Tropospheric gas composition and cloud structure of the Jovian north equatorial belt

Abstract

High spatial resolution Voyager infrared interferometer spectrometer spectra of the North Equatorial Belt (NEB) reveal longitudinal variability of 5‐μm brightness temperatures of order 100°C. These observations are used to investigate spatial variations in the gas composition and cloud structure of the NEB. We use an anisotropic multiple scattering radiative transfer model to calculate synthetic spectra for comparison with the IRIS observations. The spectral dependence of cloud extinction from 180 to 2300 cm−1 is modeled using Mie theory. The entire spectral range of the IRIS observations (180–2300 cm−1) is used to constrain the cloud properties and vertical structure of the NEB. Within the model, cloud base locations vary with assumed gas abundances according to thermochemical equilibrium. We find that spatial variations in the abundance profiles of the condensible species, para hydrogen profiles and cloud optical depths can be used as tracers of the local and large‐scale dynamics. Based on the spectral dependence of NH3 cloud extinction that is required to fit the IRIS observations, we conclude that the bulk of the NH3 cloud extinction is provided by large particles, effective radii ≈100 μm; however, a small particle mode may also be present. We find that the observed 5‐μm brightness temperature structure can be reproduced by spatial variations in cloud opacity and water relative humidity. NEB hot spots, due to their low cloud opacity, provide a unique opportunity to study the deep cloud structure in the Jovian atmosphere. Cloud opacity is required at P > 4 bars (coincident with the location of the thermochemically predicted H2O cloud) to reproduce the observed continuum level near 2130 cm−1, as well as to model the overall shape of the continuum between 2100 and 2300 cm−1. Water relative humidity is found to vary spatially above the base of the water cloud increasing from ≈15% in hot spots to 100% in colder spectral ensembles. The variation of relative humidity is strongly correlated with the variation of cloud opacity, suggesting dynamic depletion of water vapor above the cloud forming level as the most plausible model to explain the spatial variation in the water profile within the NEB.