Cosmic dust
Encyclopedia
Dust
Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...
composed of particles in space which are a few molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s to 0.1 µm
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
in size. Cosmic dust can be further distinguished by its astronomical location; for example: intergalactic dust
Intergalactic dust
Intergalactic dust is cosmic dust in between galaxies in intergalactic space. Evidence for intergalactic dust has been suggested as early as 1949, and study of it grew throughout the late 20th century. There are large variations in the distribution of intergalactic dust...
, interstellar dust, interplanetary dust (such as in the zodiacal cloud) and circumplanetary dust (such as in a planetary ring
Planetary ring
A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region.The most notable planetary rings known in Earth's solar system are those around Saturn, but the other three gas giants of the solar system possess ring systems of their...
).
In our own Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
, interplanetary dust causes the zodiacal light
Zodiacal light
Zodiacal light is a faint, roughly triangular, whitish glow seen in the night sky which appears to extend up from the vicinity of the sun along the ecliptic or zodiac. Caused by sunlight scattered by space dust in the zodiacal cloud, it is so faint that either moonlight or light pollution renders...
. Sources include comet dust
Comet dust
Comet dust refers to cosmic dust that originates from a comet. Comet dust can provide clues to comets' origin.-Dust and comet origin:The models for the origin of comets are: 1) the interstellar model, 2) the solar system model, 3) primordial rubble piles 4) aggregation of planetesimals in the dust...
, asteroidal dust, dust from the Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...
, and interstellar dust passing through our solar system.
The terminology has no specific application for describing materials found on the planet Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, other than in the most general sense that all elements
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
with an atomic number higher than helium are believed to be formed in the core of stars via stellar nucleosynthesis
Stellar nucleosynthesis
Stellar nucleosynthesis is the collective term for the nuclear reactions taking place in stars to build the nuclei of the elements heavier than hydrogen. Some small quantity of these reactions also occur on the stellar surface under various circumstances...
and supernova nucleosynthesis
Supernova nucleosynthesis
Supernova nucleosynthesis is the production of new chemical elements inside supernovae. It occurs primarily due to explosive nucleosynthesis during explosive oxygen burning and silicon burning...
events. As such all elements that exist can be indiscriminately considered to be a form of "cosmic dust". In October 2011, scientists reported that cosmic dust contains complex organic
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
matter ("amorphous organic solids with a mixed aromatic-aliphatic structure") that could be created naturally, and rapidly, by stars.
Study and importance
Cosmic dust was once solely an annoyance to astronomers, as it obscures objects they wish to observe. When infrared astronomyInfrared astronomy
Infrared astronomy is the branch of astronomy and astrophysics that studies astronomical objects visible in infrared radiation. The wavelength of infrared light ranges from 0.75 to 300 micrometers...
began, those previously annoying dust particles were observed to be significant and vital components of astrophysical processes.
For example, cosmic dust can drive the mass loss when a star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
is nearing the end of its life, play a part in the early stages of star formation
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...
, and form planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
s. In our own solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
, dust plays a major role in the zodiacal light
Zodiacal light
Zodiacal light is a faint, roughly triangular, whitish glow seen in the night sky which appears to extend up from the vicinity of the sun along the ecliptic or zodiac. Caused by sunlight scattered by space dust in the zodiacal cloud, it is so faint that either moonlight or light pollution renders...
, Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
's B Ring spokes, the outer diffuse planetary ring
Planetary ring
A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region.The most notable planetary rings known in Earth's solar system are those around Saturn, but the other three gas giants of the solar system possess ring systems of their...
s at Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
, Saturn, Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
and Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
, and comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
s.
The study of dust is a many-faceted research topic that brings together different scientific fields: physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
(solid-state, electromagnetic theory
Electromagnetism
Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...
, surface physics, statistical physics
Statistical physics
Statistical physics is the branch of physics that uses methods of probability theory and statistics, and particularly the mathematical tools for dealing with large populations and approximations, in solving physical problems. It can describe a wide variety of fields with an inherently stochastic...
, thermal physics
Thermal physics
Thermal physics is the combined study of thermodynamics, statistical mechanics, and kinetic theory. This umbrella-subject is typically designed for physics students and functions to provide a general introduction to each of three core heat-related subjects...
), fractal mathematics, chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
(chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
s on grain surfaces), meteoritics
Meteoritics
Meteoritics is a science that deals with meteorites and other extraterrestrial materials that further our understanding of the origin and history of the solar system. A specialist who studies meteoritics is known as a meteoriticist....
