Ground truth of (sub-)micrometre cometary dust - Results of MIDAS onboard Rosetta

Abstract : The investigation of comet 67P by Rosetta has allowed the comprehensive characterisation of pristine cometary dust particles ejected from the nucleus. Flying alongside the comet at distances as small as a few kilometres, and with a relative velocity of only centimetres per second, the Rosetta payload sampled almost unaltered dust. A key instrument to study this dust was MIDAS (the Micro-Imaging Dust Analysis System), a dedicated atomic force microscope that scanned the surfaces of hundreds of (sub- )micrometre sized particles in 3D with resolutions down to nanometres. This offers the unique opportunity to explore the morphology of smallest cometary dust and expand our current knowledge about cometary material. Here we give an overview of dust collected and analysed by MIDAS and highlight its most important features. These include the ubiquitous agglomerate nature of the dust, which is found at all size scales from the largest (>10 μm) through to the smallest (<1 μm) dust particles. The sub-units show characteristic sizes and shapes that are compared with model predictions for interstellar dust. Our findings constrain key parameters of the evolution of the early Solar System. We will discuss which dust growth model is favoured by the observed morphology and how the results restrict cometary formation. Finally, dust particles detected by MIDAS resemble primitive interplanetary dust which is a strong argument for a common cometary origin.