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The origin of micrometeorites (MMs) from asteroids and comets is well-established, but the relative contribution from these two classes remains poorly resolved. Likewise, determining the precise origin of individual MMs is an open challenge. Here, cosmic-ray exposure ages are used to resolve the spatial origins of 12 MMs collected from urban areas and Antarctica. Their 26Al and 10Be concentration, produced during cosmic-ray irradiation in space, were measured by accelerator mass spectrometry. These data are compared to results from a model simulating the transport and irradiation of the MM precursors in space. This model, for the first time, considers a variety of orbits, precursor particle sizes, compositions and densities and incorporates non-isotropic solar and galactic cosmic-ray flux profiles, depth-dependent production rates, as well as spherical evaporation during atmospheric entry. While the origin for six MMs remains ambiguous, two MMs show a preferential tendency towards an origin in the Inner Solar System (Near Earth Objects to the Asteroid Belt) and four towards an origin in the Outer Solar System (Jupiter Family Comets to the Kuiper Belt). These findings challenge the notion that dust originating from the Outer Solar System is unlikely to survive long-term transport and delivery to the terrestrial planets. This article is part of the theme issue 'Dust in the Solar System and beyond'. [ABSTRACT FROM AUTHOR]

We review the mineralogy, petrology, and abundance of petrological type 1 extraterrestrial material. Such material has been completely altered by aqueous processing on its parent bodies. As well as the four meteorite groups that contain type 1 members (CI, CM, CR, and CY), we summarize data from the 2019 fall Flensburg and a recent reanalysis of the "meteorite" Bench Crater found on the Moon, along with fine‐grained micrometeorites, interplanetary dust particles, and xenoliths in meteorites. Type 1 materials exhibit a remarkably high diversity of alteration conditions (temperature, water‐to‐rock [W/R] ratios, and fluid composition) and starting mineralogy. Type 1 material comprises a significant component of the modern extraterrestrial flux to the Earth and was likely common throughout the solar system during the whole course of its history, pointing to both widespread accretion with ices and heating of parent bodies. Type 1 materials are composed predominantly of various phyllosilicates, carbonates, sulfides, and magnetite. Some type 1 materials appear to be part of a "CM clan" typified by serpentine‐rich phyllosilicate compositions and an oxygen isotope composition that falls in the 16O‐rich part of the CM field. Others span a wide range in δ18O (>30‰) and fall on or above the terrestrial fractionation line (+ve Δ17O). Positive Δ17O values are unusual for carbonaceous meteorites but are relatively common in type 1 materials. The wide variation in oxygen isotopes, as well as in textures, mineralogy, and bulk chemistry, points to multiple parent bodies that may originate in the inner and/or outer solar system. Cometary materials, or transition objects such as Main Belt comets or type D asteroids, are likely the source of much of the type 1 materials on Earth but relating them to specific parents requires more study. [ABSTRACT FROM AUTHOR]

This study explores the long‐duration (0.8–2.3 Ma), time‐averaged micrometeorite flux (mass and size distribution) reaching Earth, as recorded by the Transantarctic Mountains (TAM) micrometeorite collection. We investigate a single sediment trap (TAM65), performing an exhaustive recovery and characterization effort and identifying 1,643 micrometeorites (between 100 and 2,000 μm). Approximately 7% of particles are unmelted or scoriaceous, of which 75% are fine‐grained. Among cosmic spherules, 95.6% are silicate‐dominated S‐types, and further subdivided into porphyritic (16.9%), barred olivine (19.9%), cryptocrystalline (51.6%), and vitreous (7.5%). Our (rank)‐size distribution is fit against a power law with a slope of −3.9 (R2 = 0.98) over the size range 200–700 μm. However, the distribution is also bimodal, with peaks centered at ~145 and ~250 μm. Remarkably similar peak positions are observed in the Larkman Nunatak data. These observations suggest that the micrometeorite flux is composed of multiple dust sources with distinct size distributions. In terms of mass, the TAM65 trap contains 1.77 g of extraterrestrial dust in 15 kg of sediment (<5 mm). Upscaling to a global annual estimate gives 1,555 (±753) t/year—consistent with previous micrometeorite abundance estimates and almost identical to the South Pole Water Well estimate (~1,600 t/year), potentially indicating minimal variation in the background cosmic dust flux over the Quaternary. The greatest uncertainty in our mass flux calculation is the accumulation window. A minimum age (0.8 Ma) is robustly inferred from the presence of Australasian microtektites, while the upper age (~2.3 Ma) is loosely constrained based on 10Be exposure dating of glacial surfaces at Roberts Butte (6 km from our sample site). Plain Language Summary: Each year the Earth collects large amounts of extraterrestrial material, with a significant fraction in the form of submillimeter cosmic dust, originating from asteroids and comets. This material adds elements such as nickel and sodium to the Earth's atmosphere, which effect the ionosphere. In the past, cosmic dust may have also played an important role in the origin of life as well as potentially contributing to mass extinction events. Here we examine an ancient (>0.8‐million‐year‐old) micrometeorite collection found among the Transantarctic Mountains. We estimate the quantity of cosmic dust falling to Earth over this time period to be between (approximately) 800 and 2,300 tons per year. In addition, we demonstrate that the cosmic dust flux contains at least two dominant sources with different size distributions. This could reflect the relative contributions from inner solar system asteroids and outer solar system comets. Key Points: Antarctic micrometeorite collections are time‐averaged windows of the cosmic dust flux, in terms of mass, size and compositionThe near‐Earth cosmic dust complex has a bimodal size distribution, as inferred from micrometeorite collections at the Earth's surfaceThe average flux of cosmic dust to Earth over the Quaternary, inferred from the TAM65 trap lies between ~800 and 2,300 t/yr [ABSTRACT FROM AUTHOR]