, as well as every branch of astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
and astrophysics
Astrophysics
Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior...
. 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
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
, in molecular cloud
Molecular cloud
A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen ....
s, in the circumstellar dust
Circumstellar dust
Circumstellar Dust is astronomical dust around a star. It can be in the form of a spherical shell or a disk, e.g. an accretion disk. Circumstellar dust can be responsible for significant extinction and is usually the source of an infrared excess for stars that have it.The motion of circumstellar...
of young stellar object
Young stellar object
Young stellar object denotes a star in its early stage of evolution.This class consists of two groups of objects: protostars and pre–main sequence stars. Sometimes they are divided by mass - massive YSO , intermediate mass YSO and brown dwarfs....
s, and in planetary systems such as our own solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
, 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 photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s. Once cosmic dust is detected, the scientific problem to be solved is an inverse problem
Inverse problem
An inverse problem is a general framework that is used to convert observed measurements into information about a physical object or system that we are interested in...
to determine what processes brought that encoded photon-like object (dust) to the detector. Parameters such as the particle's initial motion, material properties, intervening plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
and magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
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
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
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
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
airplanes
Fixed-wing aircraft
A fixed-wing aircraft is an aircraft capable of flight using wings that generate lift due to the vehicle's forward airspeed. Fixed-wing aircraft are distinct from rotary-wing aircraft in which wings rotate about a fixed mast and ornithopters in which lift is generated by flapping wings.A powered...
and collected from surface deposits on the large Earth ice-masses (Antarctica and Greenland/the Arctic) and in deep-sea sediments. Don Brownlee
Donald E. Brownlee
Don Brownlee is a professor of astronomy at the University of Washington and the principal investigator for NASA's STARDUST mission. His primary research interests include astrobiology, comets, cosmic dust, and porsche evangelism....
at the University of Washington
University of Washington
University of Washington is a public research university, founded in 1861 in Seattle, Washington, United States. The UW is the largest university in the Northwest and the oldest public university on the West Coast. The university has three campuses, with its largest campus in the University...
in Seattle first reliably identified the extraterrestrial nature of collected dust particles in the later 1970s. Another source is the meteorites, which contain stardust extracted from them (see below). 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.
Aerogel
Aerogel is a synthetic porous material derived from a gel, in which the liquid component of the gel has been replaced with a gas. The result is a solid with extremely low density and thermal conductivity...
.
Dust detectors in the past flew on the HEOS-2, Helios, Pioneer 10
Pioneer 10
Pioneer 10 is a 258-kilogram robotic space probe that completed the first interplanetary mission to Jupiter, and became the first spacecraft to achieve escape velocity from the Solar System. The project was managed by the NASA Ames Research Center and the contract for the construction of the...
, Pioneer 11
Pioneer 11
Pioneer 11 is a 259-kilogram robotic space probe launched by NASA on April 6, 1973 to study the asteroid belt, the environment around Jupiter and Saturn, solar wind, cosmic rays, and eventually the far reaches of the solar system and heliosphere...
, Giotto, and Galileo space missions, on the Earth-orbiting LDEF, EURECA, and Gorid satellites, and some scientists have utilized the Voyager 1
Voyager 1
The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...
and 2
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...
spacecrafts as giant Langmuir probe
Langmuir probe
A Langmuir probe is a device named after Nobel Prize winning physicist Irving Langmuir, used to determine the electron temperature, electron density, and electric potential of a plasma. It works by inserting one or more electrodes into a plasma, with a constant or time-varying electric potential...
s to directly sample the cosmic dust. Presently dust detectors are flying on the Ulysses, Cassini, Proba
PROBA
PROBA is a satellite launched by ISRO in 2001 as part of the ESA's MicroSat program. This small boxlike system, with solar panel collectors on its surface, has remarkable image-making qualities. It hosts two Earth Observation instruments dubbed CHRIS and HRC...
, Rosetta
Rosetta (spacecraft)
Rosetta is a robotic spacecraft of the European Space Agency on a mission to study the comet 67P/Churyumov–Gerasimenko. Rosetta consists of two main elements: the Rosetta space probe and the Philae lander. The spacecraft was launched on 2 March 2004 on an Ariane 5 rocket and will reach the comet by...
, Stardust
Stardust (spacecraft)
Stardust is a 300-kilogram robotic space probe launched by NASA on February 7, 1999 to study the asteroid 5535 Annefrank and collect samples from the coma of comet Wild 2. The primary mission was completed January 15, 2006, when the sample return capsule returned to Earth...
