Micron-sized Dust Particles Research Articles

The cosmic dust aggregation experiment CODAG

For the simulation of the first stage of preplanetary dust aggregation, we developed the cosmic dust aggregation experiment (CODAG). With CODAG, we intend to study the aggregational behaviour of a cloud of micron-sized dust particles due to Brownian motion of the grains. For a realistic simulation of the processes in the young solar system, the dust grains have to be dispersed in a rarefied gas so that mutual collisions are ballistic. Fast sedimentation of the grains in the Earth's gravitational field leads to unrealistic collision velocities and to a rapid loss of particles to the container walls. Therefore, CODAG was designed to work in a microgravity environment. In this paper, we present an overview of the experimental design of CODAG which was recently flown in a Get Away Special container during the STS-95 mission.

Micron‐sized dust particles detected in the outer solar system by the Voyager 1 and 2 plasma wave instruments

During the Voyager 1 and 2 flybys of the outer planets it has been demonstrated that the plasma wave instrument can detect small dust particles striking the spacecraft. In this paper, we examine the Voyager plasma wave data for dust impacts in the interplanetary medium at heliocentric radial distances ranging from 6 to 60 astronomical units (AU). The results show that a small but persistent level of dust impacts exists out to at least 30 to 50 AU. The average number density of these particles is about 2 × 10−8 m−3, and the average mass of the impacting particles is believed to be a few times 10−11 g, which corresponds to particle diameters in the micron range. Possible sources of these particles are planets, moons, asteroids, comets, and the interstellar medium. Of these, comets appear to be the most likely source. The number densities are only weakly dependent on ecliptic latitude, which indicates that the particles probably do not originate from planets, moons, or asteroids. Comparisons with interstellar dust fluxes measured in the inner regions of the solar system by the Ulysses spacecraft indicate that the particles are not of interstellar origin.

Dust measurements in the Jovian magnetosphere

Dust measurements have been obtained with the dust detector onboard the Galileo spacecraft inside a distance of about 60RJ from Jupiter (Jupiter radius, RJ = 71,492 km) during two periods of about 8 days around Galileo's closest approaches to Ganymede on 27 June and on 6 Sept 1996. The impact rate of submicrometer‐sized particles fluctuated by a factor of several hundred with a period of about 10 hours, implying that their trajectories are strongly affected by the interaction with the Jovian magnetic field. Concentrations of small dust impacts were detected at the times of Ganymede closest approaches that could be secondary ejecta particles generated upon impact of other particles onto Ganymede's surface. Micrometer‐sized dust particles, which could be on bound orbits about Jupiter, are concentrated in the inner Jovian system inside about 20RJ from Jupiter.

Generating a jet of deagglomerated small particles in vacuum

We describe a dust jet generator, which is a device for the deagglomeration of micron-sized and submicron-sized dust particles. In contrast to other mechanisms, it principally works without any gas and generates a dust jet with a speed up to 100 m/s relative to the surrounding gas. Compressed dust powder is fed onto the front side of a fast rotating cogwheel. Upon contact with the cogwheel, the dust is deagglomerated and the single particles are accelerated at a tangent to the circumference. Design details of the mechanism and characteristics of the dust jet are presented. We will use the dust jet for the investigation of collisions of individual particles with surfaces or other particles. Other possible applications of the dust jet generator are proposed.

Crystalline structures of strongly coupled dusty plasmas in dc glow discharge strata

Strongly coupled dusty plasmas are formed by suspending micron-sized dust particles in strata of a dc glow neon discharge. We have observed for the first time an ordered structure of the negatively charged particles trapped in the strata region. Image analysis reveals the crystalline structure, which is consistent with a large value of the Coulomb coupling parameter.

