The morphology of cometary dust: Subunit size distributions down to tens of nanometres

International audienceThe Rosetta orbiter carried a dedicated analysis suite for cometary dust. One of the key instruments was MIDAS (Micro-Imaging Dust Analysis System), an atomic force microscope that scanned the surfaces of hundreds of (sub-)micrometre particles in 3D with resolutions down to nanometres. This provided the opportunity to study the morphology of the smallest cometary dust; initial investigation revealed that the particles are agglomerates of smaller subunits [1] with different structural properties [2]. To understand the (surface-) structure of the dust particles and the origin of their smallest building blocks, a number of particles were investigated in detail and the size distribution of their subunits determined [3].Here we discuss the subunit size distributions ranging from tens of nanometres to a few micrometres. The differences between the subunit size distributions for particles collected pre-perihelion, close to perihelion, and during a huge outburst are examined, as well as the dependence of subunit size on particle size. A case where a particle was fragmented in consecutive scans allows a direct comparison of fragment and subunit size distributions. Finally, the small end of the subunit size distribution is investigated: the smallest determined sizes will be reviewed in the context of other cometary missions, interplanetary dust particles believed to originate from comets, and remote observations. It will be discussed if the smallest subunits can be interpreted as fundamental building blocks of our early Solar System and if their origin was in our protoplanetary disc or the interstellar material

How MIDAS improved our understanding of micrometre- sized cometary dust

International audienceThe MIDAS atomic force microscope on the Rosetta orbiter was an instrument developed to investigate, for the first time, the morphology of nearly unaltered cometary dust. It acquired the 3D topography of about 1-50 µm sized dust particles with resolutions down to a few nanometres. These images showed the agglomerate character of the dust and confirmed that the smallest subunit sizes were less than 100 nm. MIDAS acquired the first direct proof of a fractal dust particle, opening a new approach to investigate the history of our early Solar System and of comets

Fractal cometary dust – a window into the early Solar System

The ESLAB 50 Symposium - spacecraft at comets from 1P/Halley to 67P/Churyumov-GerasimenkoInternational audienceThe properties of dust in the protoplanetary disk are key to understanding the formation of planets in our Solar System. Many models of dust growth predict the development of fractal structures that evolve into non-fractal, porous dust pebbles representing the main component for planetesimal accretion. In order to understand comets and their origins, the Rosetta orbiter followed comet 67P/Churyumov-Gerasimenko for over two years and carried a dedicated instrument suite for dust analysis. One of these instruments, the MIDAS atomic force microscope, recorded the 3D topography of micro- to nanometre sized dust. All particles analysed to date have been found to be hierarchical agglomerates. Most show compact packing, however, one is extremely porous. This paper contains a structural description of a compact aggregate and the outstanding porous one. Both particles are tens of micrometres in size and show rather narrow subunit size distributions with noticeably similar mean values of 1.48+0.13−0.59 μm for the porous particle and 1.36+0.15−0.59 μm for the compact. ompact. The porous particle allows a fractal analysis, where a density-density correlation function yields a fractal dimension of Df = 1.70 ± 0.1. GIADA, another dust analysis instrument on-board Rosetta, confirms the existence of a dust population with a similar fractal dimension. The fractal particles are interpreted as pristine agglomerates built in the protoplanetary disk and preserved in the comet. The similar subunits of both fractal and compact dust indicate a common origin which is, given the properties of the fractal, dominated by slow agglomeration of equally sized aggregates known as cluster-cluster agglomeration

How MIDAS improved our understanding of micrometre- sized cometary dust

International audienceThe MIDAS atomic force microscope on the Rosetta orbiter was an instrument developed to investigate, for the first time, the morphology of nearly unaltered cometary dust. It acquired the 3D topography of about 1-50 µm sized dust particles with resolutions down to a few nanometres. These images showed the agglomerate character of the dust and confirmed that the smallest subunit sizes were less than 100 nm. MIDAS acquired the first direct proof of a fractal dust particle, opening a new approach to investigate the history of our early Solar System and of comets

