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Micrometeorites are 50µm to 2mm extraterrestrial particles collected on the Earth’s surface. They are micrometeoroids, which have survived entry through the Earth's atmosphere. They differ from meteorites in being smaller, more plentiful and different in composition and are a subset of cosmic dust, which also includes the smaller interplanetary dust particles (IDPs).[1] Micrometeorites enter the Earth’s atmosphere with high velocities (at least 11 km/s) and undergo frictional heating as they collide with air molecules. Individual micrometeorites weigh between 10-9 and 10-4 g and collectively contribute most of the extraterrestrial material that has come to the present day Earth.[2] Fred Whipple first coined the term “micro-meteorite” to describe dust-sized objects that fall to the Earth.[3] Sometimes meteoroids and micrometeoroids entering the earth's atmosphere are visible as "shooting stars", whether or not they reach the ground and survive as meteorites and micrometorites.

Introduction

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Micrometeorite (MM) textures vary as their original structural and mineral compositions are modified by the degree of heating that they experience entering the atmosphere—a function of their initial speed and angle of entry. They range from unmelted particles that retain their original mineralogy (Fig.1 a,b), to partially melted particles (Fig.1 c, d) to round melted cosmic spherules (Fig.1 e, f, g, h, Fig. 2) some of which have lost a large portion of their mass through vaporization (Fig.1 i). Classification is based on composition and degree of heating.[4][5]

Figure 1. Cross sections of different micrometeorite classes: a) Fine-grained unmelted; b) Coarse-grained Unmelted; c) Scoriaceous; d) Relict-grain Bearing; e) Porphyritic; f) Barred olivine; g) Cryptocrystalline; h) Glass; i) CAT; j) G-type; k) I-type; and l) Single mineral. Except for G- and I-types all are silicate rich, called stony MMs. Scale bars are 50µm.
Figure 2. Light microscope images of stony cosmic spherules. Largest spherule is about 300µm in diameter.

The extraterrestrial origins of micrometeorites are determined by microanalyses which show that:

  • The metal they contain is similar to that found in meteorites.[6] 2)
  • Some have wüstite, a high-temperature iron oxide found in meteorite fusion crusts.[7] 3)
  • Their silicate minerals have major and trace elements ratios similar to those in meteorites.[8][9]
  • The abundances of cosmogenic manganese (53Mn) in iron spherules and of cosmogenic beryllium (10Be), aluminum (26Al), and solar neon isotope in stony MMs are extraterrestrial[10][11]
  • The presence of pre-solar grains in some MMs[12] and deuterium excesses in ultra-carbonaceous MMs[13] indicates that they are not only extraterrestrial but that some of their components formed before our solar system.

About 30,000 ± 20,000 tons/yr[2] of cosmic dust enters the upper atmosphere each year of which ~10%, (2700 ± 1400 tons/yr reaches the surface as particles.[14] The mass deposited is roughly 50 times higher than that estimated for meteorites (~50 ton/yr),[15] and the huge number of particles entering the atmosphere each year (~1017 > 10µm) suggests that large MM collections contain particles from all dust producing objects in the solar system including asteroids, comets, and fragments from our Moon and Mars. Large MM collections provide information on the size, composition, atmospheric heating effects and types of materials accreting on Earth while detailed studies of individual MMs give insights into their origin, the nature of the carbon, amino acids and pre-solar grains they contain.

Collection sites

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Micrometeorites have been collected from deep-sea sediments, sedimentary rocks and polar sediments; they are currently collected primarily from polar snow and ice. Because of their low concentrations on the Earth’s surface, MMs are sought in environments that concentrate these materials relative to terrestrial particles.

Ocean sediments

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Melted micrometeorites (cosmic spherules) were first collected from deep-sea sediments during the 1873 to 1876 expedition of the HMS Challenger. In 1891, Murray and Renard found “two groups [of micrometeorites]: first, black magnetic spherules, with or without a metallic nucleus; second, brown-coloured spherules resembling chondr(ul)es, with a crystalline structure.”[16] In 1883, they had suggested that these spherules were extraterrestrial because they were found far from terrestrial particle sources, they did not resemble magnetic spheres produced in furnaces of the time, and their nickel-iron (Fe-Ni) metal cores did not resemble metallic iron found in volcanic rocks. The spherules were most abundant in slowly accumulating sediments, particularly red clays deposited below the carbonate compensation depth, a finding that supported a meteoritic origin.[17]

