Flattened loose particles from numerical simulations compared to particles collected by Rosetta

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 dimensions Df. 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 initial Df (a family of compact aggregates with Df close to 2.5–3 and some fluffier aggregates with Df ≈ 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.