In situ imaging of μm and sub-μm-sized grains in a cometary environment by atomic force microscopy

The micro-imaging dust analysis system (MIDAS) is an essential element among the scientific payload on the international Rosetta mission to comet 46P/Wirtanen. The MIDAS instrument based on an atomic force microscope (AFM) collects small particles drifting outwards from the nucleus surface. AFM is able to image small structures in 3D at nanometer-scale resolution. These images provide morphological and statistical information like grain size distribution on the dust population. In order to support the development of the flight hardware, optimisation of the control functions and consolidation of a proper scheme of data interpretation, laboratory studies with similar instruments were carried out. The obtained data demonstrate the capabilities of this technique. For the first time an instrument is able to observe the smallest (nm-sized) grains which are predicted by models and were to a certain extent deduced from previous measurements on the Giotto and Vega missions to comet 1P/Halley. On larger (μm-sized) particles the complex morphology will be visualised with high precision in 3D, and if present, within these aggregates crystalline materials with defined crystal faces can be identified.