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

Abstract

The 14 N/ 15 N 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 14 N/ 15 N between N 2 and HCN can be explained primarily by the photolytic fractionation of 14 N 14 N and 14 N 15 N. The [HC 14 N]/[HC 15 N] 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 atoms cm s .