Temperature changes and energy inputs in giant planet atmospheres: What we are learning from H₃⁺

Since its discovery at Jupiter in 1988, emission from H₃ ⁺ has been used as a valuable diagnostic tool in our understanding of the upper atmospheres of the giant planets. One of the lasting questions we have about the giant planets is why the measured upper atmosphere temperatures are always consistently hotter than the temperatures expected from solar heating alone. Here, we describe how H₃⁺ forms across each of the planetary disks of Jupiter, Saturn and Uranus, presenting the first observations of equatorial H₃⁺ at Saturn and the first profile of H₃⁺ emission at Uranus not significantly distorted by the effects of the Earth's atmosphere. We also review past observations of variations in temperature measured at Uranus and Jupiter over a wide variety of time scales. To this, we add new observations of temperature changes at Saturn, using observations by Cassini. We conclude that the causes of the significant level of thermal variability observed over all three planets is not only an important question in itself, but that explaining these variations could be the key to answering the more general question of why giant planet upper atmospheres are so hot.