Cometary dust: structure at the nanometre scale

International audienceThe Rosetta orbiter carried three dedicated dust analysis instruments to investigate the properties of the comet andits dust at smallest scales. The images with the highest resolutions were obtained by the MIDAS (Micro-ImagingDust Analysis System) atomic force microscope [1,2]. It collected dust particles of one to tens of micrometresin size and imaged their surface in 3D. Nominal images had approximately hundreds of nanometres resolutionand were used to study the particle morphology at the micrometre scale. It was shown that the majority ofcollected particles were fragile agglomerates [3] with a moderate packing density of subunits at the surface [4].Exceptions were one extremely porous particle with a fractal structure that is suggested to be pristinely preservedfrom early agglomeration processes in our Solar System [4], and the particles of about one micrometre size thatshow less fragility. To study these smallest detected particles a special scanning mode, called ‘reverse imagingmode‘, was developed that reached resolutions down to eight nanometres [5]. In the reverse imaging mode dustparticles were picked up with the tip and imaged with the help of a sharp spike on a calibration sample. Theresulting images opened the possibility to identify the agglomerate structure of the dust down to the nanometrescale and to determine smallest features with mean sizes of about 100 nanometres. Whether these smallestfeatures are surface related or true subunits comprising the dust will be discussed on the basis of comparisons tosmallest subunit sizes identified by indirect Rosetta measurements and by investigations of other cometary material.References:[1] W. Riedler, K. Torkar, H. Jeszenszky, et al., MIDAS – The Micro-Imaging Dust Analysis System for theRosetta Mission. Space Science Reviews 128, 2007.[2] M.S. Bentley, H. Arends, B. Butler, et al., MIDAS: Lessons learned from the first spaceborne atomic forcemicroscope. Acta Astronautica 125, 2016.[3] M.S. Bentley, R. Schmied, T. Mannel et al., Aggregate dust particles at comet 67P/Chruyumov-Gerasimenko,Nature, 537, 2016.[4] T. Mannel, M.S. Bentley, R. Schmied et al., Fractal cometary dust – a window into the early Solar system,MNRAS, 462, 2016.[5] T. Mannel, M.S. Bentley, P.D. Boakes, et al., MIDAS results: classification and extension to the nanometrescale, submitted to Astronomy & Astrophysics, Rosetta special issue, 2018

Cometary dust: structure at the nanometre scale

International audienceThe Rosetta orbiter carried three dedicated dust analysis instruments to investigate the properties of the comet andits dust at smallest scales. The images with the highest resolutions were obtained by the MIDAS (Micro-ImagingDust Analysis System) atomic force microscope [1,2]. It collected dust particles of one to tens of micrometresin size and imaged their surface in 3D. Nominal images had approximately hundreds of nanometres resolutionand were used to study the particle morphology at the micrometre scale. It was shown that the majority ofcollected particles were fragile agglomerates [3] with a moderate packing density of subunits at the surface [4].Exceptions were one extremely porous particle with a fractal structure that is suggested to be pristinely preservedfrom early agglomeration processes in our Solar System [4], and the particles of about one micrometre size thatshow less fragility. To study these smallest detected particles a special scanning mode, called ‘reverse imagingmode‘, was developed that reached resolutions down to eight nanometres [5]. In the reverse imaging mode dustparticles were picked up with the tip and imaged with the help of a sharp spike on a calibration sample. Theresulting images opened the possibility to identify the agglomerate structure of the dust down to the nanometrescale and to determine smallest features with mean sizes of about 100 nanometres. Whether these smallestfeatures are surface related or true subunits comprising the dust will be discussed on the basis of comparisons tosmallest subunit sizes identified by indirect Rosetta measurements and by investigations of other cometary material.References:[1] W. Riedler, K. Torkar, H. Jeszenszky, et al., MIDAS – The Micro-Imaging Dust Analysis System for theRosetta Mission. Space Science Reviews 128, 2007.[2] M.S. Bentley, H. Arends, B. Butler, et al., MIDAS: Lessons learned from the first spaceborne atomic forcemicroscope. Acta Astronautica 125, 2016.[3] M.S. Bentley, R. Schmied, T. Mannel et al., Aggregate dust particles at comet 67P/Chruyumov-Gerasimenko,Nature, 537, 2016.[4] T. Mannel, M.S. Bentley, R. Schmied et al., Fractal cometary dust – a window into the early Solar system,MNRAS, 462, 2016.[5] T. Mannel, M.S. Bentley, P.D. Boakes, et al., MIDAS results: classification and extension to the nanometrescale, submitted to Astronomy & Astrophysics, Rosetta special issue, 2018