Since the first collection of the HMS Challenger, cosmic spherules have been recovered from ocean sediments using cores, box cores, clamshell grabbers, and magnetic sleds.[18] Among these a magnetic sled, called the "Cosmic Muck Rake", retrieved thousands of cosmic spherules from the top 10 cm of pelagic red clays on the Pacific Ocean floor.[19]

Terrestrial sediments

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Terrestrial sediments also contain micrometeorites. These have been found in samples that:

The oldest MMs are totally altered iron spherules found in 140 to 180 million-year-old hardgrounds.[21]

Polar depositions

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Micrometeorites found in polar sediments are much less weathered than those found in other terrestrial environments, as evidenced by little etching of interstitial glass, and the presence of large numbers of glass spherules and unmelted micrometeorites, particle types that are rare or absent in deep-sea samples.[4] The MMs found in Polar Regions have been collected from Greenland snow,[26] Greenland cryoconite,[27][28][29] Antarctic blue ice[30] Antarctic aeolian (wind-driven) debris,[31][32][33] ice cores,[34] the bottom of the South Pole water well,[14][4] Antarctic sediment traps[35] and present day Antarctic snow.[13]

Click here to see an eight-minute movie of MMs being collected from the bottom of the South Pole drinking water well.

Classification and origins of micrometeorites

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Classification

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Modern classification of meteorites and micrometeorites is complex; the 2007 review paper of Krot et al.[36] summarizes modern meteorite taxonomy. Linking individual micrometeorites to meteorite classification groups requires a comparison of their elemental, isotopic and textural characteristics.

Comet vs asteroid origin of micrometeorites

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Whereas most meteorites likely originate from asteroids, the contrasting makeup of micrometeorites suggests that they originate from comets.

Fewer than 1% of MMs are achondritic and are similar to HED meteorites, which are thought to be from the asteroid, 4 Vesta.[37][38] Most MMs are compositionally similar to carbonaceous chondrites,[39][40][41] whereas approximately 3% of meteorites are of this type.[42] The dominance of carbonaceous chondrite-like MMs and their low abundance in meteorite collections suggests that most MMs derive from sources different than those for most meteorites. Since most meteorites probably derive from asteroids, an alternative source for MMs might be comets. The idea that MMs might originate from comets originated in 1950.[3] However, until recently the greater-than-25-km/s entry velocities of micrometeoroids, measured for particles from comet streams, cast doubts against their survival as MMs.[10][43] However, recent dynamical simulations[44] suggest that 85% of cosmic dust could be cometary. Furthermore, analyses of particles returned from the comet, Wild 2, by the Stardust spacecraft show that these particles have compositions that are consistent with many micrometeorites.[45][46]

