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Paper: Comet Polarimetry: Solved Problems and New Mysteries
Volume: 449, Astronomical Polarimetry 2008: Science from Small to Large Telescopes
Page: 345
Authors: Kolokolova, L.
Abstract: Comets are objects of particular interest as relics of the epoch of solar-system formation which preserve materials as they were in the protosolar nebula. It is well known that the light from comets is polarized due to the scattering of sunlight on cometary dust particles. The polarization shows smooth dependencies on scattering (phase) angle and wavelength, which can be used to reveal the properties of comet dust and, thus, to shed light on conditions in the early solar system when comets were formed. The last decade was very fruitful in comet polarimetry and many longstanding problems have been solved. The main progress was achieved by modeling comet dust as an ensemble of large aggregates of submicron particles. This model allowed us not only to explain the angular and spectral dependence of comet polarization but also to reproduce correctly the comet dust albedo, color, angular change in brightness, and thermal emission spectrum. The combination of polarimetric and thermal data appeared to be especially successful. It allowed us to answer a longstanding question of why there are two types of comets, with high (≥25%) and low (≤15%) maximum polarization, and to connect this difference with the orbital characteristics of the comets and, finally, with their age and formation region. Although many of the problems in comet polarimetry have been solved, new data on the wavelength dependence of polarization revealed a new mystery: two new types of comets: one with polarization degree that increases with wavelength and the other with polarization degree that decreases with wavelength. A tentative explanation of this phenomenon can be related to a specific of the structure of aggregates dust particles. Another interesting mystery is comet circular polarization. It has recently become possible to observe comet circular polarization with high accuracy and spatial resolution. Circular polarization has been detected in recent bright comets at the level 0.1 - 2%. It has been found changing smoothly with phase angle and distance from the nucleus. This paper discusses possible explanations of comet circular polarization, including scattering of light by homochiral organic molecules embedded in comet dust particles.
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