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Fieldwork is a pedagogical cornerstone of many geoscience degrees. During the academic years 2019–20 and 2020–21, the worldwide COVID-19 pandemic made outdoors fieldwork difficult, resulting in an urgent need to develop virtual alternatives. However, there is still more to learn about the impact of teaching fieldwork virtually on the student learning experience. This study aims to assess the student learning experience during immersive and interactive three-dimensional virtual fieldwork and establish the value of digital techniques to improve the inclusivity of geosciences degrees. Quantitative and qualitative data were collected to assess students’ attitudes to virtual fieldwork in comparison to outdoor fieldwork in terms of accessibility, inclusivity and their learning experience. Our results show overall positive student responses to virtual fieldwork, with over half stating it adequately replicated the learning experience of outdoor fieldwork. Students also value outdoor fieldwork for the degree of autonomy it provides, and idea-sharing with peers; yet simultaneously the majority believed outdoor fieldwork is inherently exclusionary. This study concludes that virtual fieldwork, taught using interactive 3D virtual outcrops set within virtual worlds, replicates the outdoor fieldwork learning experience as closely as possible. However, students missed some fundamental and important aspects of outdoor fieldwork, as being outside and the social interactions with peers and staff that are specific to on-location fieldwork. This study recommends the use of virtual fieldwork in addition to residential on-location fieldwork, as for a significant number of students virtual fieldwork may be a better way of accessing this valued pedagogy of the geosciences. Furthermore, virtual fieldwork has the potential to make geosciences more inclusive and attractive to a wider range of students.

Fusion crusts form during the atmospheric entry heating of meteorites and preserve a record of the conditions that occurred during deceleration in the atmosphere. The fusion crust of the Winchcombe meteorite closely resembles that of other stony meteorites, and in particular CM2 chondrites, since it is dominated by olivine phenocrysts set in a glassy mesostasis with magnetite, and is highly vesicular. Dehydration cracks are unusually abundant in Winchcombe. Failure of this weak layer is an additional ablation mechanism to produce large numbers of particles during deceleration, consistent with the observation of pulses of plasma in videos of the Winchcombe fireball. Calving events might provide an observable phenomenon related to meteorites that are particularly susceptible to dehydration. Oscillatory zoning is observed within olivine phenocrysts in the fusion crust, in contrast to other meteorites, perhaps owing to temperature fluctuations resulting from calving events. Magnetite monolayers are found in the crust, and have also not been previously reported, and form discontinuous strata. These features grade into magnetite rims formed on the external surface of the crust and suggest the trapping of surface magnetite by collapse of melt. Magnetite monolayers may be a feature of meteorites that undergo significant degassing. Silicate warts with dendritic textures were observed and are suggested to be droplets ablated from another stone in the shower. They, therefore, represent the first evidence for intershower transfer of ablation materials and are consistent with the other evidence in the Winchcombe meteorite for unusually intense gas loss and ablation, despite its low entry velocity. Science and Technology Facilities Council (STFC): ST/V000799/1

Fractionation effects related to evaporation and condensation had a major impact on the current elemental and isotopic composition of the Solar System. Although isotopic fractionation of moderately volatile elements has been observed in tektites due to impact heating, the exact nature of the processes taking place during hypervelocity impacts remains poorly understood. By studying Fe in microtektites, here we show that impact events do not simply lead to melting, melt expulsion and evaporation, but involve a convoluted sequence of processes including condensation, variable degrees of mixing between isotopically distinct reservoirs and ablative evaporation during atmospheric re-entry. Hypervelocity impacts can as such not only generate isotopically heavy, but also isotopically light ejecta, with δ56/54Fe spanning over nearly 5‰ and likely even larger variations for more volatile elements. The mechanisms demonstrated here for terrestrial impact ejecta modify our understanding of the effects of impact processing on the isotopic evolution of planetary crusts., Fe isotopic composition of the distal ejecta of a terrestrial impact crater records both evaporation and condensation, refining the nature of the isotopic fractionation taking place during hypervelocity impacts in the Solar System.

