Titan's atmosphere contains almost 5% methane. The current understanding based on the Voyager and Cassini readings is that hydrogen is released in atomic and molecular form from the photochemical conversion of methane along with trace amount of more complex, less saturated hydrocarbons.
It is also found that the hydrogen is disappearing at a much rapid rate. Based on the data from Cassini's composite infrared spectrometer and ion and neutral mass spectrometer, scientists developed a model of densities of hydrogen in different parts of the atmosphere and the surface. It was predicted that hydrogen will be distributed fairly equally at all parts of atmosphere but according to the new model, they found a disparity in the hydrogen densities that lead to a flow down to the surface at a rate of about 10000 trillion trillion hydrogen molecules per second. This is about the same rate at which the molecules escaping out of the upper atmosphere. They are currently not sure where did the hydrogen escaped but a plausible explanation is that the hydrogen is reacting with the water ice on Titan surface and creating a film of an hydrocarbon rich organic compound of at least a few millimeters to centimeters thick, but possibly much deeper in some places. The ice remains covered up even as liquid methane and ethane flow all over Titan's surface and fill up lakes and seas much as liquid water does on Earth making it hard to detect the composition of the film.
- Darrell F. Strobel, Molecular hydrogen in Titan’s atmosphere: Implications of the measured tropospheric and thermospheric mole fractions, Icarus, Volume 208, Issue 2, 2010, DOI: 10.1016/j.icarus.2010.03.003.