Why is there so much methane on Titan?

This Cassini-Huygens article ponders the abundance of methane on Titan. Titan's atmosphere is mostly nitrogen but there are also methane and many other organic compounds. On Earth, life refreshes the methane supply as it is a by-product of metabolism. This is not likely to be the source of methane on Titan but if, as on Earth, sunlight is continuously destroying methane, how is methane getting into Titan's atmosphere?

WebElements: the periodic table on the WWW [http://www.webelements.com/]

Copyright 1993-2011 Mark Winter [The University of Sheffield and WebElements Ltd, UK]. All rights reserved.