Dust properties determined from backscattering in the interplanetary and interstellar medium

The structure and composition of comet dust were investigated through the dependence of polarization and angular albedo as a function of phase angle. The investigations concentrated on small phase angles (comet near opposition). Polarization measurements over wide ranges of phase angle were made for three comets. The polarization data showed no color dependence from 0.5 μm to 0.8 μm. Data from two comets confirmed that the position angle of the polarization lies parallel to the scattering plane at small phase angles. All polarization data of comets at small phase angles were merged to show that the angular dependence of polarization repeats from comet to comet. Mie models were unable to explain the polarization observations, even though the parameters were varied over wide ranges. Infrared observations of the reflected solar spectrum and the dust thermal spectrum allowed a determination of the angular albedo of comet dust. The angular albedo at all phase angles describes the dust phase functions. A large phase coefficient is characteristic of class C asteroids, asteroids of low albedo. The comet infrared data supports the conclusions of the polarization study. An attempt to observe a polarization signature of comet dust in the Orion Nebula could not be completed because of instrumental problems. Instrumental improvements are suggested