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A Model for the Polarimetric and Photometric Characteristics of the Moon at Moderate Phase Angles Based on Realistic Assumptions on Regolith Microstructure

Yu. I. Velikodsky, V. V. Korokhin, and L. A. Akimov

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There are known two potential mechanisms for explaining positive polarization that is observed for atmosphereless celestial bodies at large phase angles. The first one, Fresnel's reflection, may take place due to existing of large (in comparison with wavelength) smooth surfaces in regolith (glasses). The second one, Rayleigh's scattering, supposes the presence of particles with sizes smaller than wavelength, i.e. with submicron sizes. It is known that typical size of regolith particles is about tens of micrometers. The particles are mainly aggregates of smaller grains of micron- submicron sizes. Such particles cannot produce a pure Rayleigh polarization. On the other hand, Fresnel's reflection yields too large phase angles of the positive polarization maximum and cannot explain this effect by oneself.

Taking into account that the principal contribution to scattering (and, hence, to positive polarization) may produce small particles with subwavelength sizes we propose a combinative heuristic model using the Rayleigh-Gans approximation, Fresnel's reflection on large surfaces, shadow-hiding effect, and multiple scattering on micro- and macro-scales. The model shows a good agreement with observational data for the Moon for both brightness and polarization degree phase dependences (Figs. 5 and 6) with the same set of parameters. Also the model explains Umov's law and decreasing of the phase angle of polarization maximum with albedo increasing, which is observed for the Moon and asteroids.

Phase dependence of brightness was approximated at moderate phase angles (Fig. 6), where a good approximation with exponential function exists (Akimov, 1988). At large phase angles the model yields higher values of brightness than the exponent. It is known that phase dependence at large phase angles differs from Akimov's exponent, and our model can explain this difference. To check this we plan to study phase dependence at large phase angles by absolute observations and investigation of mare-highlands contrast.

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2004/10/09