Titan's upper atmosphere composition and temperature from Cassini-ultraviolet imaging spectrograph stellar and solar occultations

This PhD project focuses on the study of the upper atmosphere of Titan from the analysis of Cassini-UVIS data and laboratory measurements of absorption cross sections.A characterization of the UVIS instrument and observations was necessary. Data from the Far UltraViolet (FUV) and Extreme UltraViolet (EUV) channels of UVIS were analyzed and corrected for instrument effects. From the analysis of 8 solar occultations in EUV, N2 and CH4 number density profiles were derived with an inversion regularization method. Temperatures were obtained from the N2 profiles assuming an isothermal upper atmosphere. Stellar occultations in FUV were modeled and a density retrieval technique characterized. The possibility of detection for different molecules (some of them not detected by this technique before) was analyzed. Then, using a Levenberg-Marquardt minimization algorithm, column density profiles for different hydrocarbons and nitriles, and optical depth of aerosols were obtained from simulated data. The column densities and optical depth were inverted with a regularization method to obtain number density profiles for the molecules and extinction profiles for the aerosols. The procedure was finally applied to 2 stellar occultations measured by UVIS. The species studied are CH4, C2H2, HCN, C2H4, C4H2, C6H6, HC3N, CH3, and aerosols (AER). The profiles from the stellar and solar occultations were obtained for different times and locations. The temperatures derived were analyzed as a function of geographical and temporal variables---latitude, longitude, date of observation, etc.---without a clear correlation with any of them, although a trend of decreasing temperature towards the poles could be observed. The globally averaged temperature obtained is (144 +/- 2) K. Atmospheric variability was discussed on the light of these results.Benzene (C6H6) is an important molecule detected in Titan's atmosphere because it is thought to be intermediate between the gas and solid particle formation. Measurements of absorption in the ultraviolet by benzene gas, at temperatures covering the range from room temperature to 215 K, were performed in different international facilities. From them, benzene absorption cross sections were derived and analyzed in terms on the transitions observed. No significant variation with measurement temperature was observed. Implications of this results for the identification of benzene in Titan's thermosphere by UVIS were discussed. The absorption cross sections were used in the derivation of C6H6 abundances in Titan's thermosphere commented above.In summary the analysis of UVIS observations presented contribute to the characterization of the upper atmosphere through N2 density profiles, thermospheric temperatures, density profiles of minor species and extinction profiles from aerosols. This observational data will help to constrain and contrast photochemical models. The abundance profiles and temperatures given for different geographical and temporal coordinates can be used to further study the atmospheric variability. As a whole, the results of this work are expected to help in the understanding of Titan's upper atmospheric composition and dynamics. This knowledge, combined with information about Titan's lower atmosphere and surface, will help to understand the evolution of organic molecules in this neighboring abiotic celestial body