A new process in post-emplacement ignimbrite degassing is proposed from observations of unusual structures in a high-grade ignimbrite in Sardinia. The structures consist of fiamme decorated above and below by vesicles, termed here Fiamme Degassing Structures (FDS). The relative volumes of vesicles and fiamme have a positive, linear correlation, and the ratio between the volume of vesicles above and below fiamme (asymmetry ratio) is 1 wt% at

NASA's strategy in exploring Mars has been to follow the water, because water is essential for life, and it has been found that there are many locations where there was once liquid water on the surface. Now perhaps, to narrow down the search for life on a barren basalt‐dominated surface, there needs to be a refocusing to a strategy of “follow the nutrients.” Here we model the entry of metallic micrometeoroids through the Martian atmosphere, and investigate variations in micrometeorite abundance at an analogue site on the Nullarbor Plain in Australia, to determine where the common limiting nutrients available in these (e.g., P, S, Fe) become concentrated on the surface of Mars. We find that dense micrometeorites are abundant in a range of desert environments, becoming concentrated by aeolian processes into specific sites that would be easily investigated by a robotic rover. Our modeling suggests that micrometeorites are currently far more abundant on the surface of Mars than on Earth, and given the far greater abundance of water and warmer conditions on Earth and thus much more active weather system, this was likely true throughout the history of Mars. Because micrometeorites contain a variety of redox sensitive minerals including FeNi alloys, sulfide and phosphide minerals, and organic compounds, the sites where these become concentrated are far more nutrient rich, and thus more compatible with chemolithotrophic life than most of the Martian surface. Plain Language Summary NASA's exploration program has allowed the scientific community to demonstrate clearly that Mars had a watery past, so the search for life needs to move on to identifying the places where water and nutrients coincided. We have investigated the relative abundance of micrometeorites on Mars compared to the Earth because these contain key nutrients that the earliest life forms on Earth used, and because their contained minerals can be used to investigate past atmospheric chemistry. We suggest that micrometeorites should be far more abundant on the Martian surface than on Earth's, and that wind‐driven modification of sediments is expected to concentrate micrometeorites, and their contained nutrients, in gravel beds and cracks in exposed bedrock.

Large airbursts, the most frequent hazardous impact events, are estimated to occur orders of magnitude more frequently than crater-forming impacts. However, finding traces of these events is impeded by the difficulty of identifying them in the recent geological record. Here, we describe condensation spherules found on top of Walnumfjellet in the Sør Rondane Mountains, Antarctica. Affinities with similar spherules found in EPICA Dome C and Dome Fuji ice cores suggest that these particles were produced during a single-asteroid impact ca. 430 thousand years (ka) ago. The lack of a confirmed crater on the Antarctic ice sheet and geochemical and 18O-poor oxygen isotope signatures allow us to hypothesize that the impact particles result from a touchdown event, in which a projectile vapor jet interacts with the Antarctic ice sheet. Numerical models support a touchdown scenario. This study has implications for the identification and inventory of large cosmic events on Earth., SCOPUS: ar.j, info:eu-repo/semantics/published

We analysed the heterogenous fine-grained (sub-μm) matrix of a small (58 × 93 μm), unmelted and minimally heated (

International audience; Cosmic spherules are micrometeorites that melt at high altitude as they enter Earth's atmosphere, and their oxygen isotope compositions are partially or completely inherited from the upper atmosphere, depending on the amount of heating experienced and the nature of their precursor materials. In this study, the three oxygen isotope compositions of 137 cosmic spherules are determined using 277 in situ analyses by ion probe. Our results indicate a possible correlation between increasing average δ18O compositions of silicate‐dominated (S‐type) spherules along the series scoriaceous < porphyritic < barred < cryptocrystalline < glass < CAT (calcium‐aluminum‐titanium) spherules (~12‰, 20‰, 22‰, 25‰, 26‰, and 50‰). This is consistent with the evolution of oxygen isotopes by mass fractionation owing to increased average entry heating and thus suggests mass fractionation dominates changes in isotopic composition, with atmospheric exchange being less significant. The Δ17O values of spherules, therefore, are mostly preserved and suggest that ~80% of particles are samples of C‐type asteroids. The genetic relationships between different S‐types can also be determined with scoriaceous, barred, and cryptocrystalline spherules mostly having low Δ17O values (≤0‰) mainly derived from carbonaceous chondrite (CC)‐like sources, while porphyritic spherules mostly have positive Δ17O (>0‰) and are largely derived from ordinary chondrite (OC)‐like sources related to S (IV)‐type asteroids. Glass and CAT spherules have variable Δ17O values indicating they formed by intense entry heating of both CC and OC‐like materials. I‐type cosmic spherules have a narrow range of δ17O (~20–25‰) and δ18O (~38–48‰) values, with Δ17O (~0‰) suggesting their oxygen is obtained entirely from the Earth's atmosphere, albeit with significant mass fractionation owing to evaporative heating. Finally, G‐type cosmic spherules have unexpected isotopic compositions and demonstrate little mass fractionation from a CC‐like source. The results of this study provide a vital assessment of the wider population of extraterrestrial dust arriving on Earth.

