Earth Planets Space, Vol. 63 (No. 10), pp. 1077-1085, 2011
1Department of Physics, P.O. Box 64, FIN-00014, University of Helsinki, Finland
2Astronomical Institute of Kharkov National University, 35 Sumskaya St., Kharkov, 61022, Ukraine
(Received November 16, 2010; Revised February 28, 2011; Accepted March 14, 2011; Online published February 2, 2012)
At small phase angles, the whole coma aperture-averaged linear polarization of comets nearly coincides with that of C-type asteroids. However, the experimental study of light scattering by independent dust particles and regoliths, consisting of exactly the same particles, shows that their negative polarization branches are substantially different. Therefore, the similarity in the linear polarization of comets and C-type asteroids has to be interpreted in terms of a random coincidence rather than evidence for common properties of dust particles. Imaging polarimetry of comets shows two features with a distinctive behavior of the linear polarization: jets and circumnucleus haloes. Dust particles in jets produce only positive polarization through all the phase angles; whereas, dust particles in the halo reveal a significant negative polarization branch with Pmin = -6%. By comparison with the experimental study of light sc/SUBattering, such a difference in the negative polarization could indicate common properties of dust in the circumnucleus haloes and C-type asteroids. The high negative polarization can be confidently attributed to weakly absorbing particles. The real part of the refractive index Re(m) is 1.5-1.6, and the imaginary part is limited to Im(m) ≤ 0.02. The morphology of dust particles in the circumnucleus haloes can be rather fluffy with a material density of about 0.8 g/cm3. The power-law index for the size distribution is estimated to be about a = 1.5-2.
Key words: Comets, asteroids, negative polarization, discrete dipole approximation.