The clouds of Jupiter: Results of the Galileo Jupiter Mission Probe Nephelometer Experiment

The results of the nephelometer experiment conducted aboard the probe of th e Galileo mission to Jupiter are presented. The tenuous clouds and sparse p articulate matter in the relatively particle-free 5-mu m "hot spot" region of the probe's descent were documented from about 0.46 bar to about 12 bars . Three regions of apparent coherent structure were noted, in addition to m any indications of extremely small particle concentrations along the descen t path. From the first valid measurement at about 0.46 bar down to about 0. 55 bar, a feeble decaying lower portion of a cloud, corresponding with the predicted ammonia particle cloud, was encountered. A denser, but still very modest, particle structure was present in the pressure regime extending fr om about 0.76 bar to a distinctive base at 1.34 bars and is compatible with the expected ammonium hydrosulfide cloud. No massive water cloud was encou ntered, although below the second structure, a small, vertically thin layer at about 1.65 bars may be detached from the cloud above, but may also be w ater condensation, compatible with reported measurements of water abundance from other Galileo Mission experiments. A third small signal region, exten ding from about 1.9 to 4.5 bars, exhibited quite weak but still distinctive structure and, although the identification of the light scatterers in this region is uncertain, may also be a water cloud, perhaps associated with la teral atmospheric motion and/or reduced to a small mass density by atmosphe ric subsidence or other causes. Rough descriptions of the particle size dis tributions and cloud properties in these regions have been derived, althoug h they may be imprecise because of the small signals and experimental diffi culties. These descriptions document the small number densities of particle s, the moderate particle sizes, generally in the slightly submicron to few micron range, and the resulting small optical depths, mass densities due to particles, column particle number loading, and column mass loading in the atmosphere encountered by the Galileo probe during its descent.