title: Comet 67P/Churyumov-Gerasimenko sheds dust coat accumulated over the past four years.
abstract: Comets are composed of dust and frozen gases. The ices are mixed with the refractory material either as an icy conglomerate, or as an aggregate of pre-solar grains (grains that existed prior to the formation of the Solar System), mantled by an ice layer. The presence of water-ice grains in periodic comets is now well established. 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 aggregates, which supports the icy-conglomerate model. It is known that comets build up crusts of dust that are subsequently shed as they approach perihelion. Micrometre-sized interplanetary dust particles collected in the Earth's stratosphere and certain micrometeorites are assumed to be of cometary origin. 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 surface. The larger grains (exceeding 50 micrometres across) are fluffy (with porosity over 50 per cent), and many shattered when collected on the target plate, suggesting that they are agglomerates of entities in the size range of interplanetary dust particles. Their surfaces are generally rich in sodium, which explains the high sodium abundance in cometary meteoroids. The particles collected to date therefore probably represent parent material of interplanetary dust particles. This argues against comet dust being composed of a silicate core mantled by organic refractory material and then by a mixture of water-dominated ices. At its previous recurrence (orbital period 6.5 years), the comet's dust production doubled when it was between 2.7 and 2.5 astronomical units from the Sun, indicating that this was when the nucleus shed its mantle. Once the mantle is shed, unprocessed material starts to supply the developing coma, radically changing its dust component, which then also contains icy grains, as detected during encounters with other comets closer to the Sun.
authors.affiliation: European Space Agency, Scientific Support Office, Keplerlaan 1, Postbus 299, 2200 AG Noordwijk, The Netherlands.
authors.forename: Rita
authors.initials: R
authors.lastname: Schulz
authors.affiliation: Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Hilchenbach
authors.affiliation: Institut d'Astrophysique Spatiale, CNRS/Université Paris Sud, Bâtiment 121, 91405 Orsay, France.
authors.forename: Yves
authors.initials: Y
authors.lastname: Langevin
authors.affiliation: Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany.
authors.forename: Jochen
authors.initials: J
authors.lastname: Kissel
authors.affiliation: Finnish Meteorological Institute, Observation services, Erik Palménin aukio 1, FI-00560 Helsinki, Finland.
authors.forename: Johan
authors.initials: J
authors.lastname: Silen
authors.affiliation: Laboratoire de Physique et Chimie de l'Environnement et de l'Espace (LPC2E), CNRS/Université d'Orléans, 45071 Orléans, France.
authors.forename: Christelle
authors.initials: C
authors.lastname: Briois
authors.affiliation: Centre de Sciences Nucléaires et de Sciences de la Matière, CNRS/IN2P3-Université Paris Sud-UMR8609, Batiment 104, 91405 Orsay campus, France.
authors.forename: Cecile
authors.initials: C
authors.lastname: Engrand
authors.affiliation: Universität der Bundeswehr, LRT-7, Werner Heisenberg Weg 39, 85577 Neubiberg, Germany.
authors.forename: Klaus
authors.initials: K
authors.lastname: Hornung
authors.affiliation: Institut d'Astrophysique Spatiale, CNRS/Université Paris Sud, Bâtiment 121, 91405 Orsay, France.
authors.forename: Donia
authors.initials: D
authors.lastname: Baklouti
authors.affiliation: 1] Laboratoire de Physique et Chimie de l'Environnement et de l'Espace (LPC2E), CNRS/Université d'Orléans, 45071 Orléans, France [2] Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), UMR CNRS 7583, Université Paris Est Créteil et Université Paris Diderot, Institut Pierre Simon Laplace, 94000 Créteil, France.
authors.forename: Anaïs
authors.initials: A
authors.lastname: Bardyn
authors.affiliation: Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), UMR CNRS 7583, Université Paris Est Créteil et Université Paris Diderot, Institut Pierre Simon Laplace, 94000 Créteil, France.
authors.forename: Hervé
authors.initials: H
authors.lastname: Cottin
authors.affiliation: Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany.
authors.forename: Henning
authors.initials: H
authors.lastname: Fischer
authors.affiliation: Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), UMR CNRS 7583, Université Paris Est Créteil et Université Paris Diderot, Institut Pierre Simon Laplace, 94000 Créteil, France.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Fray
authors.affiliation: Centre de Sciences Nucléaires et de Sciences de la Matière, CNRS/IN2P3-Université Paris Sud-UMR8609, Batiment 104, 91405 Orsay campus, France.
authors.forename: Marie
authors.initials: M
authors.lastname: Godard
authors.affiliation: University of Turku, Department of Physics and Astronomy, Tuorla Observatory Väisäläntie 20, 21500 Piikkiö, Finland.
authors.forename: Harry
authors.initials: H
authors.lastname: Lehto
authors.affiliation: Center for Space and Habitability (CSH), University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland.
authors.forename: Léna
authors.initials: L
authors.lastname: Le Roy
authors.affiliation: Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany.
authors.forename: Sihane
authors.initials: S
authors.lastname: Merouane
authors.affiliation: Université Grenoble Alpes/CNRS, Institut de Planétologie et d'Astrophysique de Grenoble, 414 Rue de la Piscine, Domaine Universitaire, 38000 Grenoble, France.
authors.forename: François-Régis
authors.initials: FR
authors.lastname: Orthous-Daunay
authors.affiliation: Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany.
authors.forename: John
authors.initials: J
authors.lastname: Paquette
authors.affiliation: Finnish Meteorological Institute, Observation services, Erik Palménin aukio 1, FI-00560 Helsinki, Finland.
authors.forename: Jouni
authors.initials: J
authors.lastname: Rynö
authors.affiliation: Department of Chemistry, Materials and Surfaces, SP Technical Research Institute of Sweden, Box 857, 50115 Borås, Sweden.
authors.forename: Sandra
authors.initials: S
authors.lastname: Siljeström
authors.affiliation: Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany.
authors.forename: Oliver
authors.initials: O
authors.lastname: Stenzel
authors.affiliation: Laboratoire de Physique et Chimie de l'Environnement et de l'Espace (LPC2E), CNRS/Université d'Orléans, 45071 Orléans, France.
authors.forename: Laurent
authors.initials: L
authors.lastname: Thirkell
authors.affiliation: Institut für Statistik und Wahrscheinlichkeitstheorie, Technische Universität Wien, Wiedner Hauptstrasse 7, 1040 Wien, Austria.
authors.forename: Kurt
authors.initials: K
authors.lastname: Varmuza
authors.affiliation: University of Turku, Department of Physics and Astronomy, Tuorla Observatory Väisäläntie 20, 21500 Piikkiö, Finland.
authors.forename: Boris
authors.initials: B
authors.lastname: Zaprudin