Source of Nitrogen Isotope Anomaly in Hcn in the Atmosphere of Titan

The N/N ratio for N2 in the atmosphere of Titan was recently measured to be a factor of 2 higher than the corresponding ratio for HCN. Using a one-dimensional photochemical model with transport, we incorporate new isotopic photoabsorption and photodissociation cross sections of N2, computed quantum-mechanically, and show that the difference in the ratio of N/N between N2 and HCN can be explained primarily by the photolytic fractionation of NN and N N. The [HCN]/[HCN] ratio produced by N2 photolysis alone is 23. This value, together with the observed ratio, constrains the flux of atomic nitrogen input from the top of the atmosphere to be in the range atoms cm s . 9 2 1 (1–2) # 10 Subject headings: atmospheric effects — methods: numerical — molecular processes — planetary systems — planets and satellites: individual (Titan) — radiative transfer