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 two higher than the corresponding ratio for HCN. Using a onedimensional photochemical model with transport, we incorporate new isotopic photoabsorption and photodissociation cross sections of N_{2}, computed quantummechanically, 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, constrain the flux of atomic nitrogen input from the top of the atmosphere to be in the range 12×10^{9} atoms cm^{2} s^{1}.

There remains a major challenge to explain the low ^{14}N/^{15}N ratio for N_{2} 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 NH_{3}, (2) the isotopic fractionation of ^{15}N in NH_{3} 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 N_{2} which would contribute to the isotopic enrichment of ^{15}N in the N_{2} atmosphere.
 Publication:

AAS/Division for Planetary Sciences Meeting Abstracts #39
 Pub Date:
 October 2007
 Bibcode:
 2007DPS....39.4705L