Constraints on the Tropospheric Cloud Structure of Jupiter from Spectroscopy in the 5-µm Region: A Comparison between Voyager/IRIS, Galileo-NIMS, and ISO-SWS Spectra

Abstract

The 5-μm window in the spectrum of Jupiter is one of the few regions that gives access to the vertical structure of the troposphere between pressure levels of approximately 4 and 8 bar. Voyager/IRIS spectra obtained in 1979 during the Voyager flybys have been analyzed intensively over the past two decades. Most of these analyses concluded on a vertical cloud structure consisting of three layers: an ammonia cloud layer with a base around 0.5 bar, an ammonium-hydrosulfide layer at about 2 bar, and a water cloud layer with a base somewhere between 5 and 7 bar.

However, the recent Galileo entry probe measurements of the vertical atmospheric structure in a hot spot region show a quite different picture. The data indicate only two cloud layers, at about 0.7 bar and 1.34 bar, and no significant cloud opacity between 2 and 22 bar, where the Probe measurements ended. New data from the Galileo/NIMS and ISO/SWS experiments agree with the Probe findings. The inclusion of cloud opacity between 5 and 7 bar in the model atmosphere results in a poor fit to the NIMS and ISO data.

A comparison between Voyager/IRIS spectra and the ISO/SWS and Galileo/NIMS spectra shows a slope in the continuum level of the IRIS spectra for wavelengths shorter than 5 μm, not present in either SWS spectrum or the NIMS spectra. We show that to fit the IRIS spectra, an additional opacity somewhere between 3 and 8 bar is needed in the model atmosphere. We suggest that, most likely, a calibration problem occurred with the Voyager/IRIS data in this spectral region, even though temporal variations in the deep atmospheric structure of Jupiter cannot be ruled out altogether.