THE GAS COMPOSITION AND VERTICAL CLOUD STRUCTURE OF JUPITER'S TROPOSPHERE DERIVED FROM FIVE MICRON SPECTROSCOPIC OBSERVATIONS.

Abstract

Spectroscopic observations of Jupiter at 5 microns were analyzed in order to derive the gas composition and vertical cloud structure for the 2 to 6 bar portion of the Jovian troposphere. Two infrared data sets were used. The first one consisted of high spectral resolution observations of Jupiter between -40 and +40 latitude acquired from the Kuiper Airborne Observatory. The second data set consisted of high spatial resolution measurements of Jupiter's belts and zones using the Voyager 1 Infrared Interferometer Spectrometer and Radiometer (IRIS). A spectrum synthesis program was used to calculate the emergent radiance from Jupiter's atmosphere between 1800 and 2250 cm⁻¹. The temperature-pressure profile and spectroscopic line parameters for seven molecules were specified. Gas mole fractions were adjusted until the calculated spectrum agreed with the observations within error limits. Molecular hydrogen is an important absorber at 5 microns. Absorption coefficients were generated as functions of frequency and temperature. Unit optical depth at 5 microns due to H₂ takes place between 5 and 7 bars in Jupiter's atmosphere. The airborne spectrum was used to infer the mole fractions of NH₃, PH₃, CH₄, CH₃D, CO, GeH₄, and H₂O for the 2 to 6 bar portion of Jupiter's troposphere. Elemental ratios were calculated and compared with model predictions. The N/H ratio is 1.5 ± 0.2 times the Lambert (1978) values for the Sun. The P/H ratio is 1.0 ± 0.1 times the Anders and Ebihara (1982) meteoritic value. The C/H ratio is 3.6 ± 0.7 times solar. The D/H ratio is 1.2 x 10⁻⁵. The mole fractions of CO and GeH₄ are (1.0 ± 0.3) x 10⁻⁹ and (7.0 ± 4) x 10⁻¹⁰, respectively. The mole fraction of H₂O was found to be the same in Jupiter's belts and zones, except for a factor of 2 depletion in the North Equatorial Belt Hot Spots. The H₂O mole fraction for the 2 to 4 bar region is (4.0 ± 1.0) x 10⁻⁶. This value increases with depth to (3.0 ± 2.0) x 10⁻⁵ at 6 bars. The H₂O ice cloud would be located near 2 bars. The O/H ratio at P = 6 bars is depleted by a factor of 40 with respect to the Sun. The thermal emission signature at 5 microns of optically thick clouds was used to develop a one-dimensional cloud model for Jupiter. The belt-to-zone variation in 5 micron flux is attributed to a massive cloud layer at 2 bars, T = 200 K, composed of NH₄SH and H₂O ice. A lower cloud at 5 bars is inconsistent with the IRIS data. Continuum absorption by H₂ determines the penetration depth at 5 microns, not a lower cloud layer.