| アブストラクト |
The degree of linear polarization of sunlight scattered by comets is sensitive to physical properties of cometary dust, such as chemical and mineral composition, size, and morphology of dust particles. Polarization is a function of the wavelength and geometry of light scattering. The latter dependence is expressed with the phase angle that is the relative position of the Sun, comet, and detector. We show that both dependencies of the polarization carry considerable information on target particles and can be used for remote sensing of comets. For instance, the phase dependence of the linear polarization in comets reveals two stable features: the branch of negative polarization at small phase angles (<30 degress) and a branch of positive polarization at large phase angles (>30 degrees). However, the amplitude of the negative polarization is sensitive to the absorption and size of dust particles; whereas, using the amplitude of the positive polarization branch, one can retrieve the geometric albedo of dust particles and, so, estimate their number density. In addition, the phase angle of the polarization maximum unambiguously depends on material absorption.However, reliable interpretation of polarimetric measurements of comets requires satisfying two important conditions. (1) In reasonably short time, one needs to measure the angular profile of the polarization, so that all measurements would correspond to essentially the same target particles. (2) The coma is inhomogeneous, so it is important to carry out polarimetry with high spatial resolution; whereas, an analysis of spatially averaged data canlead to misleading conclusions for “average” dust. Obviously, both conditions can be satisfied only within polarimetric measurements on board a space probe orbiting a given comet. |
