An updated view of Titan’s upper atmosphere from Cassini/UVIS airglow observations: Constraints on atmospheric structure and haze.

Titan’s atmosphere is one of the most complex environments in the solar system. The intense photochemistry of the main atmospheric gases (N₂ & CH₄) drives a rapidly expanding molecular growth of organic species that terminates with the formation of photochemical hazes. However, many aspects of this atmosphere remain elusive: observations with the Cassini-Huygens space mission reveal that Titan’s upper atmosphere is temporally variable through unidentified mechanisms, while the processes driving the gas to haze transition are largely unknown due to the lack of constraints on the haze microphysical properties in the upper atmosphere. Here we discuss observations obtained with the Cassini UltraViolet Imaging Spectrograph (UVIS) from 2004 to 2017 that provide a detailed view of the upper atmosphere. Spectra from the FUV detector reveal the dominance of emissions from the de-excitation of molecular and atomic nitrogen, resonant scattering of Lyman-a photons by atomic hydrogen and scattering by the atmospheric gases and the photochemical haze. We use detailed forward models of the observed emissions to characterize the upper atmosphere and get constraints on the abundance profiles (and their variability) of the N₂, CH₄ and H gases and the microphysical properties of the haze particles. Our results demonstrate that the observed gaseous emissions closely follow a temporal evolution throughout the Cassini mission that is consistent with the solar-cycle variability, while the haze scattering observations reveal marked differences between the evening and morning terminators. We discuss the implications of our retrievals on the thermal structure of the upper atmosphere and the haze microphysical growth.