Are there Dust-Flakes in Cometary Atmospheres?

Recent comet observations show that the inner comae (within 5000 km from the nucleus) has unusual light scattering features. Particularly, the color, polarization, and polarimetric color are much different than in the surrounding cometary atmosphere and cannot be explained by the light scattering from simple spherical particles. More complex particle geometries have been proposed. Stephens and Gustafson (1991, Icarus 94, 209) showed through a laboratory study that thin flakes can form on surfaces of sublimating dust/ice mixtures. Microwave analog to light scattering measurements has shown that such flakes can produce unusual combinations of color, polarization and polarimetric color and may be inner coma particle candidates. This paper presents the results of our theoretical modeling of the light scattering by flake-like dust particles. The modeling is based on our approximate light scattering solution for a thin plate with arbitrary refractive index. When the scattering model is compared to microwave test measurements an overall good agreement is found. We investigate the dependence of the polarization, color, and polarimetric color on the refractive index and geometrical dimensions of the flakes. We show how the light scattering by flakes can explain observational features of inner comae.