37th DPS Meeting, 4-9 September 2005
Session 41 Titan 1
Oral, Wednesday, September 7, 2005, 4:30-6:00pm, Music Concert Hall

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[41.02] Vertical profiles and latitudinal variations of nitrile abundances in Titan's atmosphere derived from Cassini/CIRS limb and nadir data

N. A. Teanby, P. G. J. Irwin, R. de Kok (Oxford University), C. A. Nixon (Goddard Space Flight Center), A. Coustenis, B. Bézard (Observatoire de Paris-Meudon), S. B. Calcutt, N. E. Bowles (Oxford University), F. M. Flasar (Goddard Space Flight Center), L. Fletcher, C. Howett, F. W. Taylor (Oxford University), CIRS Team

The Cassini/Huygens spacecraft entered orbit around Saturn on 1\mbox{st} July 2004. Since then, the Composite InfraRed Spectrometer (CIRS) has successfully returned thousands of infrared spectra of Titan at resolutions from 0.5--15~cm-1 in the mid- and far-IR (10--1400~cm-1 or 1000--7~\mum). This spectral region is rich in features from many nitrile compounds, which allows their abundance to be retrieved.

Nitrile species provide the possibility of tracing atmospheric circulation on Titan. One Titan year lasts 30 earth years. Therefore, with photochemical lifetimes ranging from under a year to tens of years, nitrile abundances can be used to probe time scales relevant to atmospheric motion. The observed variations can then be compared to predictions from dynamical and photochemical models.

We have used the limb sounding capabilities of CIRS to retrieve vertical profiles of nitrile species, including HCN and HC3N. First, the segment of the mid-IR spectrum from 1240-1360~cm-1 was used to retrieve stratospheric temperature. Second, sub-spectra were extracted from the 10--800~cm-1 region and used to obtain vertical profiles of nitriles. The limiting vertical resolution of these profiles is determined by the field of view size at the tangent height, which varies between 10 and 50~km. Profiles from the equator and north polar region will be presented.

We also discuss the latitude variation of nitrile compounds derived from over 18000 nadir spectra selected from 2.5~cm-1 resolution mapping sequences taken from July 2004 to April 2005 and covering 90S to 60N. Contribution functions for these observations peak around 3~mbar, well into the stratosphere. HCN, HC3N, and C2N2 all display a marked increase toward the north. HCN displays a 4 fold increase from 0--60N. A simple 1D numerical model coupled with the HCN variation implies a downwelling velocity of 0.3~mms-1. The nadir determinations will be compared with vertical profiles derived from the limb data.

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