A negative branch of polarization for comets and atmosphereless celestial bodies and the light scattering by aggregate particles

Optical observations of comets and atmosphereless celestial bodies show tha t a change of sign of the linear polarization of scattered light from negat ive to positive at phase angles less than approximate to 20 degrees is typi cal of the cometary coma, as well as of the regolith of Mercury, the Moon, planetary satellites, and asteroids. To explain a negative branch of polari zation, this research suggests a unified approach to the treatment of comet ary-dust particles and regolith grains as aggregate forms. A composite stru cture of aggregate particles resulting in the interaction of composing stru ctural elements (monomers) in the light-scattering process is responsible f or the negative polarization at small phase angles, if the monomer sizes ar e comparable to the wavelength. The characteristics of single scattering of light calculated for aggregates of this kind turned out to be close to the properties observed for cometary dust. Unlike the cometary coma, the regol ith is an optically semi-infinite medium, where the interaction between par ticles is significant. To find the reflectance characteristics of regolith, the radiative-transfer equation should be solved for a regolith layer. In this case, the interaction between scatterers can be modeled to a certain e xtent by representing the regolith grains as aggregate structures consistin g of several or many elements. Although real regolith grains are much large r than the particles considered here, laboratory measurements have shown th at it is precisely the surface irregularities comparable to the wavelength that cause a negative branch of polarization. The main observed features of the phase and spectral dependence of the linear polarization of light scat tered from comets and atmosphereless celestial bodies, which are due to the difference of the elementary scatterers in composition, size, and structur e, can be successfully explained using the aggregate model of particles.