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

✍ Scribed by Christopher R. Glein; Steven J. Desch; Everett L. Shock

• Publisher: Elsevier Science
• Year: 2009
• Tongue: English
• Weight: 331 KB
• Volume: 204
• Category: Article
• ISSN: 0019-1035
• DOI: 10.1016/j.icarus.2009.06.020

✦ Synopsis

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 2 O-H 2 system, with the assumption that the bulk D/H ratio of the system is equivalent to that of the H 2 O 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.