Hydrocarbon and nitrile species in Titan′s upper atmosphere from Cassini/UVIS stellar occultation observations

Abstract:

We present vertical profiles (18 in total) of hydrocarbon and nitrile species in Titan’s upper atmosphere, based on stellar occultation observations obtained by the Ultraviolet Imaging Spectrograph (UVIS) during the lifetime of the Cassini mission. Photochemistry and haze formation originate in photolysis of methane in the upper atmosphere of Titan. Cassini/UVIS Far-UV stellar occultation observations are effective in extracting the atmospheric composition and structure. Most of the occultations, however, require correction for pointing motion, seriously complicating the reduction process. To address the problem, Fan et al. (2019) presented a new method with an instrument simulator using forward modeling to include the effect of pointing motion, and applied the Markov Chain Monte-Carlo (MCMC) algorithm in the retrieval. Data from T52, a flyby with large pointing motion, were successfully analyzed. Some major species (e.g., C₂H₆) are not well-constrained at most of the altitudes due to overlapping spectral features with CH₄. Applying the new method to all available observations, latitude-dependent vertical profiles of atmospheric composition in Titan’s upper atmosphere have been obtained.

Reference: S. Fan, D. E. Shemansky, C. Li, P. Gao, L. Wan and Y. L. Yung (2019). Retrievals of Abundances of Hydrocarbons and Nitrile Species in Titan’s upper Atmosphere. Earth and Space Science 6, 1057.