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We analyze data from the final two years of the Cassini mission to retrieve the distributions of methane, ethane, acetylene, ethylene, and benzene in Saturn's upper stratosphere and mesosphere from stellar occultations observed by the Ultraviolet Imaging Spectrograph (UVIS), spanning pole-to-pole. These observations represent the first 2D snapshot with latitude and depth of Saturn's photochemical production region around northern summer solstice. Using UVIS occultations and CIRS limb scans, we derive temperature-pressure profiles and atmospheric structure models for each occultation latitude. W detect a strong meridional trend in the homopause pressure level, which ranges from approximately 0.05 microbar around the subsolar point to around 5 microbar at the poles, implying much weaker mixing at the poles than near the subsolar point. This trend could be explained by upwelling at low latitudes and downwelling at high latitudes, requiring vertical wind speeds under 2 cm/s. Photochemical product distributions follow this trend and also show a clear seasonal trend at pressures between 0.01 and 10 microbar, with higher abundances in the summer hemisphere. We compare the observed distributions with results from 1D seasonal photochemical models, with and without ion chemistry, to explore the impact of ion chemistry. We find that ion chemistry is particularly important for matching the observed C6H6 distribution, while its impact on other species is less pronounced. The best agreement between the models and the observations is obtained in the summer hemisphere. Disagreements between model and observations 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. Finally, we compare C2H2 profiles from UVIS occultations with CIRS limb scans, finding good agreement where they overlap., Comment: 66 pages, 11 figures

During the Grand Finale stage of the Cassini mission, organic-rich ring material was discovered to be flowing into Saturn's equatorial upper atmosphere at a surprisingly large rate. Through a series of photochemical models, we have examined the consequences of this ring material on the chemistry of Saturn's neutral and ionized atmosphere. We find that if a substantial fraction of this material enters the atmosphere as vapor or becomes vaporized as the solid ring particles ablate upon atmospheric entry, then the ring-derived vapor would strongly affect the composition of Saturn's ionosphere and neutral stratosphere. Our surveys of Cassini infrared and ultraviolet remote-sensing data from the final few years of the mission, however, reveal none of these predicted chemical consequences. We therefore conclude that either (1) the inferred ring influx represents an anomalous, transient situation that was triggered by some recent dynamical event in the ring system that occurred a few months to a few tens of years before the 2017 end of the Cassini mission, or (2) a large fraction of the incoming material must have been entering the atmosphere as small dust particles less than ~100 nm in radius, rather than as vapor or as large particles that are likely to ablate. Future observations or upper limits for stratospheric neutral species such as HC$_3$N, HCN, and CO$_2$ at infrared wavelengths could shed light on the origin, timing, magnitude, and nature of a possible vapor-rich ring-inflow event., Comment: accepted in Icarus

The co-orbital satellites of Saturn, Janus and Epimetheus, swap radial positions every 4.0 years. Since \textit{Cassini} has been in orbit about Saturn, this has occurred on 21 January in 2006, 2010, and 2014. We describe the effects of this radial migration in the Lindblad resonance locations of Janus within the rings. When the swap occurs such that Janus moves towards Saturn and Epimetheus away, nonlinear interference between now-relocated density waves launches a solitary wave that travels through the rings with a velocity approximately twice that of the local spiral density wave group velocity in the A ring and commensurate with the spiral density wave group velocity in the B ring., Comment: 15 pages, 8 figures, accepted for publication in Icarus