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R. Schulz, M. Hilchenbach, Y. Langevin, J. Kissel, J. Silén, C. Briois, C. Engrand, K. Hornung, K. Varmuza et al.:
"Comet 67P/Churyumov-Gerasimenko sheds dust coat accumulated over the past four years";
Nature, Nature 518 (2015), February; S. 216 - 218.

Comets are composed of dust and frozen gases. The ices are mixed with the refractory material either as an icy conglomerate1, or as an aggregate of pre-solar grains (grains that existed prior to the formation of the Solar System), mantled by an ice layer2, 3. The presence of water-ice grains in periodic comets is now well established4, 5, 6. Modelling of infrared spectra obtained about ten kilometres from the nucleus of comet Hartley 2 suggests that larger dust particles are being physically decoupled from fine-grained water-ice particles that may be aggregates7, which supports the icy-conglomerate model. It is known that comets build up crusts of dust that are subsequently shed as they approach perihelion8, 9, 10. Micrometre-sized interplanetary dust particles collected in the Earth´s stratosphere and certain micrometeorites are assumed to be of cometary origin11, 12, 13. Here we report that grains collected from the Jupiter-family comet 67P/Churyumov-Gerasimenko come from a dusty crust that quenches the material outflow activity at the comet surface14. The larger grains ...

http://dx.doi.org/10.1038/nature14159

http://www.nature.com/nature/journal/vaop/ncurrent/full/nature14159.html