Seasonal variation of hydrocarbon abundances on Saturn: Comparisons of coupled ion-neutral photochemical models with Cassini UVIS, CIRS, and INMS data

The long-lived Cassini mission provided unprecedented insight into the variation of photochemically produced species in Saturn’s upper atmosphere as a function of altitude, latitude, and time. A recent analysis of stellar occultations observed in 2016-2017 by the Cassini Ultraviolet Imaging Spectrograph (UVIS) (see abstract by Z. Brown and colleagues from this meeting) has proven particularly instrumental in highlighting seasonal behavior in the upper stratosphere at pressures that are difficult to probe remotely by other methods. These observations exhibit variations with latitude that correlate with the actinic solar flux that drives photochemistry, as is expected from seasonal photochemical models, as well as highlight some unexpected behavior, such as trends in the methane homopause pressure level with latitude. To gain physical insights into the observed behavior, we have developed a time-variable photochemical model for Saturn’s stratosphere and thermosphere. The model considers solar-driven coupled ion-neutral photochemistry and accounts for meridional and temporal variations in incident solar flux due to orbital and seasonal geometry, ring shadowing, and solar-cycle variations. Comparisons of the models with Cassini data (including a data set complementary to that of UVIS, consisting of Cassini Composite Infrared Spectrometer (CIRS) retrievals of hydrocarbon abundances deeper in the stratosphere) provide important clues to seasonally variable chemistry, dynamics, and haze formation in Saturn’s stratosphere. The model-data comparisons confirm the importance of ion chemistry in producing benzene and polycyclic aromatic hydrocarbons in Saturn’s stratosphere and point to potential missing chemistry in the model, including auroral-induced ion chemistry, and an as-yet-unidentified process that increases C₂H₂ production in Saturn’s upper stratosphere above model predictions. We describe the model results, compare them with available spatially resolved observations, and discuss the implications. We also present low-latitude results that include the very large influx of external material, presumably from Saturn’s rings, that was observed by the Cassini Ion and Neutral Mass Spectrometer (INMS) during the Grand Finale stage of the mission; we show how that incoming material affects the abundance of neutral and ionized species in Saturn’s upper atmosphere.

This work was supported by the NASA Solar System Workings program, grant number 80NSSC20K0462.