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The MIDAS (Micro-Imaging Dust Analysis System) atomic force microscope onboard the Rosetta comet orbiter investigated and measured the 3D topography ofa few hundred nm to tens of $\mu$m sized dust particles of67P/Churyumov-Gerasimenko with resolutions down to a few nanometers, givinginsights into the physical processes of our early Solar System. We analyze theshapes of the cometary dust particles collected by MIDAS on the basis of arecently updated particle catalog with the aim to determine which structuralproperties remained pristine. We develop a set of shape descriptors and metricssuch as aspect ratio, elongation, circularity, convexity, and particlesurface/volume distribution, which can be used to describe the distribution ofparticle shapes. Furthermore, we compare the structure of the MIDAS dustparticles and the clusters in which the particles were deposited to those foundin previous laboratory experiments and by Rosetta/COSIMA. Finally, we combineour findings to calculate a pristineness score for MIDAS particles anddetermine the most pristine particles and their properties. We find that themorphological properties of all cometary dust particles at the micrometer scaleare surprisingly homogeneous despite originating from diverse cometaryenvironments (e.g., different collection targets that are associated withcometary activities/source regions and collection velocities/periods). We nextfind that the types of clusters found by MIDAS show good agreement with thosedefined by previous laboratory experiments, however, there are some differencesto those found by Rosetta/COSIMA. Based on our result, we rate 19 out of 1082MIDAS particles at least moderately pristine, i.e., they are not substantiallyflattened by impact, not fragmented, and/or not part of a fragmentationcluster.

Cometary dust collected by MIDAS on board Rosetta II. Particle shape descriptors and pristineness evaluation