H3+: the driver of giant planet atmospheres

We present a review of recent developments in the use of Inline Formula molecular ion as a probe of physics and chemistry of the upper atmospheres of giant planets. This ion is shown to be a good tracer of energy inputs into Jupiter (J), Saturn (S) and Uranus (U). It also acts as a ‘thermostat’, offsetting increases in the energy inputs owing to particle precipitation via cooling to space (J and U). Computer models have established that Inline Formula is also the main contributor to ionospheric conductivity. The coupling of electric and magnetic fields in the auroral polar regions leads to ion winds, which, in turn, drive neutral circulation systems (J and S). These latter two effects, dependent on Inline Formula, also result in very large heating terms, approximately 5×1012 W for Saturn and greater than 1014 W for Jupiter, planet-wide; these terms compare with approximately 2.5×1011 W of solar extreme UV absorbed at Saturn and 1012 W at Jupiter. Thus, Inline Formula is shown to play a major role in explaining why the temperatures of the giant planets are much greater (by hundreds of kelvin) at the top of the atmosphere than solar inputs alone can account for.