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The properties of the smallest subunits of cometary dust contain informationon their origin and clues to the formation of planetesimals and planets.Compared to IDPs or particles collected during the Stardust mission, dustcollected in the coma of comet 67P/Churyumov-Gerasimenko during the Rosettamission provides a resource of minimally altered material with known originwhose structural properties can be used to further the investigation of ourearly Solar System. A novel method is presented to achieve the highest spatialresolution of imaging possible with the MIDAS Atomic Force Microscope on-boardRosetta. 3D topographic images with resolutions of down to 8\,nm are analysedto determine the subunit sizes of particles on the nanometre scale. Threemorphological classes can be determined, namely (i) fragile agglomerateparticles of sizes larger than about 10\,$\mathrm{\mu m}$ comprised bymicrometre-sized subunits that may be again aggregates and show a moderatepacking density on the surface of the particles; (ii) a fragile agglomeratewith a size about few tens of micrometres comprised by micrometre-sizedsubunits suggested to be again aggregates and arranged in a structure with afractal dimension less than two; (iii) small, micrometre-sized particlescomprised by subunits in the hundreds of nanometres size range that showsurface features suggested to again represent subunits. Their differential sizedistributions follow a log-normal distribution with means about 100\,nm andstandard deviations between 20 and 35\,nm. All micrometre-sized particles arehierarchical dust agglomerates of smaller subunits. The arrangement, appearanceand size distribution of the smallest determined surface features arereminiscent of those found in CP IDPs and they represent the smallest directlydetected subunits of comet 67P.

Dust of comet 67P/Churyumov-Gerasimenko collected by Rosetta/MIDAS: classification and extension to the nanometre scale