Jupiter's Cloud Structure from Galileo Imaging Data

Abstract

The vertical structure of aerosols on Jupiter is inferred from data obtained by the NASA Galileo Solid State Imaging system during the first six orbits of the spacecraft. Images at 889 nm (a strong methane band), 727 nm (a weaker methane band), and 756 nm (continuum) taken at a variety of lighting and viewing angles are used. The images are displayed and described in the companion paper by Vasavadaet al.(1998,Icarus135, 265–275). Conservative scattering cloud particles with laterally uniform single scattering properties are assumed in the analysis and are shown to be consistent with the data at these wavelengths. Particles are bright, and the darkest locations on Jupiter correspond to the smallest optical thickness of aerosols. Optical depths and vertical positions of aerosol layers vary from place to place and are the retrieved quantities in the analysis. Only mid and low latitudes are sampled in this data set. A stratospheric haze with an optical depth of roughly a tenth and an upper tropospheric haze with an optical depth of 2 to 6 exist over all regions. Both are consistent with previous conclusions based on data of lower spatial resolution (e.g., Westet al.1986,Icarus65, 161–217). The new data show that these layers contain little lateral structure on scales smaller than the planetary jets. On scales of the jets and ovals, the top and bottom of the upper tropospheric haze vary in elevation. The concentration of particles (optical depth per pressure interval) varies less than does the total optical depth. Near the base of the upper tropospheric haze is a third cloud component, usually at pressure p= 750 ± 200 mb, which is less than a scale height in geometric thickness. Its optical depth varies from zero to about 20 on regional scales and often varies by 50% on scales of a few tens of kilometers. Optical depth variations in this cloud are the principal cause of the features in Jupiter's atmosphere seen at red and longer wavelengths. It is probably composed of ammonia. The expected NH_4SH cloud has not been identified in this work, perhaps because it exists only at locations where it is concealed beneath higher clouds. Our retrievals also cannot rule out a pervasive deep haze without small-scale structure. Finally, in one region northwest of the Great Red Spot, a deeper cloud is identified. Parts of it lie at a pressure greater than four bars. It is associated with a rapidly changing storm system with optical depth of several tens (or more) and a range of cloud heights between p> 4 bars top∼ 400 mb. It is probably composed of water.

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