A Pole-to-pole Map of Hydrocarbons in Saturn's Upper Stratosphere and Mesosphere

We analyze data from the final two years of the Cassini mission to retrieve the distributions of methane (CH4), ethane (C2H6), acetylene (C2H2), ethylene (C2H4), and benzene (C6H6) in Saturn’s upper stratosphere and mesosphere from stellar occultations observed by the Ultraviolet Imaging Spectrograph (UVIS), spanning latitudes from pole to pole. These observations represent the first two-dimensional snapshot of the photochemical production region with latitude and depth for these five light hydrocarbons around the northern summer solstice. To support this analysis, we combine temperature-pressure profiles retrieved from the UVIS occultations and limb scans observed by the Cassini Composite Infrared Spectrometer (CIRS) with the CH4 profiles to provide atmospheric structure models for each occultation location that span the middle and upper atmosphere. We detect a strong meridional trend in the homopause pressure level that implies much weaker mixing at the poles than near the subsolar point. This is shown by the homopause pressure level, which ranges from ∼0.05μbar around the subsolar point to ∼5μbar at the poles, with the corresponding values of the eddy diffusion coefficient Kzz ranging from ∼1000 m2 s−1 to 1-10 m2 s−1, respectively, at the 2 μbar level. This trend could be explained by upwelling at low latitudes and downwelling at high latitudes in both hemispheres and we estimate that vertical wind speeds of less than 2 cm s−1 would be required. The distributions of the photochemical products follow the homopause trend but they also show a clear seasonal trend at pressures between 0.01 and 10 μbar, with higher abundances in the summer hemisphere than in the winter hemisphere. We compare the observed distributions with results from one-dimensional seasonal photochemical models, with and without ion chemistry, to explore the impact of ion chemistry on the results. The best agreement between the models and the observations is obtained in the summer hemisphere, whereas disagreements in the winter hemisphere and auroral region may be due to the lack of transport by global circulation and auroral electron and ion precipitation in our photochemical models. Ion chemistry is particularly important for matching the observed C6H6 distribution, whereas differences between the neutral only and ion chemistry models are more modest for the other species. We also compare the C2H2 profiles retrieved from the UVIS occultations to those retrieved from the CIRS limb scans and find good agreement between the retrievals at pressures where they overlap.