, and the New Horizons
New Horizons
New Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, Hydra and S/2011 P 1. Its estimated arrival date at the Pluto-Charon system is July 14th, 2015...
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 the cosmic dust clouds, allowing us to peer into regions of star formation and the centers of galaxies. NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
's Spitzer Space Telescope
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...
is the largest infrared telescope ever launched into space. The Spitzer Space Telescope (formerly SIRTF, the Space Infrared Telescope Facility) was launched into space by a Delta rocket from Cape Canaveral, Florida on 25 August 2003. During its mission, Spitzer will obtain images and spectra by detecting the infrared energy, or heat, radiated 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. The findings from the Spitzer already revitalized the studies of cosmic dust. A recent report from a Spitzer team shows some evidence that cosmic dust is formed near a supermassive black hole.
Radiative properties of cosmic dust
A dust particle interacts with electromagnetic radiationElectromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
in a way that depends on its cross section
Cross section (geometry)
In geometry, a cross-section is the intersection of a figure in 2-dimensional space with a line, or of a body in 3-dimensional space with a plane, etc...
, the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of the electromagnetic radiation, and on the nature of the grain: its refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
, size, etc. The radiation process for an individual grain is called its emissivity
Emissivity
The emissivity of a material is the relative ability of its surface to emit energy by radiation. It is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature...
, dependent on the grain's efficiency factor. Furthermore, we have to specify whether the emissivity process is extinction
Extinction (astronomy)
Extinction is a term used in astronomy to describe the absorption and scattering of electromagnetic radiation by matter between an emitting astronomical object and the observer. Interstellar extinction—also called Galactic extinction, when it occurs in the Milky Way—was first...
, scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
, absorption
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
, 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 means that light is redirected slightly by diffraction
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
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 nebula
Reflection nebula
In Astronomy, reflection nebulae are clouds of dust which are simply reflecting the light of a nearby star or stars. The energy from the nearby star, or stars, is insufficient to ionize the gas of the nebula to create an emission nebula, but is enough to give sufficient scattering to make the dust...
e, 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
Astrophysical X-ray source
Astrophysical X-ray sources are astronomical objects with physical properties which result in the emission of X-rays.There are a number of types of astrophysical objects which emit X-rays, from galaxy clusters, through black holes in active galactic nuclei to galactic objects such as supernova...
would possess diffuse haloes, due to the dust.
Stardust
Stardust grains (see also presolar grainsPresolar grains
Presolar grains are isotopically-distinct clusters of material found in the fine-grained matrix of primitive meteorites, such as chondrites, whose differences from the surrounding meteorite suggest that they are older than the solar system...
) are contained within meteorites, from which they are extracted in terrestrial laboratories. So-called carbonaceous chondrite
Carbonaceous chondrite
Carbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 7 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites...
s are especially fertile reservoirs of stardust. Each stardust grain existed before the earth was formed. The meteorites have preserved the previously interstellar stardust grains since that time. Stardust is a scientific term; not just a poetic one, referring to refractory dust grains that condensed from cooling ejected gases from individual presolar stars. Many different types of stardust have been identified by laboratory measurements of the highly unusual isotopic composition of the chemical elements that comprise each stardust grain. Many new aspects of nucleosynthesis
Nucleosynthesis
Nucleosynthesis is the process of creating new atomic nuclei from pre-existing nucleons . It is thought that the primordial nucleons themselves were formed from the quark–gluon plasma from the Big Bang as it cooled below two trillion degrees...
have been discovered from those isotopic ratios. An important property of stardust is the hard, refractory, high-temperature nature of the grains. Prominent are silicon carbide
Silicon carbide
Silicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...
, graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
, aluminium oxide
Aluminium oxide
Aluminium oxide is an amphoteric oxide with the chemical formula 23. It is commonly referred to as alumina, or corundum in its crystalline form, as well as many other names, reflecting its widespread occurrence in nature and industry...
, aluminium spinel
Spinel
Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4. Balas ruby is an old name for a rose-tinted variety.-Spinel group:...
