Meteorite Samples Research Articles

Toward the Origins of Quadruple Sulfur Isotope Anomalies in Modern Sulfate: A Multitracer Approach and Implications for Paleo- and Planetary Atmospheres

Sulfur isotope mass-independent fractionation signatures (S-MIF) have been widely found in modern sulfate aerosols. Their mechanistic origins and potential implications for atmospheric sulfate formation chemistry are however poorly understood. Of particular importance is that the mystery of modern sulfate S-MIF hinders precise interpretations of S-MIF in geological sediment and meteorite samples. Here, we examine several proposed origins of modern sulfate S-MIF using a multiple-tracer approach. Quadruple sulfur isotopes of sulfate aerosols collected from Guangzhou, a megacity in South China, were measured along with various chemical and isotopic tracers for high-altitude air masses, biomass burning, and mineral dusts in the same samples. Similar to previous studies, we found that negative Δ36S values in modern sulfate aerosols are linked to combustions, and to a larger extent, to high-temperature processes that may involve symmetry-dependent recombination reactions of sulfur. The origins of Δ33S anomalies widely observed in modern sulfate aerosols remain elusive and require further investigation before we can unambiguously use this proxy to quantify sulfur emission and transformation pathways in the modern atmosphere. From a comparative planetology perspective, we highlight that a complete understanding of S-MIF in modern aerosols is crucial for tracking paleo- and planetary atmospheres by multiple sulfur isotope measurements in Phanerozoic sediments and Martian meteorites because their S-MIF magnitudes are similar.

The importance of acid-processed meteoric smoke relative to meteoric fragments for crystal nucleation in polar stratospheric clouds

Abstract. The crystal formation of nitric acid trihydrate (NAT) in the absence of water ice is important for a subset of polar stratospheric clouds (PSCs) and thereby ozone depletion. It has been suggested that either fragmented meteoroids or meteoric smoke particles (MSPs), or possibly both, are important as heterogeneous nuclei of these crystals. Previous work has focused on the nucleating ability of meteoric material in nitric acid in the absence of sulfuric acid. However, it is known that when immersed in stratospheric sulfuric acid droplets, metal-containing meteoric material particles partially dissolve and components can reprecipitate as silica and alumina that have different morphologies to the original meteoric material. Hence, in this study, we experimentally and theoretically explore the relative role that sulfuric acid-processed MSPs and meteoric fragments may play in NAT nucleation in PSCs. We compared meteoric fragments that had recently been prepared (by milling a meteorite sample) to a sample annealed under conditions designed to simulate heating during entry into the Earth's atmosphere. Whilst the addition of sulfuric acid decreased the nucleating ability of the recently milled meteoric material relative to nucleation in binary nitric acid-water solutions (at similar NAT saturation ratio), the annealed meteoric fragments nucleated NAT with a similar effectiveness in both solutions. However, combining our results with measured fluxes of meteoric material to the Earth, sedimentation modelling and recent experiments on fragmentation of incoming meteoroids suggests that it is unlikely for there to be sufficient fragments to contribute to the nucleation of crystalline NAT particles. We then considered silica formed from sulfuric acid-processed MSPs. Our previous work showed that nanoparticulate silica (radius ∼6 nm) is a relatively poor promoter of nucleation compared with micron-scaled silica particles, which were more effective. Both materials have similar chemical and structural (crystallographically amorphous) properties, indicating that size is critical. Here, we account for surface curvature of primary grains using the Classical Nucleation Theory (CNT) to explore this size dependence. This model is able to explain the discrepancy in nucleation effectiveness of fumed silica and fused quartz by treating their nucleating activity (contact angle) as equal but with differing particle size (or surface curvature), assuming interfacial energies that are physically reasonable. Here, we use this CNT model to present evidence that nucleation of NAT on acid-processed MSPs, where the primary grain size is tens of nanometres, is also effective enough to contribute to NAT crystals in early season PSCs where there is an absence of ice. This study demonstrates that the modelling of crystal nucleation in PSCs and resulting ozone depletion relies on an accurate understanding of the transport and chemical processing of MSPs. This will affect estimated sensitivity of stratospheric chemistry to rare events such as large volcanic eruptions and long-term forecasting of ozone recovery in a changing climate.

