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Saturn’s Upper Atmosphere Revealed by Cassini Stellar Occultations

Presentation #508.01D in the session Origin and Evolution of Giant Planet Systems II (Oral Presentation)

Published onOct 23, 2023
Saturn’s Upper Atmosphere Revealed by Cassini Stellar Occultations

In 2016 and 2017, the Cassini spacecraft captured a series of stellar occultations over latitudes from 86°N to 86°S. Taken together, these observations provide a 2D snapshot of Saturn’s mesosphere and thermosphere. In this dissertation talk, I present the analysis of these occultations from the Ultraviolet Imaging Spectrograph (UVIS), discussing their implications for dynamics, chemistry, and energy balance in Saturn’s upper atmosphere.

The EUV channel probes H2 densities and temperatures in the thermosphere. The observed temperatures shed light on the outer planet “energy crisis.” Although enough energy is deposited at Saturn’s auroral regions to heat the thermosphere to the temperatures observed at all latitudes, previous models predicted that Saturn’s strong Coriolis forces and ion drag inhibited redistribution toward the equator. Peak temperatures are cooler than model predictions and occur at lower pressures and latitudes (55-60°), resulting in shallower pressure gradients. We estimate horizontal winds under the assumption of modified geostrophy, finding slower and broader zonal jets than predicted. These findings support auroral joule heating with equatorward redistribution as the source of Saturn’s thermospheric temperatures.

To expand our investigation of winds, we examine signatures of gravity waves found in the majority of thermospheric temperature profiles. Upon breaking, gravity waves deposit energy and momentum into the atmosphere. By characterizing these waves, we infer 2D values for zonal wave drag. The addition of this drag enhances meridional transport in both hemispheres away from the auroral regions to speeds approaching 100 m/s.

Finally, we present results from the UVIS FUV channel, which probes light hydrocarbons in the mesosphere, and compare these to photochemical model predictions. We observe an expected seasonal trend of enhanced abundance in photochemical products in the summer hemisphere As expected, benzene abundances are better matched by a model that includes ion chemistry. Surprisingly, benzene is observed in the ring shadowed region between 60° and 90°S latitudes, despite the lack of insolation to drive photochemistry. We also find enhanced abundances of C2H2, C2H4 and C2H6 at auroral latitudes, suggesting that auroral processes are important to hydrocarbon production. We observe a pronounced trend in the depth of the methane homopause with latitude, implying suppressed mixing at high latitudes. This could be explained by a two-lobed circulation cell with downwelling at high latitudes and upwelling near subsolar latitudes to explain this phenomenon.

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