Making Methane

There’s a problem with Titan, the largest moon of Saturn. There’s a fair amount of methane in its atmosphere and in its big lakes. But under the sunlight, the methane shouldn’t last very long — only a few million years. That means that fresh methane is being added to the atmosphere, although scientists haven’t found the source.
Titan is the second-largest moon in the solar system — bigger than the planet Mercury. It’s enveloped by an atmosphere that’s denser and colder than the air on Earth.
The atmosphere contains a variety of hydrocarbons — mostly methane, with a bit of ethane, acetylene, and others. The ethane and methane form clouds and fill lakes around Titan’s poles.
The problem, though, is that methane in the upper atmosphere gets destroyed by sunlight. This process is so efficient that it would take only a few million years to deplete all the methane in Titan’s atmosphere today. So there must be a source of fresh methane somewhere on the big moon.
One possibility is volcanoes t