Abstract. The results of the nephelometer experiment conducted aboard the probe of the Galileo mission to Jupiter are presented. The tenuous clouds and sparse particulate matter in the relatively particle-free 5-/xm "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 many indications of extremely small particle concentrations along the descent 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 from 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 encountered, 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 water condensation, compatible with reported measurements of water abundance from other Galileo Mission experiments. A third small signal region, extending 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 lateral atmospheric motion and/or reduced to a small mass density by atmospheric subsidence or other causes. Rough descriptions of the particle size distributions and cloud properties in these regions have been derived, although they may be imprecise because of the small signals and experimental difficulties. These descriptions document the small number densities of particles, 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.
IntroductionOne of the principal objectives of the Galileo Mission to Jupiter was to determine the locations, horizontal and vertical extent, microphysical properties, and composition of the clouds of Jupiter. A nephelometer (NEP) instrument measuring light scattering from the ambient atmosphere at five scattering angles was included as part of the experiments package on the Galileo Mission probe that entered the Jovian atmosphere on December 7, 1995. The purpose of this experiment was to establish the vertical location and to attempt to document the microphysical properties of the clouds along the probe descent trajectory. It was hoped that the data would yield values of parameters required for radiation modeling studies, such as cloud particle size distributions, opacities, scattering and extinction cross sections, and local total mass den-