Automatic Bulk Composition Analysis of Lunar Basalts: Novel Big-Data Algorithm for Energy-Dispersive X-ray Spectroscopy

The bulk composition of lunar basaltic meteorites and clasts provides crucial information for understanding their petrogenesis and thus lunar thermal evolution. Meanwhile, the basalt type of Chang’E-5 based on the bulk TiO2 contents remains debatable. Modal recombination based on mineral volume fraction, densities, and average compositions is currently the most popular method to determine the bulk composition of lunar samples. Yet, the latter two parameters can be biased markedly by ubiquitous compositional variations in pyroxene, olivine, and plagioclase. To rectify these issues and provide more accurate classifications, this study devises a novel big-data algorithm that analyzes maps of energy-dispersive X-ray spectroscopy (EDS) data of lunar basalts. The algorithm starts by labeling each point through a newly devised mineral classifier, then uses the mean of all points per mineral to represent average composition, and finally recalculates the true density per mineral to replace standard density. The accuracy of this mineral classifier is demonstrated by tests on a database of lunar minerals. The accuracy and precision of EDS mapping were verified by test analysis on certified reference minerals. Measurements on a lunar meteorite sample with a known composition, NWA 4734, are comparable to those measured using inductively coupled plasma optical emission spectrometry and confirm the reliability of the bulk composition algorithm. To demonstrate its utility for comprehensive understanding of petrographic features, the high-efficiency algorithm was applied to Chang’E-5 basalts. The results reveal that these basalts are characterized by low-Ti and low-Mg features, thus distinct from previous Apollo and Luna samples.

Asteroids’ reflectance from Gaia DR3: Artificial reddening at near-UV wavelengths

Context. Observational and instrumental difficulties observing small bodies below 0.5 μm make this wavelength range poorly studied compared with the visible and near-infrared. Furthermore, the suitability of many commonly used solar analogues, essential in the computation of asteroid reflectances, is usually assessed only in visible wavelengths, while some of these objects show spectra that are quite different from the spectrum of the Sun at wavelengths below 0.55 μm. Stars HD 28099 (Hyades 64) and HD 186427 (16 Cyg B) are two well-studied solar analogues that instead present spectra that are also very similar to the spectrum of the Sun in the wavelength region between 0.36 and 0.55 μm. Aims. We aim to assess the suitability in the near-ultraviolet (NUV) region of the solar analogues selected by the team responsible for the asteroid reflectance included in Gaia Data Release 3 (DR3) and to suggest a correction (in the form of multiplicative factors) to be applied to the Gaia DR3 asteroid reflectance spectra to account for the differences with respect to the solar analogue Hyades 64. Methods. To compute the multiplicative factors, we calculated the ratio between the solar analogues used by Gaia DR3 and Hyades 64, and then we averaged and binned this ratio in the same way as the asteroid spectra in Gaia DR3. We also compared both the original and corrected Gaia asteroid spectra with observational data from the Eight Color Asteroid Survey (ECAS), one UV spectrum obtained with the Hubble Space Telescope (HST) and a set of blue-visible spectra obtained with the 3.6 m Telescopio Nazionale Galileo (TNG). By means of this comparison, we quantified the goodness of the obtained correction. Results. We find that the solar analogues selected for Gaia DR3 to compute the reflectance spectra of the asteroids of this data release have a systematically redder spectral slope at wavelengths shorter than 0.55 μm than Hyades 64. We find that no correction is needed in the red photometer (RP, between 0.7 and 1 μm), but a correction should be applied at wavelengths below 0.55 μm, that is in the blue photometer (BP). After applying the correction, we find a better agreement between Gaia DR3 spectra, ECAS, HST, and our set of ground-based observations with the TNG. Conclusions. Correcting the near-UV part of the asteroid reflectance spectra is very important for proper comparisons with laboratory spectra (minerals, meteorite samples, etc.) or to analyse quantitatively the UV absorption (which is particularly important to study hydration in primitive asteroids). The spectral behaviour at wavelengths below 0.5 μm of the selected solar analogues should be fully studied and taken into account for Gaia DR4.

