According to data collected by ESA's Huygens probe, the origin of Titan's atmospheric methane may be in the subsoil.
ESA researchers have made models based on the data collected by Huygenes. According to this, it seems that methane is trapped in a kind of ice that is under the skin of the Titan, and that it emerges from the chemical processes caused by light. It is a slow process, but it is believed that occasionally volcanic eruptions occur in which liquid methane is released. Now, through Cassini's tools, they hope to see one of these eruptions.