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, c loud structure and the origin of the atmospheres of Jupiter and Saturn. Ava ilable 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 th e depletion of condensible volatiles to great depths, which is attributed t o local meteorology. Somewhat similar depletion of water may be present in the 5-micron bright regions of Saturn also. The supersolar abundances of he avy 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 beyo nd, tend to support the icy planetesimal model of the formation of the gian t planets and their atmospheres. However, much work remains to be done, esp ecially in the area of laboratory studies, including identification of poss ible new microwave absorbers and modelling, in order to resolve the controv ersy surrounding the large discrepancy between Jupiter's global ammonia abu ndance, hence the nitrogen elemental ratio, derived from the earth-based mi crowave observations and that inferred from the analysis of the Galileo pro be-orbiter radio attenuation data for the hotspot. We look forward to the o bservations from Cassini-Huygens spacecraft which are expected to result no t only in a rich harvest of information for Saturn, but a better understand ing of the formation of the giant planets and their atmospheres when these data are combined with those that exist for Jupiter. (C) 1999 Elsevier Scie nce Ltd, All rights reserved.