Quantitative elemental mapping of chondritic meteorites using laser ablation-inductively coupled plasma-time of flight-mass spectrometry (LA-ICP-TOF-MS)

Fast-(semi)quantitative elemental mapping using state-of-the-art laser ablation-inductively coupled plasma-time of flight-mass spectrometry (LA-ICP-TOF-MS) was applied to a set of chondritic meteorite samples.

In Situ Argon Isotope Analyses of Chondrule-Forming Materials in the Allende Meteorite: A Preliminary Study for 40Ar/39Ar Dating Based on Cosmogenic 39Ar

The argon isotopic compositions of chondrule-forming minerals of the Allende (CV3) meteorite were examined to evaluate the possibility of in situ 40Ar/39Ar dating of planetary surface rocks based on cosmogenic 39Ar without neutron irradiation in a reactor. The investigated Allende meteorite sample (ME-247H: 50 mm × 45 mm × 5 mm; 28.85 g) contains at least three textural types of chondrules: barred olivine chondrule (BOC), porphyritic olivine chondrule (POC), and unclassified chondrule (UC). Most chondrules contain olivine, low-Ca pyroxene, clinopyroxene, and plagioclase as primary phases, with minor amounts of nepheline and sodalite formed during aqueous alteration of the CV3 parent body of the early solar system. In situ argon isotope analyses on selected chondrule-forming minerals in petrographic sections of two BOCs, two POCs, and one UC using a Nd:YAG pulse laser confirmed a significant amount of cosmogenic 39Ar that formed by a 39K (n, p) 39Ar reaction in an extraterrestrial environment. Laser step-heating analyses of five bulk chondrules irradiated in a reactor revealed a plateau age (3.32 ± 0.06 Ga) from one of the five chondrules. The age spectra of all chondrules show the younger age in the low-temperature fractions, resulting in the integrated ages from 2.7 to 3.2 Ga. These results suggest that the Allende meteorite experienced argon isotopic homogenization at 3.3 Ga and the argon loss in part after the 3.3 Ga. Apparent ages of chondrule-forming minerals that were calculated using the J values of nephelines in one BOC and two POCs do not show any consistent relationship among the three types of chondrules (BOC, POC, and UC). This might be attributed to the fact that the isotopic heterogeneity among minerals took place during the heterogeneous argon loss stage after the 3.3 Ga event.

Al-Mg and U-Pb chronological records of Erg Chech 002 ungrouped achondrite meteorite

