Saturn’s Variable Thermosphere

Our knowledge of Saturn’s neutral thermosphere is far superior to that of the other giant planets due to Cassini Ultraviolet Imaging Spectrograph (UVIS) observations of 15 solar occultations and 26 stellar occultations analyzed to date. These measurements yield H2 as the dominant species, with an upper limit on the H mole fraction of 5%. Inferred temperatures near the lower boundary are ~150 K, rising to an asymptotic value of ~400 K at equatorial latitudes and increasing with latitude to polar values in the range of 550–600 K. The latter is consistent with a total estimated auroral power input of ~10 TW generating Joule and energetic particle heating of ~5–6 TW that is more than an order of magnitude greater than solar EUV/FUV heating. This auroral heating would be sufficient to solve the “energy crisis” of Saturn’s thermospheric heating if it can be efficiently redistributed to low latitudes. The inferred structure of the thermosphere yields poleward-directed pressure gradients on equipotential surfaces consistent with auroral heating and poleward increasing temperatures. A gradient wind balance aloft with these pressure gradients implies westward, retrograde winds ~500 m s−1 or Mach number ~0.3 at mid-latitudes. The occultations reveal an expansion of the thermosphere peaking at or slightly after equinox, anti-correlated with solar activity, and apparently driven by lower thermospheric heating of unknown cause. The He mole fraction remains unconstrained, as no Cassini UVIS He 58.4 nm airglow measurements have been published.