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-H-2 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 enclogenic N-2, providing a new hypothesis for the origin of Titan's atmospheric N-2: the hydrothermal oxidation of N-15-enriched NH3. A primordial origin for CH4 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 CH4 in comets, and the N-15/N-14 ratio of NH3 in cornets and that of N-2 in one of Enceladus' Plumes. (C) 2009 Elsevier Inc. All rights reserved.