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Oxygen compounds in Titan's stratosphere as observed by Cassini CIRS |
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| Authors: | R de Kok PGJ Irwin E Lellouch B Bézard S Vinatier CA Nixon C Howett NE Bowles FW Taylor |
| Institution: | a Atmospheric, Oceanic & Planetary Physics, Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK b Observatoire de Paris, LESIA, F-92195 Meudon, France c Université Pierre et Marie Curie-Paris 6, UMR 8109, F-75005 Paris, France d Department of Astronomy, University of Maryland, College Park, MD 20742, USA e NASA/Goddard Space Flight Center, Code 693, Greenbelt, MD 20771, USA |
| Abstract: | We have investigated the abundances of Titan's stratospheric oxygen compounds using 0.5 cm−1 resolution spectra from the Composite Infrared Spectrometer on the Cassini orbiter. The CO abundance was derived for several observations of far-infrared nadir spectra, taken at a range of latitudes (75° S-35° N) and emission angles (0°-60°), using rotational lines that have not been analysed before the arrival of Cassini at Saturn. The derived volume mixing ratios for the different observations are mutually consistent regardless of latitude. The weighted mean CO volume mixing ratio is 47±8 ppm if CO is assumed to be uniform with latitude. H2O could not be detected and an upper limit of 0.9 ppb was determined. CO2 abundances derived from mid-infrared nadir spectra show no significant latitudinal variations, with typical values of 16±2 ppb. Mid-infrared limb spectra at 55° S were used to constrain the vertical profile of CO2 for the first time. A vertical CO2 profile that is constant above the condensation level at a volume mixing ratio of 15 ppb reproduces the limb spectra very well below 200 km. This is consistent with the long chemical lifetime of CO2 in Titan's stratosphere. Above 200 km the CO2 volume mixing ratio is not well constrained and an increase with altitude cannot be ruled out there. |
| Keywords: | Titan Atmospheres composition |
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