The effect of ammonia ice on the outgoing thermal radiance from the atmosphere of Jupiter

Abstract

We examine the effects of NH 3 ice particle clouds in the atmosphere of Jupiter on outgoing thermal radiances. The cloud models are characterized by a number density at the cloud base, by the ratio of the scale height of the vertical distribution of particles ( H p) to the gas scale height ( H g), and by an effective particle radius. NH 3 ice particle-scattering properties are scaled from laboratory measurements. The number density for the various particle radius and scale height models is inferred from the observed disk average radiance at 246 cm −1, and preliminary lower limits on particle sizes are inferred from the lack of apparent NH 3 absorption features in the observed spectral radiances as well as the observed minimum flux near 2100 cm −1. We find lower limits on the particle size of 3 μm if H p/ H g = 0.15, or 10 μmif H p/ H g = 0.50 or 0.05. NH 3 ice particles are relatively dark near the far-infrared and 8.5-μm atmospheric windows, and the outgoing thermal radiances are not very sensitive to various assumptions about the particle-scattering function as opposed to radiances at 5 μm, where particles are relatively brighter. We examined observations in these three different spectral window regions which provide, in principle, complementary constraints on cloud parameters. Characterization of the cloud scale height is difficult, but a promising approach is the examination of radiances and their center-to-limb variation in spectral regions where there is significant opacity provided by gases of known vertical distribution. A blackbody cloud top model can reduce systematic errors due to clouds in temperature sounding to the level of 1K or less. The NH 3 clouds provide a substantial influence on the internal infrared flux field near the 600-mbar level.