微陨石
Micrometeorite.jpg
南极的雪地收集的微陨石。

微陨石是在地球表面收集到来自地球之外的小天体,大小范围从50微米至2毫米。微陨石是进入地球大气层而幸存下来的流星尘。它们从大小、组成都与陨石不同,并且数量、种类更为丰富,其中也包括较小的星际尘埃的颗粒(IDPs)[1],是宇宙尘的一部分。流星体以高速(至少11Km/s)进入地球的大气层,经过加热和大气的磨擦和压缩。目前已经在地球上搜集到,来自地球之外个别微陨石的质量在10−9和 10−4公克之间[2]弗雷德·惠普尔首先创造了微陨石这个名称来描述落在地球上如灰尘大小的天体[3]。有时,陨石和微陨石在进入地球大气层时是被看见的流星,但不论它们能否墬落到地球表面被找到,陨石和微陨石依然都存在着。

介绍

微陨石(MM)的纹理会因为原始结构在进入大气层时的加热过程,随着它们的初始速度和进入角度的函数而改变。它们的范围从保留原始矿物学未熔解的颗粒(图1a、b)到部分熔解的微粒(图1c、d)和完全熔解成圆形的宇宙球体(图1e、f、g、h),其中一些经过汽化阶段已经失去了很大部分的质量(图1i)。分类是依据它们的组成和经历的温度[4][5]

图1.不同的微陨石截面分类:a)未溶解的微细颗粒;b)未熔解的粗糙颗粒;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. 除了G和I型的,所有的都富含硅酸盐,称为石质微陨石。尺度棒是50µm。
图2.石质宇宙球体的光学显微镜影像。

由显微分析显示微陨石确实是来自地球之外:

  • 它们所含的金属类似于陨石中的[6]
  • 有些wüstite,一种耐高温的铁氧化物,在陨石熔融的外壳中发现[7]
  • 它们的硅酸盐矿物的比例类似于在陨石中找到的常量和微量元素的比例[8][9]
  • 在铁球体的宇宙锰(53Mn同位素),铝(26Al)内的宇宙铍(10Be同位素),和太阳氖(氖同位素)的丰度与来自外星的石质微流星体相似[10][11]
  • 在微陨石中存在一些前太阳系颗粒[12],和在超碳微陨石中的过量[13],表明它们不仅是天外来客,并且有些还是在太阳系形成之前的原件。

估计每年有30,000 ± 20,000 (t/年)[2]的宇宙尘进入上层大气层,但估计只有少于10%(2700 ± 1400t/yr)的能够成为颗粒降落到地球表面[14]。因此,微陨石的沉积量大约比陨石的量高出50倍,而陨石的量是50t/yr[15],并且每年有为数庞大的微粒(~1017 > 10 µ m)进入大气层,这表明收集的微陨石尘埃微粒来自太阳系的所有天体,包括小行星、彗星,和来自我们的月球和火星的片段碎屑。大微陨石的收集能提供的讯息包括大小、组成、大气的热效应和在地球增生的物质类型,对个别微陨石的详细研究可以洞察它们的起源、原始的氨基酸和包含的前太阳颗粒[16]

收集场所

微陨石的收集来自深海沉积物、沉积岩、和极地沉积物;目前的收集主要来自极地的冰和雪。由于微陨石在地球表面的低浓度,必须在能够浓缩这些材料的环境中寻找它们。

海洋沉积物

在1873年至1876年,英国皇家海军挑战者号首先从深海收集熔化的微陨石(宇宙球体)。在1891年,Murray和Renard发现"两种微陨石族群:第一种是黑色的磁球体,有或没有金属的核心;第二种,有着结晶的结构,棕色的类似陨石球粒的球体"[17]。在1883年,他们建议这些球体是来自地球之外,因为他们发现这些颗粒远不同于地球上其它粒子,它们不像当时从熔炉中山产出来的磁球,而且它们的镍-铁(Fe-Ni)金属核心也不像在火成岩中发现的金属铁。这些球体在缓慢累积的沉积物中有最大的丰度,特别是沉积在碳酸盐补偿深度之下的红黏土,这些发现支持起源于陨石[18]。除了这些球体和铁-镍金属核心,一些大于300µm的球体核心包含铂族元素[19]

自从HMS Challenger的第一次收集之后,宇宙球体已经使用岩心、砂心、蛤壳状挖泥器、磁雪撬等从海洋沉积物中反复的收集到[20]。其中的磁雪橇,被称为"宇宙淤泥的耙子",从太平洋海床顶部的红黏土收集到数以千计的10cm宇宙球体[21]

