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 System1,2. Cometary dust was previously considered to comprise irregular, fluffy agglomerates on the basis of interpretations of remote observations in the visible and infrared3,4,5,6 and the study of chondritic porous interplanetary dust particles7 that were thought, but not proved, to originate in comets. Although the dust returned by an earlier mission8 has provided detailed mineralogy of particles from comet 81P/Wild, the fine-grained aggregate component was strongly modified during collection9. 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 process10 and would produce aggregates that stick more easily at higher masses and velocities than homogeneous dust particles11. 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 release12.
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Rosetta is an ESA mission with contributions from its member states and NASA. We also thank the Rosetta Science Ground Segment and Mission Operations Centre for their support in acquiring the presented data. MIDAS became possible through support from funding agencies including the European Space Agency’s PRODEX programme, the Austrian Space Agency, the Austrian Academy of Sciences and the German funding agency DARA (later DLR). A.-C.L.-R. acknowledges support from the French Space Agency, CNES. M.S.B. and T.M. acknowledge funding from the Austrian Science Fund (FWF): P 28100-N36. T.M. also acknowledges the Steiermärkische Sparkasse and the Karl-Franzens Universität Graz for their financial support. P.E. acknowledges support from the NASA Astrobiology Institute. R.S. thanks F. Hofer and H. Plank for discussions and the Austrian Research Promotion Agency (FFG) for financial support. All data presented here will be made available in the ESA Planetary Science Archive (http://www.cosmos.esa.int/web/psa/rosetta).
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Monthly Notices of the Royal Astronomical Society (2019)