Experiments on pre-planetary grain growth

In the solar nebula, the formation of planetesimals and cometesimals is believed to be due to inelastic collisions between dust grains and due to van der Waals and other attractive surface forces. The low velocity collisions result from Brownian motion for the micron-sized pre-planetary dust particles, from differential sedimentation and gas drag-induced orbital decay for mm-sized dust aggregates, and from decoupling from gas turbulence for larger bodies. Modelling these processes is extremely difficult as there exists no theoretical description on the outcome of multiple aggregate-aggregate collisions including physical processes as coagulation, restructuring, compaction, and fragmentation. To investigate these problems from the experimental side, we developed several simulation experiments with analogous micron-sized particles. One set of experiments investigates individual grain-target collisions to determine the collisional and sticking properties of small grains. Another type of experiments deals with the production of fractal dust aggregates which are then used to observe single and multiple collisional events.

Dust colors, dust release rates, and dust-to-gas ratios in the comae of six comets

In an attempt to find correlations between properties of comets, we determine the dust colors, dust release rates, and dust-to-gas mass ratios in the comae of Comets Bowell (1980 b = 1982 I), Bradfield (1979 l = 1979 X), Brorsen-Metcalf (1989 o = 1989 X), Giacobini-Zinner (1984 e = 1985 XIII), Levy (1990 c = 1990 XX), and Stephan-Oterma (1980 g = 1980 X) using a uniform and systematic analysis of the continuum fluxes measured at optical wavelengths. We assume that the continuum fluxes observed in these comets are from light scattered by micrometer-sized dust particles. The color of the dust is blue, relative to the Sun, in Comets Bradfield (1979 X) and Brorsen-Metcalf (1989 X); both comets have a low dust-to-gas mass ratio (). Comets Giacobini-Zinner (1985 XIII) and Stephan-Oterma (1980 X) have an intermediate dust-to-gas mass ratio (), whereas Comets Levy (1990 XX) and Bowell (1982 I) have a high dust-to-gas mass ratio (). The continuum flux from dust particles in comets with intermediate to high dust-to-gas mass ratios is in general red, relative to the Sun. However, the color changes to blue in these comets when the dust-to-gas mass ratios are low relative to their peak values.

CHARGED DUST DYNAMICS IN THE SOLAR SYSTEM

▪ Abstract In most space environments, dust particles are exposed to plasmas and UV radiation and, consequently, carry electrostatic charges. Their motion is influenced by electric and magnetic fields in addition to gravity, drag, and radiation pressure. On the surface of the Moon, in planetary rings, or at comets, for example, electromagnetic forces can shape the spatial and size distribution of micron-sized charged dust particles. The dynamics of small charged dust particles can be surprisingly complex, leading to levitation, rapid transport, energization and ejection, capture, and the formation of new planetary rings.This review briefly discusses the most important processes that determine the charge state of dust particles immersed in plasmas and the resulting dynamics on exposed dusty surfaces and in planetary magnetospheres.

Emission properties and structural ordering of strongly coupled dust particles in a thermal plasma

Micron-sized dust particles are found to form a Coulomb liquid-like structure in a bulk of a thermal plasma under atmospheric pressure and a temperature of 1700 K. The particles are charged positively and have about 10 3 electron charges. The work function of the electron is found to be 2.1 eV.

The electrical charging of micron-sized dust particles in a capacitively coupled RF plasma

The electrical charge of micron-sized particles in a capacitively coupled RF discharge is determined using a relatively simple theoretical model. The specific properties of the RF sheath are taken into account. The results are compared to charge measurements of particles which have been injected into the active discharge.

Micron‐sized particles detected in the vicinity of Jupiter by the Voyager plasma wave instruments

Wideband waveform data obtained by the plasma wave instruments onboard the Voyager 1 and 2 spacecraft have been used to study micron‐sized dust particles in the vicinity of Jupiter. The technique used was developed during the fly by s of Saturn, Uranus, and Neptune, and makes use of the fact that a particle striking the spacecraft at 10–20 km/s causes a voltage pulse in the plasma wave receiver. The waveform of the voltage pulse is much different than the waveform of plasma waves and provides a highly reliable method of detecting micron‐sized dust particles. Although the dust impact rate observed in the vicinity of Jupiter is much lower than the rates at Saturn, Uranus, and Neptune, the particles are easily detectable. Approximately 1200 48‐second frames of wideband waveform data were examined in the vicinity of Jupiter. Dust impact signatures were found in approximately 20% of these frames. The peak impact rates are about 1 impact per second, and the peak number densities are about 10−5 m−3. Most of the impacts occurred near the equatorial plane at radial distances less than about 35 RJ from Jupiter. Analysis of the detection threshold indicates that the particles have masses greater than 10−11 g, which corresponds to particles with diameters of a few micrometers or larger.

