54,177 research outputs found
The Extraterrestrial Dust Flux: Size Distribution and Mass Contribution Estimates Inferred From the Transantarctic Mountains (TAM) Micrometeorite Collection
No full textThis 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)
Isotopically Heavy Micrometeorites—Fragments of CY Chondrite or a New Hydrous Parent Body?
No full textCosmic dust grains sample a diverse range of solar system small bodies. This includes asteroids that are not otherwise represented in our meteorite collections. In this work we obtained 3D images of micrometeorite interiors using tomography before collecting destructive high-precision oxygen isotope measurements. These data allow us to link textures in unmelted micrometeorites to known chondrite groups. In addition to identifying particles from ordinary chondrites, CR and CM chondrites we report two micrometeorites derived from an anomalous O-16-poor source (delta O-17: +16.4 parts per thousand, delta O-18: +28.4 parts per thousand, and Delta O-17: +1.4 parts per thousand). Their compositions overlap with a previously reported micrometeorite (TAM50-25) from Suttle et al. (2020), (EPSL: 546:116444). These particles represent hydrated carbonaceous chondrite material derived either from a new group or from the CY chondrites (thereby extending the isotopic range of this group). In either scenario they demonstrate close petrographic and isotopic connections to the CO-CM chondrite clan. Furthermore, their position in O-isotope space makes them the most likely candidate for the parent body of the anomalous "group 4" cosmic spherules previously reported by Suavet et al. (2010), (EPSL: 293:313-320) and several subsequent isotopic studies. We conclude that the "group 4" cosmic spherules originate from hydrated C-type asteroid parents
The Thermal Decomposition of Fine-grained Micrometeorites, Observations from Mid-IR Spectroscopy
No full textWe analysed 44 fine-grained and scoriaceous micrometeorites. A bulk mid-IR spectrum (8–13 lm) for each grain was collected
and the entire micrometeorite population classified into 5 spectral groups, based on the positions of their absorption
bands. Corresponding carbonaceous Raman spectra, textural observations from SEM-BSE and bulk geochemical data via
EMPA were collected to aid in the interpretation of mid-IR spectra. The 5 spectral groups identified correspond to progressive
thermal decomposition. Unheated hydrated chondritic matrix, composed predominantly of phyllosilicates, exhibit smooth,
asymmetric spectra with a peak at 10 lm. Thermal decomposition of sheet silicates evolves through dehydration, dehydroxylation,
annealing and finally by the onset of partial melting. Both CI-like and CM-like micrometeorites are shown to pass
through the same decomposition stages and produce similar mid-IR spectra. Using known temperature thresholds for each
decomposition stage it is possible to assign a peak temperature range to a given micrometeorite. Since the temperature thresholds
for decomposition reactions are defined by the phyllosilicate species and the cation composition and that these variables
are markedly different between CM and CI classes, atmospheric entry should bias the dust flux to favour the survival of CIlike
grains, whilst preferentially melting most CM-like dust. However, this hypothesis is inconsistent with empirical observations
and instead requires that the source ratio of CI:CM dust is heavily skewed in favour of CM material. In addition, a small
population of anomalous grains are identified whose carbonaceous and petrographic characteristics suggest in-space heating
and dehydroxylation have occurred. These grains may therefore represent regolith micrometeorites derived from the surface
of C-type asteroids. Since the spectroscopic signatures of dehydroxylates are distinctive, i.e. characterised by a reflectance
peak at 9.0–9.5 lm, and since the surfaces of C-type asteroids are expected to be heated via impact gardening, we suggest that
future spectroscopic investigations should attempt to identify dehydroxylate signatures in the reflectance spectra of young carbonaceous
asteroid families
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Dynamics And Distribution Of Cyanophages And Their Effect On Marine Synechococcus Spp
No full textCyanophages infecting marine Synechococcus cells were frequently very abundant and were found in every seawater sample along a transect in the western Gulf of Mexico and during a 28-month period in Aransas Pass, Tex. In Aransas Pass their abundance varied seasonally, with the lowest concentrations coincident with cooler water and lower salinity. Along the transect, viruses infecting Synechococcus strains DC2 and SYN48 ranged in concentration from a few hundred per milliliter at 97 m deep and 83 km offshore to ca. 4 x 10(5) ml(-1) near the surface at stations within 18 km of the coast. The highest concentrations occurred at the surface, where salinity decreased from ca. 35.5 to 34 ppt and Synechococcus concentrations were greatest. Viruses infecting strains SNC1, SNC2, and 838BG were distributed in a similar manner but were much less abundant (<10 to >5 x 10(3) ml(-1)). When Synechococcus concentrations exceeded ca. 10(3) ml(-1), cyanophage concentrations increased markedly (ca. 10(2) to > 10(5) ml(-1)), suggesting that a minimum host density was required for efficient viral propagation. Data on the decay rate of viral infectivity d (per day), as a function of solar irradiance I (millimoles of quanta per square meter per second), were used to develop