Micrometeorites from the northern ice cap of the Novaya Zemlya archipelago,Russia: The first occurrence

Abstract— Glacial deposits at the margins of the ice cap of the northern island of the Novaya Zemlya archipelago, Russia, contain numerous spherules and rare scoriaceous particles thought to be extraterrestrial. The 1 Kyr old glacier has decreased in volume and coverage during the last 40 years, leaving the spherules contained in the ice at the margins of the glacier where they can be easily collected. The spherules are similar in their appearance, texture, and mineralogy to cosmic spherules found in deep‐sea sediments in Greenland and Antarctica. Silicate spherules have typical bar‐like textures (75%) or porphyritic textures (15%), while other spherules are glassy (7%). The spherules from Novaya Zemlya are altered only slightly. There are spherules consisting of iron oxides, metal cores with iron oxide rims, a continuous network of iron oxide dendrites in a glass matrix, and particles rich in chromite (3%). Some spherules contain metal droplets and relict forsterite and low‐Ca pyroxene. Silicate spherule compositions match compositions of other cosmic spherules. Both Nova Zemlya and other cosmic spherules are close to carbonaceous chondrite matrices in patterns of variations for Ca, Mg, Si, and Al, which might suggest that their predecessor was similar to carbonaceous chondrite matrices. Unmelted micrometeorites are generally depleted in Ca and Mg and enriched in Al relative to cosmic spherules. The depletion of the micrometeorites in Ca and Mg can be connected with their terrestrial alteration (Kurat et al. 1994), while the Al enrichment seems to be primary.     

The classification of micrometeorites

Abstract— Due to their small size, the mineralogical and chemical properties of micrometeorites (MMs) are not representative of their parent bodies on the centimeter to meter scales that are used to define parent body groups through the petrological study of meteorites. Identifying which groups of MM are derived from the same type of parent body is problematic and requires particles to be rigorously grouped on the basis of mineralogical, textural, and chemical properties that reflect the fundamental genetic differences between meteorite parent bodies, albeit with minimal bias towards preconceived genetic models. Specifically, the interpretation of MMs requires a rigorous and meaningful classification scheme. At present the classification of MMs is, however, at best ambiguous. A unified petrological‐chemical classification scheme is proposed in the current study and is based on observations of several thousand MMs collected from Antarctic ice.     

The entry heating and abundances of basaltic micrometeorites

Basaltic micrometeorites (MMs) derived from HED‐like parent bodies have been found among particles collected from the Antarctic and from Arctic glaciers and are to date the only achondritic particles reported among cosmic dust. The majority of Antarctic basaltic particles are completely melted cosmic spherules with only one unmelted particle recognized from the region. This paper investigates the entry heating of basaltic MMs in order to predict the relative abundances of unmelted to melted basaltic particles and to evaluate how mineralogical differences in precursor materials influence the final products of atmospheric entry collected on the Earth's surface. Thermodynamic modeling is used to simulate the melting behavior of particles with compositions corresponding to eucrites, diogenites, and ordinary chondrites in order to evaluate degree of partial melting and to make a comparison between the behavior of chondritic particles that dominate the terrestrial dust flux and basaltic micrometeroids. The results of 120,000 simulations were compiled to predict relative abundances and indicate that the phase relations of precursor materials are crucial in determining the relative abundances of particle types. Diogenite and ordinary chondrite materials exhibit similar behavior, although diogenite precursors are more likely to form cosmic spherules under similar entry parameters. Eucrite particles, however, are much more likely to melt due to their lower liquidus temperatures and small temperature interval of partial melting. Eucrite MMs, therefore, usually form completely molten cosmic spherules except at particle diameters <100 μm. The low abundance of unmelted basaltic MMs compared with spherules, if statistically valid, is also shown to be inconsistent with a low velocity population (12 km s^?1) and is more compatible with higher velocities which may suggest a near‐Earth asteroid source dominates the current dust production of basaltic MMs.     