An Asteroidal Dust Ring of Micron-Sized Particles Trapped in the 1:1 Mean Motion Resonance with Jupiter

We have studied the orbital evolution of micrometer-sized asteroidal dust particles under the influence of planetary gravitational perturbations, radiation pressure, Poynting-Robertson (PR) drag, and solar wind drag. It is found that a significant fraction of the 2-μm diameter particles, with β = 0.26, are thrown into the 1:1 mean motion resonance zone with Jupiter when they are released from their parent bodies. These particles stay trapped in this resonance for thousands of years and eventually get out of the resonance due to close encounters with Jupiter. Because of ongoing collisions in the asteroid belt, there is a continuous supply of dust particles around this size from the asteroidal belt contributed to the 1:1 resonance region. Therefore, there must exist a dust ring of asteroidal particles around 2 μm in diameter locked in the 1:1 resonance with Jupiter. We also discuss the structure of the ring, the dynamics of how particles are trapped, how they evolve, and how they get out of the resonance.

Dust in the planetary system

A continuous spectrum of particulates ranging from m-sized objects down to molecular sizes is found almost everywhere in the planetary system: in interplanetary space and surrounding planets. Remote observations and in situ analysis provide information on their abundance and properties. Zodiacal dust particles displayed their low eccentricity, low inclination orbits most beautifully in the recent Galileo data. Interstellar particles entering the solar system have been found recently by the Ulysses dust detector. Dust streams have been detected in the outer solar system, with Jupiter as their potential origin. Rings were observed around all giant planets by the Voyager spacecraft. Micron sized dust particles are constituents of most rings. Because of various dynamic effects acting on small particles dust rings are short-lived phenomena or have to be continuously replenished. Descriptions of the ring systems are given and the significance of dust is discussed.

Lorentz and gravitational resonances on circumplanetary particles

Micron-sized circumplanetary dust particles are subject to various non-gravitational perturbations, principally solar radiation pressure and electromagnetic forces, which are typically a few percent as strong as the planetary gravity. Individually, these perturbations can cause some orbital evolution, but when the perturbations act in concert the excursions can be much larger. We demonstrate this effect for a single example, the coupling between resonances and drag forces. Throughout this work, we emphasize the parallels between satellite-satellite gravitational resonances and their electromagnetic counterparts (Lorentz resonances).

Discovery of Jovian dust streams and interstellar grains by the Ulysses spacecraft

Within 1 AU from Jupiter, the dust detector aboard the Ulysses spacecraft during the flyby on February 8, 1992 recorded periodic bursts of submicron dust particles with durations ranging from several hours to two days and occurring at about monthly intervals. These particles arrived at Ulysses in collimate streams radiating from close to the line-of-sight direction to Jupiter, suggesting a Jovian origin for the periodic bursts. Ulysses also detected a flux of micron-sized dust particles moving in high-velocity retrograde orbits. These grains are identified here as being of interstellar origin.

Plasma wave observations at Neptune

During the Voyager 2 flyby of Neptune, the plasma wave instrument detected many familiar phenomena. These include radio emissions, electron plasma oscillations in the solar wind upstream of the bow shock, electrostatic turbulence at the bow shock, electrostatic electron cyclotron waves and upper hybrid resonance (UHR) waves, whistler mode noise, and dust impacts. The radio emissions occur in a broad continuum-like spectrum extending from about 5 kHz to above 50 kHz, and are emitted in a disk-like beam along the magnetic equatorial plane. The radio emissions are believed to be generated by mode conversion from UHR waves at the magnetic equator. The inner magnetosphere has relatively low plasma wave intensities, generally less than 100 μV/m. At the ring plane crossing, many small micron-sized dust particles were detected striking the spacecraft. The maximum impact rate was about 280 impacts per second at the inbound ring plane crossing, and about 110 impacts per second at the outbound ring plane crossing. Most of the particles are concentrated in a dense disk, about one thousand km thick, near the equatorial plane. A broader, more tenuous distribution of dust also extends along the entire trajectory inside of 6 R N including the northern polar region.

