The14N/15N ratio for N2 in the atmosphere of Titan was recently measured to be a factor of two 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 14N/15N between N2 and HCN can be explained primarily by the photolytic fractionation of 14N14N and 14N15N. The [HC14N]/[HC15N] ratio produced by N2 photolysis alone is 23. This value, together with the observed ratio, constrain the flux of atomic nitrogen input from the top of the atmosphere to be in the range 1-2×109 atoms cm-2 s-1.----- There remains a major challenge to explain the low 14N/15N ratio for N2 in the atmosphere of Titan. In view of our success in explaining the isotopic composition of HCN, we believe that the resolution of the nitrogen isotopic fraction lies in (1) the photochemistry of NH3, (2) the isotopic fractionation of 15N in NH3 photolysis, and (3) hydrodynamic escape. It is conceivable that some of the nitrogen in the primitive Titan might have been sequestered as HCN subduction to the interior of Titan, followed by conversion back to N2 which would contribute to the isotopic enrichment of 15N in the N2 atmosphere.
AAS/Division for Planetary Sciences Meeting Abstracts #39
- Pub Date:
- October 2007