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=== Size ===
=== Size ===
The majority of dust from comet activity is sub-micrometer<ref>{{cite book |last1=Mukai |first1=T. |last2=Mukai |first2=S. |last3=Kikouchi |first3=S. |title=Symposium on the Diversity and Similarity of Comets, ESA SP-278 |date=1987 |publisher=European Space Agency |pages=427–30 |chapter=Variation of Grain Properties and the Dust Outbursts }}</ref> to roughly micrometer in size.<ref>{{cite book |last1=Grun |first1=E. |last2=Massonne |first32=L. |last3=Schwehm |first3=G. |title=Symposium on the Diversity and Similarity of Comets, ESA SP-278 |date=1987 |publisher=European Space Agency |pages=305–14 |chapter=New Properties of Cometary Dust }}</ref><ref>{{cite book |last1=Fernandez |first1=J. |title=Comets: Nature, Dynamics, Origins, and their Cosmogonical Relevance |date=2005 |publisher=Springer |page=66}}</ref> However, this fraction is short-lived, as [[radiation pressure]] causes them to blow out of the Solar System<ref name="southworth">{{cite journal |last1=Southworth |first1=R. |title=Distribution of the Zodiacal Particles |journal=Annals New York Academy of Science |date=11 Nov 1964 |volume=119 |page=54|doi=10.1111/j.1749-6632.1965.tb47423.x }}</ref><ref name="fechtig">{{cite book |last1=Fechtig |first1=H. |title=Comets |date=1982 |publisher=University of Arizona Press |location=Tucson |page=370 |chapter=Cometary Dust In The Solar System }}</ref> or [[Poynting-Robertson effect|spiral inwards]].<ref name="whipplemoc">{cite book |last=Whipple |first=Fred |date=1986 |title=The Mystery of Comets |isbn=9780521324403 |publisher=Cambridge University Press |page=143 | }</ref>
The majority of dust from comet activity is sub-micrometer<ref>{{cite book |last1=Mukai |first1=T. |last2=Mukai |first2=S. |last3=Kikouchi |first3=S. |title=Symposium on the Diversity and Similarity of Comets, ESA SP-278 |date=1987 |publisher=European Space Agency |pages=427–30 |chapter=Variation of Grain Properties and the Dust Outbursts }}</ref> to roughly micrometer in size.<ref>{{cite book |last1=Grun |first1=E. |last2=Massonne |first32=L. |last3=Schwehm |first3=G. |title=Symposium on the Diversity and Similarity of Comets, ESA SP-278 |date=1987 |publisher=European Space Agency |pages=305–14 |chapter=New Properties of Cometary Dust }}</ref><ref>{{cite book |last1=Fernandez |first1=J. |title=Comets: Nature, Dynamics, Origins, and their Cosmogonical Relevance |date=2005 |publisher=Springer |page=66}}</ref> However, this fraction is short-lived, as [[radiation pressure]] causes them to blow out of the Solar System<ref name="southworth">{{cite journal |last1=Southworth |first1=R. |title=Distribution of the Zodiacal Particles |journal=Annals New York Academy of Science |date=11 Nov 1964 |volume=119 |page=54|doi=10.1111/j.1749-6632.1965.tb47423.x }}</ref><ref name="fechtig">{{cite book |last1=Fechtig |first1=H. |title=Comets |date=1982 |publisher=University of Arizona Press |location=Tucson |page=370 |chapter=Cometary Dust In The Solar System }}</ref> or [[Poynting-Robertson effect|spiral inwards]].<ref name="whipplemoc">{cite book |last=Whipple |first=Fred |date=1986 |title=The Mystery of Comets |isbn=9780521324403 |publisher=Cambridge University Press |page=143 | }</ref><ref name="">{cite book |last=Dermott |first=S |date=2001 |title=Interplanetary Dust |isbn= |publisher=SpringerVerlag |pages=569-39 |chapter=Ch. Orbital evolution of interplanetary dust } GrünE Gustafsonb dermottS fechtigH eds. </ref>
<ref name="">{cite book |last=Dermott |first=S |date=2001 |title=Interplanetary Dust |isbn= |publisher=SpringerVerlag |pages=569-39 |chapter=Ch. Orbital evolution of interplanetary dust }</ref>GrünE Gustafsonb dermottS fechtigH eds.


The next size class is large, "fluffy"<ref name="southworth"/><ref name="fechtig"/> or "cluster-type"<ref name="zolensky">{{cite conference |last1=Zolensky |first1=M. |last2=Lindstrom |first2=D. |title=Mineralogy of 12 large 'chondritic' interplanetary dust particles |conference=1991 LPSC |date=Mar 1991 |pages=161–69}}</ref> aggregates of the above grains. These are typically 20-100 micrometers, a size not arbitrary but observed<ref>{{cite book |last1=Ney |first1=E. |title=Comets |date=1982 |publisher=University of Arizona Press |location=Tucson |page=323 |chapter=Optical and Infrared Observations of Bright Comets in the Range 0.5 um to 20 um }}</ref> as the porous aggregates tend to fracture<ref>{{cite book |last1=Simpson |first1=J. |last2=Rabinowitz |first2=D. |last3=Tuzzolino |first3=A. |last4=Ksanfomality |first4=L. |title=ESA Proceedings of the 20th ESLAB Symposium on the Exploration of Halley's Comet. Volume 2: Dust and Nucleus |date=1986 |publisher=European Space Agency |pages=11–16 |chapter=Halley's comet dust particle mass spectra, flux distributions and jet structures derived from measurements on the Vega-1 and Vega-2 spacecraft }}</ref> or compact.<ref name="zolensky"/><ref>{cite journal |last1=Leinert |first1=C |last2=Roser |first2=S |last3=Buitrago |first3=J |date=1983 |title=How to maintain the spatial distribution of interplanetary dust |journal=Astronomy & Astrophysics |volume=118 |pages=345-57 }</ref><ref>{cite journal |last1=Mukai |first1=T |last2=fechtig |first2=H |date=Jun 83 |title=Packing efficient of fluffy particles |journal=Planetary and SpaceScience |volume=31 |issue=6 |pages=655-58 }</ref>
The next size class is large, "fluffy"<ref name="southworth"/><ref name="fechtig"/> or "cluster-type"<ref name="zolensky">{{cite conference |last1=Zolensky |first1=M. |last2=Lindstrom |first2=D. |title=Mineralogy of 12 large 'chondritic' interplanetary dust particles |conference=1991 LPSC |date=Mar 1991 |pages=161–69}}</ref> aggregates of the above grains. These are typically 20-100 micrometers, a size not arbitrary but observed<ref>{{cite book |last1=Ney |first1=E. |title=Comets |date=1982 |publisher=University of Arizona Press |location=Tucson |page=323 |chapter=Optical and Infrared Observations of Bright Comets in the Range 0.5 um to 20 um }}</ref> as the porous aggregates tend to fracture<ref>{{cite book |last1=Simpson |first1=J. |last2=Rabinowitz |first2=D. |last3=Tuzzolino |first3=A. |last4=Ksanfomality |first4=L. |title=ESA Proceedings of the 20th ESLAB Symposium on the Exploration of Halley's Comet. Volume 2: Dust and Nucleus |date=1986 |publisher=European Space Agency |pages=11–16 |chapter=Halley's comet dust particle mass spectra, flux distributions and jet structures derived from measurements on the Vega-1 and Vega-2 spacecraft }}</ref> or compact.<ref name="zolensky"/><ref>{cite journal |last1=Leinert |first1=C |last2=Roser |first2=S |last3=Buitrago |first3=J |date=1983 |title=How to maintain the spatial distribution of interplanetary dust |journal=Astronomy & Astrophysics |volume=118 |pages=345-57 }</ref><ref>{cite journal |last1=Mukai |first1=T |last2=fechtig |first2=H |date=Jun 83 |title=Packing efficient of fluffy particles |journal=Planetary and SpaceScience |volume=31 |issue=6 |pages=655-58 }</ref>


