Jupiter's Atmospheric Composition and Cloud Structure Deduced from Absorption Bands in Reflected Sunlight

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  • 1 NASA Goddard Institute for Space Studies, Goddard Space Flight Center, New York, NY 10025
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Abstract

The spectrum of sunlight reflected by Jupiter is analyzed by comparing observations of Woodman et al. (1979) with multiple-scattering computations. The analysis yields information on the vertical cloud structure at several latitudes and on the abundance of CH4 and NH3 in the atmosphere of Jupiter.

The abundance of CH4, is (1.8±0.4) × 10−3 for [CH4]/[H2], which corresponds to a carbon abundance 2±0.4 times that in the atmosphere of the sun for currently accepted values of the solar composition. The quoted limits for the abundance include the effects of uncertainties in the cloud and haze structure. The abundance of NH3 is (2.8±1.0) × 10−4 for [NH3]/[H2] in the region between 1 bar and 3–5 bars, corresponding to a nitrogen abundance 1.5±0.5 times that in the atmosphere of the sun. Thus nitrogen is at least as abundant on Jupiter as on the sun, and it may exceed the abundance in the solar atmosphere by a factor as great as that for carbon. These abundances suggest that all ices (and rocks) are overabundant on Jupiter by a factor approximately 2 or more, providing an important constraint on models for the formation of Jupiter from the primitive solar nebula.

Clouds of mean visible optical depth approximately 10 exist in both belts and zones at a pressure level of several hundred millibars. The pressure level of the clouds, the gaseous NH3 abundance, the mean temperature profile and the Clausius-Clapeyron relation together suggest that these clouds are predominantly ammonia crystals and place the cloud bottom at 600–700 mb. Beneath this “ammonia” cloud region is an optically thick cloud layer at 3–5 bars; this cloud may be composed of H20. The region between these two cloud layers is relatively transparent. Thus NH4SH clouds, assumed to be optically thick in all previous multi-layered cloud models for Jupiter, are optically thin or broken, if they exist.

A diffuse distribution of aerosols (“haze”) exists between approximately 150 and 400–500 mb, i.e., above the main ammonia cloud region. These aerosols are at least 1 μm in diameter. The ultraviolet absorption occurs in both the haze region and the ammonia cloud region. The decreasing absorption with increasing wavelength is due to an increasing single scattering albedo rather than a decreasing aerosol optical depth as in the “Axel dust” model. Thus the spectral variation of albedo reflects a changing bulk absorption coefficient of the material composing the aerosols and is diagnostic of the aerosol composition.

Ratio spectra of the North Tropical Zone (NTrZ) and North Equatorial Belt (NEB) imply that the scatterers in the 150–500 mb haze region (which may include ammonia “cirrus") reach to higher altitudes over the NTrZ than over the NEB. But the tops of the more optically dense main “cloud” layer appear to reach to higher altitudes over the NEB, implying that the usual picture of the zones as regions of rising motions and enhanced ammonia cloudiness is too simple. The total optical thickness of aerosols in the haze and cloud regions is greater in the zone than in the belt, but there is more ultraviolet-absorbing aerosol in the belt. Ten parameters are needed to describe the vertical distribution of aerosol properties to satisfy only the spectra of Woodman et al., suggesting that the atmospheric dynamics and cloud physics an Jupiter are extremely complex.

Abstract

The spectrum of sunlight reflected by Jupiter is analyzed by comparing observations of Woodman et al. (1979) with multiple-scattering computations. The analysis yields information on the vertical cloud structure at several latitudes and on the abundance of CH4 and NH3 in the atmosphere of Jupiter.

The abundance of CH4, is (1.8±0.4) × 10−3 for [CH4]/[H2], which corresponds to a carbon abundance 2±0.4 times that in the atmosphere of the sun for currently accepted values of the solar composition. The quoted limits for the abundance include the effects of uncertainties in the cloud and haze structure. The abundance of NH3 is (2.8±1.0) × 10−4 for [NH3]/[H2] in the region between 1 bar and 3–5 bars, corresponding to a nitrogen abundance 1.5±0.5 times that in the atmosphere of the sun. Thus nitrogen is at least as abundant on Jupiter as on the sun, and it may exceed the abundance in the solar atmosphere by a factor as great as that for carbon. These abundances suggest that all ices (and rocks) are overabundant on Jupiter by a factor approximately 2 or more, providing an important constraint on models for the formation of Jupiter from the primitive solar nebula.

Clouds of mean visible optical depth approximately 10 exist in both belts and zones at a pressure level of several hundred millibars. The pressure level of the clouds, the gaseous NH3 abundance, the mean temperature profile and the Clausius-Clapeyron relation together suggest that these clouds are predominantly ammonia crystals and place the cloud bottom at 600–700 mb. Beneath this “ammonia” cloud region is an optically thick cloud layer at 3–5 bars; this cloud may be composed of H20. The region between these two cloud layers is relatively transparent. Thus NH4SH clouds, assumed to be optically thick in all previous multi-layered cloud models for Jupiter, are optically thin or broken, if they exist.

A diffuse distribution of aerosols (“haze”) exists between approximately 150 and 400–500 mb, i.e., above the main ammonia cloud region. These aerosols are at least 1 μm in diameter. The ultraviolet absorption occurs in both the haze region and the ammonia cloud region. The decreasing absorption with increasing wavelength is due to an increasing single scattering albedo rather than a decreasing aerosol optical depth as in the “Axel dust” model. Thus the spectral variation of albedo reflects a changing bulk absorption coefficient of the material composing the aerosols and is diagnostic of the aerosol composition.

Ratio spectra of the North Tropical Zone (NTrZ) and North Equatorial Belt (NEB) imply that the scatterers in the 150–500 mb haze region (which may include ammonia “cirrus") reach to higher altitudes over the NTrZ than over the NEB. But the tops of the more optically dense main “cloud” layer appear to reach to higher altitudes over the NEB, implying that the usual picture of the zones as regions of rising motions and enhanced ammonia cloudiness is too simple. The total optical thickness of aerosols in the haze and cloud regions is greater in the zone than in the belt, but there is more ultraviolet-absorbing aerosol in the belt. Ten parameters are needed to describe the vertical distribution of aerosol properties to satisfy only the spectra of Woodman et al., suggesting that the atmospheric dynamics and cloud physics an Jupiter are extremely complex.

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