Bioavailable Fe is an essential nutrient for phytoplankton that allows organisms to flourish and drawdown atmospheric CO2 affecting global climatic condition. In marine locales remote from the continents extraterrestrial-dust provides an important source of Fe and thus moderates primary productivity. Here we provide constraints on partitioning of extraterrestrial Fe between seawater and sediments from observations of dissolution and alteration cosmic spherules recovered from the deepsea sediments and Antarctica. Of the ~ 3000–6000 t/a extraterrestrial dust that reaches Earth surface, ~ 2–5 % material survives in marine sediments whilst the remainder is liberated into seawater. Both processes contributes ~ (3–10) × 10−8 molFe m−2 yr−1. Also, Fe contribution due to evaporation of survived particle is estimated to be ~ 10 % of Fe contribution to meteoric smoke. Changes in extraterrestrial-dust flux vary not only the amount of Fe by up to three orders of magnitude, but also the partitioning of Fe between surface and abyssal waters depending on entry velocity and evaporation.

The early stages of atmospheric entry are investigated in four large (250–950 μm) unmelted micrometeorites (three fine‐grained and one composite), derived from the Transantarctic Mountain micrometeorite collection. These particles have abundant, interconnected, secondary pore spaces which form branching channels and show evidence of enhanced heating along their channel walls. Additionally, a micrometeorite with a double‐walled igneous rim is described, suggesting that some particles undergo volume expansion during entry. This study provides new textural data which links together entry heating processes known to operate inside micrometeoroids, thereby generating a more comprehensive model of their petrographic evolution. Initially, flash heated micrometeorites develop a melt layer on their exterior; this igneous rim migrates inwards. Meanwhile, the particle core is heated by the decomposition of low‐temperature phases and by volatile gas release. Where the igneous rim acts as a seal, gas pressures rise, resulting in the formation of interconnected voids and higher particle porosities. Eventually, the igneous rim is breached and gas exchange with the atmosphere occurs. This mechanism replaces inefficient conductive rim‐to‐core thermal gradients with more efficient particle‐wide heating, driven by convective gas flow. Interconnected voids also increase the likelihood of particle fragmentation during entry and, may therefore explain the rarity of large fine‐grained micrometeorites among collections.

We analysed 44 fine-grained and scoriaceous micrometeorites. A bulk mid-IR spectrum (8–13 μm) 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 μm. 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 CI-like 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 μm, 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.

Drawings, illustrations, and field sketches play an important role in Earth Science since they are used to record field observations, develop interpretations, and communicate results in reports and scientific publications. Drawing geology in the field furthermore facilitates observation and maximizes the value of fieldwork. Every geologist, whether a student, academic, professional, or amateur enthusiast, will benefit from the ability to draw geological features accurately. This book describes how and what to draw in geology. Essential drawing techniques, together with practical advice in creating high quality diagrams, are described the opening chapters. How to draw different types of geology, including faults, folds, metamorphic rocks, sedimentary rocks, igneous rocks, and fossils, are the subjects of separate chapters, and include descriptions of what are the important features to draw and describe. Different types of sketch, such as drawings of three-dimensional outcrops, landscapes, thin-sections, and hand-specimens of rocks, crystals, and minerals, are discussed. The methods used to create technical diagrams such as geological maps and cross-sections are also covered. Finally, modern techniques in the acquisition and recording of field data, including photogrammetry and aerial surveys, and digital methods of illustration, are the subject of the final chapter of the book. Throughout, worked examples of field sketches and illustrations are provided as well as descriptions of the common mistakes to be avoided.

Advances in technology have enabled new methods in the acquisition and recording of field data in geology and its presentation within publications. These techniques compliment, rather than replace, traditional field observations. This chapter describes the use of photogrammetry and aerial drone surveys in constructing three-dimensional models of geological features, which provide valuable data when combined with field notes on lithology. Digital methods in the analysis and processing of images are discussed together with methods in digital drawing and painting to produce publication-ready diagrams for Earth Science. Photographs for use in publications should be corrected to ensure optimal contrast and brightness.

