This project created interesting insights into the properties of comets at the micro- and nanometer scale, and shed light on the dust evolution in our early Solar System. It furthered our understanding how small dust particles came together to form larger agglomerates, and ultimately comets and planets.

General structure of micrometre-sized cometary dust
(Bentley et al. 2016, https://doi.org/10.1038/nature19091)

The publication of the morphological properties of dust particles of comet 67P at the micrometre scale in Nature created a landmark for the description of micrometre-sized cometary dust particles. Most of the detected particles are over ten micrometres in size and disaggregate during scanning into smaller, a few micrometre-sized fragments. Individually collected small particles which resist fragmentation upon scanning are scarce, but mirror the agglomerate-like surface structure of the large particles. Dust at comet 67P consists of agglomerates that are suggested to be hierarchical on a scale probably covering at least hundreds of nanometres to several micrometres. The shape of the subunits is similar on all scales and can be described as bulbous.

Qualitative comparisons with the results based on IDP research, Stardust and earlier cometary mission data analysis, or remote observations show good agreement: cometary dust particles are, at least to a large part, agglomerates. They have a hierarchical structure based on bulbous subunits whose smallest grain sizes lie in the nanometre range. This conception about the structure of cometary dust particles can now be used, e.g., for (re-)interpretation of polarimetric observations, input for theories about the early Solar System, or physical models of cometary nuclei in general and comet 67P in particular.

Fractal cometary dust
(Mannel et al. 2016,https://doi.org/10.1093/mnras/stw2898)

The most exceptional particle collected by MIDAS is particle E. It is an extremely porous agglomerate of approximately 1.5 µm sized subunits. On the MIDAS target it forms a monolayer in most places, allowing to locate the majority of subunits since only few of them are hidden by overlaying subunits. A sophisticated structural analysis of the particle revealed that it might be the residue of a deposited particle with fractal structure of dimension Df = 1.7 ± 0.1. In good agreement with this result, the GIADA instrument also infers the existence of a whole population of fractal particles with a fractal dimension of Df ≈ 1.8. It is conceivable that comets, or at least comet 67P, do not only contain the already suggested agglomerate and solid particles, but also an until now undiscovered population of rather fragile particles with a fractal dimension less than two.

The co-existence of the fractal particle with the compact particles and the notably similar subunit sizes of the two particle types suggests that all particles were built from a general pool of subunits, starting with a fractal structure, and undergoing a subsequent process that compacted the majority of particles. This tracking of the particle structure in the protoplanetary disc is a great input for Solar System evolution theories and will greatly help further understanding our early Solar System. In particular, the survival of the fragile population necessitates a gentle comet formation process. Blum et al. 2017 (https://doi.org/10.1093/mnras/stx2741, co-authored by M.S. Bentley and T. Mannel) and Fulle and Blum 2017 note that the only formation scenario consistent with Rosetta findings is a gentle gravitational collapse of a bound clump of mostly compacted dust particles called pebbles. It is concluded that the fractal particles must be stored in the pores between the pebbles which relates the maximal fractal particle size to the pebble size. The emerging image is that of a comet formed in a streaming instability of mm- to cm-sized pebbles (see Blum et al. 2017, https://doi.org/10.1093/mnras/stx2741).

The discovery of fractal cometary dust fuelled a chain of papers about the (im-)possibility of fractal cometary dust particles and their implications. E.g.:

  • J. Blum et al. 2017 (https://doi.org/10.1093/mnras/stx2741, co-authored by M.S. Bentley and T. Mannel) used them as cornerstone for a new, Rosetta result based model how comets formed in the early solar system.
  • D. Bockelée-Morvan et al. 2017, https://doi.org/10.1093/mnras/sty533 calculated the properties of fractal dust particles in Virthis-H observations.
  • L. Ellerbroek et al. 2017 (https://doi.org/10.1093/mnras/stx1257, co-authored by M.S. Bentley and T. Mannel) examined particle deposition on MIDAS targets in the laboratory to investigate the possibility of collecting a false fractal particle
  • M. Fulle et al. 2017 (https://doi.org/10.1093/mnras/stx971) use fractal dust particles to constrain the collisional history of comets.

Cometary dust classification and smallest building blocks

The final classification of the dust particles, as well as the identification of their smallest building blocks was done in T. Mannel et al. 2019 (https://doi.org/10.1051/0004-6361/201834851).

Three morphological classes can be determined:

  • fragile agglomerate particles of sizes larger than about 10 µm comprised of micrometer-sized subunits that may themselves be aggregates and show a moderate packing density on the surface of the particles.
  • (ii) A fragile agglomerate with a size of about a few tens of micrometers comprised of micrometer-sized subunits that are suggested to be aggregates themselves and are arranged in a structure with a fractal dimension lower than two.
  • (iii) Small micrometer-sized particles comprised of subunits in the size range of hundreds of nanometers that show surface features that are again suggested to represent subunits.

The smallest subunits of cometary dust particles are closely investigated. They are spherical in shape and their differential size distributions follow a log-normal distribution with means of about 100 nm and standard deviations between 20 and 35 nm. The arrangement, appearance, and size distribution of the smallest determined surface features are reminiscent of those found in chondritic porous interplanetary dust particles.

Further results of this project are