The absence of endogenic methane on Titan and its implications for the origin of atmospheric nitrogen

We calculate the D/H ratio of CH₄ from serpentinization on Titan to determine whether Titan's atmospheric CH₄ was originally produced inside the giant satellite. This is done by performing equilibrium isotopic fractionation calculations in the CH₄-H₂O-H₂ system, with the assumption that the bulk D/H ratio of the system is equivalent to that of the H₂O in the plume of Enceladus. These calculations show that the D/H ratio of hydrothermally produced CH₄ would be markedly higher than that of atmospheric CH₄ on Titan. The implication is that Titan's CH₄ is a primordial chemical species that was accreted by the moon during its formation. There are two evolutionary scenarios that are consistent with the apparent absence of endogenic CH₄ in Titan's atmosphere. The first is that hydrothermal systems capable of making CH₄ never existed on Titan because Titan's interior has always been too cold. The second is that hydrothermal systems on Titan were sufficiently oxidized so that C existed in them predominately in the form of CO₂. The latter scenario naturally predicts the formation of endogenic N₂, providing a new hypothesis for the origin of Titan's atmospheric N₂: the hydrothermal oxidation of ¹⁵N-enriched NH₃. A primordial origin for CH₄ and an endogenic origin for N₂ are self-consistent, but both hypotheses need to be tested further by acquiring isotopic data, especially the D/H ratio of CH₄ in comets, and the ¹⁵N/¹⁴N ratio of NH₃ in comets and that of N₂ in one of Enceladus' plumes.