Larger particles are [[micrometeoroids]].<ref name="reachsykes03">{{cite book |last1=Reach |first1=W. |last2=Sykes |first2=M. |last3=Kelley |first3=M. |title=Workshop on Cometary Dust in Astrophysics |date=2003 |publisher=Lunar and Planetary Institute |location=Houston |chapter=Large Particles From Short-Period Comets }}</ref><ref>{{cite book |last1=Kelley |first1=M. |last2=Reach |first2=W. |last3=Woodward |first3=C. |title=Deep Impact as a World Observatory Event: Synergies in Space, Time, and Wavelength |date=2009 |publisher=Springer-Verlag |location=Berlin Heidelberg |isbn=978-3-540-76959-0 |page=125 |chapter=A Search for Deep Impact’s Large Particle Ejecta }}</ref>, not dust.<ref>{cite journal |last1=Beech |first1=M |last2=Steel |first2=D |date=1995 |title=On the definition of the term 'meteoroid' |journal=Quart. Journ. Roy. Ast. Soc. |volume=36 |page=281-84 } Sec. 4 Lower size limit: Meteoroid or dust? </ref><ref name="rubingrossman10">{cite journal |last1=Rubin |first1=A |last2=Grossman |first2=J |date=Mar 2010 |title=Mtrt n mtrd: Nw cmprhnsv dfntns |journal=Meteoritics & Planetary Science |volume=45 |issue=1 |pages=114-22 } "...i prctc th trm ims oftn appld t objs smlr thn aprxmtl 100 um. These size ranges need to be modified." "By this definition, IDPs are particles smaller than 10um.""</ref> In the absence of a definition from the IAU,<ref>{cite journal |last=Millman |first=P |date=1961 |title=A Report on Meteor Terminology |journal=Journ. Roy. Ast. Soc. Canada |volume=55 |issue=6 |page=265 } "particle sizes in general smaller than micrometeorites" </ref><ref>{cite web |https://www.iau.org/static/resolutions/IAU1961_French.pdf |title=Resolutions Adopted A. By The General Assembly |accessdate=30 Jun 2020 } Sec. "Commission 22 (Meteors and Meteorites/Météores et des Meteorites)" </ref> groups devised their own definitions of dust: smaller than 100 micrometers,<ref name="greenbergli">{cite journal |last1=Greenberg |first1=M |last2=Li |first2=A |date=1997 |title=Morphological structural and chemical composition of cometary nuclei and dust |journal=Space Science Reviews |volume=90 |pages=149-61 } "tenth micron particles" "very fluffy aggregates"</ref> 50,<ref>{cite conference |last=1Klöck |first1=W |last2=Staderman |first2=F |date=94 |title=Mineralogical and chemical relationships of interplanetary dust particles, micrometeorites, and meteorites in |conference=LPI Technical Report 94-02 Workshop on the analysis of interplanetary dust particles |zolensky M ed. } "50 um"</ref> 40,<ref>{cite journal |last1=Levasseur-regourd |first1=A |last2=mukai |last3=lasue |first3= |last4=okada |date=2007 |title=physical properties of comet and interplanetary dust |journal=Planetary Space Science |volume=55 |issue=9 |pages=1010-20 } "a radius of 20 um for the upper cut-off" </ref> 30,<ref name="grunkrugersrama">{cite journal |last1=Grun |first1=E |last2=Krüger |first2=H |last3=Srama |first3=R |date=2019 |title=The Dawn of Dust Astronomy |journal=SSR |volume=215 |page=number 46 } S.3 Multifaceted Scientific Dust Observations "<~ 30 micrometer" </ref> and 20 microns,<ref name="levasseurregourdea07"/> and <10 μm.<ref>{cite book |last1=Bradley |first1=J |last2=Sandford |first2=S |last3=Walker |first3=R |date=1988 |title=Meteorites and the Early Solar System |isbn= |publisher=University of Arizona Press |page=861 |chapter=11.1 Interplanetary Dust Particles } "~ 10 um i diamtr" "~ 10-3 cm in dia" </ref><ref>{cite journal |last1=Love |first1=S |last2=Brownlee |first2=D |date=Jan 1991 |title=Heating and thermal transformation of micrometeoroids entering the Earth's atmosphere |journal=Icarus |volume=89 |pages=26-43 } "10 um" </ref><ref>{cite journal |last1=Coulson |first1=D |last2=Wickramasinghe |first2=N |date=21 Aug 2003 |title=Frictional and radiation heating of micron-sized meteoroids in the Earth's upper atmosphere |journal=Mon. Not. Roy. Ast. Soc. |volume=343 |issue=4 |pages=1123-30 } "~10 um"</ref><ref name="rubingrossman"/> Some of these dust/micrometeorite definitions are approximate or ambiguous,<ref>{cite journal |last1=Brownlee |first1=D |last2=Tsou |first2=P |last3=Aléon |first3=J |last4=et al. |date=2006 |title= |journal=Science |volume=314 |page=1711 } "~0.1-mm micrometeorites, and 10-um interplanetary dust" </ref><ref>{cite book |last=Rehder |first=D |date=2010 |title=Chemistry in Space |isbn=978-3-527-32689-1 |publisher=Wiley-VCH |chapter=5.3.3 Intrplntr Ds Ptcls (Prsl Grs) } "<100um; typically 0.1-20um" </ref><ref name="folcocordier15">{cite book |last1=Folco |first1=L |last2=Cordier |first2=C |date=2015 |title=EMU Notes in Mineralogy |isbn= |publisher= |chapter=9. Micrometeorites } "10 um (Rubin and Grossman, 2010)""in the <100 um size fraction, i.e. across the transition between micrometeorites and IDPs" </ref> some overlapping or self-conflicting.<ref>{cite conference |last=Rietmeijer |first=F |date=Oct 2002 |title=Mesospheric Metal abundances and Meteoric Dust: Analysis of surviving Meteoroids |conference=34th COSPAR Scientific Assembly/2nd World Space Congress |issue=196 } "stratospheric interplanetary dust particles (IDPs) (2-100 microns)" "debris from progenitors ~30 to ~1,000 microns" </ref><ref name="levasseurregourdea07">{cite journal |last1=Levasseur-Regourd |first1=A |last2=Mukai |first2=T |last3=Lasue |first3=J |last4=Okada |first4=Y |date=Jun 2007 |title=Physical properties of comet and interplanetary dust |journal=Planetary Space Science |volume=55 |issue=9 |pages=1010-20 |doi=10.1016/j.pss.2006.11.014 } "20 um for the upper cut-off" "50 um for the upper cut-off" </ref><ref name="grunkrugersrama"/>
Larger particles are [[micrometeoroids]]<ref name="reachsykes03">{{cite book |last1=Reach |first1=W. |last2=Sykes |first2=M. |last3=Kelley |first3=M. |title=Workshop on Cometary Dust in Astrophysics |date=2003 |publisher=Lunar and Planetary Institute |location=Houston |chapter=Large Particles From Short-Period Comets }}</ref><ref>{{cite book |last1=Kelley |first1=M. |last2=Reach |first2=W. |last3=Woodward |first3=C. |title=Deep Impact as a World Observatory Event: Synergies in Space, Time, and Wavelength |date=2009 |publisher=Springer-Verlag |location=Berlin Heidelberg |isbn=978-3-540-76959-0 |page=125 |chapter=A Search for Deep Impact’s Large Particle Ejecta }}</ref>, not dust.<ref>{cite journal |last1=Beech |first1=M |last2=Steel |first2=D |date=1995 |title=On the definition of the term 'meteoroid' |journal=Quart. Journ. Roy. Ast. Soc. |volume=36 |page=281-84 } Sec. 4 Lower size limit: Meteoroid or dust? </ref><ref name="rubingrossman10">{cite journal |last1=Rubin |first1=A |last2=Grossman |first2=J |date=Mar 2010 |title=Mtrt n mtrd: Nw cmprhnsv dfntns |journal=Meteoritics & Planetary Science |volume=45 |issue=1 |pages=114-22 } "...i prctc th trm ims oftn appld t objs smlr thn aprxmtl 100 um. These size ranges need to be modified." "By this definition, IDPs are particles smaller than 10um.""</ref> In the absence of a definition from the IAU,<ref>{cite journal |last=Millman |first=P |date=1961 |title=A Report on Meteor Terminology |journal=Journ. Roy. Ast. Soc. Canada |volume=55 |issue=6 |page=265 } "particle sizes in general smaller than micrometeorites" </ref><ref>{cite web |https://www.iau.org/static/resolutions/IAU1961_French.pdf |title=Resolutions Adopted A. By The General Assembly |accessdate=30 Jun 2020 } Sec. "Commission 22 (Meteors and Meteorites/Météores et des Meteorites)" </ref> groups devised their own definitions of dust: smaller than 100 micrometers,<ref name="greenbergli">{cite journal |last1=Greenberg |first1=M |last2=Li |first2=A |date=1997 |title=Morphological structural and chemical composition of cometary nuclei and dust |journal=Space Science Reviews |volume=90 |pages=149-61 } "tenth micron particles" "very fluffy aggregates"</ref> 50,<ref>{cite conference |last=1Klöck |first1=W |last2=Staderman |first2=F |date=94 |title=Mineralogical and chemical relationships of interplanetary dust particles, micrometeorites, and meteorites in |conference=LPI Technical Report 94-02 Workshop on the analysis of interplanetary dust particles |zolensky M ed. } "50 um"</ref> 40,<ref>{cite journal |last1=Levasseur-regourd |first1=A |last2=mukai |last3=lasue |first3= |last4=okada |date=2007 |title=physical properties of comet and interplanetary dust |journal=Planetary Space Science |volume=55 |issue=9 |pages=1010-20 } "a radius of 20 um for the upper cut-off" </ref> 30,<ref name="grunkrugersrama">{cite journal |last1=Grun |first1=E |last2=Krüger |first2=H |last3=Srama |first3=R |date=2019 |title=The Dawn of Dust Astronomy |journal=SSR |volume=215 |page=number 46 } S.3 Multifaceted Scientific Dust Observations "<~ 30 micrometer" </ref> and 20 microns,<ref name="levasseurregourdea07"/> and <10 μm.<ref>{cite book |last1=Bradley |first1=J |last2=Sandford |first2=S |last3=Walker |first3=R |date=1988 |title=Meteorites and the Early Solar System |isbn= |publisher=University of Arizona Press |page=861 |chapter=11.1 Interplanetary Dust Particles } "~ 10 um i diamtr" "~ 10-3 cm in dia" </ref><ref>{cite journal |last1=Love |first1=S |last2=Brownlee |first2=D |date=Jan 1991 |title=Heating and thermal transformation of micrometeoroids entering the Earth's atmosphere |journal=Icarus |volume=89 |pages=26-43 } "10 um" </ref><ref>{cite journal |last1=Coulson |first1=D |last2=Wickramasinghe |first2=N |date=21 Aug 2003 |title=Frictional and radiation heating of micron-sized meteoroids in the Earth's upper atmosphere |journal=Mon. Not. Roy. Ast. Soc. |volume=343 |issue=4 |pages=1123-30 } "~10 um"</ref><ref name="rubingrossman10"/> Some of these dust/micrometeorite definitions are approximate or ambiguous,<ref name="brotsoual06"/><ref>{cite book |last=Rehder |first=D |date=2010 |title=Chemistry in Space |isbn=978-3-527-32689-1 |publisher=Wiley-VCH |chapter=5.3.3 Intrplntr Ds Ptcls (Prsl Grs) } "<100um; typically 0.1-20um" </ref><ref name="folcocordier15">{cite book |last1=Folco |first1=L |last2=Cordier |first2=C |date=2015 |title=EMU Notes in Mineralogy |isbn= |publisher= |chapter=9. Micrometeorites } "10 um (Rubin and Grossman, 2010)""in the <100 um size fraction, i.e. across the transition between micrometeorites and IDPs" </ref> some overlapping or self-conflicting.<ref>{cite conference |last=Rietmeijer |first=F |date=Oct 2002 |title=Mesospheric Metal abundances and Meteoric Dust: Analysis of surviving Meteoroids |conference=34th COSPAR Scientific Assembly/2nd World Space Congress |issue=196 } "stratospheric interplanetary dust particles (IDPs) (2-100 microns)" "debris from progenitors ~30 to ~1,000 microns" </ref><ref name="levasseurregourdea07">{cite journal |last1=Levasseur-Regourd |first1=A |last2=Mukai |first2=T |last3=Lasue |first3=J |last4=Okada |first4=Y |date=Jun 2007 |title=Physical properties of comet and interplanetary dust |journal=Planetary Space Science |volume=55 |issue=9 |pages=1010-20 |doi=10.1016/j.pss.2006.11.014 } "20 um for the upper cut-off" "50 um for the upper cut-off" </ref><ref name="grunkrugersrama"/>


