Source of Nitrogen Isotope anomaly in HCN in the Atmosphere of Titan

Abstract

The 14N/ 15N ratio for N 2 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 N 2, computed quantum-mechanically, and show that the difference in the ratio of 14N/ 15N between N 2 and HCN can be explained primarily by the photolytic fractionation of 14N 14N and 14N 15N. The [HC 14N]/[HC 15N] ratio produced by N 2 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 (1-2) × 10 9 atoms cm -2 s -1.