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تسجيل مستخدم جديدThe footprint of cometary dust analogs: I. Laboratory experiments of low-velocity impacts and comparison with Rosetta data
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Cometary dust provides a unique window on dust growth mechanisms during the onset of planet formation. Measurements by the Rosetta spacecraft show that the dust in the coma of comet 67P/Churyumov-Gerasimenko has a granular structure at size scales from sub-um up to several hundreds of um, indicating hierarchical growth took place across these size scales. However, these dust particles may have been modified during their collection by the spacecraft instruments. Here we present the results of laboratory experiments that simulate the impact of dust on the collection surfaces of COSIMA and MIDAS, instruments onboard the Rosetta spacecraft. We map the size and structure of the footprints left by the dust particles as a function of their initial size (up to several hundred um) and velocity (up to 6 m/s). We find that in most collisions, only part of the dust particle is left on the target; velocity is the main driver of the appearance of these deposits. A boundary between sticking/bouncing and fragmentation as an outcome of the particle-target collision is found at v ~ 2 m/s. For velocities below this value, particles either stick and leave a single deposit on the target plate, or bounce, leaving a shallow footprint of monomers. At velocities > 2 m/s and sizes > 80 um, particles fragment upon collision, transferring up to 50 per cent of their mass in a rubble-pile-like deposit on the target plate. The amount of mass transferred increases with the impact velocity. The morphologies of the deposits are qualitatively similar to those found by the COSIMA instrument.
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The structure of cometary dust is a tracer of growth processes in the formation of planetesimals. Instrumentation on board the Rosetta mission to comet 67P/Churyumov- Gerasimenko captured dust particles and analysed them in situ. However, these depos
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We compare low velocity impacts that ricochet with the same impact velocity and impact angle into granular media with similar bulk density, porosity and friction coefficient but different mean grain size. The ratio of projectile diameter to mean grai
n length ranges from 4 in our coarsest medium to 50 in our finest sand. Using high speed video and fluorescent markers, we measure the ratio of pre- to post-impact horizontal and vertical velocity components, which we refer to as coefficients of restitution, and the angle of deflection caused by the impact in the horizontal plane. Coefficients of restitution are sensitive to mean grain size with the ratio associated with the horizontal velocity component about twice as large for our coarsest gravel as that for our finest sand. This implies that coefficients for hydro-static-like, drag-like and lift-like forces, used in empirical force laws, are sensitive to mean grain size. The coefficient that is most strongly sensitive to grain size is the lift coefficient which decreases by a factor of 3 between our coarsest and finest media. The deflection angles are largest in the coarser media and their size approximately depends on grain size to the 3/2 power. This scaling is matched with a model where momentum transfer takes place via collisions with individual grains. The dependence of impact mechanics on substrate size distribution should be considered in future models for populations of objects that impact granular asteroid surfaces.
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s have been proposed, from localized physical mechanisms on the surface/sub-surface (see review in Belton (2010)) to purely dynamical processes involving the focusing of gas flows by the local topography (Crifo et al. 2002). While the latter seems to be responsible for the larger features, high resolution imagery has shown that broad streams are composed of many smaller features (a few meters wide) that connect directly to the nucleus surface. We monitored these jets at high resolution and over several months to understand what are the physical processes driving their formation, and how this affects the surface. Using many images of the same areas with different viewing angles, we performed a 3-dimensional reconstruction of collimated jets, and linked them precisely to their sources on the nucleus. Results.We show here observational evidence that the Northern hemisphere jets of comet 67P arise from areas with sharp topographic changes and describe the physical processes involved. We propose a model in which active cliffs are the main source of jet-like features, and therefore the regions eroding the fastest on comets. We suggest that this is a common mechanism taking place on all comets.
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