The origin of belt/zone contrasts in the atmosphere of Jupiter and their correlation with 5-mu m opacity

The commonly held interpretation of the observed banded structure of the jo vian atmosphere is that "zones" are regions of dense clouds and thus high r eflectivity and low thermal emission, while "belts" are relatively cloud-fr ee regions of low reflectivity but high thermal emission. However, it has n ot been possible with existing data to identify unambiguously which of a nu mber of possible jovian cloud layers (J. S. Lewis 1969, Icarus 10, 365-378) is mainly responsible for these variations. New data from the Near Infrared Mapping Spectrometer (NIMS) aboard the Gali leo spacecraft have been analyzed which have good spectral coverage at both near-IR and 5-mum wavelengths over wide areas. These new data provide comp elling evidence that the main source of the near-IR reflectivity variabilit y, observed to be anticorrelated with 5-mum brightness, is due to opacity c hanges of cloud layers lying at pressures between I and 2 bar, and not to v ariability of the higher altitude ammonia cloud, the other main likely cand idate. However, whether this main variable cloud layer is composed of ammon ium hydrosulfide, the upper levels of a deeper water ice cloud, or somethin g else is unknown. We also find that most of the particles in the upper tro posphere must be rather small, in order that their reflectivity drops rapid ly with wavelength, but we deduce that there must also be a component of la rger sized particles, probably composed of ammonia ice, to account for the correlation between the 5- and 45-mum emission observed by Voyager. However , although a significant opacity of large ammonia particles does provide a good fit to the observed cloud absorption feature near 3-mum, these particl es introduce a number of unobserved features at other wavelengths; thus we conclude that their opacity must be small. (C) 2001 Academic Press.