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

Context. The properties of the smallest subunits of cometary dust contain information on their origin and clues to the formation of planetesimals and planets. Compared to interplanetary dust particles or particles collected during the Stardust mission, dust collected in the coma of comet 67P/Churyumov-Gerasimenko (67P) during the Rosetta mission provides a resource of minimally altered material with known origin whose structural properties can be used to further the investigation of the early solar system. Aims. The cometary dust particle morphologies found at comet 67P on the micrometer scale are classified, and their structural analysis is extended to the nanometer scale. Methods. We present a novel method for achieving the highest spatial resolution of imaging obtained with the MIDAS Atomic Force Microscope on board Rosetta. 3D topographic images with resolutions down to 8 nm were analyzed to determine the subunit sizes of particles on the nanometer scale. Results. Three morphological classes can be determined: (i) fragile agglomerate particles of sizes larger than about 10 mu m comprised of micrometer-sized subunits that may themselves be aggregates and show a moderate packing density on the surface of the particles. (ii) A fragile agglomerate with a size of about a few tens of micrometers comprised of micrometer-sized subunits that are suggested to be aggregates themselves and are arranged in a structure with a fractal dimension lower than two. (iii) Small micrometer-sized particles comprised of subunits in the size range of hundreds of nanometers that show surface features that are again suggested to represent subunits. Their differential size distributions follow a log-normal distribution with means of about 100 nm and standard deviations between 20 and 35 nm. Conclusions. The properties of the dust particles found by MIDAS represent an extension of the dust results of Rosetta to the microand nanometer scale. All micrometer-sized particles are hierarchical dust agglomerates of smaller subunits. The arrangement, appearance, and size distribution of the smallest determined surface features are reminiscent of those found in chondritic porous interplanetary dust particles. They represent the smallest directly detected subunits of comet 67P.