H. Höfner

Comet 67P/Churyumov–Gerasimenko: Close-up on Dust Particle Fragments

The COmetary Secondary Ion Mass Analyser instrument on board ESAs Rosetta mission has collected dust particles in the coma of comet 67P/Churyumov-Gerasimenko. During the early-orbit phase of the Rosetta mission, particles and particle agglomerates have been imaged and analyzed in the inner coma at distances between 100 km and 10 km off the cometary nucleus and at more than 3 AU from the Sun. We identified 585 particles of more than 14 μm in size. The particles are collected at low impact speeds and constitute a sample of the dust particles in the inner coma impacting and fragmenting on the targets. The sizes of the particles range from 14 μm up to sub-millimeter sizes and the differential dust flux size distribution is fitted with a power law exponent of -3.1. After impact, the larger particles tend to stick together, spread out or consist of single or a group of clumps, and the flocculent morphology of the fragmented particles is revealed. The elemental composition of the dust particles is heterogeneous and the particles could contain typical silicates like olivine and pyroxenes, as well as iron sulfides. The sodium to iron elemental ratio is enriched with regard to abundances in CI carbonaceous chondrites by a factor from ˜1.5 to ˜15. No clear evidence for organic matter has been identified. The composition and morphology of the collected dust particles appear to be similar to that of interplanetary dust particles.

High-molecular-weight organic matter in the particles of comet 67P/Churyumov–Gerasimenko

The presence of solid carbonaceous matter in cometary dust was established by the detection of elements such as carbon, hydrogen, oxygen and nitrogen in particles from comet 1P/Halley. Such matter is generally thought to have originated in the interstellar medium, but it might have formed in the solar nebula—the cloud of gas and dust that was left over after the Sun formed. This solid carbonaceous material cannot be observed from Earth, so it has eluded unambiguous characterization. Many gaseous organic molecules, however, have been observed; they come mostly from the sublimation of ices at the surface or in the subsurface of cometary nuclei. These ices could have been formed from material inherited from the interstellar medium that suffered little processing in the solar nebula. Here we report the in situ detection of solid organic matter in the dust particles emitted by comet 67P/Churyumov–Gerasimenko; the carbon in this organic material is bound in very large macromolecular compounds, analogous to the insoluble organic matter found in the carbonaceous chondrite meteorites. The organic matter in meteorites might have formed in the interstellar medium and/or the solar nebula, but was almost certainly modified in the meteorites’ parent bodies. We conclude that the observed cometary carbonaceous solid matter could have the same origin as the meteoritic insoluble organic matter, but suffered less modification before and/or after being incorporated into the comet.

Externally excited planar dust acoustic shock waves in a strongly coupled dusty plasma under microgravity conditions

The formation and dissipation of an externally excited planar dust acoustic shock wave in a three-dimensional uniform dust cloud has been observed under microgravity conditions. The experiment has been performed in the dc gas discharge chamber ?Plasma Kristall-4? (Fortov et al 2005 Plasma Phys. Control. Fusion 47 B537) on board the A300 Zero-G airplane. The shock Mach number and compression factor reached 3.5 and 6, correspondingly, with a shock width of about the interparticle distance. Due to the utilization of the polarity-switching dc discharge mode and application of the Rankine?Hugoniot relations, the dust particle electrostatic pressure was determined and the Hugoniot percussive adiabat for the dust subsystem was derived. The obtained data were simulated using thermodynamic properties of highly nonideal Debye?H?ckel (Yukawa) systems. Comparison of the experimental and simulated data has demonstrated that the screening length in a dense dusty plasma is not determined by the total ion number density, but rather by those ?effective? ions which are not bounded by negatively charged dust grains. Thus, this work presents a new experimental approach for the investigation of the dense dusty plasma clouds.

Determination of the ion-drag force in a complex plasma

The ion-drag force acting on dust particles in the positive column of a dc discharge is measured in the pressure range of 20-120 Pa. The force is obtained by a method which does not require a priori knowledge of the particle charge, but uses the charge gradient determined from the same experiment. The method depends only on two experimentally determined quantities: the particle drift velocity and the electric field. The comparison of experimental results with theoretical models is presented and discussed.