Achondrite meteorites are remnants of the earliest planetary differentiation processes in the Solar System. They have been used to anchor short-lived radiochronometers to absolute ages determined from long-lived radiochronometers. More specifically, when comparing the isotopic systematics of the short-lived 26Al-26Mg chronometer anchored to absolute U-corrected Pb-Pb ages, inferences about the distribution of 26Al (half-life of ∼717 000 yr) in the protoplanetary disk can be evaluated. The ungrouped achondrite Erg Chech (EC) 002 has a distinct mineralogy and more evolved elemental composition compared to basaltic achondrites. In situ and solution 26Al-26Mg chronometry and 53Mn-53Cr chronometry suggest that EC 002 formed within ∼0.7 to 2.2 Ma after the formation of Ca-Al-rich inclusions (CAIs), making it the oldest known sample of igneous crust in the Solar System (Barrat et al., 2021; Anand et al., 2022; Zhu et al., 2022; Fang et al., 2022). Here we present the U-corrected Pb-Pb age and 26Al-26Mg age obtained by MC-ICPMS solution analysis of the same mineral separate samples of EC 002. In addition, six merrillite grains were analyzed by in-situ SIMS to determine their Pb-Pb individual ages.The U isotope composition of EC 002 exhibits internal heterogeneities between leached pyroxene (238U/235U = 137.766 ± 0.027) and the bulk rock (238U/235U = 137.8190 ± 0.0074). The Pb isotope composition of progressively leached pyroxenes are characterized by radiogenic 206Pb/204Pb ratios (ranging from 41 to 23487). Using the U isotope composition of the leached pyroxenes, the resulting age of the 207Pb/206Pb-204Pb/206Pb isochron is 4565.87 ± 0.30 Ma (2σ). The weighted mean of the Pb-Pb ages of seven SIMS analyses of merrillites are 4564.3 ± 5.2 Ma (2σ). These similar ages (within uncertainty) indicate rapid cooling and the absence of significant thermal events after ∼4559 Ma on the parent body of EC 002. The 26Al-26Mg isochron through a bulk rock, pyroxene, fine-grained and four plagioclase fractions defines an initial 26Al/27Al ratio of [8.89 ± 0.79] × 10−6 corresponding to a formation age of 1.83 ± 0.12 Ma after CAIs ([5.23 ± 0.13] × 10−5; Jacobsen et al., 2008). The initial 26Al abundance is consistent with previous MC-ICP-MS 26Al-26Mg reported systematics for EC 002 (Fang et al., 2022), but 0.46 ± 0.13 Myr older than the in situ SIMS 26Al-26Mg age previously reported by Barrat et al. (2021).When anchored to the absolute Pb-Pb age of CV3 CAIs (4567.30 ± 0.16 Ma; Connelly et al., 2012), the Al-Mg model age of EC 002 is 4565.47 ± 0.20 Ma, slightly younger than its U-corrected Pb-Pb age.The concordance of the Pb-Pb and 26Al-26Mg ages of ungrouped CC achondrites when anchored to EC 002 suggest that 26Al was homogeneously distributed between the NC and CC reservoirs at the time of their parent body accretion. Furthermore, the presence of internal U isotope heterogeneities found between mineral and whole-rock samples of EC 002 supports the need of U isotope analysis of meteoritic samples dated using the Pb-Pb chronometer.

SIMS relative sensitivity factors for Al/Mg in synthetic and Madagascar hibonite

We synthesized three compositions of hibonite, differing in their levels of MgO and TiO2 that were chosen to cover most of the range of compositions of natural meteoritic hibonite. The goal was to evaluate the appropriateness of the use of terrestrial Madagascar hibonite as a standard in the SIMS analysis of initial 26Al/27Al ratios in calcium‑aluminum-rich inclusions, and especially its use for the determination of the 27Al/24Mg relative sensitivity factor (RSF). Concern exists because of the high levels of FeO and rare earth elements (REE) in the terrestrial mineral relative to meteoritic samples. Our results show that, provided the specific Madagascar hibonite samples in a given lab are carefully characterized in terms of mineral chemistry (including Fe, Th, and REE) via electron microprobe analysis, the terrestrial mineral gives RSFs that are within 2% of those determined for the synthetic samples. The 27Al/24Mg SIMS/EPMA RSF based on the synthetic hibonite compositions alone is 0.779 ± 0.003; combining all synthetic and Madagascar hibonite analyses yields a RSF of 0.777 ± 0.003. We cannot rule out that RSFs might be somewhat different using different SIMS instruments, or among individual SIMS sessions, so RSFs should be evaluated for each SIMS session.

Spectral Evolution of Dark Asteroid Surfaces Induced by Space Weathering over a Decade

The surfaces of airless bodies like asteroids in the solar system are known to be affected by space weathering. Experiments simulating space weathering are essential for studying the effects of this process on meteorite samples, but the problem is that the time spent to reproduce space weathering in these experiments is billions of times shorter than the actual phenomenon. In 2010 December, the T-type asteroid 596 Scheila underwent a collision with an impactor a few tens of meters in size. A decade later, there is an opportunity to study how the surface layer of this asteroid is being altered by space weathering after the impact. To do so, we performed visible spectrophotometric and near-infrared spectroscopic observations of 596 Scheila. The acquired spectrum is consistent with those observed shortly after the 2010 impact event within the observational uncertainty range. This indicates that the surface color of dark asteroids is not noticeably changed by space weathering over a 10 yr period. This study is the first to investigate color changes due to space weathering on an actual asteroid surface in the solar system. Considering that fresh layers are regularly created on asteroid surfaces by collisions, we suggest a genetic link between D/T-type and dark (low albedo) X-complex asteroids and very red objects such as 269 Justitia, 732 Tjilaki, and 203 Pompeja. New observations show that 203 Pompeja has an X-type-like surface, with some local surface areas exhibiting a very red spectrum.

