Titan's Upper Atmospheric Structure Derived From Voyager Ultraviolet Spectrometer Observations

Astronomy and Astrophysics – Astronomy

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Scientific paper

The Voyager 1 Ultraviolet Spectrometer (UVS) observations are the only direct measurements we have of Titan's upper atmosphere. Previous analysis of the two UVS solar occultations yielded densities for N_2, CH_4, and C_2H_2, as well as the thermospheric temperature. These results serve as the upper atmospheric boundary conditions in models of Titan's atmosphere; however, there are discrepancies between the observations and models, and the previous analysis itself is known to be internally inconsistent. We have undertaken a reanalysis of the UVS solar occultations to resolve these differences and to extract the maximum amount of information from the data. In so doing, we have developed a detailed model of the UVS detector and an analysis method tailored to retrieving multiple species from an occultation of a finite-sized source such as the sun. Our analysis has yielded density profiles for nine species in Titan's upper atmosphere and a new measurement of the thermospheric temperature. We find higher N_2 densities and lower CH_4 and C_2H_2 densities than those previously determined. We also find a thermospheric temperature of 150-155 K instead of 176-196 K as in the early analysis. Densities for C_2H_4, C_2H_6, C_4H_2, C_2N_2, HCN, and HC_3N are retrieved in the upper atmosphere for the first time. These results provide improved and expanded constraints for the atmospheric models and should provide scientists with a better view of Titan's upper atmosphere, which is important in planning for the upcoming Cassini mission.

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