title: Aggregate dust particles at comet 67P/Churyumov-Gerasimenko.
abstract: Comets are thought to preserve almost pristine dust particles, thus providing a unique sample of the properties of the early solar nebula. The microscopic properties of this dust played a key part in particle aggregation during the formation of the Solar System. Cometary dust was previously considered to comprise irregular, fluffy agglomerates on the basis of interpretations of remote observations in the visible and infrared and the study of chondritic porous interplanetary dust particles that were thought, but not proved, to originate in comets. Although the dust returned by an earlier mission has provided detailed mineralogy of particles from comet 81P/Wild, the fine-grained aggregate component was strongly modified during collection. Here we report in situ measurements of dust particles at comet 67P/Churyumov-Gerasimenko. The particles are aggregates of smaller, elongated grains, with structures at distinct sizes indicating hierarchical aggregation. Topographic images of selected dust particles with sizes of one micrometre to a few tens of micrometres show a variety of morphologies, including compact single grains and large porous aggregate particles, similar to chondritic porous interplanetary dust particles. The measured grain elongations are similar to the value inferred for interstellar dust and support the idea that such grains could represent a fraction of the building blocks of comets. In the subsequent growth phase, hierarchical agglomeration could be a dominant process and would produce aggregates that stick more easily at higher masses and velocities than homogeneous dust particles. The presence of hierarchical dust aggregates in the near-surface of the nucleus of comet 67P also provides a mechanism for lowering the tensile strength of the dust layer and aiding dust release.
authors: Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria.
authors: Mark S
authors: MS
authors: Bentley
authors: Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria.
authors: Roland
authors: R
authors: Schmied
authors: Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria.|Physics Institute, University of Graz, Universitätsplatz 5, 8010 Graz, Austria.
authors: Thurid
authors: T
authors: Mannel
authors: Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria.
authors: Klaus
authors: K
authors: Torkar
authors: Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria.
authors: Harald
authors: H
authors: Jeszenszky
authors: European Space Research and Technology Centre, Future Missions Office (SREF), Noordwijk, The Netherlands.
authors: Jens
authors: J
authors: Romstedt
authors: UPMC (Sorbonne Université); CNRS/INSU; LATMOS-IPSL, BC 102, 4 place Jussieu, 75005 Paris, France.
authors: Anny-Chantal
authors: AC
authors: Levasseur-Regourd
authors: Institut für Planetologie, Universität Münster, Wilhelm-Klemm-Strasse 10, 48149 Münster, Germany.
authors: Iris
authors: I
authors: Weber
authors: Institut für Planetologie, Universität Münster, Wilhelm-Klemm-Strasse 10, 48149 Münster, Germany.
authors: Elmar K
authors: EK
authors: Jessberger
authors: Leiden Observatory, Postbus 9513, 2300 RA Leiden, The Netherlands.|Space Policy Institute, George Washington University, 20052 Washington DC, USA.
authors: Pascale
authors: P
authors: Ehrenfreund
authors: Department of Lithospheric Research, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria.|Natural History Museum, Burgring 7, 1010 Vienna, Austria.
authors: Christian
authors: C
authors: Koeberl
authors: Department of Physics and Technology, UiT The Arctic University of Norway, N-9037 Tromsø, Norway.
authors: Ove
authors: O
authors: Havnes