Linking remote sensing, in situ and laboratory spectroscopy for a Ryugu analog meteorite sample

In 2022 JAXA issued an Announcement of Opportunity (AO) for receiving Hayabusa2 samples returned to Earth. We responded to the AO submitting a proposal based on using a multi-prong approach to achieve two main goals. The first goal is to address the subdued contrast of remote-sensing observations compared to measurements performed under laboratory conditions on analog materials. For this we will link the hyperspectral and imaging data collected from the spacecraft and the in-situ observations from the MASCOT lander instruments (MARA and MASCam) with laboratory-based measurements of Hayabusa2 samples using bi-directional reflectance spectroscopy under simulated asteroid surface conditions from UV to MIR/FIR achieved using three Bruker Vertex 80 V spectrometers in the Planetary Spectroscopy Laboratory. The second goal is the investigation of the mineralogy and organic matter of the samples collected by Hayabusa2, to better understanding the evolution of materials characterizing Ryugu and in general of protoplanetary disk and organic matter, investigating the aqueous alteration that took place in the parent body, and comparing the results with data collected from pristine carbonaceous chondrite analog meteorites. Spectral data will be complemented by Raman spectroscopy under simulated asteroid surface conditions, X-ray diffraction, would also allow us to define the bulk mineralogy of the samples as well as investigate the presence and nature of organic matter within the samples. In situ mineralogical and geochemical characterization will involve a pre-characterization of the sample fragments through scanning electron microscopy low voltage electron dispersive X-ray (EDX) maps, and micro IR analyses of the fragments. If allowed, a thin section of one grain will be used for electron microprobe analyses to geochemically characterize its mineralogical composition. To train our data collection and analysis methods on a realistic sample, we selected a piece of the Mukundpura meteorite, as one of the closer analogs to Ryugu’s surface (Ray et al., Planetary and Space Science, 2018, 151, 149–154). The Mukundpura chunk we selected for this study measures 3 mm in its maximum dimension, and we chose it so to have a test sample of the same size as the Hayabusa2 grain we requested in our proposal to JAXA’s AO. The test gave us confidence that we can measure with good SNR measurements in bi-directional reflectance for samples around 3 mm in size (see Figures 3, 4 below). To address our second goal the spectral data was complemented by Raman spectroscopy measured again under simulated asteroid surface conditions in our Raman Mineralogy and Biodetection Laboratory at DLR.

Coordinated neutron and X‐ray computed tomography of meteorites: Detection and distribution of hydrogen‐bearing materials

AbstractThe presence and distribution of hydrogen‐bearing materials in meteorites are important constraints on processes in the early solar system, and the delivery of volatile constituents to growing planets. Here, we show that coordinated neutron and X‐ray computed tomography, NXCT, can reveal the presence and distributions of hydrogen‐bearing materials in meteorites, and thus help constrain the presence and actions of water in the early solar system. NXCT is nearly nondestructive of meteorite samples. Neutron fluence in NXCT is approximately seven orders of magnitude less than in typical instrumental neutron activation analysis, and so produces little residual radioactivity and currently undetectable changes in isotope ratios. Heating during NXCT is minimal, but NXCT will overprint the record of cosmic ray exposure held in natural thermoluminescence. Two meteorites were examined. EET 87503 is a howardite, a regolith breccia inferred to be from the asteroid 4 Vesta, and contains fragments of eucrite basalt, diogenite pyroxenite, and H‐rich carbonaceous chondrites. With NXCT, the chondrite fragments within the meteorite piece can be clearly located and characterized, in preparation for possible extraction and detailed analyses. Graves Nunataks (GRA) 06100 is a CR2 chondrite meteorite that contains abundant iron metal and H‐bearing silicates from aqueous alteration. In NXCT, H‐bearing altered material is clearly distinguished from metal, and its distribution in three dimensions is revealed as a constraint on the processes of alteration.