, and other such grains that would condense at high temperature from a cooling gas, such as in stellar winds or in the decompression of the inside of a supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
. They differ greatly from the solids formed at low temperature within the interstellar medium. Also important are their extreme isotopic compositions, which are expected to exist nowhere in the interstellar medium. This also suggests that the stardust condensed from the gases of individual stars before the isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
s could be diluted by mixing with the interstellar medium. These allow the source stars to be identified. For example, the heavy elements within the SiC grains are almost pure S-process
S-process
The S-process or slow-neutron-capture-process is a nucleosynthesis process that occurs at relatively low neutron density and intermediate temperature conditions in stars. Under these conditions the rate of neutron capture by atomic nuclei is slow relative to the rate of radioactive beta-minus decay...
isotopes, fitting their condensation within AGB
Asymptotic Giant Branch
The asymptotic giant branch is the region of the Hertzsprung-Russell diagram populated by evolving low to medium-mass stars. This is a period of stellar evolution undertaken by all low to intermediate mass stars late in their lives....
star red giant winds inasmuch as the AGB stars are the main source of S-process nucleosynthesis and have atmospheres observed by astronomers to be highly enriched in dredged-up s process elements. Another dramatic example comes from the supernova condensates, usually shortened by acronym to SUNOCON to distinguish them from other stardust condensed within stellar atmospheres. SUNOCONs show evidence that they condensed containing abundant radioactive 44Ti, which has a 65 year half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
. It was thus still alive when the SUNOCON condensed within the expanding supernova interior but would have been extinct after mixing with the interstellar gas. Its discovery proved the prediction from 1975 to identify SUNOCONs in this way. But SiC SUNOCONs are only about 1% as numerous as are SiC stardust.
Exciting as stardust is, it is but a modest fraction of the condensed cosmic dust. It seems that stardust is less than 0.1% of the mass of total interstellar solids. Its interest lies in the new information that it has brought to the sciences of stellar evolution
Stellar evolution
Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...
and nucleosynthesis
Nucleosynthesis
Nucleosynthesis is the process of creating new atomic nuclei from pre-existing nucleons . It is thought that the primordial nucleons themselves were formed from the quark–gluon plasma from the Big Bang as it cooled below two trillion degrees...
.
A fascinating aspect to human culture is the study within terrestrial laboratories of solids that existed before the earth existed. This was once thought impossible, especially in the decades when cosmochemists were confident that the solar system began as a hot gas virtually devoid of any remaining solids, which would have been vaporized by high temperature. The very existence of stardust shows that this historic picture was incorrect.
Some bulk properties of cosmic dust
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
ranging from fluffy to compact. The composition, size, and other properties depends on where the dust is found, and conversely, a compositional analysis of a dust particle can reveal much about the dust particle's origin. General diffuse interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
dust, dust grains in dense clouds, planetary rings dust, and circumstellar dust
Circumstellar dust
Circumstellar Dust is astronomical dust around a star. It can be in the form of a spherical shell or a disk, e.g. an accretion disk. Circumstellar dust can be responsible for significant extinction and is usually the source of an infrared excess for stars that have it.The motion of circumstellar...
, are each different in their characteristics. 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.
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
range between 1 and 3 g/cm³, with an average density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
at about 2.0 g/cm³.
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).
- Cometary dustComet dustComet dust refers to cosmic dust that originates from a comet. Comet dust can provide clues to comets' origin.-Dust and comet origin:The models for the origin of comets are: 1) the interstellar model, 2) the solar system model, 3) primordial rubble piles 4) aggregation of planetesimals in the dust...
is generally different (with overlap) from asteroidal dust. Asteroidal dust resembles carbonaceous chondritic meteoritesCarbonaceous chondriteCarbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 7 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites...
, and cometary dust resembles interstellar grains, which can include the elements: silicates, polycyclic aromatic hydrocarbonPolycyclic aromatic hydrocarbonPolycyclic aromatic hydrocarbons , also known as poly-aromatic hydrocarbons or polynuclear aromatic hydrocarbons, are potent atmospheric pollutants that consist of fused aromatic rings and do not contain heteroatoms or carry substituents. Naphthalene is the simplest example of a PAH...
s, and water ice.
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 starCarbon star
A carbon star is a late-type star similar to a red giant whose atmosphere contains more carbon than oxygen; the two elements combine in the upper layers of the star, forming carbon monoxide, which consumes all the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds,...
s. Stars that have evolved off the main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
and have entered the giant
Giant star
A giant star is a star with substantially larger radius and luminosity than a main sequence star of the same surface temperature. Typically, giant stars have radii between 10 and 100 solar radii and luminosities between 10 and 1,000 times that of the Sun. Stars still more luminous than giants are...
phase of their evolution are a major source of dust grains in galaxies. Star dust, sung and written in the popular media, is a colloquial term referring to the birthplace of most dust grains in the Universe. If one indeed traces the origin of the elements out of which human bodies are made, they are star dust.
