Analysis of four solar occultations by Titan’s atmosphere with the infrared instrument of VIMS : Haze, CH4, CH3D and CO vertical profiles
Résumé
Titan, the largest satellite of Saturn, has a dense atmosphere mainly composed of nitrogen, with methane at percent level and minor species. It is also covered by a thick and global photochemical organic haze. In the last two decades, the observations made by the Cassini orbiter and the Huygens probe have greatly improved our knowledge of Titan's system. The surface, the haze, the clouds and the chemical species can be studied and characterized with several instruments simultaneously. On the other hand, some compounds of its climatic cycle remains poorly known. This is clearly the case of the methane cycle which is however a critical component of Titan's climate and of its evolution. We have reanalyzed four solar occultations by Titan's atmosphere observed with the infrared part of the VIMS (Visual Infrared Mapping Spectrometer) instrument. These observations were already analyzed by Maltagliati et al. (2015) but, here, we use significantly improved methane spectroscopic data (Rey et al. (2018)). We retrieve the haze properties, not treated previously, the mixing ratios of methane, deuterated methane and CO in the stratosphere and in the low mesosphere. The methane mixing ratio in the stratosphere is much lower (about 1.1%) than expected from Huygens measurements (about 1.4 to 1.5%). This is consistent with previous results obtained with other instruments. However, features in the methane vertical profiles clearly demonstrate that there are interactions between the methane distribution and the atmosphere circulation. We also retrieve the haze extinction profiles and the haze spectral behaviour. We find that aerosols are aggregates with a fractal dimension D f ≃ 2.3 ± 0.1 rather than D f ≃ 2 as previously though. Our analysis also reveals noticeable changes in their size distribution and their morphology with altitude and time. These changes are also clearly connected to the atmosphere circulation and concerns the whole stratosphere and the transition between the main Article number, page 1 of 37 Rannou, Coutelier, Rey and Vinatier: Solar occultations by Titan's atmosphere and the detached haze layers. We finally display the vertical profiles of CH 3 D and CO for the four observations. Although the latter retrievals have large error bars due to noisy data, we can derive values in agreement with other works.
| Origine | Fichiers produits par l'(les) auteur(s) |
|---|---|
| Licence |