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Titan's Atmosphere Comes from Ammonia, Huygens Data Say
http://www.physorg.com/news3111.html

"I think what's clear from the data is that Titan has accreted or acquired significant amounts of ammonia, as well as water," Lunine said. "If ammonia is present, it may be responsible for resurfacing significant parts of Titan."

He predicts that Cassini instruments will find that Titan has a liquid ammonia-and-water layer beneath its hard, water-ice surface. Cassini will see -- Cassini radar has likely already seen -- places where liquid ammonia-and-water slurry erupted from extremely cold volcanoes and flowed across Titan's landscape. Ammonia in the thick mixture released in this way, called "cryovolcanism," could be the source of molecular nitrogen, the major gas in Titan's atmosphere.

"One thing that Titan could not have done during its history is to have a liquid layer that then froze over, because during the freezing process, Titan's rotation rate would have gone way, way up," Lunine said. "So either Titan has never had a liquid layer in its interior -- which is very hard to countenance, even for a pure water-ice object, because the energy of accretion would have melted water -- or that liquid layer has been maintained up until today. And the only way you maintain that liquid layer to the present is have ammonia in the mixture."