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Comet dust refers to cosmic dust that originates from a
comet A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are ...
. Comet dust can provide clues to comets' origin. When the Earth passes through a comet dust trail, it can produce a meteor shower.


Physical characteristics


Size

The majority of dust from comet activity is sub-micrometer to roughly micrometer in size. However, this fraction is short-lived, as
radiation pressure Radiation pressure is the mechanical pressure exerted upon any surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of any wavelength that is a ...
causes them to blow out of the Solar System or spiral inwards. The next size class is large, "fluffy" or "cluster-type" aggregates of the above grains. These are typically 20-100 micrometers, a size not arbitrary but observed as the porous aggregates tend to fracture or compact. Larger particles are micrometeoroids, not dust. "...in practice the term is most often applied to objects smaller than approximately 100 um. These size ranges need to be modified." "By this definition, IDPs are particles smaller than 10um."" In the absence of a definition from the
IAU The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach ...
, groups devised their own definitions of dust: smaller than 100 micrometers, "tenth micron particles" "very fluffy aggregates" 50, 40, 30, S.3 Multifaceted Scientific Dust Observations "<~ 30 micrometer" and 20 microns, and <10 μm. Some of these dust/micrometeorite definitions are approximate or ambiguous, "10 um (Rubin and Grossman, 2010)""in the <100 um size fraction, i.e. across the transition between micrometeorites and IDPs" some overlapping or self-conflicting. "20 um for the upper cut-off" "50 um for the upper cut-off" The IAU released a formal statement in 2017. Meteoroids are 30 micrometers to 1 meter, dust is smaller, and the term "micrometeoroid" is discouraged (though not micrometeorite). The IMO noted the new definition, but still displays a prior definition on their site. The Meteoritical Society site retains its prior definition, 0.001 cm. The AMS has posted no rigorous definition.


Composition

Dust is generally
chondritic A chondrite is a stony (non- metallic) meteorite that has not been modified, by either melting or differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar System accreted to form prim ...
in composition. Its monomers contain mafic silicates, such as olivine and pyroxene. Silicates are rich in high-condensation temperature forsterite and
enstatite Enstatite is a mineral; the magnesium endmember of the pyroxene silicate mineral series enstatite (MgSiO3) – ferrosilite (FeSiO3). The magnesium rich members of the solid solution series are common rock-forming minerals found in igneous and m ...
. As these condense quickly, they tend to form very small particles, not merging droplets. As with chondritic meteoroids, particles contain Fe(Ni) sulfide and GEMS (glass with embedded metal and sulfides) Various amounts of organics ( CHON) are present. Though organics are cosmically abundant, and were widely predicted to exist in comets, they are spectrally indistinct in most telescopes. Organics were only confirmed via mass spectrometry during the Halley flybys. Some organics are in the form of PAHs ( Polycyclic Aromatic Hydrocarbons). Very small inclusions of
presolar grains Presolar grains are interstellar solid matter in the form of tiny solid grains that originated at a time before the Sun was formed. Presolar stardust grains formed within outflowing and cooling gases from earlier presolar stars. The stellar nuc ...
(PSGs) may be found.


