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Published August 1, 2007 | Published
Journal Article Open

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


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 (1-2) × 10^9 atoms cm^(-2) s^(-1).

Additional Information

© 2007 The American Astronomical Society. Received 2007 May 7; accepted 2007 June 15; published 2007 July 12. This research was supported by NASA grant NNG06GF33G and Cassini grant JPL.1256000 to the California Institute of Technology, and Australian Research Council Discovery Program grants DP0558962 and DP0773050 to the Australian National University. Special thanks are due to D. Shemansky for providing a high-resolution solar spectrum. We also thank K. Dere, F. Mills, I. Ribas, and T. Woods for helpful discussion on the issue of the solar EUV flux, and N. Heavens and R. L. Shia for reading the manuscript. We thank W. DeMore, C. Miller, and H. Waite for valuable discussions on isotopic fractionation.

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