Time-history of the isotopic ratios in Titan's atmosphere

Saturn's moon Titan is the only moon with a significant atmosphere, with a surface pressure of 1.5 times the surface pressure of Earth's atmosphere. The main constituents of Titan's atmosphere are molecular nitrogen (98{\%}) and methane (1.4{\%}). Diffusion, escape and chemical processes fractionate the isotopic ratios $^{14}$N/$^{15}$N in N$_{2}$, $^{12}$C/$^{13}$C in CH$_{4}$, and D/H in CH$_{4}$. Diffusion and escape result in a preferential loss of the lighter isotopes. Photochemistry may result in loss of the lighter isotopes in CH$_{4}$, but has been shown to have the opposite effect for N$_{2}$. A model constructed (Mandt et al. 2009) to track the isotopic ratios as a function of geologic time found that the $^{14}$N/$^{15}$N in N$_{2}$ could not have evolved from the terrestrial ratio to its current value as a result of atmospheric escape, and that the $^{12}$C/$^{13}$C measured in CH$_{4}$ limited the length of time for methane to have consistently resided in the atmosphere to less than 200 million years. Recent re-analysis of the Huygens GCMS data from Titan's surface (Niemann et al. 2010) has revised the $^{14}$N/$^{15}$N in N$_{2}$ and $^{12}$C/$^{13}$C in CH$_{4}$, resulting in significant changes to model results for the evolution of methane. We will present these updated results on the evolution of Titan's atmosphere and discuss the implications for the history of Titan. Mandt, K. E., et al., 2009, Planetary and Space Science, 57, 1917-1930. H.B.Niemann, et al., 2010. J. Geophys. Res., in press