Presentation #306.03 in the session New Chemicals, New Clouds, New Toys for Giants.
By sampling latitudes from pole to pole in 2016 and 2017, Cassini stellar occultations have provided a global snapshot of temperatures and densities in Saturn’s upper atmosphere as a function of latitude and depth; this view is critical to understanding the dynamics, chemistry and energy balance of this region of the atmosphere. Here, we compare the distribution of the observed hydrocarbons (C2H2, C2H4, C2H6, and C6H6) to photochemical model predictions and investigate the impact of ion chemistry and seasonal effects (including ring shadow) on the results. With the exception of C6H6 and auroral latitudes, the observations and the model are generally in good agreement and follow an expected seasonal trend with an enhanced abundance of photochemical products near the subsolar point at 30 degrees north latitude at pressures less than 10 microbar. Models with neutral chemistry only and those that also include ion chemistry both underpredict the abundances of C2H2, C2H4 and C2H6 at auroral latitudes. In the absence of auroral production, both models also underpredict the abundance of C6H6 at all latitudes, in particular near auroral latitudes and at the equator. Including ion chemistry increases the abundance of benzene, but does not eliminate the disagreement with the observations. These findings suggest that auroral production may be an important component of hydrocarbon production. We previously reported a surprising trend in the depth of the methane homopause with latitude, which is much deeper at high latitudes than would arise from Saturn’s oblateness alone, possibly from downwelling at high latitudes and upwelling near subsolar latitudes. The observed methane distribution implies suppressed mixing at high latitudes, represented by a latitude-dependent Kzz coefficient in the photochemical model. We discuss the impact of a latitude-dependent homopause at deeper pressures and compare the hydrocarbon abundances retrieved from Cassini/UVIS observations with Cassini/CIRS retrievals of C2H6 and C2H2 in the middle atmosphere.