Comet dust

Source: Wikipedia, the free encyclopedia.

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

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][48]

Dust and comet origin

Microscopic view of comet dust particle

The models for the origin of comets are:[49]

  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 the 1970s.[50][51]

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.

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.[52]

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][53] (less often, "agglomerates"[54]) of 30 to hundreds of micrometers. These are fluffy,[19][55] due to the imperfect packing of cluster-type (large) dust particles, and their subsequent, imperfect packing into aggregates.[56]

The next size category is pebbles, of millimeters to centimeters scale.

geological meaning.[61] In turn, the next-larger geological term, "cobble," has been skipped by Rosetta scientists.[62]

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.[63][64][65]

The building blocks of comets are the putative cometesimals,[66] analogous to planetesimal. Whether the actual cometesimals/planetesimals were pebble-scale,[67] boulder-scale,[68] or otherwise has been a key topic in Solar System and exoplanet research.[55][69][70][71]

(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.[72] The larger solids are more properly called "debris"[73][74][64] or, for all non-gases, the general "particles"[75][76][44] or "grains."[77][56][22]

Comet 2P/Encke

Main article: Comet Encke

Encke is officially a dust-poor, gas-rich comet.[6][78][79] Encke actually emits most of its solid mass as meteoroids or "rocks,"[6] not dust. ISO measured no infrared evidence of a classical cometary dust tail due to small particles.[80]

