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

Abstract

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-

Jupiter's Cloud Structure as Constrained by Galileo Probe and HST Observations

“…Particle radii of 0.5-0.75 µm and an optical depth ∼1.6 at λ = 0.5 µm provide a reasonable fit to the observed modulations (Sromovsky et al 1998). It appears that the Nephelometer signals (Ragent et al 1998) may have recorded the very bottom of this upper cloud layer (Fig. 1, right).…”

“…From an analysis of solar modulations in the Net Flux Radiometer (NFR) signals, Sromovsky et al (1998) concluded that there were 1-2 optical depths (at 0.5 µm) of particulates no bigger than about 0.75 µm above the first Probe observation at ∼440 mb. The Nephelometer (Ragent et al 1998) detected very few particulates between 440 mb and 700 mb, where an ammonia cloud had been expected. Both the Nephelometer and the NFR detected a cloud at ∼1.2 bars, with optical depth ranging from τ ∼ 1 at 5 µm (Sromovsky et al 1998) to τ = 1.5-1.8 at 0.904 µm (Ragent et al 1998).…”

“…The Nephelometer (Ragent et al 1998) detected very few particulates between 440 mb and 700 mb, where an ammonia cloud had been expected. Both the Nephelometer and the NFR detected a cloud at ∼1.2 bars, with optical depth ranging from τ ∼ 1 at 5 µm (Sromovsky et al 1998) to τ = 1.5-1.8 at 0.904 µm (Ragent et al 1998). Few cloud particles were detected below this second cloud.…”

Jupiter’s Deep Cloud Structure Revealed Using Keck Observations of Spectrally Resolved Line Shapes

“…Only very thin clouds were found in the Galileo Probe hot spot (Ragent et al 1998), consistent with condensation via weak turbulent updrafts within the descending branch of an equatorially trapped Rossby wave (Friedson 2005;Wong et al 2015). Models of NIMS spectra by Nixon et al (2001) include water clouds in at least some hot spots, and RoosSerote et al (2004) showed that NIMS spectra of hot spots cannot rule out water clouds whose opacity is entirely restricted to P 5 > bar.…”

Jupiter's ammonia clouds—localized or ubiquitous?

“…Fig. 1 shows results of ECCM calculations for Jupiter, with 1 Â solar and 3 Â solar condensible volatile abundances in the left panel, and greatly depleted condensible volatiles in the right panel (Atreya et al, 1999) as the Galileo Probe entered one of the driest places-the Sahara Desert of Jupiter The right panel simulates the LCL of clouds detected by the Galileo probe nephelometer at 1.3 bar, and more tenuous ones at 1.6 and 0.55 bar (Ragent et al, 1998). Although the H 2 S, and H 2 O mixing ratios in the Probe Entry Site (PES) at pressures less than 9 bar are unknown, their extrapolated values from measured mixing ratios at 9-11 bar (Niemann et al, 1998;Atreya et al, 1999;Atreya et al, 2003), together with the NH 3 profile inferred from the attenuation of Galileo probe radio signal ( Fig.…”