Astronomers know that the dust is formed in the envelopes of late-evolved stars from specific observational signatures. In infrared light, emission at 9.7 micrometres is a signature of silicate dust in cool evolved oxygen-rich giant stars. Emission at 11.5 micrometres indicates the presence of silicon carbide dust in cool evolved carbon-rich giant stars. These help provide evidence that the small silicate particles in space came from the ejected outer envelopes 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
Nova
A nova is a cataclysmic nuclear explosion in a star caused by the accretion of hydrogen on to the surface of a white dwarf star, which ignites and starts nuclear fusion in a runaway manner...
and supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
ejecta, and b) R Coronae Borealis variable
R Coronae Borealis variable
A R Coronae Borealis variable is an eruptive variable star that varies in luminosity in two modes, one low amplitude pulsation , and one irregular unpredictably sudden fading by 1 to 9 magnitudes...
stars which seem to eject discrete clouds containing both gas and dust.
Most dust in our solar system is highly processed dust, recycled from the material out of which our solar system formed and subsequently collected in the planetesimals, and leftover solid material such as comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
s and asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
s, and reformed in each of those bodies' collisional lifetimes. During our solar system's formation history, the most abundant element was (and still is) H2. The metallic elements: magnesium, silicon, and iron, which are the principal ingredients of rocky planets, condensed into solids at the highest temperatures. The range of elements of the solar nebula between H2 and (Mg, Si, Fe) is not known well (Wood, J., 1999). Some molecules such as CO, N2, NH3, and free oxygen, existed in a gas phase. Some molecules, for example, graphite (C) and SiC condensed into solid grains. Some molecules also formed complex organic compounds and some molecules formed frozen ice mantles, of which either could coat the "refractory" (Mg, Si, Fe) grain cores.
The formation of these molecules was determined, in large part, by the temperature of the solar nebula. Since the temperature of the solar nebula decreased with heliocentric distance, scientists can infer a dust grain's origin(s) with knowledge of the grain's materials. Some materials could only have been formed at high temperatures, while other grain materials could only have been formed at much lower temperatures. The materials in a single interplanetary dust particle often show that the grain elements formed in different locations and at different times in the solar nebula. Most of the matter present in the original solar nebula has since disappeared; drawn into the Sun, expelled into interstellar space, or reprocessed, for example, as part of the planets, asteroids or comets.
Due to their highly-processed nature, 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
Solar nebula
In cosmogony, the nebular hypothesis is the most widely accepted model explaining the formation and evolution of the Solar System. There is evidence that it was first proposed in 1734 by Emanuel Swedenborg. Originally applied only to our own Solar System, this method of planetary system formation...
and before our solar nebula's formation. Examples of embedded elements in cosmic dust are GEMS
Glass with Embedded Metal and Sulfides
Glass with embedded metal and sulfides are tiny spheroids in cosmic dust particles with bulk compositions that are approximately chondritic. They form the building blocks of anhydrous interplanetary dust particles in general, and cometary IDPs, in particular...
, chondrule
Chondrule
Chondrules are round grains found in chondrites. Chondrules form as molten or partially molten droplets in space before being accreted to their parent asteroids...
s, and CAIs.
From the solar nebula to Earth
We can follow the trail to the right in the diagram to the IDPs that contain the most volatile and primitive elements. The trail takes us first from interplanetary dust particles to chondritic interplanetary dust particles. Planetary scientists classify chondritic IDPs in terms of their diminishing degree of oxidation so that they fall into three major groups: the carbonaneous, the ordinary, and the enstatite chondrites. As the name implies, the carbonaceous chondrites are rich in carbon, and many have anomalies in the isotopic abundances of H, C, N, and O (Jessberger, 2000). From the carbonaceous chondrites, we follow the trail to the most primitive materials. They are almost completely oxidized and contain the most low condensation temperature elements ("volatile" elements) and the largest amount of organic compounds. Therefore, dust particles with these elements are thought to be formed in the early life of our solar system. Why? The volatile elements have never seen temperatures above about 500 K, therefore, one can conclude that the IDP grain "matrix" consists of some very primitive solar system material. Such a scenario is true in the case of comet dust.
