INFRARED-ABSORPTION BY SHAPE DISTRIBUTIONS OF NH3 ICE PARTICLES - AN APPLICATION TO THE JOVIAN ATMOSPHERE

We study how distribution of small NH3 ice particles over shapes affects the strength of resonant absorption features at 9.4 and 26-mu-m. The T-matrix approach is used to compute optical cross sections for shape distributions of 0.5- and 1-mu-m volume-equivalent radius spheroids in random orientation. It is found that the maximum of the resonant absorption for the shape distributions is 1.5-2 times smaller that that for equal-volume spherical particles, the absorption peak being shifted towards longer wavelengths. The results of our computations support the conclusion of West et al. (1989) that, apparently, small NH3 ice particles cannot be the principal component of the Jovian troposphere in the 300- to 500-mbar region.