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Cosmic Dust is composed of particles in space.

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Cosmic dust is composed of particles in space which are a few molecules to 0.1 mm in size. Cosmic dust can be further distinguished by its astronomical location; for example: intergalactic dust, interstellar dust, circumplanetary dust, dust clouds around other stars, and the major interplanetary dust components to our own zodiacal dust complex (seen in visible light as the zodiacal light): Comet dust, asteroidal dust plus some of the less signficant contributors: Kuiper belt dust, interstellar dust passing through our solar system, and beta-meteoroids.

Cosmic dust..
Porous chondrite interplanetary dust particle. Courtesy of E.K. Jessberger, Institut fÜr Planetologie, MÜnster, Germany, and Don Brownlee, University of Washington, Seattle, under a cc-a-2.5 license.

Cosmic dust was once solely an annoyance to astronomers, as it obscures objects they wish to observe. When infrared astronomy began, those so-called annoying dust particles were observed to be significant and vital components of astrophysical processes.

For example, the dust can drive the mass loss when a star is nearing the end of its life, play a part in the early stages of star formation, and form planets. In our own Solar System, dust plays a major role in the zodiacal light, Saturn's B Ring spokes, the outer diffuse planetary rings at Jupiter, Saturn, Uranus and Neptune, the resonant dust ring at the Earth, and comets.

The study of dust is a many-faceted research topic that brings together different scientific fields: physics (solid-state, electromagnetic theory, surface physics, statistical physics, thermal physics), (fractal mathematics), chemistry (chemical reactions on grain surfaces), meteoritics, as well as every branch of astronomy and Astrophysics. These disparate research areas can be linked by the following theme: the cosmic dust particles evolve cyclically; chemically, physically and dynamically. The evolution of dust traces out paths in which the universe recycles material, in processes analogous to the daily recycling steps with which many people are familiar: production, storage, processing, collection, consumption, and discarding. Observations and measurements of cosmic dust in different regions provide an important insight into the universe's recycling processes; in the clouds of the diffuse interstellar medium, in Molecular clouds, in the circumstellar dust of Young stellar objects, and in planetary systems such as our own Solar System, where astronomers consider dust as in its most recycled state. The astronomers accumulate observational 'snapshots’ of dust at different stages of its life and, over time, form a more complete movie of the universe's complicated recycling steps.

The detection of cosmic dust points to another facet of cosmic dust research: dust acting as photons. Once cosmic dust is detected, the scientific problem to be solved is an inverse problem to determine what processes brought that encoded photon-like object (dust) to the detector. Parameters such the particle's initial motion, material properties, intervening plasma and magnetic field determined the dust particle's arrival at the dust detector. Slightly changing any of these parameters can give significantly different dust dynamical behavior. Therefore one can learn about where that object came from, and what is (in) the intervening medium.

Detection methods of Cosmic dust.

Cosmic dust can be detected by indirect methods utilizing the radiative properties of cosmic dust.

Cosmic dust can also be detected directly ('in-situ') using a variety of collection methods and from a variety of collection locations. At the Earth, generally, an average of 40 tons per day of extraterrestrial material falls to the Earth. The Earth-falling dust particles are collected in the Earth's atmosphere using plate collectors under the wings of stratospheric-flying NASA airplanes and collected from surface deposits on the large Earth ice-masses (Antarctica and Greenland / the Arctic) and in deep-sea sediments. Don Brownlee at the University of Washington in Seattle first reliably identified the extraterrestrial nature of collected dust particles in the later 1970s.

In interplanetary space, dust detectors on planetary spacecraft have been built and flown, some are presently flying, and more are presently being built to fly. The large orbital velocities of dust particles in interplanetary space (typically 10-40 km/s) make intact particle capture problematic. Instead, in-situ dust detectors are generally devised to measure parameters associated with the high-velocity impact of dust particles on the instrument, and then derive physical properties of the particles (usually mass and velocity) through laboratory calibration (i.e. impacting accelerated particles with known properties onto a laboratory replica of the dust detector). Over the years dust detectors have measured, among others, the impact light flash, acoustic signal and impact ionisation. Recently the dust instrument on Stardust captured particles intact in low-density aerogel.

