THE CHEMICAL ATMOSPHERIC COMPOSITION OF THE GIANT PLANETS

For a long time it was believed that the atmospheres of the giant plan ets, dominated by molecular hydrogen and helium, were similar in compo sition to the primordial nebula from which they formed. However, this image has strongly evolved over the past twenty years, due to new deve lopments of ground-based infrared spectroscopy coupled with the succes s of the Voyager space mission. Significant differences were measured in the abundances of helium, deutrium and carbon of the four giant pla nets. The variation in the C/H and D/H ratios have given support to th e ''nucleation'' formation scenario, in which the four giant planets f irst accreted a nucleus of about ten terrestrial masses, big enough to bind gravitationally the surrounding gaseous nebula; the helium deple tion in Saturn has been interpreted as a differentiation effect in Sat urn's interior; the apparent helium excess in Neptune, coupled with th e recent unexpected detection of CO and HCN in this planet, might impl y the presence of molecular nitrogen. In the case of Jupiter and Satur n, disequilibrium species have been detected (CO, PH3, GeH4, AsH3), wh ich are tracers of vertical dynamical motions. In the future, signific ant progress in our knowledge of the Jovian composition, including the noble gases, should be obtained with the mass spectrometer of the Gal ileo probe. The ISO mission is expected to provide new far-infrared sp ectroscopic data which should lead to the detection of new minor speci es and a better determination of the D/H ratio.