PK-4: Complex Plasmas in Space—The Next Generation

PK-4 is an experiment designed to investigate complex plasmas in a combined dc/RF discharge under microgravity conditions on board of the International Space Station. The dc discharge is produced in a glass tube with a length of 35 cm and a diameter of 3 cm. In addition, an RF discharge can be applied by external RF coils. The setup is especially suited for studying the liquid phase of the complex plasmas, e.g., flow phenomena such as turbulence or nozzles, and forces acting on the microparticles. Experiments in the laboratory and in parabolic flights have been used to determine the charge of the microparticles as well as the ion drag force acting on them

Dissipative dark solitons in a dc complex plasma

The observation of dark solitons in a three-dimensional dc complex plasma is presented. The experiments are performed using neon gas at a pressure of 18 Pa and melamine-formaldehyde particles with a diameter of 3.43 μm. The waves are excited by a short pulse produced by the circular electrical-manipulative electrode built in inside the discharge glass tube. The wave speed is measured to be of the order of 15 mm/s. The propagation time of the observed dark soliton is approximately 10–15 times longer than the damping time.

Parabolic flight experiments with PK-4

PK-4 is an experiment designed to investigate complex plasmas (low-temperature plasmas containing microparticles, e.g. dust grains) in a combined dc/rf discharge under microgravity conditions on board of the International Space Station. Within the 35th and 36th ESA parabolic flight campaigns first experiments under microgravity conditions in a specially designed experiment set-up have been performed. The particle flow inside the tube, the appearance of dust waves, and lane formation in interpenetrating particle clouds have been observed.

Mechanical and electrostatic experiments with dust particles collected in the inner coma of comet 67P by COSIMA onboard Rosetta

The in situ cometary dust particle instrument COSIMA (COmetary Secondary Ion Mass Analyser) onboard ESAs Rosetta mission has collected about 31 000 dust particles in the inner coma of comet 67P/Churyumov–Gerasimenko since August 2014. The particles are identified by optical microscope imaging and analysed by time-of-flight secondary ion mass spectrometry. After dust particle collection by low speed impact on metal targets, the collected particle morphology points towards four families of cometary dust particles. COSIMA is an in situ laboratory that operates remotely controlled next to the comet nucleus. The particles can be further manipulated within the instrument by mechanical and electrostatic means after their collection by impact. The particles are stored above 0°C in the instrument and the experiments are carried out on the refractory, ice-free matter of the captured cometary dust particles. An interesting particle morphology class, the compact particles, is not fragmented on impact. One of these particles was mechanically pressed and thereby crushed into large fragments. The particles are good electrical insulators and transform into rubble pile agglomerates by the application of an energetic indium ion beam during the secondary ion mass spectrometry analysis. This article is part of the themed issue ‘Cometary science after Rosetta’.

Autowaves in a dc complex plasma confined behind a de Laval nozzle

Experiments to explore stability conditions and topology of a dense microparticle cloud supported against gravity by a gas flow were carried out. By using a nozzle-shaped glass insert within the glass tube of a dc discharge plasma chamber a weakly ionized gas flow through a de Laval nozzle was produced. The experiments were performed using neon gas at a pressure of 100?Pa and melamine-formaldehyde particles with a diameter of 3.43??m. The capturing and stable global confining of the particles behind the nozzle in the plasma were demonstrated. The particles inside the cloud behaved as a single convection cell inhomogeneously structured along the nozzle axis in a tube-like manner. The pulsed acceleration localized in the very head of the cloud mediated by collective plasma-particle interactions and the resulting wave pattern were studied in detail.

Cooperative phenomena in laminar fluids: observation of streamlines

Complex plasmas are an ideal model system to investigate laminar fluids as they allow to study fluids at the kinetic level. At this level we are able to identify streamlines particle by particle. This gives us the ability to research the behaviour of these streamlines as well as the behaviour of each individual particle of the streamline.We carried out our experiments in a modified GEC‐RF‐Reference cell. We trapped the particles within two glass rings and forced them to form a circular flow by using several stripe electrodes. In this flow the particles behave like an ideal fluid and form streamlines. By putting an obstacle into the flow we reduce the cross‐section. To pass through this constricted cross‐section some streamlines have to reconnect. After the obstacle the streamlines split up again. An analysis how streamlines split up and reconnect as result of external pressure on the fluid in our system is presented here.Streamlines also occur if two clouds of particles penetrate each other. We call this ...