Dust detectors in the past flew on the HEOS-2, Helios, Pioneer 10, Pioneer 11, Giotto, and Galileo space missions, on the Earth-orbiting LDEF, Eureca, and Gorid satellites, and some scientists have utilized the Voyager 1,2 spacecraft as giant Langmuir probes to directly sample the cosmic dust. Presently dust detectors are flying on the Ulysses, Cassini, Proba, Rosetta, Stardust, and the New Horizons spacecraft. The collected dust at Earth or collected further in space and returned by sample-return space missions is then analyzed by dust scientists in their respective laboratories all over the world. One large storage facility for cosmic dust exists at the NASA Houston JSC.

Some bulk properties of cosmic dust.

interplanetary dust particle.
Smooth chondrite interplanetary dust particle. Courtesy of E.K. Jessberger, Institut für Planetologie, Münster, Germany, and on Brownlee, University of Washington, Seattle, under a cc-a-2.5 license.

Cosmic dust is made of dust grains and aggregates of dust grains. These particles are irregularly-shaped with porosity ranging from fluffy to compact. The composition, size, and other properties depends on where the dust is found. General diffuse interstellar medium dust, dust grains in dense clouds, planetary rings dust, and circumstellar dust, are all different. For example, grains in dense clouds have acquired a mantle of ice and on average are larger than dust particles in the diffuse interstellar medium. Interplanetary dust particles (IDPs) are generally larger still.

interplanetary dust particles.
Major elements of 200 stratospheric interplanetary dust particles. Courtesy of Thomas Stephan, Institut für Planetologie, Münster, Germany, under a cc-a-2.5 license.

Other specific dust properties:

  • In circumstellar dust, astronomers have found molecular signatures of CO, silicon carbide, amorphous silicate, polycyclic aromatic hydrocarbons, water ice, and polyformaldehyde, among others. (In the diffuse interstellar medium, there is evidence for silicate and carbon grains.).
  • In collected IDPs (asteroidal plus cometary}, the elemental composition is one of the following types, proportionally:
    • chondritic, 60%;.
    • Iron-sulfur-nickel, 30%;.
    • Mafic silicates, which are iron-magnesium-rich silicates, (olivine and pyroxene), 10%.
  • Cometary dust is generally different (with overlap) from asteroidal dust. Asteroidal dust resembles carbonaceous chondritic meteorites, and cometary dust resembles interstellar grains which can include elements, silicates, polycyclic aromatic hydrocarbons, and water ice.

Most of the influx of extraterrestrial matter that falls onto the Earth is dominated by meteoroids with diameters in the range 50 to 500 micrometers, of average density 2.0 g/cm³ (with porosity about 40%).

The densities of most stratospheric-captured IDPs range between 1 and 3 g/cm³, with an average density at about 2.0 g/cm³.

Typical IDPs are fine-grained mixtures of thousands to millions of mineral grains and amorphous components. We can picture an IDP as a "matrix" of material with embedded elements which were formed at different times and places in the Solar nebula and before our solar nebula's formation. Examples of embedded elements in cosmic dust are GEMS, chondrules, and CAIs.

A good argument can be made that, given the gas-to-dust ratio in the interstellar medium, a large fraction of heavy elements (other then hydrogen and helium) must be tied up in dust grains, the assembled elements for the molecules most likely being carbon, nitrogen, oxygen, magnesium, silicon, sulphur, iron, and compounds of these.

Radiative properties of cosmic dust.

A dust particle interacts with electromagnetic radiation in a way that depends on its cross section, the wavelength of the electromagnetic radiation, and on the nature of the grain: its Refractive index, size, etc. The radiation process for an individual grain is called its emissivity, dependent on the grain's efficiency factor. Furthermore, we have to specify whether the emissivity process is extinction, scattering, or absorption. In the radiation emission curves, several important signatures identify the composition of the emitting or absorbing dust particles.

