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

Abstract

We calculate the D/H ratio of CH ₄ from serpentinization on Titan to determine whether Titan's atmospheric CH ₄ was originally produced inside the giant satellite. This is done by performing equilibrium isotopic fractionation calculations in the CH ₄-H ₂O-H ₂ system, with the assumption that the bulk D/H ratio of the system is equivalent to that of the H ₂O in the plume of Enceladus. These calculations show that the D/H ratio of hydrothermally produced CH ₄ would be markedly higher than that of atmospheric CH ₄ on Titan. The implication is that Titan's CH ₄ 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 CH ₄ in Titan's atmosphere. The first is that hydrothermal systems capable of making CH ₄ 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 CO ₂. The latter scenario naturally predicts the formation of endogenic N ₂, providing a new hypothesis for the origin of Titan's atmospheric N ₂: the hydrothermal oxidation of ¹⁵N-enriched NH ₃. A primordial origin for CH ₄ and an endogenic origin for N ₂ are self-consistent, but both hypotheses need to be tested further by acquiring isotopic data, especially the D/H ratio of CH ₄ in comets, and the ¹⁵N/ ¹⁴N ratio of NH ₃ in comets and that of N ₂ in one of Enceladus' plumes.