Fossils record the evolution of life on Earth and are crucial in the dating of sedimentary rocks through biostratigraphy. This chapter examines the methods used to record the taxonomy and sedimentological context of fossils within field sketches. The taxonomy and geological range of important fossil groups are described as well as their occurrence within sediments. Blocking-in sketches accurately is particularly important when drawing fossils since their quasi-regular shapes can lead to drawings that are overly schematic. Small-scale morphological features are often important in the interpretation of fossils. Four worked examples of sketches of fossils are given together with descriptions of their interpretation to illustrate techniques to record palaeontological specimens.

Geological maps record the exposure of different rock units at the surface and are crucial in determining the stratigraphy and structure of a region. Creating and interpreting geological maps are important skills for an Earth Scientist. This chapter describes the methods used in geological mapping, including the tactics involved during fieldwork and the conventions and symbols used to record data. The chapter illustrates the use of sketch maps during fieldwork to record high resolution observations in areas with complex geological relationships. Four examples of sketch map are given to demonstrate how they can be used in field notes to record spatially constrained observations.

Illustrations and drawings have been used in geology to record field data and communicate science since the inception of the subject. In this chapter the history of illustration and drawing in geology is described and shows the evolution of pictorial representation in Earth Science from the earliest examples of the fifteenth century to the present time. The impact of new technologies and the changes in emphasis brought about by modern technology is also considered in relation to drawing as a means to record data. The chapter argues that drawing remains the best means to document geological features and still has an important place in Earth Science.

Metamorphic rocks are formed by the transformation of pre-existing lithologies under the influence of pressure and temperature. Often metamorphic rocks contain complicated folds and cleavages that make them difficult to record in field sketches and to interpret. In this chapter the methods used to draw field sketches of complex structures in metamorphic rocks are described. The important features of superimposed folds are discussed and methods to assist in the observation and interpretation of these structures are described. Three worked examples of field sketches of metamorphic rocks are given to illustrate how their structures can be recorded accurately and interpreted.

Igneous rocks are a fundamental component of the Earth and are commonly encountered during geological fieldwork. This chapter describes techniques used to draw field sketches of intrusions and volcanic features, such as lava flows, volcanic craters, and pyroclastic sequences. The most important features of igneous rocks to record are discussed. Recording the nature of contacts is particularly important in drawing igneous outcrops, in particular cross-cutting relationships that relate to emplacement timing. Four worked examples of field sketches of igneous geology are provided to illustrate concepts in their description in during fieldwork. These include the summit crater of Vesuvius, lava flows from Mount Etna, and pyroclastic deposits from Santorini.

Geological cross-sections are graphical representations of the distribution of rock units in vertical planes cut through landscapes. Cross-sections extrapolate the boundaries between rock units above and below ground. This chapter provides practical advice on how to create geological cross-sections through the interpretation of maps with an emphasis on recognizing the key features that reveal the orientation of boundaries and structures. Topography lines can be sketched with reasonable accuracy rather than transferred from maps. The use of sketch cross-sections as an interpretative diagram is introduced, and examples given to illustrate how these can be used in field notes to assist in the analysis of geological structures.

Sedimentary rocks are the commonest rocks found on the surface of the Earth’s crust and record much of the history of both our planet and life on Earth. This chapter describes how to draw outcrops of sedimentary rocks in the field and the most important features of these rocks to record and describe. The stratigraphy and interpretation of sedimentary rocks is also considered in the chapter and includes a description of common sedimentary structures. The use of sedimentary facies in evaluation of depositional environment is introduced. Five worked examples of field sketches of sedimentary outcrops are given to illustrate how to make accurate and detailed observations of sediments. Examples include how to draw unconformities, sedimentary structures, lithologies, and graphic logs.

Natural outcrops of rock often have significant topography that makes them difficult to record in field sketches. This chapter describes the methods used to draw outcrops in three dimensions. The concepts of perspective and its influence on the appearance of strata are described as well as the tactics that can be used to simplify the drawing of natural outcrops. Three worked examples of field sketches are included in the chapter to illustrate how best to record the geology exposed on surfaces with significant relief. These include sketching of large three-dimensional outcrops and drawing of coastal cliffs. Common mistakes in drawing outcrops with relief are described.

Schematic diagrams are invaluable in Earth Science in the communication of concepts in both publications and teaching. These diagrams can also be used during fieldwork to help develop interpretations of structures, palaeoenvironments, and geological processes. This chapter describes methods used to create block diagrams of structures and landscapes, surface diagrams of folds and faults, and process diagrams used to summarize interpretations of a wide range of processes. Schematic diagrams can and should be included in field notes. Process diagrams represent a cartoon of interpretative thoughts. Four worked examples of schematic diagrams are provided to illustrate their uses and construction.