Tomography: A window on the role of sulfur in the structure of micrometeorites

Abstract– To determine the role played by sulfides in the formation of vesicles and FeNi metal beads, we mapped the locations and tabulated the numbers of sulfides, metal beads, and vesicles in 1583 sectioned micrometeorites (MMs) using conventional microscopy and in 190 whole MMs using synchrotron computed microtomography (SCMT). Both the section and the SCMT images show that sulfides melt, coalesce, and migrate to the MMs surface. The decomposition of sulfides may occur during all these stages. Given the sulfide morphologies and compositions that we see in section, we think the breakdown of Ni sulfides produces the FeNi beads. The SCMT images show that metal beads are common in melted MMs, >50% have them. Vesicles in porphyritic and scoriaceous MMs are also probably formed as sulfides decompose. Not only do sulfides abut the vesicles but also the temperatures at which sulfides decompose overlap those at which MM surfaces first melt and temporarily seal, suggesting that S gases could produce most of these vesicles. As the vesicle shapes and patterns of distribution differ among MM classes, tomography can be used to nondestructively screen for specific types of MMs. Tomography is a powerful tool for visualizing the three‐dimensional distribution of metal beads, sulfides, mean densities, and vesicles in MMs.     

X‐ray computed tomography: Morphological and porosity characterization of giant Antarctic micrometeorites

Giant micrometeorites (MMs; 400–2000 µm) are exceedingly rare and scientifically valuable. Three‐dimensional nondestructive characterization by X‐ray computed tomography (X‐CT) provides information on the petrography and thus petrogenesis of MMs and serves as a guide to maximize subsequent multi‐analytical studies on such precious planetary materials. Here, we discuss the results obtained by X‐CT on 22 giant MMs and the classification based on their 3‐D density contrast images. Scoriaceous and unmelted MMs have distinct porosity ranges (10–40 vol% versus 0–25 vol%, respectively). We observe a porosity variation inside scoriaceous MMs, which allows their atmospheric entry flight history to be resolved. For the first time, spinning entry is explicitly demonstrated for four partially melted MMs. Furthermore, we are able to resolve the thermal gradient in a single particle, based on porosity variation (seen as a progressive increase in pore abundance and size with higher peak temperatures). Moreover, we explore parent body alteration through the 3‐D analysis of pores distribution, showing that shock fabrics are either absent or weakly developed in our data set. Finally, owing to the detection of pseudomorphic chondrules, we estimate that the intensively aqueously altered C1 or CI‐like material could represent 18% of the MM flux at this size fraction (400–1000 µm).     

A nuclear microprobe study of the distribution and concentration of carbon and nitrogen in Murchison and Tagish Lake meteorites,Antarctic micrometeorites,and IDPs: Implications for astrobiology

Abstract— Using a nuclear microprobe, we measured the carbon and nitrogen concentrations and distributions in several interplanetary dust particles (IDPs) and Antarctic micrometeorites (MMs), and compared them to 2 carbonaceous chondrites: Tagish Lake and Murchison. We observed that IDPs are richest in both elements. All the MMs studied contain carbon, and all but the coarse‐grained and 1 melted MM contained nitrogen. We also observed a correlation in the distribution of carbon and nitrogen, suggesting that they may be held in an organic material. The implications for astrobiology of these results are discussed, as small extraterrestrial particles could have contributed to the origin of life on Earth by delivering important quantities of these 2 bio‐elements to the Earth's surface and their gas counterparts, CO₂ and N₂, to the early atmosphere.     

Accumulation mechanisms of micrometeorites in an ancient supraglacial moraine at Larkman Nunatak,Antarctica

