  Our knowledge of Saturns 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 ~ 400K 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 ~ 10TW
generating Joule and energetic particle heating of ~ 5-6TW 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 Saturns 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 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.
