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Control/Tracking Number: 07-RC-231-AAS-DPS
Activity: Research Contributed
Current Date/Time: 7/27/2007 4:42:15 PM
Source of Nitrogen Isotope Anomaly in HCN in the Atmosphere of Titan
Author Block: Mao-Chang Liang
1
, A. Heays
2
, B. Lewis
2
, S. Gibson
2
, Y. Yung
3
1
Academia Sinica and Caltech,
2
The Australian National University, Australia,
3
Caltech.
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 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, constrain 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
.
-----
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. https://www.abstractsonline.com/submit/SubmitPrinterFriendlyVersion.asp?Contr...
7/27/2007

Oasis, The Online Abstract Submission System Page 2 of 2
:
Category (Complete) : 31. Titan: Atmosphere (Chemistry, Clouds, and Dynamics)
Facility Keywords (Complete) : None selected
Presentation Preference (Complete) : 1 - Oral Preferred
Additional Information (Complete) :
Newsworthy?
: No
I have a video for Press Officer review, : No
Status: Complete
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7/27/2007