a relationship (d = 0.2610I-0.00718; r(2) = 0.69) for conservatively estimating the destruction of infectious viruses in the mixed layer of two offshore stations. Assuming that virus production balances losses and that the burst size is 250, ca. 5 to 7% of Synechococcus cells would be infected daily by viruses. Calculations based on contact rates between Synechococcus cells and infectious viruses produce similar results (5 to 14%). Moreover, balancing estimates of viral production with contact rates for the farthest offshore station required that most Synechococcus cells be susceptible to infection, that most contacts result in infection, and that the burst size be about 324 viruses per lytic event. In contrast, in nearshore waters, where ca. 80% of Synechococcus cells would be contacted daily by infectious cyanophages, only ca. 1% of the contacts would have to result in infection to balance the estimated virus removal rates. These results indicate that cyanophages are an abundant and dynamic component of marine planktonic communities and are probably responsible for lysing a small but significant portion of the Synechococcus population on a daily basis.National Science Foundation OCE-9018833U.S. Office of Naval Research N00014-92-J-1676Marine Scienc
Flying too close to the Sun – The viability of perihelion-induced aqueous alteration on periodic comets
No full textComets are typically considered to be pristine remnants of the early solar system. However, by definition they evolve significantly over their lifetimes through evaporation, sublimation, degassing and dust release. This occurs once they enter the inner solar system and are heated by the Sun. Some comets (e.g. 1P/Halley, 9P/Tempel and Hale-Bopp) as well as chondritic porous cosmic dust – released from comets – show evidence of minor aqueous alteration resulting in the formation of phyllosilicates, carbonates or other secondary phases (e.g. Cu-sulphides, amphibole and magnetite). These observations suggest that (at least some) comets experienced limited interaction with liquid water under conditions distinct from the alteration histories of hydrated chondritic asteroids (e.g. the CM and CR chondrites). This synthesis paper explores the viability of perihelion-induced heating as a mechanism for the generation of highly localised subsurface liquid water and thus mild aqueous alteration in periodic comets. We draw constraints from experimental laboratory studies, numerical modelling, spacecraft observations and microanalysis studies of cometary micrometeorites. Both temperature and pressure conditions necessary for the generation and short-term (hour-long) survival of liquid water are plausible within the immediate subsurface (<0.5 m depth) of periodic comets with small perihelia (<1.5 A.U.), low surface permeabilities and favourable rotational states (e.g. high obliquities and/or slow rotational periods). We estimate that solar radiant heating may generate liquid water and perform aqueous alteration reactions in 3–9% of periodic comets. An example of an ideal candidate is 2P/Encke which has a small perihelion (0.33 A.U.), a high obliquity and a short orbital period. This comet should therefore be considered a high priority candidate in future spectroscopic studies of comet surfaces. Small quantities of phyllosilicate generated by aqueous alteration may be important in cementing together grains in the subsurface of older dormant comets, thereby explaining observations of unexpectedly high tensile strength in some bodies. Most periodic comets which currently pass close to the Sun are dormant, having experienced surface heating, significant cometary activity and dust release in the past. These bodies may be responsible for the partially hydrated cometary micrometeorites we find at the Earth's surface and their aqueous alteration histories may have been produced by perihelion-induced subsurface heating. This is in contrast to radiogenic and impact heating that operated during the early solar system on asteroids. This study has implications for the alteration history of the active asteroid Phaethon, the target of JAXA's DESTINY+ mission
X-ray computed tomography: Morphological and porosity characterization of giant Antarctic micrometeorites
No full textGiant 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)
Electron Cristallography of planetary materials: impactites and micrometeorites
No full textElectron crystallography have evolved in the last years into a technique able to furnish fast and reliable structural
information from nanocrystals. 3D electron diffraction (ED) methods provide diffraction data from nm-sized
domains, which are suitable for ab-initio structure solution. Moreover, it is now possible to derive a phase and
orientation map with nanometric resolution by recording a sequence of ED patterns while scanning an area.
Therefore, we have access to the crystal structure and to the phase and topotactic relations between the
crystalline grains at a scale of few nanometers at the same time.
Cutting-edge electron diffraction methods guarantee a new opportunity for understanding the kinetic and the
thermodynamic history of a geological sample with a cryptocrystalline habit. We will show specific applications of
this analysis to impact rocks shocked by a hypervelocity impacts of cometary and asteroidal bodies on Earth crust.
The investigation at the nanoscale with ED methods shows evidence of coesite formation directly from quartz and
not from a dense amorphous phase during shock unloading as previously thought.
