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

The commonly held interpretation of the observed banded structure of the jovian atmosphere is that zones are regions of dense clouds and thus high reflectivity and low thermal emission, while belts are relatively cloud-free regions of low reflectivity but high thermal emission. However, it has not been possible with existing data to identify unambiguously which of a number 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 Galileo spacecraft have been analyzed which have good spectral coverage at both near-IR and 5-mum wavelengths over wide areas. These new data provide compelling evidence that the main source of the near-IR reflectivity variability, observed to be anticorrelated with 5-mum brightness, is due to opacity changes of cloud layers lying at pressures between I and 2 bar, and not to variability of the higher altitude ammonia cloud, the other main likely candidate. However, whether this main variable cloud layer is composed of ammonium hydrosulfide, the upper levels of a deeper water ice cloud, or something else is unknown. We also find that most of the particles in the upper troposphere must be rather small, in order that their reflectivity drops rapidly with wavelength, but we deduce that there must also be a component of larger 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 particles introduce a number of unobserved features at other wavelengths; thus we conclude that their opacity must be small. (C) 2001 Academic Press.