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).<ref name="iau17">{cite web |https://www.iau.org/static/science/scientific_bodies/commissions/f1/meteordefinitions_approved.pdf |title=Definitions of terms in meteor astronomy |accessdate= }</ref> The IMO noted the new definition,<ref>{cite web |https://imo.net/definitions-of-terms-in-meteor-astronomy-iau |title=Definitions of terms in meteor astronomy (IAU) |author=Perlerin, V. |accessdate=30 Jun 2020 }</ref> but still displays a prior definition on their site.<ref name="">{cite web |https://www.imo.net/resources/glossary/ |title=Glossary |accessdate=30 Jun 2020 }</ref> The Meteoritical Society site retains its prior definition, 0.001 cm.<ref>{cite web |https://meteoritical.org/about-meteorites/dust |title=Dust |=Benoit P |accessdate=30 Jun 2020 } "0.001 cm in diameter" </ref> The AMS has posted no rigorous definition.<ref>{cite web |https://https://www.amsmeteors.org/meteor-showers/meteor-faq/ |title=METEOR FAQS |accessdate=30 Jun 2020 }</ref><ref>{cite web |https://amsmeteors.org/glossary |title=Glossary |accessdate=30 Jun 2020 }</ref>
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).<ref name="iau17">{cite web |https://www.iau.org/static/science/scientific_bodies/commissions/f1/meteordefinitions_approved.pdf |title=Definitions of terms in meteor astronomy |accessdate= }</ref> The IMO noted the new definition,<ref>{cite web |https://imo.net/definitions-of-terms-in-meteor-astronomy-iau |title=Definitions of terms in meteor astronomy (IAU) |author=Perlerin, V. |accessdate=30 Jun 2020 }</ref> but still displays a prior definition on their site.<ref name="">{cite web |https://www.imo.net/resources/glossary/ |title=Glossary |accessdate=30 Jun 2020 }</ref> The Meteoritical Society site retains its prior definition, 0.001 cm.<ref>{cite web |https://meteoritical.org/about-meteorites/dust |title=Dust |=Benoit P |accessdate=30 Jun 2020 } "0.001 cm in diameter" </ref> The AMS has posted no rigorous definition.<ref>{cite web |https://https://www.amsmeteors.org/meteor-showers/meteor-faq/ |title=METEOR FAQS |accessdate=30 Jun 2020 }</ref><ref>{cite web |https://amsmeteors.org/glossary |title=Glossary |accessdate=30 Jun 2020 }</ref>
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=== Composition ===
=== Composition ===