A second field of application is the identification of nanocrystalline phases in micrometeorites. We show the
determination of magnetite and pyroxene crystals in a hydrated chondritic micrometeorite (CP94-050-052). These
phases have been determined with a 3D ED data collection with a 150 nm beam by diffracting only on the
nanocrystalline grains of interest, avoiding any contribution by the surrounding matrix. This research was
supported through Programma Nazionale delle Ricerche in Antartide (ID# PNRA16_00029)
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Melting experiments of an L6 ordinary chondrite: Implications for the formation of alkali-rich achondrites
No full textWe conducted high-pressure (1 GPa) melting experiments (1100–1400 °C) on the equilibrated ordinary chondrite DAV 01001 (L6) to investigate partial melting scenarios of planetary embryo in the early solar system. At 1100 °C, no melting of the silicate phase is observed, and the initial chondritic texture is preserved, but the metallic-sulphidic phases formed two immiscible Fe–Ni and S-rich liquids. Melting of silicate minerals began at 1200 °C, progressing from plagioclase to high-Ca and low-Ca pyroxene and olivine. As melting advanced, the formation of new olivine and low-Ca pyroxene resulted in the production of trachy-andesitic melt at 1200 °C, basaltic trachy-andesitic melt at 1300 °C, and andesitic melt at 1400 °C. These silicate melts have chemical similarities with some anomalous achondrites (e.g., GRA 60128/9). At the same time, minerals of new formation resemble those of primitive achondrites (e.g., brachinites, ureilites, IAB silicate inclusions, acapulcoites and lodranites). The rapid mineral-liquid re-equilibration suggests that basaltic liquids can form only above 1400 °C and that relatively high degrees of melting (>20 %) and crystallisation are necessary to explain the observed diversity of achondritic lithologies. These findings suggest that partial melting and recrystallization processes within planetary embryos could have played a critical role in the early solar system, contributing to the early differentiation of planetary bodies and the diversity of achondritic lithologies, including (but not limited to) alkali-rich achondrites
3D electron diffraction for the mineralogical characterization of micro-meteorites and impact micro ejecta
No full textThe investigation of cutting-edge topics in Earth and planetary sciences
often requires the study of cryptocrystalline polyphasic materials, typically
characterized by nano-scale mineral intergrowths associated with
high-pressure/high-temperature transformations, fast and non-equilibrium
processes and small amounts of available specimens. Conventional optical
imaging and X-ray crystallographic tools may be not sufficient for the proper
characterization of such samples. The development of efficient probes able to
investigate the nanoworld is therefore crucial to further our understanding of the
mineralogical and geochemical processes that regulate Earth and extraterrestrial
environments. Over the last ten years, electron diffraction (ED) evolved from a
qualitative method restricted to few TEM users, to a robust protocol for phase
identification and ab-initio structure determination. Such changes have been
possible due to the development of automatic and semi-automatic routines for 3D
data collection (Gemmi et al., 2019). This methodology is in principle equivalent to
single-crystal X-ray diffraction, but can be performed on crystals 10 to 1000 times
smaller. In this contribution, we show recent applications of ED in planetary
sciences. In particular, how ED allowed the mineralogical screening of the
carbonaceous chondrite CM Paris (Pignatelli et al., 2018) and of a hydrated
chondritic micrometeorite (CP94-050-052) through the polytypic description of
sub-micrometric phyllosilicate grains. Moreover, we will present an extensive
petrographic and crystallographic study of quartz-coesite mineralogical association
in impact ejecta from Kamil Crater, Egypt (Folco et al., 2018), and from the
Australasian tektite strewn field (Campanale et al., 2019). We believe that the
extensive application of modern ED techniques on micro-to-nanometer
extraterrestrial samples has the potential for significant breakthroughs in our
understanding of the Solar System’s formation and evolution. Also, it will allow the
thorough exploitation of the evidence already enclosed in the micrometeorite
collection recovered within the Progetto Nazionale Ricerche in Antartide (PNRA)
and in the forthcoming European space missions. ED will also significantly support
other sources of information based on remote sensing and spectroscopy and will
therefore ensure better constraints in numerical modeling studies
Isotopically Heavy Micrometeorites—Fragments of CY Chondrite or a New Hydrous Parent Body?
No full textCosmic dust grains sample a diverse range of solar system small bodies. This includes asteroids that are not otherwise represented in our meteorite collections. In this work we obtained 3D images of micrometeorite interiors using tomography before collecting destructive high‐precision oxygen isotope measurements. These data allow us to link textures in unmelted micrometeorites to known chondrite groups. In addition to identifying particles from ordinary chondrites, CR and CM chondrites we report two micrometeorites derived from an anomalous 16O‐poor source (δ17O: +16.4‰, δ18O: +28.4‰, and Δ17O: +1.4‰). Their compositions overlap with a previously reported micrometeorite (TAM50‐25) from Suttle et al. (2020), https://doi.org/10.1016/j.epsl.2020.116444 (EPSL: 546:116444). These particles represent hydrated carbonaceous chondrite material derived either from a new group or from the CY chondrites (thereby extending the isotopic range of this group). In either scenario they demonstrate close petrographic and isotopic connections to the CO‐CM chondrite clan. Furthermore, their position in O‐isotope space makes them the most likely candidate for the parent body of the anomalous “group 4” cosmic spherules previously reported by Suavet et al. (2010), https://doi.org/10.1016/j.epsl.2010.02.046 (EPSL: 293:313‐320) and several subsequent isotopic studies. We conclude that the “group 4” cosmic spherules originate from hydrated C‐type asteroid parents
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