Laboratory measurements of the single-scattering properties of ammonia ice crystals
This work presents scattering measurements and photographs of ammonia ice crystals grown at temperatures from 130 to 180 Kelvins. The prime candidate for the material making up the visible clouds of Jupiter and Saturn is ammonia ice. Spacecraft observations of these planets have constrained the single-scattering properties of the cloud particles. To further investigate the nature of these particles, ammonia ice crystals were grown under conditions simulating the atmosphere of Jupiter and Saturn.
The experimental apparatus used to make these measurements includes a glass-walled cylindrical chamber which permits measurement of the scattered light over a wide range of scattering angles and a temperature control system which uses a liquid nitrogen reservoir combined with heaters. The chamber is illuminated by a tungsten lamp through a rapidly spinning filter/polarizer wheel which yields measurements of intensity and linear polarization in each of three colors. A photographic record of the crystals is obtained with a microscope objective, and six linear array detectors measure the scattered light.
A variety of crystal shapes and phase functions were seen. A representative selection of scattering measurements and photographs are presented. The data do not resemble theoretical calculations for ammonia cubes, tetrahedra, or octahedra. They do appear similar to microwave analog measurements of the scattering properties of a mix of particle shapes as well as of fluffy particles. The ammonia measurements fall into two groups: one has wavelength-dependent polarization and for size parameters up to about seven the scattering properties can be fit by Mie theory. The second group has wavelength-independent phase functions, implying size parameters of 10 to 50, and has a characteristic signature of polarization varying from $-$10% to +10%.
The data can be used to rule out some models for Jupiter's and Saturn's atmospheres and to guide future modelling efforts. For Jupiter, models with a cloud of ammonia crystals of size parameter equal to about 5 (in the red) are suggested. For Saturn, a model is suggested that has a thin layer of small ammonia crystals (in the Mie range) over a thicker ammonia cloud with the wavelength-independent polarization that is characteristic of larger crystals.