Scattering properties and location of the jovian 5-micron absorber from Galileo/NIMS limb-darkening observations

The upper jovian atmosphere is particularly transparent at wavelengths near 5μm. Levels well below the cloud layers, which are situated between 0.5 and 2 bar, can be sounded. Large spatial variations of the brightness are observed, which are directly related to the opacity of the overlying cloud layer. Yet, the nature of the 5-μm absorber in the jovian atmosphere has been subject of much debate. The cloud layer has been modelled many times as a thin, non-scattering layer, the opacity adjusted to fit the overall radiance level. This has proven to work well for individual spectra.Data from the Galileo near infrared mapping spectrometer (NIMS), covering the 0.7-5.2μm range, include a number of observations of the same areas, separated by several hours, at different emission angles. Should the 5μm absorber be a thin absorbing layer then, apart from a change in radiance level, the overall shape of the 5-μm spectrum is also expected to change significantly with emission angle. However, comparison of the 5-μm spectra measured by NIMS of the same location but at different viewing angles reveals that while the overall radiance level decreases with increasing emission angle, the shape of the spectra remain unchanged.In this paper we present atmospheric models that include scattering to explain this effect. We show that the 5-μm absorbing cloud particles must be significantly scattering (ω=0.9±0.05) in order to explain these observations, and find that the base of the cloud layer must reside at pressures less than 2 bar. Furthermore, we show that the scattering within this cloud has important consequences on the retrieval of gas abundances from spectra in the 5-μm region.