title: H3+: the driver of giant planet atmospheres.
abstract: We present a review of recent developments in the use of H3+ 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 H3+ 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 H3+, also result in very large heating terms, approximately 5 x 10(12) W for Saturn and greater than 10(14) W for Jupiter, planet-wide; these terms compare with approximately 2.5 x 10(11) W of solar extreme UV absorbed at Saturn and 10(12) W at Jupiter. Thus, H3+ 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.
authors.affiliation: Atmospheric Physics Laboratory, Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK. s.miller@ucl.ac.uk
authors.forename: Steve
authors.initials: S
authors.lastname: Miller
authors.forename: Tom
authors.initials: T
authors.lastname: Stallard
authors.forename: Chris
authors.initials: C
authors.lastname: Smith
authors.forename: George
authors.initials: G
authors.lastname: Millward
authors.forename: Henrik
authors.initials: H
authors.lastname: Melin
authors.forename: Makenzie
authors.initials: M
authors.lastname: Lystrup
authors.forename: Alan
authors.initials: A
authors.lastname: Aylward