We can learn more about these particles' origin, by examining their surfaces. If we examine, in the laboratory, dust particles' density of solar flare tracks, their amorphous rims, and the spallogenic isotopes from cosmic rays (Flynn, 1996), then we have good clues for how long a particle has been traveling in space. Nuclear damage tracks are caused by the ion flux from solar flares. Solar wind ions impacting on the particle's surface produce amorphous radiation damaged rims on the particle's surface. And spallogenic nuclei are produced by galactic and solar cosmic rays. A dust particle that originates in the Kuiper Belt at 40 AU would have many more times the density of tracks, thicker amorphous rims and higher integrated doses than a dust particle originating in the main-asteroid belt.
Dust grain destruction
How are the interstellar grains destroyed? There are several ultravioletUltraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
processes which lead to grain "explosions". In addition, evaporation, sputtering
Sputtering
Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. It is commonly used for thin-film deposition, etching and analytical techniques .-Physics of 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 due to shock-waves resulting from supernovae/novae explosions (and others are formed afterwards). Some of the dust is ejected out of the protostellar
Protostar
A protostar is a large mass that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. The protostellar phase is an early stage in the process of star formation. For a one solar-mass star it lasts about 100,000 years...
disk in the strong stellar winds that occur during a protostar's active T Tauri
T Tauri star
T Tauri stars are a class of variable stars named after their prototype – T Tauri. They are found near molecular clouds and identified by their optical variability and strong chromospheric lines.-Characteristics:...
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 cloud
Molecular cloud
A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen ....
s and has mantles added and removed many times before the grain core is destroyed.
Some "dusty" clouds in the universe
Our solar system has its own interplanetary dust cloudInterplanetary dust cloud
The interplanetary dust cloud is cosmic dust which pervades the space between planets in the Solar System and in other planetary systems...
; 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 nebulaReflection nebulaIn Astronomy, reflection nebulae are clouds of dust which are simply reflecting the light of a nearby star or stars. The energy from the nearby star, or stars, is insufficient to ionize the gas of the nebula to create an emission nebula, but is enough to give sufficient scattering to make the dust...
- supernova remnantSupernova remnantA supernova remnant is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar material it sweeps up and shocks along the way.There are two...
- molecular cloudMolecular cloudA molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen ....
- HII regionH II regionAn H II region is a large, low-density cloud of partially ionized gas in which star formation has recently taken place. The short-lived, blue stars forged in these regions emit copious amounts of ultraviolet light, ionizing the surrounding gas...
s - photodissociation regionPhotodissociation regionPhotodissociation regions are predominantly neutral regions of the interstellar medium in which far ultraviolet photons strongly influence the gas chemistry and act as the most important source of heat...
s
Distinctions between those types of nebula are that different radiation processes are at work. For example, H II regions, like the Orion Nebula
Orion Nebula
The Orion Nebula is a diffuse nebula situated south of Orion's Belt. It is one of the brightest nebulae, and is visible to the naked eye in the night sky. M42 is located at a distance of and is the closest region of massive star formation to Earth. The M42 nebula is estimated to be 24 light...
, 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
Crab Nebula
The Crab Nebula is a supernova remnant and pulsar wind nebula in the constellation of Taurus...
, are characterized as nonthermal emission (synchrotron radiation
Synchrotron radiation
The electromagnetic radiation emitted when charged particles are accelerated radially is called synchrotron radiation. It is produced in synchrotrons using bending magnets, undulators and/or wigglers...
).
Some of the better known dusty regions in the universe are the diffuse nebulae in the Messier catalog, for example: M1
Crab Nebula
The Crab Nebula is a supernova remnant and pulsar wind nebula in the constellation of Taurus...
, M8, M16
Eagle Nebula
The Eagle Nebula is a young open cluster of stars in the constellation Serpens, discovered by Jean-Philippe de Cheseaux in 1745-46. Its name derives from its shape which is resemblant of an eagle...
, M17
Omega Nebula
The Omega Nebula, also known as the Swan Nebula, Checkmark Nebula, Lobster Nebula, and the Horseshoe Nebula is an H II region in the constellation Sagittarius. It was discovered by Philippe Loys de Chéseaux in 1745. Charles Messier catalogued it in 1764...
, M20
Trifid Nebula
The Trifid Nebula is an H II region located in Sagittarius. Its name means 'divided into three lobes'...
, M42
Orion Nebula
The Orion Nebula is a diffuse nebula situated south of Orion's Belt. It is one of the brightest nebulae, and is visible to the naked eye in the night sky. M42 is located at a distance of and is the closest region of massive star formation to Earth. The M42 nebula is estimated to be 24 light...
, 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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