Abstract
Context.Cometary dust particles are remnants of the primordial accretion of refractory material that occurred during the initial formation stages of the solar system. Understanding their physical structure can help constrain their accretion process.Aims.The in situ study of dust particles that were collected at slow speeds by instruments on board the Rosetta space mission, including GIADA, MIDAS, and COSIMA, can be used to infer the physical properties, size distribution, and typologies of the dust.Methods.We have developed a simple numerical simulation of aggregate impact flattening to interpret the properties of particles collected by COSIMA. The aspect ratios of flattened particles from simulations and observations are compared to distinguish between initial families of aggregates that are characterized by different fractal dimensionsDf. This dimension can differentiate between certain growth modes: the diffusion limited cluster–cluster aggregates (DLCA,Df≈ 1.8), diffusion limited particle–cluster aggregates (DLPA,Df≈ 2.5), reaction limited cluster–cluster aggregates (RLCA,Df≈ 2.1), and reaction limited particle–cluster aggregates (RLPA,Df≈ 3.0).Results.The diversity of aspect ratios measured by COSIMA is consistent with either two families of aggregates with different initialDf(a family of compact aggregates withDfclose to 2.5–3 and some fluffier aggregates withDf≈ 2) or aggregates formed by a single type of aggregation process, such as DLPA. In that case, the cohesive strength of the dust particles must span a wide range to explain the range of aspect ratios observed by COSIMA. Furthermore, variations in cohesive strength and velocity may play a role in the detected higher aspect ratio range (>0.3).Conclusions.Our work allows us to explain the particle morphologies observed by COSIMA and those generated by laboratory experiments in a consistent framework. Taking into account all observations from the three dust instruments on board Rosetta, we favor an interpretation of our simulations based on two different families of dust particles with significantly distinct fractal dimensions that are ejected from the cometary nucleus.
Highlights
Comets are believed to preserve pristine dust grains and to provide information about their aggregation processes in the early solar system (e.g., Weidenschilling 1997; Blum 2000)
We have shown that simple numerical simulations of aggregate flattening can be used to infer the initial properties of particles collected by COmetary Secondary Ion Mass Analyzer (COSIMA) on-board Rosetta
The diversity of aspect ratios measured in COSIMA images appears consistent with several hypotheses on the initial properties of the collected particles
Summary
Comets are believed to preserve pristine dust grains and to provide information about their aggregation processes in the early solar system (e.g., Weidenschilling 1997; Blum 2000). Analyses of data from the Giotto mission to comet 1P/Halley and of foil impacts and aerogel tracks retrieved by the Stardust mission in the coma of comet 81P/Wild 2 have given clues to the presence of low-density dust particles that are built up of agglomerates, possibly with different tensile strengths and porosities (e.g., Fulle et al 2000; Hörz et al 2006; Burchell et al 2008). Understanding the structure of cometary dust particles can give clues to these early Solar System processes (Blum & Wurm 2008; Fulle & Blum 2017). Three instruments were devoted to the study of dust particles: (i) the COmetary Secondary Ion Mass Analyzer (COSIMA; Kissel et al 2007) collected dust particles of 10–100 μm size on 1 cm targets, imaged them with a microscope operating under grazing incidence illumination with a resolution of about 14 μm, and analyzed them
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