A Facile Route for the Formation of Complex Nitrogen-Containing Prebiotic Molecules in the Interstellar Medium.

Prebiotic molecules have often been identified in the interstellar medium and meteorite samples. However, we still have only a fragmentary knowledge of the mechanism of the evolutionary process of these prebiotic molecules. With the aid of state-of-the-art vacuum ultraviolet (VUV)-infrared (IR) spectroscopy and ab initio calculations, we reveal a new pathway leading to the formation of the biorelevant molecules carrying amine groups or peptide bonds via the single-photon ionization induced Michael/cyclization reaction of acrylonitrile (AN)-alcohol heterodimer complexes in the gas phase. In the reactions, not only N-H nitrilium cations with H+-N≡C-R Lewis structure but also cyclic amine cations with a peptide bond can be formed when the AN reacts with alcohols of increasing molecular size (such as ethanol, propanol, or butanol). This study suggests the possibility of unsaturated nitriles being reduced by ionized alcohols in space, which can further drive sequential Michael addition/cyclization reactions to form more complex biorelevant molecules.

A Meteor Spectroscopic Survey in the Nullarbor

We report the start of science operations of a meteor spectroscopic survey in the Nullarbor region, Western Australia. The observation program consists of well-proven observatories developed as part of the All-Sky Meteor Orbit System (AMOS) project. These comprise high-sensitivity all-sky imaging units, as well as spectroscopic instruments observing brighter meteors. They are co-located with Desert Fireball Network (DFN) instruments, which themselves provide high-resolution astrometry for fireballs. There are two goals for this program. One is to keep a constant watch on meteor activity by always having one AMOS sub-network in the dark. The second is to provide spectroscopic coverage for recovered meteorites by the DFN, establishing essential calibration points between meteoritic samples and fireball spectra.

Determining the noble gas cosmic ray exposure ages of 23 meteorites (8 chondrites and 15 achondrites) from modeling and empirical methods

AbstractWe present He‐Ne‐Ar isotope data for 23 meteorite samples mainly recovered in Antarctica (six ordinary chondrites [OC], two CV chondrites, eight eucrites, one diogenite, and six ureilites), which are used to compute radiogenic gas retention ages and cosmic ray exposure (CRE) ages using both empirical and modeling approaches. For all samples where both 40K‐40Ar and U,Th‐4He retention ages could be derived, we find that U,Th‐4He ages are systematically lower than 40K‐40Ar ages, likely reflecting preferential diffusive loss of He relative to Ar. There is good agreement between empirically derived CRE ages calculated by (22Ne/21Ne)cos‐3Hecos and (22Ne/21Ne)cos‐21Necos approaches; where discrepancies occur, the (22Ne/21Ne)cos‐3Hecos approach systematically yields lower CRE ages, also likely due to 3He loss. Overall, CRE ages derived from the empirical and modeling approaches show excellent agreement, within ∼10%. CRE ages derived for OC (4–24 Myr), CV chondrites (12–26 Myr), eucrites (4–45 Myr), the diogenite (30 Myr), and ureilites (<10 Myr) are in line with previous investigations of these meteorite groups. Some ureilites and one eucrite exhibit remarkably high cosmogenic 22Ne/21Ne > 1.24, as previously observed in various other rare achondrites. These samples likely contain solar cosmic ray‐produced Ne (SCR‐Ne) in addition to the commonly found galactic cosmic ray‐produced Ne (GCR‐Ne), implying low pre‐atmospheric shielding and limited ablation upon atmospheric entry. The presence of SCR‐Ne complicates the determination of the pure GCR‐22Ne/21Ne, hampering its use as a shielding indicator. Nonetheless, we suggest that a first‐order correction for SCR‐Ne contribution can be used to derive a range of potential CRE ages for each sample.