References

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  1. ^ Brownlee, D.E.; Bates, B.; Schram, L. (1997), "The elemental composition of stony cosmic spherules", Meteoritics and Planetary Science, 32: 157–175
  2. ^ a b Love, S.G.; Brownlee, D.E. (1993). "A direct measurement of the terrestrial mass accretion rate of cosmic dust". Science. 262: 550–553.
  3. ^ a b Whipple, Fred (1950), "The Theory of Micro-Meteorites", National Academy of Sciences: 687–695
  4. ^ a b c Taylor, S.; Lever, J.H.; Harvey, R.P. (2000), "Numbers, Types and Compositions of an Unbiased Collection of Cosmic Spherules", Meteoritics & Planetary Science, 35: 651–666
  5. ^ Genge, M.J.; Engrand, C.; Gounelle, M.; Taylor, S. (2008), "The Classification of Micrometeorites", Meteoritics and Planetary Sciences, 43: 497–515
  6. ^ Smales, A.A.; Mapper, D.; Wood, A.J. (1958), "Radioactivation analysis of "cosmic" and other magnetic spherules", Geochemica et Cosmochemica, Acta 13: 123–126
  7. ^ a b Marvin, U.B.; Einaudi, M.T. (1967), "Black, Magnetic Spherules from Pleistocene and Recent beach sands", Geochim. Cosmochim., Acta 31: 1871–1884
  8. ^ Blanchard, M.B.; Brownlee, D.E.; Bunch, T.E.; Hodge, P.W.; Kyte, F.T. (1980), "Meteoroid ablation spheres from deep-sea sediments", Earth Planet. Sci., Lett. 46: 178–190
  9. ^ Ganapathy, R.; Brownlee, D.E.; Bunch, T.E.; Hodge, P.W. (1978), "Silicate spherules from deep-sea sediments: Confirmation of extraterrestrial origin", Science, 201: 1119–1121
  10. ^ a b Raisbeck G.M., F., D. and M. (). 21,, G.M.; Yiou, F.; Bourles, D.; Maurette, M. (1986), "10Be and 26Al in Greenland cosmic spherules: Evidence for irradiation in space as small objects and a probable cometary origin", Meteoritics, 21: 487–488{{citation}}: CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)
  11. ^ Nishiizumi, K.; Arnold, J.R.; Brownlee, D. E.; et al. (1995), "10Be and 26Al in individual cosmic spherules from Antarctica", Meteoritics, 30: 728–732 {{citation}}: Cite has empty unknown parameter: |month= (help); Explicit use of et al. in: |last4= (help)
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  13. ^ a b Duprat, J.E.; Dobrica, C.; Engrand, J.; Aleon, Y.; et al. (2010), "Extreme Deuterium excesses in ultracarbonaceous Micrometeorites from Central Antarctic Snow", Science, 328: 742–745 {{citation}}: Explicit use of et al. in: |last5= (help)
  14. ^ a b Taylor, S.; Lever, J.H.; Harvey, R.P. (1998), "Accretion rate of cosmic spherules measured at the South Pole", Nature, 392: 899–903
  15. ^ Zolensky M., P., P., and I. () , In . Eds. ; ,, M.; Bland, M.; Brown, P.; Halliday, I. (2006), "Flux of extraterrestrial materials", in Lauretta, Dante S.; McSween, Harry Y. (eds.), Meteorites and the Early Solar System II, Tucson: University of Arizona Press{{citation}}: CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link)
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  17. ^ Murray, J.; Renard, A.F. (1883), "On the microscopic characters of volcanic ashes and cosmic dust, and their distribution in deep-sea deposits", Proc. Roy. Soc., 12, Edinburgh: 474–495
  18. ^ Brunn, A.F.; Langer, E.; Pauly, H. (1955), "Magnetic particles found by raking the deep-sea bottom", Deep-Sea Research, 2: 230–246
  19. ^ Brownlee, D.E.; Pilachowski, L.B.; Hodge, P.W. (1979), "Meteorite mining on the ocean floor (abstract)", Lunar Planet. Sci., X: 157–158
  20. ^ Crozier, W.D. (1960), "Black, magnetic spherules in sediments", Jounal of Geophysical Research, 65: 2971–2977
  21. ^ a b Czajkowski, J.; Englert, P.; Bosellini, A.; Ogg, J.G. (1983), "Cobalt enriched hardgrounds- new sources of ancient extraterrestrial materials", Meteoritics, 18: 286–287
  22. ^ Jehanno, C.; Boclet, D.; Bonte, Ph.; Castellarin, A.; Rocchia, R. (1988), "Identification of two populations of extraterrestrial particles in a Jurassic hardground of the Southern Alps", Proc. Lun. Planet. Sci. Conf., 18: 623–630
  23. ^ Mutch, T.A. (1966), "Abundance of magnetic spherules in Silurian and Permian salt samples", Earth and Planetary Science Letters, 1: 325–329
  24. ^ Taylor, S.; Brownlee, D.E. (1991), "Cosmic spherules in the geologic record", Meteoritics, 26: 203–211
  25. ^ Fredriksson, K.; Gowdy, R. (1963), "Meteoritic debris from the Southern California desert", Geochim. Cosmochim, Acta, 27: 241–243
