A CRITICAL-REVIEW OF THEORETICAL-MODELS OF NEGATIVELY POLARIZED-LIGHTSCATTERED BY ATMOSPHERELESS SOLAR-SYSTEM BODIES

About a dozen physical mechanisms and models aspire to explain the neg ative polarization of light scattered by atmosphereless celestial bodi es. This is too large a number for the reliable interpretation of obse rvational data. Through a comparative analysis of the models, our main goal is to answer the question: Does any one model have an advantage over the others? Our analysis is based on new laboratory polarimetric and photometric data as well as on theoretical results. We show that t he widely used models due to Hopfield and Wolff cannot realistically e xplain the phase-angle dependence of the degree of polarization observ ed at small phase angles. The so-called interference or coherent backs cattering mechanism is the most promising model. Models based on that mechanism use well-defined physical parameters to explain both negativ e polarization and the opposition effect. They are supported by labora tory experiments, particularly those showing enhancement of negative p olarization with decreasing particle size down to the wavelength of li ght. According to the interference mechanism, pronounced negative bran ches of polarization, like those of C-class asteroids, may indicate a high degree of optical inhomogeneity of light-scattering surfaces at s mall scales. The mechanism also seems appropriate for treating the neg ative polarization and opposition effects of cometary dust comae, plan etary rings, and the zodiacal light.