The role of H3+ in planetary atmospheres

Dr.Steve Miller
Reader in Science Communication and Planetary Science, Science and Technology Studies/Physics and Astronomy, University College London, Gower Street, London WC1E 6BT

Spectroscopic studies of the upper atmospheres of the giant planets using infrared wavelengths sensitive to the H3+ molecular ion show that this species plays a critical role in determining the physical conditions there. For Jupiter, we propose that the recently detected H3+ electrojet holds the key to the mechanism by which the equatorial plasmasheet is kept in (partial) co-rotation with the planet, and that this mechanism also provides a previously unconsidered source of energy which helps explain why the jovian thermosphere is considerably hotter than expected. For Saturn, we show that the H3+ auroral emission is about 1% that of Jupiter because of the lower ionospheric/thermospheric temperature and the lower flux of ionising particles precipitated there; it is probably unnecessary to invoke additional chemistry in the auroral/polar regions. For Uranus, we report further evidence that its emission intensity is controlled by the cycle of solar activity. And we propose that H3+ emission may just be detectable using current technology from some of the giant extra-solar planets which have been detected orbiting nearby stars, such as tau Bootes.