陆地沉积物

相关条目

参考资料

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进阶读物

  • Castaing, R.; Fredriksson, K., Analysis of Cosmic Spherules with an X-Ray Microanalyser, Geochimica et Cosmochemica Acta, 1958, 14: 114–117, Bibcode:1958GeCoA..14..114C, doi:10.1016/0016-7037(58)90099-1 
  • Dobrica, E.; Engrand, C.; Duprat, J.; Gounelle, M., A statistical overview of Concordia Antarctic micrometeorites, 73rd Meteoritical Society, 2010, 73: pdf 5213, Bibcode:2010M&PSA..73.5213D 
  • Duprat, J. E.; Engrand, C.; Maurette, M.; Gounelle, M.; Hammer, C.; 等, Micrometeorites from Central Antarctic snow: The CONCORDIA collection, Advances in Space Research, 2007, 39 (4): 605–611, Bibcode:2007AdSpR..39..605D, doi:10.1016/j.asr.2006.05.029 
  • Engrand, C.; McKeegan, K. D.; Leshin, L. A., Oxygen isotopic composition of individual minerals in Antarctic micrometeorites: Further links to carbonaceous chondrites, Geochimica et Cosmochemica Acta, 1999, 63 (17): 2623–2636, Bibcode:1999GeCoA..63.2623E, doi:10.1016/S0016-7037(99)00160-X 
  • Flynn, G. J., Atmospheric entry heating: a criterion to distinguish between asteroidal and cometary sources of interplanetary dust, Icarus, 1989, 77 (2): 287–310, Bibcode:1989Icar...77..287F, doi:10.1016/0019-1035(89)90091-2 
  • Genge, M. J.; Grady, M. M.; Hutchison, R., The textures and compositions of fine-grained Antarctic micrometeorites: Implications for comparisons with meteorites, Geochimica et Cosmochemica Acta, 1997, 61 (23): 5149–5162, Bibcode:1997GeCoA..61.5149G, doi:10.1016/S0016-7037(97)00308-6 
  • Goodrich, C. A.; Delaney, J. S., Fe/Mg-Fe/Mn relations of meteorites and primary heterogeneity of primitive achondrite parent bodies, Geochimica et Cosmochemica Acta, 2000, 64: 149–160., Bibcode:2000GeCoA..64..149G, doi:10.1016/S0016-7037(99)00107-6 
  • Gounelle, M.; Chaussidon, M.; Morbidelli, A.; Engrand, C; Zolensky, ME; McKeegan, KD; 等, A unique basaltic micrometeorite expands the inventory of solar system planetary crusts, Proc. Natl. Acad. Sci. U.S.A., 2009, 106 (17): 6904–6909, Bibcode:2009PNAS..106.6904G, PMC 2678474, PMID 19366660, doi:10.1073/pnas.0900328106 
  • Grun, E.; Zook, H. A.; Fechtig, H.; Geise, R. H., Collisional balance of the meteoritic complex, Icarus, 1985, 62 (2): 244–272, Bibcode:1985Icar...62..244G, doi:10.1016/0019-1035(85)90121-6 
  • Harvey, R. P.; Maurette, M., The origin and significance of cosmic dust from the Walcott Neve, Antarctica, Proceedings of Lunar and Planetary Science, 1991, 21: 569–578 
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  • Herzog, G. F.; Xue, S.; Hall, G. S.; Nyquist, L. E.; Shih, C. -Y.; Wiesmann, H.; Brownlee, D. E., Isotopic and elemental composition of iron, nickel, and chromium in type I deep-sea spherules: implications for origin and composition of the parent micrometeoroids, Geochimica et Cosmochimica Acta, 1999, 63 (9): 1443–1457, Bibcode:1999GeCoA..63.1443H, doi:10.1016/S0016-7037(99)00011-3 
  • Imae, N.; Taylor, S.; Iwata, N., Coarse-grained relict minerals in Antarctic micrometeorites: Links to chondrites and comets, Geochimica et Cosmochemica Acta, Accepted 
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  • Matrajt, G.; Pizzarello, S.; Taylor, S.; Brownlee, D., Concentration and variability of the AIB amino acid in polar micrometeorites: Implications for the exogenous delivery of amino acids to the primitive Earth, Meteoritics and Planetary Science, 2004, 39 (11): 1849–1858, Bibcode:2004M&PS...39.1849M, doi:10.1111/j.1945-5100.2004.tb00080.x 
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  • Taylor, S.; Matrajt, G.; Guan, Y., Fine-grained precursors dominate the micrometeorite flux, Meteoritics and Planetary Sciences, 2012, 47 (4): 550–564, Bibcode:2012M&PS...47..550T, doi:10.1111/j.1945-5100.2011.01292.x 
  • Van Ginneken, M.; Folco, L.; Cordier, C.; Rochette, P., Chondritic micrometeorites from the Transantarctic Mountains, Meteoritics and Planetary Sciences, 2012, 47 (2): 228–247, Bibcode:2012M&PS...47..228V, doi:10.1111/j.1945-5100.2011.01322.x 

外部链接

  • Meteoroids Page at NASA's Solar System exploration
  • Current meteorite news articles
  • International Meteorite Collectors Association News and information about meteorite collecting and authentication ethics
  • Planetary Science Research Discoveries: meteorite articles and photographs
  • The British and Irish Meteorite Society
  • Types of extraterrestrial material available for study
  • The Natural History Museum's meteorite catalogue database
  • Meteoritical Society
  • Earth Impact Database