  26. ^ Langway, C.C. (1963), "Sampling for extra-terrestrial dust on the Greenland Ice Sheet", Union Geodesique et Geophysique Internationale, Association Internationale d’Hydrologie Scientific, 61, Berkeley Symposium: 189–197
  27. ^ Wulfing, E.A. (1890), "Beitrag zur Kenntniss des Kryokonit", Neus Jahrb. Für Min., etc., 7: 152–174
  28. ^ Maurette, M.; Hammer, C.; Brownlee, D.E.; et al. (1986), "Placers of cosmic dust in the blue ice lakes of Greenland", Science, 233: 869–872 {{citation}}: Explicit use of et al. in: |last4= (help)
  29. ^ Maurette, M.; Jehanno, C.; Robin, E.; Hammer, C. (1987), "Characteristics and mass distribution of extraterrestrial dust from the Greenland ice cap", Nature, 328: 699–702
  30. ^ Maurette, M.; Olinger, C.; Christophe Michel-Levy, M.; et al. (1991), "A collection of diverse micrometeorites recovered from 100 tonnes of Antarctic blue ice", Nature, 351: 44–47 {{citation}}: Explicit use of et al. in: |last4= (help)
  31. ^ Koeberl, C.; Hagen, E.H. (1989), "Extraterrestrial spherules in glacial sediment from the Transantarctic Mountains, Antarctica: Structure, mineralogy and chemical composition", Geochimica et Cosmochimica, Acta 53: 937–944
  32. ^ Hagen, E.H.; Koeberl, C.; Faure, G. (1990), "Extraterrestrial spherules in glacial sediment, Beardmore Glacier area, Transantarctic Mountain", Anatarctic Research Series, 50: 19–24 {{citation}}: Cite has empty unknown parameter: |month= (help)
  33. ^ Koeberl, C.; Hagen, E.H. (1989), "Extraterrestrial spherules in glacial sediment from the Transantarctic Mountains, Antarctica: Structure, mineralogy and chemical composition.", Geochimica et Cosmochimica, Acta 53, : 937–944{{citation}}: CS1 maint: extra punctuation (link)
  34. ^ Yiou, F.; Raisbeck, G.M. (1987), "Cosmic spherules from an Antarctic ice core", Meteoritics, 22: 539–540
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  36. ^ Krot, A.N.; Keil, K.; Scott, E.R.D.; Goodrich, C.A.; Weisberg, M.K. (2007). "1.05 Classification of Meteorites". In Holland; Turekian, Karl K. (eds.). Treatise on Geochemistry. Vol. 1. Elsevier Ltd. pp. 83–128. doi:10.1016/B0-08-043751-6/01062-8. ISBN 978-0-08-043751-4. {{cite book}}: More than one of |editor= and |editor1-last= specified (help)
  37. ^ Taylor, S.; Herzog, G.F.; Delaney, J.S. (2007), "Crumbs from the crust of Vesta: Achondritic cosmic spherules from the South Pole water well", Meteoritics and Planetary Sciences, 42: 223–233
  38. ^ Cordier, C.; Folco, L.; Taylor, S. (2011), "Vestoid cosmic spherules from the South Pole Water Well and Transantarctic Mountains (Antarctica): A major and trace element study", Geochimica et Cosmochemica, Acta 75: 1199–1215
  39. ^ Kurat, G.; Koeberl, C.; Presper, T.; et al. (1994), "Petrology and geochemistry of Antarctic micrometeorites", Geochimica et Cosmochimica, Acta 58, : 3879–3904 {{citation}}: Explicit use of et al. in: |last4= (help)CS1 maint: extra punctuation (link)
  40. ^ Beckerling, W.; Bischoff, A. (1995), "Occurance and composition of relict minerals in micrometeorites from Greenland and Antarctica-implications for their origins", Planetary and Space Science, 43: 435–449
  41. ^ Greshake, A.; Klock, W.; Arndt, P.; et al. (1998), "Heating experiments simulating atmospheric entry heating of micrometeorites: Clues to their parent body sources", Meteoritics & Planetary Science, 33: 267–290 {{citation}}: Explicit use of et al. in: |last4= (help)
  42. ^ Sears, D.W.G. (1998), "The Case for Rarity of Chondrules and Calcium-Aluminum-rich Inclusions in the Early Solar System and Some Implications for Astrophysical Models", Astrophysical Journal, 498: 773–778
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  45. ^ Brownlee, D.E.; et al. (2006), "Comet 81P/Wild 2 Under a Microscope", Science, 314: 1711–1716 {{citation}}: Explicit use of et al. in: |last2= (help)
  46. ^ Joswiak, D.J.; Brownlee, D.E.; Matrajt, G.; et al. (2012), "Comprehensive examination of large mineral and rock fragments in Stardust tracks: Mineralogy, analogous extraterrestrial materials, and source regions", Meteoritics & Planetary Science, 47: 471–524 {{citation}}: Explicit use of et al. in: |last4= (help)CS1 maint: extra punctuation (link)

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