Color, comet dust

For all comets visited by spacecraft, the measured albedo is <5%, making comet nuclei among the darkest objects in the solar system. This coloration is due to black crusts that cover most of the nuclei. The materials now comprising the crusts were presumably spewed out of the interior at speeds below the escape velocity, so they can be considered lag deposits. The parts of surfaces covered by such deposits are inert, and active areas releasing gas and dust constitute only a small percentage of a comet s total surface area. 

Figure 2. Correlation between the BC-RC color and equivalent width for dust-rich comets ( ) and gas-...

The separation of the continuum from emission features is a difficult problem. There are numerous unidentified emissions [57] that can form a pseudocontinuum of gas-rich comets. It is not clear whether the continuum is reached in the continuum bands therefore, we can conclude that at least a part of the gas-rich comets have a low polarization at large phase angles and blue color mainly due to a low spectral and spatial resolution of observations. Thus, the existence of two taxonomical classes of comets with different physical properties of their dust particles still remains an open question. 

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... 

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