Dust is generally [[Chondrite|chondritic]] in composition. Its monomers contain mafic silicates, such as [[olivine]] and [[pyroxene]].<ref>{cite journal |last1=Bradley |first1=J |last2=Brownlee |first2=D |last3=Veblen |first3=D |date=83 |title=Pyroxene whiskers and platelets in interplanetary dust: evidence of vapor phase growth |journal=Nature |volume=301 |page=473 }</ref> Silicates are rich in high-condensation temperature [[forsterite]] and [[enstatite]]<ref name="brotsoual06">{cite journal |last1=Brownlee |first1=D |last2=Tsou |first2=P |last3=Aléon |first3=J |last4=et al. |date=2006 |title=81P/Wild 2 Under a Microscope |journal=Science |volume=314 |page=1711 }</ref> As these condense quickly, they tend to form very small particles, not merging droplets.
Dust is generally [[Chondrite|chondritic]] in composition. Its monomers contain mafic silicates, such as [[olivine]] and [[pyroxene]].<ref>{cite journal |last1=Bradley |first1=J |last2=Brownlee |first2=D |last3=Veblen |first3=D |date=83 |title=Pyroxene whiskers and platelets in interplanetary dust: evidence of vapor phase growth |journal=Nature |volume=301 |page=473 }</ref> Silicates are rich in high-condensation temperature [[forsterite]] and [[enstatite]].<ref name="brotsoual06">{cite journal |last1=Brownlee |first1=D |last2=Tsou |first2=P |last3=Aléon |first3=J |last4=et al. |date=2006 |title=81P/Wild 2 Under a Microscope |journal=Science |volume=314 |page=1711 }</ref> As these condense quickly, they tend to form very small particles, not merging droplets.


As with chondritic meteoroids, particles contain [[Meteoritic iron|Fe(Ni)]] sulfide <ref name="zolze06">{cite journal |last1=Zolensky |first1=M |last2=Zega |first2=T |last3=Yano |first3=H |last4=Wirick |first4=S |last5=Westphal |first5=A |last6=Weisberg |first6=M |last7=et al. |first7= |date=15 Dec 2006 |title=Mineralogy and Petrology of Comet 81P/Wild 2 Nucleus Samples |journal=Science |volume=314 |page=1735 }</ref><ref>{cite journal |last1=Zolensky |first1=M |last2=Thomas |first2=K |date=Nov 1995 |title=Iron and iron-nickel sulfides in chondritic interplanetary dust particles |journal=Geochimica et Cosmochimica Acta |volume=59 | issue=22 |page=4707 }</ref> and GEMS (glass with embedded metal and sulfides)<ref name="zolze06"/>
As with chondritic meteoroids, particles contain [[Meteoritic iron|Fe(Ni)]] sulfide<ref name="zolze06">{cite journal |last1=Zolensky |first1=M |last2=Zega |first2=T |last3=Yano |first3=H |last4=Wirick |first4=S |last5=Westphal |first5=A |last6=Weisberg |first6=M |last7=et al. |first7= |date=15 Dec 2006 |title=Mineralogy and Petrology of Comet 81P/Wild 2 Nucleus Samples |journal=Science |volume=314 |page=1735 }</ref><ref>{cite journal |last1=Zolensky |first1=M |last2=Thomas |first2=K |date=Nov 1995 |title=Iron and iron-nickel sulfides in chondritic interplanetary dust particles |journal=Geochimica et Cosmochimica Acta |volume=59 | issue=22 |page=4707 }</ref> and GEMS (glass with embedded metal and sulfides)<ref name="zolze06"/>
Various amounts of organics [[CHON]] are present.<ref>{cite journal |last1=Kissel |first1=J |last2=Sagdeev |first2=R |last3=Bertaux |first3=J |last4=et al. |date=1986 |title= |journal=Nature |volume=321 |page=280 }</ref><ref>{cite journal |last1=Kissel |first1=J |last2=Brownlee |first2=D |last3=Büchler |first3=K |last4=et al. |date=1986 |title= |journal=Nature |volume=321 |page=336 }</ref><ref>{cite journal |last1=KisselJ |first1= |last2=KrugerF |first2= |date=1987 |title= |journal=Nature |volume=326 |pages=755-60 }</ref> 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 [[Halley armada|the Halley flybys]].<ref>{cite journal |last1=Lawler |first1=M |last2=Brownlee |first2=D |date=1992 |title=CHON as a component of dust from comet Halley |journal=Nature |volume=359 |pages=810–12 }</ref><ref name="levasseuragarwal18">{cite journal |last1=Levasseur-Regourd |first1=A |last2=Agarwal |first2=A |last3=Cottin |first3=H |last4=Engrand |first4=C |last5=Flynn |first5=G |last6=Fulle |first6=M |last7=Gombosi |first7=T |last8=et al. |date=2018 |title=Cometary Dust |journal=Space Science Reviews |volume=214 |page=number 64 }</ref> Some organics are in the form of PAHs ([[Polycyclic Aromatic Hydrocarbons]]).<ref>{cite journal |last1=Clemett |first1=S |last2=Maechling |first2=C |last3=Zare |first3=R |last4=Swan |first4=P |last5=Walker |first5=R |date=1993 |title= |journal=Science |volume=262 |page=721 }</ref><ref name="greenbergli"/><ref>{cite journal |last=Lisse |first1=C |last2=et al. |first2= |date=2006 |title= |journal=Science |volume=313 |page=635 }</ref><ref>{cite journal |last1=Sandford |first1=S |last2=et al. |first2= |date=2006 |title= |journal=Science |volume=314 |page=1720 }</ref><ref>{cite journal |last1=Keller |first1=L |last2=et al. |first2= |date=2006 |title= |journal=Science |volume=314 |page=1728 }</ref>
Various amounts of organics ([[CHON]]) are present.<ref>{cite journal |last1=Kissel |first1=J |last2=Sagdeev |first2=R |last3=Bertaux |first3=J |last4=et al. |date=1986 |title= |journal=Nature |volume=321 |page=280 }</ref><ref>{cite journal |last1=Kissel |first1=J |last2=Brownlee |first2=D |last3=Büchler |first3=K |last4=et al. |date=1986 |title= |journal=Nature |volume=321 |page=336 }</ref><ref>{cite journal |last1=KisselJ |first1= |last2=KrugerF |first2= |date=1987 |title= |journal=Nature |volume=326 |pages=755-60 }</ref> 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 [[Halley Armada|the Halley flybys]].<ref>{cite journal |last1=Lawler |first1=M |last2=Brownlee |first2=D |date=1992 |title=CHON as a component of dust from comet Halley |journal=Nature |volume=359 |pages=810–12 }</ref><ref name="levasseuragarwal18">{cite journal |last1=Levasseur-Regourd |first1=A |last2=Agarwal |first2=A |last3=Cottin |first3=H |last4=Engrand |first4=C |last5=Flynn |first5=G |last6=Fulle |first6=M |last7=Gombosi |first7=T |last8=et al. |date=2018 |title=Cometary Dust |journal=Space Science Reviews |volume=214 |page=number 64 }</ref> Some organics are in the form of PAHs ([[Polycyclic aromatic hydrocarbon|Polycyclic Aromatic Hydrocarbons]]).<ref>{cite journal |last1=Clemett |first1=S |last2=Maechling |first2=C |last3=Zare |first3=R |last4=Swan |first4=P |last5=Walker |first5=R |date=1993 |title= |journal=Science |volume=262 |page=721 }</ref><ref name="greenbergli"/><ref>{cite journal |last=Lisse |first1=C |last2=et al. |first2= |date=2006 |title= |journal=Science |volume=313 |page=635 }</ref><ref>{cite journal |last1=Sandford |first1=S |last2=et al. |first2= |date=2006 |title= |journal=Science |volume=314 |page=1720 }</ref><ref>{cite journal |last1=Keller |first1=L |last2=et al. |first2= |date=2006 |title= |journal=Science |volume=314 |page=1728 }</ref>