Drawings of landscapes are often used to record large-scale geological structures in Earth Science. This chapter describes how to draw geological features exposed within landscapes with a focus on the methods used to produce accurate and detailed field sketches. The tactics in drawing large fields of view, landscapes with significant topography, and vegetated landscapes with sparse outcrops are described. Three worked examples of landscape field sketches are provided to illustrate how best to record tectonic structures and volcanic features exposed over topography. Topographic features such as ravines and ridges can be added as lines to add additional value. Common mistakes considered include insufficient detail and schematic vegetation.

Rocks and minerals are naturally occurring solid materials that make up planetary bodies such as the Earth and are highly diverse. This chapter describes how to record the mineralogy and petrology of rocks using drawings and includes methods used to graphically record hand-specimens of rocks and crystals. Four worked examples of drawings of hand-specimens are included to illustrate techniques and the important features to record. Practical advice on how to identify rock types and minerals is also given. The outline of the specimen is important since it guides the placement of other features. The petrology must be simplified to make drawing practical.

Drawing and illustrating in Earth Science has a fundamentally different focus to artistic illustration and requires different techniques. The methods used in drawing geology are described in this chapter, including the basic techniques of general drawing and those methods that are specific to geology. The chapter examines which outcrops should be drawn and introduces three rules that should be applied to every field sketch. The methods to ensure field sketches of outcrops are accurate and detailed are described, with practical advice on how to apply line style, colour, and shading in geological diagrams. Finally, aspects of drawing such as posture and position are described that will help improve the quality of sketches.

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 enhances the survival of MMs and increases 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 micrometeorite 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 475 µm survive unmelted on Mars. Approximately 10% of particles experience temperatures below ~800 K, i.e. 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 suggests a much higher fraction of organic material surviving to the martian surface.

We report the discovery of a partially altered microchondrule within a fine-grained micrometeorite.This object is circular

This study explores the petrology of five giant (>400 μm) hydrated fine-grained micrometeorites from the Transantarctic Mountain (TAM) micrometeorite collection. For the first time, the extent and mechanisms of aqueous alteration in unmelted cosmic dust are evaluated and quantified. We use a range of criteria, previously defined for use on hydrated chondrites, including phyllosilicate fraction, matrix geochemistry and micro textures. Collectively, these micrometeorites represent ∼2.22 mm2 of intensely altered hydrated chondritic matrix (with petrologic subtypes of

Comets 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 ( 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.

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

We report on the discovery of microtektites (microscopic impact glass spherules) in a glacial moraine near Larkman Nunatak in the Transantarctic Mountains, Antarctica. The microtektites were identified based on their physical and chemical properties. Major and trace element compositions of the particles suggest that they may be related to the Australasian strewn field. This would further extend the current strewn field ∼800 km southward. Depletion in volatiles and enrichment in refractory elements in Larkman Nunatak microtektites fit the volatilization trend defined by Australasian microtektites, suggesting that they may represent a new highly vapor fractionated end-member thereof. This observation is supported by their low vesicularity and absence of mineral inclusions. This discovery has significant implications for the formation of microtektites (i.e. their evolution with respect to the distance from the source crater). Finally, the discovery of potentially old (i.e. 0.8 Ma) microtektites in moraine has implications for the stability of the East Antarctic Ice Sheet in the Larkman Nunatak area over the last ∼1 Ma and, as a consequence, the high efficiency of such moraines as traps for other extraterrestrial materials (e.g. micrometeorites and meteoritic ablation debris)., SCOPUS: ar.j, info:eu-repo/semantics/published

