Abstract Micrometeorites (MMs) provide constraints on the flux and sources of extraterrestrial dust falling on Earth as well as recording the processes occurring during atmospheric entry. Collections of micrometeorites have been recovered from a wide variety of environments including Antarctic moraine, rock traps, ice and snow and on roof tops in urban areas. Studies of the mineralogy and composition of MMs suggest that most particles (>98%) >50 ​μm in diameter have asteroidal sources, whilst ~50% of particles smaller than 50 ​μm are likely to be derived from comets. The relative abundance of S(IV)-type asteroid materials, similar to ordinary chondrites increases with size, although C-type asteroidal materials, similar to carbonaceous chondrites dominate over all. Although MMs provide excellent evidence on the nature and abundance of extraterrestrial dust at the Earth’s orbit they are not without bias and uncertainty. Mineralogical and compositional change during atmospheric entry makes the exact nature of their precursors uncertain complicating evaluation of source beyond basic classes of material. This is particularly true at larger sizes when complete melting to form cosmic spherules occurs, however, unmelted MMs >50 ​μm in size are also often thermally altered. Mixing with atmospheric oxygen and mass fractionation by evaporation furthermore complicates the use of oxygen isotope compositions in identifying parent bodies. All MM collections are suggested to exhibit biases owing to: (1) collection method, (2) terrestrial weathering, (3) terrestrial contamination, and (4) erosion and deposition by terrestrial surface processes. Even in the least biased collections, those collected by dedicated melting of Antarctic snow, erosive loss of material is suggested here to make fluxes uncertain by factors of up to ~2. The abundance of asteroid-derived MMs observed in collections contradicts models of the orbital evolution of interplanetary dust to Earth, which suggests >70% should be provided by comets.

中文翻译：

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微陨石：对地球外星尘埃的通量、来源和大气进入的洞察

摘要 微陨石 (MMs) 限制了落在地球上的外星尘埃的通量和来源，并记录了大气进入过程中发生的过程。已经从各种环境中回收了微陨石的集合，包括南极冰碛、岩石陷阱、冰雪和城市地区的屋顶。对 MM 的矿物学和组成的研究表明，大多数直径大于 50 μm 的粒子 (>98%) 具有小行星来源，而约 50% 的小于 50 μm 的粒子可能来自彗星。类似于普通球粒陨石的 S(IV) 型小行星材料的相对丰度随着尺寸的增加而增加，尽管类似于碳质球粒陨石的 C 型小行星材料占主导地位。尽管 MM 为地球轨道上外星尘埃的性质和丰度提供了极好的证据，但它们并非没有偏见和不确定性。进入大气期间的矿物学和成分变化使得其前体的确切性质变得不确定，这使得对超出基本材料类别的来源的评估变得复杂。当完全熔化形成宇宙小球时，在较大尺寸下尤其如此，但是，尺寸 >50 μm 的未熔化 MM 也经常发生热改变。通过蒸发与大气氧混合和质量分馏进一步使氧同位素组成在识别母体中的使用复杂化。由于以下原因，建议所有 MM 收集都表现出偏差：(1) 收集方法，(2) 陆地风化，(3) 陆地污染，(4) 陆地表面过程的侵蚀和沉积。即使在偏差最小的收集中，那些通过南极雪的专门融化收集的，这里也建议材料的侵蚀性损失使通量不确定性高达约 2 倍。在收藏中观察到的大量小行星衍生的 MM 与行星际尘埃到地球的轨道演化模型相矛盾，这表明 > 70% 应该由彗星提供。