The distribution of hydrocarbons in Saturn’s mesosphere from Cassini’s Grand Finale

In 2017, Cassini observed a series of stellar occultations of Saturn over the course of 6 weeks. This Grand Finale suite of observations was designed to provide a snapshot of Saturn’s upper atmosphere in both depth and latitude, from pole to pole. By analyzing Grand Finale occultations in the FUV, we retrieved density profiles of light hydrocarbons in the upper stratosphere and mesosphere. Here, we explore meridional trends in the abundances of these light hydrocarbons, which include CH4, C2H2, C2H4, C2H6 and C6H6. Since the density of methane is too large to be meaningfully changed by photochemistry, it should be a good tracer of dynamics. We investigate the density of methane with latitude, finding that the abundance of methane in the mesosphere decreases with latitude away from the equator, with the lowest abundances in the winter hemisphere. Since this implies a deeper homopause at high latitudes, the trend could be of dynamical origin. Looking at other species, we compare our results to model predictions by Moses and Greathouse (2005). We find elevated mesospheric levels of C2H2 and C2H6 in the summer hemisphere and an overall trend similar to what they predict at the 2.2e-4 mbar level. Finally, we report on our search for evidence of exogenously-derived hydrocarbons in the mesosphere.