Meteorite fusion crust formation is a brief event in a high-temperature (2000- 12,000 K) and high-pressure (2-5 MPa) regime. We studied fusion crusts and bulk samples of 10 ordinary chondrite falls and 10 ordinary chondrite finds. The fusion crusts show a typical layering and most contain vesicles. All fusion crusts are enriched in heavy Fe isotopes, with d 56 Fe values up to +0.35& relative to the solar system mean. On average, the d 56 Fe of fusion crusts from finds is +0.23&, which is 0.08& higher than the average from falls (+0.15&). Higher d 56 Fe in fusion crusts of finds correlate with bulk chondrite enrichments in mobile elements such as Ba and Sr. The d 56 Fe signature of meteorite fusion crusts was produced by two processes (1) evaporation during atmospheric entry and (2) terrestrial weathering. Fusion crusts have either the same or higher d 18 O (0.9-1.5&) than their host chondrites, and the same is true for D 17 O. The differences in bulk chondrite and fusion crust oxygen isotope composition are explained by exchange of oxygen between the molten surface of the meteorites with the atmosphere and weathering. Meteorite fusion crust formation is qualitatively similar to conditions of chondrule formation. Therefore, fusion crusts may, at least to some extent, serve as a natural analogue to chondrule formation processes. Meteorite fusion crust and chondrules exhibit a similar extent of Fe isotope fractionation, supporting the idea that the Fe isotope signature of chondrules was established in a high-pressure environment that prevented large isotope fractionations. The exchange of O between a chondrule melt and an 16 O-poor nebula as the cause for the observed nonmass dependent O isotope compositions in chondrules is supported by the same process, although to a much lower extent, in meteorite fusion crusts.

I-type cosmic spherules are micrometeorites that formed by melting during atmospheric entry and consist mainly of iron oxides and FeNi metal. I-types are important because they can readily be recovered from sedimentary rocks allowing study of solar system events over geological time. We report the results of a study of the mineralogy and petrology of 88 I-type cosmic spherules recovered from Antarctica in order to evaluate how they formed and evolved during atmospheric entry, to constrain the nature of their precursors and to establish rigorous criteria by which they may be conclusively identified within sediments and sedimentary rocks. Two textural types of I-type cosmic spherule are recognised: (1) metal bead-bearing (MET) spherules dominated by Ni-poor (100 and suggest that metal from H-group ordinary, CM, CR and iron meteorites may form the majority of particles. Oxidation during entry heating increases in the series MET < magnetite-poor OX < magnetite-rich OX spherules owing to differences in particle size, entry angle and velocity. Magnetite-poor OX spherules are shown to form by crystallisation of non-stoichiometric wüstite at the liquidus followed by sub-solidus decomposition to magnetite, whilst in magnetite-rich OX spherules magnetite crystallises directly at the liquidus. Magnetite rims found on most particles are suggested to form by oxidation during sub-solidus flight. The separation of metal beads due to deceleration is proposed to have been minor with most OX spherules shown to have been in equilibrium with metal beads containing >80 wt% Ni comprising a particle mass fraction of

Scoriaceous micrometeorites are highly vesicular extraterrestrial dust particles that have experienced partial melting during atmo - spheric entry. We report the occurrence of clusters of anhedral relict forsterite crystals within these particles that testify to in situ frag - mentation. The absence of similar clusters within unmelted micro - meteorites suggests that fragmentation occurs during atmospheric entry rather than by parent body shock reprocessing. Clusters of broken forsterite crystals are shown to form as a result of fracturing owing to thermal stress developed during entry heating and require thermal gradients of >200 K μm –1 in order for differential thermal expansion to exceed the critical shear strength of olivine. Thermal gradients of this magnitude significantly exceed those resulting from thermal conduction and require the endothermic decomposition of phyllosilicates. Fragmented relict forsterite within scoriaceous micro - meteorites, therefore, indicate that the precursor grains were similar to CI and CM2 chondrites and retained phyllosilicate prior to atmo- spheric entry and thus were not dehydrated on the parent asteroid by shock or thermal metamorphism. Explosive fragmentation of hydrous asteroids during collisions, therefore, does not significantly bias the interplanetary dust population.

International audience; High-precision oxygen isotopic compositions of eighteen large cosmic spherules (> 500 mu m diameter) from the Atacama Desert, Chile, were determined using IR-laser fluorination - Isotope Ratio Mass spectrometry. The four discrete isotopic groups defined in a previous study on cosmic spherules from the Transantarctic Mountains (Suavet et al., 2010) were identified, confirming their global distribution. Approximately 50% of the studied cosmic spherules are related to carbonaceous chondrites, 38% to ordinary chondrites and 12% to unknown parent bodies. Approximately 90% of barred olivine (BO) cosmic spherules show oxygen isotopic compositions suggesting they are related to carbonaceous chondrites. Similarly, similar to 90% porphyritic olivine (Po) cosmic spherules are related to ordinary chondrites and none can be unambiguously related to carbonaceous chondrites. Other textures are related to all potential parent bodies. The data suggests that the textures of cosmic spherules are mainly controlled by the nature of the precursor rather than by the atmospheric entry parameters. We propose that the Po texture may essentially be formed from a coarse-grained precursor having an ordinary chondritic mineralogy and chemistry. Coarse-grained precursors related to carbonaceous chondrites (i.e. chondrules) are likely to either survive atmospheric entry heating or form V-type cosmic spherules. Due to the limited number of submicron nucleation sites after total melting, ordinary chondrite-related coarse-grained precursors that suffer higher peak temperatures will preferentially form cryptocrystalline (Cc) textures instead of BO textures. Conversely, the BO textures would be mostly related to the fine-grained matrices of carbonaceous chondrites due to the wide range of melting temperatures of their constituent mineral phases, allowing the preservation of submicron nucleation sites. Independently of the nature of the precursors, increasing peak temperatures form glassy textures.