Dust particles can scatter light nonuniformly. Forward-scattered light means that light is redirected slightly by Diffraction off its path from the star/sunlight, and back-scattered light is reflected light.

The scattering and extinction ("dimming") of the radiation gives useful information about the dust grain sizes. For example, if the object(s) in one's data is many times brighter in forward-scattered visible light than in back-scattered visible light, then we know that a significant fraction of the particles are about a micrometer in diameter.

The scattering of light from dust grains in long exposure visible photographs is quite noticeable in reflection nebulas, and gives clues about the individual particle's light-scattering properties. In x-ray wavelengths, many scientists are investigating the scattering of x-rays by interstellar dust, and some have suggested that astronomical x-ray sources would possess diffuse haloes, due to the dust.

Dust grain formation.

The large grains start with the silicate particles forming in the atmospheres of cool stars, and carbon grains in the atmospheres of cool Carbon stars. Stars, which have evolved off the main sequence, and which have entered the giant phase of their evolution, are a major source of dust grains in galaxies.

Astronomers know that the dust is formed in the envelopes of late-evolved stars from their observations. An observed (infrared) 9.7 micrometre emission silicate signature for cool evolved (oxygen-rich giant) stars. And an observed (infrared) 11.5 micrometre emission silicon carbide signature for cool evolved (carbon-rich giant) stars. These help provide evidence that the small silicate particles in space came from the outer envelopes (ejecta) of these stars.

It is believed that conditions in interstellar space are generally not suitable for the formation of silicate cores. The arguments are that: given an observed typical grain diameter a, the time for a grain to attain a, and given the temperature of interstellar gas, it would take considerably longer than the age of the universe for interstellar grains to form. Furthermore, grains are seen to form in the vicinity of nearby stars in real-time, meaning in a) nova and supernova ejecta, and b) R Coronae Borealis, which seem to eject discrete clouds containing both gas and dust.

Dust grain destruction.

How are the interstellar grains destroyed? There are several ultraviolet processes which lead to grain "explosions". In addition, evaporation, sputtering (when an atom or ion strikes the surface of a solid with enough momentum to eject atoms from it), and grain-grain collisions have a major influence on the grain size distribution.

These destructive processes happen in a variety of places. Some grains are destroyed in the supernovae/novae explosion (and others are formed afterwards). Some of the dust is ejected out of the protostellar disk in the strong stellar winds that occur during a protostar's active T Tauri phase and may be destroyed when passing through shocks, e.g. in Herbig-Haro objects. Plus there are some gas-phase processes in a dense cloud where ultraviolet photons eject energetic electrons from the grains into the gas.

Dust grains incorporated into stars are also destroyed, but only a relatively small fraction of the mass of a star-forming cloud actually ends up in stars. This means a typical grain goes through many Molecular clouds and has mantles added and removed many times before the grain core is destroyed.

Some "dusty" clouds in the universe.

Our solar sytem has its own interplanetary dust cloud; extrasolar systems too.

There are different types of nebulae with different physical causes and processes. One might see these classifications:

  • diffuse nebula.
  • infrared (IR) Reflection nebula.
  • Supernova remnant.
  • Molecular cloud.
  • HII regions.
  • photodissociation regions.

Distinctions between those types of nebula are that different radiation processes are at work. For example, H II regions, like the Orion Nebula, where a lot of star-formation is taking place, are characterized as thermal emission nebulae. Supernova remnants, on the other hand, like the Crab Nebula, are characterized as nonthermal emission (synchrotron radiation).

Some of the better known dusty regions in the universe are the diffuse nebula in the Messier catalog, for example: M1, M8, M16, M17, M20, M42, M43 Messier Catalog

Some larger 'dusty' catalogs that you can access from the NSSDC, CDS, and perhaps other places are:

  • Sharpless (1959) A Catalogue of HII Regions.
  • Lynds (1965) Catalogue of Bright Nebulae.
  • Lunds (1962) Catalogue of Dark Nebulae.
  • van den Bergh (1966) Catalogue of Reflection Nebulae.
  • Green (1988) Rev. Reference Cat. of Galactic SNRs.



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