Bullet-Shaped Magnetite Biomineralization Within a Magnetotactic Deltaproteobacterium: Implications for Magnetofossil Identification

Magnetite produced by magnetotactic bacteria (MTB) provides stable paleomagnetic signals because it occurs as natural single‐domain magnetic nanocrystals. MTB can also provide useful paleoenvironmental information because their crystal morphologies are associated with particular bacterial groups and the environments in which they live. However, identification of the fossil remains of MTB (i.e., magnetofossils) from ancient sediments or rocks is challenging because of their generally small sizes and because the growth, morphology, and chain assembly of magnetite within MTB are not well understood. Nanoscale characterization is, therefore, needed to understand magnetite biomineralization and to develop magnetofossils as biogeochemical proxies for paleoenvironmental reconstructions. Using advanced transmission electron microscopy, we investigated magnetite growth and chain arrangements within magnetotactic Deltaproteobacteria strain WYHR‐1, which reveals how the magnetite grows to form elongated, bullet‐shaped nanocrystals. Three crystal growth stages are recognized: (i) initial isotropic growth to produce nearly round ~20 nm particles, (ii) subsequent anisotropic growth along the [001] crystallographic direction to ~75 nm lengths and ~30-40 nm widths, and (iii) unidirectional growth along the [001] direction to ~180 nm lengths, with some growing to ~280 nm. Crystal growth and habit differ from that of magnetite produced by other known MTB strains, which indicates species‐specific biomineralization. These findings suggest that magnetite biomineralization might be much more diverse among MTB than previously thought. When characterized adequately at species level, magnetofossil crystallography, and apomorphic features are, therefore, likely to become useful proxies for ancient MTB taxonomic groups or species and for interpreting the environments in which they lived.This study was supported financially by the National Natural Science Foundation of China (grants no. 41920104009, 41890843, and 41621004), The Senior User Project of RVKEXUE2019GZ06 (Center for Ocean Me Mega‐Science, Chinese Academy of Sciences), and the Australian Research Council (grant DP160100805

Constraining the Origin of Impact Craters on Al Foils from the Stardust Interstellar Dust Collector

Preliminary examination (PE) of the aerogel tiles and Al foils from the Stardust Interstellar Dust Collector has revealed multiple impact features. Some are most likely due to primary impacts of interstellar dust (ISD) grains, and others are associated with secondary impacts of spacecraft debris, and possibly primary impacts of interplanetary dust particles (IDPs) [1, 2]. The current focus of the PE effort is on constraining the origin of the individual impact features so that definitive results from the first direct laboratory analysis of contemporary ISD can be reported. Because crater morphology depends on impacting particle shape and composition, in addition to the angle and direction of impact, unique particle trajectories are not easily determined. However, elemental analysis of the crater residues can distinguish real cosmic dust from the spacecraft debris, due to the low cosmic abundance of many of the elements in the spacecraft materials. We present here results from the elemental analysis of 24 craters and discuss the possible origins of 4 that are identified as candidate ISD impact

Coordinated Microanalyses of Seven Particles of Probable Interstellar Origin from the Stardust Mission

Stardust, a NASA Discovery-class mission, was the first sample-return mission to return solid samples from beyond the Moon. Stardust was effectively two missions in one spacecraft: it returned the first materials from a known primitive solar system body, the Jupiter-family comet Wild 2; Stardust also returned a collector that was exposed to the contemporary interstellar dust stream for 200 days during the interplanetary cruise. Both collections present severe technical challenges in sample preparation and in analysis. By far the largest collection is the cometary one: approximately 300 micro g of material was returned from Wild 2, mostly consisting of approx. 1 ng particles embedded in aerogel or captured as residues in craters on aluminum foils. Because of their relatively large size, identification of the impacts of cometary particles in the collection media is straightforward. Reliable techniques have been developed for the extraction of these particles from aerogel. Coordinated analyses are also relatively straightforward, often beginning with synchrotron-based x-ray fluorescence (S-XRF), X-ray Absorption Near-Edge Spectoscopy (XANES) and x-ray diffraction (S-XRD) analyses of particles while still embedded in small extracted wedges of aerogel called ``keystones'', followed by ultramicrotomy and TEM, Scanning Transmission X-ray Microscopy (STXM) and ion microprobe analyses (e.g., Ogliore et al., 2010). Impacts in foils can be readily analyzed by SEM-EDX, and TEM analysis after FIB liftout sample preparation. In contrast, the interstellar dust collection is vastly more challenging. The sample size is approximately six orders of magnitude smaller in total mass. The largest particles are only a few pg in mass, of which there may be only approx.10 in the entire collection. The technical challenges, however, are matched by the scientific importance of the collection. We formed a consortium carry out the Stardust Interstellar Preliminary Examination (ISPE) to carry out an assessment of this collection, partly in order to characterize the collection in sufficient detail so that future investigators could make well-informed sample requests. The ISPE is the sixth PE on extraterrestrial collections carried out with NASA support. Some of the basic questions that we asked were: how many impacts are there in the collector, and what fraction of them have characteristics consistent with extraterrestrial materials? What is the elemental composition of the rock-forming elements? Is there crystalline material? Are there organics? Here we present coordinated microanalyses of particles captured in aerogel, using S-FTIR, S-XRF, STXM, S-XRD; and coordinated microanalyses of residues in aluminum foil, using SEMEDX, Auger spectroscopy, STEM, and ion microprobe. We discuss a novel approach that we employed for identification of tracks in aerogel, and new sample preparation techniques developed during the ISPE. We have identified seven particles - three in aerogel and four in foils - that are most consistent with an interstellar origin. The seven particles exhibit a large diversity in elemental composition. Dynamical evidence, supported supported by laboratory simulations of interstellar dust impacts in aerogel and foils, and numerical modeling of interstellar dust propagation in the heliosphere, suggests that at least some of the particles have high optical cross-section, perhaps due to an aggregate structure. However, the observations are most consistent with a variety of morphologie

