Retrieval of Chemical Abundances in Titan's Upper Atmosphere From Cassini UVIS Observations With Pointing Motion

摘要

Cassini/Ultraviolet Imaging Spectrograph (UVIS) Far‐UV observations of stellar occultations at Titan are well suited for probing its atmospheric composition and structure. However, due to instrument pointing motion, only 5 out of tens of observations have been analyzed. We present an innovative retrieval method that corrects for the effect of pointing motion by forward modeling the Cassini/UVIS instrument response function with the pointing motion value obtained from the SPICE C‐kernel along the spectral dimension. To illustrate the methodology, an occultation observation made during flyby T52 is analyzed, when the Cassini spacecraft had insufficient attitude control. A high‐resolution stellar model and an instrument response simulator that includes the position of the point source on the detector are used for the analysis of the pointing motion. The Markov chain Monte Carlo method is used to retrieve the line‐of‐sight abundance profiles of eleven species (CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 , HCN, C 4 H 2 , C 6 N 2 , C 6 H 6 , haze, HC 3 N, and C 2 N 2 ) in the spectral vector fitting process. We obtain tight constraints on all of the species aside from C 2 H 6 , C 2 N 2 , and C 6 N 2 , for which we only retrieved upper limits. This is the first time that the T52 occultation was used to derive abundances of major hydrocarbon and nitrile species in Titan's upper and middle atmosphere, as pointing motion prohibited prior analysis. With this new method, nearly all of the occultations obtained over the entire Cassini mission could yield reliable profiles of atmospheric composition, allowing exploration of Titan's upper atmosphere over seasons, latitudes, and longitudes.