CO in the stratospheres of Saturn, Uranus and Neptune

The question of the origin of oxygenated compounds in the atmosphere of the giant planets was raised since the detection of H2O and CO2 in the stratospheres of the four planets (Feuchtgruber et al. 1997, 1999, Lellouch et al. 1999, Burgdorf et al. 2006). The tropospheric cold trap implies an external origin for both molecules.