Coherent backscattering and opposition phenomena exhibited by some atmosphere- less solar system bodies Janna M. Dlugach Main Astronomical Observatory of the National Academy of Sciences of Ukraine, Kyiv, Ukraine Michael I. Mishchenko NASA Goddard Institute for Space Studies, New York, USA

Opposition effects for some solar system bodies

Angular profiles of the enhancement factor and the degree of linear polarization Angular profiles of the enhancement factor (dotted curves) and the degree of linear polarization (solid curve) for a semi-infinite homogeneous medium composed of sparsely distributed non absorbing Rayleigh scatters, illuminated by normally incident unpolarized light (Mishchenko et al., 2000).

Main characteristics of the coherent back- scattering peak HWHM = The angular widths of the observed backscattering intensity peaks are consistent with the results of theoretical computations for particle sizes of the order of the wavelength, packing densities ranging from several percent to approximately 40%, and particle compositions ranging from water ice (Europa, Saturn’s rings) to silicates (44 Nysa and 64 Angelina). The observed amplitudes of the peaks are consistent with the theory of coherent backscattering by a regolithic layer composed of submicrometer-sized grains.

Scattering by a spherical volume randomly filled with small spheres

Current numerical capability Computational technique : the numerically exact superposition T-matrix method and the Fortran-90 multiple sphere T-matrix (MSTM) code developed for use on parallel computer clusters. N = 1875, m = 1.31, kR = 31, kr = 2, ρ = 0.5 (Mackowski and Mishchenko 2011)

Brightness and polarization opposition effects m = 1.31, kR = 40, kr = 2 (Dlugach, Mishchenko, Liu, Mackowski 2011)

Opposition effects for Saturn’s rings and Europa Franklin and Cook 1965 Thompson and Lockwood 1992Mishchenko et al Franklin and Cook 1965 Lyot 1929

Conclusion The photometric and polarimetric observational data for several high-albedo solar system objects reveal coexisting brightness and polarization opposition effects of comparable angular widths and with angular profiles consistent with the exact solutions of the Maxwell equations. The results of our theoretical analysis point to the conclusion that both these effects are caused by the coherent backscattering of sunlight by regolithic layers composed of microscopic grains.

Acknowledgments Thank you for your attention! This research was partially sponsored by the NASA Radiation Sciences Program managed by Hal Maring and by the NASA Remote Sensing Theory Program managed by Lucia Tsaoussi. We acknowledge support from the National Academy of Sciences of Ukraine under the Main Astronomical Observatory GRAPE/GPU/GRID Computer Cluster Project.