Spectroscopic studies of the atmospheres of giant planets, Titan, and comets

Spectroscopic observations of outer planets, Titan, and comets from Earth-based and spaceborne observatories during the last three decades have produced a long list of molecules, atoms, radicals, ions, and dimers. The detections of these constituents are chronologically presented. We have reviewed spectroscopic studies on the auroral emissions and airglows of the giant planets, and briefly summarized the spectroscopic observations of dimers in the atmospheres of the giant planets and Titan. We also highlighted the recent detections of new molecular emissions and absorptions in the spectra of the giant planets, Titan, and recent bright comets from spaceborne or ground-based observatories. In order to present the limitations of current methods on the analyses of spectroscopic observations, we discussed issues on radiative transfer calculations including absorptions and emissions of molecules and particles in these atmospheres. We also reviewed current thoughts and theories on how to understand spectroscopically obtained molecular abundances and isotopic ratios in the atmospheres in terms of the origin and evolution of the solar system.