We report the discovery of a large accumulation of micrometeorites (MMs) in a supraglacial moraine at Larkman Nunatak in the Grosvenor Mountains of the Transantarctic Range in Antarctica. The MMs are present in abundances of ~600 particles kg⁻¹ of moraine sediment and include a near-complete collection of MM types similar to those observed in Antarctic blue ice and within bare-rock traps in the Antarctic. The size distribution of the observed particles is consistent with those collected from snow collections suggesting the moraine has captured a representative collection of cosmic spherules with significant loss of only the smallest particles (<100 μm) by wind. The presence of microtektites with compositions similar to those of the Australasian strewn field suggests the moraine has been accumulating for 780 ka with dust-sized debris. On the basis of this age estimate, it is suggested that accumulation occurs principally through ice sublimation. Direct infall of fines is suggested to be limited by snow layers that act as barriers to accumulation and can be removed by wind erosion. MM accumulation in many areas in Antarctica, therefore, may not be continuous over long periods and can be subject to climatic controls. On the basis of the interpretation of microtektites as Australasian, Larkman Nunatak deposit is the oldest known supraglacial moraine and its survival through several glacial maxima and interglacial periods is surprising. We suggest that stationary ice produced by the specific ice flow conditions at Larkman Nunatak explains its longevity and provides a new type of record of the East Antarctic ice sheet.     

Atmospheric entry heating of micrometeorites at Earth and Mars: Implications for the survival of organics

The atmospheric entry heating of micrometeorites (MMs) can significantly alter their pre‐existing mineralogy, texture, and organic material. The degree of heating depends predominantly on the gravity and atmospheric density of the planet on which they fall. For particles falling on Earth, the alteration can be significant, leading to the destruction of much of the pre‐entry organics; however, the weaker gravity and thinner atmosphere of Mars enhance the survival of MMs and increase the fraction of particles that preserve organic material. This paper investigates the entry heating of MMs on the Earth and Mars in order to examine the MM population on each planet and give insights into the survival of extraterrestrial organic material. The results show that particles reaching the surface of Mars experience a lower peak temperature compared to Earth and, therefore, experience less evaporative mass loss. Of the particles which reach the surface, 68.2% remain unmelted on Mars compared to only 22.8% on Earth. Due to evaporative mass loss, unmelted particles that reach the surface of Earth are restricted to sizes <70 μm whereas particles >475 μm survive unmelted on Mars. Approximately 10% of particles experience temperatures below ~800 K, that is, the sublimation temperature of refractory organics found in MMs. On Earth, this fraction is significantly lower with less than 1% expected to remain below this temperature. Lower peak temperatures coupled with the larger sizes of particles surviving without significant heating on Mars suggest a much higher fraction of organic material surviving to the Martian surface.     

Nitrogen and noble gases in micrometeorites

Abstract— Micrometeorites (MMs) currently represent the largest steady‐state mass flux of extraterrestrial matter to Earth and may have delivered a significant fraction of volatile elements and organics to the Earth's surface. Nitrogen and noble gases contents and isotopic ratios have been measured in a suite of 17 micrometeorites recovered in Antarctica (sampled in blue ice at Cap Prudhomme) and Greenland (separated from cryoconite) that have experienced variable thermal metamorphism during atmospheric entry. MMs were pyrolized using a CO₂ laser and the released gases were analyzed for nitrogen and noble gas abundances and isotopic ratios by static mass spectrometry after specific purification. Noble gases are a mixture of cosmogenic, solar, atmospheric, and possibly chondritic components, with atmospheric being predominant in severely heated MMs. δ¹⁵N values vary between ?240 ± 62‰ and +206 ± 12‰, with most values being within the range of terrestrial and chondritic signatures, given the uncertainties. Crystalline MMs present very high noble gas contents up to two orders of magnitude higher than carbonaceous chondrite concentrations. In contrast, nitrogen contents between 4 ppm and 165 ppm are much lower than those of carbonaceous chondrites, evidencing either initially low N content in MMs and/or degradation of phases hosting nitrogen during atmospheric entry heating and terrestrial weathering. Assuming that the original N content of MMs was comparable to that of carbonaceous chondrites, the contribution of nitrogen delivery by these objects to the terrestrial environment would have been probably marginal from 3.8 Gyr ago to present but could have been significant (?10%) in the Hadean, and even predominant during the latest stages of terrestrial accretion.     