Surface microstructures of lunar soil returned by Chang’e-5 mission reveal an intermediate stage in space weathering process

The lunar soils evolution over time is mainly caused by space weathering that includes the impacts of varying-sized meteoroids and charged particles implantation of solar/cosmic winds as well. It has long been established that space weathering leads to the formation of outmost amorphous layers (50–200 nm in thickness) embedded nanophase iron (npFe0) around the mineral fragments, albeit the origin of the npFe0 remains controversial . The Chang’e-5 (CE-5) mission returned samples feature the youngest mare basalt and the highest latitude sampling site , providing an opportunity to seek the critical clues for understanding the evolution of soils under space weathering. Here, we report the surface microstructures of the major minerals including olivine, pyroxene, anorthite, and glassy beads in the lunar soil of CE-5. Unlike the previous observations, only olivine in all crystals is surrounded by a thinner outmost amorphous SiO2 layer (∼10 nm thick) and embedded wüstite nanoparticles FeO (np-FeO, 3–12 nm in size) instead of npFe0. No foreign volatile elements deposition layer and solar flare tracks can be found on the surface or inside the olivine and other minerals. This unique rim structure has not been reported for any other lunar, terrestrial, Martian, or meteorite samples so far. The observation of wüstite FeO and the microstructures support the existence of an intermediate stage in space weathering for lunar minerals by thermal decomposition.

MagNet: Automated Magnetic Mineral Grain Morphometry Using Convolutional Neural Network

AbstractMorphometry (i.e., the quantitative determination of grain size and shape information) is an essential component of all rock and environmental magnetic studies. Electron microscopy is often used to image magnetic mineral grains, but the current lack of systematic image processing tools makes it challenging to quantify key morphological features of magnetic minerals in natural samples. Here, we present an easy‐to‐use machine learning framework MagNet for automated morphological recognition of magnetic mineral grains in microscopic images. This framework, based on a convolutional neural network, performs well in the recognition and classification of magnetofossil nanoparticles in transmission electron microscopy images after training and testing. MagNet is open‐source and can easily be extended to process different types of mineral images. This tool has the potential, therefore, to extract key quantitative information of magnetic mineral populations within heterogeneous terrestrial and meteoritic samples for the interpretations of Earth and planetary processes.

Are there any pristine comets? Constraints from pebble structure

ABSTRACT We show that if comets (or any small icy planetesimals such as Kuiper belt objects) are composed of pebble piles, their internal radiogenic as well as geochemical heating results in considerably different evolutionary outcomes compared to similar past studies. We utilize a 1D thermophysical evolution code, modified to include state-of-the-art empirical measurements of pebble thermal conductivity and compression, the latter obtained through a new laboratory experiment presented here for the first time. Results indicate that due to the low pebble thermal conductivity, the peak temperatures attained during evolution are much higher than in any previous study given the same formation time. Assuming meteoritic radiogenic abundances, we find that only extremely small, sub-kilometre comets have the potential to retain the primordial, uniform, and thermally unprocessed composition from which they formed. Comets with radii in excess of about 20 km are typically swept by rapid and energetically powerful aqueous hydration reactions. Across the full range of comet sizes and formation times, evolutions result in the processing and differentiation of various volatile species, and a radially heterogeneous nucleus structure. Our computations however also indicate that the assumed fraction of radionuclides is a pivotal free parameter, because isotopic analyses of the only available cometary samples suggest that no 26Al was ever present in comet 81P/Wild 2. We show that if comets formed early in the protoplanetary disc (within 1–3 Myr), the radionuclide abundances indeed must be much smaller than those typically assumed based on meteoritic samples. We discuss the importance of our findings for the formation, present-day attributes and future research of comets.

