  The Cometary Secondary Ion Mass Analyzer (COSIMA) onboard ESA's Rosetta
orbiter has revealed that dust particles in the coma of Comet
67P/Churyumov-Gerasimenko are aggregates of small grains. We study the
morphological, elastic, and electric properties of dust aggregates in the coma
of Comet 67P/Churyumov-Gerasimenko using optical microscopic images taken by
the COSIMA instrument. Dust aggregates in COSIMA images are well represented as
fractals in harmony with morphological data from MIDAS (Micro-Imaging Dust
Analysis System) and GIADA (Grain Impact Analyzer and Dust Accumulator) onboard
Rosetta. COSIMA's images, together with the data from the other Rosetta's
instruments such as MIDAS and GIADA do not contradict the so-called rainout
growth of $10~\mu\mathrm{m}$-sized particles in the solar nebula. The elastic
and electric properties of dust aggregates measured by COSIMA suggest that the
surface chemistry of cometary dust is well represented as carbonaceous matter
rather than silicates or ices, consistent with the mass spectra, and that
organic matter is to some extent carbonized by solar radiation, as inferred
from optical and infrared observations of various comets. Electrostatic lofting
of cometary dust by intense electric fields at the terminator of its parent
comet is unlikely, unless the surface chemistry of the dust changes from a
dielectric to a conductor. Our findings are not in conflict with our current
understanding of comet formation and evolution, which begin with the
accumulation of condensates in the solar nebula and follow with the formation
of a dust mantle in the inner solar system.