Final Reports of the Stardust ISPE: Seven Probable Interstellar Dust Particles

The Stardust spacecraft carried the first spaceborne collector specifically designed to capture and return a sample of contemporary interstellar dust to terrestrial laboratories for analysis [1]. The collector was exposed to the interstellar dust stream in two periods in 2000 and 2002 with a total exposure of approximately 1.8 10(exp 6) square meters sec. Approximately 85% of the collector consisted of aerogel, and the remainder consisted of Al foils. The Stardust Interstellar Preliminary Examination (ISPE) was a consortiumbased effort to characterize the collection in sufficient detail to enable future investigators to make informed sample requests. Among the questions to be answered were these: How many impacts are consistent in their characteristics with interstellar dust, with interplanetary dust, and with secondary ejecta from impacts on the spacecraft? Are the materials amorphous or crystalline? Are organics detectable? An additional goal of the ISPE was to develop or refine the techniques for preparation, analysis, and curation of these tiny samples, expected to be approximately 1 picogram or smaller, roughly three orders of magnitude smaller in mass than the samples in other small particle collections in NASA's collections - the cometary samples returned by Stardust, and the collection of Interplanetary Dust Particles collected in the stratosphere

Simulating asteroid impacts and meteor events by high-power lasers : from the laboratory to spaceborne missions

Meteor plasmas and impact events are complex, dynamic natural phenomena. Simulating these processes in the laboratory is, however, a challenge. The technique of laser induced dielectric breakdown was first used for this purpose almost 50 years ago. Since then, laser-based experiments have helped to simulate high energy processes in the Tunguska and Chicxulub impact events, heavy bombardment on the early Earth, prebiotic chemical evolution, space weathering of celestial bodies and meteor plasma. This review summarizes the current level of knowledge and outlines possible paths of future development.Czech Science FoundationCzech Academy of Sciences Program of Regional Cooperatio

Microanalysis of Hypervelocity Impact Residues of Possible Interstellar Origin

The NASA Stardust spacecraft deployed two collector trays, one dedicated to the collection of dust from Comet Wild 2, and the other for the capture of interstellar dust (ISD). The samples were returned successfully to Earth in 2006, and now provide an unprecedented opportunity for laboratory-based microanalysis of materials from the outer solar system and beyond. Results from the cometary sample studies have demonstrated that Wild 2 contains much more refractory condensate material and much less pristine extra-solar material than expected, which further indicates that there was significant transport of inner solar system materials to the Kuiper Belt in the early solar system [1]. The analysis of the interstellar samples is still in the preliminary examination (PE) phase, due to the level of difficulty in the definitive identification of the ISD features, the overall low abundance, and its irreplaceable nature, which necessitates minimally invasive measurements [2]. We present here coordinated microanalysis of the impact features on the Al foils, which have led to the identification of four impacts that are possibly attributable to interstellar dust. Results from the study of four ISD candidates captured in aerogel are presented elsewhere [2]

Mars: new insights and unresolved questions

Mars exploration motivates the search for extraterrestrial life, the development of space technologies, and the design of human missions and habitations. Here, we seek new insights and pose unresolved questions relating to the natural history of Mars, habitability, robotic and human exploration, planetary protection, and the impacts on human society. Key observations and findings include: – high escape rates of early Mars’ atmosphere, including loss of water, impact present-day habitability; – putative fossils on Mars will likely be ambiguous biomarkers for life; – microbial contamination resulting from human habitation is unavoidable; and – based on Mars’ current planetary protection category, robotic payload(s) should characterize the local martian environment for any life-forms prior to human habitation.Some of the outstanding questions are:– which interpretation of the hemispheric dichotomy of the planet is correct; – to what degree did deep-penetrating faults transport subsurface liquids to Mars’ surface; – in what abundance are carbonates formed by atmospheric processes; – what properties of martian meteorites could be used to constrain their source locations; – the origin(s) of organic macromolecules; – was/is Mars inhabited; – how can missions designed to uncover microbial activity in the subsurface eliminate potential false positives caused by microbial contaminants from Earth; – how can we ensure that humans and microbes form a stable and benign biosphere; and – should humans relate to putative extraterrestrial life from a biocentric viewpoint (preservation of all biology), or anthropocentric viewpoint of expanding habitation of space?Studies of Mars’ evolution can shed light on the habitability of extrasolar planets. In addition, Mars exploration can drive future policy developments and confirm (or put into question) the feasibility and/or extent of human habitability of space