Basaltic micrometeorites (MMs) derived from HED-like parent bodies have been found amongst particles collected from the Antarctic and from Arctic glaciers and are to date the only achondritic particles reported amongst cosmic dust. The majority of Antarctic basaltic particles are completely melted cosmic spherules with only one unmelted particle recognised 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 modelling is used to simulate the melting behaviour 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 behaviour 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 behaviour, 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

Micrometeorites (MMs) are extraterrestrial dust particles that survive atmospheric entry and can be recovered from sedimentary rocks. Fossil MMs allow events beyond the Earth, such as the collisional breakup of asteroids, to be identified. Here the effects of vesicle formation during melting of dust are investigated through numerical modeling and observations of Antarctic MMs. Vesicle formation is shown to cause a parachute effect that causes rapid deceleration, decreasing peak temperature. Vesicular parachuting enhances the abundance of melted MMs formed from phyllosilicate-bearing C-type asteroid dust on the Earth surface by a factor of 2. Micrometeorites recovered from the geological record, therefore, are biased toward breakup events involving hydrated C-type asteroids, whilst those involving phyllosilicate-poor particles are diluted by the enhanced background flux of hydrous dust. The parachute effect is also likely to increase the delivery of 3He to ocean sediments by C-type asteroid dust.

Carbonatites are familiar to students of petrology as rare igneous rocks formed predominantly of carbonate, whose only modern expression is a single active volcano that erupts strongly alkaline carbonate lavas with no direct match in Earth’s geological record (see Lengai movie in the electronic version of this chapter or on the MSA RiMG website). Based on their Sr-Nd-Pb isotopic data, stable isotopic compositions, noble gases, and experimental phase equilibria, they are derived from the mantle, showing almost no sign of contamination by the crust. As liquids, carbonate melts have remarkable physical properties, which set them apart from the alkaline silicate melts with which they are often temporally associated. They show very high solubilities of many elements considered rare in silicate magmas, and they have the highest known melt capacities for dissolving water and other volatile species like halogens at crustal pressures. They are highly efficient transport agents of carbon from the mantle to the crust, remaining mobile over extraordinary ranges of temperature, and their very low viscosity should enhance connectivity along grain boundaries in the mantle where they are implicated in geochemical enrichment processes related to metasomatism. Most carbonatites have unambiguous origins in the mantle and the limit to their depth is not known, but the likelihood that they may exist in the lower mantle (Kaminsky et al. 2009, 2012; Stoppa et al. 2009) needs to be appraised since they may exert a fundamental control on the mobility and long-term storage of deep carbon in Earth. Ultimately the stability of carbonate melt is an extension of the stability of carbonate minerals (Hazen et al. 2013a,b) subject critically to the mantle oxidation state (Luth 1993; Frost and McCammon 2008); carbonate-melts have also been predicted in the oceanic low-velocity zone and deep mantle (Hauri et al. 1993 …

EnVision is an ambitious but low-risk response to ESA's call for a medium-size mission opportunity for a launch in 2022. Venus is the planet most similar to Earth in mass, bulk properties and orbital distance, but has evolved to become extremely hostile to life. EnVision's 5-year mission objectives are to determine the nature of and rate of change caused by geological and atmospheric processes, to distinguish between competing theories about its evolution and to help predict the habitability of extrasolar planets. Three instrument suites will address specific surface, atmosphere and ionosphere science goals. The Surface Science Suite consists of a 2.2 m 2 radar antenna with Interferometer, Radiometer and Altimeter operating modes, supported by a complementary IR surface emissivity mapper and an advanced accelerometer for orbit control and gravity mapping. This suite will determine topographic changes caused by volcanic, tectonic and atmospheric processes at rates as low as 1 mm a -1. The Atmosphere Science Suite consists of a Doppler LIDAR for cloud top altitude, wind speed and mesospheric structure mapping, complemented by IR and UV spectrometers and a spectrophotopolarimeter, all designed to map the dynamic features and compositions of the clouds and middle atmosphere to identify the effects of volcanic and solar processes. The Ionosphere Science Suite uses a double Langmiur probe and vector magnetometer to understand the behaviour and long-term evolution of the ionosphere and induced magnetosphere. The suite also includes an interplanetary particle analyser to determine the delivery rate of water and other components to the atmosphere. © 2011 Springer Science+Business Media B.V.

