As methane is broken down by light-induced chemical reactions over a timescale of tens of millions of years, it can't just be a remnant of the atmosphere present when Titan itself was formed, and it must be replenished quite regularly. |
Methane is only around for a short time geologically. The time scale is somewhere between 10 and 100 million years. |
This model is in agreement with the observations made so far by both the Huygens probe that landed on Titan on 14 January 2005 and the remote sensing instruments on board the Cassini spacecraft. |