The Molecular Ion H_3~+ in Emission in Planetary Atmospheres

摘要

The molecular ion H_3~+ was detected by spectroscopy twenty years ago, for the first time outside of the laboratory, in the upper atmosphere of the polar zones of Jupiter. This detection made possible temperature, abundance measurements, imaging of the ionic emission, and triggered its search in the atmosphere of the other giant planets, leading to a positive detection in Saturn and Uranus. These measurements, mainly in the ν_2 band around 3.7 μm and its overtone 2ν_2 at 2.1 μm, revealed a wealth of information on the planetary magnetospheres and the auroral phenomenon. On the hot Jupiters, H_3~+, likely excited through EUV radiation, could be an important target to prove the existence of a gaseous planet and to monitor the escape processes of the atmosphere. The attempts of detection in exoplanets have been so far unsuccessful with the current limits of detection. From the experience gained on the role played by this ion in the energy balance of the giant planets of the Solar System, can be inferred its role of thermostat in the upper atmosphere and the ionosphere of giant extrasolar planets as main cooling agent contributing to their stability. However, the distance to the star is an important parameter for H_3~+ to be able to form. Occultation spectroscopy with more transiting planets known should be the most promising method for this search.