Light scattering, comet dust

The wavelength dependence of the light scattered by dust can be presented as the color. In the case of comets this is determined traditionally as the... 

Figure 3 Light scattered off a ring of dust surrounding the star HR 4796A. Presumably the dust is generated by a Kuiper Belt distribution of comets with internal and external truncations. Image taken by NASA s Hubble Space Telescope. The figure is courtesy of B. Smith (University of Hawaii), G. Schneider (University of Arizona) and NASA.

The analysis of cometary observations suggests the existence of very fluffy dust aggregates. Differences are observed in the light-scattering properties, e.g. stracture of the comae, polarization phase curves maxima and minima, polarization wavelength dependence. They could be a clue to the temporal evolution of the physical properties of the dust particles, with collisional processes as well as evaporation of icy mantles and organic compoimds. Table 1 presents some polarization properties of dust particles in comets, asteroids, in the interplanetary dust cloud, and on Mars, as retrieved by remote sensing. 

The split Comet A2 (LINEAR) showed a short-term variability of the polarization parameters on a time scale of a day. Inexplicable cases of polarization-plane rotation and a decrease in degree of polarization have been found for the comet [82]. The cause of such changes is rmclear. It might be interpreted as the result of solar light scattering by aligned particles. Comet A2 (LINEAR) was observed in the active phase and displayed a series of sporadic outbursts due to disruption of its nucleus. It is impossible to eliminate the short-time changes of dust properties in the comet due to separation of small short-lived splinters that may not be directly observed. 

In this paper, we provide both a brief historical review of the polarimetric researches of comets and analysis of the observed angular and spectral characteristics of linear and circular polarization of the scattered solar light by dust particles in order to reveal principal regularities in the polarimetric properties of cometary dust. 

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