CO concentration in the upper stratosphere and mesosphere of Titan from VIMS dayside limb observations at 4.7 µm

During the last 30 years, many works have focused on the determination of the CO abundance in Titan's atmosphere, but no measurement above 300km has been done yet, due to the faint emission of CO in the upper atmosphere. Nevertheless, such a study is particularly awaited as a conrmation of photochemical models that predict a uniform volume mixing ratio of CO in the whole atmosphere. Moreover, given that CO is the main atmospheric reservoir of oxygen, its actual abundance has implications on the origins of Titan's atmosphere. In this work, we analyse a set of Cassini VIMS daytime limb observations of Titan at 4.7 m, which is dominated by solar-pumped non-LTE emission of CO rovibrational bands. In order to study this non-LTE emission, we have developed a non-LTE model for CO vibrational levels. Retrieval of the CO concentration is performed following a bayesian approach and using the calculated non-LTE populations. The data set analysed is constituted by 45 limb scanning sequences - about two thousand spectra - obtained by VIMS in 2006 and 2007. CO relative abundance proles from 200 to 500km are obtained, for each set analysed. The average prole shows little variation with altitude ranging from 50 ppm at 200km to 60 ppm at 500 km. The measurements are however well compatible with a uniform volume mixing ratio prole of 55100 ppm, thus compatible with the prediction of photochemical models of a well-mixed vertical prole.