Saturn’s Upper Atmosphere in the Ultraviolet: Temperature and Compositional Trends from Cassini UVIS with Implications for Energy Balance and Dynamics

Abstract

This study presents a comprehensive analysis of 40 stellar occultations observed during the Cassini Grand Finale mission, providing unprecedented insights into Saturn's atmosphere. By offering simultaneous latitude coverage and seasonal constraints, these observations enable a detailed exploration of composition and temperature variations across the middle and upper atmosphere. This work has produced the first 2D temperature map of the thermosphere with latitude and pressure, which reveals a temperature distribution that differs in important ways from model predictions. The Saturn Thermosphere Ionosphere Model (STIM) predicted that temperatures would peak around 630 K near 10^-8 bar at the poles. Contrary to these predictions, the observed temperature map indicates that peak temperatures are around 550 K, concentrated near auroral latitudes around 10^-12 bar. This observation suggests that auroral heating plays a pivotal role in determining thermospheric temperatures, highlighting the importance of equatorward transport, which requires further investigation. In-depth analysis using pressure-temperature profiles and H2 density data enabled the derivation of kronopotential and wind fields. These results revealed robust westward zonal jets, slower (peaking at 800 m/s) and closer to the equator (30 to 85 degrees latitude at 0.1 microbar) than predicted. Meridional winds generated by ion drag showed peak equatorward speeds of 20–40 m/s. Gravity wave signatures in the thermosphere were analyzed to derive 2D fields of gravity wave drag. The addition of gravity wave drag reduced the speed of zonal jets and increased the meridional winds by 20 to 50 m/s in the southern and northern hemispheres, respectively, demonstrating their role in enhancing meridional wind speeds. These findings align with model predictions by the Saturn Thermosphere Ionosphere model based on wave drag parameterized from near-equatorial observations by the Cassini Ion Neutral Mass Spectrometer and underscores the significance of gravity waves in facilitating the equatorward transport of energy from the auroral region, a key factor in Saturn's energy distribution. This study also presents the first 2D maps of key hydrocarbons in the upper stratosphere and mesosphere from the same dataset. The creation of atmospheric structure models was facilitated by the use of the temperature data described above in conjunction with temperatures derived from earlier observations by the Composite Infrared Spectrometer (CIRS). A significant discovery is the pronounced meridional variation in the homopause pressure level, with a deeper homopause around the poles compared to the subsolar point. Fits to the observed CH4 mixing ratios necessitate a latitude-dependent Kzz profile in both the mesosphere as well as for CIRS observations deeper in the stratosphere. A novel finding is that the homopause closely aligns with the thermosphere base. This interesting result is likely caused by an interplay of heating and dynamics, which could ultimately arise due to forces either above or below the homopause. For example, it is possible that the heating observed in the thermosphere influences the hydrocarbon distributions by impeding CH4 mixing upward along along the steep temperature gradient, with the latitudinal trend set by missing in the thermosphere. However, this study favors an explanation driven by a global circulation pattern with upwelling near the subsolar latitude and downwelling at high latitudes in the mesosphere. This is based on the latitudinal trends in Kzz needed to fit the observed CH4 abundances and is supported by modeling in the upper stratosphere. Vertical wind speeds of about 1 cm/s (ranging from -0.3 cm/s to 1.8 cm/s) are estimated based on a simple scale argument. These speeds are larger than those observed in the stratosphere but slower than expected in the thermosphere. The research detects a clear seasonal trend in photochemical product abundances at pressures below about 0.01 mbar, with higher abundances in the summer hemisphere, as predicted by photochemical modeling. Additionally, this study confirms the importance of ion chemistry on the formation of C6H6 at Saturn, which was predicted by studies at Titan. The inclusion of ion chemistry significantly improves model fit to C6H6 but does not entirely account for observed discrepancies, especially in in the winter auroral oval, which was permanently shadowed at the time of these observations. This, along with abundances of C2H6, C2H2, C2H4 much higher than the model predicted, emphasizes the significance of auroral chemistry, which was not included in the model. This work underscores the importance of comprehensive models that consider both auroral chemistry and circulation to advance our understanding of Saturn's upper atmosphere dynamics. The findings presented here in the mesosphere and thermosphere contribute valuable insights to the study of middle and upper atmosphere processes and provides valuable insights regarding the applicability of these mechanisms to other outer solar giants and to giant planets around other stars.