Electrically charged filter materials

Filters made from ordinary textile fibres are too coarse to remove micrometre-sized dust particles, which are responsible for much respiratory disease. If, however, an electric charge is applied to the fibres, a filter can provide very good respiratory protection. Fibres can be charged by triboelectric exchange, by corona or by induction; and charge levels can give rise to electric fields in the interstices of the filter, approaching the breakdown field of the air. Some of the effect of the charge is lost as the filters become loaded with dust, but charge stability during storage is high, with shelf lives of years being attainable.

Newly developed techniques for the analysis of micrometer-sized interplanetary dust particles and comet grains

Electron transparent thin sections (30–100 nm thick) of interplanetary dust particles and other fine-grained meteoritic materials are produced using an ultramicrotome equipped with a diamond knife. An analytical electron microscope (AEM) is imployed to examine indigenous physical properties (e.g. porosity), mineralogy, and petrography. Large data sets of quantitative point count analyses obtained from thin sections enable direct mineralogical comparison of IDPs and Halley.

A numerical model of the Uranian dust rings

The tenuous dust rings of Uranus ( τ ≈ 10 −5) discovered by Voyager 2 are not readily associated with any of the other observed components of the Uranus ring system. Their short lifetime due to exospheric drag and other disruptive processes requires continuous production of the dust particles. Our numerical simulations of the Uranus rings show that the dust rings could be the visible component of low optical depth moonlet belts. These belts would be made up of objects from 10 to 10 5 cm which serve as sources of the dust via micrometeoroid bombardment and collisional release of ejecta. Dust is created within the main rings of Uranus through the same processes. The dust bands cannot be the transient remnants of large impacts onto unseen moons because the impacts occur too infrequently since the dust band lifetimes are very short (≤100 years). We propose that the dust bands are the result of continuous ejection and reabsorption of micron-sized regolith material within hypothesized moonlet belts ( τ MB ≤ 10 −3). Some of the ejecta is removed from the moonlet belt giving rise to a continuous sheet of micron-sized dust particles. Numerical simulations of such a system in a Markov chain formalism reproduce the observed characteristics of the Uranian dust rings. The broad sheet of dust which extends approximately from the δ ring to Cordelia could be dust ejected from Cordelia by continuous meteoroid erosion. This dust would then radially evolve until accreting onto δ ring particles. Our simulations show that the surface of Cordelia would have to be relatively hard, or regolith-free, for enough dust to be excavated to account for the observed optical depth. In addition, we would expect a similar sheet between Ophelia and the ε ring, and none is observed. We therefore propose that this sheet is also the visible component of one or more particularly broad moonlet belts, with Cordelia the largest member of the moonlet belt distribution.

First Plasma Wave Observations at Neptune

The Voyager 2 plasma wave instrument detected many familiar plasma waves during the encounter with Neptune, including electron plasma oscillations in the solar wind upstream of the bow shock, electrostatic turbulence at the bow shock, and chorus, hiss, electron cyclotron waves, and upper hybrid resonance waves in the inner magnetosphere. Low-frequency radio emissions, believed to be generated by mode conversion from the upper hybrid resonance emissions, were also observed propagating outward in a disklike beam along the magnetic equatorial plane. At the two ring plane crossings many small micrometer-sized dust particles were detected striking the spacecraft. The maximum impact rates were about 280 impacts per second at the inbound ring plane crossing, and about 110 impacts per second at the outbound ring plane crossing. Most of the particles are concentrated in a dense disk, about 1000 kilometers thick, centered on the equatorial plane. However, a broader, more tenuous distribution also extends many tens of thousands of kilometers from the equatorial plane, including over the northern polar region.