Seasonal variations of chemical species and haze in Titan's upper atmosphere

Seasonal variation is significant in Titan's atmosphere due to the large change of solar insolation resulting from Titan's 26.7° axial tilt relative to the plane of Saturn's orbit. Here we present an investigation of hydrocarbon and nitrile species in Titan's upper atmosphere at 400-1200 km, which includes the mesosphere and the lower thermosphere, over more than one fourth of Titan's year (2006-2014, LS=318°-60°), using eighteen stellar occultation observations obtained by Cassini/UVIS. Vertical profiles of eight chemical species (CH4, C2H2, C2H4, C2H6, C4H2, C6H6, HCN, HC3N) and haze particles are retrieved from these observations using an instrument forward model, which considers the technical issue of pointing motion. The Markov-chain Monte Carlo (MCMC) algorithm is used to obtain the posterior probability distributions of parameters in the retrieval, which inherently tests the extent to which species profiles can be constrained. The results show that no change of the species profiles is noticeable before the equinox, while the decrease of atmospheric temperature and significant upwelling in the summer hemisphere are found five terrestrial years afterwards. Altitude of the detached haze layer decreases towards the vernal equinox then it disappears, and no reappearance is identified within the time range of our data, which is consistent with observations from Cassini/ISS. This study provides observational constraints on the seasonal change of Titan's upper atmosphere, and suggests further investigations of the atmospheric chemistry and dynamics therein.