Atmospheric composition and cloud structure in Jovian 5-mu m hotspots fromanalysis of Galileo NIMS measurements

NIMS is the Near-Infrared Mapping Spectrometer on board the Galileo spacecr aft in jovian orbit. We have selected four maps of warm-to-hot regions of t he North Equatorial Belt (NEB) for study, analyzing the spectra emerging in the low-opacity 5-mum window Two methods for calculating the spectrum have been used. The first is a full-scattering radiative transfer forward model that is slow but accurate. The second method calculates spectra by interpo lating on a grid of spectra precalculated using the first method for a rang e of model atmospheres. This method of forward calculation is more suited t o analysis of large data sets where application of the full radiative trans fer in every instance would be computationally prohibitive. The faster meth od is verified against the first before being used alone. A retrieval (inve rsion) algorithm is then used to match model spectra to data and obtain val ues for cloud opacities and gas mixing ratios. We first sum spectra with similar peak radiances to produce mean spectra re presentative of brighter and darker (at 5 mum) regions of the maps. These c oadded spectra are then analyzed with the fast retrieval code to obtain the average variations in atmospheric parameters from the center to the edges of the hotspots. These analyses confirm that 5-mum hotspots are relatively cloud free, and that a medium level(1.5-bar) cloud layer of large NH4SH par ticles is the main absorber at these wavelengths. Variations in water vapor relative humidity and high (0.5-bar) ammonia cloud opacity are also derive d. We then analyze single spectra over wide areas to produce spatial maps of p arameter variations. We find that models that do not include a deep water c loud (similar to4 bar) do not match all the spectra to within the noise lev el. A deep water cloud therefore seems to be present in localized areas, to ward the edges of the hotspot regions. We examine these findings in the lig ht of results from other Galileo instruments, concluding that the deep clou d observed by the SSI instrument at several locations is likely to be the d eep water cloud required by the NIMS data. e zool Academic Press.