Electron and X-ray Microanalysis of Planetary Materials: from Comet 81P/Wild2 to the Surface of Mars

This thesis concerns the electron and X-ray microanalysis of planetary materials: from Comet 81P/Wild2 to the surface of Mars. Advanced techniques in electron microscopy and X-ray spectroscopy have been developed for the microanalysis of the nakhlite martian meteorites and Comet 81P/Wild2 samples from the Stardust Mission. Electron microprobe analysis and a Focussed Ion Beam - Scanning Electron Microscope (FIB-SEM) technique for Transmission Electron Microscopy (TEM) was used to analyse the secondary mineral assemblages in the nakhlites. Fracture-filling assemblages in the nakhlites are found to be dominated by an amorphous, hydrated Fe-silicate - a ‘gel’. The gel decreases in Mgat/Mgat+Feat ratio going up the expected depth profile of the nakhlites. Other phases, especially 2:1 smectites - 1:1 phyllosilicate and carbonate are associated with the gel. Newly discovered 1:1 phyllosilicate, suggested to be serpentine, is also found in the mesostasis of Lafayette. A model is proposed describing the formation of the nakhlites’ secondary assemblages by an impact-induced hydrothermal system based on the mineralogical and geochemical differences between different samples. A suite of Stardust cometary samples have also been analysed using FIB-TEM and microfocus X-ray spectroscopy that includes: X-ray Fluorescence Spectroscopy (XRF), X-ray Absorption Near-Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) at the Diamond synchrotron. Attempts have been made to distinguish the cometary material from that formed by capture heating in aerogel via the identification of ferric-oxides at track entrances. Finally, the mineralogy and morphology of a terminal particle from Stardust track #154 was studied by analytical TEM. The results show that Comet Wild2 contains a unique Al-diopside-bearing grain, having affinities with the minerals found in refractory objects from the inner Solar System. Upon comparison with different early Solar System materials, the grain’s mineral assemblage most closely resembles Al-rich chondrules. This adds to the refractory inventory identified in Comet 81P/Wild2

Electron and X-ray microanalysis of planetary materials : from Comet 81P/Wild2 to the surface of Mars

This thesis concerns the electron and X-ray microanalysis of planetary materials: from Comet 81P/Wild2 to the surface of Mars. Advanced techniques in electron microscopy and X-ray spectroscopy have been developed for the microanalysis of the nakhlite martian meteorites and Comet 81P/Wild2 samples from the Stardust Mission. Electron microprobe analysis and a Focussed Ion Beam - Scanning Electron Microscope (FIB-SEM) technique for Transmission Electron Microscopy (TEM) was used to analyse the secondary mineral assemblages in the nakhlites. Fracture-filling assemblages in the nakhlites are found to be dominated by an amorphous, hydrated Fe-silicate - a ‘gel’. The gel decreases in Mgat/Mgat+Feat ratio going up the expected depth profile of the nakhlites. Other phases, especially 2:1 smectites - 1:1 phyllosilicate and carbonate are associated with the gel. Newly discovered 1:1 phyllosilicate, suggested to be serpentine, is also found in the mesostasis of Lafayette. A model is proposed describing the formation of the nakhlites’ secondary assemblages by an impact-induced hydrothermal system based on the mineralogical and geochemical differences between different samples. A suite of Stardust cometary samples have also been analysed using FIB-TEM and microfocus X-ray spectroscopy that includes: X-ray Fluorescence Spectroscopy (XRF), X-ray Absorption Near-Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) at the Diamond synchrotron. Attempts have been made to distinguish the cometary material from that formed by capture heating in aerogel via the identification of ferric-oxides at track entrances. Finally, the mineralogy and morphology of a terminal particle from Stardust track #154 was studied by analytical TEM. The results show that Comet Wild2 contains a unique Al-diopside-bearing grain, having affinities with the minerals found in refractory objects from the inner Solar System. Upon comparison with different early Solar System materials, the grain’s mineral assemblage most closely resembles Al-rich chondrules. This adds to the refractory inventory identified in Comet 81P/Wild2.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Hydrothermal evolution of the morphology, molecular composition, and distribution of organic matter in CR (Renazzo-type) chondrites

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