Cosmogenic production rates and recoil loss effects in micrometeorites and interplanetary dust particles

We present a purely physical model to determine cosmogenic production rates for noble gases and radionuclides in micrometeorites (MMs) and interplanetary dust particles (IDPs) by solar cosmic‐rays (SCR) and galactic cosmic‐rays (GCR) fully considering recoil loss effects. Our model is based on various nuclear model codes to calculate recoil cross sections, recoil ranges, and finally the percentages of the cosmogenic nuclides that are lost as a function of grain size, chemical composition of the grain, and the spectral distribution of the projectiles. The main advantage of our new model compared with earlier approaches is that we consider the entire SCR particle spectrum up to 240 MeV and not only single energy points. Recoil losses for GCR‐produced nuclides are assumed to be equal to recoil losses for SCR‐produced nuclides. Combining the model predictions with Poynting‐Robertson orbital lifetimes, we calculate cosmic‐ray exposure ages for recently studied MMs, cosmic spherules, and IDPs. The ages for MMs and the cosmic‐spherule are in the range <2.2–233 Ma, which corresponds, according to the Poynting‐Robertson drag, to orbital distances in the range 4.0–34 AU. For two IDPs, we determine exposure ages of longer than 900 Ma, which corresponds to orbital distances larger than 150 AU. The orbital distance in the range 4–6 AU for one MM and the cosmic spherule indicate an origin either in the asteroid belt or release from comets coming either from the Kuiper Belt or the Oort Cloud. Three of the studied MMs have orbital distances in the range 23–34 AU, clearly indicating a cometary origin, either from short‐period comets from the Kuiper Belt or from the Oort Cloud. The two IDPs have orbital distances of more than 150 AU, indicating an origin from Oort Cloud comets.     

Fine‐grained precursors dominate the micrometeorite flux

Abstract– We optically classified 5682 micrometeorites (MMs) from the 2000 South Pole collection into textural classes, imaged 2458 of these MMs with a scanning electron microscope, and made 200 elemental and eight isotopic measurements on those with unusual textures or relict phases. As textures provide information on both degree of heating and composition of MMs, we developed textural sequences that illustrate how fine‐grained, coarse‐grained, and single mineral MMs change with increased heating. We used this information to determine the percentage of matrix dominated to mineral dominated precursor materials (precursors) that produced the MMs. We find that at least 75% of the MMs in the collection derived from fine‐grained precursors with compositions similar to CI and CM meteorites and consistent with dynamical models that indicate 85% of the mass influx of small particles to Earth comes from Jupiter family comets. A lower limit for ordinary chondrites is estimated at 2–8% based on MMs that contain Na‐bearing plagioclase relicts. Less than 1% of the MMs have achondritic compositions, CAI components, or recognizable chondrules. Single mineral MMs often have magnetite zones around their peripheries. We measured their isotopic compositions to determine if the magnetite zones demarcate the volume affected by atmospheric exchange during entry heating. Because we see little gradient in isotopic composition in the olivines, we conclude that the magnetites are a visual marker that allows us to select and analyze areas not affected by atmospheric exchange. Similar magnetite zones are seen in some olivine and pyroxene relict grains contained within MMs.     

Crumbs from the crust of Vesta: Achondritic cosmic spherules from the South Pole water well

Abstract— Ten glass cosmic spherules (CS) from the South Pole water well collection were analyzed by electron microprobe. Nine of them have Fe/Mn and Fe/Mg ratios in the range typical of chondrites. One of them (SP37‐3), along with up to six other previously analyzed CS, have nonchondritic Fe/Mn and Fe/Mg ratios that agree well with values typical of either (basaltic) howardite, eucrite, and diogenite (HED) meteorites or Martian basalts, but not of lunar samples. SP37‐3 also contains an anorthite relic grain. Anorthite has not previously been reported in cosmic spherules, but is well known in HED meteorites. The much greater frequency of HEDs among hand‐sized meteorites suggests but does not prove that HED precursors are more likely for the nonchondritic spherules. We estimate that HED‐like micrometeorites constitute ～0.5 ± 0.4% of the total population of micrometeorites in the South Pole water well, a fraction that translates to a flux of 1.6 ± 0.3 × 10^?8g HED micrometeorites/m²‐y. The ratio of HED‐like objects to carbonaceous objects is about 100 times less in micrometeorites than among hand‐size specimens. We infer that the comparative mechanical weakness of carbonaceous precursor materials tends to encourage spherule formation.     

