A pole-to-pole pressure-temperature map of Saturn's thermosphere from Cassini Grand Finale data

The complete latitudinal coverage of Saturn's thermospheric temperatures from Cassini Grand Finale data shows a more uniform spatial behaviour than model predictions, indicating an efficient redistribution of auroral energy from the poles towards the equator that could contribute to solving the giant planet's energy crisis puzzle. Temperatures of the outer planet thermospheres exceed those predicted by solar heating alone by several hundred degrees. Enough energy is deposited at auroral regions to heat the entire thermosphere, but models predict that equatorward distribution is inhibited by strong Coriolis forces and ion drag(1,2). A better understanding of auroral energy deposition and circulation are critical to solving this so-called energy crisis. Stellar occultations observed by the Ultraviolet Imaging Spectrograph instrument during the Cassini Grand Finale were designed to map the thermosphere from pole to pole. We analyse these observations, together with earlier observations from 2016 and 2017, to create a two-dimensional map of densities and temperatures in Saturn's thermosphere as a function of latitude and depth. The observed temperatures at auroral latitudes are cooler and peak at higher altitudes and lower latitudes than predicted by models, leading to a shallower meridional pressure gradient. Under modified geostrophy(3), we infer slower westward zonal winds that extend to lower latitudes than predicted, supporting equatorward flow from approximately 70 degrees to 30 degrees latitude in both hemispheres. We also show evidence of atmospheric waves in the data that can contribute to equatorward redistribution of energy through zonal drag.