A Snapshot of Photochemical Products in Saturn’s Mesosphere

Photochemistry in Saturn’s upper atmosphere begins with methane photolysis in the mesosphere; circulation and differences in solar insolation affect the distribution of the photochemical products. Mapping hydrocarbon abundances with latitude on timescales short compared to a Saturn season allows constraints to be placed on chemistry and dynamics, where the latter in particular is not well-understood in Saturn’s upper atmosphere. Stellar occultations observed by the Ultraviolet Imaging Spectrograph (UVIS) during the Cassini Grand Finale provide a unique view of Saturn’s upper atmosphere. They span 86 North to 86 South latitude, offering relatively good latitudinal coverage and vertical resolution within pressures of 100 to 0.01 microbar. Having analyzed these occultations, we report on the abundance of five light hydrocarbons as a function of latitude and pressure. In order to compare the observed abundances to photochemical model predictions, we determine volume mixing ratios for each species. We fit a model atmosphere to temperatures from the UVIS EUV channel in the thermosphere and CIRS limb scans in the mesosphere as well as to H2 and hydrocarbon densities from the UVIS EUV and FUV channels. We compare the results with CIRS retrievals of ethane and acetylene in the middle atmosphere. Hydrocarbon abundances are greater in the summer hemisphere nearer to subsolar latitudes, where methane photolysis and hydrocarbon production are expected to peak. There is an unexpectedly large meridional trend in the depth of the homopause, which is likely to be dynamical in origin. We report on the distribution of individual species with latitude and pressure. At lower pressures the trends in hydrocarbon abundances with latitude follow that of the homopause depth, suggesting that dynamics, including upwelling and downwelling, may have an important effect on Saturn’s mesosphere. Comparing these to photochemical model predictions, which do not include the effects of advection or auroral chemistry, provides important information on these processes.