Very small inclusions of [[presolar grains]] (PSGs) may be found.<ref name="brotsoual06"/><ref>{cite journal |last1=Keller |first1=L |last2=Bajt |first2=S |last3=Baratta |first3=G |last4=Borg |first4=J |last5=Bradley |first5=J |last6=Brownlee |first6=D |last7=et al. |first7= |date=15 Dec 2006 |title=IR Spectroscopy of Comet 81P/Wild 2 Samples Returned by Stardust |journal=Science |volume=314 |page=1728 }</ref>
Very small inclusions of [[presolar grains]] (PSGs) may be found.<ref name="brotsoual06"/><ref>{cite journal |last1=Keller |first1=L |last2=Bajt |first2=S |last3=Baratta |first3=G |last4=Borg |first4=J |last5=Bradley |first5=J |last6=Brownlee |first6=D |last7=et al. |first7= |date=15 Dec 2006 |title=IR Spectroscopy of Comet 81P/Wild 2 Samples Returned by Stardust |journal=Science |volume=314 |page=1728 }</ref>
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{{main|Accretion (astrophysics)}}
{{main|Accretion (astrophysics)}}


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Dust particles, aided by ices and organics, form "aggregates" <ref name="brotsoual06"/><ref name="zolze06"/><ref>{cite journal |last1=Lorek |first1=S |last2=Gundlach |first2=B |last3=Lacerda |first3=P |last4=Blum |first4=J |date=2016 |title=Comet Formation in Collapsing Pebble Clouds: Wh |journal=Astronomy & Astrophysics |volume= | } "dust grains form fractal aggregates" </ref> (less often, "agglomerates"<ref>{cite journal |last1=Mannel |first1=T |last2=Bentley |first2=M |last3=Schmied |first3=R |last4=Jeszenszky |first4=H |last5=Levasseur-Regourd |first5=A |last6=Romstedt |first6=J |last7=Torkar |first7=K |date=10/11/16 |title=Fractal comet dust- a window into the early Solar System |journal=Mon. Not. Roy. Ast. Soc. |volume=462 |issue=S1 |pages=S304-11 }</ref>) of 30 to hundreds of micrometers. These are fluffy,<ref name="greenbergli"/><ref name="weissmanCII">{cite book |last1=Weissman |first1=P |last2=Asphaug |first2=E |last3=Lowry |first3=S |date=2004 |title=Comets II |isbn= |publisher=University of Arizona Press |page=p337 |chapter=Structure and Density of Comet Nuclei } Tucson "fluffy aggregate" </ref> due to the imperfect packing of cluster-type (large) dust particles, and their subsequent, imperfect packing into aggregates.<ref name="woodeniz17">{cite journal |last1=Wooden |first1=D |last2=Ishii |first2=H |last3=Zolensky |first3=M |date=5/2017 |title=CmtryDs:thDvst o prmtv rfctr grns |journal=PTRAS A:ME |volume=375 |issue=2097 } 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" </ref>
Dust particles, aided by ices and organics, form "aggregates"
<ref name="brotsoual06">{cite journal |last1=Brownlee |first1=D |last2=Tsou |first2=P |last3=aléon|first3=J |last4=alexander |first4=C |last5=araki |first5=T |date=15 Dec 2006 |title=Comet 81P/Wild 2 Under a Microscope |journal=Science |volume=314 |page=1711 }"particles"aggregates" </ref>
<ref name="zolze06">{cite journal |last1=Zolensky M |last2=Zega T |last3=Yano H |last4=Wirick S |last5=Westphal A |last6=Weisberg M ea |date=15 Dec 2006 |title=Mineralogy and Petrology of Comet 81P/Wild 2 Nucleus Samples |journal=Science |volume=314 |page=1735} 'loose aggregates ("traveling sand piles")' "weakly constructed" </ref>
⚫
<ref>{cite journal |last1=Lorek |first1=S |last2=Gundlach |first2=B |last3=Lacerda |first3=P |last4=Blum |first4=J |date=2016 |title=Comet Formation in Collapsing Pebble Clouds: Wh |journal=Astronomy & Astrophysics |volume= | } "dust grains form fractal aggregates" </ref> (less often, agglomerates)<ref>{cite journal |last1=Mannel |first1=T |last2=Bentley |first2=M |last3=Schmied |first3=R |last4=Jeszenszky |first4=H |last5=Levasseur-Regourd |first5=A |last6=Romstedt |first6=J |last7=Torkar |first7=K |date=10/11/16 |title=Fractal comet dust- a window into the early Solar System |journal=Mon. Not. Roy. Ast. Soc. |volume=462 |issue=S1 |pages=S304-11 }</ref> of 30 to hundreds of micrometers. These are fluffy,<ref name="greenbergli"/><ref name="weissmanCII">{cite book |last1=Weissman |first1=P |last2=Asphaug |first2=E |last3=Lowry |first3=S |date=2004 |title=Comets II |isbn= |publisher=University of Arizona Press |page=p337 |chapter=Structure and Density of Comet Nuclei } Tucson "fluffy aggregate" </ref> due to the imperfect packing of cluster-type (large) dust particles, and their subsequent, imperfect packing into aggregates.<ref name="woodeniz17">{cite journal |last1=Wooden |first1=D |last2=Ishii |first2=H |last3=Zolensky |first3=M |date=5/2017 |title=CmtryDs:thDvst o prmtv rfctr grns |journal=PTRAS A:ME |volume=375 |issue=2097 } 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" </ref>