Dust and comet origin

The models for the origin of comets are: # the interstellar model, # the Solar System model, # primordial rubble piles, # aggregation of planetesimals in the dust disk around the UranusNeptune region, # cold shells of material swept out by the protostellar wind. Bulk properties of the comet dust such as density as well as the chemical composition can distinguish between the models. For example, the isotopic ratios of comet and of interstellar dust are very similar, indicating a common origin. The 1) interstellar model says that ices formed on dust grains in the dense cloud that preceded the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
. The mix of ice and dust then aggregated into a comet without appreciable chemical modification. J. Mayo Greenberg first proposed this idea in the 1970s. In the 2) Solar System model, the ices that formed in the interstellar cloud first vaporized as part of the
accretion disk An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction, uneven irradiance, magnetohydrodynamic effects, and other fo ...
of gas and dust around the protosun. The vaporized ices later resolidified and assembled into comets. So the comets in this model would have a different composition than those comets that were made directly from interstellar ice. The 3) primordial rubble pile model for comet formation says that comets agglomerate in the region where Jupiter was forming. Stardust's discovery of crystalline silicates in the dust of comet Wild 2 implies that the dust formed above
glass temperature The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle "glassy" state into a viscous or rubb ...
(> 1000 K) in the inner disk region around a hot young star, and was radially mixed in the solar nebula from the inner regions a larger distance from the star or the dust particle condensed in the outflow of evolved red giants or supergiants. The composition of the dust of comet Wild 2 is similar to the composition of dust found in the outer regions of the accretion disks around newly-forming stars. A comet and its dust allow investigation of the Solar System beyond the main planetary orbits. Comets are distinguished by their orbits; long period comets have long elliptical orbits, randomly inclined to the plane of the Solar System, and with periods greater than 200 years. Short period comets are usually inclined less than 30 degrees to the plane of the Solar System, revolve around the Sun in the same counterclockwise direction as the planets orbit, and have periods less than 200 years. A comet will experience a range of diverse conditions as it traverses its orbit. For long period comets, most of the time it will be so far from the Sun that it will be too cold for evaporation of ices to occur. When it passes through the terrestrial planet region, evaporation will be rapid enough to blow away small grains, but the largest grains may resist entrainment and stay behind on the
comet nucleus The nucleus is the solid, central part of a comet, once termed a ''dirty snowball'' or an ''icy dirtball''. A cometary nucleus is composed of rock, dust, and frozen gases. When heated by the Sun, the gases sublime and produce an atmosphere su ...
, beginning the formation of a dust layer. Near the Sun, the heating and evaporation rate will be so great, that no dust can be retained. Therefore, the thickness of dust layers covering the nuclei of a comet can indicate how closely and how often a comet's perihelion travels are to the Sun. If a comet has an accumulation of thick dust layers, it may have frequent perihelion passages that don't approach the Sun too closely. A thick accumulation of dust layers might be a good description of all of the short period comets, as dust layers with thicknesses on the order of meters are thought to have accumulated on the surfaces of short-period comet nuclei. The accumulation of dust layers over time would change the physical character of the short-period comet. A dust layer both inhibits the heating of the cometary ices by the Sun (the dust is impenetrable by sunlight and a poor conductor of heat), and slows the loss of gases from the nucleus below. A comet nucleus in an orbit typical of short period comets would quickly decrease its evaporation rate to the point that neither a coma or a tail would be detectable and might appear to astronomers as a low-albedo near-Earth asteroid.


Further assemblages and bodies

Dust particles, aided by ices and organics, form "aggregates" (less often, "agglomerates") of 30 to hundreds of micrometers. These are fluffy, Tucson "fluffy aggregate" due to the imperfect packing of cluster-type (large) dust particles, and their subsequent, imperfect packing into aggregates. Discussion meeting issue “Cometary science after Rosetta” compiled and edited by Geraint H. Jones, Alan Fitzsimmons, Matthew M. Knight, and Matt G. G. T. Taylor "grains" "particles" "hierarchical aggregates" "'clusters'" "compact porous aggregates""highly porous aggregates" The next size category is pebbles, of millimeters to centimeters scale. "cm-sized pebbles" Pebbles were inferred at 103P/Hartley 2, "dust, ice, and hundreds of discrete millimeter to decimeter sized particles." and imaged directly at 67P/Churyumov-Gerasimenko. Astrophysical use of the word "pebble" differs from its geological meaning. In turn, the next-larger geological term, "cobble," has been skipped by
Rosetta Rosetta or Rashid (; ar, رشيد ' ; french: Rosette  ; cop, ϯⲣⲁϣⲓⲧ ''ti-Rashit'', Ancient Greek: Βολβιτίνη ''Bolbitinē'') is a port city of the Nile Delta, east of Alexandria, in Egypt's Beheira governorate. The R ...
scientists. Even larger bodies are "boulders" (decimeter-scale and above) or "chunks." These are rarely seen in the coma, as gas pressure is often insufficient to lift them to significant altitude or escape velocity. " ejected chunks with diameter bigger than few meters""chunks up to the radius of 0.4 m" The building blocks of comets are the putative cometesimals, analogous to planetesimal. Whether the actual cometesimals/planetesimals were pebble-scale, boulder-scale, or otherwise has been a key topic in Solar System and exoplanet research.


(Mis)Use of the term "dust"

At best, "dust" is a collective noun for the non-gas portion of the coma and tail(s). At worst, the term is an
English usage Linguistic prescription, or prescriptive grammar, is the establishment of rules defining preferred usage of language. These rules may address such linguistic aspects as spelling, pronunciation, vocabulary, syntax, and semantics. Sometimes infor ...
, understood well by astronomers in the field, but not to the general public, teachers, and scientists from other fields. The larger solids are more properly called "debris" or, for all non-gases, the general "particles" or "grains."


Comet 2P/Encke

Encke is officially a dust-poor, gas-rich comet. Encke actually emits most of its solid mass as meteoroids or "rocks," not dust. ISO measured no infrared evidence of a classical cometary dust tail due to small particles.} "abundant large particles near the comet pose a significant hazard to spacecraft. There is no evidence of a classical cometary dust tail due to small particles."


References

{{Comets, nonobject=yes Dust Cosmic dust