Quantitative verification of 1:35 diluted fused glass disks with 10 mg sample sizes for the wavelength-dispersive X-ray fluorescence analysis of the whole-rock major elements of precious geological specimens

Herein, an improved wavelength-dispersive X-ray fluorescence (WD-XRF) analytical technique for determining the major elements (including SiO2, TiO2, Al2O3, TFe2O3, MnO, MgO, CaO, Na2O, K2O, and P2O5) in precious geological samples, particularly extraterrestrial basaltic rock samples, was developed. For WD-XRF analysis, undersized (approximately 11 mm in diameter) glass disks were prepared by fusing 10 mg of a powdered sample with 350 mg of lithium borate flux at a sample-to-flux ratio of 1:35. Specialized PtAu crucibles and molds were crafted for glass disk preparation to facilitate automatic fluxer mounting. This procedure enabled the automatic preparation of undersized glass disks, with repeatability between sample batches. Calibration curves were constructed for 30 certified reference materials (CRMs), including felsic, intermediate, mafic, and ultramafic igneous rocks. Regular-sized CRM disks with large sample sizes were prepared at the same sample-to-flux ratio as the test samples to avoid issues caused by CRM heterogeneity. The influence of various factors, including glass disk thickness and homogeneity, releasing agent amount, and interference between lines (Br Lα with Al Kα, Rh Lγ2 and Rh Lγ3 with K Kα, and K Kβ with Ca Kα), on analytical accuracy was investigated. Two homogeneous synthetic reference glass samples (ARM-2 and ARM-3) were pulverized and used to verify the analytical results for small samples using the WD-XRF method, which could eliminate the uncertainty caused by sample homogeneity. Subsequently, three CRMs (JB-1b, JA-3, and JR-1) were used to validate the accuracy and precision of the method. Finally, two lunar meteorite samples (NWA4898 and NWA4734) were assayed by the proposed method, and comparison with inductively coupled plasma optical emission spectroscopy results confirmed the reliability of the method. The proposed small sample protocol, which involves the destructive pretreatment of precious and limited extraterrestrial basaltic samples, has great application potential for precious samples such as lunar soil samples.

“The ice-organic-silicate contents of small solar system bodies: indicators for a comet to asteroid evolutionary pathway”

ABSTRACT Comets and asteroids have traditionally been separated categories, but main belt comets skew this view, portraying a possible intermediate stage between these two endmembers. Investigating the relationship between these bodies can improve our understanding of the formation and evolution of the Solar System and help to identify potentially interesting parent bodies from within our solar system, for future sample return missions. Furthermore, elucidating the ice-organic-silicate ratios of potential meteorite parent bodies can help to explain the observed isotopic ratios and petrography of meteorite samples. While the ice-organic-silicate ratios of particular bodies have been estimated, there has been no study undertaken which compares different types of bodies in terms of their ice-organic-silicate ratios. Therefore, this study presents a geophysical-chemical mass balance model, to estimate the ice-organic-silicate ratios of comets, main belt comets and asteroids. The results drawn from the model form a diagonal trend upon an ice-organic-silicate ternary diagram, in which comets and main belt comets plot together at generally higher ice contents, with asteroids typically plotting at lower ice contents. However, an overlap between all three body types is observed and supports the scenario in which comets, main belt comets and asteroids are genetically linked.

Petrochemistry and Oxygen Isotope of Karimati (L5) Chondrite, a 2009 Fall in Uttar Pradesh, India

Abstract Karimati, a single stone, weighing ~1 kg, fell on the mid-day of May 28, 2009, in Karimati village, Tahsil Hamirpur, Uttar Pradesh, India. Based on textures and mineralogy, Karimati is classified as brecciated, L5 (S2), ordinary chondrite. Petrologically, the chemical group, L has been assigned based on the typical mean compositions of olivine (Fa:25.5) and low-Ca pyroxene (Fs:21.6). The bulk chemical composition also resembles L chondrite as Mg/Si ratio ~0.82. Finally, the oxygen isotopic ratios of bulk meteorite sample yielded δ17O (+3.78%o and +4.07%o) and δ18O (+4.95%o and +5.52%o), respectively further confirmed Karimati as L-chondrite. The Karimati appears to have been subjected to single fragmentation in the atmospheric flight and it is evident from similar characters of all the lateral faces except the base of the pyramid in terms of its uniform colour, homogeneous fusion crust and shallow regmaglypts. The metamorphic equilibration temperature for Karimati chondrite using two different thermometers (olivine-chromite and two pyroxene thermometry) has been estimated and ranges 655°C and 770°C, respectively with a mean value of ~710°C and corresponds to the Type 5 thermal equilibration event.