Cosmic dustalso called extraterrestrial dust, space dust, or star dustis dust that occurs in outer space or has fallen onto Earth. Most cosmic dust particles measure between a few molecules and , such as micrometeoroids (<30 μm) and meteoroids (>30 μm). Cosmic dust can be further distinguished by its astronomical location: intergalactic dust, interstellar dust, interplanetary dust (as in the zodiacal cloud), and circumplanetary dust (as in a planetary ring). Information on the nature of dust in locations beyond the Solar System is mostly obtained by methods of observational astronomy, such as photometry, polarimetry and infrared spectroscopy.Whittet 2022, pp. 1-3 – 1-11 Direct methods are also available for the collection and study of space dust within the Solar System. For example, the Stardust spacecraft collected cometary dust, and also detected some particles of probable interstellar origin, returning samples to Earth in 2006.
In the Solar System, interplanetary dust causes the zodiacal light. Solar System dust also includes cometary dust, planetary dust (e.g. from Mars), lunar regolith, asteroidal dust, dust from the Kuiper belt, and interstellar dust passing through the Solar System. Thousands of tons of cosmic dust are estimated to reach Earth's surface every year, with most grains having a mass between 10−16 kg (0.1 pg) and 10−4 kg (0.1 g). The density of the dust cloud through which the Earth is traveling is approximately 10−6 dust grains/m3.
A major fraction of the dust in space is "stardust", consisting of refractory minerals such as silicates, graphite and amorphous carbon. These solids condense as circumstellar dust around evolved stars such as red giants, carbon stars, novae and supernovae, and are swept into interstellar space by the stellar winds and outflows of the parent star.Whittet 2022, pp. 8-1 – 8-37 The dust also contains organic compounds (amorphous organic solids with a mixed aromatic–aliphatic structure) that could be created naturally, and rapidly, in the outflows of carbon stars. or by processes occurring in the interstellar medium.Whittet 2022, pp. 9-1 – 9-41
Study and importance
Cosmic dust was once solely an annoyance to astronomers, as it obscures objects they wished to observe. When infrared astronomy began, the dust particles were observed to be significant and vital components of astrophysical processes. Their analysis can reveal information about phenomena like the formation of the Solar System. For example, cosmic 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 the 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, and comets.
The interdisciplinary study of dust brings together different scientific fields: physics (solid-state, electromagnetic theory, surface physics, statistical physics, thermal physics), fractal mathematics, surface chemistry on dust grains, 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 the 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.
Parameters such as 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
A wide range of methods is available to study cosmic dust. Cosmic dust can be detected by remote sensing methods that use the radiative properties of cosmic dust particles, cf. Zodiacal light measurement.
Cosmic dust can also be detected directly ('in-situ') using a variety of collection methods and from a variety of collection locations. Estimates of the daily influx of extraterrestrial material entering the Earth's atmosphere range between 5 and 300 tonnes.
NASA collects samples of star dust particles in the Earth's atmosphere using plate collectors under the wings of stratospheric-flying airplanes. Dust samples are also 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 latter 1960s/Brownlee, D. E., & Hodge, P. W. (1969). Results of a large volume micrometeorite collection at an altitude of 115.000 feet.. Meteoritics, 4, 264. Another source is the meteorites, which contain stardust extracted from them. Stardust grains are solid refractory pieces of individual presolar stars. They are recognized by their extreme isotopic compositions, which can only be isotopic compositions within evolved stars, prior to any mixing with the interstellar medium. These grains condensed from the stellar matter as it cooled while leaving the star.
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, Galileo, Ulysses and Cassini space missions, on the Earth-orbiting LDEF, EURECA, and Gorid satellites, and some scientists have used the Voyager 1 and 2 spacecraft as giant Langmuir probes to directly sample the cosmic dust. Presently dust detectors are flying on the Ulysses, 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.
Infrared light can penetrate cosmic dust clouds, allowing us to peer into regions of star formation and the centers of galaxies. NASA's Spitzer Space Telescope was the largest infrared space telescope, before the launch of the James Webb Space Telescope. During its mission, Spitzer obtained images and spectra by detecting the thermal radiation emitted by objects in space between wavelengths of 3 and 180 micrometres. Most of this infrared radiation is blocked by the Earth's atmosphere and cannot be observed from the ground. Findings from the Spitzer have revitalized the studies of cosmic dust. One report showed some evidence that cosmic dust is formed near a supermassive black hole.
Another detection mechanism is polarimetry. Dust grains are not spherical and tend to align to interstellar magnetic fields, preferentially polarizing starlight that passes through dust clouds. In nearby interstellar space, where interstellar reddening is not intense enough to be detected, high precision optical polarimetry has been used to glean the structure of dust within the Local Bubble.
In 2019, researchers found interstellar dust in Antarctica which they relate to the Local Interstellar Cloud. The detection of interstellar dust in Antarctica was done by the measurement of the radionuclides iron-60 and manganese-53 by highly sensitive Accelerator mass spectrometry.
Radiation properties
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. Further specifications regarding the emissivity process include extinction, scattering, absorption, or polarisation. 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 is light that is redirected slightly off its path by diffraction, 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 in one's data is many times brighter in forward-scattered visible light than in back-scattered visible light, then it is understood 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 nebulae, 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.
Presolar grains
Presolar grains are contained within meteorites, from which they are extracted in terrestrial laboratories. The term "stardust" or "presolar stardust" is sometimes used to distinguish grains from a single star in comparison to aggregated interstellar dust particles, though this distinction is not universally applied. Presolar material was a component of the dust in the interstellar medium before its incorporation into meteorites. The meteorites have stored those presolar grains ever since the meteorites first assembled within the planetary accretion disk more than four billion years ago. Carbonaceous chondrites are especially fertile reservoirs of presolar material. Presolar grains definitionally existed before the Earth was formed. Presolar grain (and, less frequently, "stardust" or "presolar stardust") is the scientific term referring to refractory dust grains that condensed from cooling ejected gases from individual presolar stars and incorporated into the cloud from which the Solar System condensed.
Many different types of presolar grains have been identified by laboratory measurements of the highly unusual isotopic composition of the chemical elements that comprise each presolar grain. These refractory mineral grains may earlier have been coated with volatile compounds, but those are lost in the dissolving of meteorite matter in acids, leaving only insoluble refractory minerals. Finding the grain cores without dissolving most of the meteorite has been possible, but difficult and labor-intensive.