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

摘要

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 ~(15)N-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 ~(15)N/~(14)N ratio of NH_3 in comets and that of N_2 in one of Enceladus' plumes. All rights reserved.