  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.
