The footprint of cometary dust analogues - I. Laboratory experiments of low-velocity impacts and comparison with Rosetta data

Cometary dust provides a unique window on dust growth mechanisms duringthe onset of planet formation. Measurements by the Rosetta spacecraftshow that the dust in the coma of comet 67P/Churyumov-Gerasimenko has agranular structure at size scales from sub-μmup to several hundredsof μm, indicating hierarchical growth took place across these sizescales. However, these dust particles may have been modified duringtheir collection by the spacecraft instruments. Here, we present theresults of laboratory experiments that simulate the impact of dust onthe collection surfaces of the COSIMA (Cometary Secondary Ion MassAnaylzer) and MIDAS (Micro-Imaging Dust Analysis System) instrumentsonboard the Rosetta spacecraft. We map the size and structure of thefootprints left by the dust particles as a function of their initialsize (up to several hundred μm) and velocity (up to 6 ms-1). We find that in most collisions, only part of the dustparticle is left on the target; velocity is the main driver of theappearance of these deposits. A boundary between sticking/bouncing andfragmentation as an outcome of the particle-target collision is found atv ˜ 2 m s-1. For velocities below this value, particleseither stick or leave a single deposit on the target plate, or bounce,leaving a shallow footprint of monomers. At velocities >2 ms-1and sizes >80 μm, particles fragment upon collision,transferring up to 50 per cent of their mass in a rubble-pile-likedeposit on the target plate. The amount of mass transferred increaseswith the impact velocity. The morphologies of the deposits arequalitatively similar to those found by the COSIMA instrument.