The next size category is pebbles, of millimeters to centimeters scale.<ref name="blumgund17">{cite journal |last1=Blum |first1=J |last2=Gundlach |first2=B |last3=Krause |first3=M |last4=Fulle |first4=M |last5=Johansen |first5=A |last6=Agarwal |first6=J |last7=vonBorstel |first7=I |last8=et al. |date=Jul 2017 |title= |journal=Mon. Not. Roy. Ast. Soc. |volume=469 |issue=S2 |pages=S755-73 }</ref><ref>{cite journal |last1=Kretke |first1=K |last2=Levison |first2=H |date=Dec 2015 |title=Evidence for Pbs in Cms |journal=Icarus |volume=262 |pages=9-13 |doi=10.1016/j.icarus.2015.08.017 }</ref><ref name="fullealto16">{cite journal |last1=Fulle |first1=M |last2=Altobelli |first2=N |last3=Buratti |first3=B |last4=Choukroun |first4=M |last5=Fulchignoni |first5=M |last6=Grün |first6=E |last7=Taylor |first7=M |last8=et al. |date=Nov 2016 |title=Unexpected and significant findings in comet 67P/Churyumov-Gerasimenko:an interdisciplinary view |journal=Mon. Not. Roy. Ast. Soc. |volume=462 |pages=S2-8 } "cm-sized pebbles" </ref> Pebbles were inferred at 103P/Hartley 2,<ref name="hermalyn">{cite journal |last1=HermalynB |first1= |last2=farnhamT |first2= |last3=collinsS |first3= |last4=kelleyM |first4= |last5=ahearnM |first5= |last6=bodewitsD |first6= |last7=carcichB |first7= |last8=et al. |date=2013 |title=The detection, localization, and dynamics of large icy particles surrounding Comet 103P/Hartley 2 |journal=Icarus |volume=222 |issue=2 |pages=625-33} "dust, ice, and hundreds of discrete millimeter to decimeter sized particles." </ref> and imaged directly at 67P/Churyumov-Gerasimenko<ref name="fullealto16"/><ref name="blumgund17"/> Astrophysical use of the word "pebble" [[Separation of knowledge|differs from]] its [[Udden-Wentworth_scale|geological meaning]].<ref>{cite journal |last1=Dones |first1=L |last2=Brasser |first2=R |last3=Kaib |first3=N |last4=Rickman |first4=H |date=2015 |title= |journal=Space Science Reviews |volume=197 |pages=191-69 } "so the astrophysical use of the word "pebble" differs from its geological meaning." </ref> In turn, the next-larger geological term, "cobble," has been skipped by Rosetta scientists.<ref>{cite journal |last1=Pajola |first1=M |last2=et al. |first2= |date=2016 |title= |journal=Mon. Not. Roy. Ast. Soc. |volume=462 |pages=S242–52 |doi=10.1093/mnras/stw2720 } "Since inside the Rosetta team the word ‘cobbles’ has never been used, while it has been used ‘pebble’ ... we suggest here to use the word ‘pebble’ for the 0.25 m > size > 0.002 m range. Below 0.002 m the term ‘particle’ is used." </ref>
The next size category is pebbles, of millimeters to centimeters scale.<ref name="blumgund17">{cite journal |last1=Blum |first1=J |last2=Gundlach |first2=B |last3=Krause |first3=M |last4=Fulle |first4=M |last5=Johansen |first5=A |last6=Agarwal |first6=J |last7=vonBorstel |first7=I |last8=et al. |date=Jul 2017 |title= |journal=Mon. Not. Roy. Ast. Soc. |volume=469 |issue=S2 |pages=S755-73 }</ref><ref>{cite journal |last1=Kretke |first1=K |last2=Levison |first2=H |date=Dec 2015 |title=Evidence for Pbs in Cms |journal=Icarus |volume=262 |pages=9-13 |doi=10.1016/j.icarus.2015.08.017 }</ref><ref name="fullealto16">{cite journal |last1=Fulle |first1=M |last2=Altobelli |first2=N |last3=Buratti |first3=B |last4=Choukroun |first4=M |last5=Fulchignoni |first5=M |last6=Grün |first6=E |last7=Taylor |first7=M |last8=et al. |date=Nov 2016 |title=Unexpected and significant findings in comet 67P/Churyumov-Gerasimenko:an interdisciplinary view |journal=Mon. Not. Roy. Ast. Soc. |volume=462 |pages=S2-8 } "cm-sized pebbles" </ref> Pebbles were inferred at 103P/Hartley 2,<ref name="hermalyn">{cite journal |last1=HermalynB |first1= |last2=farnhamT |first2= |last3=collinsS |first3= |last4=kelleyM |first4= |last5=ahearnM |first5= |last6=bodewitsD |first6= |last7=carcichB |first7= |last8=et al. |date=2013 |title=The detection, localization, and dynamics of large icy particles surrounding Comet 103P/Hartley 2 |journal=Icarus |volume=222 |issue=2 |pages=625-33} "dust, ice, and hundreds of discrete millimeter to decimeter sized particles." </ref> and imaged directly at 67P/Churyumov-Gerasimenko.<ref name="fullealto16"/><ref name="blumgund17"/> Astrophysical use of the word "pebble" [[Separation of knowledge|differs from]] its [[Udden-Wentworth_scale|geological meaning]].<ref>{cite journal |last1=Dones |first1=L |last2=Brasser |first2=R |last3=Kaib |first3=N |last4=Rickman |first4=H |date=2015 |title= |journal=Space Science Reviews |volume=197 |pages=191-69 } "so the astrophysical use of the word "pebble" differs from its geological meaning." </ref> In turn, the next-larger geological term, "cobble," has been skipped by [[Rosetta (spacecraft)|Rosetta]] scientists.<ref>{cite journal |last1=Pajola |first1=M |last2=et al. |first2= |date=2016 |title= |journal=Mon. Not. Roy. Ast. Soc. |volume=462 |pages=S242–52 |doi=10.1093/mnras/stw2720 } "Since inside the Rosetta team the word ‘cobbles’ has never been used, while it has been used ‘pebble’ ... we suggest here to use the word ‘pebble’ for the 0.25 m > size > 0.002 m range. Below 0.002 m the term ‘particle’ is used." </ref>


