LIGHT SCATTERING FROM AMMONIA AND WATER CRYSTALS
Researchers analyzing the upper clouds of Jupiter and Saturn are unable to theoretically reproduce the data returned by Pioneers 10 and 11 and Voyagers 1 and 2 with an approach based on Mie theory. Ammonia crystals are believed to be an important constituent of Jupiter's upper clouds, but both their shape and scattering properties were unknown at the start of this work. Ammonia crystals and water crystals were grown in a cold chamber at temperatures 20°C below their freezing points (0°C and -77.7°C, respectively). The H₂O crystals formed had shapes in agreement with published growth habit diagrams. The NH₃ crystals formed were usually irregular in shape, but regular four-sided pyramids were commonly observed. This four-sided pyramidal shape is in agreement with ammonia's primitive cubic crystal structure. Ammonia crystals could not be formed at temperatures above -95°C due to nucleation problems. A scattering measuring instrument was constructed with fifteen separate lens-detector combinations aimed at a common point in the center of the cold chamber. A laser beam (6328Å wavelength) traversed the chamber center, illuminating any crystal aerosal clouds present. A computer was used to rapidly sample the outputs of the fifteen detectors and to drive a photoelectric modulator to change the slow speed polarization properties of the laser beam. The measurements resulted in a determination of the single scattering phase function and degree of linear polarization for the crystal species present. Water crystals were found to have scattering properties similar to that reported by previous researchers. The H₂O crystal scattering possesses a smaller backscatter peak and smaller polarization features than is common for water spheres of similar size. A negative polarization of 5% occurred in the forward scattering hemisphere and a positive polarization of 10% in the rear. Ammonia particles were observed to have a backscattering peak four times higher than for water crystals. The NH₃ particle light scattering produced very little polarization of the scattered light. A small (∼ 4%) negative polarization occurred in the forward scattering hemisphere. Work is continuing here to make scattering measurements using blue light illumination nearly simultaneous with the red HeNe laser wavelength illumination.
Crystal optics., Light -- Scattering., Jupiter (Planet) -- Atmosphere.
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