The Photochemistry of NH3 in the Jovian Atmosphere

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  • 1 Kitt Peak National Observatory, Tucson, Ariz. 85717
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Abstract

A quantitative study of the photochemistry of NH3 above the Jovian tropopause is given. The NH3, density distribution is described by two relevant vertical scales: Hav the scale height of the background atmosphere, and (K/Jnet½ the “photomechanical” scale height, where K is the eddy diffusion coefficient and Jnet> the net destruction rate of NH3.In the case of slow mixing, substantial photochemical destruction of NH3 occurs and the NH3 density profile departs significantly from a mixed distribution. Comparison of the observed and calculated UV albedos of Jupiter suggests a value of K≈2×104 cm2 sec −1 is appropriate in the lower stratosphere. Radiative excitation of NH2 radicals to the A state is unimportant in the NH3 photochemistry. A slow circulation that transports N2H4 to the hotter, dense regions of the eep atmosphere, where it undergoes thermal decomposition to NH2 radicals which react with H2 to form fresh NH3, is invoked to explain the continued presence of NH3 on Jupiter. It is suggested that condensation of N2H4 is a source of the upper semi-transmitting cloud layer postulated by Axel to explain the low observed UV geometric albedo.

Abstract

A quantitative study of the photochemistry of NH3 above the Jovian tropopause is given. The NH3, density distribution is described by two relevant vertical scales: Hav the scale height of the background atmosphere, and (K/Jnet½ the “photomechanical” scale height, where K is the eddy diffusion coefficient and Jnet> the net destruction rate of NH3.In the case of slow mixing, substantial photochemical destruction of NH3 occurs and the NH3 density profile departs significantly from a mixed distribution. Comparison of the observed and calculated UV albedos of Jupiter suggests a value of K≈2×104 cm2 sec −1 is appropriate in the lower stratosphere. Radiative excitation of NH2 radicals to the A state is unimportant in the NH3 photochemistry. A slow circulation that transports N2H4 to the hotter, dense regions of the eep atmosphere, where it undergoes thermal decomposition to NH2 radicals which react with H2 to form fresh NH3, is invoked to explain the continued presence of NH3 on Jupiter. It is suggested that condensation of N2H4 is a source of the upper semi-transmitting cloud layer postulated by Axel to explain the low observed UV geometric albedo.

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