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.<ref>{cite journal |last1=Poulet |first1=F |last2=Lucchetti |first2=A |last3=Bibring |first3=J |last4=Carter |first4=J |last5=Gondet |last6=et al. |date=2016 |title=Origin of the local structures at the Philae landing site and implications on the |journal=Mon. Not. Roy. Ast. Soc. |volume=462 |page=S23 }</ref><ref name="pajolalucc17">{cite journal |last1=Pajola |first1=M |last2=Luccheti |first2=A |last3=Fulle |first3=M |last4=Mottola |first4=S |last5=Hamm |first5=M |last6=Da Deppo |first6=V |date=17 |title=The Pebbles/boulders size distribution on Sais: Rosetta's final landing site on Comet 67P/Churyumov-Gerasimenko |journal=Mon. Not. Roy. Ast. Soc. |volume=469 |page=S636 } " ejected chunks with diameter bigger than few meters""chunks up to the radius of 0.4 m" </ref><ref>{cite journal |last1=Güttler |first1=C |last2=Mannel |first2=T |last3=Rotundi |first3=A |last4=Merouane |first4=S |last5=Fulle |first5=M |last6=Bockelee-Morvan |first6=D |last7=lasue |first7=J |last8=ea |date=2019 |title=Synthesis of the morphological description of cometary dust at comet 67P/Churyumov-Gerasimenko |journal=Astronomy & Astrophysics |volume=630 |page=A24 } "small, decimeter-sized boulders"</ref>
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.<ref>{cite journal |last1=Poulet |first1=F |last2=Lucchetti |first2=A |last3=Bibring |first3=J |last4=Carter |first4=J |last5=Gondet |last6=et al. |date=2016 |title=Origin of the local structures at the Philae landing site and implications on the |journal=Mon. Not. Roy. Ast. Soc. |volume=462 |page=S23 }</ref><ref name="pajolalucc17">{cite journal |last1=Pajola |first1=M |last2=Luccheti |first2=A |last3=Fulle |first3=M |last4=Mottola |first4=S |last5=Hamm |first5=M |last6=Da Deppo |first6=V |date=17 |title=The Pebbles/boulders size distribution on Sais: Rosetta's final landing site on Comet 67P/Churyumov-Gerasimenko |journal=Mon. Not. Roy. Ast. Soc. |volume=469 |page=S636 } " ejected chunks with diameter bigger than few meters""chunks up to the radius of 0.4 m" </ref><ref>{cite journal |last1=Güttler |first1=C |last2=Mannel |first2=T |last3=Rotundi |first3=A |last4=Merouane |first4=S |last5=Fulle |first5=M |last6=Bockelee-Morvan |first6=D |last7=lasue |first7=J |last8=ea |date=2019 |title=Synthesis of the morphological description of cometary dust at comet 67P/Churyumov-Gerasimenko |journal=Astronomy & Astrophysics |volume=630 |page=A24 } "small, decimeter-sized boulders"</ref>


The building blocks of comets are the putative cometesimals,<ref>{cite journal |last=A'Hearn |first=M |date=2006 |title=Whence Comets? |journal=Science |volume=314 |page=1708 }</ref> analogous to planetesimal. Whether the actual cometesimals/planetesimals were pebble-scale,<ref>{cite journal |last1=Lorek |first1=S |last2=Lacerda |first2=P |last3=Blum |first3=J |date=2018 |title=Local growth of dust- and ice-mixed aggregates as building blocks in the solar nebula |journal=Astronomy & Astrophysics |volume=611 |page=A18 }</ref> boulder scale,<ref>{cite journal |last1=Weissman |first1=P |last2=A'Hearn |first2=M |date=Nov 2015 |title=Accretion of Cometary Nuclei in the Solar Nebula: Boulders, Not Pebbles |conference=2015 AAS-47th DPS Meeting |issue=309.05 }</ref> or otherwise has been a key topic in Solar System and exoplanet research.<ref name="weissmanCII"/><ref>{cite journal |last1=Fulle |first1=M |last2=Blum |first2=J |date=2017 |title=Fractal dust constrains the collisional history of comets |journal=Mon. Not. Roy. Ast. Soc. |volume=469 |page=S39 }</ref><ref>{cite journal |last1=Lambrechts |first1=M |last2=Johansen |first2=A |date=2018 |title=Forming the cores of giant planets from the radial pebble flux in protoplanetary disks |journal=Astronomy & Astrophysics |volume= |page= }</ref><ref>{cite journal |last1=Levasseur-Regourd |first1=A |last2=Baruteau |first2=C |last3=Lasue |first3=J |last4=Milli |first4=J |last5=Renard |first5=J |date=2019 |title=Linking studies of tiny meteoroids, zodiacal dust, cometary dust and circumstellar disks |journal= |volume= | }</ref>
The building blocks of comets are the putative cometesimals,<ref>{cite journal |last=A'Hearn |first=M |date=2006 |title=Whence Comets? |journal=Science |volume=314 |page=1708 }</ref> analogous to [[planetesimal]]. Whether the actual cometesimals/planetesimals were pebble-scale,<ref>{cite journal |last1=Lorek |first1=S |last2=Lacerda |first2=P |last3=Blum |first3=J |date=2018 |title=Local growth of dust- and ice-mixed aggregates as building blocks in the solar nebula |journal=Astronomy & Astrophysics |volume=611 |page=A18 }</ref> boulder-scale,<ref>{cite journal |last1=Weissman |first1=P |last2=A'Hearn |first2=M |date=Nov 2015 |title=Accretion of Cometary Nuclei in the Solar Nebula: Boulders, Not Pebbles |conference=2015 AAS-47th DPS Meeting |issue=309.05 }</ref> or otherwise has been a key topic in Solar System and exoplanet research.<ref name="weissmanCII"/><ref>{cite journal |last1=Fulle |first1=M |last2=Blum |first2=J |date=2017 |title=Fractal dust constrains the collisional history of comets |journal=Mon. Not. Roy. Ast. Soc. |volume=469 |page=S39 }</ref><ref>{cite journal |last1=Lambrechts |first1=M |last2=Johansen |first2=A |date=2018 |title=Forming the cores of giant planets from the radial pebble flux in protoplanetary disks |journal=Astronomy & Astrophysics |volume= |page= }</ref><ref>{cite journal |last1=Levasseur-Regourd |first1=A |last2=Baruteau |first2=C |last3=Lasue |first3=J |last4=Milli |first4=J |last5=Renard |first5=J |date=2019 |title=Linking studies of tiny meteoroids, zodiacal dust, cometary dust and circumstellar disks |journal= |volume= | }</ref>
===(Mis)Use of the term "dust"===
===(Mis)Use of the term "dust"===