Survival of organic phases in porous IDPs during atmospheric entry: A pulse‐heating study

Abstract— In this study, we have performed pulse‐heating experiments at different temperatures for three organic molecules (a polycyclic aromatic hydrocarbon [PAH], a ketone, and an amino acid) absorbed into microporous aluminum oxide (Al₂O₃) in order to imitate the heating of the organic molecules in interplanetary dust particles (IDPs) and micrometeorites (MMs) during atmospheric entry and to investigate their survival. We have shown that modest amounts (a few percent) of these organic molecules survive pulse‐heating at temperatures in the 700 to 900 °C range. This suggests that the porosity in IDPs and MMs, combined with a sublimable phase (organic material, water), produces an ablative cooling effect, which permits the survival of organic molecules that would otherwise be lost either by thermal degradation or evaporation during atmospheric entry.     

Small Antarctic micrometeorites: A mineralogical and in situ oxygen isotope study

Abstract— We have investigated the texture, bulk chemistry, mineralogy, as well as the anhydrous minerals oxygen isotopic composition of 67 small Antarctic micrometeorites (AMMs) collected at Cap Prudhomme, Antarctica, and belonging to the currently poorly studied size fraction 25–50 μm. When compared to larger (50–400 μm) micrometeorites collected at the same site in Antarctica with the same techniques, no significant differences are found between the two populations. We therefore conclude that the population of Cap Prudhomme AMMs is homogeneous over the size range 25–400 μm. In contrast, small AMMs have different textures, mineralogy, and oxygen isotopic compositions than those of stratospheric interplanetary dust particles (IDPs). Because small AMMs (<50 μm) overlap in size with IDPs, the differences between these two important sources of micrometeorites can no longer be attributed to a variation of the micrometeorite composition with size. Physical biases introduced by the collection procedures might account for these differences.     

Concentration and variability of the AIB amino acid in polar micrometeorites: Implications for the exogenous delivery of amino acids to the primitive Earth

Abstract— Micrometeorites (MMs) are extraterrestrial particles ranging in size from 25 μm to 2 mm that survive atmospheric entry and are collected on the Earth's surface. They represent the largest mass flux (MF) of extraterrestrial material (30,000 ± 20,000 t/yr) to the present‐day Earth. Studies of large collections of MMs suggest that about 20% have not been heated to high temperatures and that they contain organic carbon. Since non‐protein amino acids have been found in some carbonaceous meteorites, they might also be found in unmelted MMs. However, previous searches for amino acids in MMs were inconclusive. We combined a new extraction method for amino acids with a highly sensitive analytical method to detect and quantitate amino acids in MMs collected at the South Pole. We found the non‐protein amino acid α‐amino isobutyric acid (AIB) in one of our samples. The non‐detection of this amino acid in the other samples analyzed suggests that there are amino acid‐containing and amino acid‐free MMs, with ?14% of the MMs containing AIB. Since the MF of MMs is much higher than that of carbonaceous chondrites (CMs), amino acids in these small particles would represent an important source of exogenous delivery of organic molecules. Therefore, the results are discussed on the basis of their implications for astrobiology.     

Mineralogy,chemistry, and oxygen isotopes of refractory inclusions from stratospheric interplanetary dust particles and micrometeorites

