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As we (laymen) are told, comets lose gas and dust when they descent into the denser regions of the solar system. Seemingly so much and so large parts that they can even cause meteor showers like in the case of the Leonids from 55P/Temple-Tuttle. Nevertheless we were also told, that the surface of 67P is covered with several centimeres of dust over a much harder surface. Where does the dust come from? The solar wind should have blown away any lose dust long ago, no? Is there a process which regenerates dust on the surface every cycle?

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Your assumption that solar wind should have blown the dust away is only valid for comets that have a close perihelion. But you are correct in that there is a process which regenerates dust.

From Wikipedia

A comet will experience a range of diverse conditions as it traverses its orbit. For long period comets, most of the time it will be so far from the Sun that it will be too cold for evaporation of ices to occur. When it passes through the terrestrial planet region, evaporation will be rapid enough to blow away small grains, but the largest grains may resist entrainment and stay behind on the comet nucleus, beginning the formation of a dust layer. Near the Sun, the heating and evaporation rate will be so great, that no dust can be retained. Therefore, the thickness of dust layers covering the nuclei of a comet can indicate how closely and how often a comet's perihelion travels are to the Sun. If a comet has an accumulation of thick dust layers, it may have frequent perihelion passages that don't approach the Sun too closely.

A thick accumulation of dust layers might be a good description of all of the short period comets, as dust layers with thicknesses of order meters are thought to have accumulated on the surfaces of short period comet nuclei. The accumulation of dust layers over time would change the physical character of the short-period comet. A dust layer both inhibits the heating of the cometary ices by the Sun (the dust is impenetrable by sunlight and a poor conductor of heat), and slows the loss of gases from the nucleus below. A comet nucleus in an orbit typical of short period comets would quickly decrease its evaporation rate to the point that neither a coma or a tail would be detectable and might appear to astronomers as a low-albedo near-Earth asteroid.

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  • $\begingroup$ So Earth orbit isn't close enough to clear the core from dust, like in 67P's case. Interesting. Will we learn about the particle size in the coma (and the cutoff-size where no larger particles can be found)? Rosetta has a microscope aboard for such things I think. $\endgroup$ – Vroomfondel Jan 19 '15 at 15:24

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