At best, "dust" is a collective noun for non-gas portions of the coma and tail(s). At worst, the term is an [[Informal language|English]] usage, understood well by astronomers in the field, but not to the general public, teachers, and scientists from other fields.<ref name="borovicka">{cite journal |last=Borovička |first=J |date=2016 |title=About the definition of meteoroid, asteroid, and related terms |journal=WGN, the Journal of the IMO |volume=44 |page=31 }</ref> The larger solids are more properly called "debris"<ref name="pajolalucc17"/> or, for all non-gases, the general "particles"<ref>{cite journal |last1=Stern |first1=S |last2=Jackson |first2=A |last3=Boice |first3=D |date=1994 |title=Numerical simulations of particle orbits around 2060 Chiron |journal=Astronomical Journal |volume=107 |issue=2 |pages765-71 }</ref><ref>{cite journal |last1=Economou |first1=T |last2=Green |first2=S |last3=Brownlee |first3=D |last4=Clark |first4=B |date=13Ic222 |title=DFMI measurements during Stardust-NExT Flyby of Comet 9P/Tempel 1 |journal=Icarus |volume=222 |issue=2 |pages=526-39 } "clouds of particles resulting from fragmentation of larger aggregates emitted" </ref><ref name="levasseuragarwal18"/> or "grains."<ref>{cite journal |last1=Rotundi |first1=A |last2=Sierks |first2=H |last3=Delle Corte |first3=V |last4=Fulle |first4=M |last5=et al. |date=23 Jan 2015 |title= |journal=Science |volume=347 |issue=6220 |page=3905 } "grains"</ref><ref name="woodeniz17"/><ref name="grunkrugersrama"/>
At best, "dust" is a collective noun for the non-gas portion of the coma and tail(s). At worst, the term is an [[Informal language|English usage]], understood well by astronomers in the field, but not to the general public, teachers, and scientists from other fields.<ref name="borovicka">{cite journal |last=Borovička |first=J |date=2016 |title=About the definition of meteoroid, asteroid, and related terms |journal=WGN, the Journal of the IMO |volume=44 |page=31 }</ref> The larger solids are more properly called "debris"<ref name="pajolalucc17"/> or, for all non-gases, the general "particles"<ref>{cite journal |last1=Stern |first1=S |last2=Jackson |first2=A |last3=Boice |first3=D |date=1994 |title=Numerical simulations of particle orbits around 2060 Chiron |journal=Astronomical Journal |volume=107 |issue=2 |pages765-71 }</ref><ref>{cite journal |last1=Economou |first1=T |last2=Green |first2=S |last3=Brownlee |first3=D |last4=Clark |first4=B |date=13Ic222 |title=DFMI measurements during Stardust-NExT Flyby of Comet 9P/Tempel 1 |journal=Icarus |volume=222 |issue=2 |pages=526-39 } "clouds of particles resulting from fragmentation of larger aggregates emitted" </ref><ref name="levasseuragarwal18"/> or "grains."<ref>{cite journal |last1=Rotundi |first1=A |last2=Sierks |first2=H |last3=Delle Corte |first3=V |last4=Fulle |first4=M |last5=et al. |date=23 Jan 2015 |title= |journal=Science |volume=347 |issue=6220 |page=3905 } "grains"</ref><ref name="woodeniz17"/><ref name="grunkrugersrama"/>


===Comet 2P/Encke===
===Comet 2P/Encke===
Line 73: Line 69:
{{main|Comet Encke}}
{{main|Comet Encke}}


Encke is officially a dust-poor, gas-rich comet. <ref name="whipplemoc"/><ref>{cite journal |last=Sekanina |first=Z |date=1988 |title=OAoPCE I A24-84 |journal=Astronomical Journal |volume=95 |issue=3 |page=911 } "very low dust content"extremely low dust content"</ref>
Encke is officially a dust-poor, gas-rich comet.<ref name="whipplemoc"/><ref>{cite journal |last=Sekanina |first=Z |date=1988 |title=Outgassing Asymmetry of Periodic Comet Encke I- Apparitions 1924-1984 |journal=Astronomical Journal |volume=95 |issue=3 |page=911 } "very low dust content"extremely low dust content"</ref> Encke actually emits most of its sold mass as meteoroids or "rocks,"<ref name="whipplemoc"/> not dust. There is no evidence of a classical cometary dust tail due to small particles.<ref>{{cite journal |last1=Reach |first1=W |last2=Sykes |first2=M |last3=Lien |first3=D |last4=Davies |first4=J |journal=Icarus |date=2000 |title=The Formation of Encke Meteoroids and Dust Trail |volume=148 |page=80}} "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." </ref>



== References ==
== References ==

Revision as of 15:35, 1 July 2020

Comet dust refers to cosmic dust that originates from a comet. 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[1] to roughly micrometer in size.[2][3] However, this fraction is short-lived, as radiation pressure causes them to blow out of the Solar System[4][5] or spiral inwards.[6][7]

The next size class is large, "fluffy"[4][5] or "cluster-type"[8] aggregates of the above grains. These are typically 20-100 micrometers, a size not arbitrary but observed[9] as the porous aggregates tend to fracture[10] or compact.[8][11][12]

Larger particles are micrometeoroids[13][14], not dust.[15][16] In the absence of a definition from the IAU,[17][18] groups devised their own definitions of dust: smaller than 100 micrometers,[19] 50,[20] 40,[21] 30,[22] and 20 microns,[23] and <10 μm.[24][25][26][16] Some of these dust/micrometeorite definitions are approximate or ambiguous,[27][28][29] some overlapping or self-conflicting.[30][23][22]

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).[31] The IMO noted the new definition,[32] but still displays a prior definition on their site.[33] The Meteoritical Society site retains its prior definition, 0.001 cm.[34] The AMS has posted no rigorous definition.[35][36]

Composition

Dust is generally chondritic in composition. Its monomers contain mafic silicates, such as olivine and pyroxene.[37] Silicates are rich in high-condensation temperature forsterite and enstatite.[27] As these condense quickly, they tend to form very small particles, not merging droplets.

As with chondritic meteoroids, particles contain Fe(Ni) sulfide[38][39] and GEMS (glass with embedded metal and sulfides)[38]

Various amounts of organics (CHON) are present.[40][41][42] 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.[43][44] Some organics are in the form of PAHs (Polycyclic Aromatic Hydrocarbons).[45][19][46][47][48]

Very small inclusions of presolar grains (PSGs) may be found.[27][49]

Dust and comet origin

Microscopic view of comet dust particle

The models for the origin of comets are:[50]

  1. the interstellar model,
  2. the Solar System model,
  3. primordial rubble piles,
  4. aggregation of planetesimals in the dust disk around the Uranus–Neptune region,
  5. 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 mix of ice and dust then aggregated into a comet without appreciable chemical modification. J. Mayo Greenberg first proposed this idea in 1986.

In the 2) Solar System model, the ices that formed in the interstellar cloud first vaporized as part of the accretion disk 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 (> 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.[51]

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, 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

Main article: Accretion (astrophysics)

Dust particles, aided by ices and organics, form "aggregates" [27][38][52] (less often, "agglomerates"[53]) of 30 to hundreds of micrometers. These are fluffy,[19][54] due to the imperfect packing of cluster-type (large) dust particles, and their subsequent, imperfect packing into aggregates.[55]

The next size category is pebbles, of millimeters to centimeters scale.[56][57][58] Pebbles were inferred at 103P/Hartley 2,[59] and imaged directly at 67P/Churyumov-Gerasimenko.[58][56] Astrophysical use of the word "pebble" differs from its geological meaning.[60] In turn, the next-larger geological term, "cobble," has been skipped by Rosetta scientists.[61]

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.[62][63][64]

The building blocks of comets are the putative cometesimals,[65] analogous to planetesimal. Whether the actual cometesimals/planetesimals were pebble-scale,[66] boulder-scale,[67] or otherwise has been a key topic in Solar System and exoplanet research.[54][68][69][70]

(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, understood well by astronomers in the field, but not to the general public, teachers, and scientists from other fields.[71] The larger solids are more properly called "debris"[63] or, for all non-gases, the general "particles"[72][73][44] or "grains."[74][55][22]

Comet 2P/Encke

Main article: Comet Encke

Encke is officially a dust-poor, gas-rich comet.[6][75] Encke actually emits most of its sold mass as meteoroids or "rocks,"[6] not dust. There is no evidence of a classical cometary dust tail due to small particles.[76]

References

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  76. ^ Reach, W; Sykes, M; Lien, D; Davies, J (2000). "The Formation of Encke Meteoroids and Dust Trail". Icarus. 148: 80. "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."
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