Introduction: Following low temperature aqueous alteration (<200 °C), a number of CM carbonaceous chondrites experienced post-hydration heating (>300 °C) on their parent body [1-3]. However, the timing, duration and mechanism of the dehydration events remain poorly defined. Constraining the thermal history of the carbonaceous chondrites is an important step to understanding the evolution of water-rich C-complex asteroids, and the distribution and transport of volatiles in the early solar system. Iron sulfides are widespread in carbonaceous chondrites and are highly sensitive to both aqueous alteration and thermal metamorphism. Dehydrated CM chondrites contain a generation of recrystallized sulfides with compositions and textural features diagnostic of their metamorphic heating [4-6]. Here, we have systematically characterized Fesulfide grains within the matrices of dehydrated CM chondrites that experienced a range of peak temperatures (~300 - >750 °C) to constrain the timescales of metamorphism and infer the thermal history of the parent body. Samples & Methods: The composition and morphology of coarse (10 -150 µm) Fe-sulfide grains within polished sections of Jbilet Winselwan (CM2, Stage II, ~300 - 500°C), Dhofar (Dho) 1434 (CMan, Stage III, ~500 - 750°C) and Pecora Escarpment (PCA) 02012 (CM2, Stage IV, >750°C) were characterized using a ZEISS EVO 15LS scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). Backscattered electron (BSE) images, element maps and spot analyses were acquired at 20 kV and 1.5 nA. Results: In Jbilet Winselwan, most (~80%) of the analysed grains are pyrrhotite with inclusions of pentlandite (PoPn), which form either ~5 µm sized oriented laths or patches 10 - 50 µm in size. The pentlandite patches themselves appear to contain ~1 µm sized pyrrhotite inclusions. We also observed individual grains of pyrrhotite (Po) and pentlandite (Pn) in Jbilet Winselwan. In Dho 1434, the grains are either PoPn, pentlandite with inclusions of metal (PnM), or pyrrhotite with inclusions of both pentlandite and metal (PoPnM). The majority of Fe-sulfide grains within PCA 02012 are PoPn and PoPnM grains, although we also found one individual Po grain, and one pyrrhotite grain with metal inclusions (PoM). Unlike the oriented laths observed in Jbilet Winselwan, all of the metal and pentlandite inclusions within Dho 1434 and PCA 02012 are irregular in shape, randomly oriented, and <~15 µm in size. Discussion: Kimura et al. [5] used Fe-sulfide textures and compositions to classify the dehydrated CM chondrites into three categories: A) unheated meteorites, with pentlandite commonly occuring along the boundary of pyrrhotite grains; B) mildly heated (~Stage II-III), characterized by blebs or lamellae of pentlandite in all pyrrhotite grains; C) severely heated (~Stage III-IV), with abundant Ni-rich metal inclusions and an absence of pentlandite exsolution textures. The evolving Fe-sulfide assemblages with increasing peak metamorphic temperature reflect exsolution of pentlandite from a monosulfide solid solution ([Fe,Ni]1-xS) at temperatures <610 °C, and thermal decompositon of pentlandite into Ni-rich metal at temperatures >610 °C [5,6]. The PoPn grains within Jbilet Winselwan are consistent with the mildly heated Category B Fe-sulfides. However, the small pyrrhotite inclusions within the larger patches of pentlandite are similar to the "snowflake texture" in CM and CR chondrites described by Singerling and Brearley [7], who interpreted them as primary Fe-sulfides that condensed from the solar nebula prior to accretion. We therefore suggest that Jbilet Winselwan contains distinct lithologies that experienced varying degrees of metamorphism [2], including at least one lithology that was never heated to temperatures >300°C and still retains primary Fe-sulfide grains from the nebula. In Dho 1434 and PCA 02012, the Fe-sulfide textures are consistent with both the mildly heated Category B and strongly heated Category C sulfides. This does not appear to be attributable to brecciation as the Fe-sulfide grains of different categories are not associated with a particular lithology in either of the sections. Instead, the range of Fesulfide textures could either result from different Fe-sulfide compositions prior to heating, or indicate localized variations (on the order of 1000's of µm) in peak metamorphic temperature. The presence of PnM and PoPnM grains, where pentlandite has only partially broken down into metal, suggests the metamorphic event was too rapid to allow for equilibrium to be maintained. Rapid and potentially heterogeneous heating is more consistent with impact heating rather than heating via solar radiation or radiogenic decay. [ABSTRACT FROM AUTHOR]