Micrometeorites (MMs) are extraterrestrial dust particles, in the size range 25–400 μm, recovered from the Earth’s surface. They have experienced a wide range of heating during atmospheric entry from completely molten spherules to particles heated to temperatures

It is widely accepted that Earth’s early atmosphere contained less than 0.001 per cent of the present-day atmospheric oxygen (O2) level, until the Great Oxidation Event resulted in a major rise in O2 concentration about 2.4 billion years ago1. There are multiple lines of evidence for low O2 concentrations on early Earth, but all previous observations relate to the composition of the lower atmosphere2 in the Archaean era; to date no method has been developed to sample the Archaean upper atmosphere. We have extracted fossil micrometeorites from limestone sedimentary rock that had accumulated slowly 2.7 billion years ago before being preserved in Australia’s Pilbara region. We propose that these micrometeorites formed when sand-sized particles entered Earth’s atmosphere and melted at altitudes of about 75 to 90 kilometres (given an atmospheric density similar to that of today3). Here we show that the FeNi metal in the resulting cosmic spherules was oxidized while molten, and quench-crystallized to form spheres of interlocking dendritic crystals primarily of magnetite (Fe3O4), with wüstite (FeO)+metal preserved in a few particles. Our model of atmospheric micrometeorite oxidation suggests that Archaean upper-atmosphere oxygen concentrations may have been close to those of the present-day Earth, and that the ratio of oxygen to carbon monoxide was sufficiently high to prevent noticeable inhibition of oxidation by carbon monoxide. The anomalous sulfur isotope (Δ33S) signature of pyrite (FeS2) in seafloor sediments from this period, which requires an anoxic surface environment4, implies that there may have been minimal mixing between the upper and lower atmosphere during the Archaean.

Melting of micrometeorites (MMs) due to atmospheric entry heating causes significant changes in the textures, mineralogies and compositions of particles that obscure their primary properties and greatly complicate interpretation of these extraterrestrial materials. Despite the abundance of melted MMs, the nature of melting processes in these materials is poorly constrained. In this study, mineralogical, textural, and compositional data on 77 MMs with igneous rims are presented, which suggest that fusion of micrometeorites during atmospheric entry occurs initially by surface melting. Textural and mineralogical evidence are presented that demonstrate unequivocally that igneous rims crystallized from a melt surrounding a largely unmelted core and establish melting as a gradational process. The compositions of igneous rims on fine-grained MMs (fgMMs) are broadly similar to those of the unmelted core except for depletions in volatile and moderately volatile elements produced by partial evaporation and suggest the formation of the rims by melting of the fine-grained matrix core. Enrichments in Fe/Si, Ni/Si, and Mn/Si in igneous rims compared with unmelted cores within fgMMs are suggested to occur due to the migration of Fe–S eutectic liquids from the core of the particle into the surface melt layer. The sulphide liquids are probably generated in the core of the particle under reducing conditions resulting by the pyrolysis of carbonaceous materials. The presence of igneous rims on fgMMs is enabled by the thermal decomposition of phyllosilicates and, therefore, indicates that fgMMs were hydrated particles prior to atmospheric entry. In contrast the compositions of igneous rims on coarse-grained MMs (cgMMs) are difficult to reconcile with those of the unmelted core and instead closely resemble those of fgMMs. The igneous rims of cgMMs are, therefore, suggested to form by melting of fine-grained matrix that was present on the exterior of these particles prior to atmospheric entry. Coarse-grained MMs with igneous rims thus were originally composite micrometeoroids, which consist of both fine-grained and coarse-grained materials, and are thought to be samples of chondrule-like objects. The abundance of composite MMs to cgMMs allows a first estimate of the mean chondrule radius within the parent bodies of MMs of ∼625 μm to be made.

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.