Abstract— Calcium, aluminum-rich inclusions (CAIs) are characteristic components in carbonaceous chondrites. Their mineralogy is dominated by refractory oxides and silicates like corundum, perovskite, spinel, hibonite, melilite, and Ca-pyroxene, which are predicted to be the first phases to have condensed from the cooling solar nebula. Allowing insights into processes occurring in the early solar system, CAIs in carbonaceous and ordinary chondrites were studied in great detail, whereas only a few refractory inclusions were found and studied in stratospheric interplanetary dust particles (IDPs) and micrometeorites. This study gives a summary of all previous studies on refractory inclusions in stratospheric IDPs and micrometeorites and will present new data on two Antarctic micrometeorites. The main results are summarized as follows: (a) Eight stratospheric IDPs and six micrometeorites contain Ca, Al-rich inclusions or refractory minerals. The constituent minerals include spinel, perovskite, fassaite, hibonite, melilite, corundum, diopside and anorthite. (b) Four of the seven obtained rare-earth-element (REE) patterns from refractory objects in stratospheric IDPs and micrometeorites are related to Group III patterns known from refractory inclusions from carbonaceous chondrites. A Group II related pattern was found for spinel and perovskite in two micrometeorites. The seventh REE pattern for an orthopyroxene is unique and can be explained by fractionation of Gd, Lu, and Tb at highly reducing conditions. (c) The O-isotopic compositions of most refractory objects in stratospheric IDPs and micrometeorites are similar to those of constituents from carbonaceous chondrites and fall on the carbonaceous chondrites anhydrous minerals mixing line. In fact, in most cases, in terms of mineralogy, REE pattern and O-isotopic composition of refractory inclusions in stratospheric IDPs and micrometeorites are in good agreement with a suggested genetic relation of dust particles and carbonaceous chondrites. Only in the case of one Antarctic micrometeorite does the REE pattern obtained for an orthopyroxene point to a link of this particle to enstatite chondrites.     

The origins of I‐type spherules and the atmospheric entry of iron micrometeoroids

The Earth's extraterrestrial dust flux includes a wide variety of dust particles that include FeNi metallic grains. During their atmospheric entry iron micrometeoroids melt and oxidize to form cosmic spherules termed I‐type spherules. These particles are chemically resistant and readily collected by magnetic separation and are thus the most likely micrometeorites to be recovered from modern and ancient sediments. Understanding their behavior during atmospheric entry is crucial in constraining their abundance relative to other particle types and the nature of the zodiacal dust population at 1 AU. This article presents numerical simulations of the atmospheric entry heating of iron meteoroids to investigate the abundance and nature of these materials. The results indicate that iron micrometeoroids experience peak temperatures 300–800 K higher than silicate particles explaining the rarity of unmelted iron particles which can only be present at sizes of <50 μm. The lower evaporation rates of liquid iron oxide leads to greater survival of iron particles compared with silicates, which enhances their abundance among micrometeorites by a factor of 2. The abundance of I‐types is shown to be broadly consistent with the abundance and size of metal in ordinary chondrites and the current day flux of ordinary chondrite‐derived MMs arriving at Earth. Furthermore, carbonaceous asteroids and cometary dust are suggested to make negligible contributions to the I‐type spherule flux. Events involving such objects, therefore, cannot be recognized from I‐type spherule abundances in the geological record.     

Rare,metal micrometeorites from the Indian Ocean

Metal in various forms is common in almost all meteorites but considerably rare among micrometeorites. We report here the discovery of two metal micrometeorites, i.e., (1) an awaruite grain similar to those found in the metal nodules of CV chondrites and (2) a metal micrometeorite of kamacite composition enclosing inclusions of chromite and merrillite. This micrometeorite appears to be a fragment of H5/L5 chondrite. These metal micrometeorites add to the inventory of solar system materials that are accreted by the Earth in microscopic form. They also strengthen the argument that a large proportion of material accreted by the Earth that survives atmospheric entry is from asteroidal sources.     

Chondrules in Antarctic micrometeorites

Abstract— Previous studies of unmelted micrometeorites (>50 μm) recovered from Antarctic ice have concluded that chondrules, which are a major component of chondritic meteorites, are extremely rare among micrometeorites. We report the discovery of eight micrometeorites containing chondritic igneous objects, which strongly suggests that at least a portion of coarse‐grained crystalline micrometeorites represent chondrule fragments. Six of the particles are identified as composite micrometeorites that contain chondritic igneous objects and fine‐grained matrix. These particles suggest that at least some coarse‐grained micrometeorites (cgMMs) may be derived from the same parent bodies as fine‐grained micrometeorites. The new evidence indicates that, contrary to previous suggestions, the parent bodies of micrometeorites broadly resemble the parent asteroids of chondrulebearing carbonaceous chondrites.