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Author(s): Rita Schulz [sup.1] , Martin Hilchenbach [sup.2] , Yves Langevin [sup.3] , Jochen Kissel [sup.2] , Johan Silen [sup.4] , Christelle Briois [sup.5] , Cecile Engrand [sup.6] , Klaus [...], Grains collected from comet 67P/Churyumov-Gerasimenko by the Rosetta mission come from a dusty crust that is predicted to be imminently shed as the comet nears the Sun; the grains are high in sodium and fluffy, not icy, suggesting that they are the precursors of interplanetary dust particles. Rosetta's analysis of cometary dust Since August 2014, the Rosetta spacecraft has been in orbit around comet 67P/Churyumov-Gerasimenko, monitoring the evolution of the comet as it advances towards the Sun. Here Rita Schulz et al. present initial results from Rosetta's COSIMA instrument, which is collecting cometary dust grains for optical and compositional analysis. They describe grains collected from the dusty crust that quenches material outflow at the comet surface. This dust is part of the mantle that is shed as the comet nears the Sun, eventually revealing an icy layer beneath. The larger grains (>50 [mu]m diameter) are fluffy and many have shattered when collected on the target plate, suggesting that they are agglomerates of interplanetary dust particles. Comets are composed of dust and frozen gases. The ices are mixed with the refractory material either as an icy conglomerate.sup.1, or as an aggregate of pre-solar grains (grains that existed prior to the formation of the Solar System), mantled by an ice layer.sup.2,3. The presence of water-ice grains in periodic comets is now well established.sup.4,5,6. 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 aggregates.sup.7, which supports the icy-conglomerate model. It is known that comets build up crusts of dust that are subsequently shed as they approach perihelion.sup.8,9,10. Micrometre-sized interplanetary dust particles collected in the Earth's stratosphere and certain micrometeorites are assumed to be of cometary origin.sup.11,12,13. 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 surface.sup.14. 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.sup.15. 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.sup.2,3. 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.sup.14, 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.sup.4,5.

Author(s): Nicolas Fray (corresponding author) [1]; Anas Bardyn [1, 2]; Herv Cottin [1]; Kathrin Altwegg [3]; Donia Baklouti [4]; Christelle Briois [2]; Luigi Colangeli [5]; Ccile Engrand [6]; Henning Fischer [...]