, and this time a clear glimpse of a cloud-free south polar region. Though clouds are sparse, Titan's atmosphere is clearly dynamic and its upper layers of haze are finely structured. So far, Titan is exhibiting the complexity in body and soul befitting a moon revered for its enormous potential to teach and inform. We should have expected no less. Soon after leaving Titan, we slipped by the icy, airless moon Dione for a magnified view of the puzzling bright wisps, discovered by Voyager 24 years ago, that criss-cross its trailing hemisphere and for which Dione is renowned. In a tale of two moons, what Titan has jealously guarded Dione has freely given. By dint of a few detailed images, we found the mother lode on Dione that we came to find: the wisps of Dione are not thick ice deposits as some had previously surmised but a complex pattern of fine, braided fractures whose clean icy cliff faces give the region its bright white appearance from afar. This is surely one of our most surprising findings thus far. Now, with the Huygens probe successfully deployed and safely on its way to an historic atmospheric entry and touchdown on Titan in two weeks' time, we ready ourselves for a first close encounter with Iapetus, up ahead and only a day away. With a dramatically split countenance -- one side as white as anything else we see around Saturn, and the other as dark as dark gets -- this is one of Saturn's most intriguing bodies and one of the most unusual in all the solar system. Was Iapetus the site of a major cataclysmic event long ago that caused sub-surface material, unusually dark and rich in organic substances, to gush out onto its surface and coat its leading hemisphere? Or did it sweep up, in its never-ending race around Saturn, the remains of debris that originated on an even more distant moon, like Phoebe, and slowly evolved inward towards the orbit of Iapetus? This is a question that burns in the minds of some of us. And if Iapetus proves as cooperative as Dione, we may soon know the answer. Carolyn Porco Cassini Imaging Team Leader CICLOPS Space Science Institute Boulder, CO

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