References

  1. ^ Mukai, T.; Mukai, S.; Kikouchi, S. (1987). "Variation of Grain Properties and the Dust Outbursts". Symposium on the Diversity and Similarity of Comets, ESA SP-278. European Space Agency. pp. 427–30.
  2. ^ Grun, E.; Massonne; Schwehm, G. (1987). "New Properties of Cometary Dust". Symposium on the Diversity and Similarity of Comets, ESA SP-278. European Space Agency. pp. 305–14.
  3. ^ Fernandez, J. (2005). Comets: Nature, Dynamics, Origins, and their Cosmogonical Relevance. Springer. p. 66.
  4. ^
    S2CID 85917931
    .
  5. ^ a b Fechtig, H. (1982). "Cometary Dust In The Solar System". Comets. Tucson: University of Arizona Press. p. 370.
  6. ^
    ISBN 9780521324403
    .
  7. ^ Dermott, S (2001). "Ch. Orbital evolution of interplanetary dust". In Grün E; Gustafson B; Dermott S; Fechtig H (eds.). Interplanetary Dust. SpringerVerlag. pp. 569–39.
  8. ^ a b Zolensky, M.; Lindstrom, D. (Mar 1991). Mineralogy of 12 large 'chondritic' interplanetary dust particles. 1991 LPSC. pp. 161–69.
  9. ^ Ney, E. (1982). "Optical and Infrared Observations of Bright Comets in the Range 0.5 um to 20 um". Comets. Tucson: University of Arizona Press. p. 323.
  10. ^ Simpson, J.; Rabinowitz, D.; Tuzzolino, A.; Ksanfomality, L. (1986). "Halley's comet dust particle mass spectra, flux distributions and jet structures derived from measurements on the Vega-1 and Vega-2 spacecraft". ESA Proceedings of the 20th ESLAB Symposium on the Exploration of Halley's Comet. Volume 2: Dust and Nucleus. European Space Agency. pp. 11–16.
  11. Bibcode:1983A&A...118..345L
    .
  12. doi:10.1016/0032-0633(83)90006-5
    .
  13. ^ Reach, W.; Sykes, M.; Kelley, M. (2003). "Large Particles From Short-Period Comets". Workshop on Cometary Dust in Astrophysics. Houston: Lunar and Planetary Institute.
  14. ISBN 978-3-540-76959-0
    .
  15. Bibcode:1995QJRAS..36..281B
    . Sec. 4 Lower size limit: Meteoroid or dust?
  16. ^
    S2CID 129972426
    . "...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.""
  17. ^ Millman, P (1961). "A Report on Meteor Terminology". J. R. Astron. Soc. Canada. 55 (6): 265. "particle sizes in general smaller than micrometeorites"
  18. ^ "Resolutions Adopted A. By The General Assembly" (PDF). Retrieved 30 Jun 2020. Sec. "Commission 22 (Meteors and Meteorites/Météores et des Meteorites)"
  19. ^
    S2CID 189789755
    . "tenth micron particles" "very fluffy aggregates"
  20. ^ Klöck, W; Staderman, F (1994). Mineralogical and chemical relationships of interplanetary dust particles, micrometeorites, and meteorites in. LPI Technical Report 94-02 Workshop on the analysis of interplanetary dust particles. "50 um"
  21. doi:10.1016/j.pss.2006.11.014
    . "a radius of 20 um for the upper cut-off"
  22. ^
    S2CID 208527737
    . S.3 Multifaceted Scientific Dust Observations "<~ 30 micrometer"
  23. ^
    doi:10.1016/j.pss.2006.11.014
    . "20 um for the upper cut-off" "50 um for the upper cut-off"
  24. ^ Bradley, J; Sandford, S; Walker, R (1988). "11.1 Interplanetary Dust Particles". Meteorites and the Early Solar System. University of Arizona Press. p. 861. "~ 10 um i diamtr" "~ 10-3 cm in dia"
  25. doi:10.1016/0019-1035(91)90085-8
    . "10 um"
  26. doi:10.1046/j.1365-8711.2003.06478.x
    . "~10 um"
  27. ^
    S2CID 141128
    .
  28. ISBN 978-3-527-32689-1
    . "<100um; typically 0.1-20um"
  29. ^ Folco, L; Cordier, C (2015). "9. Micrometeorites". EMU Notes in Mineralogy. "10 um (Rubin and Grossman, 2010)""in the <100 um size fraction, i.e. across the transition between micrometeorites and IDPs"
  30. ^ Rietmeijer, F (Oct 2002). Mesospheric Metal abundances and Meteoric Dust: Analysis of surviving Meteoroids. 34th COSPAR Scientific Assembly/2nd World Space Congress. "stratospheric interplanetary dust particles (IDPs) (2-100 microns)" "debris from progenitors ~30 to ~1,000 microns"
  31. ^ "Definitions of terms in meteor astronomy" (PDF).
  32. ^ Perlerin, V. (26 September 2017). "Definitions of terms in meteor astronomy (IAU)". Retrieved 30 Jun 2020.
  33. ^ "Glossary". Retrieved 30 Jun 2020.
  34. ^ Benoit, P. "Dust". Archived from the original on 17 June 2020. Retrieved 30 Jun 2020. "0.001 cm in diameter"
  35. ^ "METEOR FAQS". Retrieved 30 Jun 2020.
  36. ^ "Glossary". Retrieved 30 Jun 2020.
  37. S2CID 4303275
    .
  38. ^
    S2CID 25539280
    .
  39. doi:10.1016/0016-7037(95)00329-0
    .
  40. S2CID 122405233
    .
  41. S2CID 186245081
    .
  42. S2CID 4358568
    .
  43. S2CID 4314100
    .
  44. ^
    S2CID 189791473
    .
  45. S2CID 24398934
    .
  46. S2CID 3024593
    .
  47. S2CID 2727481
    .
  48. ^
    S2CID 35413527
    .
  49. ^ Science News 149, June 1, 1996, pp. 346–347.
  50. ^ Greenberg, J (1977). "From dust to comets". Comets, asteroids, meteorites: Interrelations, evolution and origins, Proceedings of the Thirty-Ninth International Colloquium. University of Toledo. p. 491.
  51. ISBN 0816507694
    .
  52. Bibcode:2007prpl.conf..539M
    .
  53. doi:10.1051/0004-6361/201526565
    . "dust grains form fractal aggregates"
  54. doi:10.1093/mnras/stw2898
    .
  55. ^ a b Weissman, P; Asphaug, E; Lowry, S (2004). "Structure and Density of Comet Nuclei". Comets II. University of Arizona Press. p. 337. Tucson "fluffy aggregate"
  56. ^
    PMID 28554979
    . 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"
  57. ^
    S2CID 119230851
    .
  58. S2CID 117797138
    .
  59. ^
    doi:10.1093/mnras/stw1663
    . "cm-sized pebbles"
  60. doi:10.1016/j.icarus.2012.09.030
    . "dust, ice, and hundreds of discrete millimeter to decimeter sized particles."
  61. S2CID 123931232
    . "so the astrophysical use of the word "pebble" differs from its geological meaning."
  62. hdl:10261/150386
    . "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."
  63. doi:10.1093/mnras/stw1959
    .
  64. ^
    doi:10.1093/mnras/stx1620
    . " ejected chunks with diameter bigger than few meters""chunks up to the radius of 0.4 m"
  65. S2CID 119074609
    . "small, decimeter-sized boulders"
  66. S2CID 43461600
    .
  67. doi:10.1051/0004-6361/201630175
    .
  68. Bibcode:2015DPS....4730905W
    .
  69. doi:10.1093/mnras/stx971
    .
  70. ^ Lambrechts, M; Johansen, A (2018). "Forming the cores of giant planets from the radial pebble flux in protoplanetary disks". Astronomy & Astrophysics.
  71. S2CID 212628560
    .
  72. ^ Borovička, J (2016). "About the definition of meteoroid, asteroid, and related terms". WGN, the Journal of the IMO. 44: 31.
  73. ^ Hadjuk, A (1991). "Evolution of Cometary Debris: Physical Aspects". Comets in the Post-Halley Era, Vol. 1. Kluwer. pp. 593–606.
  74. S2CID 52036763
    .
  75. doi:10.1086/116896
    .
  76. doi:10.1016/j.icarus.2012.09.019
    . "clouds of particles resulting from fragmentation of larger aggregates emitted"
  77. S2CID 206634190
    . "grains"
  78. doi:10.1086/113965
    .
  79. doi:10.1086/114689
    . "very low dust content"extremely low dust content"
  80. S2CID 18509697
    .} "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."
Retrieved from "https://en.wikipedia.org/w/index.php?title=Comet_dust&oldid=1193879245"