A comparison of the atmospheres of Jupiter and Saturn: deep atmospheric composition, cloud structure, vertical mixing, and origin.

We present our current understanding of the composition, vertical mixing, cloud structure and the origin of the atmospheres of Jupiter and Saturn. Available observations point to a much more vigorous vertical mixing in Saturn's middle-upper atmosphere than in Jupiter's. The nearly cloud-free nature of the Galileo probe entry site, a 5-micron hotspot, is consistent with the depletion of condensable volatiles to great depths, which is attributed to local meteorology. Somewhat similar depletion of water may be present in the 5-micron bright regions of Saturn also. The supersolar abundances of heavy elements, particularly C and S in Jupiter's atmosphere and C in Saturn's, as well as the progressive increase of C from Jupiter to Saturn and beyond, tend to support the icy planetesimal model of the formation of the giant planets and their atmospheres. However, much work remains to be done, especially in the area of laboratory studies, including identification of possible new microwave absorbers, and modelling, in order to resolve the controversy surrounding the large discrepancy between Jupiter's global ammonia abundance, hence the nitrogen elemental ratio, derived from the earth-based microwave observations and that inferred from the analysis of the Galileo probe-orbiter radio attenuation data for the hotspot. We look forward to the observations from Cassini-Huygens spacecraft which are expected to result not only in a rich harvest of information for Saturn, but a better understanding of the formation of the giant planets and their atmospheres when these data are combined with those that exist for Jupiter.

Search for organic molecules in the outer solar system.

Recent developments of millimeter astronomy have led to the discovery of more and more complex molecules in the interstellar medium. In a similar way, attempts have been made to detect complex molecules in the atmospheres of the most primitive bodies of the Solar System, i.e. outer planets and comets, as well as in Titan's atmosphere. An important progress has been achieved thanks to the continuous development of infrared astronomy, from the ground and from space vehicles. In particular, an important contribution has come from the IRIS-Voyager infrared spectrometer with the detection of prebiotic molecules on Titan, and some complex organic molecules on Jupiter and Saturn. Another important result has been the observation of carbonaceous material in the immediate surroundings of Comet Halley's nucleus. In the near future, the search for organic molecules in the outer Solar System should benefit from the developments of large millimeter antennae, and in the next decade, from the operation of infrared Earth-orbiting spacecrafts (ISO, SIRTF).